2211251b0Nature221518719690329125112530028-0836196910.1038/2211251b0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v221/n5187issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue2211251b0Biological Activity of Synthetic Prostaglandins
AU  - RAMWELL, P. W.
AU  - SHAW, JANE E.Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545.
AU  - COREY, E. J.
AU  - ANDERSEN, N.Department of Chemistry, Harvard University, Boston, Massachusetts.THE total synthesis of prostaglandin E1 and prostaglandin F1a has recently been achieved by methods which also make available various derivatives and steroisomers of the prostaglandin series1,2. Biological studies on the synthetic prostaglandins, and certain unnatural synthetic stereoisomers, have been undertaken first to provide biological confirmation of the chemical synthesis and, second, to explore the metabolism and pharmacological activity of various stereoisomers in areas where the prostaglandins may be of clinical use. These include the regulation of gastric secretion3,4, blood platelet aggregation5, blood pressure, lipolysis, carbohydrate metabolism6 and corticosteroidogenesis7.This communication is a preliminary account of the results obtained on intestinal and reproductive smooth muscle preparations and on the arterial blood pressure of the rat. Unexpected biological potency of certain unnatural synthetic stereoisomers was found.
Prostaglandins ?1 and Fla are designated as 11?, 15(S)-dihydroxy-9-cwo-13-trans-prostenoic and 9?, 11?, 15(S)-trihydroxy-13-[pound]rans-prostenoic acids respectively. The synthetic compounds were in each case racemic mixtures (natural antipode plus an equal amount of its mirror image). The racemic mixture is indicated by the prefix rac, and the mirror image of the natural prostaglandin by the prefix ent. The structures of natural prostaglandins EX, ?? and Fia are shown for reference in stereochemical detail including the absolute configuration. Changes in configuration at 01? and C15 from those which are found in natural prostaglandin ?1 are indicated by the term epi (for example, 11, 15-epi prostaglandin E! has the structure shown).
Prostaglandin E1A, (PGE1,) Prostaglandin A1 (PGA1)
Prostaglandin Fla (PGF,1 a) II, \5-e/)i Prostaglandin E,
Contractions of preparations of isolated tissue suspended on 0.5-2.0 ml. baths oxygenated with air were recorded with an isotonic transducer (Phipps and Bird ST-2) ; the rat uterus in de Jalon's solution at 20 C, guinea-pig ileum in Tyrode solution at 30 C and the rabbit jejunum in Tyrode solution at 37 C were used in the absence of any drug antagonist. 2 + 2 Latin square design was used for bioassay. Arterial blood pressure of rats weighing 400 g, anaesthetized with pentobarbitone and treated with pentolinium, was recorded with a strain gauge (Statham P23AC) ; injections were made into the jugular vein in a volume of 0-1 ml. saline; not more than two compounds were tested in any one animal. The concentration of prostaglandins E1? Ax and Fla necessary to contract the isolated smooth muscle preparations to 50 per cent of maximum and to elicit a 10 per cent change in blood pressure is indicated in Table 1.
The biological activities of the various natural and synthetic prostaglandins are summarized in Table 2. Only a few micrograms of the individual synthetic compounds were available; these studies have therefore been limited initially to simple pharmacological preparations.
Table l. CONCENTRATION OP PROSTAGLANDIN STANDARDS USED ON PHARMACOLOGICAL PREPARATIONS
 PGEX PGAj PGFia
Rat uterus (ng/ml.) Guinea-pig ileum (ng/ml.) Rabbit jejunum (ng/ml.) Rat blood pressure (jug/kg) 2 10
0-125 20 2,500
0-125 4 2,000
0-25
Table 2. CROSS ASSAY OP AUTHENTIC AND SYNTHETIC PGE, PGF AND PGA COMPOUNDS
Prostaglandins Rat uterus Guinea-pig ileum Rabbit jejunum Rat blood pressure
Natural PGEi Itac-PGEx .Efai-PGEi 1-00 0-44 [plusmn]0-06 0-001 1-00 0-5 1-00 0-4 0-0013 1-00 0-57
Natural 15-epi PGEi Rac-1i5-epi PGEi Rac-ll-epi PGEX Rac-ll, 15-epi PGEi 0-008 + 0-001 0-04 + 0-007[ast] 0-13 [plusmn]0-04 1-00 [plusmn]0-2[ast] 0-02 0-06 0-036 0-1 0-12 + 0-02 5-4[plusmn]l-6 0-05 0-05 0-05 0-04
Natural PGAX Eoc-PGAi Natural 15-epi PGAX Rac-15-epi PGAa 1-00 0-73 + 0-06 0-42 1-00 ? 1-00 0-53
2-00 1-00 1-00 0-16 0-16
Natural PGFlW Jtac-PGF1a 1-00 0-45 [plusmn]0-09 1-00 0-56 1-00 0-4 __
[ast] A log dose-response curve deviates significantly from that of natural PGEi. Potency expressed relative to parent prostaglandin (= 1.00) within each group ([plusmn] S.E. when available, n = 4).
