LAUSR

Background: Sacituzumab Govitecan (SG) is an antibody-drug conjugate targeting Trop-2 and has been approved for use in breast and bladder cancers. Trop-2 has been shown to be overexpressed in multiple solid tumors including prostate cancer. Here we report an interim analysis of longitudinal liquid biopsies collected in a phase II clinical trial of SG to identify biomarkers of response and resistance. Methods: We collected longitudinal blood samples from 20 patients for circulating tumor cell (CTC) and cell-free DNA (cfDNA) analysis as part of an open label Phase II trial of SG in patients with mCRPC (ClinicalTrials.gov Identifier: NCT03725761). Eligibility criteria for the trial included rising PSA and radiographic progression on abiraterone acetate or enzalutamide in the mCRPC setting. Liquid biopsies were collected at baseline, C1D8, and then every 3 cycles until disease progression. cfDNA was sequenced using a custom 822 gene targeted capture panel (IDT). CTCs were isolated with antibodies to either EpCAM or Trop-2 followed by parallel enumeration, EpCAM and Trop-2 protein phenotyping and RNA extraction for AR/NEPC marker transcriptional profiling. Results: Although no PSA50 responses were observed, the 6-month rPFS rate was 45% (64% in chemotherapy naïve patients). For this analysis, these patients will be termed responding patients. The number of Trop-2-postive CTCs at baseline was significantly higher in responding (24.3 ±13.4 per 7.5 mL blood) than non-responding (10.0±3.8) patients. During treatment the number of Trop-2 positive CTCs in responders significantly decreased by C3D1 to 10.0 ±3.8. Interestingly, while the number of Trop-2 positive CTCs continued to decline to 2.8±1.99 cells/7.5 mL, EpCAM-positive CTCs increased from 12.5±10.52 cells/7.5 mL to 23.2±19.49 at end of trial. Gene expression analysis demonstrated high expression of genes specific to adenocarcinoma including AR-splice variants, TMPRSS2, KLK2 and KLK3 in the majority of patients, with conversion to AR-variant negative CTCs identified in responders. Preliminary cfDNA analysis identified one patient with a mutation acquired in the gene encoding topoisomerase 1 (TOP1) upon progression on SG, with additional analysis ongoing. Conclusions: Patients who responded to SG had a more Trop-2 positive CTCs with adenocarcinoma characteristics with expression of AR pathway genes. Longitudinal analysis of liquid biopsy biomarkers identified multiple mechanisms of acquired SG resistance. Decline in Trop2-CTC population with parallel expansion of EpCAM-CTC population in multiple patients suggests loss of Trop2 surface protein expression as resistance mechanism, while identification of a cfDNA TOP1 mutation in one patient at time of progression is consistent with acquired resistance to the topoisomerase inhibitor payload as has been reported in metastatic triple negative breast cancer. Citation Format: Jamie M. Sperger, Amy K. Taylor, Yue Shi, Charlotte N. Stahlfeld, Kyle Helzer, Matthew Bootsma, Katherine R. Kaufmann, Marina N. Sharifi, Christos E. Kyriakopoulos, Susan Slovin, Scott Tagawa, Scott Dehm, Shuang G. Zhao, Joshua M. Lang. Liquid biopsy biomarker analysis of a phase II trial of sacituzumab govitecan in castrate resistant metastatic prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B016.