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Alpha chain unsaturated derivatives of misoprostol
PROSTAGLANDINS
Alpha
Chain Unsaturated Derivatives of Misoprostol
Paul W. Collins, Alan F. Gasiecki, Richard M. Weier, Steven W. Kramer, Peter H. Jones, Gary W. Gullikson, Robert G. Bianchi and Raymond F. Bauer. G a s t r o i n t e s t i n a l Diseases Research G.D. Searle & Co. 4901 Searle Parkway Skokie, IL 60077
Department
Abstract Misoprostol, a 1 5 - d e o x y - 1 6 - h y d r o x y - 1 6 - m e t h y l analog of PGE4, is an effective agent for the treatment of peptic ulcer disease. Efforts to impede metabolic degradation of the alpha chain of misoprostol led to the d i s c o v e r y of a s~cond clinical candidate in this series. Enisoprost, a a-Z derivative of misoprostol, is more potent as a gastric a n t i s e c r e t o r y agent and longer acting than misoprostol. These findings prompted further work to determine the effects that two double bonds in the alpha chain might have on the activity profile of misoprostol. The most promising structure in this series was a i:I mixture of 3E,5Z and 3Z,5Z dienes which was about three times more potent than misoprostol in inhibiting gastric secretion in dogs, while the separation of the diarrheogenic effect was s i g n i f i c a n t l y improved. Chromatographic separation of the mixture was very difficult, but small amounts of each isomer were obtained by HPLC, and preliminary a n t i s e c r e t o r y studies indicated that most of the activity resided in the 3E,5Z isomer. A stereospecific synthesis of the 3E,5Z isomer was carried out to provide sufficient quantities for complete pharmacological assessment. The 3E,5Z diene was about three times more potent than misoprostol in inhibiting gastric acid secretion in dogs and also in producing diarrhea in rats. Introduction Misoprostol (Scheme i), a 1 5 - d e o x y - 1 6 - h y d r o x y - 1 6 - m e t h y l analog of naturally occurring PGE1, represents a major advance in the development of synthetic p r o s t a g l a n d i n s for the treatment of peptic ulcer disease and related conditions.(1) It is currently approved and marketed in a p p r o x i m a t e l y eighteen countries and is under c o n s i d e r a t i o n for approval in most other markets.
Key Words: enisoprost, gastric antisecretory, diarrhea, p a l l a d i u m (O) coupling, ~ chain dienes, cuprate, nickel boride.
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PROSTAGLANDINS
Scheme I SYNTHETIC DEVELOPMENT OF MISOPROSTOL O ..~,.~-"~ COOH ----
~
is
H(~
Hg
0
Pi E1
H(~
OH 16-H, 16-OH
CH 3
H(~
OH 16-METHYL,16-OH MISOPROSTOL
Discovery of Misoprostol The discovery of misoprostol arose from a chemical program begun at Searle in the early seventies to synthesize analogs of PGE1, with the objective o~ improving its oral gastric antisetretory activity, side effect profile and duration of action. The first breakthrough in that moving the 15-hydroxy 16-position significantly prostaglandin side effects antisecretory activity.(2) The key compound,
18
this program was the finding group of PGE I to the adjacent reduced many ~f the typical yet did not decrease gastric
15-deoxy-16-hydroxy
PGE 1 methyl ester
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(Scheme I), was a p p r o x i m a t e l y equlpotent to PGE 1 in inhibiting gastric secretion by intravenous administration. Side effects such as trembling, emesis and diarrhea, which are u s u a l l y observed with PGE 1 and synthetic 1 5 - h y d r o x y prostaglandins, were absent at effective a n t i s e c r e t o r y doses of the 16-hydroxy compound. However, it was only w e a k l y active by oral a d m i n i s t r a t i o n and its d u r a t i o n of action was quite short. Enzymatic studies indicated that the 16-hydroxy group was also a substrate for the d e h y d r o g e n a s e enzyme which inactivates 15-hydroxy prostaglandins. In an effort to block this oxidative degradation, the 16-methyl, 15,15-dimethyl, and the 1 7 , 1 7 - d i m e t h y l analogs were prepared. The addition of a methyl group at carbon 16 d r a m a t i c a l l y increased oral potency and d u r a t i o n of action and completed the synthetic d e v e l o p m e n t of m i s o p r o s t o l . ( 3 , 4 ) In contrast, addition of two methyl groups at either carbons 15 or 17 greatly d i m i n i s h e d gastric a n t i s e c r e t o r y activity.(4) Discovery
o f Enisoprost
Following the d i s c o v e r y of misoprostol, our research was directed toward o p t i m i z i n g the desirable properties of this compound. For instance, we began to examine various ways of b l o c k i n g or impeding E-oxidation of the alpha side chain of m i s o p r o s t o l in order to further improve its duration of action. Efforts in this area eventually led to the d i s c o v e r y of a second clinical candidate in this series, enisoprost. Enisoprost is the 4-cis (4Z) u n s a t u r a t e d analog of misoprostol and currently is in Phase I clinical study. The decision to explore the effects of a cis double bond at the "unnatural" 4,5 position of teh-6-molecule was based on reports that this m o d i f i c a t i o n impeded E-oxidation (5) but did not alter gastric a n t i s e c r e t o r y activity (6) of other p r o s t a g l a n d i n compounds. The c o r r e s p o n d i n g 4-trans (4E) and 4,5 acetylene d e r i v a t i v e s were also prepared (7) because the influence of t:~ese m o d i f i c a t i o n s on gastric a n t i s e c r e t o r y activity and E-oxidation of p r o s t a g l a n d i n s was not known. The three d e r i v a t i v e s were tested in histamine stimulated Heidenhain pouch (HP) dogs by intravenous administration. The introduction of a Z double bond at the 4,5 position of m i s o p r o s t o l resulted in a three-fold increase in gastric a n t i s e c r e t o r y potency. In contrast, the presence of an E double bond at this p o s i t i o n reduced activity by 30 times, while, surprisingly, the acetylenic derivative was devoid of gastric a n t i s e c r e t o r y activity at a dose 300 times the ED~0 of misoprostol. (7) The 4Z compound, enisoprost, wag then compared to misoprostol in gastric fistula dogs by intragastric a d m i n i s t r a t i o n (Table I). Enisoprost was a p p r o x i m a t e l y 5 times more potent than misoprostol in this assay. Also, studies in meal stimulated Pavlov pouch dogs indicated that enisoprost had a longer duration of action than misoprostol.
