Phototrexate is a photoswitchable potent inhibitor of human dihydrofolate reductase (DHFR). Phototrexate is a photochromic analog of Methotrexate. Antifolate activity is switched on (cis-Phototrexate) by UVA (375 nm) light; it can be switched off (trans-Phototrexate) by blue or white light and by thermal relaxation in the dark.cis-Phototrexate (IC50 = 6 nM) significantly reduces the viability of HeLa cells compared withtrans-Phototrexate (IC50 = 34 μM).