All the selective calcium channel blockers are well absorbed after oral administration, although there are marked differences in oral bioavailability that relate to differences in first-pass metabolism.^3,4 Verapamil and isradipine undergo fairly extensive first-pass metabolism, whereas diltiazem, nifedipine, and nicardipine do not. Protein binding percentages are higher with the dihydropyridines than with either diltiazem or verapamil.³ With nifedipine and possibly other dihydropyridines, protein binding is concentration dependent, allowing for the possibility of protein-binding interactions, although none of clinical significance has been reported. With verapamil and diltiazem, protein binding is independent of drug concentrations, making displacement interactions unlikely.

Because the older calcium channel blockers have relatively short half-lives, extended-release formulations have been developed to permit once- or twice-daily administration. Isradipine, felodipine, and amlodipine have substantially longer elimination half-lives, although only amlodipine's half-life allows once-daily administration.^5,6

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