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PROSTAGLANDINS
Table I
Pharmacological Comparison of Misoprostol and Enisoprost with 3,5 Diene Analogues Compound
EDsopglKg IG Gastric Antisecretory Effects in D o g s
Activity Ratio
Diarrheal Effectsin Rats
EDH Diarrhea EDs0Antisecretory
Misopro~ol Enisopro~ 4,5c is
.28 .05
382 62
1364 1240
5 5F.JZ
.08 .08
642 131
8025 1638
Diarrhea is a major side effect of natural and synthetic prostaglandins in animals (8) and man. (9-11) Previous studies in rats established that misoprostol had a much greater separation of gastric antisecretory activity from diarrheogenic activity than standard 15-hydroxy prostaglandins. (7) To compare the relative separation of these two effects for enisoprost, diarrheogenic activity was determined in rats and an activity ratio calculated (Table I). Enisoprost was more diarrheogenic than misoprostol, but showed about the same degree of separation of the two activities as misoprostol because of its greater antisecretory potency. Alpha Chain Diene Derivatives o__ffMisoprostol Based on these findings, we decided to investigate the effects that two double bonds in the alpha chain might have on the intensity and duration of the gastric antisecretory activity of misoprostol. Seven diene derivatives of misoprostol were prepared with variations in both position and configuration of the double bonds. (12) The intravenous gastric antisecretory activities of these compounds were determined in histamine stimulated Heidenhain Pouch (HP) dogs at a test dose of 1.0 pg/kg (Table II). Comparative data are given for misoprostol and enisoprost. Compounds 4, 5 and 7 displayed good gastric antisecretory activity in this assay but on
20
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followup evaluation, only the conjugated diene 5 which is approximately a i:I mixture of 3Z,5Z (3-cis, 5-cis) and 3E, 5Z (3-trans, 5-cis) isomers, showed a-n--inter--6-{ting level of a ~ t y . --T~e intragastric antisecretory potency of 5 was approximately equal to that of enisoprost and about three times that of misoprostol (Table I).
Table II
Gastric Antiseereto~ Activity of Alpha Chain Dienes in Dogs o
Compound
Structure
Percent Inhibition of Total Acid Output (TAO) at lpg/kg I.V.
Misopro~ol Enisoprost 2E, 5Z 2E, 5E
70 92 0 68
5
3£/Z, 5Z
83
6 7 8 9
3E, SE 3Z, 5E 2E, 4Z 2E, 4E
21 96 41 53
3 4
The diarrheogenic activity of 5 in rats was quite low, being somewhat less than misoprostol and about ten times less than enisoprost. Comparison of activity ratios of diarrheal to antisecretory EDen values clearly demonstrates that the separation of these activities is considerably greater for 5 than for either misoprostol or enisoprost (Table I). The very desirable pharmacological profile of 5 prompted a more thorough investigation of the chromatographic separation of its geometric isomers. Numerous systems were tried but an efficient and practical method was not found. However, one HPLC system (silver nitrate treated Partisil SCX) did provide small amounts (3-4 mg) of the
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PROSTAGLANDINS
individual geometric isomers. Preliminary intravenous antisecretory studies in HP dogs indicated that the 3Z,5Z isomer was weakly active and that the 3E,5Z isomer was responsible for most of the activity of 5. A stereospecific synthesis of the active 3E/5Z isomer, 5E/Z, was carried out in order to determine its antisecretory/diarrheal profile. Synthesis of the 3E/5Z Diene (5E/Z) The key intermediate in the preparation of 5E/Z was the alpha chain truncated compound 10 (Scheme II). There are several methods available for coupling acetylenes or acetylenic derivatives with vinyl halides, alkenyl copper compounds or alkenyl boranes to provide conjugated enynes. (13-15) Thus our synthetic strategy centered upon the coupling of I0 with a suitable four carbon ester ii in which the E stereochemistry of the double bond has been established and "X" is either halogen, copper, or boron. The conjugated enyne 12 could then be converted to 5E/Z by selective catalytic hydrogenation of the 5-yne and removal of protecting groups. SCHEME II SYNTHETIC STRATEGY
>o+ + 8
X~,,,,,~:ooc..
