Breast Cancer Treatment

Introduction to CDK4/6 Inhibitors

The American Society of Clinical Oncology recommends the use of CDK4/6 inhibitors, such as palbociclib, ribociclib, and abemaciclib, in hormone receptor-positive, HER2-negative breast cancer, due to their similar mechanism of action as CDK4/6 inhibitors 1, 2

The National Comprehensive Cancer Network recommends palbociclib dosing at 125 mg once daily for 21 days followed by 7 days off treatment 2
Abemaciclib is dosed at 150 mg twice daily continuously, with a unique feature of having three active metabolites (M2, M20, M18) with similar potency, and is more selective for CDK4 over CDK6, uniquely inhibiting CDK9 1, 2
Ribociclib is administered at 600 mg orally once daily for 21 consecutive days followed by 7 days off treatment, in combination with an aromatase inhibitor or fulvestrant, for patients with hormone receptor-positive, HER2-negative advanced or metastatic breast cancer 3

Clinical trials have shown that abemaciclib can be used as monotherapy after progression on endocrine therapy and chemotherapy, with benefit demonstrated in the adjuvant setting (monarchE trial) 1, 2
The monarchE trial demonstrated the efficacy of abemaciclib in the adjuvant setting, whereas the PALLAS and PENELOPE-B trials failed to show benefit for palbociclib 1
Ribociclib has demonstrated significant improvements in both progression-free survival (PFS) and overall survival (OS) with letrozole, with a PFS of 25.3 months vs 16.0 months with letrozole alone (HR 0.56), and an objective response rate of 43% vs 29% 3
Ribociclib has also demonstrated significant improvements in PFS in premenopausal women, with a PFS of 24 months vs 13 months with endocrine therapy alone, and an OS improvement of 70% vs 46% at 3.5 years (HR 0.71) 3

The selection of a specific CDK4/6 inhibitor should consider the patient's comorbidities, with cardiac issues requiring avoidance of ribociclib due to QT prolongation, GI issues requiring avoidance of abemaciclib due to diarrhea, and severe neutropenia risk requiring consideration of abemaciclib due to less hematologic toxicity 1
Monitoring requirements include additional ECG and electrolyte monitoring for ribociclib, and CBC monitoring, especially during first cycles, for all CDK4/6 inhibitors 2
Common grade 3/4 adverse events include neutropenia (62%), leukopenia (21.3%), and abnormal liver function tests (10.2%) 3
The National Comprehensive Cancer Network recommends monitoring complete blood count every 2 weeks for the first 2 cycles, then at the beginning of each subsequent 4 cycles, and liver function tests every 2 weeks for the first 2 cycles, then at the beginning of each subsequent 4 cycles 3

Abemaciclib showed benefit in the monarchE trial, ribociclib showed benefit in the NATALEE trial, and palbociclib showed no benefit in the PALLAS and PENELOPE-B trials, in the adjuvant setting 1
Network meta-analyses have found no meaningful difference in overall survival among the three drugs in the metastatic setting 1
The National Comprehensive Cancer Network recommends ribociclib + aromatase inhibitor (letrozole/anastrozole) as the preferred option for patients with HR+/HER2- metastatic breast cancer and no prior AI exposure or relapse >12 months after stopping adjuvant AI, with a median PFS of 25.3 months vs 16.0 months with AI alone 3
The European Society for Medical Oncology suggests that for patients who relapsed on adjuvant AI or within 12 months of stopping adjuvant AI, ribociclib + fulvestrant is recommended, with an overall survival benefit 4

Advanced age alone should not exclude patients from CDK4/6 inhibitor therapy, although older patients (≥75 years) may experience more toxicity, including higher incidence of hematological adverse events, greater risk of fatigue, diarrhea, neutropenia, and hepatotoxicity, and more frequent dose reductions or treatment interruptions 5, 6
Ovarian function suppression/ablation may be added to the treatment regimen, including an LH-RH agonist 7, 3, 5
Rechallenge with CDK4/6 inhibitors after progression may be possible after a treatment-free interval of 12 months, based on evidence regarding rechallenge with other therapies 5
After progression on a CDK4/6 inhibitor, recommended testing includes somatic PIK3CA and ESR1 mutations, and germline BRCA1/2 and PALB2 mutations, with evidence-based second-line options including fulvestrant-alpelisib, exemestane-everolimus, tamoxifen-everolimus, fulvestrant-everolimus, single-agent endocrine therapy, chemotherapy, and PARP inhibitors for tumors with germline BRCA mutations 5