Abstract Background: Trastuzumab + pertuzumab + taxane and ado-trastuzumab emtansine (T-DM1) are standard first and second-line therapies for HER2+ metastatic breast cancer. Lapatinib is approved in combination with capecitabine in trastuzumab-resistant patients and in combination with letrozole in hormone receptor positive HER2+ pts for whom endocrine treatment is indicated. In Italy, L is also approved in combination with trastuzumab for hormone receptor-negative HER2+ pts. There are only few data on the activity of lapatinib in pts who received prior pertuzumab and /or T-DM1. Methods: We retrospectively analysed HER2+ metastatic breast cancer pts who received lapatinib after prior pertuzumab and/or T-DM1 from July 2013 to June 2018. Objective response rate (ORR) was assessed according to RECIST 1.1. Progression-free survival (PFS) was calculated from lapatinib-based therapy starting to disease progression (PD) or last follow-up. Overall Survival (OS) was calculated from lapatinib-based therapy starting to death or last follow up. Results: Data from 32 HER2+ mBC treated with lapatinib-based therapy were recorded: 30 pts (94%) received lapatinib combined with capecitabine, 1 pt received lapatinib combined with letrozole and 1 pt received lapatinib combined with trastuzumab. All patients had been treated with prior T-DM1 and 9 (28%) with prior pertuzumab for metastatic breast cancer. Setting of treatment with lapatinib-based therapy was: 2° line (n=2, 6%), 3° line (n=17, 53%) and >4° line (n=13, 41%). Patients characteristics were as follows: median age 55 years (range 35-71), hormone receptor positive 66% (n=21), stage IV at diagnosis 34% (n=11), visceral involvement at lapatinib-based therapy starting 91% (n=29). As of June 2018, lapatinib-based therapy was ongoing for 4 pts and 28 pts discontinued treatment due to disease progression (median number of courses for these pts was 8.5, range 2-27). ORR in evaluable pts was: complete response (n=1, 3%), partial response (n=13, 41%), stable disease (n=9, 28%), disease progression (n=9, 28%). Median PFS was 6.4 months (95% CI 3.0-9.8). As of June 2018, 14 pts (44%) were alive, median OS was 11.8 months (95% CI 9.9-13.6). Conclusion: These results confirm that lapatinib-based therapy retains clinical efficacy in HER2+ metastatic breast cancer pts treated with prior pertuzumab and/or T-DM1 and represents a valid therapeutic option in this setting. Citation Format: Frezzini S, Giarratano T, Dieci MV, Giorgi CA, Griguolo G, Vernaci G, Menichetti A, Mantiero M, Tasca G, Faggioni G, Falci C, Miglietta F, Mioranza E, Angelini S, Ghiotto C, Conte P, Guarneri V. Lapatinib-based therapies after pertuzumab and/or T-DM1 for HER2+ metastatic breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-18-24.