In the present study we found no evidence that benefit from adjuvant capecitabine-containing chemotherapy compared to conventional chemotherapy is associated with BRCA1-like status in early-stage TNBC patients. Therefore, it is unlikely that the BRCA1-like classifier will help in selecting patients for adjuvant capecitabine-enriched chemotherapy.
Our hypothesis of differential treatment effects was based on previous observations made by Van Rossum et al. . Their retrospective analysis of the randomized GAIN study suggested that patients with BRCA1-like tumors have a selective treatment benefit from a conventional dose-dense chemotherapy regimen, containing capecitabine, if compared with non-myeloablative, intensified dose-dense chemotherapy. There are three important differences between the GAIN study and the FinXX study. The main difference is that the FinXX study compared two regimens that only differed by the addition of capecitabine (yes/no) and a slightly lower dose of docetaxel in the capecitabine arm , while the GAIN study compared miscellaneous regimens that differed in drug dose and drug combinations . Second, the GAIN study accrued only node-positive patients, while the FinXX study also recruited high risk node-negative patients. And lastly, the capecitabine dose and schedule differed between the studies (10 cycles of capecitabine 1000 mg/m2 twice daily administered on day 1–14 in a three weekly schedule in the GAIN trial versus 6 cycles of capecitabine 900 mg/m² twice daily on day 1–15 every 3 weeks in the FinXX study). Interestingly, we observed a significant capecitabine benefit for the non-BRCA1-like group (RFS and OS P value < 0.01) and less evidence of benefit for the BRCA1-like group (RFS P value 0.42; OS P value 0.59), in contrast to the observations of Van Rossum et al. . Whether or not these two observations represent inconsistent patterns is difficult to determine due to the small sample size in these unplanned subgroup analyses. The patterns are actually reversed, so unless the direction of effects changes with more data, narrower confidence intervals will only make both studies less consistent with each other.
Two biological processes may explain why BRCA1-like status is not predictive for benefit of adjuvant capecitabine treatment. First, although capecitabine causes DNA-damage , it may not specifically result in DNA damage that is dependent on a proficient homologous recombination machinery resulting in error-free DNA repair . In fact, capecitabine and its active form 5-fluorouracil lead to (1) DNA base pair mismatches which are repaired by the DNA mismatch repair (MMR) pathway  and (2) inhibition of DNA replication, leading to abasic sites that are repaired by base excision repair (BER) proteins . Second, there are some data suggesting that capecitabine might result in better outcome in TNBC patients with grade 2 tumors, but not grade 3 . Since BRCA1-like tumors are enriched for high-grade tumors, our findings of no predictive value of BRCA1-like status for benefit of capecitabine support these observations.
Our results are in line with previous preclinical findings of Quinn et al who observed no differential dose-response effect of capecitabine in BRCA1-mutated compared with wild-type BRCA1 human BC cells , and with Alli et al who found a 5-fold lower sensitivity to 5-fluoro-uracil of BRCA1-deficient compared to wild-type BRCA1 murine mammary epithelial cells . Moreover, our findings are consistent with the recent observations of Asleh et al. . In the same dataset, they found no significant association of improved outcome and signatures associated with DNA damage repair, including HRD and BRCAness, using an 800-gene panel in 111 early-stage TNBC patients treated with adjuvant capecitabine in the FinXX trial. Additionally, we demonstrated 78.6% concordance between our DNA-based CNV BRCA1-like classifier and the RNA-based NanoString BRCAness signature used by Asleh et al. . Taken together, these data indicate that benefit of adjuvant capecitabine-containing chemotherapy could not be predicted by deficiency in homologous recombination. Currently, the NordicTrip (ClinicalTrials.gov Identifier: NCT04335669) is an ongoing translational clinical trial in early-stage TNBC prospectively comparing the effect on pathologic complete response (pCR) rate of adding capecitabine to epirubicin plus cyclophosphamide followed by carboplatin plus paclitaxel neoadjuvant chemotherapy, stratified for HRD positive versus HRD negative/ HRD-intermediate. Results of this study have to be awaited to further clarify the value of HRD as a predictive biomarker for benefit of capecitabine-containing chemotherapy in early-stage TNBC.
The main strength of our study is the study design, i.e., a prospective, randomized controlled trial with collection of archival material. This prospective-retrospective design is the first choice to assess a putative predictive biomarker in case a prospective randomized clinical trial is not feasible, because such trials require huge numbers of patients, are costly and take many years to complete . An additional strength, in contrast to the exploratory analyses of Asleh et al. using an RNA 800-gene panel without predefined cutoff for the BRCAness signature , is that we evaluated a single biomarker with a predefined cutoff based on prior biological and empirical evidence [21, 23–25]. This confirmatory objective is required to establish the implementation of a predictive biomarker or to refute it .
A limitation of the present study is the small sample size, which is due to the fact that the FinXX trial was powered to evaluate the main effect of capecitabine among patients with any biological type of breast cancer rather than a treatment-marker interaction in the subgroup of TNBC patients. In addition, the number of patients was further reduced by the failure to obtain BRCA1-like status for all TNBC patients for several reasons. However, our patient group accounts for 129 (63.9%) of the 202 accrued TNBC patient in the FinXX trial, which is within the recommended range of sample size for a study to evaluate predictive biomarkers . Furthermore, the included patients did not differ meaningfully from all accrued TNBC patients for the evaluated clinical variables. Lastly, whether BRCA1-like status has predictive value for capecitabine-containing chemotherapy only in individuals with residual disease after neoadjuvant chemotherapy is still an open question. Collaborate efforts to assess this in the CREATE-X trial are ongoing.
Our findings may have implications for treatment decisions in early-stage TNBC patients. Currently, the addition of the polyadenosine 5’diphosphoribose polymerase (PARP) inhibitors in the (neo) adjuvant treatment of these TNBC patients is an emerging area of investigation [43–46]. The OlympiA trial is one very important and large trial that evaluates the efficacy of adjuvant treatment with olaparib, a PARP inhibitor, compared to placebo in patients with non-metastatic, germline BRCA mutated (gBRCAm) high risk HER2-negative primary breast cancer (ClinicalTrials.gov Identifier: NCT02032823). An unanswered question remains what the added value of adjuvant treatment with PARP inhibitors would be compared to the current clinical practice of capecitabine-containing treatment. In our present study, we demonstrate that the efficacy of adjuvant capecitabine treatment in early-stage TNBC patients does not differ by BRCA1-like status. Therefore, the added value of PARP inhibition to capecitabine-containing adjuvant chemotherapy in gBRCAm-associated or BRCA1-like TNBC patients that did not achieve a pathological complete remission on standard neoadjuvant chemotherapy needs to be investigated.