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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.

Gemcitabine, a novel pyrimidine nucleoside antimetabolite, has shown clinical antitumor activity against several tumors (breast, small-cell and non-small-cell lung, bladder, pancreatic, and ovarian). We have developed a drug-resistance profile for gemcitabine using eight drug-resistant P388 leukemias in order to identify potentially useful guides for patient selection for further clinical trials of gemcitabine and possible noncrossresistant drug combinations with gemcitabine. Multidrug-resistant P388 leukemias (leukemias resistant to doxorubicin or etoposide) exhibited no crossresistance to gemcitabine. Leukemias resistant to vincristine (not multidrug resistant), cyclophosphamide, melphalan, cisplatin, and methotrexate were also not crossresistant to gemcitabine. Only the leukemia resistant to 1-beta-D-arabinofuranosylcytosine was crossresistant to gemcitabine. The results suggest that (1) it may be important to exclude or to monitor with extra care patients who have previously been treated with 1-beta-D-arabinofuranosylcytosine and (2) the lack of crossresistance seen with gemcitabine may contribute to therapeutic synergism when gemcitabine is combined with other agents.