Predicting response to neoadjuvant therapy (NAT) in patients (pts) with early-stage breast cancer (BC) using circulating tumor DNA (ctDNA) testing.

569 Background: Recent studies have shown the utility of ctDNA testing for monitoring response and predicting long term outcomes in pts with BC receiving NAT. This pilot, single institution study (CIPHER, NCT05333874) evaluated whether trend of ctDNA during NAT can serve as an early indicator of treatment response and inform disease management in the adjuvant setting. Methods: This study included 35 pts with stage II-III triple negative (TN) and HER2+ BC and longitudinal ctDNA testing performed during standard of care NAT. A clinically validated, personalized, tumor-informed 16-plex PCR assay (Signatera) was used for the detection and quantification of ctDNA in blood samples collected at diagnosis (pre-treatment), every 3 weeks on NAT, and post-surgery. ctDNA status and dynamics were correlated with clinicopathological features and surgical outcomes. Results: At diagnosis (N=30), ctDNA was detected in 95% (18/19) of pts with TNBC [median 8.89 (0.09-180.01) mean tumor molecules (MTM)/mL] and 91% (10/11) of pts with HER2+ BC [median 1.55 (0.29-453.86) MTM/mL], including 73% (8/11) pts with HR+/HER2+ BC. ctDNA detection rate was higher in pts with clinical T3-4 disease compared to T1-2 (TNBC: 4/4, 100% vs 14/15, 93%; HER2+ BC: 4/4, 100% vs 6/7, 86%). Similarly, higher median pre-treatment ctDNA levels were observed in pts with clinical T3-4 compared to T1-2 (TNBC 61.49 vs 9.28 MTM/mL; HER2+ BC 14.94 vs 1.18 MTM/mL). Pts with early ctDNA clearance (<6 weeks of NAT) had a higher proportion of favorable surgical outcomes (pCR or <1 cm, node negative) compared to pts with late (>6 weeks) or no clearance during NAT (17/20, 85% vs 4/6, 67%). Post-surgical ctDNA-positivity was observed in 5% (1/19) vs 67% (4/6) pts with early vs late/no clearance during NAT, respectively. Post-surgery ctDNA detection rate was higher in pts with stage III disease compared to stage II (4/12, 33% vs 1/13, 8%). Interestingly, 20% (2/10) pts with stage III disease vs 9% (1/11) with stage II disease had detectable ctDNA after surgery despite favorable surgical findings. Higher pre-treatment ctDNA levels were associated with higher likelihood of ctDNA positivity post-surgery: <1 MTM/mL: 0/4, 0%; 1-10 MTM/mL: 1/9; 11%; >10 MTM/mL: 3/11, 27% (p=0.0025 χ2). ctDNA positivity post-surgery led to change in disease management for two pts. The first pt (inflammatory HR-HER2+ BC), despite achieving pCR, remained ctDNA positive after NAT and surgery, which informed a change in adjuvant treatment. For the second pt (TNBC), ctDNA positivity post-surgery led to detection of radiographically confirmed asymptomatic metastatic relapse and treatment change. Conclusions: ctDNA dynamics during NAT can facilitate real-time assessment of treatment response and can serve as an early indicator of pathologic response, predict ctDNA status after surgery, and inform disease management for BC pts at high risk of recurrence. Clinical trial information: NCT05333874 .