Excellent In vivo Bystander Activity of Fludarabine Phosphate against Human Glioma Xenografts that Express the Escherichia coli Purine Nucleoside Phosphorylase Gene

Abstract

Escherichia coli purine nucleoside phosphorylase (PNP) expressed in tumors converts relatively nontoxic prodrugs into membrane-permeant cytotoxic compounds with high bystander activity. In the present study, we examined tumor regressions resulting from treatment with E. coli PNP and fludarabine phosphate (F-araAMP), a clinically approved compound used in the treatment of hematologic malignancies. We tested bystander killing with an adenoviral construct expressing E. coli PNP and then more formally examined thresholds for the bystander effect, using both MuLv and lentiviral vectoring. Because of the importance of understanding the mechanism of bystander action and the limits to this anticancer strategy, we also evaluated in vivo variables related to the expression of E. coli PNP (level of E. coli PNP activity in tumors, ectopic expression in liver, percentage of tumor cells transduced in situ, and accumulation of active metabolites in tumors). Our results indicate that F-araAMP confers excellent in vivo dose-dependent inhibition of bystander tumor cells, including strong responses in subcutaneous human glioma xenografts when 95 to 97.5% of the tumor mass is composed of bystander cells. These findings define levels of E. coli PNP expression necessary for antitumor activity with F-araAMP and demonstrate new potential for a clinically approved compound in solid tumor therapy.

Efficient electrogene therapy for pancreatic adenocarcinoma treatment using the bacterial purine nucleoside phosphorylase suicide gene with fludarabine

“…Despite better experimental conditions, namely a more convenient EP device fitted with a 6-electrode array (vs. only 2 in the present study), a 1-month supply of the prodrug (vs. 2 recurrent 5-day injections in our experiment) and the use of an immunocompetent tumor model (25), their electrogene therapy was less effective than ours. This improvement is likely the result of the higher bystander effect of the ePNP/ MePdR system (4,5,40) outlining the potential of such a suicide gene.…”

Activators of viral gene expression in polarized epithelial monolayers identified by rapid-throughput drug screening

“…12 Viral gene transfer by newer generation 'suicide' genes designed specifically for an improved bystander effect are approximately 10-fold below the thresholds necessary for eliciting tumor cures, as judged by studies of lentiviral transduced tumor cell lines. 36,37 Similarly, use of durable expression vectors such as AAV in clinical trials of cystic fibrosis and other inherited diseases suggests gene transfer efficiencies approaching, but still below thresholds necessary for phenotypic improvement. Coadministration of compounds such as those described here could be studied for their ability to potentiate viral transduction in these preclinical settings in vivo.…”

Advances in the mechanisms of action of cancer-targeting oncolytic viruses (Review)

“…The active metabolite of TK, PNP and 5-FU can directly kill infected tumor cells. Furthermore, they promote a strong bystander effect on neighboring tumor cells that are not actively infected (59,65,66). In a previous study by Leveille et al (67), recombinant vesicular stomatitis virus (VSV-MD51) expressing the cytosine deaminase/uracil phosphoribosyl-transferase suicide gene with the 5-FC prodrug was combined for the treatment of cancer.…”