Background of the invention Female sexual dysfunction is not a well defined clinical entity, but can be characterized as the equivalence of male erectile dysfunction i. e. the inability of the female to reach sexual satisfaction. In contrast to the mechanism and etiology of male erectile dysfunction which have been extensively studied, only little is known about female sexual dysfunction; however, a reasonable hypothesis is inadequate vasodilatory capacity of the external genitals. Thus local drugs that restore the vasodilatory capacity of the genitals to the appropriate stimuli can be anticipated to improve the female sexual function. The prevalence of female sexual dysfunction is largely unknown, and dependent on age, but generally it can be estimated that such dysfunction is common e. g. amongst post menopausal women. Currently there is no adequate treatment of female sexual dysfunction other than hormonal treatment in post menopausal women.

While it is understood that many drugs with vasodilatory capacity administered e. g. orally may have effect on the genitals, and that certain systemic drugs e. g. sfidenafd used for male erectile dysfunction, may have beneficial effect also for the treatment of female sexual dysfunction, it is a clear advantage if the drugs can be administered locally in the genital tract to avoid systemic side-effects.

Summary of the invention We have now unexpectedly found that certain prostaglandin analogues of the E-type, more specifically agonists of the EP2 prostanoid receptor, exert good relaxant effect in the penile cavernous tissue, and in blood vessels while exhibiting markedly reduced pain inducing effect typical for many prostaglandins. Since the female external genitals embryologically correspond to the male genitals such as the penis, and expansive cavernous tissue is present in the genitals of both sexes it is very likely that the effect of certain prostaglandin analogues is fundamentally similar in the male and female.

Such drugs may therefore also be used to treat female sexual dysfunction.

In particular we have found that PGE2 analogues substituted in carbon 18,19 or 20 with hydroxyl (OH) exert beneficial effect, and especially one analogue, namely 19R- OH-PGE2, exhibited a surprisingly advantageous effect with respect to relaxation of cavernous tissue and blood vessels without inducing pain as studied in an animal model.

Thus, in a first aspect, the present invention provides a composition for the treatment of female sexual dysfunction, comprising a prostaglandin which is a selective EP2 or EP4 receptor agonist, or an active derivative, including the free acid, or a salt or an ester thereof optionally together with a physiologically acceptable carrier or vehicle.

In a second aspect, the invention provides a method of treating female sexual dysfunction, which method comprises administering a therapeutically active and physiologically acceptable amount of the composition to an individual in need thereof.

In a third aspect, the invention provides the use of a selective EP2 or EP4 receptor agonist, or an active derivative including the free acid, a salt or an ester thereof for the preparation of a medicament for the treatment of female sexual dysfunction.

Brief description of the Figures Fig. 1 is a diagram showing the concentration-response relation for PIGE, and 19R- hydroxy-PGE2 in isolated preparation of human corpus cavernosum precontracted with 10 M norepinephrine. Fig. 2 is a diagram showing the vasodilatory effect of PGE1 and 19R-hydroxy-PGE2 on rabbit penile vasculature precontracted with 10-6 M norepinephrine.

Detailed description of the invention Engorgement of the genital tissue is primarily based on marked relaxation of both arteries and cavernous tissue as demonstrated e. g. in the penis. PGEi and the EP2 receptor agonists cause vasodilation and relax the expansive tissue to about the same extent and thus promote an erectile response. However, PIGE, being a natural prostaglandin with low selectivity for a specific prostanoid receptor has a marked irritant effect and causes pain as evident in clinical trials performed with alprostadil e. g.

Padha-Natharn, 19973 and as will also be shown in the experimental part further below.

According to the invention, however, prostaglandin analogues with selectivity primarily for the EP2 receptor which induce the same beneficial effect but with significantly less irritant effect are much more preferable as it can be predicted that such analogues will cause no or only minimal pain in clinical practice. To facilitate the understanding of the invention, a general description of prostaglandins is first given.

The prostaglandins are fatty acids usually derived from the precursors eicosatrienoic, eicosatetraenoic or eicosapentaneoic acid through metabolic steps involving oxygenation. The prostaglandins typically carry a cyclopentane ring to which two carbon chains link, the upper chain usually being called the alpha chain and the lower the omega chain. The alpha chain is a 7 carbon chain with a terminal carboxylic acid moiety whereas the omega chain is an 8 carbon methyl-terminated aliphatic chain.

Depending on the number of double bonds in these chains subscripts of 1-3 are used.

In prostaglandins with subscript 1, e. g. PAGEL, the double bond is situated between carbons 13 and 14 in the omega chain, and it exhibits trans configuration in naturally occuring prostaglandins. In prostaglandins with subscript 2, e. g. PGE2, an additional double bond in the cis configuration exists between carbons 5 and 6 in the alpha chain, and finally in prostaglandins with subscript 3 a third double bond is situated between carbons 17 and 18 in the omega chain. This double bond also exhibits cis configuration in naturally occuring prostaglandins. All naturally occuring prostaglandins carry a-hydroxyl group in carbon 15, which is essential for biological activity. The substituents and the configuration of the cyclopentane ring determine whether the prostaglandin is of the A, B, C, D, E, F, G, H, I or J type as depicted in Scheme 1 below. The prostaglandins that have been used to exemplify the present invention are of the E-type, and the chemical structures of these prostanoids are depicted in Scheme 2. Except for 18RS-OH-PGE2-methyl ester the prostaglandin analogues were also used as acids.

