WO2022234110A1

WO2022234110A1 - Gamma-aminobutyric acid derivatives for use in cancer therapy

Info

Publication number: WO2022234110A1
Application number: PCT/EP2022/062321
Authority: WO
Inventors: Manuel Vicente SALINAS-MARTIN
Original assignee: Plus Vitech, S.L.
Priority date: 2021-05-07
Filing date: 2022-05-06
Publication date: 2022-11-10

Abstract

The present invention relates to the treatment of cancer using a gamma-aminobutyric acid derivative, or pharmaceutically acceptable salt or prodrug thereof.

Description

CANCER THERAPY FIELD OF THE INVENTION The present invention relates to the treatment of cancer using a gamma- aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt or prodrug thereof. BACKGROUND TO THE INVENTION The invention concerns a class of gamma-aminobutyric acid derivatives. The drug pregabalin, which is a member of this class, is currently authorized for the treatment of peripheral and central neuropathic pain in adults, for the treatment of partial seizures with secondary generalization, and for the treatment of generalized anxiety disorder (GAD) in adults. This class of gamma-aminobutyric acid derivatives, as exemplified by pregabalin, has been shown in clinical studies to be safe for use in patients. Accordingly, pregabalin has been authorised for use in humans for the aforementioned uses, including treatment of peripheral and central neuropathic pain in adults. However, there has been no data published indicating that gamma-aminobutyric acid derivatives including pregabalin may be efficacious in the treatment of cancer. Aprepitant and its prodrug fosaprepitant are neurokinin 1 (NK1) inhibitors that have been approved for treating nausea and vomiting, for example acute or delayed chemotherapy-induced nausea and vomiting, or post-operative nausea and vomiting. Aprepitant has also been investigated for use in treating a variety of other diseases, including depression and cancer. SUMMARY OF THE INVENTION It has now surprisingly been found that a class of gamma-aminobutyric acid derivatives represented by formula (I), including pregabalin, are effective in the treatment of cancer. In particular, as described in the Examples below, such compounds have now been shown to prevent or inhibit cancer cell proliferation and tumour growth. These compounds have been shown to promote apoptosis of cancer cell lines in vitro and to inhibit the growth of cancer cells in vitro. The efficacy of compounds of formula (I) in the inhibition of cancer cell activity has been demonstrated with a variety of different cancer cell lines including breast cancer, lung cancer, colon cancer, melanoma, lymphoma, leukemia, pancreatic cancer, renal cancer, prostate cancer and bladder cancer cells. In addition, the inventors have surprisingly found that when the gamma- aminobutyric acid derivatives (including pregabalin) are administered to cancer cells in combination with an NK1 receptor antagonist (such as aprepitant or maropitant), the combination shows a synergistic effect. The combination of gamma-aminobutyric acid derivative and NK1 receptor antagonist is more effective in inhibiting or preventing cell proliferation, and in some cases promoting apoptosis of cancer cells, than would be expected from the individual efficacies of the two components. Accordingly, the present invention provides a gamma-aminobutyric acid derivative for use in a method of treating cancer in a patient in need thereof, wherein said gamma- aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof:

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group. The present invention further provides a pharmaceutical composition for use in a method of treating cancer in a patient in need thereof, comprising a gamma-aminobutyric acid derivative together with a pharmaceutically acceptable excipient, wherein the gamma- aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt or prodrug thereof:

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group. The invention further provides a method of treating cancer in a patient in need thereof, which method comprises administering to said patient a gamma-aminobutyric acid derivative or a composition comprising a gamma-animobutyric acid derivative together with a pharmaceutically acceptable excipient, wherein said gamma-aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt or prodrug thereof:

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group. In particular, the invention provides a method of treating cancer in a patient in need thereof, which method comprises administering to said patient a gamma-aminobutyric acid derivative or a composition comprising a gamma-animobutyric acid derivative as defined herein, in combination with an NK1 receptor antagonist. In a preferred aspect of this embodiment, the NK1 receptor is aprepitant, fosaprepitant or maropitant, or a pharmaceutically acceptable salt or prodrug thereof. In a further preferred aspect of this embodiment, the NK1 receptor antagonist is aprepitant, or a pharmaceutically acceptable salt or prodrug thereof. The invention further provides the use of a gamma-aminobutyric acid derivative in the manufacture of a medicament for the treatment of cancer in a patient in need thereof, wherein said gamma-aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt or prodrug thereof:

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a representative microscopic image of a population of breast cancer cells after incubation for a period of 32 hours in the presence of 80 µM pregabalin. Figure 2 shows a representative microscopic image of a population of colon cancer cells after incubation for a period of 32 hours in the presence of 80 µM pregabalin. DETAILED DESCRIPTION The present invention is concerned with the treatment of cancer using a gamma- aminobutyric acid derivative which is a compound of formula (I), or pharmaceutically acceptable salt or prodrug thereof. In particular, the invention is concerned with the treatment of cancer using a combination of a gamma-aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt or prodrug thereof, in combination with an NK1 receptor antagonist. The invention is also concerned with the treatment of cancer using a combination of a gamma-aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt or prodrug thereof, in combination with an anti-tumour agent. Gamma-aminobutyric acid derivatives The compound used in the present invention is a gamma-aminobutyric acid derivative of formula (I), or a pharmaceutically acceptable salt or prodrug thereof

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group. An alkyl group may be a straight-chain or branched-chain alkyl group. C_1-6 alkyl includes methyl, ethyl, propyl, butyl, pentyl and hexyl. C_3-6 cycloalkyl includes cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. Preferably an alkyl group is unsubstituted. Preferably R₁ is a C_1-6 alkyl group, more preferably a -(CH₂)_0-2-iC₄H₉ group and most preferably an -iC₄H₉ group (i.e., an isobutyl group). Preferably R₂ is hydrogen. Preferably R₃ is hydrogen. A particularly preferred compound of formula (I) is one in which R₁ is an -iC₄H₉ group and R₂ and R₃ are both hydrogen. A particularly preferred gamma-aminobutyric acid derivative is the compound of formula (I) in which R₁ is an -iC₄H₉ group and R₂ and R₃ are both hydrogen. Compounds of formula (I) can contain one or several asymmetric carbon atoms. The invention includes the individual diastereomers or enantiomers, and the mixtures thereof. The individual diastereomers or enantiomers may be prepared or isolated by methods already well-known in the art. Most preferably the compound of formula (I) is pregabalin, i.e. (3S)-3- (aminomethyl)-5-methylhexanoic acid, or a pharmaceutically acceptable salt thereof, or a prodrug thereof. For example, the gamma-aminobutyric acid derivative is pregabalin. Pregabalin has the following structure:

