In Vivo Gene Therapy of Cancer with E. coli Purine Nucleoside Phosphorylase

Abstract

We have developed a new strategy for the gene therapy of cancer based on the activation of purine nucleoside analogs by transduced E. coli purine nucleoside phosphorylase (PNP, E.C. 2.4.2.1). The approach is designed to generate antimetabolites intracellularly that would be too toxic for systemic administration. To determine whether this strategy could be used to kill tumor cells without host toxicity, nude mice bearing human malignant D54MG glioma tumors expressing E. coli PNP (D54–PNP) were treated with either 6-methylpurine-2′-deoxyriboside (MeP-dR) or arabinofuranosyl-2-fluoroadenine monophosphate (F-araAMP, fludarabine, a precursor of F-araA). Both prodrugs exhibited significant antitumor activity against established D54–PNP tumors at doses that produced no discernible systemic toxicity. Significantly, MeP-dR was curative against this slow growing solid tumor after only 3 doses. The antitumor effects showed a dose dependence on both the amount of prodrug given and the level of E. coli PNP expression within tumor xenografts. These results indicated that a strategy using E. coli PNP to create highly toxic, membrane permeant compounds that kill both replicating and nonreplicating cells is feasible in vivo, further supporting development of this cancer gene therapy approach.

Overview summary

In the present manuscript, we describe tumor regression with E. coli PNP in a mouse xenograft model and define prodrug dosing schedules that mediate their effects without apparent harm to the animals. The therapeutic benefit of MeP-dR and another analog, F-araAMP, to animals bearing tumors expressing E. coli PNP was measured. Expression of E. coli PNP at levels of approximately 200 nmoles of MeP-dR converted/hr/mg tumor tissue led to profound tumor sensitization in that exposure of tumors to MeP-dR for less than one tumor doubling time (3 doses of drug) led to complete regressions and cures. The effect was dose dependent, both in terms of the amount of prodrug administered and the level of expression of E. coli PNP.