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Cicaprost is a potent, chemically and metabolically stable PGI2-mimetic. Pharmacodynamic effects were observed after oral administration of approximately 10 micrograms in man when plasma levels were in the low pg-range. The present report describes the development of a selective antiserum and a tracer with high specific activity and their use for the RIA determination of Cicaprost in biological samples. Cicaprost-[3H]-methylester with a specific activity of 819 GBq/mmol was used as a tracer. RIA-analyses were carried out with 0.05-0.5 ml plasma adjusted to pH 2 with 1 N HCl and extracted with 2.5 ml diethylether. Separation of antiserum bound and unbound Cicaprost was achieved by the charcoal method. Extraction recovery of Cicaprost was approximately 90% at pH approximately 2. The detection limit of the assay was 10-20 pg/ml plasma. Coefficients of variations were 6, 3 and 9% (within-day, n = 5) and 25, 12 and 10% (day-to-day, n = 11) at 50, 100 and 200 pg/ml. HPLC-chromatograms of plasma extracts did not reveal any peak apart from Cicaprost, demonstrating the specificity of the method. The present RIA for Cicaprost exhibits high specificity and sensitivity and will be used for further bioanalyses in pharmacokinetic study.

In a Phase I study, the tolerability, pharmacodynamics and pharmacokinetics of cicaprost have been investigated in 6 male volunteers given 5, 10, 15 and 20 micrograms as tablets of the beta-cyclodextrin clathrate. Individual inhibition of platelet aggregation and changes in facial colour (measured by chromametry) were dose-dependent and reached a maximum 30 to 60 min post-dose. The maximum inhibition of platelet aggregation was about 40%. After 3 to 4 h pre-treatment values had returned. Blood pressure remained within the normal range. The peak plasma level of cicaprost was reached within 15 to 90 min after drug intake. Both Cmax- and AUC were individually dose-dependent. The terminal half-life in plasma of cicaprost was approx. 1 h, and its total clearance amounted to 4-7 ml.min-1.kg-1. The time courses of the plasma levels and of the pharmacodynamic actions were in agreement. Interindividual differences were observed in the occurrence of unwanted effects (e.g. headache). Thus, cicaprost is an orally available PGI2-mimetic, for which effects on platelet aggregation and vascular perfusion have been demonstrated in healthy volunteers after doses of 5 to 15 micrograms.

The development of a radioimmunoassay for ZK 36 374, a chemically stable prostacyclin analogue, and its application to the pharmacokinetics of the drug in the rat is described. Antiserum against ZK 36 374 was raised in rabbits by immunization with ZK 36 374 coupled to bovine serum albumin via the carboxylic group. ZK 36 374 is extracted from plasma samples and purified from matrix constituents by means of octadecyl silyl cartridges. At a plasma concentration of 2.5 ng/ml intra- and interassay variations are 6 and 7%, respectively. The limit of detection is 0.5 ng/ml. After intravenous administration of 200 micrograms/kg to female Wistar rats a biphasic decline of the drug concentration in the plasma was observed with half-lives of 5 min and 3 h. Oral doses of 200 and 2000 micrograms/kg were very rapidly absorbed reaching maximum plasma levels of 3 and 32 ng/ml as early as 10-15 min p.admin.. Bioavailability at the two dose levels was calculated to be 8 and 13% of dose. Disposition was biphasic, as well, with a somewhat prolonged beta-phase after the highest dose.

A new analytical principle has been developed combining the features of both radioimmunoassay and GC/MS. Its application in eicosanoid analysis was tested with the prostacyclin analogue, iloprost. The iloprost antibody, generally employed in RIA measurements, was coupled to Sepharose 4B and used as stationary phase for extraction of the drug. Variations in recovery were corrected by using deuterated iloprost as an internal standard. The samples were derivatized and quantitated by negative-ion chemical ionization-mass spectrometry. Reproducibility was 2.3 % at the 50 pg/ml level and the limit of detection was 5 pg for 1 ml samples. With plasma volumes of up to 20 ml, 0.25 pg/ml could be determined. Antibody/GC/MS proved superior to radioimmunoassay due to its higher specificity and sensitivity and superior to GC/MS with conventional clean-up procedures because of a higher sample capacity.

The development of a sensitive radioimmunoassay for measuring concentrations of nileprost , a stable prostacyclin analogue, is described. The RIA method, furthermore, is applied to evaluate the pharmacokinetics of nileprost in the rat. Antiserum against nileprost was raised in rabbits by immunizing with the drug coupled to bovine serum albumin via the carboxylic group. Nileprost is extracted from plasma samples using a reversed-phase Sep- PakR cartridge. The limit of detection is less than 10 pg/ml. At the 1.25 ng/ml level intra- and inter-assay variations of 6 and 16% were calculated. Following intravenous injection of 200 micrograms/kg to female Wistar rats, a three-phasic decline in plasma levels was observed with half-lives of 10 min, 30 min and 7.4 h. Oral doses of 200 and 2,000 micrograms/kg were very rapidly absorbed exhibiting maximum concentrations in the plasma of 20 and 30 ng/ml as early as 5 min postadministration . Bioavailability of nileprost was calculated to be 21 and 13% of the dose. The disposition half-life was 5-7 h in both cases.