PROJECT SUMMARY Serine is an essential nutrient for tumor growth, and there is significant interest in starving cancer cells of serine for cancer therapy. For example, we have recently found that luminal/ER+ breast tumors, which account for approximately half of all breast cancer fatalities, are unable synthesize serine de novo (i.e., they are auxotrophic for serine) and are therefore particularly vulnerable to serine deprivation. Dietary serine starvation is currently the only method of reducing serine availability in vivo, but this approach will be difficult to implement in humans due to the extreme dietary modifications it requires. Furthermore, dietary serine starvation can only reduce circulating serine levels by 50%, which may not be sufficient to inhibit the growth of many tumors. Therapeutic enzymes are an alternative method of manipulating nutrient levels in vivo that have proven to be effective treatments for cancer and other diseases. We hypothesized that a therapeutic serine degrading enzyme might be a more effective method of achieving in vivo serine starvation for cancer therapy. To test this hypothesis, we have developed a novel therapeutic serine degrading enzyme, engineered human serine dehydratase (eSDH), that is capable of reducing circulating serine levels by greater than 90% in mice without the need for any dietary changes. Our preliminary data suggests that prolonged serine depletion with eSDH is well-tolerated by mice and capable of inhibiting tumor growth in multiple mouse models. The overarching goals of this proposal are to optimize eSDH to generate an enzyme that is suitable for subsequent clinical development and to evaluate it as a potential cancer therapeutic in pre-clinical models. To achieve these goals, we propose experiments that will 1) engineer a more selective and stable optimized eSDH enzyme with enhanced pharmacological properties 2) assess the physiological impact and potential side- effects of enzymatic serine depletion, 3) evaluate the efficacy of eSDH against tumors that are auxotrophic for serine, and 4) further investigate our preliminary finding that eSDH treatment induces anti-tumor immunity. A targeted therapeutic approach for serine auxotrophic tumors that also induces anti-tumor immunity could provide an effective treatment modality for patients with luminal breast cancer and other malignancies.