WO1994021256A1

WO1994021256A1 - Compositions for the treatment of eye diseases

Info

Publication number: WO1994021256A1
Application number: PCT/SK1994/000001
Authority: WO
Inventors: Alica Huxley; Georg Mathis
Original assignee: Ciba Vision Ag
Priority date: 1993-03-19
Filing date: 1994-03-17
Publication date: 1994-09-29
Also published as: AU6295094A; IL108924A0; ZA941921B

Abstract

The present invention relates to the use of a compound of formula (I), wherein R1 and R2 are each independently of the other C1-C4alkyl, or of a salt thereof, in the preparation of a pharmaceutical composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, for treating vasospastic constitutions of the eye or for treating diabetic retinopathy, and also to corresponding ophthalmic compositions.

Description

Compositions for the treatment of eve diseases

The present invention describes the use of a compound in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure. In particular, the use of a compound of formula (I) given below, or a salt thereof, in the preparation of a pharmaceutical composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, for treating vasospastic constitutions of the eye or for treating diabetic retinopathy, and also ophthalmic compositions comprising a compound of formula (I), or a salt thereof, are described.

The term glaucoma covers pathological symptoms of the eye that are attributable to elevated intra-ocular pressure. Obstruction of the outflow of aqueous humour often causes an increase in intra-ocular pressure. Chronically increased intra-ocular pressure has a damaging effect on the optic nerve and the retina, which can result not only in a restricted field of vision but also in blindness.

Accordingly, the search for active ingredients that are able significantly to reduce intra- ocular pressure is regarded as very important. In U.S. Patent No. 4906613 renin inhibitors are described as being suitable agents for the treatment of glaucoma. U.S. Patent No. 5 134 124 names a specific renin inhibitor that is suitable as an agent for the treatment of glaucoma and for the treatment of diabetic retinopathy.

Extensive pharmacological studies have shown that compounds of formula (I)

wherein Rj and R₂ are each independently of the other C₁-C alkyl, or their salts, are suitable to a surprising degree for reducing intra-ocular pressure. This effect is achieved both by the topical application of the active ingredient and by its systemic administration.

The compounds of formula (I) and salts thereof, together with other compounds in which Rj and R have other meanings and/or which are substituted by the same or different substituents in the central part of the molecule of formula (I), have already been described in European Patent Application EP 236734. The suitability of those compounds as renin inhibitors is also disclosed in that Application.

In contrast, the present invention relates to the use of a compound of formula (I), or a salt thereof, in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure, wherein Ri and R₂ are each independently of the other C₁-C alkyl. C₁-C alkyl is, for example, methyl, ethyl, n-propyl, isopropyl, 1-methylpropyl, 2-methylpropyl, n-butyl and tert-butyl. The configurations of the centres of asymmetry in the compounds of formula (I) are to be understood as being as shown and have already been described in European Patent Application EP 236734. The methine carbon atom in the 2-position of formula (I) may be in the R-, S- or R,S-configur- ation. Also preferred is the use of a compound of formula (I), or a salt thereof, in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure, wherein R_λ is C alkyl and R is C to C₄-alkyl. Additionally preferred is the use of a compound of formula (I), or a salt thereof, in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure, wherein R_j is C_r to C₄-alkyl and R₂ is alkyl. Also preferred is the use of a compound of formula (I), or a salt thereof, in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure, wherein Rj and R₂ are each independently of the other C₄alkyl. Also preferred is the use of a compound of formula (I), or a salt thereof, in the preparation of a pharmaceutical composition for the treatment of eye diseases associated with increased intra-ocular pressure, wherein Ri is tert-butyl and R₂ is n-butyl, and the methine carbon atom in the 2-position has the R- or the S -configuration.

A compound of formula (I) may be in the form of a salt, especially a pharmaceutically acceptable salt. Because a compound of formula (I) has, for example, at least one basic centre, it can form an acid addition salt. Such pharmaceutically acceptable salts are formed, for example, with strong inorganic acids, such as mineral acids, for example sulfuric acid, phosphoric acid or a hydrohalic acid, for example hydrochloric acid, with strong organic carboxylic acids, such as, for example, unsubstituted or halo-substituted C]-C alkanecarboxylic acids, for example acetic acid, propionic acid, chloroacetic acid or trifluoroacetic acid, saturated or unsaturated dicarboxylic acids, for example oxalic, malonic, succinic, maleic, fumaric, phthalic or terephthalic acid, hydroxycarboxylic acids, for example ascorbic, glycolic, lactic, malic, tartaric or citric acid, amino acids, for example aspartic or glutamic acid, or aromatic organic carboxylic acids, for example benzoic acid, or with organic sulfonic acids, such as unsubstituted or substituted, for example halo-substituted, - alkanesulfonic or aiylsulfonic acids, for example methane- or p-toluene-sulfonic acid. Corresponding acid addition salts can also be formed with any additional basic centre that may be present.

