Cysteine-mediated bortezomib resistance is governed by α-ketoacid availability.

Dissecting how metabolite availability impacts drug sensitivity is critical for understanding therapy resistance. While proteasome inhibitors like the boronic acid proteasome inhibitor bortezomib are cornerstones of therapy for multiple myeloma, the clinical utility of these drugs is often limited by development of resistance, the metabolic drivers of which remain poorly understood. In this study, we find that α-ketoacids, including pyruvate, increase sensitivity to boronic acid proteasome inhibitors independent of their conventional roles in metabolism. Instead, α-ketoacids directly react with intracellular cysteine to form thiazolidines, sequestering cysteine away from forming conjugates with boronic acid proteasome inhibitors and detoxifying these drugs. Preventing cysteine-mediated detoxification through α-ketoacid supplementation increases the efficacy of boronic acid proteasome inhibitors, leading to greater proteasome inhibition and cytotoxicity that is reversed by cysteine supplementation. These findings suggest that modulating available cysteine through α-ketoacid interactions can impact effective levels of some drugs in cells, and represent a potential strategy to overcome resistance and maximize the efficacy of boronic acid proteasome inhibitors.