Abstract Alopecia remains the one side-effect of chemotherapy for which there is no therapeutic intervention. We have recently developed a multi-course chemotherapy rat model of transplantable chloroleukemia (MIAC51) in which to study Chemotherapy-[[Unsupported Character - Codename ­]]Induced Alopecia (CIA). This model provides the opportunity to study CIA and the cancer cells throughout multiple courses of chemotherapy. In the present study, we investigated the effect of calcitriol in a proprietary delivery system, Cytotech Labs API 31543 to protect from CIA using the following chemotherapy regimens: cyclophosphamide; cyclophosphamide and doxorubicin; cyclophosphamide, doxorubicin and cytarabine; cyclophosphamide, paclitaxel and etoposide; doxorubicin, paclitaxel and etoposide. Animals were injected intraperitoneally with 1×105 MIAC51 on day 5. Thereafter, 0.2µg of API 31543 or vehicle control was applied topically over the head area daily starting on day 6 for 6 consecutive days for the first course. Rats were isolated for 6 hours. Subsequently, the treated area was cleaned with soap and water and rats were returned to their litters. For the first cycle, chemotherapy regimens were administered on day 13, intraperitoneally in a total volume of 0.1 mL. Animals positive for leukemic cells in smears were euthanized on day 23. On day 31, a second anagen cycle was induced by clipping the hair in the neck area on leukemia survivors. Animals were then treated with 0.2 µg API 31543 on day 40 for 6 days in the shaven area. Chemotherapy regimens were then administered on day 46. In both cycles, alopecia was recorded 10 days after the last dose of chemotherapy. Neonatal rats treated with 0.2µg of API 31543 in the head area exhibited localized protection. Similarly, in the adult rats 0.2µg of API 31543 exerted a localized protective effect at the site of application. In both groups, rats that received chemotherapy alone became totally alopecic. Pretreatment with API 31543 protected the hair follicles against chemotherapy without having an effect on the efficacy of chemotherapy treatment of leukemia. In summary, the results herein set a solid foundation for clinical investigation of API 31543. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2865.