Scheme I ~ PGE1-isopropyi ester 18RS-Hydroxy-PGE2-methyi ester 19R-Hydroxy-PGE2-methyl ester 20-Hydroxy-PGE2-methyi ester Scheme 2 The prostaglandins exert their pharmacologic effect through specific G protein coupled membrane receptors. There are at least 8 different receptors for the endogenous prostaglandins, namely the FP (PGF2=), EPI, EP2, EP3, EP4 (PGE2), DP (PGD2), IP (PGI2), and TP (TxA2) receptors, the most common naturally occuring ligand for respective receptor being shown within parentheses. At least for the EP3 receptor splice variants have been shown to exist. The EP2 and EP4 receptors are closely related, and probably mediate similar effects. We have therefore studied the effect only on the EP2 receptor, and regard it to represent the EP4 receptors as well in the case the compounds studied would have an effect on that receptor, too. The molecular biology and pharmacology of the prostanoid receptors have recently been reviewed by Coleman et al., 1994.

From a therapeutic point of view a problem with the endogenous prostaglandins is that they exert effects on many different prostanoid receptors. Each endogenous prostaglandin has a preference for one specific receptor type, but is not very selective and usually distinguishes poorly between the receptor subtypes, i. e. the EP receptors.

Thus PGE, and PGE2 are good ligands for all subtypes of the EP receptor.

Consequently selective effects on one of the subtypes of the EP receptor is impossible to achieve with the endogenous prostaglandins. However, certain synthetic prostaglandin analogues, e. g. butaprost, l l-deoxy-PGEl and AH13205 as well as a naturally occuring metabolite of PGE2, namely 19R-OH-PGE2, are selective EP2 prostanoid receptor agonists.

18-OH-PGE2, 19R-OH-PGE2 and 20-OH-PGE2 are effective EP2 receptor agonists with selectivity for the EP2 receptor over the EP3 receptor. The endogenous PIGE, is unselective and does not distinguish between the EP receptor subtypes sufficiently, and furthermore significantly spills over on the DP/IP receptors which e. g. the 18-, 19- and 20-OH substituted PGE2 analogues do not. However, PGEx has been included as a reference substance as it is the only prostaglandin currently in clinical use for the treatment of erectile dysfunction.

Accordingly, high selectivity or specificity to the EP2 receptor compared to other prostaglandin receptors, particularly the EP3 receptor, characterizes the compounds to be used in the method or compositions according to the present invention. It need not be said that the more specific the compound is for the EP2 receptor the better results are obtained, but a certain advantage is, of course, achieved also in cases of some interaction with other receptors. High selectivity in this connection means that the effect on the EP2 receptor is at least more than 5 times, especially more than 10 times, and in particular more than 100 or 1000 times the effect on other prostaglandin receptors.

As stated above, according to the present invention, in particular the selective EP2 receptor agonist 19R-OH-PGE2 and its carboxylic acid esters appear to be unique and ideally suited for the treatment of female sexual dysfunction. In this context it should be mentioned that 19R-OH.-PGE2 is a metabolite of PGE2 in the genital tract and can be found in large quantitiesDormally in human semen (Taylor and Kelly, 1974).

However, the physiological role of this unique metabolite is unknown. Thus, 19R-OH- PGE2 and its carboxylic acid esters constitute a very attractive medication for female sexual dysfunction since this analogue has a marked vasodilatory capacity, causes no pain, and furthermore occurs normally in the body.

With respect to 19R-OH-PGE which appears ideally suited for treatment of female sexual dysfunction, it should be observed that various modifications or substitutions of the molecule are possible as long as the new derivatives exhibit selective agonism on the E ? 2 receptor.

In Formula I, the wavy bonds represent the a (S) or ß (R) configuration, and the dashed bonds represent a single, a triple or a double bond in the cis or trans configuration. R in the formula is hydrogen, a salt moiety, e. g. an alkali or ammonium salt, a straight or branched saturated or unsaturated alkyl group, preferably with 1-10 carbons, an alicyclic ring, preferably with 3-8 carbons, arylalkyl, preferably aryl-C2_5 alkyl, or an aryl ring. X is a straight chain, saturated or unsaturated, preferably consisting of 2-5 carbons optionally interrupted by a heteroatom (O, S, N), and the chain may contain an alicyclic ring, preferably C3- C7 cycloalkyl, or an aryl or heteroaryl ring. Rl and R2, which are identical or different, are hydrogen, hydroxy, halogen, oxygen (=O or alkoxy) or an alkyl group with 1-3 carbons or analkoxy group with 1-3 carbons or an ester OCOR3, where R3 is a straight or branched, saturated or unsaturated alkyl group preferably containing 1-10, especially 1-6 carbons, or a cyclcalkyl, preferably containing 3-7 carbons, or an aryl or arylalkyl group, preferably aryl-C2-5 alkyl (e. g. benzyl).