In view of the close relationship between a compound of formula (I) in free form and in the form of a salt, hereinbefore and hereinafter any reference to a free compound of formula (I) is to be understood as including also a corresponding salt and any reference to a salt is to be understood as including also a corresponding free compound of formula (I), where appropriate and expedient.

A surprising effect is that a compound of formula (I) or a salt thereof induces a vaso- relaxing effect in the eye both in the case of topical and in the case of systemic administr¬ ation and can accordingly be used in the treatment of vasospastic constitutions of the eye.

In addition, a compound of formula (I) or a salt thereof can be used in the treatment of diabetic retinopathy.

The present invention relates to the use of a compound of formula (I) or a pharmaceut¬ ically acceptable salt thereof in the preparation of a pharmaceutical composition, especially an ophthalmic composition, for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, and for treating vasospastic constitutions of the eye or diabetic retinopathy.

The present Application relates also to a method of treating glaucoma, of reducing intra- ocular pressure, of increasing the outflow of aqueous humour from the eye, and of treating vasospastic constitutions of the eye or diabetic retinopathy, which comprises adminis¬ tering to a patient requiring such treatment a therapeutically effective amount of a compound of formula (I) or of a pharmaceutically acceptable salt thereof.

The present Application relates also to an ophthalmic composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, or for treating vasospastic constitutions of the eye or diabetic retinopathy, which composition comprises a therapeutically effective amount of a compound of formula (I) or of a pharmaceutically acceptable salt thereof, a carrier and a solubiliser.

A corresponding ophthalmic composition is advantageously applied topically to the eye, especially in the form of a solution, an ointment, a gel or a solid insert Such compositions comprise the active ingredient, for example, in a range of from approximately 0.000001 to approximately 5.0 % by weight, preferably from approximately 0.001 to approximately 1.0 % by weight, or in the range of from approximately 0.01 to approximately 0.5 % by weight. The dose of the active ingredient may depend on various factors, such as mode of administration, requirement, age and/or individual condition.

There are used for a corresponding ophthalmic composition customary pharmaceutically acceptable excipients and additives known to the person skilled in the art, for example those of the type mentioned below, especially carriers, solubilisers, tonicity enhancing agents, buffer substances, preservatives, thickeners, complexing agents and other excipients. Examples of such additives and excipients can be found in U.S. Patents Nos. 5 134 124 and 4 906613. Such compositions are prepared in a manner known per se, for example by mixing the active ingredient with the corresponding excipients and/or additives to form corresponding ophthalmic compositions. The active ingredient is preferably administered in the form of eye drops, the active ingredient being dissolved, for example, in a carrier by means of a solubiliser. The solution is, where appropriate, adjusted and/or buffered to the desired pH and, where appropriate, a tonicity enhancing agent is added. Where appropriate, preservatives and/or other excipients are added to an ophthalmic composition.

In order to prepare suitable formulations, the active ingredient, a compound of formula (I), is mixed with a carrier suitable for topical or general administration. Suitable carriers are especially water, mixtures of water and water-miscible solvents, such as lower alkanols, vegetable oils or mineral oils comprising from 0.5 to 5 % by weight hydroxyethyl- cellulose, ethyl oleate, carboxymethylcellulose, polyvinylpyrrolidone and other non-toxic water-soluble polymers for ophthalmic uses, such as, for example, cellulose derivatives, such as methylcellulose, alkali metal salts of carboxymethylcellulose, hydroxymethyl- cellulose, hydroxyethylcellulose and hydroxypropylcellulose, acrylates or methacrylates, such as salts of polyacrylic acid or ethyl acrylate, polyacrylamides, natural products, such as gelatin, alginates, pectins, tragacanth, karaya gum, xanthan gum, carrageenin, agar and acacia, starch derivatives, such as starch acetate and hydroxypropyl starch, and also other synthetic products, such as polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl methyl ether, polyethylene oxide, preferably cross-linked polyacrylic acid, such as neutral Carbopol, or mixtures of those polymers. The concentration of the carrier is, for example, from 1 to 100000 times the concentration of the active ingredient.

