The active ingredient of Viread is tenofovir disoproxil fumarate, which belongs in the nucleoside reverse transcriptase inhibitor (NRTI) class of antiretroviral drugs. Tenofovir disoproxil fumarate in cells gets converted into tenofovir diphosphate, an analogue of deoxyadenosine 5’-triphosphate, one of the four nucleotides from which DNA assembles. When viral RNA is reverse-transcribed into DNA in the cell, tenofovir diphosphate gets incorporated in the place of deoxyadenosine 5’-triphosphate into the DNA that is being synthesised. When this happens, DNA synthesis is halted, as the next nucleotide can not form a bond with tenofovir diphosphate (this DNA synthesis halt is called chain termination).
Tenofovir disoproxil fumarate was shown to decrease the levels of HIV RNA in the body, and to increase the levels of CD4+ cells (cells of our immune system and one of the main targets of HIV – their levels normally decrease after an HIV infection).