o
SIEtj
$iMe) 10
11
SiEts
o
SiMe)
I2
~{.1/.,*o
(*)
S EIZ
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PROSTAGLANDINS
The synthesis of i0 (Scheme III) was based on chemistry developed by Piancatelli (16) for the conversion of 2-furyl carbinols to hydroxycyclopentenones. Reaction of furfuraldehyde with propargyl magnesium bromide cleanly generated the carbinol 13. Modest yields (20-30%) of 14 were obtained by heating a solution of 13 and p-toluenesulfonic acid 10.5%) in an 8:1 mixture of dioxane and water at 83-85vC for 36 hours. Conversion of 14 to 15 was effected by adsorption on an alumina chromatography column for 16 hours or by stirring 14 in an ether slurry of alumina for the same period. Silylation of 15 with triethylchlorosilane in dimethylformamide and imidazole gave 16. The intermediate i0 was formed in good yield by treating 16 with the cuprate reagent derived from (E)-(tri-n-butylstannyl)-4-methyl-4-[(trimethylsilyl)oxy]-l-octene (7) followed by enolate capture with t-butyldimethyl-chlorosilane. (12)
SCHEME
III
~ . o
+ IrktgCH2C=CH
o
Dioxane / H,O ) A 14
o
Io~-
t%S~
6
SIE%
16
Silae~
o
10
SiMej
The crucial step in this sequence was the coupling of I0 with the E vinyl iodide 18 (Scheme IV). The selection of this coupling approach was based on the work of Negishi (13) who reported that simple alkynylzinc chlorides could be coupled under mild conditions with vinyl halides in the
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PROSTAGLANDINS
p r e s e n c e of a p a l l a d i u m (0) catalyst to give conjugated enyses. T r e a t m e n t of i0 in THF with n-butyl lithium at -20 C followed by addition of one equivalent of anhydrous zinc chloride generated the alkynylzinc chloride 17. To this solution at 0 C were added several equivalents of 18 and a c a t a l y t i c amount of bis(dibenzylideneacetone) p a l l a d i u m (17). The desired product 12 was obtained in 70% yield (from i0).
SCHEME IV
1).--.U ) ~ Z n C I 2)ZnCI, % ~ 0
SiE%
10
8 Si|t~
SIMe,
I ",,,,,~,.,,,,/¢ OOCH,
17
o $iMel
>o+
18
6 ~Ets
o ~Mej 12
The final step in the synthesis of 5E/Z was the selective c a t a l y t i c h y d r o g e n a t i o n of the enyne to the diene. R e d u c t i o n studies (Scheme V) were performed on the d e p r o t e c t e d compound 19 which was obtained by treatment of 12 w i t h a 3:1:1 mixture of acetic acid, t e t r a h y d r o f u r a n and water at room temperature for 24 hours. Surprisingly, h y d r o g e n a t i o n of 19 with Lindlar catalystoand quinoline in a I:I mixture of THF and cyclohexane at 0 C yielded a complex mixture of several dienes and monoenes of which the d e s i r e d product was only a minor constituent. However, use of a different catalyst, nickel boride or P-2 nickel, (18) g e n e r a t e d two products in a 4:1 ratio: the desired diene 5E/Z and the 5Z mono-olefin which was separated from 5E/Z by HPLC.
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PROSTAGLANDINS
SCHEME V
1o"+ AcOH / HzO
24 hr
SIet,
12
SIMe~
1-+)~ Lindlar
.,=~m,
(P-lNi)
(±)~
I
-
"--
x~..
