Water-soluble poly-(L-glutamic acid)-Gly-camptothecin conjugates enhance camptothecin stability and efficacy in vivo.

The therapeutic efficacy of 20(s)-camptothecin (CPT) is limited in humans by the instability of the active lactone form due to preferential binding of the carboxylate to serum albumin and by difficulty in formulation. Formation of an ester bond with an amino acid via the hydroxyl group at carbon 20 of CPT stabilizes the lactone. Linking CPT to a high molecular weight (MW) anionic polymer enhances solubility and improves distribution to the tumor through enhanced permeability and retention (EPR effect). Poly-(L-glutamic acid) (PG) is an anionic homo-polymer that can theoretically bind one molecule of a drug via the gamma carboxylic acid of each monomeric subunit. It has been used to make a water-soluble PG-paclitaxel conjugate currently in Phase II clinical trials that contains 37% paclitaxel by weight and is administered in a 10 min infusion without pre-medication. We evaluated the anti-tumor activity of PG conjugates of CPT after a single intraperitoneal injection using subcutaneous murine B-16 melanoma tumor growth as an indicator. Interposition of a glycine (gly) linker allowed CPT loading up to 50% w/w on the polymer. Increasing the PG MW from 33 to 49 kDa enhanced the efficacy without altering the maximum tolerated dose (MTD). In athymic mice bearing ectopic human colon or lung tumors, efficacy was enhanced compared to free camptothecin. Thus, as with paclitaxel, conjugation of CPT to PG enhanced pharmaceutical properties and preclinical efficacy.

Conjugation of camptothecins to poly-(L-glutamic acid).

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.

Highly efficient and versatile synthesis of libraries of constrained beta-strand mimetics.

The general approach of using a bicyclic template to produce inhibitors of the protease superfamily of enzymes has been investigated. The Diels Alder cycloaddition reaction on solid support has been found to be highly efficient for the synthesis of libraries of compounds that mimic the beta-strand secondary structure of proteins. Several potent and selective inhibitors of proteases have been discovered.