Abstract PS2-08-20: Translational analysis of cerebrospinal fluid (CSF) and plasma circulating tumor DNA (ctDNA) from breast cancer patients (pts) with leptomeningeal disease (LMD) treated with trastuzumab deruxtecan (T-DXd) in the DEBBRAH trial

Abstract Rationale: LMD represents a severe and life-threatening complication of metastatic cancer, historically associated with extremely poor outcomes. Therapeutic options are limited, and these pts are routinely excluded from clinical trials investigating novel systemic therapies. Radiological response assessment in pts with LMD is challenging and CSF ctDNA has emerged as a potential minimally invasive biomarker for the diagnosis and disease monitoring of LMD. The DEBBRAH trial was a multi-cohort phase II study assessing T-DXd in HER2-positive and HER2-low breast cancer pts with brain metastases and/or LMD, with Cohort 5 dedicated to LMD. The primary endpoint of overall survival indicated antitumour response (median OS 13.3 months) in this cohort. This translational analysis aims to evaluate intra- and extracranial T-DXd activity through serial ctDNA measurement in CSF and plasma collected from pts in DEBBRAH cohort 5, and to determine molecular features associated with response and treatment resistance. Methods: Seven pts with cytology positive LMD were enrolled in DEBBRAH cohort 5 and received T-DXd 5.4 mg/kg intravenously every 21 days. Radiological response was measured by RECIST v.1.1 for extracranial disease and RANO-BM for intracranial lesions. CSF was collected at baseline from six pts (in one case, collection occurred after cycle 1), and serially prior to each T-DXd dose up to cycle 7 in four pts (three HER2-positive and one HER2-low). Plasma was obtained at baseline and prior to cycle 2 in all pts, and at end of treatment in four pts. Both sample types were collected in Streck Cell-Free DNA BCT tubes and centrifuged to remove cellular content, prior to cell-free DNA (cfDNA) extraction. cfDNA underwent shallow WGS (median 0.5X coverage), for large scale copy number calling and tumor fraction estimation by ichorCNA. Results: CSF ctDNA was detected in all baseline samples (tumor fraction range 0.11 - 0.83), whereas plasma ctDNA was only detected in three of seven pts (tumor fraction range 0.06 - 0.82). All four pts with serial CSF samples showed a reduction in ctDNA tumor fraction during T-DXd treatment, and two of them achieved complete CSF ctDNA clearance within 1 to 2 treatment cycles. The two pts with CSF ctDNA complete clearance were HER2-positive and received 7 and 12 cycles of T-DXd, respectively. In both cases, CSF remained ctDNA-negative at the end of CSF sampling (cycle 7) and treatment was discontinued due to extracranial progression according to RECIST v.1.1. Of the remaining two pts with serial CSF samples, one HER2-positive pt achieved a 15% reduction in ctDNA from baseline, completed 28 cycles of T-DXd, and remains alive at 18.5 months. The other, a HER2-low pt, had a 59% reduction in ctDNA from baseline and completed 15 cycles of T-DXd before developing extracranial progression according to RECIST v.1.1. An increase in plasma ctDNA tumor fraction was observed in three of the four patients with samples taken at the end of treatment, suggesting progression of extracranial disease.Mutational analysis to determine molecular variants associated with treatment response and resistance is being performed, and results will be displayed at the conference. Conclusion:These findings suggest that CSF ctDNA offers superior sensitivity over plasma ctDNA for detecting LMD. Although CSF ctDNA clearance was observed, which could be associated with LMD response, these data could not be correlated clinically since LMD-specific radiological and clinical assessment tools were not used in this trial. CSF ctDNA appears to be a promising response assessment biomarker for LMD in clinical trials, and merits further study in larger cohorts, incorporating LMD-specific assessment tools. Citation Format: A. Fitzpatrick, T. Shanmugalingam, S. Haider, M. Iravani, L. Garrigós, J. García-Sáenz, P. Cortez, F. Racca, S. Blanch, M. Ruiz-Borrego, G. Martrat, M. Mancino, J. Guerrero, J. Pérez-García, M. Vaz Batista, S. Braga, A. Llombart-Cussac, J. Cortés. Translational analysis of cerebrospinal fluid (CSF) and plasma circulating tumor DNA (ctDNA) from breast cancer patients (pts) with leptomeningeal disease (LMD) treated with trastuzumab deruxtecan (T-DXd) in the DEBBRAH trial 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 PS2-08-20.