Optionally CIO can be substituted e. g. to contain a mono-or dialkyl group.

Z is an alkyl chain of 1-8, e. g. 3-8 and 2-4 carbons, saturated or unsaturated, optionally interrupted by one or more heteroatoms (O, S, N), and containing one or more, preferably one substituent Y, attached to a carbon atom within the chain or a carbon-atom in terminal position. The chain Z may be branched, containing alkyl (preferably methyl) substituents and/or contain an alicyclic ring, e. g. a cycloalkyl, or an aryl (esp. a benzene) or heteroaryl ring, in inter-or terminal position. Y, is hydroxy, sulfhydryl, amino, methylamino, dimethyiamino, C, _3 alkoxy or halogen (Cl, Br, F) or oxygen (keto). In a preferred embodiment of the invention Y is attached to carbon 18,19 or 20. In the most preferred embodiment Y is OH and attached to carbon 19. At present it is believed that the inventive concept is based on the electronegative effect introduced by Y The prostaglandins may be epimeric mixtures as, well as in the form of the individual epimers.

Description of suitable embodiments The EP2 prostanoid receptor agonists according to the present invention can be used as the normal carboxyiic acids, salts (e. g. cationic) or as ester prodrugs, preferentially as carboxylic acid esters. The active compound can be administered topically in a pharmaceutically acceptable delivery medium. Cvclodextrins may be employed for solubilisation and stabilisation. Different delivery media, e. g. liniment, creams, gels, ointments, and possibly solid state forms may be employed. Gels, creams, ointments and solid state formulations may or may not contain preservative such as benzalkonium chloride, chlorhexidine, thiomersal, parabenzoic acid, and other compounds with satisfactory antimicrobial effect. The dose interval is 0.001-1 mg, typically 0.01-0.1 mg per application.

Accordingly the new medicanient should be administered locally in the genital tract.

Such treatment should. be initiated 5-60 min before intercourse.

Exemplification of the invention The test compounds PGEI-isopropyl ester, 18RS-hydroxy-PGE2-methyl ester, 19R- hydroxy-PGE2-methyl ester and 20-hydroxy-PGE2-methyl ester were dissolved in 0.5% polysorbate-80 as a stock solution, and were diluted in 0.5% polysorbate-80 to the appropriate concentrations in the various pharmacological experiments.

Synthesis of PGEl-isopropyl ester PGEi was obtained from Chinoin, Pharmaceutical and Chemical Works Co. Ltd., Budapest, Hungary.

DBU (644 mg, 4.2 mmol) in acetonitrile (1 ml) was added dropwise to a stirred solution of PGEl (300 mg, 0.84 mmol) in actonitrile (3 ml) at 0°C. The mixture was allowed to warm to room temperature whereupon isopropyl iodide (1142 mg, 6.72 mmol) in acetonitrile (2ml) was added dropwise. After being stirred for 1 Oh (TLC monitoring), the reaction mixture was quenched with water (8ml), the mixture was extracted with ethyl acetate (2x50mI), and the extract was washed with brine (10ml),citricacid3%(10ml), and finally sodium hydrogen carbonate 5% (2x lOml). After drying with anhydrous sodium sulphate, the solvent was removed in vacuo and the residual oil was chromatographed on silicagel using ethyl acetate as eluent This afforded 230 mg of the product (69%) of the title <BR> <BR> <BR> compound as a colorless oil: Rf = 0.18 (ethyl acetate); E NMR (CDC13) õ 0.89 (m, 3H), 1.2 (d, 6H), 1.21-1.4 (m, 10H), 1.42-1.62 (dm, 6H), 2.2-2.4 (dm, 4H), 2.7-2.75 (dd, 1H), (dm, 2H), 5.0 (m, 1H), 5.5-5.7 (dm, 2H).