The solubilisers used for the compounds of formula (I) are, for example, fatty acid glycerol polyglycol esters, fatty acid polyglycol esters, polyethylene glycols, glycerol ethers or mixtures of those compounds. A specific example of an especially preferred solubiliser is a reaction product of castor oil and ethylene oxide, for example the commercial product Cremophor EL®. Reaction products of castor oil and ethylene oxide have proved to be particularly good solubilisers that are tolerated extremely well by the eye. The concentration used depends especially on the concentration of the active ingredient. The amount added must be at least sufficient to dissolve the active ingredient. For example, the concentration of the solubiliser is from 1 to 1000 times the concentration of the active ingredient.

Examples of buffer substances are acetate, ascorbate, borate, hydrogen carbonate/- carbonate, citrate, gluconate, lactate, phosphate, propionate and tris buffers. The amount of buffer substance added is, for example, that necessary to ensure and maintain a physio¬ logically tolerable pH range.

Tonicity enhancing agents are, for example, ionic compounds, such as alkali metal or alkaline earth metal halides, such as, for example, CaCl₂, KBr, KC1, LiCl, Nal, NaBr or NaCl, or boric acid. Non-ionic tonicity enhancing agents are, for example, urea, glycerol, sorbitol, mannitol, propylene glycol, or dextrose. For example, sufficient tonicity enhancing agent is added to impart to the ready-for-use ophthalmic composition an osmolality of approximately from 50 to 400 mOsmol.

Examples of preservatives are quaternary ammonium salts, such as cetrimide, benz- alkonium chloride or benzoxonium chloride, alkyl-mercury salts of thiosalicylic acid, such as, for example, thiomersal, phenylmercuric nitrate, phenylmercuric acetate or phenyl- mercuric borate, parabens, such as, for example, methylparaben or propylparaben, alcohols, such as, for example, chlorobutanol, benzyl alcohol or phenyl ethanol, guanidine derivatives, such as, for example, chlorohexidine or polyhexamethylene biguanide, or sorbic acid. Where appropriate, a sufficient amount of preservative is added to the ophthalmic composition to ensure sterility.

The ophthalmic compositions may comprise further non-toxic excipients, such as, for example, emulsifiers, wetting agents or fillers, such as, for example, the polyethylene glycols designated 200, 300, 400 and 600, or Carbowax designated 1000, 1500, 4000, 6000 and 10000. Other excipients that may be used if desired are listed below but they are not intended to limit in any way the scope of the possible excipients. They are especially complexing agents, such as disodium-EDTA or EDTA, antioxidants, such as ascorbic acid, acetylcysteine, cysteine, sodium hydrogen sulfite, butyl-hydroxyanisole, butyl- hydroxytoluene or α-tocopherol acetate, stabilisers, such as β-cyclodextrin, hydroxy- propyl-β-cyclodextrin, thiourea, thiosorbitol, sodium dioctyl sulfosucrinate or monothio- glycerol, or other excipients, such as, for example, lauric acid sorbitol ester, triethanol amine oleate or palmitic acid ester. The amount and type of excipient added is in accord¬ ance with the particular requirements and is generally in the range of from approximately 0.0001 to approximately 90 % by weight.

The present Application relates also to an ophthalmic composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, and for treating vasospastic constitutions of the eye or diabetic retinopathy, which composition comprises a therapeutically effective amount of a compound of formula (I) or of a pharmaceutically acceptable salt thereof, a solubiliser, and a therapeutically effective pharmaceutical agent which may be, for example, an antibiotic, an anaesthetic, an anti¬ phlogistic, a further agent suitable for treating intra-ocular pressure, or another drug. The degree to which intra-ocular pressure has been reduced after administration of a pharmaceutical active ingredient of formula (I) according to the present invention can be determined, for example, in animals, especially in rabbits or monkeys. Two typical experi¬ mental procedures which illustrate the present invention, but are not intended to limit it in any way, are described below.

In vivo testing on rabbits of the "Fauve de Bourgogne" type in order to determine the activity of topically-applied compositions in reducing intra-ocular pressure can be carried out, for example, as follows: the intra-ocular pressure (IOP) is measured using an applan- ation tonometer before the experiment and at regular intervals. After local anaesthesia, the suitably formulated test compound is applied topically in a precisely defined concentration (for example 0.000001 - 5 % by weight) to one eye of the animal concerned. The contra- lateral eye is treated, for example, with physiological saline solution. The measured values so obtained are evaluated statistically.