The structures of the synthetic and natural prostaglandins have been established beyond doubt by chemical and physical data1'2. Comparison of the biological activity of the natural prostaglandins and their corresponding racemic synthetic forms in effect provides information therefore as to the biological activity of the unnatural synthetic antipode. In the case of prostaglandins E! and Fia, the activity of fche racemic synthetic prostaglandins is approximately one-half that of the naturally occurring compound in all assays, showing that the unnatural antipodes are pharmacologically less active ; this is the almost universally observed situation with biologically active structures. Indeed, ent -prostaglandin E! had but one-thousandth of the activity of natural prostaglandin Ej on the isolated smooth muscle preparations. Bioassay of rac-prostaglandin A1 indicated, however, that the antipode of prostaglandin AI has significant activity on the rat uterus preparation. Moreover, although natural 15-epi prostaglandin Ex and 15-epi prostaglandin Aj are pharmacologically less active than natural prostaglandin ?1 and prostaglandin Ax respectively, their synthetic racemates, in certain assays, have been found to be more active than the natural isomer ; this suggests that the mirror image of the natural 15-epi derivatives (ent 15-epi prostaglandin E1and ent 15-epi prostaglandin A1 respectively) on some pharmacological preparations is more active than the naturally occurring isomer.
Another modification of biological activity has been observed on variation of configuration at carbons 11 and 15. Whereas both rac-11-epi prostaglandin ?1 and rac-15-epi prostaglandin E! are substantially less active than rac-prostaglandin Ex in tests with rat uterus and rabbit jejunum, rac-11, 15-epi prostaglandin Ex has been found to be more active than rac-prostaglandin E!. Extrapolating from the previous findings that the mirror image of the natural 15-epi derivatives of prostaglandin E! and prostaglandin AJ are more potent than the naturally occurring isomer, it is anticipated that the increased activity of rac-11, 15-epi prostaglandin ET over rac-prostaglandin Ex will in fact be identified with ent-11, 15-epi prostaglandin E! (mirror image). Only partial resolution of the racemate has so far been achieved, yielding ent-11, 15-epi prostaglandin Ex of 80 per cent optical purity. Preliminary assay on the isolated rat uterus indicates that this compound is 1.1-1.4 times more potent than natural prostaglandin E!. A further similarity between the pharmacological activity of rac-15-epi prostaglandin E! and rac-11, 15-epi prostaglandin E! has been established, in that the log-dose response curves of these two synthetic compounds on the isolated rat uterus preparation both deviate significantly from that of natural prostaglandin E1.
Measurement of the biological activity of racemic synthetic prostaglandins has thus revealed unusual relationships between the properties of the racemic and natural forms of certain prostaglandins.
Prostaglandin E1 itself is one of the most potent pharmacologically active substances known, and we plan to investigate in detail the potential of these more active synthetic compounds. It is also hoped that with an extended pharmacological profile of various synthetic isomers of the parent prostaglandin structures as they become available, compounds which show but one facet of the wide spectrum of activity presently associated with the prostaglandin class of compounds will be obtained.
We thank Sheila Jessup and Wendy McDonald-Gibson for assistance. This work was supported by grants from the US National Institutes of Health to Harvard University and by grants to the Worcester Foundation for Experimental Biology. Prostaglandin E1, Fla and 15-epi prostaglandin Ex were supplied by Drs J. Pike and W. P. Schneider, the Upjohn Company, and Dr B. Samuelsson, Stockholm.Corey, , E. J., Andersen, , N. H., Carlson, , R. M., Paust, , J., Vedejs, , E., Vlattas, , I., and Winter, , R. E. K., J. Amer. Chem. Soc., 90, 3245 (1968).Corey, , E. J., Vlattas, , I., Andersen, , N. H., and Harding, , K., J. Amer. Chem. Soc., 90, 3247 (1968).Robert, , A., in Prostaglandin Symposium of the Worcester Foundation for Experimental Biology (edit. by Ramwell, P. W., and Shaw, J. E.), 47 (Wiley (Interscience), New York, 1968).Shaw, , J. E., and Ramwell, , P. W., in Prostaglandin Symposium of the Worcester Foundation for Experimental Biology (edit. by Ramwell, P. W., and Shaw, J. E.), 55 (Wiley (Interscience), New York, 1968).Kloeze, , J., in Proceedings of the Second Nobel Symposium (edit. by Bergstrom, S., and Samuelsson, B.), 241 (Wiley (Interscience), New York, 1967).Bergstrom, , S., Carlson, , L. A., and Weeks, , J. R., Pharmacol. Rev., 20 (1) (1968).Flack, , J. D., Ramwell, , P. W., and Jessup, , R., Science (in the press).