The population pharmacokinetics of ribavirin were assessed in patients with chronic hepatitis C virus (HCV) infection treated with interferon alpha-2b and ribavirin in four clinical efficacy studies. The authors collected 3450 ribavirin serum concentrations from 1105 patients at different treatment weeks for inclusion in the analysis. Population factors included gender, age, body weight, serum creatinine, creatinine clearance, and previous interferon treatment history. Ribavirin apparent clearance (CL/F) was calculated from individual patients' daily doses divided by concentration values, and the influence of these factors was assessed by multiple regression. Body weight, gender, age, and serum creatinine affected CL/F. Population mean CL/F estimates were 17.9 L/h (female) and 21.5 L/h (male) assuming an age of 40 years and body weight of 70 kg. Ribavirin apparent clearance increased as a function of body weight and decreased at ages greater than 40 years. Serum creatinine had little influence on CL/F, which may reflect the relatively normal renal function of these patients. Total CL/F variability was approximately 28%. The four covariates in the model explained 27% of this variability, and were thus of limited clinical significance because of the substantial residual variability not accounted for by the model. In assessing the relationship between pharmacokinetics and pharmacodynamics, the week 4 hemoglobin nadir value was negatively associated with week 4 ribavirin concentrations. The percentage of reduction from baseline was positively associated with ribavirin concentrations, although these data were highly variable. Loss of HCV-RNA at 24 weeks after completion of treatment was considered a response to interferon and ribavirin treatment in a logistic regression analysis of clinical and pharmacokinetic variables and treatment response in the interferon-naive patients. Hepatitis C virus genotype, pretreatment HCV-RNA titer, duration of treatment period, week 4 ribavirin concentration, and patient age affected the likelihood of response. Higher ribavirin concentrations at treatment week 4 were associated with a higher response rate. Variables that have predictive value for treatment outcome in patients treated with interferon and ribavirin are similar to those previously reported for interferon monotherapy.

The performance of the p53-/- transgenic (knockout) mouse model was evaluated through review of the data from 31 short-term carcinogenicity studies with 21 compounds tested as part of the International Life Sciences Institute's (ILSI) Alternatives to Carcinogenicity Testing (ACT) project, together with data from other studies which used comparable protocols. As expected based on the hypothesis for the model, a significant number (12/16 or 75%) of the genotoxic human and/or rodent carcinogens tested were positive and the positive control, p-cresidine, gave reproducible responses across laboratories (18/19 studies positive in bladder). An immunosuppressive human carcinogen, cyclosporin A, was positive for lymphomas but produced a similar response in wild type mice. Two hormones that are human tumorigens, diethylstilbestrol and 17beta-estradiol, gave positive and equivocal results, respectively, in the pituitary with p53-deficient mice showing a greater incidence of proliferative lesions than wild type. None of the 22 nongenotoxic rodent carcinogens that have been tested produced a positive response but 2 compounds in this category, chloroform and diethylhexylphthalate, were judged equivocal based on effects in liver and kidney respectively. Four genotoxic noncarcinogens and 6 nongenotoxic, noncarcinogens were also negative. In total (excluding compounds with equivocal results), 42 of 48 compounds or 88% gave results that were concordant with expectations. The technical lessons learned from the ILSI ACT-sponsored testing in the p53+/- model are discussed.

Clinical investigation of combination therapy using SCH66336 and taxanes in cancer patients is warranted. Further, SCH66336 may be useful for sensitizing paclitaxel-resistant tumors to taxane treatment.

This simulation suggests that ribavirin dosage (in combination with pegylated interferon alfa-2b) for patients with chronic hepatitis C should be based on body weight.

This analysis confirms earlier observations of progressive pharmacokinetic changes in the patients with hepatitis C during 48 weeks of treatment. The absence of a relationship between toxicity or efficacy variables and interferon concentration or activity (within a dose level) suggests that clinical management of patients (eg, for efficacy or to manage toxicity) should be based on clinically derived dosing guidelines rather than on serum concentration or activity criteria.