2459 - Evaluating the Addition of Low-Dose Navitoclax to Single-Fraction Radiation Therapy Using Patient-Derived Solid Tumor Organoids and Fibroblast Co-Culture Models

Purpose/Objective(s):

Over 30% of cancer patients in the United States receive radiation therapy (RT) during their course of treatment, either alone or in combination with systemic therapies. While RT can be an effective modality, intrinsic radioresistance and TME-mediated effects can limit treatment response. One strategy to improve the therapeutic ratio is the use of low-dose radiosensitizing (RS) agents. Navitoclax (ABT-263), a BCL-2 family inhibitor, has demonstrated RS in HNSCC in vitro; however, its potential across other solid tumors remains poorly defined. Here, we utilized patient-derived NSCLC, CRC, and metastatic germ cell (GCT) tumor organoid (TO) and TO-fibroblast co-culture models to investigate RS potential of ABT-263. We hypothesize that the addition of low-dose ABT-263 enhances RS and reduces TO viability compared with RT alone.

Materials/Methods:

To evaluate ABT-263 RS across various tumor types, NSCLC, CRC and GCT TO models were treated with single-fraction RT (0-8Gy) alone or in combination with ABT-263 (125 nM-8 uM), with the upper dose selected to reflect the clinically achievable Cmax reported in lymphoid malignancy trials. First, TO viability was assessed five days post-treatment using CellTiter-Glo and normalized to vehicle-treated, 0 Gy controls. Drug-radiation interactions were quantified using ZIP synergy scores calculated with SynergyFinder. Next, to assess the impact of stromal interactions on observed RS, TOs were cultured either alone (MC) or in 1:1 co-culture (CC) with fibroblasts and treated with RT + ABT-263 doses demonstrating maximal RS. TOs were fluorescently labeled and the total fluorescent area was monitored using IncuCyte live-cell imaging over the course of 14 days (4 days before treatment and 10 days post-treatment), normalized to baseline (day 0).

Results:

Across models, TOs demonstrated variable baseline sensitivity to RT and ABT-263. NSCLC1 TO was highly sensitive to RT and ABT-263 alone, with no synergy observed. In contrast, CRC TO lines demonstrated enhanced RS with the combination treatment, with CRC1 showing the strongest synergistic combination at 4 Gy + 125 nM of ABT-263 (ZIP synergy score > 26). A synergistic reduction in viability was also observed in the GCT1 TO line, with maximal synergy at 8Gy + 500 nM of ABT-263 (ZIP synergy score > 40). In GCT1, RT and ABT-263 reduced total TO area as single agents (compared to the 0Gy + vehicle treated cultures) but the combined treatment produced the greatest TO area suppression in both MC and CC conditions. By day 10, RT-treated TOs expanded to ~ 3x the baseline area, whereas the combination-treated TO area remained below baseline, with a statistically significant difference in relative area between the two groups (p < 0.01) in both MC and CC contexts.

Conclusion:

Our findings suggest that ABT-263 can function as a RS in a tumor type- and context-dependent manner, supporting further investigation in physiologically relevant organoid models.