Early clinical outcomes, treatment adherence, and toxicity profile of adjuvant CDK4/6 inhibitors in hormone receptor-positive early breast cancer: Real-world data from India.

Abstract

Background: Hormone receptor-positive (HR+)/HER2-negative (HER2-) early breast cancer (EBC) is associated with a persistent risk of recurrence despite optimal local therapy and adjuvant endocrine therapy, particularly in patients with high-risk clinicopathologic features. Adjuvant cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have demonstrated significant benefits in randomized clinical trials; however, real-world data from Indian populations remain limited. This study evaluates real-world outcomes, tolerability, and treatment adherence to adjuvant CDK4/6 inhibitors in HR+/HER2 - EBC patients treated at a tertiary cancer center in India.

Methods: This single-center, retrospective observational study included consecutive adult patients with HR+/HER2 - EBC who received adjuvant endocrine therapy in combination with either ribociclib or abemaciclib following definitive surgery between January 2023 and December 2025. Demographic, clinicopathologic, treatment, toxicity, and outcome data were extracted from institutional medical records. Adverse events were graded using common terminology criteria for adverse events. The primary endpoints were treatment tolerability and invasive disease-free survival (iDFS). Secondary endpoints included overall survival, dose modifications, and treatment discontinuation rates. Survival outcomes were estimated using the Kaplan-Meier method.

Results: A total of 75 patients (median age: 52 years) were analyzed. Stage III disease was present in 69.3% of patients, including IIIA/IIIB/IIIC, and ≥4 positive lymph nodes were observed in 70.6%, representing a high-risk group. Node-negative status was seen in 10 (13.3%) patients. Ribociclib was administered to 55.3% of patients and abemaciclib to 43.4% of patients, and dose modifications were required in 14.2% and 18.1% of patients, respectively, while permanent discontinuation occurred in 3.6% of ribociclib-treated patients and none in the abemaciclib group. Grade ≥3 toxicities were infrequent. In the ribociclib group, grade 3-4 neutropenia occurred in 6.0%, anemia in 4.7%, thrombocytopenia in 2.3%, and diarrhea in 2.3% of patients. In the abemaciclib group, grade 3-4 neutropenia was observed in 6.0%, anemia in 3.0%, thrombocytopenia in 3.0%, and diarrhea in 12.1%, the latter being the most frequent severe adverse event. At a median follow-up of 6.35 months, iDFS was 100% at 6 months and 90.6% at 12 months.

Conclusion: In this real-world Indian cohort, adjuvant CDK4/6 inhibitors demonstrated favorable tolerability, low rates of severe toxicity and permanent discontinuation, and encouraging early disease control in high-risk HR+/HER2 - EBC patients. These findings support the feasibility and applicability of adjuvant CDK4/6 inhibitor therapy in routine clinical practice in India, while emphasizing the need for longer follow-up and larger multicenter studies.