Fracture risk in metastatic breast cancer patients treated with CDK 4/6 inhibitors and endocrine therapy.

PURPOSE: Investigate skeletal morbidity (SM) in metastatic breast cancer (MBC) patients undergoing CDK4/6 inhibitors (CDK4/6is) and endocrine therapy (ET).

METHODS

In this retrospective study we evaluated skeletal morbidity - defined as the occurrence of skeletal-related events (SREs) in metastatic bone and fragility fractures in non-metastatic bone - in 214 MBC patients who had received ET and CDK4/6is. As secondary aim, we compared VF progression, defined as a new fracture or worsening of a pre-existing fracture at spine CT scan, between 121 patients receiving ET alone (cohort A) and 121 patients on ET plus CDK4/6is (cohort B), balanced using propensity score.

RESULTS

Among the 147 patients (68.7%) with bone metastases, 59 (40.1%) experienced SREs including non-vertebral pathologic fractures (17 patients, 11.6%), pathologic VF progression (21 patients, 14.3%), spinal cord compression (3 patients, 2.0%), radiation to bone (18 patients, 12.2%). Considering the non-metastatic bone, 3 out of 214 patients (1.4%) experienced new non-vertebral fragility fractures, and 26 patients (12.2%) had fragility VF progression. In the comparative study, pathologic VF progression in metastatic bone was 38.3% in cohort A and 29.1% in cohort B (p = 0.093). The corresponding fragility VF progression rate in non-metastatic bone was 22.3% and 12.4% (p = 0.031).

CONCLUSIONS

A considerable proportion of women with MBC treated with CDK4/6is + ET experience SM on both metastatic and non-metastatic bone. Patients treated with CDK4/6is + ET had lower SM than those on ET alone.