Use of E. coli Purine Nucleoside Phosphorylase in the Treatment of Solid Tumors

Evaluating antitumor activity of Escherichia coli purine nucleoside phosphorylase against head and neck patient-derived xenografts

BACKGROUND

Purine nucleoside phosphorylase (PNP) gene transfer represents a promising approach to treatment of head and neck malignancies. We tested recombinant adenovirus already in phase I/II clinical testing and leading-edge patient-derived xenografts (PDX) as a means to optimize this therapeutic strategy.

METHODS

Our experiments investigated purine base cytotoxicity, PNP enzyme activity following treatment of malignant tissue, tumor mass regression, viral receptor studies, and transduction by tropism-modified adenovirus.

RESULTS

Replication deficient vector efficiently transduced PDX cells and mediated significant anticancer effect following treatment with fludarabine phosphate in vivo. Either 6-methylpurine or 2-fluoroadenine (toxic molecules generated by the PNP approach) ablated head and neck cancer cell proliferation. High levels of adenovirus-3 specific receptors were detected in human tumor models, and vector was evaluated that utilizes this pathway.

CONCLUSIONS

Our studies provide the scientific foundation necessary to improve PNP prodrug cleavage and advance a new treatment for head and neck cancer.

Molecular Basis of NDT-Mediated Activation of Nucleoside-Based Prodrugs and Application in Suicide Gene Therapy

Herein we report the first proof for the application of type II 2′-deoxyribosyltransferase from Lactobacillus delbrueckii (LdNDT) in suicide gene therapy for cancer treatment. To this end, we first confirm the hydrolytic ability of LdNDT over the nucleoside-based prodrugs 2′-deoxy-5-fluorouridine (dFUrd), 2′-deoxy-2-fluoroadenosine (dFAdo), and 2′-deoxy-6-methylpurine riboside (d6MetPRib). Such activity was significantly increased (up to 30-fold) in the presence of an acceptor nucleobase. To shed light on the strong nucleobase dependence for enzymatic activity, different molecular dynamics simulations were carried out. Finally, as a proof of concept, we tested the LdNDT/dFAdo system in human cervical cancer (HeLa) cells. Interestingly, LdNDT/dFAdo showed a pronounced reduction in cellular viability with inhibitory concentrations in the low micromolar range. These results open up future opportunities for the clinical implementation of nucleoside 2′-deoxyribosyltransferases (NDTs) in cancer treatment.

Tricyclic Nucleobase Analogs and Their Ribosides as Substrates and Inhibitors of Purine-Nucleoside Phosphorylases III. Aminopurine Derivatives

Etheno-derivatives of 2-aminopurine, 2-aminopurine riboside, and 7-deazaadenosine (tubercidine) were prepared and purified using standard methods. 2-Aminopurine reacted with aqueous chloroacetaldehyde to give two products, both exhibiting substrate activity towards bacterial (E. coli) purine-nucleoside phosphorylase (PNP) in the reverse (synthetic) pathway. The major product of the chemical synthesis, identified as 1,N2-etheno-2-aminopurine, reacted slowly, while the second, minor, but highly fluorescent product, reacted rapidly. NMR analysis allowed identification of the minor product as N2,3-etheno-2-aminopurine, and its ribosylation product as N2,3-etheno-2-aminopurine-N2--D-riboside. Ribosylation of 1,N2-etheno-2-aminopurine led to analogous N2--d-riboside of this base. Both enzymatically produced ribosides were readily phosphorolysed by bacterial PNP to the respective bases. The reaction of 2-aminopurine-N9- -D-riboside with chloroacetaldehyde gave one major product, clearly distinct from that obtained from the enzymatic synthesis, which was not a substrate for PNP. A tri-cyclic 7-deazaadenosine (tubercidine) derivative was prepared in an analogous way and shown to be an effective inhibitor of the E. coli, but not of the mammalian enzyme. Fluorescent complexes of amino-purine analogs with E. coli PNP were observed.

The use of Trichomonas vaginalis purine nucleoside phosphorylase to activate fludarabine in the treatment of solid tumors

Treatment with fludarabine phosphate (9-β-D-arabinofuranosyl-2-F-adenine 5'-phosphate, F-araAMP) leads to regressions and cures of human tumor xenografts that express Escherichia coli purine nucleoside phosphorylase (EcPNP). This occurs despite the fact that fludarabine (F-araA) is a relatively poor substrate for EcPNP, and is cleaved to liberate 2-fluoroadenine at a rate only 0.3% that of the natural E. coli PNP substrate, adenosine. In this study, we investigated a panel of naturally occurring PNPs to identify more efficient enzymes that may be suitable for metabolizing F-araA as part of experimental cancer therapy. We show that Trichomonas vaginalis PNP (TvPNP) cleaves F-araA with a catalytic efficiency 25-fold greater than the prototypic E. coli enzyme. Cellular extracts from human glioma cells (D54) transduced with lentivirus stably expressing TvPNP (D54/TvPNP) were found to cleave F-araA at a rate similar to extracts from D54 cells expressing EcPNP, although much less enzyme was expressed per cell in the TvPNP transduced condition. As a test of safety and efficacy using TvPNP, human head and neck squamous cell carcinoma (FaDu) xenografts expressing TvPNP were studied in nude mice and shown to exhibit robust tumor regressions, albeit with partial weight loss that resolved post-therapy. F-araAMP was also a very effective treatment for mice bearing D54/TvPNP xenografts in which approximately 10% of tumor cells expressed the enzyme, indicating pronounced ability to kill non-transduced tumor cells (high bystander activity). Moreover, F-araAMP demonstrated activity against D54 tumors injected with an E1, E3 deleted adenoviral vector encoding TvPNP. In that setting, despite higher F-araA cleavage activity using TvPNP, tumor responses were similar to those obtained with EcPNP, indicating factors other than F-Ade production may limit regressions of the D54 murine xenograft model. Our results establish that TvPNP is a favorable enzyme for activating F-araA, and support further studies in combination with F-araAMP for difficult-to-treat human cancers.

Purine-Metabolising Enzymes and Apoptosis in Cancer

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.