The mechanism of action of PMX is based on the inhibition of three enzymes responsible for the purine and pyrimidine synthesis—thymidylate synthase (TS), dihydrofolate reductase (DHFR), and glycinamide ribonucleotide formyltransferase [56]—which prevents the formation of DNA and RNA, which are responsible for the growth of normal and cancer cells. Here, DHFR is linked to cancer.