Figure 8. Effect of P450 3A4 activity on the time required for a 50% decrease in alfentanil venous plasma concentration after targeted infusions of variable durations. These context-sensitive half-times were determined according to the method of Hughes et al. [47] using the pharmacokinetic parameters in Table 2. For low P450 3A4 activity, the average k10(t) was used. Use of the fully (71%) inhibited k10(t) would result in even longer half-times. Half-times for fentanyl and remifentanil in normal participants were calculated using reported kinetic parameters. [50,51]