Acetylsalicylic acid (ASA) - How much, how often, and when? A clinical-pharmacological perspective.

The dose of acetylsalicylic acid (ASA) commonly used in the prevention and treatment of arteriosclerotic angiopathies is equal to or less than 100 mg daily. This choice of dose is predominantly based on molecular-pharmacological findings showing an inhibition in synthesis of the prothrombotic thromboxane (TXB2) and an irreversible inhibition in blood platelet aggregation. However, an analysis of ASA dose-effect relationships for doses of 50 - 500 mg (PO and IV) shows that doses of ASA up to 100 mg daily produce only a small or moderate inhibition in collagen/epinephrine-induced platelet aggregation and have no significant effect on the important platelet factors, PF3 and PF4. Doses of ASA 300 - 500 mg, on the other hand, inhibit platelet aggregation almost completely and, in addition, produce a 50 - 70% inhibition in PF3 and PF4 lasting at least 24 hours. There is also evidence that doses of ASA above 100 mg daily markedly inhibit thromboxane synthesis for up to 24 hours and that doses of 500 mg daily produce a clinically relevant inhibition in platelet adhesion to vessel walls for up 72 hours and prevent procoagulatory shape changes for up to 12 hours. These findings suggest that a dose of ≥ 300 mg at intervals of 2 - 3 days would be more appropriate for primary and secondary prophylaxis of arteriosclerotic angiopathies and that the benefit-risk ratio would be greater because of the increased availability of mucoprotective prostaglandins, PGI2 (prostacyclin) and the gastroprotective, PGE2. Our viewpoint, predominantly based on findings with biomarkers, could serve as a basis for further randomized controlled studies.

Angiotensin II dose-effect curves and Schild regression plots for characterization of different angiotensin II AT1 receptor antagonists in clinical pharmacology.

The 'Schild regression' method is based on the principle of assessing the rightward shift of agonist dose-effect curves in the presence of different doses/concentrations of the respective receptor antagonist and presenting their relationship in a double log plot (i.e. the 'Schild plot'). The original method was developed to quantitatively characterize antagonistic drugs in experimental pharmacology. The method was adopted for evaluation of various AT1 antagonists in humans utilizing (human) angiotensin II as the agonist. Angiotensin II (Ang II) in continuous intravenous dose-incremental administration resulted in a clearly dose-dependent increase in blood pressure. All AT1 antagonists tested after oral administration yielded concentration-dependent rightward shifts of those Ang II dose-effect curves that were quantified as dose ratio (DR). DR minus 1 (DR-1) enabled the assessment of antagonist time kinetics in humans and a quantitatively precise determination of the half-life of antagonism in vivo. Schild plots allowed for assessment of apparent Ki doses indicative of a twofold rightward shift of the Ang II effect, thus providing the means for a rational comparison of the pharmacological potency of many of these compounds, where the Ki doses obtained at 24 h after administration were in the range of 'therapeutic' doses. Schild plots of a variety of substances showed linear relations independent of whether the blockade was deemed surmountable or not. It is therefore assumed that this property does not play a role at clinical doses/concentrations. Slopes slightly below 1 in the Schild plots of all tested antagonists point to a second 'counterregulatory' vasodilatory mechanism of action of Ang II which becomes apparent with AT1 blockade in conditions of high doses/concentrations of Ang II. Concentration vs. effect relationships indicate that if assessed at the same degree of direct vascular antagonism, other effects, such as increase in plasma renin activity, may be present to a varying degree with different antagonists. Thus for irbesartan, the potency to stimulate renin release was found to be at least twice that of candesartan. These observations should stimulate further research into the relevance of these dynamic differences between the various compounds. Thus, methodologies relying on fundamental principles of experimental pharmacology can provide the clinical pharmacologist with powerful tools to measure accurately degree of antagonism and time kinetics and to investigate the nature of receptor antagonism in humans.

Pharmacodynamic studies on the angiotensin II type 1 antagonists irbesartan and candesartan based on angiotensin II dose response in humans.

The in vivo effects of two unsurmountable angiotensin II type 1 (AT1) antagonists, irbesartan (150 mg) and candesartan (8 mg), were studied in a double-blind, randomized, crossover study in 18 healthy men. The drugs' direct vascular effects were assessed as the rightward shift (dose ratio - 1) of angiotensin dose-effect curves on diastolic blood pressure (DBP). Renal and adrenal effects were assessed by plasma renin activity (PRA), aldosterone concentrations, and antagonistic concentration equivalents (n x Ki) in a radioligand rat lung receptor assay. Both drugs exerted similar substantial (> 30-fold) and long-lasting (> 2-fold 47 h after dosing) rightward shifts of the angiotensin II dose effect declining with half-lives of 15 h irbesartan and 12 h candesartan, respectively. Dose ratio - 1 versus n x Ki showed a linear relationship in Schild regression plots; both drugs increased PRA, decreased DBP, and suppressed aldosterone. The slopes of linear relationship between angiotensin antagonism (dose ratio - 1) and PRA increase were almost threefold steeper (p = 0.005) following irbesartan as compared with candesartan. The findings suggest that for the same degree of angiotensin II antagonism, irbesartan produces a greater increase in PRA than candesartan. These pharmacodynamic differences warrant further investigation and clarification.