. As used herein, a pharmaceutically acceptable salt is a salt with a pharmaceutically acceptable acid or base. Pharmaceutically acceptable acids include both inorganic acids such as hydrochloric, sulphuric, hydrosulphuric, phosphoric, diphosphoric, hydrobromic or nitric acid and organic acids such as citric, fumaric, maleic, malic, ascorbic, succinic, tartaric, benzoic, mandelic, acetic, methanesulphonic, ethanesulphonic, benzenesulphonic or p-toluenesulphonic acid. Pharmaceutically acceptable bases include alkali metal (e.g. sodium or potassium) and alkali earth metal (e.g. calcium or magnesium) hydroxides and organic bases such as alkyl amines such as meglumine, aralkyl amines or heterocyclic amines. Typically, the pharmaceutically acceptable acid or base that forms the pharmaceutically acceptable salt with the gamma-aminobutyric acid is not itself a therapeutic agent. In other words, the pharmaceutically acceptable acid or base is usually a species which does not produce a significant therapeutic effect if administered separately to the gamma-aminobutyric acid derivative. The pharmaceutically acceptable acid or base may therefore be described as a non-therapeutically active acid or a non-therapeutically active base. For example, the pharmaceutically acceptable acid or base may be a pharmaceutically acceptable acid or base which is not an anti-cancer agent (an agent having a therapeutic effect against cancer), or an anti-diabetic agent (an agent having a therapeutic effect against Type II diabetes). For example, the pharmaceutically acceptable acid or base is typically not metformin, or a derivative of metformin. As used herein, a prodrug of a compound of formula (I) is a structural analogue of a compound of formula (I) which is transformed in the body into a compound of formula (I) or a species which mimics the biological activity of the compound of formula (I). For example a prodrug of pregabalin is a structural analogue of pregabalin which is transformed in the body into pregabalin itself or a species which mimics the biological activity of pregabalin. By “in the body” is meant within the human or animal body following administration of the prodrug to the human or animal. The compound of formula (I) is not typically formulated as a prodrug. Thus, preferably, the compound used in the present invention is a compound of formula (I) or a pharmaceutically acceptable salt thereof, for example pregabalin or a pharmaceutically acceptable salt thereof. More preferably, the compound used in the present invention is a compound of formula (I), for instance pregabalin. Synthetic methods for preparing compounds of formula (I) and pharmaceutically salts thereof are well known in the art. For example, suitable methods are described in WO 98/003167, the contents of which are herein incorporated by reference in their entirety. NK1 receptor antagonists The gamma-aminobutyric acid derivative may be used in combination with an NK1 receptor antagonist. An NK1 receptor antagonist is any molecule that reduces the function or activity of NK1. The NK1 receptor antagonist may reduce function or activity of NK1 by any amount. The NK1 receptor antagonist may reduce NK1 function or activity by at least 10%, at least 20%, at least 30% at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or at least 95%, or may prevent any NK1 function or activity. The extent to which an NK1 receptor antagonist reduces NK1 function or activity may be determined by measuring NK1 function or activity in cells in the presence and absence of the NK1 receptor antagonist. The cells may be normal cells or cancer cells. For example, the cells may be cancer cells as described herein, for instance lung cancer, colon cancer or breast cancer cells. Typically, the antagonist is an agent which interacts with the NK1 receptor. An agent that interacts with the NK1 receptor is typically an agent which binds to the NK1 receptor. Example of suitable NK1 receptor antagonists which may be used in combination with the gamma-aminobutyric acid include one or more of Aprepitant, Fosaprepitant, Netupitant, Maropitant, Vestipitant, Casopitant, Vofopitant, Ezlopitant, Lanepitant, LY- 686017, L-733,060, L-732,138, L -703,606, WIN 62,577, CP-122721, TAK-637, R673, CP-100263, WIN 51708, CP-96345, L-760735, CP-122721, L-758298, L-741671, L-742 694, CP-99994 or T-2328; or a pharmaceutically acceptable salt or prodrug thereof. For instance, the NK1 receptor antagonist which may be used in combination with the gamma- aminobutyric acid include one or more of Aprepitant, Fosaprepitant, Netupitant, Maropitant, Vestipitant, Casopitant, Vofopitant, Ezlopitant, Lanepitant, LY-686017, L- 733,060, L-732,138, L -703,606, WIN 62,577, CP-122721, TAK-637, R673, CP-100263, WIN 51708, CP-96345, L-760735, CP-122721, L-758298, L-741671, L-742694, CP- 99994 or T-2328; or a pharmaceutically acceptable salt thereof. In particular, the NK1 receptor antagonist which may be used in combination with the gamma-aminobutyric acid includes one or more of Aprepitant, Fosaprepitant, and Maropitant; or a pharmaceutically acceptable salt thereof. Preferred among these is aprepitant, or a pharmaceutically acceptable salt or prodrug thereof. Pharmaceutically acceptable salts or prodrugs are as defined above in relation to the gamma-aminobutyric acid derivative. Aprepitant has the following structure:

Aprepitant is not typically formulated in the form of a pharmaceutically acceptable salt. Thus, preferably the NK1 receptor antagonist used in the present invention is aprepitant. Fosaprepitant is prodrug of aprepitant and has the following structure:

Fosaprepitant is typically provided in the form of a pharmaceutically acceptable salt, preferably in the form of the dimeglumine salt:

Thus, in a further preferred aspect of the invention, the compound used in the present invention is fosaprepitant dimeglumine. Pharmaceutically acceptable salts of fosaprepitant, such as fosaprepitant dimeglumine, are typically reconstituted in an aqueous solvent, such as saline, prior to administration, thereby providing an aqueous solution comprising fosaprepitant. Fosaprepitant is converted in vivo to aprepitant. Thus, when administered to a patient, typically intravenously, fosaprepitant is converted to aprepitant. Maropitant has the following structure.

Maropitant may be provided in the form of a pharmaceutically acceptable salt. For instance, Maropitant is often provided in the form of a citrate. Thus, in a further preferred aspect of the invention, the NK1 receptor antagonist is a pharmaceutically acceptable salt of Maropitant such as a citrate salt. Anti-tumour agent The gamma-aminobutyric acid derivative may be used in combination with an NK1 receptor antagonist. An anti-tumour agent is an agent which is capable of acting to reduce the growth of a tumour. For instance, an anti-tumour agent may slow the growth of a tumour or prevent the growth of a tumour; in some cases the anti-tumour agent may reduce the size of a tumour. The anti-tumour agent may be a kinase inhibitor, particularly a tyrosine kinase inhibitor. A tyrosine kinase inhibitor is an agent which reduces the function or activity of one or more tyrosine kinases. The tyrosine kinase may reduce function or activity of one or more tyrosine kinase enzymes by any amount. The tyrosine kinase inhibitor may tyrosine kinase function or activity by at least 10%, at least 20%, at least 30% at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or at least 95%, or may prevent any tyrosine kinase inhibitor function or activity. The extent to which a tyrosine kinase inhibitor reduces tyrosine kinase function or activity may be determined by measuring tyrosine kinase function or activity in cells in the presence and absence of the tyrosine kinase inhibitor. The cells may be normal cells or cancer cells. For example, the cells may be cancer cells as described herein, for instance lung cancer, colon cancer or breast cancer cells. Preferred examples of anti-tumour agents include Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib. Among these, a particularly preferred example of an anti-tumour agent is Imatinib. Imatinib has the following structure.

Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib may each be provided in the form of a pharmaceutically acceptable salt or prodrug. For instance, Imatinib is often provided in the form of a mesylate. Thus, in a further preferred aspect of the invention, the anti-tumour agent is a pharmaceutically acceptable salt of Imatinib such as a mesylate salt. Treatment of cancer As is shown by the Examples below, gamma-aminobutyric acid derivatives including pregabalin and its derivatives have efficacy in inhibiting the activity of cancer cells in a wide variety of cancer cell lines. Accordingly, the invention provides the use of a gamma-aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt thereof, or a prodrug thereof, for use in the treatment of cancer. The cancer may be any cancer. The cancer may be a solid tumour. The cancer may be a carcinoma, a sarcoma, a lymphoma, a leukemia, a germ cell tumour or a blastoma. In a typical embodiment the cancer is a carcinoma which is an adenocarcinoma, a squamous cell carcinoma, an adenosquamous carcinoma, an anaplastic carcinoma, a large cell carcinoma or a small cell carcinoma. The cancer may affect any tissue or organ, including skin; lymph node; breast; cervix; uterus; gastrointestinal tract; lung; ovary; prostate; colon; rectum; mouth; brain; head and neck; throat; testes; thyroid; kidney; pancreas; bone; spleen; liver; bladder; larynx; nasal passages; blood and/or bone marrow. The cancer may be an AIDS-related cancer. In one embodiment the cancer affects skin; lymph node; breast; cervix; uterus; gastrointestinal tract; lung; ovary; prostate; colon; rectum; mouth; brain; head and neck; throat; testes; thyroid; kidney; pancreas; bone; spleen; liver; larynx; nasal passages; blood and/or bone marrow. In another embodiment the cancer affects skin; lymph node; cervix; uterus; gastrointestinal tract; lung; ovary; prostate; colon; rectum; mouth; brain; head and neck; throat; testes; thyroid; kidney; pancreas; bone; spleen; liver; larynx; nasal passages; blood and/or bone marrow. For instance, the cancer may affect the lung and/or the colon. The cancer may be a metastatic cancer, which has metastasized or spread within the body. For instance, the cancer may be a cancer which has spread from one tissue or organ to another tissue or organ. The cancer may be a cancer which has spread from one to another of the following tissues and organs: skin; lymph node; breast; cervix; uterus; gastrointestinal tract; lung; ovary; prostate; colon; rectum; mouth; brain; head and neck; throat; testes; thyroid; kidney; pancreas; bone; spleen; liver; bladder; larynx; nasal passages; blood and/or bone marrow. For instance, the cancer may be a cancer which has spread from the lung and/or the colon to any of the following tissues and organs: skin; lymph node; breast; cervix; uterus; gastrointestinal tract; lung; ovary; prostate; colon; rectum; mouth; brain; head and neck; throat; testes; thyroid; kidney; pancreas; bone; spleen; liver; bladder; larynx; nasal passages; blood and/or bone marrow. In an embodiment, the cancer treated is lung cancer, breast cancer or colon cancer. Preferably, the cancer treated is lung cancer or colon cancer. In some embodiments, the cancer treated is breast cancer. The cancer may be breast carcinoma, preferably breast adenocarcinoma. For instance, the breast cancer may be ductal carcinoma in situ (DCIS), invasive ductal carcinoma (IDC), invasive lobular carcinoma (ILC), inflammatory breast cancer, lobular carcinoma in situ (LCIS), male breast cancer, luminal A disease, luminal B disease, triple-negative/basal-like breast cancer, HER2-enriched breast cancer, normal-like breast cancer, Paget’s disease of the nipple, phyllodes tumour of the breast, or metastatic breast cancer. The breast cancer may be a subtype of IDC, e.g. tubular carcinoma of the breast, medullary carcinoma of the breast, mucinous carcinoma of the breast, papillary carcinoma of the breast or cribriform carcinoma of the breast. In some embodiments, the cancer treated is colon cancer. For instance, the colon cancer may be a carcinoma of the colon (preferably an adenocarcinoma of the colon), a carcinoid tumour, a gastrointestinal stromal tumor (a soft tissue sarcoma); lymphoma; Turcot Syndrome, or Peutz-Jeghers Syndrome. The colon cancer may be metastatic colon cancer, for example a metastatic carcinoma of the colon. Particularly preferably, the cancer treated is lung cancer. The lung cancer may be lung carcinoma, preferably lung adenocarcinoma. For instance, the lung cancer may be small cell lung carcinoma (SCLC) or non-small cell lung carcinoma (NSCLC). The cancer may be metastatic lung cancer, for example metastatic small cell lung carcinoma or metastatic non-small cell lung carcinoma. Further examples of cancers which may be susceptible to treatment according to the present invention include: cancers of the skin, such as melanoma; cancers of the blood and bone marrow, such as multiple myeloma and acute and chronic leukemias, for example, lymphoblastic, myelogenous, lymphocytic, and myelocytic leukemias, hematologic carcinoma, T/NK cell leukemia, B lymphoblastic leukemia; gastric carcinoma, e.g. gastric adenocarcinoma; pancreatic carcinoma, e.g. adenocarcinoma of the pancreas; ovarian carcinoma, e.g. ovarian adenocarcinoma; endometrial carcinoma, choriocarcinoma, uterine cervix carcinoma, carcinoma of the bladder, carcinoma of the urinary bladder, transitional bladder urinary carcinoma, advanced malignancy, amyloidosis, neuroblastoma, bone marrow neuroblastoma, retinoblastoma, meningioma, hemangiopericytoma, multiple brain metastase, glioblastoma multiforms, glioblastoma, brain stem glioma, poor prognosis malignant brain tumour, malignant glioma, recurrent malignant glioma, glial carcinoma of the central nervous system, fibrosarcoma, malignant fibrous histiocytoma, Edwing’s sarcoma, human endometrial stromal sarcoma, osteosarcoma, rhabdomyocarsoma, embryonal carcinoma, anaplastic astrocytoma, anaplastic oligodendroglioma, neuroendocrine tumour, rectal adenocarcinoma, Dukes C & D colorectal cancer, unresectable colorectal carcinoma, metastatic hepatocellular carcinoma, Kaposi's sarcoma, karotype acute myeloblastic leukemia, chronic lymphocytic leukemia (CLL), Hodgkin's lymphoma, non-Hodgkin's lymphoma, cutaneous T-Cell lymphoma, Burkitt lymphoma, cutaneous B-Cell lymphoma, diffuse large B-Cell lymphoma, low grade follicular lymphoma, metastatic melanoma (localized melanoma, including, but not limited to, ocular melanoma), malignant mesothelioma, malignant pleural effusion mesothelioma syndrome, peritoneal carcinoma, papillary serous carcinoma, gynecologic sarcoma, soft tissue sarcoma, scleroderma, cutaneous vasculitis, Langerhans cell histiocytosis, Leiomyosarcoma, fibrodysplasia ossificans progressive, hormone refractory prostate cancer, resected high-risk soft tissue sarcoma, unrescectable hepatocellular carcinoma, Waldenstrom's macroglobulinemia, smoldering myeloma, indolent myeloma, fallopian tube cancer, prostate carcinoma, androgen independent prostate cancer, androgen dependent stage IV non-metastatic prostate cancer, hormone-insensitive prostate cancer, chemotherapy-insensitive prostate cancer, thyroid carcinoma, metastatic thyroid carcinoma, papillary thyroid carcinoma, metastatic papillary thyroid carcinoma, follicular thyroid carcinoma, metastatic follicular thyroid carcinoma, medullary thyroid carcinoma, and leiomyoma. Selected examples of the cancer include gastric carcinoma, e.g. gastric adenocarcinoma, more preferably, carcinoma of the colon, most preferably adenocarcinoma of the colon; carcinoma of the pancreas, most preferably adenocarcinoma of the pancreas; breast cancer, most preferably adenocarcinoma breast and/or breast carcinoma; ovarian carcinoma, most preferably adenocarcinoma of the ovary and/or ovarian carcinoma; endometrial carcinoma, choriocarcinoma; cervix carcinoma; lung carcinoma, more preferably lung adenocarcinoma, lung carcinoma non-small cell and/or lung carcinoma; small cell carcinoma of the thyroid, more preferably human papillary thyroid carcinoma; metastasizing and/or follicular thyroid carcinoma; bladder carcinoma, more preferably carcinoma of urinary bladder and/or transitional cell carcinoma; urinary bladder carcinoma; prostate carcinoma CNS glial; fibrosarcoma; malignant fibrous histiocytoma; Edwing sarcoma; human endometrial stromal sarcoma; osteosarcoma; rhabdomyosarcoma; melanoma; embryonal carcinomas, more preferably neuroblastoma, neuroblastoma bone marrow, and/or retinoblastoma; and haematological cancers, more preferably leukemia cell T/NK, lymphoblastic B leukemia, lymphoblastic T leukemia, lymphoblastic leukemia B, Burkitt lymphoma, Hodgkin lymphoma, T lymphoma and/or multiple myeloma. The cancer may be a cancer wherein the cancer cells express substance P. For instance, the cancer may be a cancer wherein at least 5% of the cancer cells express substance P. Preferably, the cancer is a cancer wherein at least 10%, or at least 20%, or at least 50% of the cancer cells express substance P. Grading systems are used in cancer biology and medicine to categorize cancer cells with respect to their lack of cellular differentiation. This reflects the extent to which the cancer cells differ in morphology from healthy cells found in the tissue from which the cancer cell originated. The grading system can be used to provide an indication of how quickly a particular cancer might be expected to grow. Typically used grades of cancer are Grades (G) X and 1 to 4. GX indicates that the cancer grade cannot be assessed. G1 (low grade) cancer cells have a similar morphology to normal, healthy, cells (i.e. they are well differentiated) and would be expected to grow slowly, and are less likely to spread. G2 (intermediate grade) cancer cells are moderately differentiated, i.e. they appear more abnormal and would be expected to grow slightly faster than G1 cells. G3 (high grade) cancer cells have a very different morphology compared to normal cells (i.e. they are poorly differentiated) and would be expected to grow faster than G1 and G2 cells. G4 (high grade) cancer cells are undifferentiated (also referred to as anaplastic) and would be expected to have the highest capacity for proliferation. The cancer to be treated may be of any grade. In some cases, the cancer is poorly differentiated or undifferentiated, for instance poorly differentiated. For instance, the cancer may be poorly differentiated colon cancer, e.g. poorly differentiated, metastatic colon cancer. The cancer may be poorly differentiated lung cancer, e.g. poorly differentiated, metastatic lung cancer. The patient to be treated in the present invention is suffering from cancer. Typically the patient to be treated is a mammal. Preferably the patient is a human. The patient may be diagnosed with cancer using routine diagnostic techniques known to those of skill in the art. A successful treatment can identified by the absence or reduction of symptoms of cancer and/or using routine diagnostic techniques. For example, the method may result in a reduction in the size of a tumour in the patient. Treatment may be curative or palliative, i.e. it may aim at curing the patient, achieving complete or partial remission, alleviating or managing symptoms and/or side effects of the disease (without curing the patient). Preferably, however, the treatment is curative. The gamma-aminobutyric acid derivatives including pregabalin and its derivatives, and combinations thereof, are shown herein to inhibit, or even to prevent, cancer cell proliferation. Accordingly, the treatment of cancer described herein may be a treatment of cancer wherein cancer cell proliferation is inhibited. For instance, the treatment of cancer may be a treatment wherein tumor growth is inhibited. Thus, the invention provides the use of a gamma-aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt thereof, or a prodrug thereof, for use in the treatment of cancer, wherein the proliferation of cancer cells is inhibited. In particular, the examples indicate that inhibition or prevention of cell proliferation caused by the compounds and combinations described herein may occur at least partly due to promotion of apoptosis. That is, the inhibition or prevention of cell proliferation may occur by a mechanism wherein death of cancer cells occurs by natural, self-induced, non- toxic cell death rather than by direct toxicity of the administered