For these reasons, RTV enhanced indinavir (IDV) and fos-amprenavir (fos-APV) combinations appear promising for salvage therapy as these two agents are pharmacokinetically enhanced by RTV, and have unique resistant mutation profiles when compared with other PIs. Additionally, since clinically limiting interaction is known to occur between fos-APV and lopinavir/ritonavir (LPV/r), the interactions between IDV, fos-APV, and RTV may be different in a manner that could be clinically beneficial in salvage therapy. Importantly, we will evaluate IDV/fos-APV/RTV at a dose of 800/700/100 mg bid as we anticipate that fos-APV may lower IDV concentrations similarly to lopinavir (LPV), although likely to a lesser extent; such that a dual benefit of enhanced efficacy and improved tolerability could be achieved. The pharmacokinetic profile of this combination regimen therefore merits evaluation to allay concerns of clinically important drug-drug interactions before introduction into clinical practice.