Abstract PD10-12: Genomic analysis from the talazoparib beyond BRCA clinical trial: Homologous recombination (HR) deficiency scores, loss-of-heterozygosity and mutations in non-BRCA1/2 mutant tumors with other HR mutations

Background: Talazoparib, a PARP1 inhibitor, is active in germline BRCA1/2 mutant advanced HER2-negative breast cancer, but its activity beyond BRCA1/2 less well understood. The Talazoparib Beyond BRCA clinical trial treated 20 patients with germline (g) or somatic (s) mutations in HR pathways genes other than BRCA1/2, which included PALB2, CHEK2, ATM, BRIP1, RAD50, ATR, PTEN, and FANCA. Talazoparib treatment was associated with a 31% overall response rate in patients with breast cancer, including tumor regression from baseline as best response in 6 out of 6 patients treated gPALB2 mutations. We now present the results of additional genomic studies to further characterize treatment responses and resistance mechanisms. Methods: Primary and metastatic tumor biopsies were assessed using the commercially-available next-generation sequencing HRD assay (Myriad). Panel gene sequencing of tumors was performed for 108 genes associated with genomic instability. Loss-of-heterozygosity (LOH) was assessed in tumors. Tumor/normal exome sequencing identified pre-treatment somatic variants. Circulating-tumor DNA at baseline and disease progression was assessed by targeted panel sequencing and plasma whole exome sequencing. Results: Of the 18 HRD assays performed, 3 failed and were thus excluded. Seven patients had HRD analysis performed on both primary and metastatic tumors. For these patients the HRD score was significantly higher in the metastasis versus the primary (means 46.2 versus 36.5, p = 0.018 by paired t-test). By panel sequencing of tumors (n=18) the most common alterations detected included mutations in PIK3CA (n=8), PALB2 (n=6), ATM (n=5), KRAS (n=4), PTEN (n=5) and TP53 (n=4). In all cases except one, the HR-associated mutation used as entry criteria were detected. LOH analysis showed that of tumors with gPALB2 mutations, 3 of the 6 had LOH for PALB2, and an additional two tumors had 2 independent PALB2 mutations suggesting bi-allelic inactivation. The one remaining tumor had an uncertain LOH result due to test failure. Thus, it is likely that most, if not all, of the tumors in our cohort with gPALB2 mutations had complete inactivation of PALB2 gene function. Other detected mutations that were associated with LOH included all sTP53 mutations (n=4), all gCHEK2 mutations (n=3), gFANCA (n=1), all sRB1 mutations (n=3) and NF1 (n=1). Of the three gATM mutations one had LOH, while the others had two independent mutations, suggestive of bi-allelic inactivation. sATM mutations were associated with LOH in a breast cancer and with 2 independent (possibly bi-allelic) mutations in a non-breast cancer. Additional results from ctDNA targeted sequencing and plasma whole exome sequencing of baseline and progression samples will be presented. Conclusions: In this proof-of-concept phase II study, talazoparib demonstrated activity in HER2-negative advanced breast cancer patients with a HR pathway mutation beyond BRCA1/2, especially in patients with gPALB2 mutations and high HRD scores. These additional genomic analyses provide evidence that HRD scores may be significantly higher in metastatic samples compared to primary tumors. Also, we detected either LOH or bi-allelic inactivation for most HR-associated mutations used to determine eligibility. These findings may inform selection of patients for PARP inhibitor monotherapy beyond BRCA1/2. Citation Format: Joshua J Gruber, Anosheh Afghahi, Alyssa Hatton, Wyatt Gross, Jessica Foran, Alex McMillan, James M Ford, Melinda L Telli. Genomic analysis from the talazoparib beyond BRCA clinical trial: Homologous recombination (HR) deficiency scores, loss-of-heterozygosity and mutations in non-BRCA1/2 mutant tumors with other HR mutations [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD10-12.