4CPS-006 Complete clinical response in metastatic breast cancer after front-line treatment with ribociclib/tamoxifen

Background and ImportanceEndocrine therapy with ovarian suppression or ablation is the standard first-line treatment for perimenopausal or premenopausal women with hormone receptor (HR) positive, HER2-negative, advanced breast cancer; however, endocrine therapy resistance and disease progression occur in most cases. Ribociclib is a selective, small molecule inhibitor of cyclin dependent kinases (CDKs) 4 and 6 has showed that alongside endocrine therapy can improve progression-free survival and achieve higher proportions of overall responses than endocrine therapy alone in premenopausal women with HR-positive, HER2-negative, advanced breast cancer.Aim and ObjectivesWe present the case of a woman patient diagnosed with stage-IV HR+/HER2- breast cancer (Ki67-25%) who achieved complete response to first-line ribociclib treatment.Material and MethodsThis was an observational retrospective study of the use of ribociclib in a 60-year-woman diagnosed with HR+/HER2- metastatic breast cancer. Data were obtained of the electronic medical records.ResultsThe premenopausal 54-aged patient was diagnosed with HR+/HER2- (Ki67-10%) localised infiltrating ductal carcinoma of left breast (1.5cm-size tumour) in July/2015. She underwent tumorectomy and received adjuvant radiotherapy and five-year tamoxifen 20 mg treatment. In March 2021, she suffered from loss of strength of left upper limb. CT-scan revealed a mass in the left axillary region between pectoral region and first rib and hypermetabolic bone lesions in the trochanter of the left femur, compatible with bone metastases. HR+/HER2- breast cancer was confirmed by tumour biopsy. Ki67 expression was 25%. In June/2021, this premenopausal 60-year woman was treated with 3-monthly 10.8 mg goserelin, daily 20 mg tamoxifen and ribociclib 600 mg once daily for 21 consecutive days followed by 7 days off treatment, resulting in a complete cycle of 28 days. In September 2021, she achieved complete metabolic response of the lesions described in the axilla and bone, without current foci of neoplastic disease. In June 2022, the last CT-scan revealed absence of neoplastic disease, therefore, she continues with the same treatment without dose modifications or delays. Side effects: treatment was well tolerated; she underwent grade I haematological toxicity.Conclusion and RelevanceThis case report documents an exceptional tumour response of a fast growing, locally advanced, bone metastatic HR+/HER2– de novo breast cancer treated by ribocic