On January 6, 2012, the FDA announced that the package insert for efavirenz has been updated to include a new dosing recommendation for HIV-infected patients with tuberculosis (TB): When efavirenz is coadministered with rifampin in patients weighing 
50 kg, it should be dosed at 800 mg once daily instead of the usual 600 mg daily. The evidence cited to support this recommendation is from drug–drug interaction trials and a pharmacokinetic modeling study. These studies suggest that coadministration reduces systemic exposure to efavirenz when that drug is given at 600 mg daily, but not when it is given at 800 mg daily; at the higher dose, systemic exposure is comparable to that achieved when efavirenz is dosed at 600 mg daily without rifampin.
Comment: With this new recommendation, the FDA aims to resolve a decade-long debate on how best to treat HIV/TB-coinfected patients. Efavirenz is metabolized by CYP450 enzymes, which are known to be induced by rifampin. When the two drugs are coadministered, the interaction could lead to suboptimal efavirenz levels, such that patients on efavirenz-containing triple therapy receive effective exposure to only two of the three drugs in their regimen. In a 2008 case study, FDA investigators concluded that the existing evidence on this interaction was inadequate to support a dosing change (J Clin Pharmacol 2008; 48:518), but 3 years later, they have changed their view.
Although the studies cited in the FDA's recommendation support the proposed approach, others do not. Our own work — and at least one additional pharmacokinetic study — have demonstrated that the drug–drug interaction could be in the opposite direction, thus increasing efavirenz exposure (Antivir Ther 2011; 16:527, Eur J Clin Pharmacol 2011; e-pub ahead of print). If this is the case, a non-individualized weight-based dosage increase could result in toxicity, possibly without any added efficacy benefit. Several studies indicate that efavirenz pharmacokinetics are influenced by CYP450 G516T and possibly other single nucleotide polymorphisms, rather than by rifampin alone. Perhaps more important, numerous studies show that patients receiving concurrent rifampin and efavirenz achieve virologic suppression even with standard efavirenz dosing (AIDS 2005; 19:1481, J Antimicrob Chemother 2006; 58:1299, Clin Infect Dis 2011; 53:716). Taken collectively, these data suggest that the rifampin–efavirenz interaction may not be consistent in all populations and that increasing the dose of efavirenz may not improve clinical outcomes.
By Tanuja N. Gengiah, M Clin Pharm, MS (Epi), and Salim S. Abdool Karim, MD, PhD
Ms. Gengiah is the Head of Pharmacy at the Centre for the AIDS Programme of Research in South Africa (CAPRISA), University of KwaZulu-Natal, Durban, South Africa. She reports no conflicts of interest.
Published in Journal Watch HIV/AIDS Clinical Care February 6, 2012
Citation(s):
Food and Drug Administration. Sustiva labeling update/dosing adjustment with rifampin. Jan 6 , 2012. (http://www.natap.org/2012/newsUpdates/010612_06.htm)