EP0509056A1

EP0509056A1 - Target-specific, cytotoxic, recombinant pseudomonas exotoxin

Info

Publication number: EP0509056A1
Application number: EP91904103A
Authority: EP
Inventors: Ira Pastan; Vijay Chaudhary; David Fitzgerald
Original assignee: US Department of Health and Human Services; US Department of Commerce
Current assignee: US Department of Health and Human Services; US Department of Commerce
Priority date: 1990-01-02
Filing date: 1990-12-27
Publication date: 1992-10-21
Also published as: WO1991009949A1; CA2072891C; JPH05502032A; JP2610740B2; AU7242491A; EP0509056A4; CA2072891A1; AU644139B2

Abstract

Exotoxine de Pseudomonas, cytotoxique, recombinée spécifique à une cible. Lesdites toxines sont obtenues par insertion de molécules de reconnaissance spécifiques sur des sites de clonage spécifiques, dans au moins le domaine III au niveau de la terminaison carboxyle de la molécule PE. Diverses modifications de la terminaison carboxyle de la molécule PE augmentant la cytotoxicité, sont présentées.Pseudomonas exotoxin, cytotoxic, recombinant target specific. Said toxins are obtained by insertion of specific recognition molecules at specific cloning sites, in at least domain III at the carboxyl termination of the PE molecule. Various modifications of the carboxyl termination of the PE molecule increasing cytotoxicity are presented.

Description

TARGET-SPECIFIC, CYTOTOXIC, RECOMBINANT PSEUDOMONAS EXOTOXIN

The present invention is related generally to the making of improved recombinant immunotoxins. More particularly, the present invention is related to the construction of a recombinant Pseudomonas exotoxin

(rPE) with specific cloning sites for the insertion of recognition molecules at least at the carboxyl end of the PE to achieve target-directed cytotoxicity. BACKGROUND OF THE INVENTION

The mechanism by which protein toxins kill cells is quite complex. Many toxins bind to receptors on the surface of mammalian cells, are internalized by endocytosis, translocate to the cytosol and, there, exert an enzymatic activity that kills the target cell. Accordingly, these toxins have separate domains for cell binding, translocation and an enzymatic activity that inactivates an essential cellular function. Pseudomonas exotoxin A (PE) is a single polypeptide chain of 613 amino acids. X-ray crystallographic studies and mutational analysis of the PE molecule have shown that PE consists of three domains: an amino terminal cell receptor binding domain (Domain I); a middle translocation domain (Domain II); and a carboxyl terminal activity domain (Domain III). Domain III catalyzes the ADP ribosylation and inactivation of elongation Factor 2(EF-2) which inhibits protein synthesis and leads to cell death. Mutational analysis of Domain I has revealed that Lysine⁵⁷ plays a major role in receptor binding. Similarly glutamic acid⁵⁵³, Tyrosine⁴⁸¹ and histidine⁴²⁶ have been shown to be important for ADP-ribosylation activity. Recently mutational analysis of domain II has shown that certain portions of this domain are absolutely required for the cytotoxicity of PE.

While constructing various chimeric toxins in which growth factors were fused to a form of PE (PE40) which was devoid of domain I, it was observed that the recombinant fusion proteins, made by attaching TGFα, interleukin-2 or interleukin-4 at the carboxyl end of PE40 had poor cytotoxic activity. Hence, an examination of the role of the carboxyl terminus of the PE molecule (domain III) was undertaken. SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to determine the role of the carboxyl terminus of the PE molecule in its cytotoxic action. It is another object of the present invention to identify specific regions at the carboxyl terminus of the PE molecule for the insertion of recognition molecules for selective killing of target cells.

It is a further object of the present invention to provide an improved, target-specific, cytotoxic recombinant PE molecule, wherein the improvement comprises the insertion of target-specific recognition molecule at least in domain III at the carboxyl terminus of the PE molecule.

A still further object of the present invention is to modify the carboxyl end of the PE to increase the potency of the chimeric toxin.

It is yet another object of the present invention to make cytotoxic PE with two recognition molecules (target ligands) wherein either the same recognition molecule is placed at two different termini, such as the amino terminus and near the carboxyl terminus, for enhanced cellular binding, or two different recognition elements are inserted, one each at two different regions of the PE molecule, to enable more effective binding of the resulting PE molecule to the cell surface having two or more different entities, such as antigens , receptors and the like to which said recognition elements could bind.