.~lZ
Results and Discussion. The gastric a~%isecretory activity of 5E/Z was determined by i n t r a g a s t ~ administration in gastric fistula (GF) dogs. The results were compared to that of misoprostol and the i:I mixture of 3Z and 3E isomers 5 (Table I). The antisecretory potency of 5E/Z was approximately equal to that of 5 and about three times that of misoprostol. These data support the results of preliminary intravenous studies in Heidenhain pouch dogs which indicated that the 3Z,5Z isomer was only weakly active and that the 3E,5Z isomer, 5E/Z, possessed most of the antisecretory activity of the geometric mixture. The separation of undesired diarrheogenic effects from gastric antisecretory activity for 5E/Z, relative to misoprostol and the geometric mixture, 5, was also determined (Table I). In contrast to 5, the single isomer, 5E/Z, was approximately 3 times more diarrheogenic than misoprostol and showed an equivalent separation of
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PROSTAGLANDINS
diarrheal and antisecretory activities as misoprostol because of its greater gastric antisecretory potency. The data suggest that, when the two geometric isomers are given as a i:i mixture, the 3Z,5Z isomer, 5Z/Z, moderates the ability of 5E/Z to induce diarrhea in rats, but does not significantly affect the antisecretory activity of 5E/Z in dogs. However, antisecretory and diarrheogenic studies with 5Z/Z are needed to confirm these preliminary conclusions. The details of the gastric antisecreory studies described throughout this manuscript are provided in previous publications. Intravenous antisecretory activity was obtained in adult female mongrel dogs prepared with Heidenhain pouches and stimulated with histamine. (4,7) Oral antisecretory data were obtained in adult female Beagles prepared with simple gastric fistulas and stimulated with histamine. (7) Diarrheal studies were performed in adult Charles River male rats fasted for 24 hr prior to the test. (7) Acknowledgment The authors thank C. Anglin, J. Casler and D. Price for technical assistance in the antisecretory and diarrheal studies, D. Honda for HPLC support, D. Calhoun for statistical assistance, L. Householder for artwork and D. Weiman for typing the manuscript. References I. Brooks, F.P., G. Watkinson, and H.C. Davies, eds., Prostaglandins in Gastroenterology, Focus on Misoprostol, Proceedings of an International Symposium, Algarve, Portugal, September 13-14, (1984) Dig. Dis. Sci., 30, Supp ii, 1985. 2. Dajani, E.Z., D.R. Driskill, R.G. Bianchi, P.W. Collins, and R. Pappo, Influence of the Position of the Side Chain Hydroxy Group on the Gastric Antisecretory and Antiulcer Actions of E I Prostaglandin Analogs. Prostaglandins :i0 733, 1975. 3. Dajani, E.Z., D.R. Driskill, R.G. Bianchi, P.W. Collins, and R. Pappo, SC-29333: A Potent Inhibitor of Canine Gastric Secretion, Am. J. Dig. Dis. 21: 1049, 1976. 4. Collins, P.W., E.Z. Dajani, D.R. Driskill, M.S. Bruhn, C.J. Jung, and R. Pappo, Synthesis and Gastric Antisecretory Properties of 15-Deoxy-16-Hydroxyprostaglandin E 1 Analogues. J. Med. Chem. 20: 1152, 1977.
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5. Green, K., B. Samuelson, and B.J. Magerlein, Decreased Rate of Metabolism Induced by a Shif~ of the D~uble Bond in Prostaglandin Fg~ from the A to the APosition, Eur. J. Bioch~m. 62: 527, 1976. 6. Walker, E.R.H. In: Chemistry, Biochemistry and Pharmacological Activity of Prostanoids, (S.M. Roberts and F. Scheinmann, eds.) Pergamon Press, New York, 1979, p.326. 7. Collins, P.W., E.Z. Dajani, R. Pappo, A.F. Gasiecki, R.G. Bianchi, and E.M. Woods, Synthesis and Gastric Antisecretory properties of 4,5-Unsaturated Derivatives of 15-Deoxy-16-Hydroxy-16-Methyl-Prostaglandin El, J__~. Med. Chem. 26: 786, 1983. 8. Dajani, E.Z., E.A.W. Roge, and R.E. Bertermann, Effects of E Prostaglandins, Diphenoxylate and Morphine on Intestinal Motility in Vivo, Eur. J. Pharmaol. 34: 105, i975. 9. Horton, E.W., I.H.M. Main, C.J. Thompson, and P.M. Wright, Effect of Orally Administered Prostaglandin E 1 on Gastric Secretion and Gastrointestinal Motility in Man. Gut 9: 655, 1968. 10. Misiewicz, J.J., S.L. Waller, N. Kiley, and E.W. Horton, Effect of Oral Prostaglandin E 1 on Intestinal Transit in Man. Lancet i: 648,1969. ii. Karim, S.M.M. and J.J. Amy, Termination of Pregnancy with Prostaglandin E 2 Methyl Ester, Prostaglandins !: 293, 1974. 12. Collins, P.w., A.F. Gasiecki, P.H. Jones, R.F. Bauer, G.W. Gul]~kson, E.M. Woods, and R.B. Bianchi, Synthesis and Gastric Antisecretory Properties of Chain Diene Derivatives of Misoprostol, J. Med. Chem. 29: 1195, 1986. 13. King, A.O., N. Okukado, and E. Negishi, Highly General Stereo-, Regio- and Chemo- Selective Synthesis of Terminal and Internal Conjugated Enynes by the Pd-catalyzed Reaction of Alkynylzinc Reagents with Alkenyl Halides. J.C.S. Chem. Comm. 683, 1977. 14. Normant, J.F., A. Commercon, and J. Villieras, Synthese D'Enynes et de Dienes Conjugues A L'Aide D'organocuivreux Vinyliques. Application a la Synthese du Bombyol. Tetrahedron Lett. 1465, 1975.
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15. Miyaura, N., K. Yamada, and A. Suzuki, A New Stereospecific Cross-Coupling by the PalladiumCatalyzed Reaction of l-Alkenyl Boranes with l-Alkenyl or l-Alkynyl Halides, Tetrahedron Lett. 3437, 1979. 16. Piancatelli, from Furans,
G., Advances Heterocycles
in Cyclopentenone 19: 1735, 1982.
Synthesis
17. Takahaski, Y., T. Ito, T.S. Sakai, and Y. Ishii, A Novel Palladium (0) Complex; Bis (dibenzylideneacetone) palladium (O). J.C.S. Chem. Comm. 1065, 1970. 18. Brown, C.H., and V.K. Ahuja, "P-2 Nickel" Catalyst with Ethylenediamine, a Novel System for Highly Stereospecific Reduction of Alkynes to cis-olefins. J.C.S. Chem. Comm. 553, 1973.
