Approximately 80% of WHO grade II and III gliomas, and secondary glioblastomas (representing 25% of adult diffuse gliomas overall) harbor mutations in the metabolic isocitrate dehydrogenase enzymes encoded by the IDH1/2 genes. These mutations, now recognized as the genetic hallmark of these cancers, result in neomorphic enzymatic activity driving overproduction of the oncometabolite 2-hydroxyglutarate (2-HG), which leads to profound reprogramming of tumor cellular metabolism. As a result, IDH1-mutant gliomas are dependent upon the canonical coenzyme NAD+ for survival. It is known that poly(ADP-ribose) polymerase (PARP) activation consumes NAD+ during base excision repair of chemotherapy-induced DNA damage. We therefore hypothesized that a strategy combining NAD+ biosynthesis inhibitors with the alkylating chemotherapeutic agent temozolomide could potentiate NAD+ depletion-mediated cytotoxicity in mutant IDH1 cancer cells.
We used several IDH mutant cancer models to study NAD+ metabolism. To investigate the impact of temozolomide (TMZ) on NAD+ metabolism, patient-derived xenografts and engineered mutant IDH1-expressing cell lines were exposed to TMZ, in vitro and in vivo, both alone and in combination with nicotinamide phosphoribosyltransferase (NAMPT) inhibitors, which block NAD+ biosynthesis.
We found that TMZ is an NAD+ stressor, specifically in IDH1 mutant cancers. Mechanistically, TMZ stimulates NAD+ consumption by PARP via activation of DNA damage repair in exposed cells. The acute time period (<3 hours) after TMZ treatment displayed a burst of NAD+ consumption driven by PARP activation, but the dynamic capacity of the base excision repair system typically is able to withstand the metabolic stress induced by these DNA repair processing demands. However, in IDH1-mutant-expressing cells, this consumption of NAD+ critically reduced the abnormally lowered basal steady-state levels of NAD+, introducing a window of hypervulnerability to NAD+ biosynthesis inhibitors. This effect was selective for IDH1-mutant cells and independent of methylguanine methyltransferase or mismatch repair status, which are known rate-limiting mediators of adjuvant temozolomide genotoxic sensitivity. Combined temozolomide and NAMPT inhibition in an in vivo IDH1-mutant cancer model exhibited enhanced efficacy compared with each agent alone.
In conclusion, we find that IDH1-mutant cancers have distinct metabolic stress responses to chemotherapy-induced DNA damage and that combination regimens targeting nonredundant NAD+ pathways yield potent anticancer efficacy in vivo. More generally, our results suggest that effective targeting of convergent metabolic pathways in genetically selected cancers could minimize treatment toxicity and improve durability of response to therapy. Highlighting the central importance of NAD+ levels in IDH mutant gliomas, our observations suggests that metabolic strategies targeting NAD+ homeostasis may be combined with standard-of-care therapies to improve efficacy against these tumors.
Harvard-Massachusetts General Hospital
D. Cahill: Consulting: Merck, Lilly.
NIH, Burroughs Wellcome Foundation