Abstract 2701: Patient-derived colorectal cancer organoid therapeutic escalation: Tracking resistance to EGFR inhibition for predicting therapeutic sensitivity

Abstract Background: Precision oncology requires translational tools predictive of clinical benefit. Epidermal growth factor receptor inhibition (EGFRi) yields a survival advantage in RAS/RAF wildtype (wt) colorectal cancer (CRC), however clinical tools do not predict individual patient response or resistance mechanisms. Patient-derived cancer organoids (PDCOs) allow for an ex vivo method to track subclonal response. Here, we examine the development of PDCO EGFRi resistance as a novel biomarker to differentiate treatment sensitivities. Methods: Fresh CRC tissue was obtained following consent to an IRB-approved protocol. Molecular profiling was performed on expanded PDCOs and in parallel to clinical hotspot sequencing. Cultures were treated with stepwise (20%) increases in panitumumab (46ug/mL per escalation) up to the physiologic Cmax (230ug/mL). A HER2 amplified culture was also assessed by escalation of trastuzumab with 20% stepwise increase to physiologic Cmax (180ug/mL). Threshold for escalation to the next dose level was a median relative growth of 20% at 96h. Time to resistance (TTR) was defined from start to persistent PDCO growth at 100% Cmax. Treatment studies assessed the longest diameter of individual spheres, assessed at 48h and compared to control using effect size (Glass's (G) Δ). TTR was compared to molecular profiles of primary resistance and clinical outcomes. Results: CRC PDCOs were derived from 10 patients (5 localized and 5 metastatic samples). PDCOs had diverse molecular profiles: 3 KRAS mutant (mt), 1 PTEN mt, 7 TP53 mt, and 7 RAS/RAF wt. TTR ranged from 26-226 days, with 90% of PDCOs lines achieving resistance. RAS mt PDCOs had a homogeneous TTR of 27.0±1.4 days. RAS wt PDCOs displayed marked variation in therapeutic response with TTR of 51.5±61.7 days (p=0.06, compared to RAS mt). A HER2-amplified PDCO (copy number 14) showed intermediate sensitivity to EGFRi (TTR 55 days), similar to dose escalation of HER2-targeting trastuzumab (TTR of 51 days). This HER2-amplified PDCO showed intermediate sensitivity to single agent trastuzumab (GΔ = 0.56), yet no sensitivity to trastuzumab following EGFRi resistance (GΔ = 0.09). Prospective clinical correlation was possible for 1 PDCO line, a RAS wt PDCO derived prior to EGFRi which showed marked sensitivity with growth and metabolic arrest achieved at 40% CMax consistent with clinical response. Conclusions: We present therapeutic dose escalation using PDCOs as a translational tool to track the clonal dynamics of resistance to EGFRi in CRC. This technique provides an objective measure of TTR to differentiate culture specific sensitivity in a clinically relevant timeframe. Molecular profiling of resistant cultures is ongoing which may provide an ex vivo tool for predicting individual resistance as cancers evade therapeutic pressure of targeted selection. Citation Format: Jeremy D. Kratz, Aishwarya Sunil, Meghan Conroy, Cheri A. Pasch, Nicole Lassen, Hannah Houtler, Kayla K. Lemmon, Linda Clipson, Daniel E. Abbott, Evie H. Carchman, Elise H. Lawson, Noelle K. Loconte, Sam J. Lubner, Kristina A. Matkowskyj, Daniel L. Mulkerin, Nataliya Uboha, Dustin A. Deming. Patient-derived colorectal cancer organoid therapeutic escalation: Tracking resistance to EGFR inhibition for predicting therapeutic sensitivity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2701.