Evaluation of MTH1 and DCTPP1 inhibitors in fluorocyclopentenylcytosine (RX-3117) resistant lung cancer cells and potential cross resistance patterns.

Fluorocyclopentenylcytosine (RX-3117) is a nucleoside analog developed to bypass gemcitabine resistance. We developed several fluorocyclopentenylcytosine- and gemcitabine-resistant non-small cell lung cancer (NSCLC) cell lines, which showed decreased accumulation of the active triphosphate. The expression of the human equilibrative nucleoside transporter and the activity of uridine cytidine kinase 2 were not altered. The cells showed cross-resistance to some cytidine analogs (e.g. ethynylcytidine and cyclopentenyl-cytosine) but not to other DNA damaging drugs, such as cisplatin, and drugs inhibiting the proteasome, a protease or cell cycle regulators. In most resistant lines, the expression of MutT Homolog-1/8-oxo-dGTP diphosphatase (MTH1) and deoxycytidine triphosphatase 1 (DCTPP1) was increased, suggesting that these enzymes degrade the triphosphates. To reverse resistance, we combined fluorocyclopentenylcytosine with inhibitors of MTH1 (TH588) and DCTPP1 (TH1217 and triptolide). Drug sensitivity was tested in resistant NSCLC variants using the SRB assay with co-treatment of fluorocyclopentenylcytosine and IC25 concentrations of inhibitors. Despite the increased expression of MTH1 and DCTPP1, treatment with TH588, TH1217, or triptolide did not re-sensitize the resistant lines, except for cells resistant to gemcitabine and to both gemcitabine and fluorocyclopentenylcytosine, which showed increased sensitivity to fluorocyclopentenylcytosine combined with the inhibitors. In conclusion, while fluorocyclopentenylcytosine resistance involves decreased triphosphate accumulation and increased expression of DNA repair enzymes, only gemcitabine- and double-resistant cells became more sensitive to enzyme inhibition, suggesting other mechanisms contribute to resistance.