Tumor organoids have been pushed forward as advanced model systems for in vitro oncology drug testing, with the eventual goal to direct personalized cancer treatments. However, drug testing efforts suffer from a large variation in experimental conditions for organoid culturing and organoid treatment. Moreover, most drug tests are restricted to whole-well viability as the sole read-out, thereby losing important information about key biological aspects that might be impacted due to the use of administered drugs. These bulk read-outs also discard potential inter-organoid heterogeneity in drug responses. To tackle these issues, we developed a systematic approach for processing organoids from prostate cancer (PCa) patient-derived xenografts (PDXs) for viability-based drug testing and identified essential conditions and quality checks for consistent results. In addition, we generated an imaging-based drug testing procedure using high-content fluorescence microscopy in living PCa organoids to detect various modalities of cell death. Individual organoids and cell nuclei in organoids were segmented and quantified using a dye combination of Hoechst 33342, propidium iodide and Caspase 3/7 Green, allowing the identification of cytostatic and cytotoxic treatment effects. Our procedures provide important insights into the mechanistic actions of tested drugs. Moreover, these methods can be adapted for tumor organoids originating from other cancer types to increase organoid-based drug test validity, and ultimately, accelerate clinical implementation.

Conclusion: Several genetic biomarkers have proven predictive value for the treatment outcome of systemically administered oxaliplatin. By extrapolation, these genetic biomarkers may also be predictive for the efficacy of intraperitoneal oxaliplatin. This should be the subject of further investigation.

3633 Background: The outcome for patients with localized rectal cancer treated with chemoradiotherapy (CMT) or radiotherapy alone (RT) and no planned subsequent surgery is unknown. Insights can be provided by study of patients that do not undergo surgery due to excessive operative risk (medically inoperable) as a result of advanced age and/or co-morbidities, or patient refusal. Methods: A retrospective analysis at 6 Australian centers. Patients with recurrent rectal cancer, metastatic disease or primary tumors that were considered unresectable were excluded from the study. Results: We identified 48 patients treated between August 1998 and June 2005 meeting our criteria. There were 13 females and 35 males with a median age of 76 years (range 49 - 94). 24 patients (50%) were deemed medically inoperable and 24 patients refused surgery (the most common reasons were the desire to avoid a stoma or fear of surgery). Treatment was with standard long course radiation plus 5-FU (either bolus or continuous infusion) in 36 patients (75%) and 12 (25%) received RT alone. A complete clinical response was documented in 23 patients (48%) and a partial clinical response was documented in 14 patients (30%). At a median follow-up of 49 months, 18 patients (37.5%) have documented disease progression including 10 of 24 in the medically inoperable group and 8 of 24 in the refused surgery group. The median progression-free survival for all patients was 30+ months. 7 had local progression only, 7 had distant progression only and 4 patients had both local and distant progression. Of the 23 deceased patients, 16 (70%) died from progressive disease and 7 (30%) from non-cancer causes, including 4 from the refused surgery group. The median survival for all patients was 35 + months. Conclusions: In patients who are not suitable candidates for surgical resection due to advanced age and/or comorbidity CRT or RT alone is a safe alternative that results in significant progression-free and overall survival times, with many patients succumbing to co-morbidities rather than progressive disease. Many of these patients refusing surgery will have long periods of disease control after treatment with CRT or RT. No significant financial relationships to disclose.