OBJECTIVES: Evaluate real-world relative dose intensity (RDI) and drug holiday RDI in patients with locally advanced or metastatic breast cancer treated with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) (ribociclib, palbociclib or abemaciclib).

METHODS: Based on Swedish national prescription data, an observational registry-based analysis was carried out. Data was extracted from the Prescribed Drug Register based on ATC code including all patients who had received at least one prescription of a CDK4/6i treatment between 01/2016 and 03/2022. As CDK4/6i treatment became more common in 2018, only patients who initiated treatment from Jan. 1^st, 2018, onwards were included in the analysis. No adjustments were made for combination therapies or line of therapy. Patients with treatment interruption by another CDK4/6i treatment or patients who switched to a different CDK4/6i treatment were censored.

RESULTS: 2572 patients who had received at least one prescription of a CDK4/6i were included in this analysis. Mean time on treatment was 16.0, 15.4 and 14.1 months, average total RDI was 0.80, 0.83 and 0.82 and drug holiday RDI was 0.92, 0.97 and 1.00 for ribociclib, palbociclib and abemaciclib, respectively.

CONCLUSIONS: Dosing adjustments of CDK4/6i used to treat advanced or metastatic breast cancer patients in Sweden appear to reflect drops in RDI observed in randomized clinical trials. This provides an opportunity for treatment cost savings if the pricing of CDK4/6i’s is linear to the administered dose.