Conjugation of Camptothecins to Poly‐(l‐Glutamic Acid)

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Abstract
Conjugation of water-insoluble cancer chemotherapeutic drugs to macromolecular polymers can lead to improved pharmaceutical properties and improved therapeutic ratios due to accumulation of the polymer-drug conjugate in tumor tissue through the enhanced permeability and retention (EPR) to macromolecules associated with tumor vasculature. Pharmaceutical shortcomings of certain active camptothecins including difficulty in formulation and instability of the active lactone form due to interactions with human albumin might be improved by conjugation to polymers. In this report, conjugations of camptothecin (CPT), 10-hydroxy-CPT, and 9-amino-CPT to poly-(l-glutamic acid) (PG) are described; coupling was accomplished either through the 20(S)-hydroxyl or 9 and 10 substituents with and without the use of a glycine linker. Studies using a PG paclitaxel conjugate (PG-TXL), which is currently in Phase I testing, demonstrated that PG enhanced aqueous solubility, prolonged plasma residence time, and greatly increased the distribution of paclitaxel to tumor tissue in a murine model. In this report, we describe the use of similar conjugation technology for CPT derivatives and demonstrate that these difficult to formulate compounds can be rendered water soluble, that their maximum tolerated doses are increased, and that they retain substantial anti-tumor activity in syngeneic and xenogeneic tumor models. Preliminary data suggest that PG with molecular weights between 37 and 50 kDa with CPT loading between 14% and 37% with or without glycine linkers display enhanced efficacy compared with nonconjugated camptothecins administered at their maximum tolerated dose.

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