PDEPT: polymer-directed enzyme prodrug therapy. I. HPMA copolymer-cathepsin B and PK1 as a model combination

Br J Cancer. 2001 Sep 28;85(7):1070-6. doi: 10.1054/bjoc.2001.2026.


Polymer-directed enzyme prodrug therapy (PDEPT) is a novel two-step antitumour approach using a combination of a polymeric prodrug and polymer-enzyme conjugate to generate cytotoxic drug selectively at the tumour site. In this study the polymeric prodrug N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer-Gly-Phe-Leu-Gly-doxorubicin conjugate PK1 (currently under Phase II clinical evaluation) was selected as the model prodrug, and HPMA copolymer-cathepsin B as a model for the activating enzyme conjugate. Following polymer conjugation (yield of 30-35%) HPMA copolymer-cathepsin B retained approximately 20-25% enzymatic activity in vitro. To investigate pharmacokinetics in vivo,(125)I-labelled HPMA copolymer-cathepsin B was administered intravenously (i.v.) to B16F10 tumour-bearing mice. HPMA copolymer-cathespin B exhibited a longer plasma half-life (free cathepsin B t(1/2alpha)= 2.8 h; bound cathepsin B t(1/2alpha)= 3.2 h) and a 4.2-fold increase in tumour accumulation compared to the free enzyme. When PK1 (10 mg kg(-1)dox-equiv.) was injected i.v. into C57 mice bearing subcutaneously (s.c.) palpable B16F10 tumours followed after 5 h by HPMA copolymer-cathepsin B there was a rapid increase in the rate of dox release within the tumour (3.6-fold increase in the AUC compared to that seen for PK1 alone). When PK1 and the PDEPT combination were used to treat established B16F10 melanoma tumour (single dose; 10 mg kg(-1)dox-equiv.), the antitumour activity (T/C%) seen for the combination PDEPT was 168% compared to 152% seen for PK1 alone, and 144% for free dox. Also, the PDEPT combination showed activity against a COR-L23 xenograft whereas PK1 did not. PDEPT has certain advantages compared to ADEPT and GDEPT. The relatively short plasma residence time of the polymeric prodrug allows subsequent administration of polymer-enzyme without fear of prodrug activation in the circulation and polymer-enzyme conjugates have reduced immunogenicity. This study proves the concept of PDEPT and further optimisation is warranted.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Area Under Curve
  • Cathepsin B / chemistry
  • Cathepsin B / pharmacokinetics
  • Cathepsin B / pharmacology*
  • Combined Modality Therapy
  • Enzyme Activation
  • Half-Life
  • Infusions, Intravenous
  • Male
  • Melanoma / drug therapy
  • Melanoma / veterinary
  • Mice
  • Mice, Inbred C57BL
  • Neoplasms, Experimental
  • Polymethacrylic Acids / chemistry
  • Polymethacrylic Acids / pharmacokinetics
  • Polymethacrylic Acids / pharmacology*
  • Prodrugs*
  • Skin Neoplasms / drug therapy
  • Skin Neoplasms / veterinary


  • Polymethacrylic Acids
  • Prodrugs
  • Duxon
  • Cathepsin B