l 5 specific amino acid positions. Of 18 patients for whom both phenotypic and genotypic analysis were performed on free virus isolated from plasma, 12 showed reduced susceptibility to ritonavir in vitro. All 18 patients possessed one or more mutations in the viral protease gene. The 82 mutation appeared to be necessary but not sufficient to confer phenotypic resistance. Phenotypic resistance was defined as a ≥5-fold decrease in viral sensitivity invitro from baseline. The clinical relevance of phenotypic and genotypic changes associated with ritonavir therapy has not been established.
Cross-Resistance to Other Antiretrovirals
Among protease inhibitors variable cross-resistance has been recognized. Serial HIVisolates obtained from six patients during ritonavir therapy showed a decrease in ritonavir susceptibility in vitro but did not demonstrate a concordant decrease in susceptibility to saquinavir in vitro when compared to matched baseline isolates. However, isolates from two of these patients demonstrated decreased susceptibility to indinavir in vitro (8-fold). Isolates from 5 patients were also tested for cross-resistance to amprenavir and nelfinavir; isolates from 2 patients had a decrease in susceptibility to nelfinavir (12- to 14-fold), and none to amprenavir. Cross-resistance between ritonavir and reverse transcriptase inhibitors is unlikely because of the different enzyme targets involved. One ZDV-resistant HIV isolate tested in vitro retained full susceptibility to ritonavir.
Pharmacokinetics
The pharmacokinetics of ritonavir have been studied in healthy volunteers and HIV-infected patients (CD4 ≥50 cells/µL). See Table 1 for ritonavir pharmacokinetic characteristics.
Absorption
The absolute bioavailability of ritonavir has not been determined. After a 600 mg dose of oral solution, peak concentrations of ritonavir were achieved approximately 2 hours and 4hours after dosing under fasting and non-fasting (514 KCal; 9% fat, 12% protein, and 79% carbohydrate) conditions, respectively.
Effect of Food on Oral Absorption
When the oral solution was given under non-fasting conditions, peak ritonavir concentrations decreased 23% and the extent of absorption decreased 7% relative to fasting conditions. Dilution of the oral solution, within one hour of administration, with 240 mL of chocolate milk, Advera® or Ensure® did not significantly affect the extent and rate of ritonavir absorption. After a single 600 mg dose under non-fasting conditions, in two separate studies, the soft gelatin capsule (n = 57) and oral solution (n = 18) formulations yielded mean ± SD areas under the plasma concentration-time curve (AUCs) of 121.7 ± 53.8 and 129.0 ± 39.3 µg•h/mL, respectively. Relative to fasting conditions, the extent of absorption of ritonavir from the soft gelatin capsule formulation was 13% higher when administered with a meal (615 KCal; 14.5% fat, 9% protein, and 76% carbohydrate).
Metabolism
Nearly all of the plasma radioactivity after a single oral 600 mg dose of 14C-ritonavir oral solution (n = 5) was attributed to unchanged ritonavir. Five ritonavir metabolites have been identified in human urine and feces. The isopropylthiazole oxidation metabolite (M-2) is the major metabolite and has antiviral activity similar to that of parent drug; however, the concentrations of this metabolite in plasma are low. In vitro studies utilizing human liver microsome
