Pharmacokinetic and pharmacodynamic study of IST-622, a novel synthetic derivative of chartreusin, by oral administration in a phase II study of patients with breast cancer

The aim of this study was to analyze the pharmacokinetics and pharmacodynamics (PK/PD) of 6-O-(3-ethoxypropionyl)-3',4'-O-exo-benzylidene-chartreusin (IST-622) and its metabolites, and to develop limited sampling models (LSM). Based on the data from 18 patients with breast cancer who were treated orally with 280 or 525 mg/m(2) of IST-622 once daily after breakfast for five consecutive days, we analyzed the relationship between the area under the plasma concentration versus time curve (AUC) and toxicities using a sigmoid E-max model and logistic regression. Plasma concentrations of IST-622 and its metabolites, 3',4'-O-exo-benzylidene-chartreusin (A-132) and 3-demethyl-3',4'-O-exo-benzylidene-chartreusin (A-132M), were measured at 1, 2, 4, 8 and 24 h after administration on day 1. The AUC was calculated using the trapezoidal method. We also developed a LSM using stepwise linear regression analysis. IST-622 was detected in very few patients, and its concentration was very low and could be disregarded. It was suggested that meals promoted absorption of IST-622. AUCs of A-132 plus A-132M showed a better correlation with the rates of decrease and nadir Counts of leukocytes, neutrophils and platelets than the AUC of each metabolite separately. Patients with the sum of AUCs more than 70 mug(.)h/ml showed severe myelotoxicities. Moreover, logistic regression analysis showed that grade 4 myelotoxicities would be seen in 30% of patients at an AUC of 65 mug(.)h/ml. We also developed an unbiased and precise LSM: AUC(0-24h) = C-8h x 17.6 - 0.95, where C-8h denotes the sum of plasma concentrations of A-132 and A-132M. Myelotoxicities showed a good correlation with AUC(0-24h), and based on the results, it was decided that the target AUC was 65 mug(.)h/ml. The LSM was very convenient for estimating AUC(0-24h) and sufficiently accurate. These results show the possibility of predicting toxicities and dose adaptation for interpatient variability using LSM.