Synthesis of 18RS-hydroxe-PGErmethyl ester 1. Preparation of Dimethyl-(2-oxo-5-heptyne)-phosphonate. (O To a stirred suspension of dry sodium hydride (3. 13 g, 124,07 mmol) in dry THF (100 ml) at room temperature was added dropwise a solution of dimethyl- (2-oxopropyl)- phosphonate (20,61 g, 124,07 mmol) in dry THF (50 ml) under N2. The reaction mixture was stirred for lh, then cooled in an ice-bath and treated with a solution of n- butyllittium (7.95 g, 124,07 mmol) in hexane, causing a dark brown solution to be formed. Stirring was continued for 1 h at 0 °C, followed by dropwise addition of 1- bromo-2-butyne. l (15 g, 112,79 mmol) in Ie (30 rnl). The reaction mixture was gradually warmed to room temperature and after 1 h (TLC monitoring) the reaction mixture was quenched with ice-water, HCI 1M to pH about 4 and extracted, twice, with ethyl acetate. The organic layer was washed with brine and chromatographed on silica gel using EtOAc as eluent. Rf=0.38 (silica gel, ethylacetate), yield 18g (73%). <BR> <BR> <BR> <BR> <BR> <BR> <BR> <P>2. (IS, 5R, 6R, 7R)-6-Formuyl-7- { (4-phenylbenzoyl) oxy}-2-oxabicyclo<BR> <BR> <BR> <BR> <BR> <BR> <BR> {3.3.0} octan-3-one-Gi) A mixture of Coreys lactone (23,06 g, 65,44 mmol)), DCC (40,50 g, 196,31 mmol)), DMSO (27,8 ml, 392,6 mmol) and phosphoric acid 85% (2,2 ml) in DME (130 ml) was stirred at room temperature for 2h (TLC monitoring). The precipitate was removed on silica-gel pad washed with DME (2 x 50 ml). The filtrate was concentrated in vacuo and used for the next step without isolation.

4. (1S, SR, 6R, 7R)-b- (3-Oxo-6-yn-IE-1-octenyi)-7- { (4-phenylbenzoyl) axy}-2- oxabicyclo {3.3.0} octan-3-one. (4) To a stirred suspension of sodium hydride (1,98 g, 78,53 mmol)) in DME (140 ml) under N2 was added dropwise the above phosphonate 2 (18,56 g, 85,07 mmol) and stirred mechanically at room temperature for 1,5 h. The mixture was then cooled to -5 °C, and a solution of the above crude Coreys aldehyde 3-was added dropwise. After 30 min at 0 °C and 2 h at room temperature (TLC monitoring), the reaction mixture was neutralised with citric acid 5% and extraced with ethyl acetate (2xl00ml). The organic layer was dried and evaporated. The residue (oil) was chromatographed on silica-gel using ethyl acetate and 10% methanol in ethyl acetate successively, giving a light yellowish oil. Rf = 0.58 (silica-gel, ethyl acetate), yield 52%.

5.. (IS, 5R, 6R, 7R)-6-(3, 6-Dioxo-lE-1-octenyl)-7-{(sphenylbenzoyl) oxy}-2-. xabicyclo {3.3.0} octan-3-one. (5) To the acetylenic solution 4 in acetonitrile: water 2: 1 (100 ml) was added mercuric oxide (13,8 g, 63,73 mmol)) and sulfuric acid 1M (25,49 ml, 25,49 mmol)). The reaction mixture was stirred magnetically. After about 1 h at room temperature (TLC monitoring) the reaction mixture was worked-up by addition of ethylacetate and HC1 IM. The organic layer was dried and evaporated. The crude oil was used for the next step without purification. Rf = 0.44 (silicagel, EtOAc), yield = 41%.

6. (lS, 5R, 6R, 7R)-6- (3RS, 6RS-Dihydroxy-lE-1-octenyl)-7- { (4-phenylbenzoyl) oxy}-2-oxabicyclo {3.3.0} octan-3-one. (6) To a stirred solution of the above diketone 5 (12,0 g, 26.1 mmol) and cerium chloride hepta hydrate (5.83 g, 15,64 mmol) at-20 °C in methanol: methylene chlorider 1: 1 was added sodium borohydride (1.48 g, 39,09 mmol)) in small portions under N2.

After 30 min (TLC monitoring). The reaction mixture was quenched with HC1 1M to pH about 4-5, and diluted with water (50 ml) and ethyl acetate (100 ml). The organic layer was separated and the water layer was washed twice with EtOAc, dried and evaporated The residue was purified on silica gel using EtOAc as eluent. The title compound 6 was obtained as colorless oil: yield 8.8 g (71%). Rf = 0,15,0,13 corresponding to the two isomers 15a and 15ß (silicagel, EtOAc).

.'H NMR (CDCI3) 8 0,9 (m, 3H), 1,4-1,8 (dm, 6H), 2,3 (d, 1I), 2, 5-2,9 (dm, 5H), 3,5 (m, 1H), 4,2 (m, 1H). 5.1 (m, 1H), 5,3 (m, 1H), 5,7 (m, 2H), 7,4 (m, 1H), 7,5 (m, 2H), 7,6-7,7. (dd, 4H), 8,1 (d,. 2H); 13C NMR (CDCL3) d 176,40 (C6H4C=0), 16591 (lactone C=O), 130,15,128,91,128,31,127,15,83,27,79,13, 55,34,85,34,01,33,43,32,95,32,09, 30,17,9,96.

