HPMA Copolymer Conjugates of Paclitaxel and Docetaxel with pH-Controlled Drug Release

In this paper we describe the synthesis, physicochemical characteristics and data on the biological activity of polymer prodrugs based on the anticancer drugs paclitaxel (PTX) and docetaxel (DTX) conjugated with a water-soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer drug carrier. The drugs were derivatized and then attached to the polymer backbone via a spacer that is stable under physiological conditions (pH 7.4) and hydrolytically degradable in mild acidic environments (e.g., endosomes, pH ∼ 5). Polymer−drug conjugates were designed to achieve prolonged blood circulation and release of the active compound in target cells. Six types of conjugates differing in the structure of the keto acid (levulic, 3-(acetyl)acrylic acid) and 4-(2-oxopropyl)benzoic acid-containing spacer or in the amount of drug bound to the HPMA copolymer were synthesized. In all the conjugates, the linkage susceptible to hydrolytic cleavage was formed by the reaction of the carbonyl group of a drug derivative with the hydrazide group-terminated side chains of the polymer. In vitro incubation of the conjugates in buffers resulted in much faster release of drugs or their derivatives from the polymer at pH 5 than at pH 7.4 with the rate depending on the detailed structure of the spacer. Conjugates containing drugs acylated with levulic acid were tested for their anticancer activity in vivo using two murine models. The PTX-containing conjugate showed better antitumor efficacy in the 4T1 model of mammary carcinoma than the parent drug and its derivative. The DTX-containing conjugate demonstrated high activity in treating EL4 T cell lymphoma. The treatment with the polymer conjugates was devoid of side toxicity. In both models, we achieved complete regression of established tumors accompanied by a durable tumor resistance in most of the cured animals.