EXTH-50. THE COMPLEXITIES OF FATTY ACID DESATURATION IN GLIOBLASTOMA

Fatty acid desaturation is an enzymatic reaction in which a double bond is introduced into an acyl chain. Monounsaturated fatty acids (MUFA) are essential components of membrane. The most abundant MUFA-synthesizing enzyme is the delta 9 desaturate called Stearoyl Co-A Desaturase (SCD and SCD5 in humans, and SCD1-4 in mice). SCD desaturates Stearoyl-CoA (C18) and palmitoyl-CoA (C16) to oleoyl-CoA (C18:1) and palmitoyl-CoA (C16:1), respectively. SCD is often upregulated and a therapeutic target in cancer. We made an unexpected discovery that that median expression of SCD is low in glioblastoma relative to normal brain due to hypermethylation and monoallelic co-deletion with the tumor suppressor PTEN in a subset of patients. Cell lines from this subset, expressed nearly undetectable SCD yet they retained residual SCD enzymatic activity. Surprisingly, these lines evolved to survive independent of SCD through unknown mechanisms. On the other hand, cell lines that escaped such genetic and epigenetic alterations expressed higher levels of SCD and were highly dependent on SCD for survival. Finally, we identify that SCD-dependent lines acquire resistance through a previously unknown mechanism that involved drug-induced target (SCD) upregulation by the transcription factor FOSB. Accordingly, FOSB inhibition blunted acquired resistance and extended survival of tumor bearing mice treated with SCD inhibitor. Our findings reveal an intriguing feature of the cancer genome that may be used to stratify PTEN deleted cancer patients for SCD inhibitor therapy. A recent study showed that some cancer cells can use another MUFA-synthesizing enzyme FADS2 to bypass the SCD reaction. However, our data shows that the SCD inhibitor- resistant GBM lines are also FADS2-independent. Our targeted and untargeted metabolomics studies revealed unexpected findings that cannot be explained by conventional wisdom, and may lead to identification of novel lipogenic targets in GBM.

EXTH-23. PRECLINICAL EFFICACY OF A TARGETED, BRAIN PENETRANT INHIBITOR OF FATTY ACID DESATURATION IN GLIOBLASTOMA

There is increasing evidence that targeting de novo fatty acid synthesis could be effective for the treatment of primary and metastatic brain tumors, but brain penetrant inhibitors of this pathway are still lacking. We have previously reported that Stearoyl CoA Desaturase (SCD), a desaturase enzyme which converts saturated fatty acids into their unsaturated counterparts is a therapeutic target in glioblastoma (GBM). We showed that SCD exerts a cytoprotective role by protecting GBM cells against lipotoxicity and is essential for maintaining self-renewal and tumor initiating properties in GBM stem cells (GSCs). Using a panel of patient derived GSCs, mouse orthotopic GSC models, isogenic astrocytes models of gliomagenesis as well as in silico analysis, we report that in addition to SCD1, a second SCD isoform is also essential for GSCs maintenance. Further we demonstrate that while EGFR/PI3K/AKT signaling promotes lipogenesis, the activation of RAS/MEK/ERK signaling creates a metabolic vulnerability and sensitizes to SCD inhibitors. We tested a first-in-class, clinically relevant SCD inhibitor and showed that this compound effectively inhibits fatty acid desaturation in GSCs in a mouse orthotopic GSC model. Importantly, using different GSCs mouse models, we demonstrate that this SCD inhibitor can effectively increase overall survival as a monotherapy. Further, SCD inhibition impairs DNA-damage repair via homologous recombination thereby sensitizing to the standard of care therapeutics for GBM, ionizing radiation and temozolomide (TMZ). Consequently, combining this inhibitor with TMZ in mouse orthotopic GSC models leads to a significantly increased overall survival. Altogether, our results provide a deeper understanding of de novo fatty acid dynamics in GBM and support the clinical testing of a new SCD inhibitor, with favorable pharmacokinetic and pharmacodynamic properties, in patients diagnosed with GBM.