Which drug is known as a protease inhibitor used in HIV treatment?

Lopinavir is indeed a protease inhibitor used in the treatment of HIV, often in combination with ritonavir to enhance its effectiveness. Ritonavir, as the answer hints, is also a protease inhibitor that plays a critical role in HIV therapy. In fact, ritonavir was one of the first protease inhibitors approved for the treatment of HIV and is used primarily for its ability to boost the plasma concentrations of other protease inhibitors when taken together.

Enfuvirtide is not a protease inhibitor; it is a fusion inhibitor that works differently by preventing the HIV virus from entering human cells. Abacavir, on the other hand, is a nucleoside reverse transcriptase inhibitor (NRTI) and functions by inhibiting reverse transcriptase, an enzyme crucial for viral replication.

Thus, ritonavir stands out as a cornerstone in antiretroviral therapy due to its ability to inhibit the HIV protease enzyme, thereby preventing the maturation of viral particles and lowering viral load in patients.