Abstract Background: Standard of care treatment for patients with locally-advanced rectal cancer includes neoadjuvant chemoradiotherapy followed by surgery. However, this treatment is toxic affecting the patient’s quality of life. Additionally, 20% of patients achieve a complete response, 15% show no response and 30-40% may develop metastasis post treatment. Therefore, predicting patient response to treatment would be beneficial to inform clinicians if treatment is not optimal. 3D patient-derived tumor organoids (PDTOs) have been used to predict patient response to treatment due to their ability to replicate the biology and drug responsiveness of the original tumor. My project aims to use rectal cancer PDTOs to develop a personalized medicine strategy to accurately predict patient clinical response prior to therapy to inform clinicians of their treatment options. Patient-derived normal organoids (PDNOs) will also be used for toxicity assessment. Methods: Rectal cancer patients that meet the required criteria were recruited. Biopsies were collected via flexible-sigmoidoscopy and processed immediately in the laboratory for PDO establishment. Post establishment, patient-derived organoids (PDOs) were tested using a chemoradiation assay to determine sensitive, partial and resistant responders. On day 0, PDOs were seeded on a 384 well plate in 5% matrigel. On day 2, PDOs were exposed to Fluorouracil (0.412uM) followed by radiation (2 and 4 Grays). On day 7, live cells were measured using CellTiter Glo 3D Cell Viability Assay (Promega) on the Cytation 5 multiplate reader. Treatment sensitivity was investigated by calculating 50% growth rate (GR50) using Graphpad Prism. Results: To date, 14 patients have been recruited and 11 (78.5%) patient biopsy samples have been collected. Out of the collected biopsy samples, 7 out of 10 (70%) have been established with one currently in progress. Chemoradiation assays have been completed for 5 of the 7 established PDOs (RC-001, RC-003, RC-006, RC-008 and RC-010). RC-001 PDTOs exhibited a cytotoxic effect indicating complete response to treatment. The remaining PDTOs illustrated a partial effect indicating partial response to treatment. RC-001, 003, 008 and 010 PDNOs achieved GR50 at 4Gy whereas RC-006 PDNOs achieved GR50 at 2.2Gy. These results indicate that the complete responder could potentially benefit from chemoradiation treatment and the partial responders and could potentially benefit from additional treatment strategies with one partial responder potentially more susceptible to adverse treatment effects. Conclusion: This project is the first of its kind in Australia to predict patient response to chemoradiotherapy using PDOs. We demonstrate varied patient response to treatment and the important information that could potentially be garnered. Our work has the potential to inform selection of optimal therapy for patients and improve rectal cancer outcomes. Citation Format: Kiki Lianos, Kelly Olsen, Aleksandra Edmundson, Danielle Siganto, Matthew Burge, David Clark, Vicki Whitehall. Development of personalized medicine strategies for rectal cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7549.
Lianos et al. (Fri,) studied this question.