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Alpha
Chain Unsaturated Derivatives of Misoprostol
Paul W. Collins, Alan F. Gasiecki, Richard M. Weier, Steven W. Kramer, Peter H. Jones, Gary W. Gullikson, Robert G. Bianchi and Raymond F. Bauer. G a s t r o i n t e s t i n a l Diseases Research G.D. Searle & Co. 4901 Searle Parkway Skokie, IL 60077
Department
Abstract Misoprostol, a 1 5 - d e o x y - 1 6 - h y d r o x y - 1 6 - m e t h y l analog of PGE4, is an effective agent for the treatment of peptic ulcer disease. Efforts to impede metabolic degradation of the alpha chain of misoprostol led to the d i s c o v e r y of a s~cond clinical candidate in this series. Enisoprost, a a-Z derivative of misoprostol, is more potent as a gastric a n t i s e c r e t o r y agent and longer acting than misoprostol. These findings prompted further work to determine the effects that two double bonds in the alpha chain might have on the activity profile of misoprostol. The most promising structure in this series was a i:I mixture of 3E,5Z and 3Z,5Z dienes which was about three times more potent than misoprostol in inhibiting gastric secretion in dogs, while the separation of the diarrheogenic effect was s i g n i f i c a n t l y improved. Chromatographic separation of the mixture was very difficult, but small amounts of each isomer were obtained by HPLC, and preliminary a n t i s e c r e t o r y studies indicated that most of the activity resided in the 3E,5Z isomer. A stereospecific synthesis of the 3E,5Z isomer was carried out to provide sufficient quantities for complete pharmacological assessment. The 3E,5Z diene was about three times more potent than misoprostol in inhibiting gastric acid secretion in dogs and also in producing diarrhea in rats. Introduction Misoprostol (Scheme i), a 1 5 - d e o x y - 1 6 - h y d r o x y - 1 6 - m e t h y l analog of naturally occurring PGE1, represents a major advance in the development of synthetic p r o s t a g l a n d i n s for the treatment of peptic ulcer disease and related conditions.(1) It is currently approved and marketed in a p p r o x i m a t e l y eighteen countries and is under c o n s i d e r a t i o n for approval in most other markets.
Key Words: enisoprost, gastric antisecretory, diarrhea, p a l l a d i u m (O) coupling, ~ chain dienes, cuprate, nickel boride.
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PROSTAGLANDINS
Scheme I SYNTHETIC DEVELOPMENT OF MISOPROSTOL O ..~,.~-"~ COOH ----
~
is
H(~
Hg
0
Pi E1
H(~
OH 16-H, 16-OH
CH 3
H(~
OH 16-METHYL,16-OH MISOPROSTOL
Discovery of Misoprostol The discovery of misoprostol arose from a chemical program begun at Searle in the early seventies to synthesize analogs of PGE1, with the objective o~ improving its oral gastric antisetretory activity, side effect profile and duration of action. The first breakthrough in that moving the 15-hydroxy 16-position significantly prostaglandin side effects antisecretory activity.(2) The key compound,
18
this program was the finding group of PGE I to the adjacent reduced many ~f the typical yet did not decrease gastric
15-deoxy-16-hydroxy
PGE 1 methyl ester
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(Scheme I), was a p p r o x i m a t e l y equlpotent to PGE 1 in inhibiting gastric secretion by intravenous administration. Side effects such as trembling, emesis and diarrhea, which are u s u a l l y observed with PGE 1 and synthetic 1 5 - h y d r o x y prostaglandins, were absent at effective a n t i s e c r e t o r y doses of the 16-hydroxy compound. However, it was only w e a k l y active by oral a d m i n i s t r a t i o n and its d u r a t i o n of action was quite short. Enzymatic studies indicated that the 16-hydroxy group was also a substrate for the d e h y d r o g e n a s e enzyme which inactivates 15-hydroxy prostaglandins. In an effort to block this oxidative degradation, the 16-methyl, 15,15-dimethyl, and the 1 7 , 1 7 - d i m e t h y l analogs were prepared. The addition of a methyl group at carbon 16 d r a m a t i c a l l y increased oral potency and d u r a t i o n of action and completed the synthetic d e v e l o p m e n t of m i s o p r o s t o l . ( 3 , 4 ) In contrast, addition of two methyl groups at either carbons 15 or 17 greatly d i m i n i s h e d gastric a n t i s e c r e t o r y activity.(4) Discovery
o f Enisoprost
Following the d i s c o v e r y of misoprostol, our research was directed toward o p t i m i z i n g the desirable properties of this compound. For instance, we began to examine various ways of b l o c k i n g or impeding E-oxidation of the alpha side chain of m i s o p r o s t o l in order to further improve its duration of action. Efforts in this area eventually led to the d i s c o v e r y of a second clinical candidate in this series, enisoprost. Enisoprost is the 4-cis (4Z) u n s a t u r a t e d analog of misoprostol and currently is in Phase I clinical study. The decision to explore the effects of a cis double bond at the "unnatural" 4,5 position of teh-6-molecule was based on reports that this m o d i f i c a t i o n impeded E-oxidation (5) but did not alter gastric a n t i s e c r e t o r y activity (6) of other p r o s t a g l a n d i n compounds. The c o r r e s p o n d i n g 4-trans (4E) and 4,5 acetylene d e r i v a t i v e s were also prepared (7) because the influence of t:~ese m o d i f i c a t i o n s on gastric a n t i s e c r e t o r y activity and E-oxidation of p r o s t a g l a n d i n s was not known. The three d e r i v a t i v e s were tested in histamine stimulated Heidenhain pouch (HP) dogs by intravenous administration. The introduction of a Z double bond at the 4,5 position of m i s o p r o s t o l resulted in a three-fold increase in gastric a n t i s e c r e t o r y potency. In contrast, the presence of an E double bond at this p o s i t i o n reduced activity by 30 times, while, surprisingly, the acetylenic derivative was devoid of gastric a n t i s e c r e t o r y activity at a dose 300 times the ED~0 of misoprostol. (7) The 4Z compound, enisoprost, wag then compared to misoprostol in gastric fistula dogs by intragastric a d m i n i s t r a t i o n (Table I). Enisoprost was a p p r o x i m a t e l y 5 times more potent than misoprostol in this assay. Also, studies in meal stimulated Pavlov pouch dogs indicated that enisoprost had a longer duration of action than misoprostol.
