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Pharmacokinetic/Pharmacodynamic Modeling and Simulation of Neutropenia during Phase I Development of Liposome-Entrapped Paclitaxel

Authors' Affiliations: ¹ Cognigen Corporation; ² Roswell Park Cancer Institute, Buffalo, New York; ³ NeoPharm, Inc., Lake Bluff, Illinois; ⁴ Fox Chase Cancer Center, Philadelphia, Pennsylvania; ⁵ H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida; and ⁶ The Cancer Institute of New Jersey, New Brunswick, New Jersey

Requests for reprints: Gerald J. Fetterly, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263. Phone: 716-845-4889; Fax: 716-845-1659; E-mail: gerald.fetterly{at}roswellpark.org.

Purpose: To evaluate the maximum tolerated dose (MTD), dose-limiting toxicities (DLT), and pharmacokinetics of liposome-entrapped paclitaxel easy-to-use (LEP-ETU) and to characterize the relationship between LEP-ETU concentrations and the time course of neutropenia in cancer patients.

Experimental Design: LEP-ETU was administered to 88 patients and 63 were evaluable for pharmacokinetic/pharmacodynamic (PK/PD) analysis following 1.5- and 3-h infusions every 3 weeks (q3w; dose range, 135-375 mg/m²). MTD was identified using a 3 + 3, up-and-down dose-finding algorithm. PK/PD modeling was done to describe the temporal relationship between paclitaxel concentrations and neutrophil count. Simulations assessed the influence of dose and schedule on neutropenia severity to help guide dose selection.

Results: The MTD of LEP-ETU was identified as 325 mg/m². DLTs occurring at 375 mg/m² consisted of febrile neutropenia and neuropathy. The C_max and area under the plasma concentration-time curve of LEP-ETU were less than proportional with increasing dose. The PK/PD model showed that LEP-ETU inhibition of neutrophil proliferation was 9.1% per 10 µg/mL of total paclitaxel concentration. The incidence of grade 4 neutropenia increased from 33% to 42% across the dose range of 275 to 325 mg/m² q3w. For a dose of 110 mg/m² given weekly, grade 4 neutropenia was estimated to be 16% compared with 42% for the same total dose administered q3w.

Conclusions: LEP-ETU can be administered safely at higher doses than Taxol. Modeling and simulation studies predict that 325 mg/m² LEP-ETU q3w provides acceptable neutropenic events relative to those observed at 175 mg/m² Taxol q3w. A 275 mg/m² dose may offer an improved therapeutic index.