Pharmacokinetic and pharmacodynamic studies following single and multiple doses of rolafagrel, a novel inhibitor of thromboxane synthase, in normal volunteers

Amine oxidases and monooxygenases in the in vivo metabolism of xenobiotic amines in humans: has the involvement of amine oxidases been neglected?

In this review, the major enzyme systems involved in vivo in the oxidative metabolism of xenobiotic amines in humans are discussed, i.e. the monooxygenases [cytochrome P450 system (CYPs) and flavin-containing monooxygenases (FMOs)] and the amine oxidases (AOs). Concerning the metabolism of xenobiotic amines (drugs in particular) by monoamine oxidases (MAOs), this aspect has been largely neglected in the past. An exception is the extensive investigation carried out on the inhibition of the metabolism of tyramine, when tyramine-containing food is ingested by subjects taking inhibitors of MAO A or of both MAO A and B. Moreover, investigations in humans on the metabolism of drug amines on the market by AOs, such as semicarbazide-sensitive amine oxidases (SSAOs) and polyamine oxidases (PAOs), are practically nonexistent, with the exception of amlodipine. In contrast to MAOs, monooxygenases (CYP isoenzymes more than FMOs) have been extensively investigated concerning their involvement in the metabolism of xenobiotics. It is possible that the contribution of AOs to the overall metabolism of xenobiotic amines in humans is underestimated or erroneously estimated, as most investigations of drug metabolism are performed using in vitro test systems optimized for CYP activity, such as liver microsomes, and most investigations of drug metabolism in vivo in humans carry out only the identification of the final, stable metabolites. However, for some drugs on the market, the involvement of MAOs in their in vivo metabolism in humans has been demonstrated recently, among these drugs citalopram, sertraline and the triptans are examples that can be mentioned.

Antiplatelet and anticoagulant effects of "HN-11 500," a selective thromboxane receptor antagonist

The antiplatelet and anticoagulant effect of a thromboxane receptor (TX receptor) antagonist developed by Nycomed (Linz) has been studied in a placebo-controlled double-blind phase I study. Sixteen healthy male volunteers received different single oral doses of "HN-11 500" (C(14)H(15)NO(5)S(2); 1, 10, 100, 200, and 400 mg). Eight volunteers received placebo. The washout period between each dosage applied was at least 12 days. Platelet aggregation induced by the thromboxane mimetic "U 46 619" (C(21)H(34)0(4)) and platelet adhesion to siliconized glass were significantly and dose-dependently inhibited. The effect lasted between 3 and 4 h (10 mg) and 8 h (400 mg), respectively, and correlated well with the pharmacokinetic data. Platelet aggregation seems to be more sensitive to monitor the effects of HN-11 500 on platelet function than platelet adhesion. Plasma levels of 300 ng/ml HN-11 500 probably leads to >90% inhibition of platelet aggregation. The template bleeding time slightly increased but did not exceed the normal range. Furthermore, there was a wide variation of results. There were no significant changes in platelet counts, platelet-induced thrombin generation time (PITT), and blood coagulation parameters. All doses of HN-11 500 were well tolerated. HN-11 500 is a potent TX receptor antagonist (TXRA), which inhibits either platelet aggregation or platelet adhesion, which has not yet been described. In clinical routine, TXRAs have to demonstrate the effectiveness in large clinical trials for different clinical indications and to compete with single or combined administrations of cyclooxygenase (COX) inhibitors, thienovridines, thromboxane synthase inhibitors, and GIIb/IIIa inhibitors.

Inhibitors of platelet signal transduction as anti-aggregatory drugs

In the treatment and prevention of cardiovascular diseases, inhibition of platelet aggregation is of fundamental importance. Inhibition of platelet aggregation can be achieved by either inhibition of membrane receptors or by interception of signalling pathways. While receptor antagonism provides high specificity, the inhibition of platelet signal transduction is more effective. The effectiveness results from the inhibition of platelets, regardless of the cause of activation. These common pathway inhibitors are either intercepting platelet activating mechanisms or amplifying the action of endogenous platelet inhibitors. The physiological anti-aggregants are the endothelial factors NO and prostacyclin, which elevate intracellular cGMP or cAMP content, respectively. By administration of NO-releasing agents, prostacyclin analogues or other cyclic nucleotide elevating drugs the platelet anti-aggregatory action of endothelial factors can be effectively mimicked. Besides antiplatelet activity these drugs also act on vascular smooth muscle causing relaxation and therefore vasodilation, an additional beneficial effect. Inhibition of phosphodiesterases causes elevation of platelet cyclic nucleotide content and thus inhibits platelet aggregation and causes vasodilation. Another relevant target for anti-aggregatory treatment is the arachidonic acid metabolic pathway. This pathway can be intercepted by blockade of either cyclooxygenase-1 (COX-1) or thromboxane synthase. Inhibition of these enzymes may be further amplified by additional antagonism of the thromboxane receptor thus not only preventing formation of thromboxane but also activation of thromboxane receptor by thromboxane precursors, which were particularly effective in clinical trials. In vivo these precursors may be metabolised to prostacyclin in the endothelium and consequently provide additional platelet anti-aggregatory activity. A rather new target for platelet anti-aggregatory treatment is the ecto-nucleotidase CD-39 which limits the plasma level of nucleotides. While several of the novel anti-aggregatory drugs were disappointing in clinical studies combinations of drugs with different effector enzymes showed potent antithrombotic efficacy.