The in vivo tests on monkeys of the species Macaca Fascicularis for determining the activity of topically-applied compositions in reducing intra-ocular pressure can be carried out, for example, as follows: the suitably formulated test compound is applied in a precisely defined concentration (for example 0.000001 - 5 % by weight) to one eye of the monkeys. The other eye is treated correspondingly, for example, with physiological saline solution. Before the start of the test, the animals are anaesthetised with intramuscular injections of, for example, ketamine. The intra-ocular pressure (IOP) is measured at regular intervals. The test is carried out and evaluated in accordance with the rules of good laboratory practice (GLP).

The effect of the present invention that is completely surprising to the person skilled in the art is that a compound according to formula (I) generally exhibits approximately twice the duration of action and also approximately twice the efficacy of the compounds described hitherto. That applies specifically to a compound of formula (II).

The Examples given below serve to illustrate the present invention further, they are not, however, intended to limit the scope thereof in any way.

R_! = tert-butyl R₂ = n-butyl, 2-(S) (π)

Rj = tert-butyl R₂ = methyl, 2-(R/S) (πi)

Rj = phenyl R₂ = n-butyl, 2-(R/S) (IV)

Example 1: Formulation of eye drops

compound of formula π 1.00 mg

CremophorEL® 50.00 mg

NaCl 9.00 mg benzalkonium chloride 0.10 mg

NaOH O.lN qs deion. water ad. 1.00 ml pH = 7.35 Example 2: Formulation of eye drops

compound of formula π 1.00 mg

Cremophor EL® 50.00 mg

NaCl 9.00 mg cetrimide 0.10 mg

NaOH 0. IN qs deion. water ad. 1.00 ml pH = 7.45

Example 3: Formulation of a compound

Using Cremophor, a 0.3 % solution of the compound of formula (II) (with 5 % Cremophor) is freshly prepared in the following manner: 250 mg of Cremophor are placed in an Erlenmeyer flask, 15 mg of active ingredient are weighed out and added thereto and dissolved directiy using external heat (for example a hair-drier). The solution is then adjusted to a pH of 7 with 0. IN NaOH and made up to a total volume of 5 ml with physio¬ logical saline solution.

Example 4: Testing of the compound of Example 3

First of all the influence of the solubiliser, in this case 5 % Cremophor in physiological saline solution, on intra-ocular pressure (IOP) is tested. The monkeys (Macaca Fasci- cularis species, both sexes) are anaesthetised before the experiment with an intramuscular injection of ketamine IMALGEN lOOO¹^ (20 mg kg). At intervals, i.e. every 20 to 30 minutes, each of the animals receives a dose of 0.5 mg/kg of IMALGEN 1000^. 6 monkeys are treated in one eye with 50 μl of the solution in blind manner. The contra- lateral eye is treated with 50 μl of physiological saline solution. At intervals (0, 30, 60, 120..360 min.) the IOP is measured using an applanation tonometer on both eyes and recorded. After a regeneration period of 1 week, the test compound is applied in an analogous manner to one eye (using 50 μl of the formulated compound solution according to Example 3). The contralateral eye is treated with 50 μl of physiological saline solution. The percentage reduction in the IOP is calculated as follows: 1/P₀-{P-P₀}-100, P₀ being the base value and P being the value actually read. Results: Treated eve

IOP (mm Hg) reduction in IOP time T (min) mean 1 SEM (%)

0 17.21 0.5 —

30 14.21 1.4* -17.417.8

60 13.01 1.0* -24.317.2

120 14.11 1.1* -17.7 + 7.5

240 15.310.7* -11.1 15.4

360 16.310.4 - 5.212.5

An asterisk after a value always indicates hereinbefore and hereinafter that the measured values (treated and contralateral eye) are compared with one another and, after evaluation using the so-called "paired student's t-test", have a statistically relevant significance of p ≤ 0.05.

Example 5: Testing of IOP-reducing activity of a 0.3 % solution of the compound of formula (II) in the normotensive rabbit model.

