A drug's persistence in plasma or in cells depends on its half life. Long half-life antiretroviral agents may allow for missed or delayed doses, if concentrations are maintained at therapeutic levels until the next dose is taken. NRTI are pro-drugs and must be activated by different phosphorilation steps to be effective against HIV. Intracellular triphosphate (TP) metabolites have been shown to be characterized by long half lives. However, data on drug persistence are limited and whether drug doses can be omitted and dosing delayed is unknown.