7. (IS, SR, 6R, 7R)-6-(3RS, 6RS-Di t butyldimetnylsilyloxy-1E-1-octenyl)-7-f (4- phenylbenzoyl)oxy}-2-oxabicyclo {3.3.0}octan-3-one. (7) To a stirred solution of the above dihydroxy compound 6 (8,6 g, 18,51 mmol) in dichloromethane was added triethyl amine (12,83 ml, 92, 56 mmol)), t-butyldimethylsilyl chloride (13,95 g, 92,56 mmol)) and 4-dimethylamino pyridine (1,13 g, 9,26 mmol). The mixture was stirred. magnetically for 15 h at room temperature. The reaction mixture was diluted with ether, filtered and the precipitate was washed with ether. The organic layer was washed with brine, dried and concentrated in vacuo. The residue was chromatographed on silica gel using 5% ether in dichloromethane. Rf = 0.58 (silica gel, 5% ether in CH2C12). Yield 12,2 g (92%) 8. (lS, 5R, 6R, 7R)-6- (3RS, 6RS-Di t butyldimethylsilyloxy-lE-1-octenyl)-7-t ( hydroxy}-2-oxabicyclo {3.3.0} octan-3-one. (8) To a stirred solution. of the above disilyl ether 7 (12 g, 17,31 mmol) in methanol was added potassium carbonate (1,2 g, 8,66 mmol). The reaction mixture was stirred at room temperature for 4 h (TLC monitoring). The mixture was neutralised with citric acid 5%, extracted twice with ethyl acetate (100 ml), dried and concentrated in vacuo.

The oil was chromatographed on silica gel using gradient elution with 5% ether in CH2Cl2 and EtOAc: acetone 1: 1 successively. Ru=0.43 (silica gel, ethyl acetate), yield = 8,86 g (74%) 9. (lS, 5R, 6R, 7R)-6- (3RS, 6RS-Di t butyldimethylsilyloxy-lE-1-octenyl)-7- { (4-t- butyldimethylsilyloxy}-2-oxabicyclo {3.3.0} octan-3-one. (9) To a stirred solution of the above dislyloxy ether 8 (6,6 g, 12,86 mmol) in CH2C12 at room temperature was added triethylaminc (7,14 ml, 51,47 mmol) t-butyldimethyl- silyl chloride (7,76 g, 51,47 mmol) and 4-dimethyl aminopyridine (0.47 g, 3,9 mmol).

The reaction mixture was worked-up as in 7. The crude product was chromatographed on silica gel using 5% ether in dichloromethane to give a pure trisilyloxy ether product 9 as an oil. Rf = 0.62 (silica gel, 5% ether in CH2C12), yield 7,8 g, (96%) 10. (1 S, SR, 6R, 7R)-6-(3RS, 6RS-Di t butyldimethylsilyloxy-1E-1-octenyl)-7-{ (t- butyldimethylsilyloxy}-2-oxabicyclo {3.3.0) octan-3-ol. (10) A solution of diisobutyl aluminium hydride (DIBAL-H) (2,9 g, 20,87 mmol) in dry THF was added dropwise to a stirred solution of the above trisilyl ether lactone 9 (7,7 g, 12,24mmol) in TG (80 ml) at-72/-80 °C. After lh (TLC monitoring), the reaction mixture was quenched with ice (15 g) and ethyl acetate (150 ml), filtered and the filtrate was concentrated in vacuo. The residue was used directly without separation in the next step. Rf= 0.27 (Silica gel, 5% ether in dichloromethane).

11.11, 15RS, 18RS-Tri-t-butyldimethylsilyloxy-PGF (11) To a stirred suspension of 4-carboxybutyl-triphenylphosphonium bromide (21,70 g, 48,95 mmol) in dry THF under N2 at-5 °C was added potassium tert-butoxide (10,99 g, 97,91 mmol) and the mixture was stirred at room temperature for 30 min. To the resultant red-orange solution of ylide at-15/-10 °C was added the lactol 10 (7,7 g, 12,24 mmol) in THF, and the mixture was stirred for 2-3 h (TLC monitoring) at room temperature. The reaction mixture was diluted with water and quenched with citric acid 15 % to pH 6-7 and extracted with EtOAc, dried and concentrated in vacuo. The resultant slurry was used directly without isolation for the next step.

18RS-Tri-t-butyldimethylsilyloxy-PGF2a methyl ester (12) Methyl iodide (8,6 g, 61,20 mmol), was added to a stirred solution of the crude product 10 (8,73 g, 12,24 mmol) and N, N-Diisopropyl ethyl amine (9.473 g, 73,44 mmol) in acetonitrile at room temperature. After 15h (TLC monitoring) the mixture was diluted with water (100 ml) and ethyl acetate (150 ml), washed with sodium hydrogen carbonate 5% (60 ml) and brine (70 mi). The organic layer was dried and evaporated in vacuo. The residue was chromatographed on silica gel using EtOAc as eluent. Rf = 0.18 (silica gel, EtOAc : hexane 1: 9).

