FIGURE 1.

MTX intracellular folate metabolic pathway. MTX enters the cell through active transport mediated by RFC1 or SLCO1B1. After entering the cell, MTX is transformed into MTX-PGs under the action of FPGS. MTX-PGs is the sequential addition of multiple glutamate residues to the γ-carboxyl group of folate and MTX to ensure effective intracellular retention. At the same time, GGH converts MTX-PGs back to MTX. Subsequently, glutamate-free MTX was exported from the cell with the participation of members of the ATP-binding box superfamily (ABCB1 and ABCG2). The main targets of MTX and MTX-PGs are DHFR, TYMS and several other enzymes involved in purine synthesis. MTX—methotrexate; MTX-PGs—MTX polyglutamates; RFC1—reduced folate carrier 1; SLCO1B1-solute carrier organic anion transporter family member 1B1; ABCB1—ATP-binding cassette subfamily B member 1; ABCG2—ATP-binding cassette subfamily G member 2; FPGS—folylpolyglutamate synthetase; GGH—gamma-glutamyl hydrolase; DHFR—dihydrofolate reductase; TYMS—thymidylate synthetase; MTHFR—methylenetetrahydrofolate reductase; MTHFD1—methylenetetrahydrofolate dehydrogenase; MTR—methylenetetrahydrofolate-homocysteine methyltransferase; MTRR—5-methylenetetrahydrofolate-homocysteine methyltransferase reductase; SHMT1—serine hydroxymethyltransferase 1; THF—tetrahydrofolate; DHF—dihydrofolate; dUMP—deoxyuridine monophosphate; and dTMP—deoxythymidine monophosphate.