agents. Accordingly, the treatment of cancer described herein may be a treatment of cancer which comprises causing apoptosis of cancer cells. Thus, the invention provides the use of a gamma- aminobutyric acid derivative which is a compound of formula (I), or a pharmaceutically acceptable salt thereof, or a prodrug thereof, for use in the treatment of cancer, wherein the treatment comprises causing apoptosis of cancer cells. Therapeutic combinations The gamma-aminobutyric acid derivative described herein may be used/administered alone or in combination with other therapeutic compositions or treatments. The gamma-aminobutyric acid derivative described herein may be used in monotherapy. By monotherapy is meant a therapy wherein the gamma-aminobutyric acid derivative or its pharmaceutically acceptable salt or prodrug is the sole species used having significant therapeutic activity, for instance anti-cancer activity. Thus, the invention provides a monotherapeutic method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma-aminobutyric acid derivative as described herein. In other words, the method may comprise administering to a patient in need thereof a gamma-aminobutyric acid derivative as described herein, and optionally no other therapeutic agent. The gamma-aminobutyric acid derivative described herein may alternatively be used in combination with any other cancer therapy or any other therapeutic agent for a cancer, such as a chemotherapeutic agent. Thus, where the gamma-aminobutyric acid derivative described herein is used in combination, it may be used in combination with an active agent which inhibits or prevents proliferation of cancer cells. In particular, the gamma-aminobutyric acid derivative described herein may be used in combination with an NK1 receptor antagonist. This is preferred, as it has been shown by the inventors in the Examples (see below) that the gamma-aminobutyric acid derivative described herein is more effective in the treatment of cancer when used in combination with an NK1 receptor antagonist than when the gamma-aminobutyric acid derivative and the NK1 receptor antagonist are used separately. In other words, the gamma-aminobutyric acid derivative described herein, when combined with an NK1 receptor antagonist, may show a synergistic efficacy in the treatment of cancer. Thus, the active agent may be an NK1 receptor antagonist, for instance aprepitant or fosaprepitant or maropitant; preferably aprepitant. Other examples of a suitable active agent which inhibit or prevent proliferation of cancer cells and which may be used in combination with the gamma-aminobutyric acid include Chlorambucil, Melphalan, Aldesleukin, 6-Mercaptopurine, 5-Fluoruracil, Ara-c, Bexarotene, Bleomycin, Capecitabine, Carboplatin, Cisplatin, Docetaxel, Doxorubicin, Epirubicin, Fludarabine, Irinotecan, Methotrexate, Mitoxantrone, Oxaliplatin, Paclitaxel, Rituximab, Vinblastine, Etoposide, Teniposide, Vincristine, Vinorelbine, Imatinib, Gefitinib, Erlotinib, Crizotinib, Sorafenib, Vemurafenib, Dabrafenib, Encorafenib, Trametinib, Cobimetinib, Binimetinib Palbociclib, Abemaciclib, Axitinib, Cabozantinib, Everolimus, Lenalidomide, Lenvatinib, Pazopanib, Regorafenib, Sunitinib, Thalidomide, Vandetanib, Netupitant, Vestipitant, Casopitant, Vofopitant, Ezlopitant, Lanepitant, LY-686017, L-733,060, L-732,138, L - 703,606, WIN 62,577, CP-122721, TAK-637, R673, CP-100263, WIN 51708, CP-96345, L-760735, CP-122721, L-758298, L-741671, L-742694, CP-99994 or T-2328. A pharmaceutically acceptable salt or prodrug of the NK1 receptor antagonist may be used. However, the NK1 receptor antagonist need not be in the form of a prodrug or pharmaceutically acceptable salt. Accordingly, in a preferred embodiment the gamma-aminobutyric acid derivative described herein, preferably pregabalin, is used in combination with an NK1 receptor antagonist, preferably aprepitant, for the treatment of cancer. The active agent may be an anti-tumour agent, particularly a kinase inhibitor, for instance a tyrosine kinase inhibitor. Preferred examples of anti-tumour agents include the kinase inhibitors Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib, and pharmaceutically aceptable salts or prodrugs thereof. A particularly suitable example of such an agent is Imatinib. In another preferred embodiment, the gamma-aminobutyric acid derivative described herein, preferably pregabalin, is used in combination with an anti-tumour agent, for the treatment of cancer. The anti-tumour agent is typically a kinase inhibitor, and is preferably selected from Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib, particularly preferably Imatinib. A pharmaceutically acceptable salt or prodrug of the anti- tumour agent may be used. However, the anti-tumour agent does not ned to be in the form of a prodrug or pharmaceutically acceptable salt. The gamma-aminobutyric acid derivative described herein may be used in combination with one or more active agents, preferably one or more therapeutic agents for the treatment of cancer, as described herein. The invention therefore provides a method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma-aminobutyric acid derivative and one or more active agents as described herein. In particular, the invention provides a method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma-aminobutyric acid derivative and one or more active agents which inhibit or prevent proliferation of cancer cells. For example, the invention provides a method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma-aminobutyric acid as defined herein in combination with one or more NK1 receptor antagonist(s). In another example, the invention provides a method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma- aminobutyric acid as defined herein in combination with one or anti-tumour agents, preferably Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib; particularly preferably Imatinib. Preferably, the method for the treatment of cancer comprises administering to a patient in need thereof pregabalin, or a pharmaceutically acceptable salt or prodrug thereof, in combination with aprepitant or a pharmaceutically acceptable salt or prodrug thereof. Where the gamma-aminobutyric acid derivative described herein is used in combination with one or more active agents, any or all of said one or more active agents may be active agents other than therapeutic agents for cancer. However, typically, the gamma-aminobutyric acid derivative described herein is not used in combination with any agent indicated for the treatment of type II diabetes. For instance, the gamma- aminobutyric acid derivative described herein is typically not used in combination with metformin. Thus, the invention provides a method for the treatment of cancer, which comprises administering to a patient in need thereof a gamma-aminobutyric acid derivative and at least one additional active agent as described herein which is not an agent indicated for the treatment of type II diabetes, for instance which is not metformin. Pharmaceutical compositions The present invention provides a pharmaceutical composition that comprises a gamma-aminobutyric acid derivative which is compound of formula (I), or a pharmaceutically acceptable salt or prodrug thereof (the “active ingredient”), for use in treating cancer. Pharmaceutical compositions according to the invention further comprise one or more pharmaceutically acceptable excipients. Suitable pharmaceutical excipients are known in the art and can readily be selected by the skilled person. The pharmaceutical excipient(s) selected will depend on the intended mode of administration of the pharmaceutical composition (e.g. oral, topical, by injection, by inhalation and so on). Typically, the pharmaceutical excipient may be selected from one or more of a carrier, a diluent, a stabiliser, a binding agent, a disaggregating agent, an isotonic agent, a buffer, a colouring agent, a flavouring agent, a sweetener, a wetting agent and in general non-toxic and pharmacologically inactive substances used in pharmaceutical formulations. In general, administration of the pharmaceutical compositions may be oral (as syrups, tablets, capsules, lozenges, controlled-release preparations, fast-dissolving preparations, oral suspensions, oral solutions etc), by injection (subcutaneous, intradermal, intramuscular, intravenous, etc.), or by inhalation (as a dry powder, a solution, a dispersion, etc.). The preferred route of administration will depend upon the specific active ingredient to be delivered, and a skilled person can easily choose an appropriate route. Typically, the compound of formula (I) (e.g. pregabalin) is delivered orally. Thus, in a preferred embodiment the pharmaceutical composition is an oral formulation. By “oral formulation” is meant that the pharmaceutical composition is suitable for oral administration. For oral administration, the pharmaceutical compositions of the present invention may take the form of, for example, tablets, lozenges or capsules prepared by conventional means with pharmaceutically acceptable excipients such as binding agents (e.g. pregelatinised maize starch, polyvinylpyrrolidone or hydroxypropyl methyl cellulose); fillers (e.g. lactose, microcrystalline cellulose or calcium hydrogenphosphate); lubricants (e.g. magnesium stearate, talc or silica); disintegrants (e.g. potato starch or sodium glycolate); or wetting agents (e.g. sodium lauryl sulphate). The tablets may be coated by methods well known in the art. Liquid preparations for oral administration may take the form of, for example, solutions, syrups or suspensions, or they may be presented as a dry product for constitution with water or other suitable vehicle before use. Such liquid preparations may be prepared by conventional means with pharmaceutically acceptable excipients such as suspending agents, emulsifying agents, non-aqueous vehicles or preservatives. The preparations may also contain buffer salts, flavouring agents, colouring agents or sweetening agents, as appropriate. For administration by injection, the pharmaceutical compositions typically take the form of an aqueous injectable solution. Examples of suitable aqueous carriers that may be employed in the injectable pharmaceutical compositions of the invention include water, buffered water and saline. In many cases, it will be preferable to include isotonic agents, for example, sugars, polyalcohols such as mannitol, sorbitol, or sodium chloride in the composition. For administration by inhalation, the pharmaceutical composition may take the form of a dry powder, which will typically comprise the active ingredient and a carrier such as lactose, and be delivered via an inhaler. Alternatively, the pharmaceutical composition may for example be formulated as aqueous solutions or suspensions and be delivered as an aerosol from a pressurised metered dose inhaler, with the use of a suitable liquefied propellant. Suitable propellants include fluorocarbon or hydrogen-containing chlorofluorocarbon or mixtures thereof, particularly hydrofluoroalkanes. The preferred route of administration will depend upon the specific active ingredient(s) to be delivered, and a skilled person can easily choose an appropriate route. For example, aprepitant and pregabalin are each preferably delivered orally. Pharmaceutical compositions comprising of the invention may be prepared by any suitable method known to those of skill in the art. Pharmaceutical compositions of the invention may comprise additional active ingredients, such as an additional therapeutic or prophylactic agent intended, for example, for the treatment of the same condition or a different one, or for other purposes such as amelioration of side effects. Thus, the pharmaceutical composition described herein may comprise, in addition to the gamma-aminobutyric acid derivative, one or more active agents as described above. The active agent is typically a species which inhibits or prevents proliferation of cancer cells. The active agent may be an NK1 receptor antagonist, for instance aprepitant or fosaprepitant or maropitant; preferably aprepitant. Other examples of an active agent which may be included in the pharmaceutical composition of the invention include Chlorambucil, Melphalan, Aldesleukin, 6-Mercaptopurine, 