It is an additional object of the present invention to provide a recombinant PE having repeat carboxyl end cytotoxic sequences for enhanced cytocidal activity. Various other objects and advantages will become evident from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and many of the attendant advantages of the invention will be better understood upon a reading of the following detailed description when considered in connection with the accompanying drawings wherein:

Figure 1 demonstrates cytotoxicity of PE and PE mutants on Swiss cells. Various dilutions of PE proteins were made in PBS containing 0.2% human serum albumin and added to 1x10⁵ Swiss 3T3 cells in 24-well plates. Sixteen hours later the cells were pulse labeled with 3H-leucine arid TCA precipitable cell associated radioactivity was determined as a measure of protein synthesis. The results are expressed as percent of control where no toxin was added. •------•

PE; O-------O PEΔ613; ♤-------♡ PEΔ 612,613; and Δ------Δ

PEΔ611-613. All the assays were done in duplicate and repeated twice.

Figure 2 shows the results of competition for the cellular uptake of recombinant PE. Swiss 3T3 mouse cells were incubated with 400 ng ³H-PE (specific activity 3.5x10⁵ DPM/μg) and increasing concentrations of purified mutant proteins for one hour at 37°C. Cell monolayers were washed and cell-associated radio- activity was determined. •-------• PE; ▲--------▲ PEglu57;

Δ--------Δ PE 612,613; O--------O PEΔ 613; ■--------■

PEgly²⁷⁶; ♤-------♤ PE 609-613; +--+ PE Δ609-

613+598-613.

Figure 3 shows immunofluorescence detection of binding and internalization of Pseudomonas exotoxin and its recombinant variants in Swiss 3T3 cells. Swiss 3T3 cells were incubated at 37°C for 30 minutes in the absence of toxin (A) or in the presence of 10 μg/ml of native Pseudomonas exotoxin (PE) (B), recombinant PE gly⁵⁷ (C) or recombinant PE 612,613 (D). Following this incubation, the cells were fixed in formaldehyde and further incubated in the continuous presence of saponin. The cells were incubated with mouse monoclonal anti PE (M40-1) (10 μg/ml), followed by affinity-purified rhodamine-labeled goat anti-mouse IgG (25 μg/ml). (Mags - x 400; bar = 10 μm).

DETAILED DESCRIPTION OF THE INVENTION The above and various other objects and advantages of the present invention are achieved by a cytotoxic recombinant Pseudomonas exotoxin (rPE) having a recognition molecule inserted at least in domain III at the carboxyl terminus of the PE for selective killing of target cells recognized by said recognition molecule without substantial cytotoxicity to other cells not recognized by said recognition molecule, and by a rPE with modified "cytotoxic sequence" with increased cell killing activity. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned hereunder are incorporated herein by reference. Unless mentioned otherwise, the techniques employed or contemplated herein are standard methodologies well known to one of ordinary skill in the art. The materials, methods and examples are illustrative only and not limiting. The term "recognition molecule" as defined herein means those molecules or ligands which recognize only target cells desired to be killed. Examples of such recognition molecules are antibodies or portions thereof that can recognize the target cells, growth factors, lymphokines, cytokineε, hormones and the like which specifically bind to molecules on the surface of the target cells.

The term "cytotoxic sequence" as used herein means those variety of amino acid sequences at the carboxyl end of the PE, the presence of which is a prerequisite for the cytocidal activity of the toxin and the repeat sequences of which may determine the level of cytotoxicity. The examples of such sequences are KDEL, RDELK and the like as will become apparent from the various embodiments of the sequences discussed herein below. MATERIALS AND METHODS

Materials

Unless mentioned otherwise, the materials and reagents used herein were obtained from commercial sources. Polymerization Chain Reaction (PCR) kit, Gene Amp Kit, was obtained from Perkin Elmer-Cetus, Norwalk, CT. Mutants and Plasmid Constructions

Mutants were created by oligonucleotide directed mutagenesis using plasmid pVC45f+T as described by

