Background: Breast cancer (BC) represents 25% of all cancers in women. Whilst the majority have early stage disease approximately 30% will develop metastatic breast cancer (MBC). In HER2 negative MBC, palliative chemotherapy is one of the main treatment options. It remains to be seen whether the use of adjuvant taxane chemotherapy leads to an increase in taxane resistance at the onset of MBC, although for patients with a relatively short disease free interval this may be the case. Cabazitaxel (CBZ) is a novel taxoid selected for development from preclinical evidence in cell lines resistant to docetaxel and paclitaxel including activity in a HER-2 positive BC tumour xenograft, with innate resistance to docetaxel. Clinically CBZ is licensed for metastatic castration-resistant prostate cancer following progression during or after docetaxel chemotherapy. A phase 3 RCT in this patient group showed a 3 month overall survival benefit for patients receiving CBZ and prednisolone compared with mitoxantrone and prednisolone. Methods: CONCEPT is an open label randomised phase 2 trial of first line chemotherapy in patients with HER-2 negative MBC where paclitaxel would be considered the standard treatment. Patients are randomised to cabazitaxel 25 mg/m² every 21 days for 6 cycles or paclitaxel 80 mg/m² weekly for 18 weeks. Eligibility includes patients who are PS 0 or 1 who may have received prior docetaxel in the adjuvant setting or be taxane-naïve. The primary endpoint is progression free survival (PFS), defined as the time between the date of randomisation and progression (according to RECIST version 1.1) or death from any cause. Secondary end-points include safety, overall survival and assessment of quality of life factors by FACT-B and EQ-5D-5L. For the current phase 2 study 90 patients will be recruited, with a 1:1 randomisation, proceeding to phase 3 of 160 patients, if the interim analysis does not show futility. To date 38 patients have been recruited from 10 centres. The IDMC met in Oct 2016 and recommended the study continue recruitment.