Background: This study evaluated whether there were racial disparities in utilization (decomposed as change in proportion of eligible patients initiating therapy over time and time to treatment initiation (TTI)) to CDK4/6 inhibitors and pertuzumab since FDA approval as first line (1L) treatment for ER+/HER2- and HER2+ metastatic breast cancer (MBC), respectively. Methods: This cohort study used the nationwide Flatiron Health electronic health record-derived de-identified database. Included women were ≥18 years old, diagnosed with de novo or recurrent MBC, who received 1L therapy, and documented as non-Hispanic White (NHW) or non-Hispanic Black/African American (NHB). Two cohorts were generated for patients with ER+/HER2- MBC diagnosed 3/2015-10/2021 and HER2+ MBC diagnosed 7/2012-9/2021. We estimated temporal trends in the proportion of NHW and NHB patients receiving respective therapy using logistic regression with natural cubic splines for time trends and tested for changes in uptake over time within each group as well as differences in trends between groups. A similar model was used for TTI among treatment initiators, using linear regression models. In addition to time and race, regression models included age, ECOG performance status, insurance, Medicaid expansion status, practice type, and proportion of black patients per practice among patients who met both cohort criteria. Results: 8318 patients (NHW=7006; NHB=1312) met ER+/HER2- cohort criteria and 2321 (NHW=1915; NHB=406) met HER2+ cohort criteria. There was a significant change over time in the proportion of NHW (p<0.0001) and NHB (p<0.0001) initiating 1L CDK4/6 inhibitors (Table). Temporal trends were also significantly different between NHW and NHB (p<0.0001). Higher ECOG performance status (OR=0.85 per unit increase, 95%CI 0.80-0.90; p<0.001) and an increasing proportion of black patients seen at practice (OR=0.45, 95%CI 0.29-0.71; p<0.001) were independent predictors of lower odds of initiating CDK4/6 inhibitors. Overall, 43% NHW vs 41% NHB received pertuzumab respectively. There was no significant difference in uptake trends for pertuzumab between NHW and NHB (p=0.41). There was no significant difference in the temporal trends of TTI for either CDK4/6 inhibitors (p=0.74) or pertuzumab (p=0.84). Conclusions: Utilization of CDK4/6 inhibitors steadily increased over time, however > 25% of eligible patients did not receive the drug, and less than half of individuals who met indication for pertuzumab were prescribed it. Racial disparities were significant for oral CDK4/6 inhibitors but not for intravenous pertuzumab.