Jinno et al, 1988, J. Biol. Chem. 263, 13203-13207 and Jinno et al, 1989, J. Biol. Chem. 264, 15953-15959 or using polymerase chain reaction (PCR) as described below. pVC45f+T carries a PE gene under a T7 promoter and also contains a T7 transcriptional terminator and a fl phage origin. The PE gene also contains an OmpA signal sequence which is cleaved upon secretion of PE into the periplasm leaving a 3 amino acid (ala asn leu) extension at the amino terminus (Chaudhary et al, 1988, Proc. Natl. Acad. Sci. USA 85, 2939-2943). For PCR mutagenesis, two oligonucleotides and a 1.0 Kb Sall- EcoRI fragment of pVC45f+T were employed. One oligonucleotide was the same as nucleotides 2216-2236 of the PE gene (Gray et al, 1984, Proc. Natl. Acad. Sci. USA 81, 2645-2649). Other oligonucleotides were complementary to the 3' end of the coding sequence PE gene, contained desired mutations and created an EcoRI site after the stop codon. Other unique restriction sites were also created without changing amino acids to identify the mutants. A 30 cycle PCR was performed with denaturation at 94°C for 2 minutes, annealing at 55C for 1 min and polymerization at 72C for 3 min with 10 seconds extension per cycle using a gene amplification thermal cycler (Perkin Elmer Cetus). After the PCR, amplified fragment was cut with EcoRI and BamHI, it was purified using low melting point agarose. PCR fragments were ligated with a 4.5 Kb dephosphoylated EcoRI-BamHI fragment of pVC45f+T. Mutants were identified by unique restriction sites which were created during mutagenesis and finally confirmed by sequencing by Snager's dideoxy-chain termination procedure using Sequenase (US Biochemical Corp.). pVC4915f+T

This plasmid contains two mutations; Codon 608, CCG and 609, CGC were changed to CCC and GGG, respectively. This mutation results in glycine at 609 in place of arginine and creates a Smal site between codons 608 and 609. This plasmid was used to clone various carboxyl terminal fragments of PE. pVC4975f+T: A 1 Kb BamHI- Pstl fragment of pVC8 (Wozniak et al, 1988, Proc. Natl. Acad. Sci. USA 85, 8880-8884) was restricted with Narl, treated with T4 DNA polymerase to make blunt ends followed by EcoRI, and a 286bp fragment waε ligated to a 4.9 Kb dephophorylated Smal-EcoRI fragment of pVC4915f+T. pVC4985f+T: A 1 Kb BamHI-Pstl fragment of pVC8 was restricted with Hinfl, treated with T4 DNA polymerase followed by EcoRI and a 237bp fragment was ligated to the 4.9 Kb Smal-EcoRI fragment of pVC45f+T. pVC4995f+T: A synthetic oligonucleotide duplex VK192/193 (not shown), containing codons 598-613 of PE with a stop codon and an EcoRI compatible 3' end, was ligated to the 4.9 Kb Smal-EcoRI fragment of pVC4915f+T. pVC4715f+T: This plasmid was created by PCR mutagenesis and contains restriction sites Stul, Ndel, Smal, EcoRV and EcoRI within the 3' end of the PE gene and encodes amino acids RPHMPGDILK in place of PREDLK at 608 to 613. These unique sites were later used to make insertions and to attach various DNA segments encoding carboxyl terminal portions of PE. PVC47195f+T

This was created by ligating oligonucleotide duplex VK191/192 to a 4.9 Kb EcoRV-EcoRI fragment of pVC4715f+T. The carboxyl terminus of this PE mutant contains amino acids RPHMPGDPDYASQPGKPPREDLK in place of amino acids 608-613 (PREDLK) of PE. Plasmids for the insertion of receptor binding domains in the carboxyl end of PE Plasmid pVC4715f+T consists of DNA sequences encoding PE from amino acids l to 607 followed by a polylinker that contains Stul, Ndel, Smal, EcoRV and Aflll sites as well as encodes amino acids RPHMPGDILK. These sequence are under T7 promoter control and also contain shine-Dalgarno region and signal sequence from OmpA of E. coli. Plasmid pVC 4715/4E f+T is similar to pVC 4715 f+T, but also contains mutations in the receptor binding domain of PE (Domain I). These mutations are Lys⁵⁷—> Glu, His^246,249—> Glu and Arg²⁴⁷—> Glu.

