Folate Depletion as a Radiosensitizing Strategy for Solid Tumors

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<h3>Purpose/Objective(s)</h3> Folate is an essential substrate for DNA synthesis, therefore important for propagation of cancer cells through cell cycle following repair of radiation induced DNA damage. We hypothesized folate depletion could improve radiosensitivity by impairing cell cycle progression, therefore potentially the efficacy of radiation therapy. <h3>Materials/Methods</h3> We investigated folate depletion as a novel radiosensitization strategy <b>in-vitro</b> using a range of cancer cell lines; melanoma (A375), pancreas (PANC1), cervix (Ca Ski) and lung (A549), to recapitulate varying degree of radiosensitivity observed in clinic. Cell lines were irradiated to a number of target doses (0, 2, 4, 8, 16 and 24 Gy) using an intensity-modulated radiation therapy-based radiobiology platform and subjected to varying levels of folate depletion, achieved with carboxypeptidase G (0, 0.00006, 0.0002, 0.0006, 0.002 and 0.006 units [U]). Cell viability was assessed with MTS assay and Loewe combination index values were calculated to assess interaction between radiation and folate depletion (< 1 synergy; = 1 additivity; > 1 antagonism). <h3>Results</h3> Reproducible, dose dependent and statistically significant reduction in cell viability seen for radiation as well as to folate depletion for A375, PANC1 and Ca Ski cell lines (<i>P</i> < 0.0001, two-way ANOVA), and similar results observed for A549 cell line although the magnitude of cell kill was less marked for radiation (<i>P</i> = 0.0013). A375 cell line demonstrated antagonism to lower carboxypeptidase G concentrations that was more pronounced for larger radiation doses, but increasing drug concentrations resulted in synergy. PANC1 cell line showed low level antagonism for 0.00006 U carboxypeptidase G, whereas at 0.0002 U synergy was seen which was more apparent for lower radiation doses, and 0.0006 U produced even better synergy (Table 1). Ca Ski cell line showed additivity with the lowest concentration of carboxypeptidase G but increasing concentrations demonstrated higher degree of synergy. A549 cell line showed antagonism with lower concentrations of drug, however, in contrast with other cell lines, high antagonism was observed with low radiation doses, and when synergy was seen, it was for larger radiation doses. <h3>Conclusion</h3> We demonstrate folate depletion as a novel radiosensitizing strategy <b>in-vitro. In-vivo</b> validation would advance this approach for further exploration to enhance efficacy of radiation therapy in solid tumors.