Since tipranavir (Aptivus) inhibits the CYP3A enzyme when it is boosted with ritonavir, patients taking it should not take the following drugs, due to dangerous interactions:

• Amiodarone (Cordarone X), due to heart problems.
• Astemizole, due to heart problems.
• Atorvastatin (Lipitor), due to increased atorvastatin levels and an increased risk of side-effects.
• Bepridil, due to heart problems.
• Cisapride, due to heart problems.
• Ergotamine tartrate (Cafergot / Migril), due to heart problems.
• Flecainide, due to heart problems.
• Halofantrine.
• Hypericin (St John’s wort), due to decreased tipranavir levels.
• Lovastatin, due to the risk of lovastatin-associated side-effects.
• Lumefantrine.
• Midazolam (Hypnovel), due to the risk of sedation or breathing problems.
• Pimozide (Orap), due to heart problems.
• Propafenone, due to heart problems.
• Quinidine (Kinidin Dureles), due to heart problems.
• Rifampicin (Rifadin / Rimactane), due to an 80% reduction in tipranavir levels.
• Simvastatin (Zocor), due to simvastatin-associated side-effects.
• Terfenadine, due to heart problems.
• Triazolam, due to the risk of sedation or breathing problems.

Tipranavir has been reported to reduce levels of saquinavir (Invirase), amprenavir (Agenerase) and lopinavir, despite the presence of ritonavir (Norvir) as a boosting agent in the drug combination^[1]. At present Boehringer Ingelheim cannot advise on the dose adjustments that would be necessary if tipranavir is used alongside one of these protease inhibitors because it needs to carry out further research. Therapeutic drug monitoring is likely to be the only way of determining whether adjusted doses are achieving adequate concentrations of protease inhibitors used alongside tipranavir.

Data collected from four drug interaction studies of tipranavir in HIV-negative people found that standard doses of AZT (zidovudine, Retrovir), tenofovir (Viread), ddI (didanosine, Videx / VidexEC) and efavirenz may be used with ritonavir-boosted tipranavir^[2]. Total exposure to AZT is reduced by 33 to 43% when taken with tipranavir but Boehringer Ingelheim has not recommended dose adjustments at this stage. However, enteric-coated ddI should be taken at least hours apart from tipranavir, due to decreases in ddI levels if it is taken with food.

Other interaction studies involving tipranavir have reported that antacids reduce the blood levels of tipranavir by one third^[3]. Tipranavir may also reduce the efficacy of oestrogen-based hormonal contraceptives. Additional forms of contraception should be used.

Additional drug interaction studies are planned.