With the release of two new classes of antiretrovirals last fall, most care providers needed to quickly get back on the learning curve to master both drugs simultaneously.
Fortunately, the new integrase inhibitor raltegravir (Isentress) has one standard dosage for all adults: one pill twice a day with or without food. It has no drug interactions and should work in any patient with HIV.
However, it takes a little more study to use the other newly approved drug, maraviroc (Selzentry).
When it was discovered that people are essentially resistant to HIV if they lack the CCR5 receptor on the surface of their CD4 cells, the hunt was on to find a CCR blocker. Maraviroc is the first of this class. But only about 70% to 80% of people with HIV have an R5 virus that uses CCR5 to gain access to the CD4 cell. About 2% to 5% have an X4 virus that uses another receptor, CXCR4. The remaining 20% to 30% have a virus population that is a mixture of R5 and X4.
The first step in using maraviroc is to test to see if a patient has R5 virus. This test is currently available through only one laboratory. It takes two to three weeks to get results and requires a viral load of at least 1,000. The test is also expensive, but at the moment it seems to be covered by insurance.
Next, the dosage may need to be raised or lower based on what other drugs the patient is taking, since maraviroc is processed through a common liver pathway called CYP 3A4. This is the pathway used by most protease inhibitors and blocked by ritonavir. Hence, the maraviroc dosage is cut in half when used in combination with a ritonavir-boosted protease inhibitor. Alternatively maraviroc must be double when used with efavirenz, which up-regulates CYP 3A4 activity. Then there are exceptions like nevirapine or boosted tipranovir, both of which cause no change in maraviroc levels. And when both a boosted PI and efavirenz are used, the boosted-PI affect wins and the dosage is halved.
Confused and concerned? Don't be. The maraviroc prescribing literature has a clear chart to follow. But don't forget to review the maraviroc dosage whenever changes are made in a treatment regimen.
At least it is as easy to take as raltegravir: one pill twice a day with or without food.
Bowers is an HIV specialist and board-certified physician in family practice, and he is a senior partner with Pacific Oaks Medical Group, one of the largest U.S. practices devoted to HIV care, in Beverly Hills, Calif.