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PROSTAGLANDINS
Table I
Pharmacological Comparison of Misoprostol and Enisoprost with 3,5 Diene Analogues Compound
EDsopglKg IG Gastric Antisecretory Effects in D o g s
Activity Ratio
Diarrheal Effectsin Rats
EDH Diarrhea EDs0Antisecretory
Misopro~ol Enisopro~ 4,5c is
.28 .05
382 62
1364 1240
5 5F.JZ
.08 .08
642 131
8025 1638
Diarrhea is a major side effect of natural and synthetic prostaglandins in animals (8) and man. (9-11) Previous studies in rats established that misoprostol had a much greater separation of gastric antisecretory activity from diarrheogenic activity than standard 15-hydroxy prostaglandins. (7) To compare the relative separation of these two effects for enisoprost, diarrheogenic activity was determined in rats and an activity ratio calculated (Table I). Enisoprost was more diarrheogenic than misoprostol, but showed about the same degree of separation of the two activities as misoprostol because of its greater antisecretory potency. Alpha Chain Diene Derivatives o__ffMisoprostol Based on these findings, we decided to investigate the effects that two double bonds in the alpha chain might have on the intensity and duration of the gastric antisecretory activity of misoprostol. Seven diene derivatives of misoprostol were prepared with variations in both position and configuration of the double bonds. (12) The intravenous gastric antisecretory activities of these compounds were determined in histamine stimulated Heidenhain Pouch (HP) dogs at a test dose of 1.0 pg/kg (Table II). Comparative data are given for misoprostol and enisoprost. Compounds 4, 5 and 7 displayed good gastric antisecretory activity in this assay but on
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followup evaluation, only the conjugated diene 5 which is approximately a i:I mixture of 3Z,5Z (3-cis, 5-cis) and 3E, 5Z (3-trans, 5-cis) isomers, showed a-n--inter--6-{ting level of a ~ t y . --T~e intragastric antisecretory potency of 5 was approximately equal to that of enisoprost and about three times that of misoprostol (Table I).
Table II
Gastric Antiseereto~ Activity of Alpha Chain Dienes in Dogs o
Compound
Structure
Percent Inhibition of Total Acid Output (TAO) at lpg/kg I.V.
Misopro~ol Enisoprost 2E, 5Z 2E, 5E
70 92 0 68
5
3£/Z, 5Z
83
6 7 8 9
3E, SE 3Z, 5E 2E, 4Z 2E, 4E
21 96 41 53
3 4
The diarrheogenic activity of 5 in rats was quite low, being somewhat less than misoprostol and about ten times less than enisoprost. Comparison of activity ratios of diarrheal to antisecretory EDen values clearly demonstrates that the separation of these activities is considerably greater for 5 than for either misoprostol or enisoprost (Table I). The very desirable pharmacological profile of 5 prompted a more thorough investigation of the chromatographic separation of its geometric isomers. Numerous systems were tried but an efficient and practical method was not found. However, one HPLC system (silver nitrate treated Partisil SCX) did provide small amounts (3-4 mg) of the
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individual geometric isomers. Preliminary intravenous antisecretory studies in HP dogs indicated that the 3Z,5Z isomer was weakly active and that the 3E,5Z isomer was responsible for most of the activity of 5. A stereospecific synthesis of the active 3E/5Z isomer, 5E/Z, was carried out in order to determine its antisecretory/diarrheal profile. Synthesis of the 3E/5Z Diene (5E/Z) The key intermediate in the preparation of 5E/Z was the alpha chain truncated compound 10 (Scheme II). There are several methods available for coupling acetylenes or acetylenic derivatives with vinyl halides, alkenyl copper compounds or alkenyl boranes to provide conjugated enynes. (13-15) Thus our synthetic strategy centered upon the coupling of I0 with a suitable four carbon ester ii in which the E stereochemistry of the double bond has been established and "X" is either halogen, copper, or boron. The conjugated enyne 12 could then be converted to 5E/Z by selective catalytic hydrogenation of the 5-yne and removal of protecting groups. SCHEME II SYNTHETIC STRATEGY
>o+ + 8
X~,,,,,~:ooc..
