PRECLINICAL EVALUATION OF POLYMER-BOUND DOXORUBICIN

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymers (Mw approximately 20 000) were synthesised to contain doxorubicin (DOX) (approximately 7 wt%) attached to the polymer backbone via biodegradable Gly-Phe-Leu-Gly peptidyl spacers (polymer 1) and optionally in addition (polymer 2) the carbohydrate moiety galactosamine (approximately 4 mol%) to facilitate liver targeting. Both polymeric DOX conjugates were degraded by isolated rat lysosomal enzymes in vitro and within rat liver following intravenous (i.v.) administration, to liberate free DOX in a time-dependent fashion. Total release of DOX occurred over 24-48 h. When administered either intraperitoneally (i.p.) at doses 2.5-50 mg DOX/kg (triplicate doses) or i.v. (single dose, 13-100 mg DOX/kg) to mice bearing L1210 leukaemia inoculated i.p., both polymers 1 and 2 showed good antitumour activity. The highest T/C seen for conjugates after i.p. treatment was > 762 compared with a T/C of 214 seen for free Dox. At 30 mg/kg given i.p. polymer 2 produced a large number of long-term survivors 7/20. A variety of dosing schedules were used to test the activity of polymer-DOX against a number of solid tumour models; M5076, P388, B16 melanoma, Walker sarcoma and the xenograft LS174T. In each case treatment with polymer-bound DOX produced an increase in survival time greater than seen with free DOX and experiments with P388 and Walker sarcoma showed remarkable tumour regression (in some cases tumours completely disappeared). B16 melanoma and M5076 were used as a liver metastatic model in particular to study the antitumour activity of the liver-targeted polymer 2. Although polymer 2 did decrease tumour burden attributable to liver metastases it was not significantly better than polymer 1 or free DOX. In these studies HPMA copolymer-DOX conjugates expressed a wide range of antitumour activities, but their overall mechanism of action is still unclear. Certainly these macromolecular produgs are subject to intracellular lysosomal activation, they have the ability to concentrate drug in solid tumours, and with incorporation of targeting residues can promote organ-specific or tumour-specific uptake. In addition covalent conjugation markedly reduces all aspects of DOX-associated toxicity.