Plasmid pVC 47195/f+T is similar to pVC4715 f+T but contains a polylinker with Stul, Ndel, Smal sites encoding amino acids RPHMPGI followed by the last 16 codons of PE that encode PDYASQPGKPPREDLK. Plasmid pVC 47195/4# f+T contains insertions of a cDNA encoding transforming growth factor a in the Ndel site of plasmids pVC 4715f+T and 4715/4Ef+T respectively. Plasmids pVC47395f+T and pVC 47195/4Ef+T are derived from 47195f+T and pVC 47195f+T by inserting TGFα sequences into the Ndel site. Plasmid pVC 47355/4Ef+t was derived from pVC47395/4Ef+T by deleting 6 amino acids, inserting TGFα and following it with 10 amino acids of the PE carboxyl end.

A deposit of pVC49415f+t and pVC47355/4Ef+t has been made at the ATCC, Rockville, MD., on December 28, 1989 under accession numbers 68198 and 68199, respectively. The deposit shall be viably maintained, replacing if it becomes non-viable during the life of the patent, for a period of 30 years from the date of the deposit, for 5 years from the last date of request for a sample of the deposit, whichever is longer, and upon issuance of the patent made available to the public without restriction in accordance with the provisions of the law. The Commissioner of the Patents and Trademarks, upon request, shall have access to the deposit. Protein Expression and Purification

Cultures of E. coli strain BL21 (ΛDE3) containing various plasmids were grown to OD650 of 0.6-0.8 and induced with 1 mM isopropyl-thiogalactoside for 90 minutes at 37 °C. Periplasmic fractions were prepared as described by Chaudhary et al, supra. By virtue of having an OmpA signal sequence, more than 90% of each of the expressed toxin proteins were secreted into the periplasm. These proteins have a residual ala asn leu sequence at the amino end left behind after the processing of the OmpA signal sequence. Periplasmic fractions were assayed for ADP ribosylation activity and cytotoxicity. Later PE mutants were purified ssing a MonoQ anion exchange column (HR5/5) attached to a Pharmacia FPLC system. PE and mutant proteins eluted at NaCl concentrations of 0.22-0.26M. Upon SDS-PAGE the toxins were more than 90% pure. Protein concentration was measured by the Bradford assay Reagent (BioRad.Richmond, CA) using bovine serum albumin as a standard. ADP Ribosylation and Cytotoxicity Assays

ADP ribosylation activity was assayed after the activation of PE and mutant proteins with 4M urea and 50 mM DTT unless otherwise stated (Collier et al, 1971, J. Biol. Chem. 246, 1496-1503). Cytotoxicity of PE mutants was determined by adding various dilutions of periplasmic proteins or purified proteins to 1 x 10⁵ Swiss 3T3 cells in 24-2311 plates as described by Jinno et al, (1988) supra, and Jinno et al, (1989) supra. ADP-ribosylation and cytotoxic activities of recombinant PE and native PE (from Swiss Serum and Vaccine Institute, Berne, Switzerland) were indistinguishable.

Toxin Binding and Internalization

The ability of various mutant PE proteins to compete the binding of ³H-labelled PE on Swiss cells as well as internalization of various mutant PE derivatives as studied by immunofluorescence has been described by Jinno et al, (1989), supra. Preparation of Target-Specific immunotoxins

PE expression vectors with the cloning sites in the carboxyl end of PE to produce selective cytotoxic molecules are illustrated here with TGFα, a recognition molecule which recognizes only EGF receptor bearing cells. These cloning sites were used to insert TGFα near the carboxyl end of PE which produced a very active molecule that killed EGF receptor bearing cells when the insertion was followed by the last 10 amino acids of PE (Table A). The details of the procedure are now described.

RESULTS The role of sequences at the carboxyl end of PE was determined by making a series of carboxyl terminal deletion mutants that removed 1,2,3,7,8,11,14 and 24 amino acids. Removal of two or more amino acids eliminated cytotoxicity without affecting ADP ribosylation activity (Table 1, Fig. 1). In fact, even eleven amino acids (603-613) could be removed without any loss of ADP ribosylation activity. However, removal of 14 amino acids resulted in a protein with low, but measurable, ADP ribosylation activity (600- 613) and removal of 24 amino acids (590-613) produced an enzymatically inactive protein. These results indicate that particular sequences at the carboxyl end of PE have a role in toxin action that are not required for ADP ribosylation activity.

