Therapeutic potential of the cytosine deaminase::uracil phosphoribosyl transferase/5‐fluorocytosine suicide system for canine melanoma

We tested the efficacy of a yeast cytosine deaminase::uracil phosphoribosyl transferase/5‐fluorocytosine (CDU/5‐FC) non‐viral suicide system on eight established canine melanoma cell lines. Albeit with different degree of sensitivity 5 days after lipofection, this system was significantly efficient killing melanoma cells, being four cell lines highly, two fairly and two not very sensitive to CDU/5‐FC (their respective IC50 ranging from 0.20 to 800 μM 5‐FC). Considering the relatively low lipofection efficiencies, a very strong bystander effect was verified in the eight cell lines: depending on the cell line, this effect accounted for most of the induced cell death (from 70% to 95%). In our assay conditions, we did not find useful interactions either with the herpes simplex thymidine kinase/ganciclovir suicide system (in sequential or simultaneous modality) or with cisplatin and bleomycin chemotherapeutic drugs. Furthermore, only two cell lines displayed limited useful interactions of the CDU/5‐FC either with interferon‐β gene transfer or the proteasome inhibitor bortezomib respectively. These results would preclude a wide use of these combinations. However, the fact that all the tested cells were significantly sensitive to the CDU/5‐FC system encourages further research as a gene therapy tool for local control of canine melanoma.