Abstract PO1-15-12: Circulating tumor DNA (ctDNA) for prediction of pathologic complete response (pCR) after 12 weeks of pembrolizumab + trastuzumab + pertuzumab in HER2-enriched early breast cancer: WSG-Keyriched-1 trial

Background ctDNA testing is emerging as an important biomarker in early breast cancer (eBC). However, its value in prediction of tumor response to de-escalated, chemotherapy-free neoadjuvant therapy (NAT) remains underexplored. In WSG-Keyriched-1 (NCT03988036), a single-arm phase 2 trial, we investigated for the first time a chemotherapy-free NAT with dual HER2 blockade and pembrolizumab in HER2- enriched HER2+ eBC. In this pre-specified translational analysis, we evaluated whether ctDNA measurement could predict pCR. Methods 48 patients (pts) with HER2 2+ (ISH+) or 3+ eBC (stage I-III) and HER2-enriched subtype by PAM50 were included. Pts received pembrolizumab (200 mg), trastuzumab biosimilar (loading dose (LD) 8 mg/kg bodyweight (BW), maintenance dose (MD) 6 mg/kg BW), and pertuzumab (LD 840 mg/kg BW, MD 420 mg/kg BW) q3w for 12 weeks. Primary objective was pCR (ypT0/is ypN0). ctDNA was analyzed in 92 plasma samples collected from 31 pts at baseline (BL) and week 3 of NAT and 30 pts at end of treatment (EOT). Sequencing libraries with unique molecular identifiers were constructed from cell-free DNA and hybridization panels with ≤50 patient-specific somatic mutations from tumor sequencing were used for enrichment. Libraries were sequenced to an ultra-high depth of 100,000×. Sequencing data was analyzed using a combination of public pipelines (megSAP and umiVar2 for mapping and deduplication) and custom in-house scripts (to extract the allele counts for patient-specific variants). To reduce the error rate, only corrected reads with ≥4 duplicates were used. p-value for ctDNA detection was calculated using the total variant count, depth and sample-specific error. Association between ctDNA with pCR and other clinical parameters were assessed with Chi-square or Fisher’s exact test and univariable logistic regression. Additionally, bivariable logistic regressions for pCR were performed with ctDNA and either cT, cN, or grade. Results ctDNA was detected (ctDNA+) in 58.0% (n=18/31) of pts at BL, 9.7% (n=3/31) at week 3, and 10% (n=3/30) at EOT. ctDNA was cleared by week 3 in 83.3% of pts (n=15/18). Compared with ctDNA-negative cases (ctDNA-) at BL, those ctDNA+ more often had cT2-3 disease (94.4%, n=17/18, vs 38.5%, n=5/13; p=.001) and lymph node involvement (61.1%, n=11/18, vs 0%, n=0/13; p< .001). 72.2% (n=13/18) of ctDNA+ and 61.5% (n=8/13) of ctDNA- cases at BL were grade 3 (p=.53). Each of the 3 pts who remained ctDNA+ at week 3 was node-positive; all pts remaining ctDNA- at week 3 were node-negative. pCR rate was 38.9% (n=7/18) in ctDNA+ vs 76.9% (n=10/13) in ctDNA- pts at BL (p=.067), 0% (n=0/3) in ctDNA+ vs 60.7% (n=17/28) in ctDNA- at week 3 (p=.081), and 33.3% (n=1/3) in ctDNA+ vs 59.3% (n=16/27) in ctDNA- at EOT (p=.565). pCR rate was highest in pts who remained ctDNA- throughout week 3 (76.9%, n=10/13), compared to pts with ctDNA cleared by week 3 (46.7%, n=7/15), and pts remaining ctDNA+ (0%, n=0/3, p=.024). ctDNA at BL was predictive for pCR in univariable analysis (odds ratio, OR 0.22, 95%CI 0.04- 0.93, area under curve, AUC=0.69). Bivariable models including ctDNA and grade yielded the highest accuracy for analyses with ctDNA at BL (AUC=0.77), week 3 (AUC=0.79), and with ctDNA change from BL to week 3 (AUC=0.87). Conclusions Absence of ctDNA prior to and during NAT, as well as early clearance of ctDNA associates with a higher pCR rate in eBC pts with HER2-enriched intrinsic subtype treated with chemotherapy-free pembrolizumab plus dual HER2 blockade. These results are in line with prior data with more intensive therapy from I-SPY 2 and NeoALTTO trials in HER2+ eBC and indicate that ctDNA analysis during NAT appears feasible for real- time monitoring of response to treatment and could be used to guide escalation/de-escalation strategies. Further trials with a larger number of pts and pre-designed follow-up are needed to confirm the role of ctDNA for prediction of tumor response and relapse. Citation Format: Monika Graeser, Sherko Kuemmel, Oleg Gluz, Christopher Schroeder, Leon Schuetz, Olga Kelemen, Stephan Ossowski, Katarzyna Jozwiak, Mattea Reinisch, Athina Kostara, Iris Scheffen, Kerstin Lüdtke-Heckenkamp, Felix Hilpert, Angela Kentsch, Carsten Ziske, Reinhard Depenbusch, Michael Braun, Jens-Uwe Blohmer, Matthias Christgen, Stephan Bartels, Hans-Heinrich Kreipe, Enrico Pelz, Ulrike Nitz, Peter Schmid, Nadia Harbeck, Andreas Hartkopf. Circulating tumor DNA (ctDNA) for prediction of pathologic complete response (pCR) after 12 weeks of pembrolizumab + trastuzumab + pertuzumab in HER2-enriched early breast cancer: WSG-Keyriched-1 trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-15-12.