On the multiplicity of platelet prostaglandin receptors. I. Evaluation of competitive antagonism by aggregometry

Methods for the evaluation of competitive interactions at receptors associated with platelet activation and inhibition using aggregometry of human PRP have been developed. The evidence supports the suggestion that PGE1 and PGI2 share a common receptor for inhibition of platelet reactivity, but only a portion (if any) of the aggregation stimulation associated with PGE2 is the result of PGE2 binding (without efficacy) to this receptor. PGE2 (at .3-20 microM) is an effective antagonist of PGE1, PGI2, and PGD2 producing a shift of about one order of magnitude in the IC50-values obtained from complete aggregation inhibition dose response curves. The antagonism of PGD2 inhibition is particularly notable, 80 nM PGE2 levels are detectable. This and other actions of PGE2 indicate another platelet receptor for PGE2. PGE1 acts at both the PGE2 and PGI2 receptor. Other substances showing PGI2-like actions only at high doses (1-30 microM), display additive responses with PGI2 indicative of decreased affinity for the I2/E1 receptor and the absence of PGE2-like aggregation stimulation activity. PGI2 methyl ester has intrinsic inhibitory action not associated with in situ ester hydrolysis. The methyl ester is dissaggregatory showing particular specificity for inhibition of release and second wave aggregation.

Synthesis and chiroptical characterization of prostacyclin diastereomers

In the mercuri- and halo-cyclizations of PGF2 alpha methyl ester and its 11,15-bis(alpha- ethoxyethyl)-ether (or other protected forms) the exo-PGI1 derivative predominates independent of reagent and degree of protection of the PGF2 alpha sample used. Diastereomerically pure samples of exo- and endo-PGI1 and prostacycline (PGI2) were prepared. PGI0 epimers were prepared: catalytic hydrogenation of PGI2 Me ester provides exclusively the endo isomer. PGI2 methyl ester was found to be stable to extensive chromatography on silica, and to storage for at least a year in anhydrous ethanol at -20 degrees C. At pH 7.4 in 2:1 H2O:EtOH, the ester has a half-life in excess of 5 hr at 25 degrees C. A reproducible small scale (0.4-3 mg) synthesis of prostacyclin uses a modification of Whittaker's iodocyclization followed by DBN treatment. This procedure, developed with 15-3H-PGF2 alpha, proved widely applicable to PGF2 alpha analogs and diastereomers. The following prostacyclins (in the Me ester and Na salt forms) bearing the 5-en-6-yl ether unit were prepared in this way: ent-PGI2, rac-PGI2, 15-epi-PGI2, ent-15-epi-PGI2, 11-epi-PGI2, 8,9,12-epi-PGI2, E-PGI2, 13,14-dihydro-PGI2, and 13,14-dihydro-15-epi-PGI2. NMR comparisons for the methyl esters reveal that of the resonances (H-5,9,UU, 15) that appear at delta 4.0 +/- 0.6 ppm, the most deshielded is H-9 so long as the 5.6-olefin is Z. The 8R,9S-6,9-oxido-Z-5,6-ene unit is most readily characterized by its strong positive dichroic absorption at 210-230 nm. CD spectroscopy not only serves to confirm the presence of this unit in analogs, but also can be used for quantitative analysis of PGI2 solutions and for monitoring the rate of hydrolytic cleavage of these enol ethers.

Aureobasidium pullulans metabolism of prostaglandins of the A-type

Aureobasidium pullulans, originally introduced as an inadvertent contaminant in solutions used for evaluating the stability of prostaglandins, proved to lead to the rapid disappearance of the cyclopentenone unit of PGA2 (as monitored by circular dichroic spectroscopy). The cyclopentenone unit is converted, in various metabolites, to a 9-keto, 9 alpha or 9 beta-hydroxy group lacking the ring unsaturation. The major EtoAc-soluble 9-hydroxy metabolite (Compound-I) was shown to be 9 alpha, 15 alpha-dihydroxy-2, 3, 4, 5-tetranor-13-trans-prostenoic acid. Similar tetranor 9-hydroxy metabolites with one additional degree of unsaturation, and with a 9 beta-hydroxy group, also occur but these have not been fully characterized. Only two of the wide range of 9-keto metabolites are fully characterized by mass spectral (MS) data: 9, 15-oxo-2, 3, 4, 5-tetranorprostanoic acid and 9, 15-oxo-2, 3, 4, 5-tetranor-13-trans-prostenoic acid. The water soluble metabolites have not been characterized further. The fully characterized metabolites together with MS data from mixtures of minor metabolites indicate that A. pullulans can perform the following transformation: beta-oxidation, dehydrogenation at C-15, reduction of the enone carbon-carbon double bonds (both delta 10,11 and delta 13,14), reduction of the 9-ketone, and possibly migration of the cyclopentyl double bond (delta 10, 11 leads to delta 11, 12). A. pullulans metabolizes 15-epimeric PGA2 equally readily with the production of similar products. PGA1 affords less 9-keto metabolites with compound I constituting 33% of the product by HPLC analysis. A. pullulans displays some enantioselectivity, PGA2 and 15-epi-PGA2 are each metabolized more rapidly than their enantiomers. Other prostaglandins appear to be less readily metabolized.

Synthesis of diastereomeric bis-unsaturated prostaglandins

With the report given herein all diastereomers of PGF2, PGE2, and PGD2 which bear the naturally recognized 15-S hydroxylated center, whether in the natural or entprostanoic acid skeleton, have been prepared by a route involving initial introduction of the carboxyl (alpha) chain (1). A major advantage of the initial alpha-ylation route is the facile reduction of the 13, 14-en-15-one system with methanolic NaBH4 which proceeds without competing 1,4-reduction. The products are thus free of 13,14-dihydro-PG2 contaminants (2). The initial pharmacological evaluation of these diastereomers will be submitted for publication in this journal (3).

Molecular basis of prostaglandin potency. II. Proton NMR studies of the conformation of prostaglandin F2alpha

The proton nmr of PGF2alpha (dissolved in aqueous tBuOH) in the presence of PrC13 and limited amounts of base shows LIS shifts for the terminal methyl indicating an average effective chain length (C-1 to C-20) like that of octanoic acid, further evidence that the alpha and omega carbons of prostaglandins are closer in space than would be expected based on a random distribution of confermers with alternate zig-zag sections of hydrocarbon chains. Assessing the exact nature of the sidechain alignment will require further experiments.