The role of the carboxyl terminal sequences in toxin action was defined by creating a series of internal deletions and substitutions (Table 2). These mutations began at amino acid 602 so that ADP ribosylating activity would not be affected and extended to position 611. It was found that several small deletions encompassing amino acids 601-604 and 606-608 did not reduce cytotoxicity. Furthermore, two substitutions which altered amino acids 603-608 as well as two other substitutions within amino acids 606-608 of PE did not reduce cytotoxicity. Therefore, the sequence of amino acids in positions 602-608 did not appear to be important for cytotoxicity. However, deletions which removed arginine at 609 (pVC 49215 and pVC 49255) greatly reduced the cytotoxic action of PE. These results, together with the experiments in Table 1 showing that deletion of amino acids 612 and 613 abolish cytotoxicity, focused our attention on amino acids 609-613 which are situated at the carboxyl terminus of PE. The role of arginine 609 was studied by either deleting it or replacing it with several different amino acids. Replacement of arginine at 609 with another basic amino acid, lysine, retained the cytotoxic activity of PE (Table 3). However, deleting arginine at 609 (pVC 49115) or replacing it with glycine, glutamic acid or leucine reduced cytotoxocity about 6-10-fold. Thus, a basic amino acid appears to be important at position 609.

To study the sequence specificity of the last five amino acids of PE, several other mutant molecules were then constructed. In two of these, the order of the acidic amino acids at positions 610 and 611 was reversed and lysine 613 deleted (Table 4, pVC 49415 and pVC 49425). These molecules were fully active whether or not position 609 was a lysine or an arginine. A molecule was also created with a leucine at position 609 and an arginine at 612 (pVC 49435) that was inactive.

Although deletion of the terminal amino acid lysine at 613 did not affect cytotoxicity, it was suspected that other mutations in this position might affect cytotoxicity in a negative manner because of the low activity of various chimeric toxins in which the ligand was placed in peptide linkage at the carboxyl terminus of PE. Therefore, lysine ⁶¹³ was converted to glutamine, asparagine or aspartate. All these mutations produced a less cytotoxic molecule (Table 5). Addition of 6 or 11 amino acids to the carboxyl terminus of PE also produced a less cytotoxic molecule (data not shown). However, replacement of lys⁶¹³ with the basic amino acid, arginine, did not decrease cytotoxicity. Thus, positions 609 and 613 both require a basic amino acid for full cytotoxic activity. There are two other lysine residues at the carboxyl end of PE; these are situated at positions 590 and 606. Both of these lysines could be converted to the uncharged amino acid glutamine without a decrease in cytotoxicity, indicating that a positively charged amino acid was not required at position 590 or 606 (Table 5).

Having shown the importance of particular amino acids at the carboxyl terminus of PE, it was determined that the five carboxyl terminal amino acids could be separated from the ADP ribosylation domain to regenerate an active toxin. As shown in Table 6, a fully active cytotoxic molecule could be generated from PEΔ 609-613 (which is not cytotoxic) by the addition of amino acids 551-613, 567-613 or 598-613 of an intact PE to the carboxyl terminus of PEΔ 609-613. Thus, the distance between the ADP ribosylation domain which ends around amino acid 600 and the essential amino acids at positions 609-613 was not critical and could be substantially increased without a decrease in cytotoxicity. Also shown in Table 6 is a PE molecule with the carboxyl terminus of RPHMPGDILK in place of PREDLK. This molecule, in which arg⁶⁰⁹ and asp⁶¹¹ were altered, was not cytotoxic. But attaching the last 16 amino acids of an intact PE molecule to give a carboxyl terminus of RPHMPGDPDYASQPGKPPREDLK restored cytotoxicity to this molecule.

