Carrier and dose effects on the pharmacokinetics of T-0128, a camptothecin analogue-carboxymethyl dextran conjugate, in non-tumor- and tumor-bearing rats.

T-0128 is a novel camptothecin (CPT) analogue (T-2513: 7-ethyl-10-aminopropyloxy-CPT)-carboxymethyl (CM) dextran conjugate via a Gly-Gly-Gly linker, with a molecular weight (MW) of 130 kDa. Our previous studies demonstrated that T-0128 has strong antitumor activity against human tumor xenografts that are highly refractory to CPT analogues attributable to the passive tumor targeting of released T-2513. This study examines the effects of carrier, dose, and tumor on T-0128 pharmacokinetics. To study carrier effect, tumor-bearing rats received one i.v. injection of fluorescein isothiocyanate (FITC)-labeled CM dextran with a different degree of substitution (DS) of the carboxymethylated groups and a different MW. Results showed that CM dextran from Dextran T-110 (MW 110 kDa) with a DS value of 0.4 is an appropriate drug carrier for T-0128 regarding plasma half-life and passive tumor targeting. To study dose and tumor effects, non-tumor- and tumor-bearing rats were treated with T-0128 doses ranging from 1 to 25 mg/kg (based on the amount of T-2513 bound to CM dextran). Dose-dependent pharmacokinetics of T-0128 were observed in both kinds of rats. The presence of tumor reduced the plasma half-life and systemic exposure of T-0128. The saturation of hepatic and splenic tissue uptake clearances (CLups), and a large contribution of the tumor CLup to the total body clearance explain these results. Overall, our data provide a rationale for the selection of the carrier for T-0128 and a need for pharmacokinetic studies to evaluate the influences of tumor on the drug disposition.