Dose Adjustments When Using Amprenavir
Amprenavir (Agenerase®), the most recently approved protease inhibitor, is often used by people who have lost responsiveness to one or more of the previously available drugs of this class. It is considered wise in this situation to try to start two new drugs at the same time so many people combine amprenavir with efavirenz (Sustiva®). Unfortunately, few people seem aware that there are significant interactions between these two drugs which require adjusting their dosage.
Efavirenz decreases amprenavir levels in the blood by about 40%, making an increased dose of amprenavir necessary. The optimum dose of amprenavir in this combination is still not clearly established, but some researchers believe it should be increased to 1,200mg three times a day (compared to the standard dose of 1,200mg twice daily). However, early study results indicate that this change in dose is unnecessary if the combination also includes 100-200mg of ritonavir or 1,250mg of nelfinavir taken twice a day along with the amprenavir and efavirenz. Check the chart below for preliminary dosing recommendations.
APV=amprenavir; EFV=efavirenz; RTV=ritonavir; NFV=nelfinavir.
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