Antony Tin

Purpose:For older patients with competing comorbidities, optimizing oncologic therapies is of paramount importance. Circulating tumor DNA (ctDNA) is a validated prognostic factor across solid tumors and may provide a strategy to identify patients for whom safe de-escalation of certain therapies is possible.Experimental Design:In this prospective, hybrid-decentralized trial (n = 43 patients; NCT05914792) that integrated clinical outcomes, patient- and caregiver-reported outcomes, and correlative tissue analysis, the primary objective was to determine if ctDNA levels were associated with tumor progression in older patients who opted to forgo breast cancer surgery in favor of primary endocrine therapy (pET).Results:ctDNA levels were highly concordant with imaging findings, and a lack of ctDNA clearance at 6 months was associated with tumor progression. In a competing risk regression adjusted for patient age, tumor stage, tumor grade, and tumor Ki-67, pretreatment ctDNA positivity was associated with a significant risk of tumor progression (HR, 30; 95% confidence interval, 4.4–209; P = 0.0011). No patients with pretreatment ctDNA negativity experienced tumor progression. In correlative analyses examining ctDNA-positive tumors progressing on pET, we identified populations of CD11+ T cell–interacting macrophages that upregulate CD109 and CD89 and secrete immunosuppressive chemokines to create a favorable environment for cancer epithelial cell proliferation.Conclusions:These findings suggest that ctDNA may be a modality to identify older patients who can safely receive long-term pET, warranting future evaluation in a randomized setting.

Overall Representativeness of the Study The study cohort is broadly representative of very elderly (≥75 years), frail patients with ER⁺ breast cancer in the United States. The high proportion of community-based enrollment, distribution of clinical stage, and histology align with SEER data, although the cohort underrepresents Hispanic patients and healthier older adults who may receive more aggressive therapy. This study cohort (n = 43; all ≥ 75 years, median age 86, median RAI 36, and 42% ECOG ≥ 2) closely mirrors the demographic and clinical features of prior primary endocrine therapy (pET) cohorts in older women with ER⁺ breast cancer. Carleton et al. (PMID 38135542) reported a median age of 82 (range up to 97), with ∼82% ER⁺/PR⁺ disease, 85% T1–T2 tumors, and 66% node-negative, which closely parallels our distribution of 83% T1–T2 and 72% N0. Similarly, large observational pET studies (PMIDs 37479944, 36326972) describe patient populations with median ages of 81-83, predominantly frail, with early-stage, unifocal, ER⁺ tumors and low nodal burden. In these series, 70-85% of patients were node-negative, most tumors were histologic grade 1–2, and selection for pET was strongly associated with comorbidity or functional dependence – characteristics directly reflected in our cohort with high RAI scores and ECOG ≥ 2. Collectively, our study population is highly representative of the real-world population of elderly, frail women receiving pET for ER⁺ breast cancer in contemporary practice, supporting external validity for this therapeutic context.

Supplementary Figure 3: Whole exome sequencing from the core biopsies of all patients with a baseline (pre-treatment) ctDNA blood draw. Plot shows the top 20 mutated genes in addition to those commonly found in breast cancer.

Supplementary Figure 5: (a) n = 6 patients, with matched core biopsy and surgical specimens, were included in the correlative studies: n = 4 patients had tumors that were responding to pET at the time of surgery while n = 2 patients had tumors that were progressing at the time of surgery. Notably, the two patients who experienced tumor progression did initially have a partial response. (b) Whole exome sequencing from the core biopsies of these six patients at the time of their initial diagnosis shows no underlying genomic differences. Plot shows the top 20 mutated genes in addition to those commonly found in breast cancer.

Supplementary Figure 1: Swimmer plot for those patients without a baseline (pre-treatment) ctDNA draw but were still enrolled in the study. Overlayed onto the plot is the type of primary endocrine therapy (most patients started on a non-steroidal aromatase inhibitor (NSAI), such as anastrozole or letrozole), any type of locoregional intervention, disease progression, and vital status.

Supplementary Figure 2: ctDNA dynamics of one patient (Study ID: 13) enrolled on the study who had 10 draws over the course of the study period. This patient experienced multiple progressions (as indicated in the plot) with intermittent durable responses to endocrine therapy and palliative radiation therapy. Accompanying the ctDNA dynamics plot is the tumor size changes based on volumetric tumor size on CT scans.

Supplementary Figure 7: Quantification of T cells and macrophages from the multiplex immunohistochemistry in stromal regions of the TME.

Supplementary Table 1: Baseline clinicopathologic characteristics of patients enrolled in the study. Abbreviations: IDC / NST = invasive ductal carcinoma or carcinoma of no special type; ILC = invasive lobular carcinoma; mDLC = mixed ductal-lobular carcinoma; ET = endocrine therapy. # RAI calculated based on patients with cancer, which would give a minimum score of 35. RAI can be calculated manually at: https://efrailty.hsl.harvard.edu/ToolRiskAnalysisIndex.html (PMID: 27893030). * For this patient, they started on anastrozole with ribociclib.

Supplementary Figure 4: (a) Magee Equations (https://path.upmc.edu/onlineTools/mageeequations.html) can be used to estimate the OncotypeDX Recurrence Score. Plot shows that there is no difference in Magee Equation score (based on the average Magee Equation score) between those patients who had baseline ctDNA positive and baseline ctDNA negative tumors. (b) Similar analysis showing the results for Magee Equation 3, which has been previously been shown to associate with response to endocrine therapy. The filled circles for a subset of patients in the ctDNA positive group indicate those who cleared their ctDNA after 6 months of endocrine therapy.