Drug interactions

There is test tube evidence that abacavir (Ziagen) enhances the effects of the protease inhibitor amprenavir (Agenerase), although this may not translate into a more effective combination in people.1

A drug called mycophenolic acid (Myfortic) enhances the antiviral effectiveness of abacavir in test tube studies.2 Although clinical trials are ongoing to determine whether mycophenolic acid can improve the anti-HIV effects of abacavir without inducing high-level toxicity, a small safety study found no significant benefit of the combination.3 At this stage, combining abacavir with mycophenolic acid is experimental and not recommended as standard treatment.

Clearance of methadone hydrochloride (Methadose) is increased by 22% when combined with abacavir, but no dose modification of methadone hydrochloride is required. However, patients taking both drugs should be monitored for methadone hydrochloride withdrawal symptoms.

Alcohol is known to increase blood levels of abacavir, but this has been found not to be clinically significant. Dose adjustments are not required.4

The lack of an interaction between abacavir and the tuberculosis drug rifampicin (Rifadin/Rimactane) has led to suggestions that triple nucleoside reverse transcriptase inhibitor (NRTI) combinations including abacavir might be useful during anti-tuberculosis therapy. The drawback in this strategy is that abacavir hypersensitivity cannot be distinguished from immune restoration inflammatory syndrome that can occur when people with tuberculosis begin antiretroviral therapy. This condition is caused when the immune system begins to recover and mounts a new or stronger response to the organisms that cause tuberculosis. The outcome is a temporary worsening of symptoms, which often includes fever, and may be mistaken for a hypersensitivity reaction.

References

  1. Drusano G et al. Nucleoside analog 1592U89 and human immunodeficiency virus protease inhibitor 141W94 are synergistic in vitro. Antimicrob Agents Chemother 42: 2153-2159, 1998
  2. Margolis DM et al. Abacavir and mycophenolic acid, an inhibitor of inosine monophosphate dehydrogenase, have profound and synergistic anti-HIV activity. J Acquir Immune Defic Syndr 21: 362-370, 1999
  3. Toukas CM et al. The use of mycophenolic acid, an inosine monophosphate dehydrogenase inhibitor, as part of salvage therapy in late stage HIV disease. Fifth International Congress on Drug Therapy in HIV Infection, Glasgow, abstract P11, 2000
  4. McDowell JA et al. Pharmacokinetic interaction of abacavir (1592U89) and ethanol in human immunodeficiency virus-infected adults. Antimicrob Agents Chemother 44: 1686-1690, 2000
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