Chemotherapy Monitoring by ctDNA in HER2- Metastatic Breast Cancer

Patients will be enrolled in the screening step at the start of the second line of chemotherapy, and will undergo blood draws for ctDNA detection: That second line of treatment will be managed by clinical and radiological evaluations (RECIST); ctDNA will not be released to clinician and patient in real time. While the included patient is being treated by second line therapy, a customized ctDNA detection based on tumor mutations (droplet-digital PCR) will be developed. Once set up, the two blood above-mentioned samples will be subjected to ctDNA detection. The SEB will then retrospectively assess whether ctDNA levels changes during the second line of treatment were indicative of the efficacy of the second line therapy. Patients for whom ctDNA was successfully detected and found informative by the SEB (Steering Executive Board) could then be included in the interventional step when starting a new line of therapy. The third blood draw for ctDNA detection will be used to compare results to the tumor evaluation performed by imaging. In the interventional step, ctDNA analyses and interpretation will be performed in real time; results made available before the cycle 2. Quantitative results will be interpreted by the laboratory committee, with two possible recommendations: * ctDNA changes at day 15 display a major drop (Mutant Allelic Frequency (MAF) or copies/ml) reduced by 40% or more compared to baseline: continuing the same chemotherapy will be recommended; * ctDNA changes at day 15 display no major drop (MAF or copies/ml) either increased, stable or reduced by less than 40% compared to baseline: changing chemotherapy will be recommended; In light of ctDNA levels changes observed during the second line for each included patient, the above-mentioned thresholds might be modified, on a case-by-case basis, by the SEB (to keep into account individual characteristics). The test will be repeated for any new line of therapy that may be initiated during the first 6 months following the accrual of each patient in the interventional step. If this strategy is considered efficient (on an individual basis), further ctDNA tests will be made available on request for the next six months. Consequently, the patients who will change chemotherapy line after the recommendation following result from C1D15 ctDNA results will have a new ctDNA test at C1D1 and C1D15 of the new chemotherapy line.