A Platform of CDK4/6 Inhibitor-Resistant Patient-Derived Breast Cancer Organoids Illuminates Mechanisms of Resistance and Therapeutic Vulnerabilities

San Antonio Breast Cancer Symposium 2021 Presentation
Authors Ariella B. Hanker, Sumanta Chatterjee, Yunguan Wang, Dan Ye, Dhivya R. Sudhan, Lauren Smith, Yilin Zhang, Vishal Kandagatla, Kuntal Majmudar, Ezequiel Renzulli, Saurabh Mendiratta, Kimberly Blackwell, Alana L. Welm, Sunati Sahoo, Nisha Unni, Cheryl Lewis, Tao Wang, Ameen Salahudeen, Carlos L. Arteaga

CDK4/6 inhibitors (CDK4/6i) in combination with antiestrogens have revolutionized the treatment of ER+ metastatic breast cancer (MBC), significantly prolonging survival. However, this combination is not curative, and tumors eventually acquire resistance. Following progression on this combination, patients are left with limited treatment options. A diverse array of mechanisms of resistance to CDK4/6i + antiestrogens have been described. However, laboratory models that capture this heterogeneity of resistance mechanisms are lacking. Patient-derived organoids (PDOs) provide a rapid, robust and reliable platform that recapitulates intra-tumor heterogeneity, partially mimics the cancer microenvironment, and accurately predicts drug response. We aspired to generate a platform of CDK4/6i-resistant breast cancer PDOs to serve as models for understanding acquired resistance to CDK4/6i + antiestrogens and identifying therapies to overcome resistance.

We successfully established 16 PDOs out of 32 biopsies (50% efficiency) of metastates from patients with ER+ MBC progressing on CDK4/6i (palbociclib or abemaciclib) + antiestrogens (letrozole or fulvestrant; median response to combination = 9 months). Our collection includes PDOs derived from lobular (n=3) and inflammatory (n=2) breast cancers and reflects racial/ethnic diversity (50% white/not Hispanic; 18.8% Hispanic; 12.5% Black; 12.5% other/unknown). Next-gen sequencing reports were available for 10 patients from which organoids were established, revealing alterations associated with CDK4/6i and/or antiestrogen resistance, including ESR1 (n=2), HER2/ERBB2 (n=2), PTEN (n=2), CCNE1 (n=1), NF1 (n=1), and ARID1A (n=1). Furthermore, one biopsy and its derived organoid lost ER expression, and 5 harbored PIK3CA activating mutations. Thus far, we have performed targeted DNAsequencing on 7 PDOs and found 13/15 (86.7%) concordance with driver mutations from tumor NGS reports. PDOs established from CDK4/6i-resistant biopsies maintained resistance to palbociclib or abemaciclib ± fulvestrant (500 nM each) in 3D cell viability assays (6 days of treatment). In contrast, control PDOs established from primary ER+ breast cancer surgical samples (n=2) were sensitive to each CDK4/6i ± fulvestrant (median viability for combination=25.6-31.5% for control vs 65.2-80.5% for resistant). GSEA analysis of RNA-seq data from control (n=2) and CDK4/6i-resistant (n=6) PDOs cultured in estrogen-depleted media ± 200 nM palbociclib revealed that palbociclib treatment resulted in downregulation of E2F target and G2M checkpoint signatures in control but not resistant PDOs. Next, we performed a high-throughput screen of 1,000 compounds in 3 resistant PDOs. One PDO showed exquisite sensitivity to G2/M cell cycle checkpoint components, including CDK1, PLK1, Aurora kinase, ATR, Chk1, and Wee1 inhibitors. Finally, treatment of 10 resistant PDOs with the CDK2/4/6 inhibitor PF-06873600 revealed that the CCNE1 (cyclin E1)-amplified PDO was highly sensitive (IC50=130 nM vs >1000 nM), supporting that CCNE1-amplified tumors are vulnerable to CDK2 inhibition.

Conclusions: PDOs can be successfully established from ER+ MBC biopsies, maintain the resistant phenotype in culture, retain driver alterations found in tumors from which they were derived, and fail to suppress E2F targets following treatment with CDK4/6i. Therefore, these PDOs represent valuable models to understand and explore diverse mechanisms of CDK4/6i resistance and therapeutic vulnerabilities.