Intelence (etravirine) may lower blood levels of Isentress (raltegravir) in some people, according to a series of case reports published in the April 27 issue of AIDS.

Isentress and Intelence, an integrase inhibitor and a non-nucleoside reverse transcriptase inhibitor, have become popular medication options for people who are heavily treatment experienced. Thus far, small studies in HIV-negative volunteers have not demonstrated that one lowers or raises blood levels of the other, but there are not substantial amounts of data involving drug combinations of both antiretrovirals in treatment-experienced HIV-positive patients.

Contrary to the studies in HIV-negative volunteers, Amélie Ménard, MD, from the Sainte Marguerite University Hospital, in Marseille, France and her colleagues report four cases of HIV-positive heavily treatment-experienced patients having significant drug interactions. In three of the cases, the patients started Isentress and Intelence at the same time. In the remaining case, the patient started taking Intelence after having been on Isentress for a while.

In all four cases, blood levels of Isentress were reduced significantly, while blood levels of Intelence stayed in the normal range. Ménard and her colleagues comment that HIV levels in the four patients did not drop as much as would have been expected after starting new regimens involving these drugs, and they believe that the reduced Isentress blood levels are the likely culprits.

The authors conclude that standard drug interaction studies involving HIV-negative volunteers may not always accurately predict what will happen in people with HIV, especially treatment-experienced patients. They recommend that providers explore therapeutic drug monitoring options to measure Isentress blood levels in patients who combine Isentress and Intelence, and that further study of these two drugs is warranted.