Furthermore, constructs were made in which a cDNA TGFα was inserted at the carboxyl end of PE with an inactive carboxyl terminus (Table A pVC 47315/4Ef+T) and an active carboxyl terminus (Table A, pVC 47355f+T and pVC 47395f+T). The constructs with good carboxyl termini were more than 50 times as cytotoxic to cells with EGF receptors (TGFα binds to the EGF receptor) as the ones with the bad carboxyl ends. This clearly indicates that for the highest cytotoxic activity, a suitable carboxyl end is an essential requirement.

Altogether the data presented herein demonstrate that the cytotoxic activity of a PE molecule that is inactive due to a deletion or modification within the carboxyl end can be restored by attaching an intact carboxyl end. Hence, it is now possible to create active chimeric molecules by inserting a binding ligand such as TGFα at 608 within the carboxyl end of PE thus retaining the last five amino acids as REDLK. Although it has been previously demonstrated that domain I of PE is the region responsible for cell binding, it was important to show that the mutations at the carboxyl end of PE that decreased cytotoxicity did not also somehow decrease cell binding. To test this, the ability of various mutant forms of PE to compete for the uptake of [³H]-PE was evaluated. As shown in Figure 2, several PE mutants that had decreased cytotoxicity due to mutations at the carboxyl terminus of PE were just as able to compete for the uptake of [³H]-PE as authentic wild type of PE. In this competition assay, PE40 which has a deletion of domain I and Peglu⁵⁷ were inactive as previously described (Jinno et al, supra).

These uptake results were confirmed using a fluorescence assay that measured the internalization of PE and various mutant PE molecules (Figure 3). In this assay, cells are incubated with various toxins for 30 minutes to allow binding and internalization into endocytic vesicleε. Molecules with a point mutation in domain I (PEglu⁵⁷) or PE40 were not internalized. In contrast, all the other PE molecules, whether or not they contained mutations at the carboxyl end of domain III, were found to have bound and internalized into endocytic vesicles and other elements in the trans- Golgi system in the perinuclear area of the cells (Figure 3, Panel B and D). These results clearly show that decreased cytotoxicity of carboxyl terminal mutants is not due to decreased receptor binding or cellular uptake of PE molecules.

In summary, the results presented herein clearly show that mutations at the carboxyl end of PE and particularly in the last five amino acids of PE result in a molecule with full ADP ribosylation activity, but greatly reduced cytotoxicity. The data show that the amino acid sequence at the carboxyl end of PE is Arg, Glu, Asp, Leu, Lys (REDLK, Table 2). The arginine at 609 can be replaced by lysine but non basic amino acids cannot be tolerated (Table 3). Lysine at 613 is not essential and can be deleted without loss of cytotoxic activity (Table 1), but it cannot be replaced with a non-basic amino acid (Table 5). Thus, having either ArgGluAspLeu or LysGluAspLeuLys at the carboxyl terminus produced a fully cytotoxic molecule (Table 4). A search of the literature for similar sequences that were present in other molecules and performed a specific biological function revealed that the sequence which retains newly formed proteins within the endoplasmic reticulum is LysAspGluLeu (Munro et al, 1987, Cell 48, 899-907). Therefore, several other mutant molecules were constructed, one of which contained the exact sequence previously described as being responsible for the retention of the protein in the lumen of the endoplasmic reticulum (Table 4). It was found that a molecule ending with LysAspGluLeu

(KDEL) was fully cytotoxic. Also a molecule ending in ArgAspGluLeu (RDEL) but not LeuAspGluArg (LDER) was fully active. These findings indicate that the successful entry of PE into the cytosol from an endocytic compartment requires interaction with the similar cellular component that helps retain proteins made by the cells within the endoplasmic reticulum. These findings also suggest that the sequence at the carboxyl end of PE acts as some type of recognition sequence to assist translocation of PE from an endocytic compartment into the cytosol. Other sequences that perform the same function would likewise increase the activity.

Of further significance was the finding that because the cell targeting ligands can be inserted at two cloning regions in the PE molecule (at the amino terminus as previously described or near the carboxyl end as described herein), the same or different targeting ligands can be inserted at these two regions thereby increasing either cell binding, cytotoxicity or both. Different targeting molecules at each of the two cloning regions would enable the chimeric toxin to bind to two different types of receptors present on the same cell. This is important because some antigens on target cells do not internalize well and are, therefore, poor targets for immunotoxins. But, if the toxin also binds to another antigen that is well internalized, specific cell killing is increased greatly.

