In a study of the interaction between alcohol and the calcium-channel blocker felodipine (Plendil), the grapefruit juice vehicle for the alcohol appeared to increase felodipine plasma concentrations. Subsequent studies have found that grapefruit juice increases plasma concentrations of several calcium-channel blockers and of some other drugs as well (DG Bailey et al, Clin Pharmacokinet, 26:91, 1994).

The ability of grapefruit juice to increase serum concentrations of drugs was first discovered during a study of the effect of ethanol on felodipine (Plendil) pharmacokinetics. Double-strength grapefruit juice used to disguise the taste of ethanol resulted in higher than expected serum concentrations of felodipine (DG Bailey et al, Clin Invest Med 1989; 12:357).

Serious adverse interactions between drugs continue to be reported. Many of these are due to inhibition or induction of cytochrome P450 (CYP) enzymes, particularly CYP3A4. CYP3A is thought to be involved in the metabolism of more than 50 percent of currently prescribed drugs.2 CYP3A4, which is more abundantly expressed than CYP3A5, accounts for most CYP3A activity in vivo.

A recent letter to the editor of the Annals of Internal Medicine documented a single case of myopathy apparently due to an interaction between simvastatin (Zocor, and others) and green tea. Since it became available generically, simvastatin has surpassed atorvastatin (Lipitor) as the best selling statin. As such, it is probably the most common cause of statin-induced myopathy, which is often a result of drug interactions.

Cilostazol, a phosphodiesterase III inhibitor that has been used in Japan since 1988, has been approved by the FDA for treatment on intermittent claudication due to occlusive peripheral arterial disease.

Felodipine (Plendil - Merck), an extended-release preparation of a dihydropyridine calcium-channel blocker was recently marketed in the USA for oral treatment of hypertension. An immediate-release formulation is available in some other countries.

Nisoldipine (Sular -Zeneca), a dihydropyridine calcium-channel blocker structurally similar to nifedipine, has been approved for marketing by the US Food and Drug Administration. It is available in an oral extended-release formulation for treatment of hypertension.

Changes caused by one drug in the absorption, distribution, metabolism or excretion of another may lead to a pharmacokinetic adverse drug interaction (DN Juurlink et al, JAMA 2003; 289:1652). Additive drug interactions, such as vasodilation caused by both sildenafil (Viagra) and nitrates, can also have adverse effects.

Sirolimus, whlch is structurally related to tacrolimus, has been approved by the FDA for prevention of acute renal transplant rejection. The manufacturer recommends that sirolimus be taken with cyclosporine and corticosteroids.

Voclosporin (Lupkynis – Aurinia), an oral calcineurin inhibitor, has been approved by the FDA for use in combination with the antimetabolite immunosuppressant mycophenolate mofetil (Cellcept, and generics) and a corticosteroid for treatment of adults with active lupus nephritis. It is the first calcineurin inhibitor to be approved in the US for this indication. Tacrolimus (Prograf, and others) and cyclosporine (Neoral, and others), the other available systemic calcineurin inhibitors, are approved for prophylaxis of organ rejection in transplant patients.