Role of proton-coupled folate transporter in pemetrexed-resistance of mesothelioma: clinical evidence and new pharmacological tools.

Annals of Oncology 2017 September [Epub ahead of print] [Link]

Giovannetti E, Zucali PA, Assaraf YG, Funel N, Gemelli M, Stark M, Thunnissen E, Hou Z, Muller IB, Struijs EA, Perrino M, Jansen G, Matherly LH, Peters GJ


Thymidylate synthase (TS) has a predictive role in pemetrexed-treatment of mesothelioma, however additional chemoresistance mechanisms are poorly understood. Here we explored the role of the reduced-folate carrier (RFC/SLC19A1) and proton-coupled folate-transporter (PCFT/SLC46A1) in antifolate-resistance in mesothelioma.
PCFT , RFC and TS RNA and PCFT protein levels were determined by quantitative-RT-PCR of frozen tissues and immunohistochemistry of tissue-microarrays, respectively, in two cohorts of pemetrexed-treated patients. Data were analyzed by t-test, Fisher/log-rank test and Cox-proportional models. The contribution of PCFT expression and PCFT -promoter methylation on pemetrexed activity were evaluated in mesothelioma cells and spheroids, through 5-aza-2′-deoxycytidine-mediated demethylation and siRNA-knockdown.
Pemetrexed-treated patients with low- PCFT had significantly lower rates of disease control, and shorter overall-survival (OS), in both the test (N = 73, 11.3 vs. 20.1 months, P = 0.01) and validation (N = 51, 12.6 vs. 30.3 months, P = 0.02) cohorts. Multivariate analysis confirmed PCFT independent prognostic role. Low-PCFT protein levels were also associated with shorter OS. Patients with both low- PCFT and high- TS levels had the worst prognosis (OS, 5.5 months), whereas associations were neither found for RFC nor in pemetrexed-untreated patients. PCFT silencing reduced pemetrexed sensitivity, while 5-aza-2′-deoxycytidine overcame resistance.
These findings identify for the first time PCFT as a novel mesothelioma prognostic biomarker, prompting prospective trials for its validation. Moreover, preclinical data suggest that targeting PCFT -promoter methylation might eradicate pemetrexed-resistant cells characterized by low- PCFT expression.