Figure 2. Effect of P450 3A4 activity on alfentanil disposition. (Left) Alfentanil plasma concentrations in a typical participant. Disposition of alfentanil (20 micro gram/kg given intravenously) is shown for the control (no treatment; triangles), rifampin treatment (squares), and troleandomycin treatment (circles). Symbols depict measured concentrations, and lines represent predicted concentrations using microconstant parameters derived from a three-compartment model. (Right) Average (+/- SE) alfentanil plasma concentrations for all nine men. Table 2shows the average pharmacokinetic parameters for all nine men.