Abstract P6-04-12: STAT3 as a therapeutic target in estrogen receptor positive breast cancer patients refractory to CDK4/6 inhibition
Authors: Kettner Nicole M , Bui Tuyen , Ha Min Jin , Eckols T Kris , Tweardy David J , Meric-Bernstam Funda , Hunt Kelly K , Tripathy Debu , Keyomarsi Khandan 
Published: March 2020

Abstract
Background: CDK4/6 inhibitors (i.e. palbociclib) in combination with endocrine therapy (ET) are currently standard of care for estrogen receptor (ER) positive breast cancer patients with advanced/metastatic disease. While this combination has proven successful in delaying progression, no improvement in long-term survival has been observed to date in the post-menopausal setting. Further, resistance is inevitable as 50-60% of patients’ metastatic lesions progress on CDK4/6 inhibitor therapy within 2-years of treatment. For patients experiencing resistance to CDK4/6 inhibitors, novel treatment strategies are needed to delay progression or to improve survival. Our recently published data shows that palbociclib-resistance is marked by significant upregulation of the IL6- STAT3 signaling pathway in ER-positive breast cancer cells. Knockdown of STAT3 in resistance cancer cells restored sensitivity to palbociclib. Additionally, matched biopsies from advanced ER-positive breast cancer patients who progressed on palbociclib showed upregulation in p-STAT3 as compared to their pre-treatment biopsy samples (14 out of 25 patients; p=0.042). Collectively, these data suggest that STAT3 is viable therapeutic target to overcome palbociclib resistance. Hence, the goal of this study is to translate these findings in vivo and provide pre-clinical rational to the efficacy of TTI-101 (an orally bioavailable STAT3 inhibitor) in PDX models of palbociclib resistance.
Methods: To identify PDX models that recapitulate palbociclib resistance observed in cell line models, we correlated two published RNA-seq data sets of a panel of 64 PDX models from breast cancer patients to our multi-omics analysis of palbociclib resistant cell lines. Four PDXs that recapitulated the resistant-transcriptome expression pattern based on Spearman’s correlation, were narrowed down for future studies. In addition, we have also established PDX models (1st-4th passage) from two sets of patients (1) those who are intrinsically resistant to palbociclib (the patients progressed at 2-3 months while on palbociclib + ET) and (2) those who have developed resistance over time (i.e. acquired resistance-patients progress at 12-18 months while on palbociclib + ET).
Results: We chose PDX model BCX94 for our preliminary studies as it showed baseline induction of p-STAT3 by immunoblot analysis. Using BCX94, we show that TTI-101 at 50mg/kg twice a day potently reduced tumor volume and improved survival of these mice. We also observed a significant reduction in tumor-derived IL-6 in circulation; as assayed by hIL-6 levels in the serum. Further, complete blood count analysis of whole blood collected at endpoint (i.e. 28 days on treatment) showed no drug toxicities. Treatment of intrinsic palbociclib resistant PDX models with TTI-101, on the other hand, showed only a brief delay in tumor progression, but no overall benefit. Conclusions: Collectively, these results indicate that TT1-101 is safely tolerated and efficacious at the dose examined in PDX models and that TTI-101 may be a suitable target for those tumors that are resistant to palbociclib. The differences in response to TTI-101 in the BCX94 versus our intrinsic PDX models, suggest that acquired vs. intrinsic resistance signatures of palbociclib may be mutually exclusive and that the IL-6-STAT3 signaling axis may be a driver in acquired palbociclib resistance, but not the intrinsic setting. Our ongoing studies are therefore geared towards testing the efficacy of IL-6-STAT3 inhibition in both settings (i.e. acquired and intrinsic) and identifying distinct therapeutic vulnerabilities of intrinsic palbociclib resistance that may be unresponsive to STAT3 inhibition.
Citation Format: Nicole M Kettner, Tuyen Bui, Min Jin Ha, T Kris Eckols, David J Tweardy, Funda Meric-Bernstam, Kelly K Hunt, Debu Tripathy, Khandan Keyomarsi. STAT3 as a therapeutic target in estrogen receptor positive breast cancer patients refractory to CDK4/6 inhibition [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-12.