5-Fluoruracil, Ara-c, Bexarotene, Bleomycin, Capecitabine, Carboplatin, Cisplatin, Docetaxel, Doxorubicin, Epirubicin, Fludarabine, Irinotecan, Methotrexate, Mitoxantrone, Oxaliplatin, Paclitaxel, Rituximab, Vinblastine, Etoposide, Teniposide, Vincristine, Vinorelbine, Imatinib, Gefitinib, Erlotinib, Crizotinib, Sorafenib, Vemurafenib, Dabrafenib, Encorafenib, Trametinib, Cobimetinib, Binimetinib Palbociclib, Abemaciclib, Axitinib, Cabozantinib, Everolimus, Lenalidomide, Lenvatinib, Pazopanib, Regorafenib, Sunitinib, Thalidomide, Vandetanib, Netupitant, Vestipitant, Casopitant, Vofopitant, Ezlopitant, Lanepitant, LY- 686017, L-733,060, L-732,138, L -703,606, WIN 62,577, CP-122721, TAK-637, R673, CP-100263, WIN 51708, CP-96345, L-760735, CP-122721, L-758298, L-741671, L-742 694, CP-99994 or T-2328, or a pharmaceutically acceptable salt or prodrug thereof. The active agent may be an anti-tumour agent, particularly an anti-tumour agent which is a kinase inhibitor, optionally selected from Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib or a pharmaceutically acceptable salt or prodrug thereof. Particularly preferably the anti-tumour agent is Imatinib, or a pharmaceutically acceptable salt or prodrug thereof. Typically, the pharmaceutical composition does not include an active agent which is indicated for the treatment of type II diabetes. For instance, the pharmaceutical composition may not comprise metformin. The pharmaceutical composition of the invention may comprise one or more active agents as described herein. However, it is generally preferred that the compositions of the invention do not contain any further active ingredients (i.e. the pharmaceutical compositions contain only one active ingredient which is the compound of formula (I) (e.g. Pregabalin), or a pharmaceutically acceptable salt or prodrug thereof. Dosages and dosage regimes Suitable dosages of the active ingredients described herein may easily be determined by a skilled medical practitioner. A suitable dose is typically an amount which induces a therapeutic response in the patient. A suitable dose is typically an amount sufficient to inhibit proliferation and/or induce apoptosis in cancer cells of the patient. The invention therefore provides a method for the treatment of cancer, which comprises administering a gamma-aminobutyric acid derivative to a patient, in an amount sufficient to inhibit proliferation and/or induce apoptosis in cancer cells of the patient. Actual dosage levels of the active ingredients in the pharmaceutical compositions described herein may be varied so as to obtain an amount of the active ingredient, which is effective to achieve the desired therapeutic response for a particular patient, composition, and mode of administration, without being toxic to the patient. The selected dosage level will depend upon a variety of pharmacokinetic factors including the activity of the particular compositions employed, the route of administration, the time of administration, the rate of excretion of the particular compound being employed, the duration of the treatment, other drugs, compounds and/or materials used in combination with the particular compositions employed, the age, sex, weight, condition, general health and prior medical history of the patient being treated, and like factors well known in the medical arts. Dosage regimens may be adjusted to provide the optimum desired response. For example, a single dose may be administered, several divided doses may be administered over time or the dose may be proportionally reduced or increased as indicated by the exigencies of the therapeutic situation. Dosage unit form as used herein refers to physically discrete units suited as unitary dosages for the subjects to be treated; each unit contains a predetermined quantity of active compound calculated to produce the desired therapeutic effect or therapeutic response in association with the required pharmaceutical carrier. Administration may be in single or multiple doses. Multiple doses may be administered via the same or different routes and to the same or different locations. Dosage and frequency may vary depending on the half-life of the drugs in the patient and the duration of treatment desired. Typically, the gamma-aminobutyric acid derivative which is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof, is administered 1 to 5 time per day, for example 3 times per day. Pregabalin has been previously used in clinical practice for the treatment of pain, to produce concentrations in blood plasma of around 2 to 8 µM. Such concentrations are generally achieved by using dosages in the region of 50-300 mg/day. The present examples, discussed below, illustrate the efficacy of pregabalin in preventing/inhibiting cancer cell proliferation and cancer tumor growth. This efficacy is demonstrated at a wide range of concentrations, particularly in the range from 10 µM to 80 µM. Accordingly, efficacy is expected at blood plasma concentrations from about 2 µM up to much higher dosages of 80 µM or more. Accordingly, the gamma-aminobutyric acid derivative is typically administered for the purposes described herein in a dosage regime intended to produce a concentration in the blood of from about 2 µM to 80 µM or more. Exemplary dosage regimes are described below. Typically, the total amount the gamma-aminobutyric acid derivative which is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof, administered per day is 10 mg to 5000 mg per day, for example 25 mg or 30 mg to 4000 mg per day, preferably 100 to 2000 mg per day. The total amount of said compound administered per day may be greater than 500 mg per day. Exemplary doses of the gamma-aminobutyric acid derivative which is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof (e.g. pregabalin) that may be useful in the invention include about 50, 100, 150 or 600 mg per day. These doses may conveniently be administered by way of two or three separate doses (e.g. 25 and 25 mg = 50mg; 50, 50 and 50 mg = 150 mg; 200, 200 and 200 mg = 600 mg). Smaller daily doses may be administered in fewer divided doses, while larger daily doses (e.g. 600 mg) are more typically administered in three divided doses (e.g. three doses of 200 mg each daily). In an exemplary dosage regime, the amount of the gamma-aminobutyric acid derivative which is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof which is administered to the patient is gradually increased over time. For instance, the total daily dosage may be increased from an initial dosage of about 50 to 150 mg per day to about 250 to 5000 mg per day over a period of one to three weeks. Determination of the proper dosage for particular situations is within the skill of the person skilled in the art. The dose is typically such as to produce an effective concentration of the gamma- aminobutyric acid derivative in the blood of the patient. For instance, the dose is typically a dose of from 1 to 150 µM in the blood of the patient, preferably from 2 to 100 µM, more preferably from 10 to 80 µM of the gamma-aminobutyric acid derivative in the blood of the patient. “Blood” in this context generally refers to the blood plasma of the patient. Use of substance P as a biomarker As is shown in Example 17 below, expression of substance P by the cancer cells treated is an indicator of whether the cancer cells will be receptive to treatment with the compounds of formula (I) described herein. Cell lines wherein a high percentage of cells expressed substance P showed a very significant inhibition of proliferation in response to treatment with pregabalin. Cell lines wherein a lower percentage of cells expressed substance P still showed an inhibition of proliferation in response to the treatment, but to a lesser extent than the cell line with greater expression of substance P. Without wishing to be bound by theory, it is speculated that pregabalin (and other gamma-aminobutyric acid derivatives) may achieve anti-tumour activity by inhibiting the release of substance P. Substance P is an agonist of the NK1 receptor. One possible mechanism for the anti-tumour effect of gamma- aminobutyric acid derivatives including pregabalin is by inhibition of the release of substance P. It is speculated that, by inhibiting the release of substance P, gamma- aminobutyric acid derivatives including pregabalin can inhibit interaction between substance P and the NK1 receptor. Reduction of this interaction may have an anti-tumour effect. Accordingly, the expression of substance P by cancer cells may be used as an indication of the likely effectiveness of treatment of the cancer using a compound of formula (I), and combinations containing such a compound. Preferably, therefore, the treatment of cancer described herein is a treatment of a cancer wherein the cancer cells express substance P. For instance, the cancer may be a cancer wherein at least 5% of the cancer cells express substance P. Preferably, the cancer may be a cancer wherein at least 10% or at least 20%, more preferably at least 50% of the cancer cells express substance P. Expression of substance P by the cancer cells in question may be determined by methods which are well known to the person skilled in the art. An example of such a method is given in example 17. The methods used are generally immunohistochemical in nature. In general, expression of substance P by cancer cells may be analysed by treating a sample of the cancer cells in question with an anti-substance P antibody (a so-called anti- SP antibody) and then detecting cells labelled with that antibody. Accordingly, described herein is a gamma-aminobutyric acid derivative for use in a method of treating cancer in a patient in need thereof, wherein said gamma-aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof, and the method of treating cancer comprises determining that the cancer cells express substance P. For instance, the method may comprise analysing the cancer cells to determine whether or not they express substance P, and confirming that the said cancer cells do express substance P. In particular, the method may comprise obtaining a sample of the cancer cells; analysing the said sample (for example by performing an immunohistochemical analysis on the sample of the cancer cells); and confirming that the cancer cells express substance P. The method may further comprise quantifying the amount of the cancer cells (e.g. the cancer cells in the sample obtained) and determining the amount of the cancer cells that express substance P. For instance, the method may further comprise determining that at least 5%, or at least 10%, or at least 20% of the cancer cells express substance P. Preferably, the method may further comprise determining that at least 50% of the cancer cells express substance P. The cancer cells analysed to determine the expression of substance P generally comprise a sample taken from the cancer. For instance, the cancer cells analysed may be a sample from a biopsy of a solid tumour. Generally, the analysis of the cancer cells is performed prior to the administration of the gamma-aminobutyric acid described herein. Also provided herein is the use of substance P as a biomarker to identify a cancer which is suitable for treatment with a gamma-aminobutyric acid derivative as described herein. The present invention is explained in more detail in the following by referring to the Examples, which are not to be construed as limiting. The Examples illustrate the efficacy of a gamma- aminobutyric acid derivative which is a compound of formula (I), pregabalin, in reducing cancer cell survival rates for a wide variety of cancers. EXAMPLES Example 1 – breast cancer cells The following example examined the effect of a compound of formula (I), pregabalin, on the survival of breast cancer cells. 3000 breast cancer cells of the cell line 13762 MATBIII (ATCC-CRL-1666) were seeded per well in a 96 well plate, and treated with pregabalin. Five different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The medium used was McCoy's 5A medium, together with 2 mM l- glutamine, penicillin (50 U/ml), streptomycin (50 µg/ml) and 10% fetal bovine serum. The cells were incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator. 3000 breast cancer cells of the cell line 4T1 (ATCC CRL-2539) were seeded per well in a 96 well plate cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated, and performed five times overall. The results are shown in Table 1, below. In the presence of pregabalin, a significant reduction in the survival of breast cancer cells is seen in concentrations between 10 and 80 µM.

