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14 Nov 2014

For many years people taking protease inhibitors (PIs) have taken ritonavir (brand name Norvir) as part of their HIV drug regimens. Ritonavir, a pharmacoenhancer, was added to ‘boost’ the level of the PI in blood and allowed for lower doses of the PI than might otherwise be required to fully suppress HIV replication.

Now there’s another booster available: cobicistat (COBI). Although similar in composition to ritonavir, COBI has a much better side-effect profile. It also has fewer drug interactions than ritonavir (which inhibits a wider range of liver enzymes). COBI is one of the four component drugs in the fixed dose combination, Stribild. It is used to boost the level of elvitegravir — a new potent integrase inhibitor.

COBI is co-formulated with elvitegravir, tenofovir and emtricitabine. It is also to be co-formulated with darunavir and also separately with Reyataz. These drugs need to be taken with a separate boosting agent, increasing pill burden and the number of co-payments. Co-formulation will simplify regimens that include these drugs.

COBI has no activity against HIV as such and is used solely as a booster. It does not interfere with the metabolism of sugars and does not appear to contribute to increases in blood sugars or cholesterol. That’s good news as many people taking ritonavir-boosted PIs experience increased cholesterol and blood sugars.

COBI does, however, slow the excretion of creatinine through the kidneys and can cause creatinine increase in blood. Creatinine level is used as a measure of the effectiveness of the kidneys in filtering waste products. So it might be thought that COBI is causing damage to the kidneys. The effect of COBI in raising creatinine level is well known and your doctor will be aware of this and will take this into account when considering your kidney function results.

COBI has been found to adversely interact with drugs to treat erectile dysfunction such as Viagra, Levitra and Cialis. It may also increase blood levels of amphetamines including ecstasy, cocaine, speed and meth.

Cobicistat is currently being considered as a potential alternative to ritonavir in other HIV drug combinations and may be approved by the PBS next year.