Abstract Background: Metastatic triple-negative breast cancer (mTNBC) is a poor prognosis subset ofbreast cancer characterized by shorter survival and higher circulating tumor DNA (ctDNA) ‘tumorfraction’ (TF) relative to other breast cancer subtypes. TF is the proportion of ctDNA originatingfrom tumors cells, and early changes in ctDNA TF may correlate with therapy response,potentially offering a rapid response biomarker for early phase studies. Furthermore, there is littleknown regarding TF dynamics in the hours immediately after initiation of systemic therapy –including whether there is a ‘surge’ in ctDNA. Methods: Banked plasma samples were identified from four completed phase Ib/II clinical trialsenrolling patients with mTNBC: heat shock protein 90 (HSP90) inhibitor onalespib with paclitaxel(NCT02474173), MEK inhibitor trametinib alone and in combination with AKT inhibitorGSK2141795/uprosertib (NCT01964924), multi-kinase inhibitor cabozantinib monotherapy(NCT02260531), and JAK1/2 inhibitor ruxolitinib monotherapy(NCT01562873). For theonalsepib+paclitaxel study, plasma was collected pre-infusion, end-of-infusion (EOI), then0.5/1/2/4/6/8/24 hours post-infusion for 1) onalespib alone (day -7); 2) paclitaxel alone (day 1);and 3) onalespib + paclitaxel (day 8). For the remaining studies, plasma was collected pre-treatment on cycle 1 day 1 (C1D1) and prior to cycle 2 day 1 (C2D1). Plasma samples underwentshallow whole genome sequencing (sWGS) and TF determination. The primary objectives wereto evaluate change in TF and association with best overall response and progression-freesurvival. Results: To evaluate acute ctDNA TF change in the onalespib+paclitaxel trial, ctDNA TF wassuccessful in 313/317 (98.7%) of available plasma samples from 14 patients. There wassignificant decline in TF from pre-infusion to 6 hours for paclitaxel alone (Wilcoxon signed rankp=0.03) but no significant change for onalespib alone or onalespib + paclitaxel from pre-infusion to 6- or 24-hours. There was significant decline in TF from pre-infusion day -7 to cycle 1, day 9(median TF 16% to 6.5%, Wilcoxon signed rank p=0.004). No significant ‘surge’ was identifiedand despite significant decline in TF over the first full cycle of therapy. To evaluate change in TFacross multiple Phase Ib/II clinical trials of targeted therapies in mTNBC, we identified 57 totalpatients across studies (onalsepib+paclitaxel trial n=8; trametinib trial n=22; cabozantinib trialn=19; ruxolitinib trial n=8) with available plasma sample prior to C1D1 (T1) and follow-up plasmasample 14-28 days after therapy initiation (T2). Change in ctDNA from T1:T2 was significantlyassociated with best overall response (ANOVA p=0.01). Clearance of ctDNA at T2 (TF=0) wasassociated with significantly longer PFS (median PFS 10.3 months clearance vs. 2.3mo noclearance; log-rank p=0.013). Similarly, TF that was stable or declined T1:T2 was associated withsignificantly improved PFS relative to TF increase (log-rank p=0.003). Conclusion: There was no significant ‘surge’ in ctDNA TF within minutes to 24-hours of infusionof a targeted therapy, cytotoxic chemotherapy, or both in combination. Stability or decline ofctDNA TF 14-28 days after initiation of targeted therapy trial was associated with objectiveresponse and prolonged PFS. This supports further research on ctDNA dynamics immediatelyafter systemic therapy initiation, particularly in the context of early phase trials for mTNBC. Citation Format: B. To, V. Prasath, D. Veney, C. Rolfo, V. Adalsteinsson, N. Lin, S. M. Tolaney, H. A. Parsons, R. Wesolowski, D. Quiroga, D. Stover. Acute and Early Circulating Tumor DNA Dynamics in Metastatic Triple-Negative Breast Cancer Targeted Therapy Phase Ib/II Clinical Trials abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-01-15.
To et al. (Tue,) studied this question.