Small Molecule Inhibitors Of Shmt1/2 Validate Serine Metabolism As A Target In The Treatment Of C-Myc Positive Solid Tumors

Many oncogenes modulate metabolic pathways and altered metabolism is one of the hallmarks of cancer. In order to sustain proliferation, cell growth and adopt to a very specific tumor microenvironment, cancer cells have to undergo metabolic reprogramming. Increased uptake of glucose, consumed in anaerobic manner, allows to maintain essential bioenergetic and biosynthetic pathways. Various reports indicated that many cancers cells are crucially dependent on serine, which could be either imported or synthesized by the serine synthesis pathway (SSP) branched from glycolysis. Serine can be converted to glycine by Serine Hydroxymethyltransferases isoforms 1 and 2 (SHMT1/2), which provide also carbon for the folate cycle. There is a growing interest in targeting SSP and SHMT1/2 have been proposed as druggable targets for the treatment of various cancers. One of the major challenges is validation of these concepts by high quality pharmacological and genetic tools, particularly in the context of high tumor heterogeneity, artificial tissue culture conditions and many branching points of tumor metabolism, which eventually result in acquired resistance. We have identified and characterized a series of sub micromolar dual SHMT1/2 inhibitors. Affinity of these compounds to protein targets has been confirmed in biochemical and binding assays and further corroborated by X-ray crystallography studies. In order to confirm efficacy of these compounds in cancer cells, both sensitive and resistant cells to the inhibition of SSP have been identified by using conditioned serine and glycine depleted media. Moreover functional roles of both paralogs: SHMT1, which fuels cytoplasmic folate cycle and SHMT2, which is responsible for the mitochondrial branch, were further confirmed by gene knockdown studies. Cell lines resistant to depletion of serine in cell culture media were characterized by elevated levels of proteins involved in the synthesis of serine, namely PHGDH, PSAT1, SHMT2, c-Myc amplification and increased 13C flux from glucose to serine and glycine. Metabolic flux analysis further indicated that treatment with presented SHMT1/2 inhibitors effectively blocked the production of glycine from glucose and serine in cancer cells. Viability studies confirmed anti-cancer efficacy of SHMT1/2 inhibitors at concentrations consistent with metabolic flux studies in the same cells. Moreover rescue experiments with media supplemented with glycine and formate, which is a crucial intermediate between mitochondrial and cytoplasmic branches of folate cycle, were sufficient to reduce activity of SHMT1/2 inhibitors. Finally synergistic studies with antifolates provided an insight how efficacy of SHMT1/2 could be exploited therapeutically also in rational combinations with approved drugs. Citation Format: Tomasz Rzymski, Anna Wrobel, Karolina Pyziak, Agnieszka Sroka, Marta Sowinska, Agnieszka Dreas, Marcin Krol, Pawel Guzik, Agnieszka Adamus, Agnieszka Przybylowicz, Katarzyna Hamara, Magdalena Sieprawska-Lupa, Artur Biela, Krzysztof Brzozka. Small molecule inhibitors of SHMT1/2 validate serine metabolism as a target in the treatment of c-Myc positive solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 442. doi:10.1158/1538-7445.AM2017-442