13.11, 15, 18RS-Tri-t-butyldimethylsilyloxy-PGE2 methyl ester (13) To a stirred solution of the compound 11 (3,3 g, 4,54 mmol) in dichloromethanne was added pyridinium chlorchromate (PCC). (3,9 g, 18,15 mmol) treated with aluminium oxide (Ig PCC was stirred with 5g aluminium oxide in acetone, the solvent was removed in vacuo giving a light yellow powder). The resulting suspension was stirred at room-temperature for 4h (TLC monitoring). The suspension was filtered on silica gel pad, washed with dichloromethane. The solvent was removed and the resulting oil was diluted with ether and washed with water (50 ml), sodium hydrogen carbonate 5% (50 ml). The solvent was removed in vacuo. The residue was chromatographed on silica gel using 5% ether in CH2C12. Rf = 0. 32 (silica gel, EtOAc: hexane 1: 9), yield 3,1 g (86%) 14.18RS-hydroxy PGE2 methyl ester (14) The protecting groups 11,15,18-tri-t-butyldimethylsilyl chloride were removed by addition of HF 4% (108 ml) to a solution of 13 (3,0 g, 3,98 mmol) in acetonitrile (300 ml). The reaction mixture was stirred ai room temperature for about 8h (TLC monitoring). The reaction mixture was worked-up by addition of EtOAc (200 ml).

The organic layer was separated and washed with sodium bicarbonate 5% and the pH was adjusted to about 6. The organic layer was washed with brine, dried and concentrated in vacuo. The residue was purified by column chromatography on silica gel using gradient elution with CH2Cl2, EtOAc and 5-10% methanol in ether successively (The stationary phase, silica gel, in the column must be washed with the eluent containing triethylamine before purification, to avoid isomerization) 18RS- hydroxy-15S-PGE2 Me ester Rf = 0.16 (5% MeOH in ether); yield 310 mg.

18RS-hydroxy-15R-PGE2 Me ester Rf = 0.20 (5% MeOH in ether); <BR> <BR> <BR> <BR> yield 248 mg.<BR> <BR> <BR> <BR> <BR> <BR> <P>18RS-hydroxy-PGE2 methyl ester 1H NMR (CDC13) 8 0,9 (t, 3H), (dm, 8H), 2,1 (m, 2H), 2,2 (m, 2H), (m, 5H), 2,7 (m, 1H), 3,6 (m, 1H, 18-CHOH), 3,7 (s, 3H), 4,05, (m, 1H, 15-CHOH), 4,2 (m, 1H, 1 I-CHOH), (dm, 2H, db), 5,6-5,8 (dm, 2H, db); 13C (NMR) 8 36,71,78,71,70,55,54, 34,30,29,30,07.

Scheme 3 Svnthesis of 19-R-hvdroxy-prostaglandin. E?-methyl ester 19R-hydroxy-prostaglandin E2 was obtained from Cayman Chemicals, Ann Arbor, Michigan, USA. Methyl iodide (9.2 mg, 0.065 mmol) in acetonitrile (1.0 ml) was added drop-wise to a stirred solution of 19R-hydroxy-prostaglandin E2 (4 mg, 0.011 mmol), and N, N-diisopropyl ethyl amine (7 mg, 0.054 mmol) in acetonitrile. More methyl iodide (4.5 mg, 0.032 mmol) in acetonitrile was added after 6 h and the stirring was continued for 12 h (TLC monitoring). The reaction mixture was quenched with water (5.0 ml) and extracted with ethyl acetate (2xlO ml), and the organic phase was washed with sodium hydrogen carbonate 5% (5 ml) and hydrochloric acid O. 5M (5 ml). After drying with anhydrous sodium sulphate, the solvent was removed in vacuo and the residue was chromatographed on silica-gel using ethyl acetate: acetone 1: 1 and acetone as eluent. This afforded 3.2 mg (72.7%) of the product as a colorless oil: Rif--0.17 (ethyl acetate: acetone: acetic acid 1: 1: 0.01) 1HNMR (CDC13) 1.25 (d, 3H), 1.5-1.7 (m, 8H), 2.1-2.6 (mm, 9H), 3.7 (s, 3H), 3.8 (m, 1H), 4.1 (m, 1H), 4-2 (rii, IH), 5.3-5.5 (dm, 2H), 5.6-5.8 (dm, 2H).

Synthesis of 20-OH-prostaglandin E-methvl ester The commercially available 20-hydroxy PGE2 (Cayman Chemicals, Ann Arbor, Michigan) (2.0 mg, 0.0054mmol) was esterified in acetonitrile (2,0 ml) with methyl iodide (4.6 mg, 0.0327 mmol) in the presence of N, N-diisopropyl ethyiamine (3.5 mg, 0.027mmol). The reaction mixture was stirred at room temperature for 10 h (TLC monitoring, silica gel, ethyl acetate). The reaction mixture was quenched with water (3,0 rnl) and extracted with ethyl acetate (2xlO ml). The organic layer was dried and concentrated in vacuo and the residual oil was chromatographed on silica gel using ethyl acetate: acetone 1: 0.5 as eluent; Rf= 0.38 (silica gel, ethyl acetate: acetone 1: 1).

