Abstract B02: Co-clinical trial of olaparib in breast and ovarian patient-derived tumor xenografts (PDX) enables the identification of response biomarkers (2016)

Abstract Background: Women with heterozygous germline mutations in BRCA1 or BRCA2 are predisposed to developing breast or ovarian cancers, since BRCA1/2 mutation results in impaired high-fidelity DNA repair by homologous recombination (HR) and subsequently genetic instability. In non-gBRCA TNBC, HR deficiency occurs at the somatic level, by means of BRCA1 mutation, BRCA1 epigenetic loss or mutation in other HR-associated genes. PARP1/2 inhibitors (PARPi) are active anti-cancer agents in gBRCA with advanced breast or ovarian cancer. However, not all HR-deficient tumors respond to PARP blockade, and eventually all develop acquired resistance. Here, we sought to identify PARPi response biomarkers using PDX derived from both the early disease and the metastatic setting. Methods: We developed a panel of PDX from patients harboring or not germline BRCA1/2 mutations, namely from 26 primary or advanced breast cancer and 2 high-grade serous metastatic ovarian cancer (HGSOC). The antitumor activity of the PARP1/2 inhibitor olaparib as single agent (50 mg/kg) was assessed in all models. To study the mechanisms of acquired resistance, the olaparib-sensitive PDXs were exposed to olaparib for &amp;gt;100 days, until individual tumors regrew. The tumor's capacity to repair DNA double strand breaks was estimated by quantification of the BRCA1 and RAD51 nuclear foci in the S/G2-phase of the cell cycle. We investigated the correlation between the tumor's BRCA1/RAD51 foci formation and sensitivity to olaparib, and also identified potential genetic modifiers of PARPi sensitivity by targeted sequencing. Results: Seven out of 28 PDX (25%) treated with single agent olaparib exhibited tumor regression or disease stabilization. Among the non-gBRCA PDX, BRCA1 hypermethylation or PALB2 mutation were present in olaparib-sensitive PDX. No genetic reversions in BRCA1/2 mutations were identified as the mechanism of olaparib resistance in gBRCA but BRCA1 foci formation was observed in 6 out of 10 resistant models (60%). Nuclear RAD51 foci formation correlated with PARPi resistance in twenty PDX models investigated, either with primary or acquired resistance. The duration of response was similar between gBRCA and non-gBRCA PDX. Acquired-resistance mechanisms involve restoration of HR functionality. Conclusions: Our study highlights that somatic HR-deficiency is frequent in TNBC and provides the basis of sensitivity to PARPi. In our gBRCA (n=12) and non-gBRCA (n=16) PDX, reactivation of HR functionality measured as RAD51 foci formation is a frequent event that is associated with PARPi resistance. Citation Format: Violeta Serra, Cristina Cruz, Marta Castroviejo, Sara Gutiérrez, Alba Llop, Alejandra Bruna, Beatriz Morancho, Ginevra Caratú, Ludmila Prudkin, Ana Vivancos, Paolo Nuciforo, Mark O'Connor, Jos Jonkers, Carlos Caldas, Joaquin Arribas, José Baselga, Isabel T. Rubio, Cristina Saura, Orland Díez, Judith Balmaña, Javier Cortés. Co-clinical trial of olaparib in breast and ovarian patient-derived tumor xenografts (PDX) enables the identification of response biomarkers. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr B02.