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Pancreatic cancer cells alter their metabolism to survive cancer-associated stress (1-4). For example, cancer cells must adapt to steep nutrient gradients that characterize the natural tumor microenvironment (TME) (5-7). In the absence of adaptive strategies, harsh metabolic conditions promote the generation of free radicals (8) and impair energy production in tumor cells. Towards this end, wild-type isocitrate dehydrogenase 1 (IDH1) activity is a metabolic requirement for cancer cells living in a harsh metabolic milieu. The cytosolic enzyme interconverts isocitrate and alpha-ketoglutarate, and uses NADP(H) as a cofactor. We show that under low nutrient conditions, the enzymatic reaction favors oxidative decarboxylation to yield NADPH and alpha-ketoglutarate. Metabolic studies showed that the IDH1 products directly support antioxidant defense and mitochondrial function in stressed cancer cells. Genetic IDH1 suppression reduced growth of pancreatic cancer cells in vitro under low nutrient conditions and in mouse models of pancreatic cancer. Surprisingly, allosteric inhibitors of mutant IDH1 proved to be potent wild-type IDH1 inhibitors under conditions specific to the TME, highlighting a natural therapeutic window. The presence of low magnesium enhanced allosteric inhibition by the drug, and ambient low glucose levels enhanced cancer cells’ dependence on wild-type IDH1. Thus, intrinsic TME conditions sensitized wild-type IDH1 to FDA-approved AG-120 (ivosidenib), and revealed the drug to be a potent single-agent therapeutic in cell culture and diverse in vivo cancer models. This work identified a potentially new repertoire of safe cancer therapies, including a clinically available compound, for the treatment of multiple wild-type IDH1 cancers (e.g., pancreatic).