Efficacy of Olaparib in Treatment-Refractory, Metastatic Breast Cancer with Uncommon Somatic BRCA Mutations Detected in Circulating Tumor DNA

Poly(ADP-ribose) polymerase inhibitors (PARPi) induce synthetic lethality in tumors with homologous recombination repair deficiency. While olaparib and talazoparib are approved for HER2-negative metastatic breast cancer with deleterious germline BRCA mutations, their efficacy in tumors harboring somatic BRCA mutations is less established because most trials focused on germline variants. Emerging data suggest potential benefit in somatic BRCA1/2 mutations, but evidence for rare variants of unknown significance is scarce. This report describes a metastatic breast cancer patient with the rare somatic BRCA1 c.4336G>T (p.E1446*) mutation detected in ctDNA who responded to olaparib, with serial liquid biopsies used to monitor response and acquired resistance.

- Large clinical trials have shown efficacy of PARPi (olaparib, talazoparib) in HER2-negative metastatic breast cancer with germline BRCA mutations (Robson et al., NEJM 2017; Litton et al., NEJM 2018). - TBCRC 048 (J Clin Oncol 2020) suggested activity of olaparib in metastatic breast cancer with mutations in homologous recombination-related genes, including somatic variants, though data are limited. - COSMIC has reported the BRCA1 E1446* mutation in ovarian cancer, but clinical implications and PARPi responsiveness in breast cancer were previously unknown. - BRCA reversion mutations detected in ctDNA predict primary/acquired resistance to PARPi in ovarian carcinoma (Lin et al., Cancer Discov 2019) and are a recognized mechanism of PARPi resistance (Li et al., Mol Cancer 2020). - ESR1 ligand binding domain mutations (e.g., L536H) confer constitutive ER activity and resistance to endocrine therapy (Brett et al., Breast Cancer Res 2021).

Study design: Single-patient case report with serial ctDNA liquid biopsy profiling and radiologic response assessment. Ethics: Written informed consent obtained; IRB approval from Seoul National University Hospital (IRB H-1805-049-944) in accordance with the Declaration of Helsinki. Sample collection: Peripheral blood (20 mL) collected into AlphaLiquid Tubes (IMBdx, Seoul, Korea) between June 2021 and July 2022. Plasma processing: Ficoll-based centrifugation at 1,500 ×g for 15 minutes followed by 16,000 ×g for 10 minutes to remove debris. DNA extraction: cfDNA isolated from 2–4 mL plasma using Maxwell RSC cfDNA Plasma Kit (Promega). Germline genomic DNA from PBMCs using Maxwell RSC Blood DNA Kit (Promega). NGS: Targeted capture with AlphaLiquid 100 panel (IMBdx); library construction with IMBdx NGS Library Prep Kit. Sequencing on Illumina NovaSeq 6000, 2×150 bp paired-end. Bioinformatics: Trimmed reads aligned to hg38 using BWA v0.7.10; mutation calling via in-house algorithm (manuscript in preparation). Pathogenicity predictions included CADD score and an in-house homologous recombination repair deleteriousness (HRD) score (0–1 scale). Clinical treatment and assessments: Off-label olaparib initiated at 300 mg twice daily (August 2021). Tumor markers (CA 15-3) trended. Radiologic responses assessed by CT scans per RECIST v1.1. Serial ctDNA analyses (AlphaLiquid100) performed to monitor variant allele frequencies and emergence of resistance mechanisms. Data availability: Patient-level data not publicly available to protect privacy; available upon reasonable request to corresponding authors.

- A rare somatic BRCA1 nonsense mutation, c.4336G>T (p.E1446*; COSMIC COSV58803047), was detected in ctDNA at 6.68% variant allele frequency (VAF) after multiple prior lines of therapy had failed. - Bioinformatic evidence suggested pathogenicity: CADD score 34; in-house HRD score 0.87 (0=benign, 1=deleterious), supporting BRCAness and PARPi sensitivity. - Off-label olaparib (300 mg BID) induced rapid clinical and radiologic response: - CA 15-3 decreased soon after initiation. - Best radiologic response (January 2022): 55% decrease in sum of target lesions (RECIST v1.1 partial response). - BRCA1 E1446* VAF fell to 0.25% on ctDNA. - Acquired resistance after approximately 8 months of therapy (April 2022): - Disease progression with 194% increase from best response; new liver lesions and left pleural effusion. - BRCA1 E1446* VAF increased to 19.16%. - Emergence of BRCA1 reversion mutations restoring the truncated region: p.E1446L, p.E1446Y, p.S1436_S1448del, p.E1446Q. - ESR1 c.1607T>A (p.L536H) increased from 0.05% to approximately 5.95% VAF. - At last ctDNA follow-up (July 2022), the primary BRCA1 E1446* and two reversion mutations (p.E1446L, p.E1446Q) persisted.

This case demonstrates that a rare somatic BRCA1 mutation (p.E1446*) can confer sensitivity to olaparib in metastatic breast cancer, aligning with the biological rationale that BRCA1/2-deficient tumors are susceptible to PARP inhibition due to defective homologous recombination repair. Despite approvals focusing on germline BRCA mutations, the observed clinical and molecular responses support extending PARPi consideration to select somatic BRCA mutations when supported by functional domain context and deleteriousness predictions. Serial ctDNA monitoring provided real-time insights into treatment effect and resistance evolution: suppression of BRCA1 E1446* during response and emergence of BRCA1 reversion mutations concomitant with clinical progression, a known mechanism restoring BRCA1 function and mediating PARPi resistance. The rise of ESR1 L536H further reflects endocrine resistance evolution in this hormone receptor–positive tumor. Together, these findings underscore the utility of liquid biopsy for target identification, response tracking, and resistance mechanism detection in heavily pretreated patients.

A patient with treatment-refractory, metastatic breast cancer harboring a rare somatic BRCA1 c.4336G>T (p.E1446*) mutation experienced a marked and durable response to off-label olaparib, followed by acquired resistance associated with BRCA1 reversion mutations detected via ctDNA. This is, to the authors' knowledge, the first report linking BRCA1 E1446* to olaparib sensitivity in breast cancer and documenting dynamic mutation changes by liquid biopsy. Advances in liquid biopsy and pathogenicity prediction can uncover actionable alterations in patients lacking tissue samples or standard options. Accumulation of similar case evidence may guide future trials assessing PARPi efficacy across somatic and rare BRCA variants and inform strategies to anticipate and manage resistance.

- Single-patient case report limits generalizability and causal inference. - Off-label use without a comparator; response assessment lacks randomized context. - Molecular profiling relied on ctDNA rather than contemporaneous tumor tissue; intratumoral heterogeneity and clonal representation may differ. - Some analyses (mutation calling algorithm, HRD score) are based on in-house methods not yet peer-reviewed. - Underlying data are not publicly available, limiting external validation.