Table 1. Percentage of cells surviving after incubation in presence of pregabalin Example 2 – Lung cancer cells The experiment described in Example 1 was repeated using 3LL (JCRB1348), and A549 (ATCC CCL-185) Lung carcinoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 2, below.

Table 2. Percentage of cells surviving after incubation in presence of pregabalin Example 3 – colon cancer cells The experiment described in Example 1 was repeated using CT26WT colon cancer cells (ATCC-CRL-6475) and HT29 colon cancer cells (ATCC-HTB-38, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 3, below. The CT26WT cancer cell line is a type of undifferentiated, metastatic carcinoma. It is therefore a suitable model of undifferentiated or poorly differentiated cancers, and metastatic cancers. It is a particularly good model for poorly differentiated and/or metastatic colon cancer.

Table 3. Percentage of cells surviving after incubation in presence of pregabalin Example 4 – Melanoma cancer cells The experiment described in Example 1 was repeated using B16F10 (ATCC CRL- 6475) and A375 (ATCC® CRL-1619) melanoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 4, below.

Table 4. Percentage of cells surviving after incubation in presence of pregabalin Example 5 Leukemia cancer cells The experiment described in Example 1 was repeated using Jurkat (ATCC TIB- 152) Acute T Cell Leukemia, and L1210 (ATCC CCL-219) Lymphocytic Leukemia cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 4, below.

Table 5. Percentage of cells surviving after incubation in presence of pregabalin Example 6 – Lymphoma cancer cells The experiment described in Example 1 was repeated using EL4 (ATCC® TIB-39) Lymphoma, and A20 (ATCC TIB-208) B Cell Lymphoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 6, below.

Table 6. Percentage of cells surviving after incubation in presence of pregabalin Example 7 – Other types of cancer cells The experiment described in Example 1 was repeated using BxPc3 (ATCC CRL- 1687) Pancreatic carcinoma, Renca (ATCC CRL-2947) Renal carcinoma, PTEN (ATCC CRL-3031) Prostate carcinoma and RT-112 (DSMZ ACC 418) Urinary bladder carcinoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin. Seven different concentrations of pregabalin were employed: 10 µM, 30 µM, 40µM, 50µM, 60 µM, 80µM and 100 µM. The experiment was repeated and performed five times overall. The results are shown in Table 7 and 8 below.

Table 7. Percentage of cells surviving after incubation in presence of pregabalin

Table 8. Percentage of cells surviving after incubation in presence of pregabalin Example 8. Pregabalin treatment induces cancer cells death by a natural process as Apoptosis Images of the cells were taken using the inverted microscope CKX41SF2 (Olympus). Figure 1 shows a representative photographic image of a population of breast cancer cells after incubation for a period of 32 hours in the presence of 80 µM pregabalin. Figure 2 shows a representative photographic image of a population of colon cancer cells after incubation for a period of 32 hours in the presence of 80 µM pregabalin. Figures 1 and 2 show the morphological changes that characterize the cells that undergo apoptosis. Specifically, cell retraction is seen in which the cell is smaller, the cytoplasm is dense and the organelles present a more compact packing. Chromatin condensation is also seen, which is the main characteristic of apoptosis and nucleus fragmentation. Another aspect is the formation of cytoplasmic vesicles and apoptotic bodies. Example 9 – lung cancer cells treated in combination with Aprepitant The following example examined the effect of a compound of formula (I), pregabalin, on the survival of lung cancer cells. The effect of aprepitant on those cells, and the effect of pregabalin in combination with aprepitant, was also examined. 3000 lung cancer cells of the cell line 3LL (JCRB1348) and 3000 lung cancer cells of the cell line A549 (ATCC CCL-185), were seeded per well in a 96 well plate, and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin. The cells were cultured according to the supplier's recommendations and incubated for up to 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator. After 24 hours, the cell viability was determined using an MTT assay. The MTT assay is a well-known colorimetric assay for assessing cell metabolic activity. In brief, the cells are exposed to a substance which is metabolized by enzymes in any remaining viable cells to a colored product. The absorbance in the relevant part of the spectrum of the incubated products are then measured and compared to the absorbance of a control cell population, incubated in the absence of drug. The proliferation of the lung cancer cells was thus determined. The cell proliferation is expressed in Table 1 below as a percentage compared to the cell proliferation in the control experiment. The experiment was repeated and performed five times overall. The values in Table 9 below are mean values obtained from the five experimental runs.

Table 9. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 9 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 9 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NKR receptor antagonist (aprepitant) is more than additive. For instance, at 24 hours, 60 µM pregabalin alone results in a reduction of cell proliferation of 9,77% compared to the control experiment. However, incubation of the lung cancer cells with 20 µM aprepitant does not prevent cell proliferation. On the contrary, greater cell proliferation is seen with aprepitant than in the absence of any drug; cell proliferation increases by 12,35%. Accordingly, if the effects of pregabalin and aprepitant were merely additive, it would be expected that the combination of 20 µM aprepitant & 60 µM pregabalin would result in a cell proliferation that is greater than is observed with 60 µM pregabalin alone. In other words, 60 µM pregabalin is expected to cause a 9,77% reduction in cell proliferation at 24 hours while 20 µM aprepitant is expected to cause an increase in cell proliferation. Thus, if the effects of pregabalin and aprepitant were merely additive, the combination would be expected to cause a reduction in cell proliferation of less than 9,77%, or no reduction at all. However, in combination they demonstrate a 47,38% reduction in cell proliferation: greater than 9,77%. It is clear, therefore, that the gamma-aminobutyric acid derivative (e.g. pregabalin) and the NK1 receptor antagonist (e.g. aprepitant) act synergistically to produce a greater reduction in cell proliferation than is expected from each component acting in isolation. Similar synergistic results are seen in the lung cancer cell line A549. Example 10 – Colon cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using CT26WT (ATCC CRL-2638), and HT29 (ATCC HTB-38) colon cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 10. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 10 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 10 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 11 – Breast cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using MATBIII (ATCC- CRL-1666), and 4T1 (ATCC CRL-2539) Breast cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37 °C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 11. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 11 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 11 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 12 – Melanoma cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using B16F10 (ATCC CRL-6475), and A375 (ATCC® CRL-1619) Melanoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37°C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 12. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 12 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 12 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 13 – Lymphoma cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using EL4 (ATCC® TIB-39), and A20 (ATCC TIB-208) Lymphoma cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37°C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 13. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 13 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 13 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 14 – Leukemia cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using Jurkat (ATCC TIB- 152), and L1210 (ATCC CCL-219) Leukemia cancer cell lines, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37°C in a 5% CO2- humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 14. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 14 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 13 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 15 – Other cancer cells treated in combination with Aprepitant The experiment described in Example 9 was repeated using BxPc3 (ATCC CRL- 1687) Pancreatic carcinoma, Renca (ATCC CRL-2947) Renal carcinoma, PTEN (ATCC CRL-3031) Prostate carcinoma and RT-112 (DSMZ ACC 418) Urinary bladder carcinoma, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37°C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and/or aprepitant. Three different drug concentrations were employed: (i) 20 µM aprepitant; (ii) 60 µM pregabalin; and (iii) 20 µM aprepitant in combination with 60 µM pregabalin.

