Abstract The development of chloroleukemia in the rat after transplantation leads to 100% mortality. Recently, two novel clones have been developed. Following intraperitoneal injection, one clone preferentially migrates and forms tumors in the lungs (CHL-L), while another clone forms tumors in the liver (CHL-Li). In vitro, proliferation of both clones were inhibited in a dose-dependent manner by a novel IV solution of API 31510. Accordingly, in the present study we examined the hypothesis that Cytotech Labs API 31510 alone or in combination with cyclophosphamide (CTX) could be effective in the treatment of both clones in vivo. In the first experiments, 24 rats were randomized into 4 groups of six rats each and injected intraperitoneally with CHL-L. In the control group, 100% developed CHL-L. In the group receiving CTX, 5 out of 6 developed CHL-L, while 1 animal remained disease-free. In the group receiving API 31510, 3 out of 6 rats remained disease-free and 3 out of 6 had a partial therapeutic response. In the group which received API 31510 and CTX, 100% of the animals remained disease-free. In a separate experiment, the same protocol was followed using CHL-Li. As observed with the other clone, animals injected with all control animals developed CHL-Li tumors. Similarly, in the group receiving CTX alone, 5 out of 6 developed CHL-Li and 1 remained disease-free. In the group receiving API 31510, 3 out of 6 animals remained disease-free and 3 out of 6 had a partial response. When combining 31510 and CTX, 5 out of 6 animals remained disease-free and 1 had a partial response. No side effects were observed in the animals receiving API 31510 as evidenced by weight gain and behavior. Taken together, these results indicate that API 31510 as a single agent was effective against both chloroma clones used and as an adjuvant to chemotherapy, API 31510 exhibited treatment efficacy in over 90% of animals treated. These data provide a strong foundation for further development of Cytotech Labs API 31510 intravenous formulation as a viable agent for cancer. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. 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 3568.