o
SIEtj
$iMe) 10
11
SiEts
o
SiMe)
I2
~{.1/.,*o
(*)
S EIZ
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PROSTAGLANDINS
The synthesis of i0 (Scheme III) was based on chemistry developed by Piancatelli (16) for the conversion of 2-furyl carbinols to hydroxycyclopentenones. Reaction of furfuraldehyde with propargyl magnesium bromide cleanly generated the carbinol 13. Modest yields (20-30%) of 14 were obtained by heating a solution of 13 and p-toluenesulfonic acid 10.5%) in an 8:1 mixture of dioxane and water at 83-85vC for 36 hours. Conversion of 14 to 15 was effected by adsorption on an alumina chromatography column for 16 hours or by stirring 14 in an ether slurry of alumina for the same period. Silylation of 15 with triethylchlorosilane in dimethylformamide and imidazole gave 16. The intermediate i0 was formed in good yield by treating 16 with the cuprate reagent derived from (E)-(tri-n-butylstannyl)-4-methyl-4-[(trimethylsilyl)oxy]-l-octene (7) followed by enolate capture with t-butyldimethyl-chlorosilane. (12)
SCHEME
III
~ . o
+ IrktgCH2C=CH
o
Dioxane / H,O ) A 14
o
Io~-
t%S~
6
SIE%
16
Silae~
o
10
SiMej
The crucial step in this sequence was the coupling of I0 with the E vinyl iodide 18 (Scheme IV). The selection of this coupling approach was based on the work of Negishi (13) who reported that simple alkynylzinc chlorides could be coupled under mild conditions with vinyl halides in the
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PROSTAGLANDINS
p r e s e n c e of a p a l l a d i u m (0) catalyst to give conjugated enyses. T r e a t m e n t of i0 in THF with n-butyl lithium at -20 C followed by addition of one equivalent of anhydrous zinc chloride generated the alkynylzinc chloride 17. To this solution at 0 C were added several equivalents of 18 and a c a t a l y t i c amount of bis(dibenzylideneacetone) p a l l a d i u m (17). The desired product 12 was obtained in 70% yield (from i0).
SCHEME IV
1).--.U ) ~ Z n C I 2)ZnCI, % ~ 0
SiE%
10
8 Si|t~
SIMe,
I ",,,,,~,.,,,,/¢ OOCH,
17
o $iMel
>o+
18
6 ~Ets
o ~Mej 12
The final step in the synthesis of 5E/Z was the selective c a t a l y t i c h y d r o g e n a t i o n of the enyne to the diene. R e d u c t i o n studies (Scheme V) were performed on the d e p r o t e c t e d compound 19 which was obtained by treatment of 12 w i t h a 3:1:1 mixture of acetic acid, t e t r a h y d r o f u r a n and water at room temperature for 24 hours. Surprisingly, h y d r o g e n a t i o n of 19 with Lindlar catalystoand quinoline in a I:I mixture of THF and cyclohexane at 0 C yielded a complex mixture of several dienes and monoenes of which the d e s i r e d product was only a minor constituent. However, use of a different catalyst, nickel boride or P-2 nickel, (18) g e n e r a t e d two products in a 4:1 ratio: the desired diene 5E/Z and the 5Z mono-olefin which was separated from 5E/Z by HPLC.
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SCHEME V
1o"+ AcOH / HzO
24 hr
SIet,
12
SIMe~
1-+)~ Lindlar
.,=~m,
(P-lNi)
(±)~
I
-
"--
x~..
.~lZ
Results and Discussion. The gastric a~%isecretory activity of 5E/Z was determined by i n t r a g a s t ~ administration in gastric fistula (GF) dogs. The results were compared to that of misoprostol and the i:I mixture of 3Z and 3E isomers 5 (Table I). The antisecretory potency of 5E/Z was approximately equal to that of 5 and about three times that of misoprostol. These data support the results of preliminary intravenous studies in Heidenhain pouch dogs which indicated that the 3Z,5Z isomer was only weakly active and that the 3E,5Z isomer, 5E/Z, possessed most of the antisecretory activity of the geometric mixture. The separation of undesired diarrheogenic effects from gastric antisecretory activity for 5E/Z, relative to misoprostol and the geometric mixture, 5, was also determined (Table I). In contrast to 5, the single isomer, 5E/Z, was approximately 3 times more diarrheogenic than misoprostol and showed an equivalent separation of
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PROSTAGLANDINS
