Vol. 4, 3077-3082, December 1998 Clinical Cancer Research 3077 Encapsulation of the Topoisomerase I Inhibitor GL147211C in Pegylated (STEALTH) Liposomes: Pharmacokinetics and Antitumor Activity in HT29 Colon Tumor Xenografts Gail T. Colbern,’ Donald J. Dykes, Charles Engbers, Randy Musterer, Alan Hiller, Erik Pegg, Renee Saville, Steve Weng, Michael Luzzio, Paul Uster, Michael Amantea, and Peter K. Working SEQUUS Pharmaceuticals, Inc., Menlo Park, California 94025 [G. T. C., C. E., R. M., A. H., E. P., R. S., S. W., P. U., M. A., P. K. W.]; Southern Research Institute, Birmingham, Alabama 35246 [D. J. D.]; and Glaxo Wellcome, Inc., Research Triangle Park, North Carolina 27709 [M. L.] ABSTRACT The topoisomerase I inhibitor GL147211C {7-[(4-meth- ylpiperazino)methyl]-10,11-(ethylenedioxy)-(20S)-camptothecin trifluoroacetate), a camptothecin analogue, has significant activity in tumor cell cytotoxicity assays in vitro and antitu- mor activity in both animal tumor models and human pa- tients. Its toxicity is significant, however, effectively limiting the amount of drug that can be administered and its clinical utility. To determine whether the therapeutic index of GL147211C could be improved, the drug was encapsulated in long-circulating, pegylated (STEALTH) liposomes (SPI- 355). The pharmacokinetics and antitumor activity of SPI- 355 were compared to those of nonliposomal GL147211C. The plasma pharmacokinetics of SPI-355 in rats were typi- cal of those of other pegylated liposomal formulations, with significantly increased blood circulation time; the dose- corrected area under the curve and Cmax of SPI-355 (10 mg/kg) were 1250- and 35-fold higher, respectively, than those ofnonliposomal GL14711C (8.72 mg/kg). The compar- ative antitumor activity of SPI-355 and nonliposomal GL1472211C was evaluated in nude mice implanted with HT29 colon carcinoma xenografts. SPI-355 was 20-fold more effective than GL147211C in inhibiting tumor growth (1 mg/kg SPI-355 and 20 mg/kg GL147211C) and produced durable complete remissions of tumors at well-tolerated dose levels that were >5-fold lower than the maximally tolerated dose of GL147211C, which induced no durable complete responses. Signs of toxicity were similar between the two drugs, but liposome encapsulation increased the Received 6/23/98; revised 9/11/98; accepted 9/15/98. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. I To whom requests for reprints should be addressed, at SEQUUS Pharmaceuticals, Inc., 960 Hamilton Court, Menlo Park, CA 94025. Phone: (650) 463-3 1 19: Fax: (650) 617-3080. toxicity of drug -4-fold, with increased weight loss and several deaths with SPI-355 (5 mg/kg SPI-355 versus 20 mg/kg GL147211C). Despite the increased toxicity seen with SPI-355, the therapeutic index of the liposomal formulation was increased -5-fold over that of nonliposomal GL147211C, suggesting that such a pegylated liposomal for- mulation could demonstrate increased therapeutic index in human patients. INTRODUCTION The camptothecin analogue GLl472l lC {7-[(4-meth- ylpiperazino)methyl]- I 0, 1 1 -(ethylenedioxy)-(20S)-campto- thecin trifluoroacetate }, provided by Glaxo Research Institute (Research Triangle Park, NC), is a potent and specific inhibitor of DNA topoisomerase I (1, 2). Topoisomerase I inhibitors stabilize the cleavable complex formed by topoisomerase I and DNA and cause single-stranded DNA breaks (1, 3). This DNA damage, however, is not toxic to the cell until DNA synthesis, when DNA replication forks encounter stabilized cleavable complexes and result in double-stranded DNA breaks (3, 4). Because of this significant cell cycle-dependent antitumor ac- tivity (1, 3), prolonged exposure to effective drug concentrations is required to maximize the fractional tumor cell kill (3, 5). The topoisomerase I inhibitor GL14721 lC has significant activity in both in vitro cytotoxicity tests and in vivo animal tumor models (2, 6). Phase I clinical trials have been published (7-9) using a 5-day infusion per cycle, with a recommended dose of 1.0-1.5 mg/m2/day. Toxicity of the nonliposomal drug (marked weight loss in mice at clinically effective doses and myelosuppression and moderate gastrointestinal toxicity in Phase I clinical trials in humans) is significant, limiting the amount of drug that can be administered and restricting expo- sure of tumor tissue to effective drug concentrations (1, 7-10). STEALTH liposomes are liposomes that contain surface- bound polyethylene glycol chains (1 1-13). Pegylated liposomes exhibit prolonged circulation times by avoiding uptake by the organs of the mononuclear phagocyte system (13-15). Mole- cubes that have been successfully incorporated into pegylated liposomes, such as doxorubicin, have shown marked increase in circulation times and impmvement in antitumor activity (16, 17). This report summarizes the results of a pharmacokinetic study in rats and efficacy studies initiated with xenograft tumors in mice to determine the antitumor activity of GL14721 lC encapsulated in long-circulating, pegylated liposomes (SPI- 355). MATERIALS AND METHODS Test Material GL14721 lC (GG21 1 or Lurtotecan; supplied by Dr. Mi- chael Luzzio at Glaxo Research Institute, Research Triangle Research. on November 29, 2021. © 1998 American Association for Cancer clincancerres.aacrjournals.org Downloaded from