Polymer conjugates of the highly potent cytostatic drug 2-pyrrolinodoxorubicin

This paper describes the synthesis and biological evaluation of a conjugate of the highly cytotoxic drug 2-pyrrolinodoxorubicin (p-DOX) with an N-(2-hydroxypropyl)methacrylamide copolymer (PHPMA) as a water-soluble biocompatible polymer carrier, utilizing the advantageous concept of polymer–drug conjugates. The conjugate of p-DOX with HPMA copolymer (PHPMA/p-DOX) was prepared by reacting the PHPMA/DOX conjugate, where the DOX was bound via a hydrazone bond, with 4-iodobutyraldehyde. The hydrazone bond between the polymer and drug is susceptible to pH-controlled hydrolysis, enabling prolonged stability in circulation and fast p-DOX release under conditions mimicking the intracellular environment. The in vitro cytostatic activity of free p-DOX was in accordance with literature, whereas its PHPMA conjugate exhibited a 1.3- to 5-fold lower cytotoxicity, depending on the cancer cell line, when compared to the free p-DOX. This is in qualitative agreement with the data obtained for DOX and its HPMA copolymer conjugates. On mice bearing T-cell EL4 lymphoma, no tumor suppression was observed from the free p-DOX at a subtoxic dose of 0.1 mg/kg, whereas the PHPMA/p-DOX conjugate significantly inhibited the initial tumor growth at approximately equitoxic doses of 0.4 and 0.8 mg p-DOX eq/kg. However, moderately elevated doses of the p-DOX equivalent in the conjugate caused toxic effects, making accurate dosage setting essential.