Table 15. Cell proliferation after incubation with pregabalin and/or aprepitant The results in Table 15 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 15 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (aprepitant) is more than additive in colon cancer cells. Example 16 – Cancer cells treated in combination with Maropitant The experiment described in Example 9 was repeated using 3LL (JCRB1348) Lung Carcinoma, A549 (ATCC CCL-185) Lung Carcinoma, HT29 (ATCC HTB-38) Colon Carcinoma, A375 (ATCC® CRL-1619) Melanoma and BxPc3 (ATCC CRL-1687) Pancreatic Carcinoma cancer cells, that were cultured according to the supplier's recommendations, incubated for 24 hours at 37°C in a 5% CO2-humidified, 95% air incubator and treated with pregabalin, and treated with pregabalin, and/or maropitant. Three different drug concentrations were employed: (i) 5 µM maropitant; (ii) 60 µM pregabalin; and (iii) 5 µM maropitant in combination with 60 µM pregabalin.

Table 16. Cell proliferation after incubation with pregabalin and/or maropitant The results in Table 16 illustrate that pregabalin alone can cause significant reduction in cancer cell proliferation and can promote cancer cell apoptosis. This is clear from the fact that cell viability assays at 24 hours, in the presence of 60 µM pregabalin, show reduced cell viability. Moreover, it is clear from the results in Table 16 that the combined effect of the gamma-aminobutyric acid derivative (pregabalin) and the NK1 receptor antagonist (Maropitant) is more than additive in cancer cells. These Examples, which are merely illustrative and are not intended to limit the scope of the invention, illustrate the efficacy of gamma-aminobutyric acid derivatives against a broad range of cancer types. For all cancer cell types tested, it has been shown that the presence of a gamma-aminobutyric acid derivative, even in an amount as low as 10 µM, can reduce the number of cancer cells surviving and proliferation. Thus, the Examples indicate that gamma-aminobutyric acid derivatives inhibit proliferation of cancer cells (i.e. tumour growth). Moreover, the Examples indicate that gamma-aminobutyric acid derivatives are capable of efficiently inducing cancer cell apoptosis. The above Examples indicate that gamma-aminobutyric acid derivatives and particularly pregabalin have activity (i.e. inhibit proliferation or promote cell death) against a wide variety of cancer types. Efficacy is shown against breast cancer, lung cancer, colon cancer, melanoma, lymphoma, leukemia, pancreatic cancer, renal cancer, prostate cancer and bladder cancer. Particularly good efficacy is shown against these cancers when pregabalin is used in combination with an NK1 receptor antagonist (aprepitant or Maropitant). In particular, the Examples illustrate that gamma-aminobutyric acid derivatives are unexpectedly highly efficacious against lung cancer and colon cancer; and particularly highly efficacious against lung cancer when used in combination with an NK1 receptor antagonist (aprepitant or Maropitant). The cancer cell apoptosis induced by pregabalin cannot be attributed to general toxicity of pregabalin, which is approved for use in humans for the treatment of peripheral and central neuropathic pain, partial seizures with secondary generalization, and generalized anxiety disorder (GAD). Example 17 – Cancer cells with different levels of expression of Substance P treated. For this example, the cell lines MATBIII (ATCC-CRL-1666; breast cancer), 4T1 (ATCC CRL-2539; breast cancer), 3LL (JCRB1348; lung cancer), A549 (ATCC CCL- 185;; lung cancer), CT26WT (ATCC CRL-2638; colon cancer), HT29 (ATCC HTB-38; Colon Carcinoma), B16F10 (ATCC CRL-6475; melanoma cancer), A375 (ATCC® CRL- 1619; melanoma cancer ), EL4 (ATCC® TIB-39; Lymphoma), A20 (ATCC TIB-208; Lymphoma), Jurkat (ATCC TIB-152; Leukemia), L1210 (ATCC CCL-219; Leukemia), BxPc3 (ATCC CRL-1687; Pancreatic Carcinoma), RenCA (ATCC CRL-2947; Renal Carcinoma), PTEN (ATCC CRL-3031; Prostate Cancer), RT112 (DSMZ ACC 418; Urinary bladder carcinoma). All cells were cultured according to the manufacturer's specifications and were subjected to different passages. In each passage, the cultures were selected based on the expression of Substance P until obtaining, for each cell line, a stable cell culture in which between 1 and 10% of the cells presented expression of SP (group +), another stable cell culture in which between 11 and 49% of the cells showed SP expression (group ++) and another stable cell culture in which between 50 and 100% of the cells they presented expression of SP (group +++). The study of SP was carried out using the immunohistochemical technique. In summary, a sample of each culture was dehydrated by treatment with increasing concentrations of ethanol and finally xylene. Subsequently said dried samples were embedded in paraffin, thus creating a block. Said paraffin blocks were cut on a microtome to a thickness of 5 µm, and the resulting sections (slices) were placed on slides suitable for conducting immunohistochemistry techniques. Subsequently, the sections were deparaffinised by immersion in xylene and then rehydrated through immersion in a series of solutions containing decreasing concentrations of ethanol and, finally, water. Subsequently, these samples were subjected to 10 times atmospheric pressure (10.1 bar) in citrate buffer at pH 6.0, in order to obtain greater exposure to antigens. The samples were then allowed to cool to room temperature over 10 minutes. Endogenous peroxidase activity was blocked by treatment with 3% hydrogen peroxide over 30 min at room temperature. After washing the samples with 0.05 M Tris buffer, they were incubated with 10% non-immune pig serum over 30 minutes at room temperature. In order to verify the expression of SP, the cell samples were incubated in the presence of anti-SP antibodies (Substance P Antibody -PA5-106934- provided by Invitrogen / ThermoFisher Scientific for use in immunohistochemistry analysis of paraffin-embedded Substance P) diluted 1: 100 at 22 ° C. Antigen retrieval was performed using citrate buffer. Samples were blocked with blocking buffer (1.5 hr, 22 ° C), incubated with Substance P polyclonal antibody (Product # PA5-106934) using a dilution of 1: 100 (1.5 hr, 22 ° C), followed by HRP (Envision System-HRP -Dako- reagents ) conjugated goat anti -rabbit, according to the provider's specifications. Immunoreactivity was visualized by light microscopy with a chromogenic solution with 3,3'-diaminobenzidine (DAB +; Dako, USA). In order to differentiate the cell nuclei, these were lightly stained with haematoxylin. Samples that were not incubated with the primary antibody, but wherein this was replaced by a non-immune serum were used as negative controls. All experiments were performed in sextuplicate. In order to evaluate the degree of immunostaining in each of the six sections, a cell-count was performed in 20 high-power fields (400x) using an Olympus microscope (model CX31). The total number of cells and the number of cells displaying immunostaining were counted in each one of the fields in order to subsequently determine the percentage of cells displaying said immunostaining. The results corresponding to the breast cancer cell lines can be seen in Table 17 below.

Table 17. Inhibition of cell proliferation according to the expression level of Substance P in breast cancer cell lines The results corresponding to the Lung cancer cell lines can be seen in Table 18 below.

Table 18. Inhibition of cell proliferation according to the expression level of Substance P in lung cancer cell lines The results corresponding to the Colon cancer cell lines can be seen in Table 19 below.

Table 19. Inhibition of cell proliferation according to the expression level of Substance P in colon cancer cell lines The results corresponding to the Melanoma cancer cell lines can be seen in Table 20 below.

Table 20. Inhibition of cell proliferation according to the expression level of Substance P in melanoma cell lines The results corresponding to the Lymphoma cancer cell lines can be seen in Table 21 below.

Table 21. Inhibition of cell proliferation according to the expression level of Substance P in lymphoma cell lines The results corresponding to the Leukemia cancer cell lines can be seen in Table 22 below.

Table 22. Inhibition of cell proliferation according to the expression level of Substance P in leukemia cell lines The results corresponding to other cancer cell lines can be seen in Table 23 below.

Table 23. Inhibition of cell proliferation according to the expression level of Substance P in other cell lines Example 18 – Pregabalin in combination with the tyrosine kinase inhibitor Imatinib. For this example, the cell lines 3LL (JCRB1348; lung cancer), CT26WT (ATCC CRL-6475; colon cancer), B16F10 (ATCC CRL-6475; melanoma), MATBIII (ATCC CRL-1666; breast cancer); PTEN (ATCC CREL-3031; prostate carcinoma); L1210 (ATCC CCL-219; lymphocytic leukemia), RenCA (ATCC CRL-2947; Renal Carcinoma), and Jurkat (ATCC TIB-152; acute T cell leukemia) were used. 3000 cancer cells of every cancer cell type were seeded per well in a 96 well plate, and treated with pregabalin, and/or the tyrosine kinase inhibitor Imatinib. Different concentrations of each drug were employed, as outlined in Table 24 below. The cells were cultured according to the supplier's recommendations and incubated for up to 24 hours at 37 °C in a 5% CO₂-humidified, 95% air incubator. After 24 hours, the cell viability was determined using an MTT assay. The MTT assay is a well-known colorimetric assay for assessing cell metabolic activity. In brief, the cells are exposed to a substance which is metabolized, by enzymes in any remaining viable cells, to a colored product. The absorbance of the incubated products is then measured in the relevant part of the spectrum. This absorbance is compared to the absorbance of a control cell population, incubated in the absence of drug. The proliferation of the cancer cells was thus determined, and the results are shown in Table 24, below. The cell proliferation is expressed in Table 24 as a percentage compared to the cell proliferation in the control experiment. The experiment was repeated and performed five times overall. The values in Table 24 are mean values obtained from the five experimental runs.

