PCN3 - REAL WORLD EVIDENCE OF METASTATIC BREAST CANCER TREATMENT: A COMPARISON OF ADVERSE EFFECTS BETWEEN PALBOCICLIB TREATMENT AND ENDOCRINE THERAPY

Abstract

Palbociclib (PC) is a CDK4/6 inhibitor for ER(+)/HER2(-), metastatic breast cancer (MBC). Studies show progression-free survival is better among palbociclib patients than among patients receiving endocrine therapy. Evidence on whether the frequency of adverse effects (AEs) differs between PC patients and patients on other endocrine therapies is lacking. Data from a health research network representing over 19M female patient-lives was used to compare: (1) the occurrence of AEs in PC+letrozole(L) patients versus patients receiving an aromatase inhibitor (AI); and (2) the occurrence of AEs in PC+fulvestrant(F) patients versus patients receiving second-line endocrine therapy (ET). Patients had to be 50+ years old, have an MBC diagnosis between 2013-2017, and be ER(+)/HER2(-). The following AEs were defined by an ICD and/or LOINC code: neutropenia, anemia, acute respiratory infections, osteoporosis, fractures, weakness, bleeding events and mood disorders. We report the RRs and 95% CIs. The mean ages of PC+L(n=755), PC+F(n=447), AIs(n=7,600) and ET(n=1,821) patients were comparable (ages 66-67). PC+L and PC+F patients had a significantly higher risk of neutropenia and anemia than AI and ET patients. Compared to AI patients, PC+L patients had a lower risk of osteoporosis (RR=0.5;0.4-0.7) and mood disorders (RR=0.8;0.7-0.9), and a higher risk of bleeding events (RR=1.4;1.1-1.9). PC+L patients did not significantly differ from AI patients with respect to acute respiratory infections, fractures, or weakness or fatigue. Compared to ET patients, PC+F patients only had a significantly lower risk of osteoporosis (RR=0.7;0.5-0.9). Adjusting for treatment history and other confounders did not meaningfully impact the results. Although studies have demonstrated benefits to PC treatment over endocrine therapies, PC patients may have an increased risk of infection and bleeding. As PC, and other newly approved therapies in the same class, become more common, further analyses should assess whether these differences raise concern for patient safety.