Background and Importance Cyclin-dependent kinase (CDK) 4/6 inhibitors, block the transition from the G1 to S phase of the cell cycle by interfering with Rb phosphorylation and E2F release, showing potent antitumour activity and manageable toxicity in HR+/HER2−breast cancer patients.

Aim and Objectives The main objective of this work is to compare Real world data (RWD) between palbociclib and ribociclib in order to investigate the continuity in treatment and the frequecy of heamatologic adverse events(AEs) before and after CDK inihibitors dose reduction(DR).

Material and Methods A court of 128 pts has been analyse from medical and pharmacy records, of these 101 treated with palbociclib and 27 with ribociclib. Patients (PTS) has been observed from 2019 to 2021 and the results were compared with those of pivot trials. The DR was defined as reducing palbociclib dose from 125 mg to 100 mg or 75 mg (≥20% DR), while in ribociclib from 600mg to 400mg or 200mg. In both cases, DR is effective in the management of AE

Results RWD shows that time to first DR is similar in both cases: 11 and 10 months respectively for palbociclib and ribociclib. If a second DR is necessary, it occurrs by th 16,5 months for palbociclib and 16.6 for ribociclib. Of 101 pts treated with palbociclib, 50 (49.5%) discontinued for progression disease (PD) and one of them for metastatic melanoma. 6/27 of pts (22.22%) in the ribociclib setting stopped for PD.In both cases, neutropenia is the prior AE to dose reduction as shown in real life and clinical trials. Its frequency decreases during the first cycle following the dose reduction, with a reduction in the severity.Other AEs observed were: haematologic disorder, hepatic cytolysis, drug intolerance, anaemia, leukocytosis, febrile neutropenia and fever.

Conclusion and Relevance As shown by the pivot trials, both the treatments are equal in term of toxicity and duration.The proportion of pts with PD appears to be superior in Palbociclib setting, even though need a deeper study with a good statistical model to confirm results..For clinician using ribociclib is much more comfortable than palboclib, due to the possibility of DM without interrupting treatment

Conflict of Interest No conflict of interest