Measurement of (15S)-15-methyl prostaglandin F2alpha by radioimmunoassay.

Antibodies against (15s)-15-methyl prostaglandin F2alpha (15-MF) were produced in rabbits immunized with a 15-MF bovine serum albumin conjugate. Tritium labeled 15-MF was prepared from (15S)-15-methyl prostaglandin E2 by reduction with tritiated sodium borohydride. Antiserum specificity and label specific activity (12 Ci/mM) were sfficient to enable development of a sensitive, accurate and relatively specific radioimmunoassay for this prostaglandin analog. The assay can detect as little as five picograms of 15-MF in 100 mul of unextracted blood plasma. Endogenous concentrations of PGF2alpha should not cause a significant blank using current assay conditions, as cross reactivity with this molecule was less than one percent. There is, however, significant affinity (45%) for the C-1 methyl ester of 15-MF. This makes the assay also applicable for measuring this molecule, although no distinction can be made between it and the free acid when neat plasma is assayed. Plasma samples from human and monkey studies were assayed for 15-MF before and after recipients were treated with pharmacologically active doses of this compound. Blank values (0 Hour) of less than 0.1 nanograms per ml were found for all pre-treatment samples while post-treatment samples contained detectable levels. Results of the human study was compared and found to be in excellent agreement with values that were obtained using a gas-liquid chromatographic - mass spectrographic assay.