“…For AZT and ddI, which, following their intracellular phosphorylation, act at the RT step (2), addition to the cells could be delayed until -5 hr (n = 5) after infection, and for the TIBO derivatives (R82150, R82913), which do not need intracellular transformations before they can interact with their target enzyme (HIV-1 RT) (4,6), the addition could be delayed by another 2 hr (n = 7). The protease inhibitor Ro31-8959 [compound XVII (14)], which interacts with a late event in the virus cycle (assembly of mature virions) (11)(12)(13)(14)(15)(16) was still effective if added as late as 21 hr after infection (n = 21). From the time-of-addition experiment, it appeared that the bicyclams JM1657 and JM2763 (n = 1 or 2) had to interact with a process following virus adsorption but preceding reverse transcription, which means virus-cell fusion and/or uncoating.…”