The relationship between ritonavir plasma levels and side-effects: implications for therapeutic drug monitoring

Objectives: To assess whether the neurological or gastrointestinal adverse effects of ritonavir correlate with parameters of ritonavir systemic exposu re. Methods: Peak (C-max) and trough (C-min) ritonavir plasma levels were compa red in 11 patients experiencing side-effects (group A) versus 10 patients w ithout side-effects (group B). Ritonavir was administered with the followin g escalation dosing scheme: 300, 400, 500 mg twice a day for 3, 4, and 5 da ys, respectively, then the full dose of 600 mg twice a day. Blood sampling was done in group A within 24 h of the occurrence of side-effects and in gr oup B after at least 3 days of the full dosage regimen. Results: Both C-max and C-min were significantly higher (Mann-Whitney U tes t) in patients with side-effects. C-max was [median (interquartile range)] 26.7 (22.7-33.3) mg/l Versus 16.2 (13.4-17.0) mg/l (P = 0.001) and C-min wa s 12.6 (9.1-13.9) versus 7.5 (4.9-8.6) mg/l (P = 0.002). Conclusion: Patients with higher ritonavir concentrations are at a higher r isk of experiencing neurological or gastrointestinal side-effects. Individu alization of the dosage regimen, e.g. a downward titration of the ritonavir dose in patients with side-effects, guided by plasma level monitoring, may result in a substantial increase in the percentage of patients tolerating ritonavir without increasing the risk of treatment failure as a result of s uboptimal systemic exposure. (C) 1999 Lippincott Williams & Wilkins.