The FDA has required the manufacturer of Plavix, an antiplatelet drug used in addition to aspirin to prevent cardiovascular events in high-risk patients,1 to add a boxed warning to the package insert about the risk of a poor response to the drug in patients with genetic polymorphisms of the cytochrome P450 enzyme CYP2C19. Clopidogrel is a prodrug and CYP2C19 is mainly responsible for its bioactivation. The Medical Letter reported last year that several studies have found higher rates of cardiovascular events, including stent thrombosis, in patients with these polymorphisms taking...

Resmetirom (Rezdiffra – Madrigal), a thyroid hormone receptor-beta agonist, has received accelerated approval from the FDA for treatment of noncirrhotic nonalcoholic steatohepatitis (NASH) with moderate to advanced fibrosis in adults. NASH has recently been renamed metabolic dysfunction-associated steatohepatitis (MASH). Resmetirom is the first drug to be approved in the US for this indication.

The FDA has approved eliglustat (Cerdelga – Genzyme), an oral glucosylceramide synthase inhibitor, for treatment of adults with type 1 Gaucher disease. Eliglustat is metabolized primarily by CYP2D6. Because patients who are CYP2D6 ultra-rapid metabolizers may not achieve therapeutic concentrations and a specific dosage cannot be recommended for indeterminate metabolizers, the FDA has approved the drug only for patients who are extensive, intermediate, or poor metabolizers of CYP2D6.

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 FDA recently cleared the AmpliChip CYP450 Test (Roche), which analyzes blood-derived DNA to detect genetic variations in the activity of cytochrome P450 (CYP) enzymes CYP2D6 and CYP2C19 and determines the metabolizer status of the patient. The test is intended to help guide clinicians in prescribing individualized drug therapy. About 25% of all drugs, including many antidepressants and antipsychotics, are substrates of either CYP2D6 or CYP2C19. The test is being promoted initially to psychiatrists.

The FDA has issued new warnings about the use of the opioid analgesics codeine and tramadol in children, particularly those <12 years old, and in breastfeeding women due to concerns about the risk of respiratory depression and death. The FDA previously issued warnings about these drugs in 2013 and 2015.

View Table: Mechanisms of Drug Interactions

Modafinil (Provigil), a benzhydryl sulfinylacetamide non-amphetamine stimulant that has been used in Europe since 1995, has now been approved by the FDA for treatment of excessive daytime sleepiness associated with narcolepsy.

Tolterodine tartrate (Detrol - Pharmacia & Upjohn) is a new muscarinic receptor antagonist now being widely promoted for treatment of urinary frequency, urgency and urge incontinence caused by bladder (detrusor) overactivity.

Metreleptin (Myalept – Amylin), a recombinant leptin analog produced in E. coli, has been approved by the FDA to treat the complications of leptin deficiency in patients with congenital or acquired generalized lipodystrophy. It has not been approved to date for the treatment of partial lipodystrophies, including those associated with the use of protease inhibitors in patients with HIV. Metreleptin is approved in Japan for the treatment of any lipodystrophy disorder.