Modeling toxicity and response in carboplatin-based combination chemotherapy

Data from women with advanced ovarian cancer (International Federation of Gynecology and Obstetrics stage III or IV) were analyzed to evaluate the pharmacokinetic/pharmacodynamic relationships of carboplatin-based combination chemotherapy. With the equation area under the plasma concentration versus time curve (AUC) = dose/(creatinine clearance + 25), carboplatin AUC was calculated in each of up to six treatment cycles in 224 women with advanced ovarian cancer who had been randomized to receive carboplatin 300 mg/m2 plus cyclophosphamide 600 mg/m2. In addition, for each patient, the predicted nadir count (obtained by rearranging the University of Maryland single-agent carboplatin dosing formula) was compared with the actual observed nadir count, received and relative received dose intensities were calculated, and carboplatin exposure intensity was defined. Relationships were sought between these treatment indices and the clinical outcomes of time to progression and survival. When combined with cyclophosphamide 600 mg/m2, any carboplatin AUC was found to be associated with greater myelotoxicity and a higher likelihood of both leukopenia and thrombocytopenia occurring than had been determined for single-agent carboplatin. Furthermore, the platelet nadir in 83% of patients was equal to or below that predicted to result from the same dose of single-agent carboplatin. There was a relatively narrow range of received dose intensities within this patient population, but carboplatin exposure intensity was calculated as being distributed over a two-fold range within the population. Therefore, received carboplatin dose intensity underestimates the range of plasma drug exposure associated with a fixed dosing regimen of carboplatin. However, there were no consistent relationships between received dose intensity, relative received dose intensity, or carboplatin exposure intensity and the clinical outcomes of time to progression or survival. The relationships between carboplatin exposure and the pharmacodynamic measures of toxicity and response are likely to require definition in each regimen that includes carboplatin and for each tumor type treated.