Table 24. Inhibition of cell proliferation by Imatinib, alone or in combination with pregabalin. This example shows that, while pregabalin alone has some inhibitory effect on the proliferation of cancer cells, its inhibitory effect is greatly magnified in the presence of the tyrosine kinase inhibitor Imatinib. The effect is not merely additive. Imatinib alone often has no inhibitory effect at all on the cancer cells. In some cases, Imatinib has an inhibitory effect but its magnitude is not sufficient to account for the effect of the combination. Example 19 – Pregabalin in combination with a kinase inhibitor selected from Trametinib, Vemurafenib, Sorafenib and Cobimetinib For this example, the cell lines A375 (ATCC CRL-1619; melanoma), A549 (ATCC CCL-185; lung carcinoma), HT29 (ATCC-HTB-38; colon cancer), 3LL (JCRB1348; lung carcinoma), and BxPc3 (ATCC CRL-1687; pancreatic carcinoma) were used. 3000 cancer cells of every cancer cell type were seeded per well in a 96 well plate, and treated with pregabalin, and/or a kinase inhibitor selected from Trametinib, Vemurafenib, Sorafenib and Cobimetinib. Different concentrations of each drug were employed, as outlined in Table 25 below. The cells were cultured according to the supplier's recommendations and incubated for up to 24 hours at 37 °C in a 5% CO₂-humidified, 95% air incubator. After 24 hours, the cell viability was determined using an MTT assay as discussed in Example 18. The proliferation of the cancer cells was thus determined, and the results are shown in Table 25, below. The cell proliferation is expressed in Table 25 as a percentage compared to the cell proliferation in the control experiment. The experiment was repeated and performed five times overall. The values in Table 25 are mean values obtained from the five experimental runs.

Table 25. Inhibition of cell proliferation by Trametinib, Vemurafenib, Sorafenib and Cobimetinib, alone or in combination with pregabalin. This example shows that, while pregabalin alone has some inhibitory effect on the proliferation of cancer cells, its inhibitory effect is greatly magnified in the presence of a kinase inhibitor selected from Trametinib, Vemurafenib, Sorafenib and Cobimetinib. Example 20 – clinical applicability of pregabalin in cancer treatment, in combination with the NK1 antagonist aprepitant A 44-year-old female patient was identified with ductal-type breast cancer, with a triple negative molecular profile (absence of expression of estrogen receptors, progesterone receptors and Her-2/Neu in the immunohistochemical study of the tumor) refractory to treatment. She presented a primary tumor focus at the level of the right breast of 25 millimeters and a metastatic tumor focus of 10 millimeters in the right humerus. She was in clinical stage IV (as defined in the American Joint Committee on Cancer (AJCC) cancer staging manual). The patient needed to be in bed more than half the day due to the presence of symptoms. She required help with most activities of daily living such as getting dressed (grade 3 according to the ECOG scale, Eastern Cooperative Oncology Group). She was treated using a treatment regime wherein Aprepitant was administered first alone, and after some days of administration was then administered in combination with pregabalin. Table 26 below shows the treatments administered, their duration, and the corresponding improvement in the ECOG status, in tumor size and the degree of response to treatment. It can be seen from this table that the combined treatment with an NK1 antagonist (aprepitant) and a drug of the γ-Aminobutyric acid family (Pregabalin), (i) improved the quality of life of the patient and ( ii) achieved the stabilization and reduction of a cancerous tumor. In terms of quality of life, the patient who initially presented a grade 3 on the ECOG scale (needs to be in bed more than half the day due to the presence of symptoms, requires help with most activities of daily living such as getting dressed), subsequently progressed to grade 2 (inability to do any work, symptoms that force you to stay in bed for several hours a day, in addition to those at night, but that do not exceed 50% of the day). She subsequently reached a grade 0 on the ECOG scale (totally asymptomatic and able to carry out normal work and activities of daily life) under treatment. The response was measured according to the RECIST criteria (Response Evaluation Criteria In Solid Tumors). Thus, the treatment achieved stabilization of the disease was achieved in the first stage and, subsequently, the remission of the cancer. During treatment, there were no major or dose-limiting adverse effects: treatment tolerance was excellent.

Table 26. Progress of stage IV breast cancer in patient treated with aprepitant and pregabalin.

Claims

Claims 1. A gamma-aminobutyric acid derivative for use in a method of treating cancer in a patient in need thereof, wherein said gamma-aminobutyric acid derivative is a compound of formula (I) or a pharmaceutically acceptable salt thereof or a prodrug thereof:

wherein R₁ is a C_1-6 alkyl, phenyl, or C_3-6 cycloalkyl group; R₂ is a hydrogen or methyl group; and R₃ is a hydrogen, methyl, or carboxyl group.

2. The gamma-aminobutyric acid derivative for use according to claim 1, wherein the compound of formula (I) is pregabalin.

3. The gamma-aminobutyric acid derivative for use according to claim 1 or 2, wherein the cancer is selected from breast cancer, lung cancer, colon cancer, melanoma, lymphoma, leukemia, pancreatic cancer, renal cancer, prostate cancer and bladder cancer.

4. The gamma-aminobutyric acid derivative for use according to any preceding claim, wherein the cancer is selected from lung cancer, breast cancer, and colon cancer.

5. The gamma-aminobutyric acid derivative for use according to any preceding claim, wherein the cancer is lung cancer.

6. The gamma-aminobutyric acid for use according to any of claims 1 to 5, wherein the method comprises administering the gamma-aminobutyric acid derivative to the patient in an amount sufficient to inhibit proliferation and/or induce apoptosis in cancer cells of the patient.

7. The gamma-aminobutyric acid for use according to any of claims 1 to 6, wherein the method comprises administering the gamma-aminobutyric acid derivative to the patient in combination with an NK1 receptor antagonist.

8. The gamma-aminobutyric acid for use according to claim 7, wherein the NK1 receptor is aprepitant, fosaprepitant or maropitant, or a pharmaceutically acceptable salt or prodrug thereof.

9. The gamma-aminobutyric acid for use according to claim 8, wherein the NK1 receptor antagonist is aprepitant or a pharmaceutically acceptable salt or prodrug thereof.

10. The gamma-aminobutyric acid for use according to claim 8, wherein the NK1 receptor antagonist is maropitant or a pharmaceutically acceptable salt or prodrug thereof.

11. The gamma-aminobutyric acid for use according to any preceding claim, wherein the method comprises administering the gamma-aminobutyric acid derivative to the patient in combination with an anti-tumour agent, preferably wherein the anti- tumour agent is a kinase inhibitor.

12. The gamma-aminobutyric acid for use according to claim 11, wherein the anti- tumour agent is selected from Imatinib, Trametinib, Vemurafenib, Sorafenib, and Cobimetinib; preferably wherein the anti-tumour agent is Imatinib.

13. The gamma-aminobutyric acid for use according to any preceding claim, wherein the method comprising administering the gamma-aminobutyric acid in a dose sufficient to provide a concentration of from 10 µM to 80 µM in the blood of the patient.

14. The gamma-aminobutyric acid for use in a method of treating cancer according to any preceding claim, wherein the proliferation of cancer cells is inhibited.

15. The gamma-aminobutyric acid for use in a method of treating cancer according to any preceding claim, which method comprises determining that the cancer cells express substance P.

16. The gamma-aminobutyric acid for use in a method of treating cancer according to claim 15 wherein the method comprises obtaining a sample of the cancer cells; analysing the said sample; and confirming that the cancer cells express substance P.

17. A pharmaceutical composition for use in a method of treating cancer as defined in any preceding claim in a patient in need thereof, comprising a gamma-aminobutyric acid derivative as defined in claim 1 or 2 together with a pharmaceutically acceptable excipient.

18. The pharmaceutical composition according to claim 17 which is an oral formulation.

19. A method of treating cancer in a patient in need thereof as defined in any one of claims 1 to 16, comprising administering to said patient a gamma-aminobutyric acid derivative as defined in claim 1 or 2 or a composition as defined in claim 17 or 18.

20. Use of a gamma-aminobutyric acid derivative as defined in claim 1 or 2 in the manufacture of a medicament for use in a method of treatment of cancer as defined in any one of claims 1 to 16 in a patient in need thereof.

21. Use of use of substance P as a biomarker to identify a cancer which is suitable for a method of treatment with a gamma-aminobutyric acid derivative as defined in any of claims 1 to 16.