The formulation of the compound, 0.3 % active ingredient + 5 % Cremophor, is carried out analogously to Example 3. The test group consists of 6 female rabbits of the "Fauve de Bourgogne" type weighing from 2 to 3 kg. The rabbits' eyes are first of all anaesthetised locally using 50 μl of NOVESINEN⁰ (1:10 dilution), then 30 μl either of the test compound or of physiological saline solution are applied once in blind manner to both eyes. Before the experiment and after the application of the active ingredient, the values of the IOP in both eyes are measured at intervals (30, 45, 60, 120, 180, 240, 300, 360 min.) using an applanation tonometer. The percentage reduction in IOP attributable to the active substance applied (0.3 % + 5 % Cremophor) is calculated as follows: 1/P₀-{P-P₀}-100. (P₀ is the base value of the IOP of the anaesthetised animal, P = value actually measured. Results: time T (min) (%) reduction in IOP mean 1 SD

15 -23.215.7*

30 -24.1 17.1*

45 -24.1 16.2*

60 -26.214.5*

120 -22.1 14.3*

180 -18.214.7*

240 -15.1 12.8*

300 - 9.413.2*

360 - 2.9 + 4.9* (duration of action ≥ 5 hours)

Example 6: Testing of IOP-reducing activity of a 0.01 % solution of the compound of formula (II) in the normotensive rabbit model analogously to Example 5. The formulation of the compound, 0.01 % active ingredient + 5 % Cremophor, is carried out in a manner analogous to Example 3. Results:

time T (min) (%) reduction in IOP mean 1 SD

15 -22.814.4*

30 -22.914.8*

45 -19.214.6*

60 -19.214.6*

120 -15.612.8*

180 -10.714.7*

240 - 2.315.4 (duration of action ≥ 3 hours)

Example 7: Testing of IOP-reducing activity of a 0.3 % solution of the compound of formula (HI) in the normotensive rabbit model analogously to Example 5. The formulation of the compound, 0.3 % active ingredient + 5 % Cremophor, is carried out analogously to Example 3. Results: time T (min) (%) reduction in IOP mean 1 SD

15 -20.1 16.5*

30 -20.215.2*

45 -20.1 16.5*

60 -17.815.7*

120 -13.014.8*

180 -11.817.2

240 - 8.215.3

300 - 4.712.6

360 - 7.1 17.7

(duration of action approximately 2 hours)

Example 8: Comparison of the IOP-reducing activity of a 0.3 % solution of the compound of formula (II) in the form of a free base, on the one hand, and in the form of a salt (methanesulfonate), on the other, in the normotensive rabbit model. The active ingredients are formulated analogously to Example 3 and the test is carried out analogously to Example 5. Results:

time T (min) (%) reduction in IOP mean 1 SD free base methanesulfonate

15 -23.215.7* -21.214.9*

30 -24.117.1* -24.213.4*

45 -24.1 16.2* -27.212.4*

60 -26.214.5* -27.212.4*

120 -22.1 14.3* -25.214.0*

180 -18.214.7* -21.1 15.8*

240 -15.1 12.8* -18.1 14.9*

300 - 9.413.2* -12.015.1

360 - 2.914.9 - 4.915.8

Example 9: Testing of the IOP-influencing activity of 5 % Cremophor mixed with physio¬ logical saline solution in the rabbit model. The test is carried out analogously to Example

5. time T (min) (%) reduction in IOP mean 1 SD

15 -1 1 10

30 -1 12.6

45 1.1 14.6

60 0.1 15.5

120 1.1 14.6

180 0.014 (no significant effects)

Example 10: Testing of the IOP-influencing activity of physiological saline solution in the rabbit model. The test is carried out analogously to Example 5.

time T (min) (%) reduction in IOP mean 1 SD

15 -4 18.5

30 -0.914.5

45 -1 14.6

60 -1.916.3

120 0.1 15.4

180 0.1 13.8 (no significant effects)

Example 11; Formulation of a comparison compound of formula (TV). 50 mg of Cremophor are weighed into a small Erlenmeyer flask, and 3 mg of the compound of formula (TV) are added thereto and dissolved with stirring and heating, for example with a hair-drier. After the addition of physiological saline solution to a total volume of 1 ml, a clear solution having a pH of 7 has been obtained.

Example 12: Testing of the IOP-reducing activity of a comparison compound of formula (IV) with 0.3 % active ingredient. The active ingredient solution is obtained in accordance with Example 11. The test is carried out analogously to Example 5. Results: time T (min) (%) reduction in IOP mean 1 SD 15 -11.913.6*

30 -11.913.6*

45 -8.9 1 3.1*

60 -7.9 16.1

120 -8.9 16.1

180 -5.9 15.3

240 -4.9 14.4

300 -2.9 13.2

(duration of action approximately 45 minutes)

Example 13: Formulation of a comparison compound of formula (V)

50 mg of Cremophor are weighed into a small Erlenmeyer flask, and 1 mg of the active ingredient is added thereto and dissolved with stirring and heating, for example with a hair-drier. After the addition of physiological saline solution to a total volume of 1 ml, a clear solution having a pH of 7 has been obtained.

