Quarcoo, Theresa Naashika; (2003) Investigations of the mechanisms of antitumour activity of PK1, the first macromolecular prodrug to reach clinical trials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Twenty seven years ago the use of water soluble synthetic polymers as targetable carriers to provide better and more effective treatment for cancer was proposed. To date, the most exploited carrier, HPMA [N-(2-Hydroxypropyl)methacrylamide], has been successfully used to specifically target tumours by taking advantage of the increased vascular permeability and the lack of effective tumour lymphatic drainage. In 1999, a phase 1 clinical trial result of the first drug-polymer conjugate, PK1, was published. PK1 comprises of doxorubicin covalently bound to HPMA via a tetrapeptide spacer, which is susceptible to cleavage by cathepsin B, a lysosomal protease found to be upregulated in most human cancers. Given the progression of PK1 into phase II testing, its mode of action and toxicological profile are poorly understood. This study set out to investigate the mechanism of antitumour activity in vitro and in vivo. PK1 was found to be toxic against a panel of tumour cell lines with varying levels of cathepsin B activity. However, cytotoxicity could always be attributed to the 0.02% free doxorubicin present in the polymer preparation. Following confirmation of PK1 internalisation by all cell lines with fluorescence microscopy, the ability of PK1 to induce DNA damage at the single cell level with the Comet assay was investigated. Induction of DNA damage was observed in the B16F10 murine melanoma model, but not in vitro. Furthermore, DNA damage to bone marrow cells paralleled that of PK1 and the parent drug, doxorubicin observed in tumour cells. In light of our results, we propose that the mechanism for the antitumour activity of PK1 may be due cleavage of the conjugate by cathepsin B at the tumour periphery and not within lysosomes as initially proposed. However, further experiments are needed to confirm our hypothesis.

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