Background: TreatER+ight is the 1st prospective observational study in Canadian HR+ HER2- advanced breast cancer patients currently receiving endocrine therapy (ET) alone or in combination with targeted therapy (TT) (NCT02753686). Methods: This exploratory analysis displays real-world evidence on baseline demographics, treatment patterns, sequencing, and method of access to therapy. More specifically this abstract focuses on treatment decisions around CDK4/6-based therapy at various lines of treatment. At data cut-off of April 1st 2019, 305 patients were enrolled from 24 sites since Mar'16 with 113 patients receiving CDK4/6is at baseline. 300 patients were evaluable for treatment analysis. Results: Baseline CharacteristicsOverall (n=305)CDK4/6 + ET (n=113)Median age, years (range)67 (23 - 92)67 (23-87)ECOG 0,1,2*95, 104, 1941,32, 4Post-menopausal*21468Pre/peri-menopausal*6530Therapy by disease severity at baselineVisceral (n = 196)Bone only (n = 101)CDK4/6 + ET7338ET5941mTOR + ET6421PI3K + ET01Current therapy on the study Overall (N = 300)CDK4/6 + ET113ET100mTOR + ET86 PI3K + ET1Current CDK therapy by line of treatmentAll CDK4/6 (N = 113)First line60Second line38Third line13 Fourth line2Overall treatment patterns by lineFirst-Line (n = 154)Second-Line (n = 111)Third-Line (n = 64)CDK 4/6i + ET855317ET582711mTOR + Exemestane82114Chemo31022Median duration on therapy in months1L2LCDK4/6 + ET8.87 (median follow-up of 9.6 mos)8.77 (median follow-up of 11.6 mos)*Data missing for some patients at the time of this analysis Conclusions: The majority of patients enrolled (66.6%) are receiving combination therapy with the most common treatment being a CDK4/6-based therapy (56.5%). Of the patients receiving CDK4/6 at baseline a majority are receiving this therapy in the 1L (53.1%) and 2L (33.6%), with the median duration on therapy of respectively 8.87 and 8.77 months. For patients receiving CDK4/6 therapy in 1L, the most common 2L treatment is either endocrine therapy single agent and in certain parts of the country everolimus + exemestane. Further sequencing information will be analyzed and shared for this abstract. Citation Format: Catherine Doyle, Nadia Califaretti, Cristiano Ferrario, Nayyer Iqbal, John Hilton, Theodore Vandenberg, Mihaela Mates, Vivian Glenns, Swati Kulkarni, Puneet Bains, Sina Haftchenary, Sabrina Perri, Stephen Chia. Exploratory analysis of TreatER+ight; a Canadian prospective real-world observational study in HR+ advanced breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-36.