Abstract 1845: Increased antitumor activity and reduced organ toxicity of doxorubicin administered using RRx-001 modified RBCs

Abstract RRx-001 is currently in a phase III clinical trial for small cell lung cancer and showed activity in a phase II trial in colorectal cancer. It has negligible side effects so could be used in combination regimens with low risk of additive toxicity. It has multiple mechanisms of action including binding to hemoglobin to alter red blood cell (RBC) properties, increasing RBC adhesion to the endothelium and increasing membrane phosphatidyl serine exposure. Importantly, RRx-001-treated RBCs are known to preferentially localize to solid tumors. Existing therapeutics that partition into RBCs could therefore be localized to cancers and have a greater therapeutic index by combining with RRx-001. Doxorubicin has been a cornerstone of treatment for many cancer types for decades and has well-known limiting side effects including cardiotoxicity and cytopenias. It partitions into RBCs at equilibrium with approximately 60% of doxorubicin becoming intracellular. Therefore, treatment with RBCs loaded with doxorubicin and modified with RRx-001 to promote localization to tumors may improve its activity and reduce its potential side effects. Previous research in mice carrying H22 tumors found that biodistribution to the tumor was higher after treatment with RBCs loaded with doxorubicin and treated with RRx-001 than with free doxorubicin or RBCs loaded with doxorubicin without RRx-001, while biodistribution to the heart and skin was decreased. H22 tumor growth was lower after treatment with RBCs loaded with doxorubicin and treated with RRx-001 than with doxorubicin alone or RBCs loaded with doxorubicin without RRx-001. Further experiments identified an approach for loading RBCs with doxorubicin and treating with RRx-001 that could be implemented clinically while achieving high doxorubicin loading efficiency given its rate of equilibration into RBCs, avoiding hemolysis, and minimizing doxorubicin precipitation. The final chosen procedure would be to draw blood with citrate anticoagulation while mixing with RRx-001 to reach 5 μM, add 2 mg/mL doxorubicin in normal saline to reach a final concentration of 0.3 mg/mL, mix for 1 hour at 37°C, and re-infuse the treated blood. For a clinical trial with a starting dose of 20 mg/m2 doxorubicin weekly, a patient with a BSA of 2 m2 would require 101 mL of blood to be drawn for mixing and re-infused 1 hour later, which would be practical even for patients with advanced cancer who are candidates for doxorubicin. For reference, a typical blood donation is approximately 450 mL. In conclusion, this describes the pre-clinical rationale and establishes a clinical procedure for administering RBCs loaded with doxorubicin and treated with RRx-001 as a way to improve its activity and reduce side effects. Citation Format: Ana Sanchez, Garett Soapes, Chris Larson, Tony Reid, Bryan Oronsky. Increased antitumor activity and reduced organ toxicity of doxorubicin administered using RRx-001 modified RBCs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1845.