Satchi, Ronit; (1999) PDEPT: Polymer directed enzyme prodrug therapy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Polymer Directed Enzyme Prodrug Therapy (PDEPT) was proposed as a novel two-step antitumour approach. Combination of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer prodrug and an HPMA copolymer-enzyme conjugate was suggested as a means of generating a potent cytotoxic agent rapidly and selectively at the tumour site. To test the feasibility of PDEPT, two HPMA copolymer-enzyme conjugates were synthesised (containing (β-lactamase and cathepsin B as model enzymes) and their in vitro and in vivo properties were evaluated. HPMA copolymer-Gly-Phe-Leu-Gly-doxorubicin (PKl) was used as a substrate for HPMA copolymer-cathepsin B to test the PDEPT combination in vitro and in vivo. Following polymer conjugation (yield of 30-35%) both enzymes retained 20- 25% of their enzymatic activity. To investigate their pharmacokinetics in vivo, 125I-labelled HPMA copolymer-enzyme conjugates were administered intravenously (i.v.) to B16F10 tumour-bearing mice. Due to selective tumour tissue accumulation by the enhanced permeability and retention (EPR) effect, the HPMA copolymer-enzyme conjugates showed a 2-3 fold increase in tumour accumulation compared to the native enzyme, and they also exhibited a longer plasma half-life. The ability of HPMA copolymer-cathepsin B to access and degrade the prodrug in vivo was determined by HPLC evaluation of doxombicin release. PKl was injected i.v. to B16F10-bearing mice and after 5 h HPMA copolymer-cathepsin B was administered. This enzyme led to a rapid increase in the rate of doxorubicin release intratumourally (3.6-fold faster than seen for PKl alone). Moreover, when the antitumour activity (doxorubicin-equivalent dose of 10 mg/Kg) was measured using this tumour model, the combination (PDEPT) had the highest activity (T/C = 168%) compared to that seen for PKl alone (T/C = 152%) or free doxorubicin (T/C = 144%). No animal weight loss or other toxicity was observed indicating the possibility of dose escalation. As conjugation of proteins to HPMA copolymers is known to reduce their immunogenicity, and antitumour activity has been demonstrated, further development of PDEPT is warranted.

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