Can the Inhibitory Quotient (IQ) be Useful in Clinical Pratice?

Two recent reports support the predictive value ofthe inhibitory quotient (IQ) for virologic response toantiretroviral therapy (Casado et al. AIDS 2003;17:262-4; Shulman et al. Antimicrob Agents Chemother2002;46:3907-16). IQ is a measure that integratesboth resistance and drug levels. It is usually expressedas the ratio of Ctrough/IC50, where IC50can be derived from real phenotype or a phenotypicinterpretation of the genotype (when a virtual phenotypeis used, then often is referred as virtual or vIQ).Casado et al., reported results using dual PI combinations(with NRTI) in heavily pretreated patients.The virologic response at 3 months was significantlybetter among patients with an IQ greater than 1 fornelfinavir and saquinavir (27 patients on dual nelfinavir/saquinavir therapy). The results were similar forindinavir and ritonavir (25 patients on this dual PItherapy, pooling 400/400 mg dosing and 800/100 mgdosing), but those results were only borderline significant.In some way it is surprising that such a low IQ(PI trough concentrations were higher than thoseneeded to inhibit only 50% of the virus replication)were still sufficient to provide a significant viral loaddrop. In many cases, achievement of higher IQvalues may be unrealistic in such a population withpre-existing resistance to PIs.