Example 14: Testing of the IOP-reducing activity of a comparison compound of formula (V) with 0.1 % active ingredient. The active ingredient solution is obtained in accordance with Example 13. The test is canned out analogously to Example 5. Results:

Eye treated untreated time T (min) IOP in mm Hg (mean) % reduc¬ tion

0 17.0 16.8 0

15 16.5 16.6 0

30 16.1 16.3 0

45 16.5 16.5 0

60 16.6 16.3 0

120 16.5 16.0 0

180 16.5 16.2 0

240 15.2 15.1 0

300 16.5 15.6 0 The comparison compound of formula (V), at a concentration of 0.1 % (with 5 % Cremophor), exhibits no IOP-reducing activity.

Claims

What is claimed is:

1. The use of a compound of formula I

wherein R_x and R₂ are each independently of the other C₁-C₄alkyl, or of a pharmaceuti¬ cally acceptable salt thereof, in the preparation of a pharmaceutical composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, for treating vasospastic constitutions of the eye or for treating diabetic retinopathy.

2. The use of a compound of formula I according to claim 1, or of a pharmaceutically acceptable salt thereof, in the preparation of a pharmaceutical composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, or for treating diabetic retinopathy.

3. The use of a compound of formula I according to claim 1, or of a pharmaceutically acceptable salt thereof, in the preparation of a pharmaceutical composition for treating glaucoma or for reducing intra-ocular pressure.

4. The use of a compound of formula I according to claim 1, or of a pharmaceutically acceptable salt thereof, the carbon atom in the 2-position of formula I having the R- or the S -configuration.

5. The use of a compound of formula I according to claim 1, or of a pharmaceutically acceptable salt thereof, wherein R_x is alkyl and R₂ is C to C -alkyl.

6. The use of a compound of formula I according to claim 1, or of a pharmaceutically acceptable salt thereof, wherein Rj is C_r to C₄-alkyl and R₂ is C₄alkyl.

7. The use of a compound of formula I according to claim 1 , or of a pharmaceutically acceptable salt thereof, wherein R_: and R₂ are each independently of the other alkyl.

8. The use of a compound of formula I according to claim 4, or of a pharmaceutically acceptable salt thereof, wherein R_x is tert-butyl and R₂ is n-butyl.

9. An ophthalmic composition for treating glaucoma, for reducing intra-ocular pressure, for increasing the outflow of aqueous humour from the eye, or for treating vasospastic constitutions of the eye or diabetic retinopathy, which composition comprises a therapeut¬ ically effective amount of a compound of formula I according to claim 1 or of a pharma¬ ceutically acceptable salt thereof, a carrier and a solubiliser.

10. An ophthalmic composition according to claim 9, wherein the therapeutically effective amount of a compound of formula I is from 0.000001 to 5 % by weight

11. An ophthalmic composition according to claim 9, wherein the therapeutically effective amount of a compound of formula I is from 0.001 to 1 % by weight

12. An ophthalmic composition according to claim 9, wherein the therapeutically effective amount of a compound of formula I is from 0.01 to 0.5 % by weight.

13. An ophthalmic composition according to claim 9, which comprises a therapeutically effective amount of a compound of formula I according to claim 1 or of a pharmaceut¬ ically acceptable salt thereof, a carrier, a solubiliser and a buffer substance.

14. An ophthalmic composition according to claim 9, which comprises a therapeutically effective amount of a compound of formula I according to claim 1 or of a pharmaceutic¬ ally acceptable salt thereof, a carrier, a solubiliser, a buffer substance and a tonicity enhancing agent

15. An ophthalmic composition according to claim 9, which comprises a therapeutically effective amount of a compound of formula I according to claim 1 or of a pharmaceutic¬ ally acceptable salt thereof, a carrier, a solubiliser, a buffer substance, a tonicity enhancing agent and a preservative.

16. A method of treating glaucoma, of reducing intra-ocular pressure, of increasing the outflow of aqueous humour from the eye, of treating vasospastic constitutions of the eye or of treating diabetic retinopathy, which comprises administering to a patient requiring such treatment a therapeutically effective amount of a compound of formula I according to claim 1 or of a pharmaceutically acceptable salt thereof.