Abstract P4-14-19: ONT-380 in the treatment of HER2+ breast cancer central nervous system (CNS) metastases (mets)

Abstract

Abstract Background: The risk of CNS involvement in patients with HER2+ metastatic breast cancer (MBC) is high. The natural history of HER2+ CNS metastases (mets) is different from other breast cancer subtypes; there is a longer time from metastatic diagnosis to CNS relapse, greater control of extracranial disease at the time of CNS relapse, and longer median overall survival from the time of CNS relapse. Local treatments, i.e., surgery and/or radiation, remain the mainstay of treatment for HER2+ CNS disease as standard systemic therapy options have limited efficacy. ONT-380, a potent HER2 selective tyrosine kinase inhibitor with minimal EGFR-like side effects, has been associated with increased survival compared to lapatinib or neratinib in an animal model of HER2+ CNS disease. Here, we describe 24 pts from two studies with response-evaluable CNS mets treated with ONT-380 in combination with other systemic therapies. Methods: Pts with asymptomatic untreated or post-treatment progressive CNS mets were enrolled in ongoing phase 1b studies of ONT-380 + ado-trastuzumab emtansine (T-DM1) or ONT-380 ± trastuzumab (T) ±capecitabine (C). All pts received treatment in 21 day cycles including ONT-380 300 mg PO BID and approved doses of either T-DM1, T or C alone, or T+C. Eligibility criteria for all pts included prior treatment with T and a taxane, and for pts receiving T and/or C, prior T-DM1. Prior lapatinib was allowed. Assessments included safety, systemic tumor response per RECIST 1.1, and CNS tumor response by MRI every 2 cycles per modified RECIST 1.1. Results: 24 pts (10 with untreated CNS mets and 14 with progressive CNS mets after local therapy) received ONT-380 plus T-DM1 (n =14), C (n=1), T (n = 5) or T+C (n = 4) for 1-8 cycles. Of these 24 pts, 14 are evaluable for response in the CNS (at least one follow-up MRI); 7 pts have not yet been rescanned, and 3 are non-evaluable. In the 14 response-evaluable pts, best CNS response has been: 1 CNS CR (T-DM1), 4 CNS PR (T-DM1 n = 2; T+C n = 1; C n=1) and 9 CNS SD (T-DM1 n = 5; T n = 3; T+C n=1). Pts with CR or PR (prior lapatinib n=2; prior pertuzumab n=2; prior T-DM1 n=2) all had > 50% decrease in CNS target lesions. One CNS non-evaluable pt (T) had a 15% increase in their target lesion and underwent surgical resection; pathology, however, revealed no viable tumor with only necrotic tissue present. Two additional CNS non-evaluable pts (T-DM1 n=1; C+T n=1) were taken off study due to systemic PD after treatment was held for AEs (Gr 3 AST/ALT elevation [n=1]; Gr 4 edema in thalamic lesion [n=1]). No other ≥ Gr 3 ONT-380 related events were reported. Systemic disease control was also seen, with a best response in 17 evaluable pts of 6 PR, 9 SD, and 2 PD. Conclusions: This case series demonstrates early clinical signs of activity of ONT-380 against HER2+ CNS mets in combination with other systemic agents. Further study of the CNS activity of ONT-380 is ongoing and further studies are being planned. Updated results will be reported. Clinical trial information: NCT01983501, NCT02025192. Citation Format: Murthy RK, Hamilton E, Borges VF, Moulder S, Aucoin N, Welch S, Chaves J, Falkson CI, Walker L, Ferrario C. ONT-380 in the treatment of HER2+ breast cancer central nervous system (CNS) metastases (mets). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-14-19.