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.

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).

The FDA has approved the proton-pump inhibitor (PPI) dexlansoprazole (Kapidex - Takeda), a delayed release formulation of the R-enantiomer of lansoprazole (Prevacid - Takeda), for treating and maintaining healing of erosive esophagitis and for treatment of heartburn associated with non-erosive gastroesophageal reflux disease (GERD).

Febuxostat (Uloric - Takeda), a xanthine oxidase inhibitor, has been approved by the FDA for chronic management of hyperuricemia in patients with gout. It is the first drug marketed for treatment of gout in 40 years. Febuxostat is structurally unrelated to allopurinol, the only other commercially available inhibitor of xanthine oxidase. Xanthine oxidase inhibitors decrease serum urate concentrations by decreasing urate synthesis.

Nedocromil sodium (Tilade - Fisons), an anti-inflammatory drug similar to cromolyn sodium (Intal - Fisons), although chemically unrelated, is now available in the USA as a metered-dose inhaler for maintenance treatment of mild to moderate asthma.

Alosetron hydrochloride (Lotronex - Glaxo Welcome), a selective serotonin receptor antagonist, has received FDA approval for treatment of irritable bowel syndrome (IBS) in women whose primary bowel symptom is diarrhea.

Telithromycin (Ketek - Aventis) has been approved by the FDA for oral treatment of mild to moderate community-acquired pneumonia, acute exacerbations of chronic bronchitis and acute bacterial sinusitis in patients age 18 and older. The drug is the first in a new class of antibiotics, the ketolides, derived from the macrolide erythromycin. Telithromycin has been marketed in Europe since 2001.

Zileuton (zye loo' ton; Zyflo - Abbott), an inhibitor of leukotriene synthesis, has been approved by the US Food and Drug Administration (FDA) for prophylaxis and maintenance treatment of chronic asthma in adults and children more than 12 years old. A related drug—zafirlukast (Accolate), a leukotriene receptor antagonist—was recently reviewed in the Medical Letter (vol. 38, page 111, December 20, 1996). Neither of these drugs is recommended for treatment of acute asthma.

Grepafloxacin (Raxar - Glaxo Wellcome), a once-daily oral fluoroquinolone, is now being marketed for treatment of community-acquired pneumonia, acute bacterial exacerbations of chronic bronchitis, uncomplicated gonorrhea, and nongonococcal urethritis and cervicitis.

Clarithromycin (Biaxin - Abbott) and azithromycin (Zithromax - Pfizer), two macrolide antibiotics chemically related to erythromycin, have been approved by the US Food and Drug Administration for treatment of respiratory, skin, and skin structure infections. Azithromycin has also been approved for treatment of nongonococcal urethritis and cervicitis caused by Chlamydia trachomatis.