diarrheal and antisecretory activities as misoprostol because of its greater gastric antisecretory potency. The data suggest that, when the two geometric isomers are given as a i:i mixture, the 3Z,5Z isomer, 5Z/Z, moderates the ability of 5E/Z to induce diarrhea in rats, but does not significantly affect the antisecretory activity of 5E/Z in dogs. However, antisecretory and diarrheogenic studies with 5Z/Z are needed to confirm these preliminary conclusions. The details of the gastric antisecreory studies described throughout this manuscript are provided in previous publications. Intravenous antisecretory activity was obtained in adult female mongrel dogs prepared with Heidenhain pouches and stimulated with histamine. (4,7) Oral antisecretory data were obtained in adult female Beagles prepared with simple gastric fistulas and stimulated with histamine. (7) Diarrheal studies were performed in adult Charles River male rats fasted for 24 hr prior to the test. (7) Acknowledgment The authors thank C. Anglin, J. Casler and D. Price for technical assistance in the antisecretory and diarrheal studies, D. Honda for HPLC support, D. Calhoun for statistical assistance, L. Householder for artwork and D. Weiman for typing the manuscript. References I. Brooks, F.P., G. Watkinson, and H.C. Davies, eds., Prostaglandins in Gastroenterology, Focus on Misoprostol, Proceedings of an International Symposium, Algarve, Portugal, September 13-14, (1984) Dig. Dis. Sci., 30, Supp ii, 1985. 2. Dajani, E.Z., D.R. Driskill, R.G. Bianchi, P.W. Collins, and R. Pappo, Influence of the Position of the Side Chain Hydroxy Group on the Gastric Antisecretory and Antiulcer Actions of E I Prostaglandin Analogs. Prostaglandins :i0 733, 1975. 3. Dajani, E.Z., D.R. Driskill, R.G. Bianchi, P.W. Collins, and R. Pappo, SC-29333: A Potent Inhibitor of Canine Gastric Secretion, Am. J. Dig. Dis. 21: 1049, 1976. 4. Collins, P.W., E.Z. Dajani, D.R. Driskill, M.S. Bruhn, C.J. Jung, and R. Pappo, Synthesis and Gastric Antisecretory Properties of 15-Deoxy-16-Hydroxyprostaglandin E 1 Analogues. J. Med. Chem. 20: 1152, 1977.
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5. Green, K., B. Samuelson, and B.J. Magerlein, Decreased Rate of Metabolism Induced by a Shif~ of the D~uble Bond in Prostaglandin Fg~ from the A to the APosition, Eur. J. Bioch~m. 62: 527, 1976. 6. Walker, E.R.H. In: Chemistry, Biochemistry and Pharmacological Activity of Prostanoids, (S.M. Roberts and F. Scheinmann, eds.) Pergamon Press, New York, 1979, p.326. 7. Collins, P.W., E.Z. Dajani, R. Pappo, A.F. Gasiecki, R.G. Bianchi, and E.M. Woods, Synthesis and Gastric Antisecretory properties of 4,5-Unsaturated Derivatives of 15-Deoxy-16-Hydroxy-16-Methyl-Prostaglandin El, J__~. Med. Chem. 26: 786, 1983. 8. Dajani, E.Z., E.A.W. Roge, and R.E. Bertermann, Effects of E Prostaglandins, Diphenoxylate and Morphine on Intestinal Motility in Vivo, Eur. J. Pharmaol. 34: 105, i975. 9. Horton, E.W., I.H.M. Main, C.J. Thompson, and P.M. Wright, Effect of Orally Administered Prostaglandin E 1 on Gastric Secretion and Gastrointestinal Motility in Man. Gut 9: 655, 1968. 10. Misiewicz, J.J., S.L. Waller, N. Kiley, and E.W. Horton, Effect of Oral Prostaglandin E 1 on Intestinal Transit in Man. Lancet i: 648,1969. ii. Karim, S.M.M. and J.J. Amy, Termination of Pregnancy with Prostaglandin E 2 Methyl Ester, Prostaglandins !: 293, 1974. 12. Collins, P.w., A.F. Gasiecki, P.H. Jones, R.F. Bauer, G.W. Gul]~kson, E.M. Woods, and R.B. Bianchi, Synthesis and Gastric Antisecretory Properties of Chain Diene Derivatives of Misoprostol, J. Med. Chem. 29: 1195, 1986. 13. King, A.O., N. Okukado, and E. Negishi, Highly General Stereo-, Regio- and Chemo- Selective Synthesis of Terminal and Internal Conjugated Enynes by the Pd-catalyzed Reaction of Alkynylzinc Reagents with Alkenyl Halides. J.C.S. Chem. Comm. 683, 1977. 14. Normant, J.F., A. Commercon, and J. Villieras, Synthese D'Enynes et de Dienes Conjugues A L'Aide D'organocuivreux Vinyliques. Application a la Synthese du Bombyol. Tetrahedron Lett. 1465, 1975.
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15. Miyaura, N., K. Yamada, and A. Suzuki, A New Stereospecific Cross-Coupling by the PalladiumCatalyzed Reaction of l-Alkenyl Boranes with l-Alkenyl or l-Alkynyl Halides, Tetrahedron Lett. 3437, 1979. 16. Piancatelli, from Furans,
G., Advances Heterocycles
in Cyclopentenone 19: 1735, 1982.
Synthesis
17. Takahaski, Y., T. Ito, T.S. Sakai, and Y. Ishii, A Novel Palladium (0) Complex; Bis (dibenzylideneacetone) palladium (O). J.C.S. Chem. Comm. 1065, 1970. 18. Brown, C.H., and V.K. Ahuja, "P-2 Nickel" Catalyst with Ethylenediamine, a Novel System for Highly Stereospecific Reduction of Alkynes to cis-olefins. J.C.S. Chem. Comm. 553, 1973.
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