Inosine Monophosphate Dehydrogenase Dependence in a Subset of Small Cell Lung Cancers

Small cell lung cancer (SCLC) is a rapidly lethal disease with few therapeutic options. We studied metabolic heterogeneity in SCLC to identify subtype-selective vulnerabilities. Metabolomics in SCLC cell lines identified two groups correlating with high or low expression of the Achaete-scute homolog-1 (ASCL1) transcription factor (ASCL1(High) and ASCL1(Low)), a lineage oncogene. Guanosine nucleotides were elevated in ASCL1(Low) cells and tumors from genetically engineered mice. ASCL1(Low) tumors abundantly express the guanosine biosynthetic enzymes inosine monophosphate dehydrogenase-1 and -2 (IMPDH1 and IMPDH2). These enzymes are transcriptional targets of MYC, which is selectively overexpressed in ASCL1(Low) SCLC. IMPDH inhibition reduced RNA polymerase I-dependent expression of pre-ribosomal RNA and potently suppressed ASCL1(Low) cell growth in culture, selectively reduced growth of ASCL1(Low) xenografts, and combined with chemotherapy to improve survival in genetic mouse models of ASCL1(Low)/MYCHigh SCLC. The data define an SCLC subtype-selective vulnerability related to dependence on de novo guanosine nucleotide synthesis.