Pharmacologicalexperiments The effect of the test compounds on cavernous tissue was tested using penile tissue.

As mentioned previously both in the male and female the genitals contain cavernous tissue, and the penile tissue can therefore be used as model for genital cavernous tissue. The general blood vessel relaxing effect of the test compounds was studied by investigating the immediate blood pressure decrease in rats upon intravenous infusion of the test compounds. The acute drop in blood pressure reflects a peripheral vasodilation in the animals brought about by the test compounds. The general irritative effect of the test compounds was studied in the cat eye which is very sensitive to noxious stimuli.

Human penile cavernous tissue was obtained fresh from surgery, and representative tissue samples were mounted in smooth muscle tissue baths containing a modified Kreb's solution consisting of NaCl 119 mM, Ksi 4.6 mM, MgCl2 1.2 mM, NaH2POa.

1.2 mM, NaHCO3 15 mM, CaCI2 1.5 mM and glucose 11 mM. The solutionalso-' contained indomethacin at a final concentration of about 3 pu. The solution was continuously bubbled by 95% 02 and 5% COz, and the temperature was kept at 37°C.

The tissue preparations were stretched by a force corresponding to 500 mg, and were given a contractile tone by the addition of norepinephrine at a concentration of 10-6 M.

Concentration-response curves were then constructed by addingcumulatively increasing concentrations of the prostaglandin analogues to the bath in a routine way.

The relaxant effect was normalized by comparing to that of carbachol in the same preparation. PGEI and 19R-OH-PGE2, respectively, were used as acids.

To study the effect of the PIGE, and 19R-OH-PGE2 on the penile vasculature, penile blood vessels of the rabbit were isolated and mounted as ring segments in a small vessel myograph (J. P. Trading, Denmark) containing a solution consisting of NaCI 119 mM, KCI 4.7 mM, CaCl2 1.5 mM, MgS041.17 mM, KH2PO4 1. 18 mM, NaHCO3 25 mM, EDTA 0.027 mM and glucose 11 mM. The solution also contained indomethacin at a final concentration of about 3 uM, and was continuously bubbled by 95% 02 and 5% CO2. The temperature was kept constant at 37 °C. The vessels were stretched and then precontracted using 104 M norepinephrine, and the vasorelaxant effect of the prostaglandin analogues was normalized by comparing it to that of papaverine in the same preparation. Cumulative concentration-response curves were constructed for the analogues.

The vasodilatory effect of the prostaglandin analogues was also studied in the rat by registering the blood pressure reducing effect. This is a relevant in vitro model for showing general vasodilatory effect of importance for inducing erection since vasodilation in penis is needed for achieving erection. Rats were anaesthetized with a mixture of ketamine and xylazin and the prostaglandin analogues were infused intravenously. The blood pressure was continuously registered in a femoral artery.

Each analogue was infused in 3 escalating doses in the same animal. None of the compounds was found to have any significant effect of the heart rate and the immediate reduction in arterial blood pressure therefore reflects an acute vasodilatory response to the prostaglandin analogues tested. In all experiments the blood pressure reduction was transient and the blood pressure increased immediately after termination of the infusion. The analogues were used both as esters and acids for the experiments.

The irritant effect of the prostaglandin analogues was tested using a behavioural model in cats. In this model the irritant effect on the eye is studied by registering the degree of eyelid closure and the: behaviour of the animals. Compounds that cause discomfort and pain in the eye cause the animals to close their eyes. The analogues were applied as carboxylic acid esters, to enhance bioavailability, topically on the eye as a single dose at different dose levels. The animals were then followed for several hours at regular intervals. Each compound and dose was tested on a group of 3-6 cats.

At least 3 days elapsed between two consecutive tests on the same animals. An arbitrary scale from 0 (absence ofirritation) to 3 (marked irritation), with half steps was used.

The relaxant effect on the human penile cavernous tissue ofPGEt, and 19R-OH-PGE2 are presented in Table I. It can be seen that PIGE, and 19R-OH-PGE2 were about equieffective and comparable to carbachol, however PGEI was slightly more potent than 19R-OH-PGE2. Since 19R-OH-PGE2 is a selective EP2 receptor agonist this finding indicates that the EP2 receptor accounts for most of the relaxant effect of PGEI. Furthermore, in Fig. l it is demonstrated that the concentration-response curves of PAGEL and 19R-OH-PGE2 are parallei and differ only with a factor of about 2-3, i. e. the latter analogue is about half to one third as potent as the former.

