The cytochrome P450 enzyme system (CYP450) is responsible for the biotransformation of drugs from active substances to inactive metabolites that can be excreted from the body.  In addition, the metabolism of certain drugs by CYP450 can alter their PK profile and result in sub-therapeutic plasma levels of those drugs over time.  In the area of anti-viral therapy, this can lead to resistance of the virus to the drug. 
The virus causing acquired immunodeficiency syndrome (AIDS) is know by various names, including human immunodeficiency virus (HIV), of which two distinct families have been indentified—HIV-1 and HIV-2.  Many inhibitors of HIV, including HIV protease inhibitors, HIV integrase inhibitors and non-nucleoside reverse transcriptase inhibitors are metabolized by CYP450.  This metabolic activity can lead to unfavorable pharmacokinetics, requiring administering more frequent and/or higher doses than are optimal. 
Many drugs, including some HIV protease inhibitors, are now paired with other agents that improve exposure of the drug, with the drug-drug interaction being commonly referred to as “boosting.”  International Publication Nos.  WO 2006/108879, WO 2007/034312 and WO 2008/010921; U.S. Patent Publication No.  US 2009/0175820; and U.S. Pat.  No.  7,919,488 describe compounds useful as pharmacokinetic enhancers. 
Ritonavir, a common boosting agent, is widely used with HIV agents and is an HIV protease inhibitor itself that exerts its boosting effect through inhibition of Cytochrome P450 3A4 (CYP3A4) and p-glycoprotein drug transporters.  Ritonavir, however, is associated with certain risks, including hepatotoxicity, hyperlipidemia and unfavorable gastrointestinal effects.