BILR 355 BS + ritonavir + emtricitabine + tenofovir disoproxil fumarate

This is a **combination antiretroviral regimen** consisting of four agents: - **BILR 355 BS**: a second-generation non-nucleoside reverse transcriptase inhibitor (NNRTI) under development for HIV-1 infection, particularly for strains resistant to currently available NNRTIs. It demonstrates high potency and a favorable resistance profile against both wild-type and NNRTI-resistant strains of HIV-1. - **Ritonavir**: a protease inhibitor mainly used in combination with other antiretrovirals to "boost" their levels through strong inhibition of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein, thereby increasing systemic exposure of coadministered drugs. - **Emtricitabine**: a nucleoside reverse transcriptase inhibitor (NRTI) that inhibits HIV reverse transcriptase by incorporating into viral DNA and causing chain termination. - **Tenofovir disoproxil fumarate**: a prodrug of tenofovir, another NRTI, which similarly inhibits reverse transcriptase and causes DNA chain termination. This combination was studied as an investigational regimen, particularly in patients with HIV-1 infections resistant to earlier NNRTIs. The mechanism of action is multifaceted across different stages of the HIV replication cycle; NNRTI and NRTIs both inhibit reverse transcriptase, while ritonavir is used to increase the concentration and efficacy of BILR 355 BS through pharmacokinetic boosting[1][3][4][5].