EFFECT OF PREIMMUNIZATION ON THE ACTIVITY OF POLYMER-DOXORUBICIN AGAINST MURINE L1210 LEUKEMIA

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymers covalently linked to doxorubicin (DOX) via degradable peptidyl spacers display considerable antitumour activity in vivo against leukaemic, and in particular, solid tumour models. The observation that repeated administration of such conjugates produces only a poor antibody response (mainly IgM) suggests that such conjugates will be useful clinically for repeated administration. In this study we have investigated the effect of pre-immunisation on the anti-tumour activity (against an i.p. L1210 tumour model) of two HPMA copolymer-DOX conjugates. Both conjugates contain DOX bound to the polymer via a Gly-Phe-Leu-Gly spacer which is degradable by lysosomal enzymes. The second contains additionally galactose residues to facilitate targeting to the liver for treatment of primary and secondary liver disease. Animals (both pre-immunized and non pre-treated) bearing i.p. L1210 were treated (i.p.) with free or polymer-bound DOX (single dose 10 mg/kg DOX equivalent) and their blood cell count, anti-conjugate antibody titer and survival were monitored. Both conjugates produced a low primary immune response (IgM titers ranged from 1/160-1/640). In L1210-bearing animals the mean survival of untreated animals was 14-18 days, animals treated with DOX (10 mg/kg) suffered acute toxicity, but animals treated with polymer-DOX showed no evidence of toxicity and there was an increase in mean survival (T/C 125-170). In certain cases long term survivors resulted. Pre-immunisation did not alter the anti-tumour response observed after treatment with either conjugate. Administration of polymer-DOX produced a transient change in the red blood-cell count. Also, there was an increase (up to 8-fold) in the white cell count in non-surviving animals attributable to the presence of L1210 cells in the circulation.