EX-VIVO INHIBITION OF BETA-THROMBOGLOBULIN RELEASE FOLLOWING ADMINISTRATION TO MAN OF ABT-299, A NOVEL PRODRUG OF A POTENT PLATELET-ACTIVATING-FACTOR ANTAGONIST

Objective and Design: ABT-299 is a prodrug that is converted by serum esterase to a potent platelet activating factor (PAF) antagonist (A-85 783). In order to evaluate the pharmacological activity of this antago nist in man the effect of ABT-299 given to healthy volunteers on ex vi vo PAF-induced beta-thromboglobulin (beta-TG) release in blood was ass essed. Subjects: 37 healthy male volunteers, age 18 to 40 (mean age of 23.6 years) and free of medication, participated in the study. Treatm ent: Subjects were administered intravenously 0.8 mg, 2 mg, or 70 mg d oses of ABT-299 (6-7 subjects per group) or placebo (9 subjects, poole d). Methods: Peripheral blood taken over 12 h after dosing was used fo r ex vivo beta-TG release and, in the case of the 70 mg dose, measurem ent of plasma drug concentration. Data were compared by Student's t-te st. Results: All three doses produced highly significant inhibition (p < 0.005 compared to predose values) of PAF-induced beta-TG release (u nits/ml plasma +/- SEM) 12 h after drug administration (54 +/- 14 vs. 405 +/- 51, n = 8; 79 +/- 23 vs. 480 +/- 127, n = 7; 21 +/- 10 vs. 327 +/- 72, n = 6, respectively) whereas there was no significant differe nce in beta-TG release in the placebo group (449 +/- 90 vs. 307 +/- 49 , n = 9). Inhibition was associated with the rapid appearance in plasm a of A-85783 and the pyridine N-oxide metabolite of A-85783. Within 2 h, the plasma concentration of the metabolite exceeded that of the par ent drug. Both the parent drug and the metabolite exhibited potent in vitro inhibition of PAF-induced beta-TG release (A(2) values of 4 and 1 nM respectively). Conclusions: These studies are the first to illust rate the utility of the beta-TG release assay for assessing ex vivo ac tivity of PAF antagonists. These studies also demonstrate that the adm inistration of ABT-299 to man results in potent, long lasting inhibiti on of PAF-mediated platelet activation, due in part to the pyridine-N- oxide metabolite, and support the potential therapeutic utility of thi s prodrug in treating PAF-mediated diseases.