Measurement of IC50, m, and IIP. (a,d) Log-log dose-response curves for zinovudine (AZT) and efavirenz (EFV). 50% human serum and 10% fetal calf serum were included in the culture medium to account for protein binding of the drugs. Each point represents the mean±s.d. from more than three experiments. The clinical concentration range for each drug is shaded. Because of the short plasma half-life of the AZT prodrug, the range between Cave and Cmax is shown. Clinical concentrations of AZT produce inhibition that is readily measured by collecting ~50,000 events (live cells). In contrast, clinical concentrations of EFV produce inhibition that can only be measured with assays having a dynamic range of >5 logs. (b,e) Linearized dose-response curves for AZT and EFV based on median effect model. IIP at Cmin, Cave or Cmax was calculated using equations (3)–(5). For EFV, this is graphically equivalent to extrapolation of the median effect plot to higher concentrations (dashed line). To verify values obtained by extrapolation, large scale infections were carried out (open symbol, see below). (c,f) Verifying the IIP of EFV in large scale infections (~2 × 106 events). In cultures with 10 μM AZT, infection events (GFP+ cells) were readily detectable. In cultures with 10μM EFV, less than three GFP+ cells were detected. This represents >5 logs of inhibition, consistent with the values predicted by extrapolation.