Background: Sacituzumab govitecan (SG) and trastuzumab deruxtecan (TDXd) are antibody drug conjugates (ADCs) that are approved for the treatment of HER2 low metastatic breast cancer (MBC). Both carry payloads which inhibit topoisomerase 1, and it is unknown whether these ADCs retain clinical activity when used sequentially to each other. Here we present a retrospective study of patients treated with both SG and TDXd at our institution and report real world progression free survival (PFS), clinical factors associated with longer PFS, and whether ADC sequence impacts outcome. Methods: All patients with MBC treated at MSKCC who had received both SG and TDXd (in any order) were eligible for inclusion. Clinicopathological and demographic data were obtained from retrospective chart review. PFS was determined clinically and calculated using Kaplan-Meier survival analysis. Univariate and multivariate associations of survival were assessed using Cox proportional hazards models. Results: Eighty-five patients met eligibility per the criteria above: 54 with estrogen receptor positive (ER+) MBC and 31 with triple negative breast cancer (TNBC). 33 patients received SG followed by TDXd (14 ER+, 19 TNBC), and 52 received TDXd followed by SG (40 ER+, 12 TNBC). Median age at time of second ADC treatment (ADC2) was 59 among all patients, 62 among SG first, and 58 among TDXd first. This cohort was heavily pre-treated, with median 5 prior therapies at the time of first ADC (ADC1) and 7 at the time of ADC2. Both the SG first and TDXd first cohorts had median 1 line of intervening therapy between ADCs. In the SG first group, median ADC2 (TDXd) PFS was 3.5 months (95%CI 2.7-7.7), and in the TDXd first group median ADC2 (SG) was 2.8 months (95%CI 2.6-3.7). On multivariate analysis, neither ADC sequence (SG first vs. TDXd first, HR 1.30, 95%CI 0.71-2.37, p=0.4) nor ER status (TNBC vs. ER+, HR 1.21, 95%CI 0.63-2.32, p=0.6) showed statistically significant differences in ADC2 PFS between these groups. Instead, later treatment line at the time of ADC2 was significantly associated with shorter ADC2 PFS (continuous, HR 1.11, 95%CI 1.01-1.23, p=0.03). To separately evaluate differences in PFS for each ADC depending upon whether it was first or follow-up therapy, median ADC1 vs. ADC2 PFS is summarized by ADC (Table). Conclusions: In this heavily pre-treated patient cohort, the clinical activity of both SG and TDXd appear modest in the ADC2 setting. Further studies are needed to confirm these findings and to determine whether payload cross-resistance may undermine the benefit of sequential ADC therapy.