It was further discovered during the modificational study of the carboxyl end of the PE that if the REDLK (single letter amino acid code) sequence is replaced with KDEL, the resulting molecule is about two fold more active. Even more spectacular was the finding that a molecule with three repeats of KDEL in place of REDLK, was three times as active (Table B) indicating that by adding KDEL or equivalent repeat sequences, chimeric toxins with enhanced cytotoxicity can be produced.

In short, the present invention for the first time shows that: 1. An appropriate carboxyl end sequence is absolutely required for cytotoxicity of the PE; 2. Deletion of as few as two amino acids from the carboxyl end of PE yields a molecule that contains full ADP ribosylation and receptor binding activity, but is nontoxic to target cells (Table 1); 3. Mutational analyses indicate that PE should possess a positively charged amino acid at 609, negatively charged amino acids at 610 and 611 and a leucine at 612;

4. Lysine at 613 can be deleted but cannot be substituted with several other amino acid residues; 5. Addition of random amino acid residues at the carboxyl end of PE produce relatively inactive molecules (data not shown).

6. Addition of at least 10 carboxyl end amino acids of PE to the PE molecules that are not cytotoxic due to mutations in the carboxyl end, restores full cytotoxic activity (Table 4);

7. Different targeting ligands at different ends (amino and carboxyl) provide the flexibility of producing better binding and cytocidal PE molecules; and

8. Repeat "cytotoxic sequences" multiply the cytotoxicity in appropriate cases.

Of course, other target-specific immunotoxins are made similar to the method described herein supra by using appropriate recognition molecules, toxins and cytotoxic sequences including such modified recombinants as TGFα-PE40 and the like.

It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.

Mutant PE proteins were expressed in E. coli and purified from the periplasm. Substitutions are shown as replacement amino acids (also see Tables 1 and 2).

Claims

WHAT IS CLAIMED IS:

1. A cytotoxic recombinant Pseudomonas exotoxin (PE) having a first recognition molecule inserted in domain III at the carboxyl terminus of the PE, the resulting PE being selectively cytotoxic only to target cells recognized by said recognition molecule.

2. The PE of claim 1 wherein a second recognition molecule is inserted in a region other than said carboxyl terminus.

3. The PE of claim 2 wherein said first and second recognition molecules are the same or different.

4. The PE of claim 1 wherein said recognition molecule is an antibody or a portion of an antibody which recognizes said target cell.

5. The PE of claim 1 wherein said recognition molecule is selected from the group consisting of a growth factor, lymphokine, cytokine and a hormone.

6. The PE of claim 1 wherein said recognition molecule is inserted after amino acid 607.

7. The PE of claim 1 wherein said carboxyl terminus has only the last 10 amino acid residues of PE following insertion at 607 and deletion of 608-613.

8. The PE of claim 1 wherein the amino acid after residue 608 comprises a cytotoxic sequence KEDL, RDELK or repeat thereof.

9. The PE of claim 8 wherein the amino acid after residue 608 comprises a cytotoxic sequence KDELKDEL.

10. The PE of claim 8 wherein the cytotoxic sequence after residue 608 is KDELKDELKDEL.

11. The PE of claim 8 wherein the repeat cytotoxic sequence yields a rPE molecule with enhanced cytotoxicity compared to a molecule without the repeat sequence.

12. A composition, comprising a cytotoxic amount of the PE of claim 1 and a pharmaceutically acceptable carrier.

13. A method for killing target cells, comprising contacting cells targeted to be killed, with a cytotoxic amount of the PE of claim 1.

14. A method for producing target-specific cytotoxic PE molecule, comprising inserting a first target- specific recognition molecule at least in domain III at the carboxyl terminus of the PE molecule.

15. The method of claim 14 further comprising replacing amino acid residues after 608 with the sequence KDEL or repeats thereof.

16. The method of claim 15 further comprising the step of inserting a second recognition molecule in a region other than said carboxyl terminus of the PE molecule.

17. The method of claim 16 wherein said second recognition molecule is the same as or different from said first recognition molecule.

18. The method of claim 17 wherein the second recognition molecule is inserted at the amino terminus of the PE molecule.