Methotrexate and Pralatrexate

Abstract

This article reviews methotrexate and the more potent, related compound, pralatrexate, for the treatment of cutaneous T-cell lymphomas, including mycosis fungoides, Sézary syndrome, and CD30+ lymphoproliferative disorders. Although these folate antagonists are traditionally viewed as antiproliferative cell cycle inhibitors, it is recognized that they inhibit DNA methylation, providing a rationale for their use as epigenetic regulators and cell proliferation inhibitors. The underlying mechanisms are outlined, key supporting data presented, followed by brief mention of recent mathematical modeling supporting the general superiority of combination therapy. Several novel examples involving folate antagonists are proposed.

Keywords: Apoptosis; Dihydrofolate reductase; Folate; Folic acid; Methotrexate; Pralatrexate; Purine synthesis; S phase.

Fig. 1

MTX inhibits DNA methylation by depleting SAM. Chain of methyl group (Me) transfer is shown underlined from N-Me-tetrahydrofolate (Me-THF) to Me-vitamin B12 to methionine (M) to SAM to Me-DNA. MTX competitively inhibits dihydrofolate reductase (DHFR) which is involved in the multistep conversion of folate to Me-THF and inhibits methionine synthase (MS) and subsequent downstream methyl transfers that normally generate SAM. Other factors shown: homocysteine (HC), methionine adenosyltransferase (MAT), S-adenosylhomocysteine (SAH), SAH hydrolase (SAHH). The upper portion of the diagram is simplified and does not show that the immediate product of DHFR is THF, which is then converted into methylene THF before generation of Me-THF, which is then converted back to THF. Methylene THF is the more proximate precursor of deoxythymidine monophosphate (dTMP) and purines. DNMT, DNA methyltransferase.

Fig. 2

MTX reduces FAS promoter methylation. SAM reverses it. Demethylation of 6 CpG sites in the FAS promoter was detected by pyrosequencing after treatment with MTX in Fas promoter methylation-high CTCL cell lines (SZ4, HH). The y-axis shows the percentage of the methylation. The upper lines are the dimethyl sulfoxide (DMSO) controls. The lower lines show the demethylating effect of MTX. The middle lines show that exogenous SAM can reverse the demethylating effect of MTX.

Fig. 3

MTX increases FAS death receptor protein expression; SAM reverses it. CTCL cell lines (HH, SZ4) show increased expression of FAS after treatment with MTX. Addition of exogenous SAM almost fully reverses this FAS upregulation. FAS protein detected by flow cytometry. Y-axis: fold change in mean fluorescence intensity. Asterisks represent statistically significant t test differences between the MTX samples and the no-treatment controls or the MTX + SAM samples (2-tailed P<.05 was considered statistically significant).