Table 1. Relaxation of human corpus cavemosum tissue induced by prostaglandins compared to carbachol (104 M). (MeaneSEM) Compound n Reduction intension(%)EC-50(Moles/I) Carbachol 8 PGEI 8 2.5 x 10-7 19R-OH-PGI.) 8 6.9 x 10' The vasodilatory effect of PGEI and the EP2 receptor agonist 19R-OH-PGE2 on rabbit penile blood vessels is demonstrated in Fig 2. It can be seen that the two prostaglandins had the same efficacy in inducing vasodilatation and were also about equipotent. This furthermore demonstrates that the EP2 receptor mediates most of the vasodilatory effect of PGEI. The vasodilatory effect of all the prostaglandin analogues as studied in the rat by investigating the immediate reduction in blood pressure upon intravenous infusion is presented in Table IL As can be seen in the table PIGE, and the three hydroxy-substituted PGE2 analogues effectively reduced the blood pressure in the anaesthetized rat, demonstrating an acute vasodilatory response.

Table TT. Reduction of blood pressure in anaesthetized rats by prostaglandin analogues with agonistic effect on the EP2 receptor. (n= 3 for each dose; MeanSEM) 2.5-3.5 ug/kg7. 5-lOug/kg25-35 ugg Prostaglandin Blood Blood Blood analogue. pressure o Reduction pressure o Reduction pressure o Reduction (mmHg) (%) (mmHg) (%) (mmHg) (%) PGEI 11420134105153551039 523 18RS-OH-PGE2 122. t28 73 1233 153 115t32 32+2 19R-OH-PGE2 115#14 13#10 103#12 31#3 91#9 43#6 20-OH-PGE2 9111 265 S510 383 916 44t0 Blood pressure o = blood pressure before infusion of prostaglandin analogue.

The irritant effects of the three prostaglandin analogues and PIGE, as studied in the feline eye are presented in Table m. All three hydroxy-substituted PGE2-analogues had significantly less irritant effect than PGEv-isopropyl ester and PIGE, acid. Most surprisingly we found that both 18RS-OH-PGE2-methyl ester and 19R-OH-PGE2- methyl ester had no or markedly reduced irritant effect, even at doses 1000 times higher than PGEI-isopropyl ester. Previously it has been shown that 19R-OH-PGE2 does not cause lid closure in rabbits but induces an irritant effect by inducing swelling of the ocular structures (Hall and Jaitly, 1977). We found no evidence of such irritation in the cat eye with 19R-OH-PGE2 or the other hydroxy-substituted PGE2 analogues. r To confirm that 19R-OH-PGE,-methyl ester, which is more hydrophilic than PGEI- isopropyl ester, indeed penetrates into the cornea and the intraocular parts of the eye we measured the intraocular pressure in 3 cats under local anaesthesia before and 1 hour after topical administration of the prostaglandin to the eye. The intraocular pressure was measured by pneumatonometry. One eye was treated with the test compound and the other eye received the vehicle only. In the eyes treated topically with 19R-OH-PGE2-methyl ester the intraocular pressure decreased from 16.3 +/-0.9 mmHg to mmEg, whereas it was 16.3+/-0. 9 mmHg and mmHg in the control eyes at the same time points. It is well known that prostaglandins reduce the intraocular pressure in cats (Bito et al., 1989), and it can thus be taken as an evidence that the drug has penetrated into the eye. Furthermore, from Table m it can also be seen that the PGE1 acid, which is a much less lipophilic compound than the 18-, 19-and 20-hydroxy-substituted PGE2-methyl ester analogues, caused significantly more irritation, and at much lower dose, than the hydroxy-substituted analogues. Thus the absence of pain and irritation after administration of 18RS-OH- <BR> <BR> <BR> PIGE2,19R-OH-PGE2 and 20-OH-PGE2 cannot be explained on the basis of increased hydrophilicity and poor bioavailability. Accordingly, selective EP2 prostanoid receptor agonists seem to be very advantageous in that they exhibit no, or markedly reduced irritant effect.

Table HI. Maximum irritative effect of prostaglandin analogues as studied in cat eyes.

The log P values were estimated based on thin layer chromatography with PGF2a isopropyl ester as reference (log P 4.5). ie= isopropyl ester, me= methyl ester.

Maximum irritation =3. MeanSEM (n= 3-6). Analogue log p-Dose: Dose: Dose: Dose: Dose: Dose: Dose: Dose: value 0.01 µg 0.1 µg 0.3 µg 1 µg 3 µg 10 µg 30 µg 100 µg Perte 4.7 0.4#0.2 1.7#0.3 2.8#0.1 - - - - - PGE, acid 0. 7 0. 60. 1-1. 60. 2-3. 00. 0 avoid OH-2'---0. 00. 0 0. 0i0. 0 0.0. 2 PGE2-me 19R-OH- PGER-me 2. 5-0. 00. 0-0. 10. 1-0. 10. 1-0. 70.4 20-OH-2.2 0. 5+-0.50. 0 1.204 Puez-me References Bito, L. Z., Camras, C. B., Gum, G. G. et al. (1989). The ocular hypotensive effect and side-effects of prostaglandins on the eyes of experimental animals. In: The ocular effects of prostaglandins and other eicosanoids (Eds.: L. Z. Bito and J. Sternschantz).

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