Abstract 4530: Pralatrexate-induced apoptosis in multiple myeloma cells correlates with RFC-1 and DHFR expression

Abstract

Abstract Pralatrexate (PDX, 10-propargyl 10-deazaaminopterin) is a novel anti-folate agent that is approved for use in relapsed peripheral T cell lymphoma (PTCL). It is distinguished from the parent drug methotrexate (MTX) by having significantly greater affinity for the reduced folate carrier (RFC)-1, the principle transporter that internalize anti-folates, as well as broader efficacy, which strongly suggests that PDX has mechanisms of action beyond inhibition of nucleotide synthesis. Given the activity of PDX in aggressive lymphoid malignancies such as PTCL, we examined the activity of this agent in preclinical models of multiple myeloma (MM), a plasma cell malignancy that is currently incurable. We examined the cytotoxic activity of anti-folates in vitro against a panel of human multiple myeloma cell lines (HMCL) and showed that PDX induced dose-dependent apoptotic cell death in a subset of these. In sensitive cell lines (MM.1s and ARH-77) PDX demonstrated significantly greater potency at 48 hours compared to MTX (LD50 PDX = 2-3 nM vs. LD50 MTX = 25-30 nM). PDX also induced apoptosis in a polyclonal population of primary myeloma cells isolated from a patient specimen, whereas MTX had no effect on viability in these cells. PDX-induced apoptosis in sensitive HMCL was characterized by cleavage and activation of caspase-3 and -9, accompanied by the loss of the anti-apoptotic Bcl-2 family protein Mcl-1. Quantitative RT-PCR analysis of a panel of folate metabolism genes, including RFC-1, folyl polyglutamate synthetase (FPGS), gamma-glutamyl hydrolase (GGH), thymidylate synthase (TS), and dihydrofolate reductase (DHFR) revealed that RFC-1 expression was significantly higher in sensitive HMCL compared to resistant. Dose-dependent up-regulation of DHFR protein expression was observed by western blot in PDX-resistant cell lines, but not in PDX-sensitive ones. Incubation of HMCL with the proteasome inhibitor bortezomib or the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA), both of which demonstrate activity in MM, did not affect expression of RFC-1 in PDX-resistant HMCL and did not synergize with PDX to induce apoptosis in vitro. These results demonstrate that PDX induces apoptosis in sensitive HMCL, and that RFC-1 expression and DHFR up-regulation are functional biomarkers for susceptibility to this anti-folate. These data support further exploration of PDX therapy in MM, including combination therapy with agents that can increase expression of RFC-1 and/or block up-regulation of DHFR. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4530. doi:10.1158/1538-7445.AM2011-4530