Fostemsavir is used in the treatment of HIV-1 infection, particularly in heavily treatment-experienced adult patients who have developed resistance to multiple antiretroviral therapies. It functions as a prodrug that is converted in the body to temsavir, which directly binds to the gp120 subunit of the HIV-1 envelope glycoprotein. This binding prevents the virus from attaching to host CD4+ T-cells, a critical first step in viral entry. By blocking viral attachment, fostemsavir helps reduce viral load and restore immune function in patients with limited treatment options. It is typically used in combination with other antiretroviral agents to enhance its effectiveness and reduce the risk of further resistance development.