Oral idarubicin pharmacokinetics--correlation of trough level with idarubicin area under curve

Abstract

Idarubicin is the first anthracycline that can be successfully administered via the oral route and thus may facilitate antineoplastic chemotherapy in an improved quality of life. These perspectives are somewhat hampered by the large variation in bioavailability between individual patients and the obvious requirement to monitor plasma concentration and area-under-the-curve values (AUC) for an appropriate adjustment of idarubicin dose. In this study we describe the pharmacokinetics of idarubicin and its main metabolite idarubicinol in 12 patients after oral application of 20 mg/m2 idarubicin on 3 consecutive days and demonstrate that the 24 h trough levels show a high correlation with AUC and may thus allow a rapid and easy determination of individual drug concentrations and an appropriate dose adjustment. The average terminal half-life was 30.5 h for idarubicin and 66.9 h for idarubicinol. The AUC for idarubicin and its main metabolite idarubicinol revealed a substantial interpatient variation with AUC values ranging from 25.7 to 114 ng x h/ml (average 58.1 ng x h/ml) for idarubicin and from 109.4-445.2 ng x h/ml (average 287.3 ng x h/ml) for idarubicinol. However, the ratio of idarubicin/idarubicinol differed only two folds from 1:3.7 to 1:7.7 with an average of 1:5.1. Both idarubicin and idarubicinol concentrations were highly reproducible, however, upon measurements after repeated applications within individual patients. Moreover, idarubicinol and idarubicin AUCs showed a good correlation with r = 0.78, indicating that the interindividual variation of idarubicin AUC reflects differences in absorption rather than in metabolism. In order to describe the interindividual bioavailability of idarubicin - represented by the respective AUC - measurement of a single data point with a high correlation with the AUC would be ideal. Our study demonstrates that the 24 h trough level shows such an excellent correlation (r = 0.96) with AUC, making it the perfect candidate for fast estimates of the individual bioavailability in a given patient. On this basis, the longitudinal measurement of the 24 h trough level may allow assessment of the impact of interindividual variations in AUC on clinical outcome and toxicity.