POLYMERS CONTAINING ENZYMATICALLY DEGRADABLE BONDS .12. EFFECT OF SPACER STRUCTURE ON THE RATE OF RELEASE OF DAUNOMYCIN AND ADRIAMYCIN FROM POLY[N-(2-HYDROXYPROPYL)-METHACRYLAMIDE] COPOLYMER DRUG CARRIERS INVITRO AND ANTITUMOR-ACTIVITY MEASURED INVIVO

N-(2-Hydroxypropyl) methacrylamide (HPMA) copolymers containing the anticancer agents adriamycin (ADR) and daunomycin (DNM) were synthesised to include different peptidyl spacers, tri- and tetra-peptides, linking drug to the polymeric carrier. The relationship between the length and detailed structure of the oligopeptide spacer and the rate of drug release was studied by incubation of polymer with the plant thiol-proteinase papain (as a model enzyme) or alternatively a mixture of lysosomal enzymes isolated from rat liver (tritosomes). The drug conjugate would be exposed to lysosomal enzymes following pinocytic internalization by cells. Tetrapeptide spacers released both anthracyclines more effectively than tripeptide spacers, Gly-Leu-Phe-Gly being cleaved the most effectively throughout (up to > 90% drug released over 48 h). In addition the rate of drug liberation was measured during incubation of polymer conjugates with human serum, and all the conjugates were almost completely stable (< 5% drug released over 24 h). To determine the relationship between polymer structure and antitumour activity, conjugates containing ADR were tested against the mouse leukemia L1210 both in vitro and in vivo. Cytotoxicity measured in vitro did not correlate with the rate of drug release seen during incubation with papain and tritosomes, the conjugate containing the spacer Gly-Phe-Leu-Gly-ADR being more cytotoxic than the more readily degradable conjugate containing Gly-Leu-Phe-Gly-ADR side-chains. Those conjugates containing tetrapeptide spacers showed greatest ability to prevent appearance of intraperitoneally inoculated L1210 tumours, (when administered intraperitoneally to DBA2 Mice at an ADR dose of 3 x 5 mg/kg on days 1, 2 and 3). However, although the tumours appeared more quickly following administration of those polymeric carriers releasing ADR more slowly, these tumours did not progress rapidly (in fact some subsequently disappeared) and indeed the slow releasing ADR conjugates containing tripeptide spacers were most effective in increasing animal lifespan.