Commentary: CDK4/6 Inhibitors, Breast Irradiation, and Aromatase Inhibitors in Breast Cancer Treatment, July 2023

After a median follow-up of 21.6 mo, the dalpiciclib group demonstrated a significantly longer median progression-free survival (PFS) compared with the placebo group (30.6 mo vs 18.2 mo; stratified hazard ratio [HR] 0.51; 95% CI 0.38-0.69; P < .0001). Overall, the dalpiciclib group demonstrated a manageable safety profile, although a higher percentage of grade 3/4 adverse events was noted with dalpiciclib than with placebo (90% vs 12%), as expected. Overall survival data for this CDK4/6 inhibitor are yet to come. These results suggest that dalpiciclib in combination with endocrine therapy is an alternative treatment for this group of patients, especially in countries where the traditionally approved CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) are not available.

The optimal sequencing of endocrine therapy (ET) after progression on CDK4/6 inhibitor–based therapy remains a challenge. In the phase 2 MAINTAIN trial, 119 patients (all of whom had HR+/HER2- metastatic breast cancer and who progressed on ET and CDK4/6 inhibitors) were randomly assigned to receive a different ET (fulvestrant or exemestane) from the previous ET they had received plus either the CDK4/6 inhibitor ribociclib or placebo. In the study by Kalinksky and colleagues, at a median follow-up of 18.2 mo, a significant improvement in PFS was observed in the switched ET-plus-ribociclib group compared with the switched ET-plus-placebo group (HR 0.57; P = .006). The phase 2 MAINTAIN trial is the first randomized trial to show the benefit of a CDK4/6 inhibitor after progression on another CDK4/6 inhibitor. It is important to note that the majority of patients in the MAINTAIN study previously received palbociclib in the first-line setting, which in recent studies has been demonstrated to be inferior to other CDK4/6 inhibitors. Therefore, it is important to confirm whether this will hold true upon progression from ribociclib or abemaciclib in the first-line setting. In addition, more data are needed to compare this approach with other ET treatment options, such as phosphoinositide 3-kinases inhibitors and oral selective estrogen receptor degraders.

There are several options for adjuvant radiation therapy for early-stage breast cancer. A meta-analysis of 14 randomized controlled trials and six comparative observational studies assessed the efficacy of whole breast irradiation (WBI) compared with partial breast irradiation (PBI) in 17,234 adults with early-stage breast cancer. Results of this meta-analysis showed that PBI was not significantly different from WBI, with similar rates of ipsilateral breast recurrence  at 5 years (relative risk [RR] 1.34; 95% CI 0.83-2.18) and 10 years (RR 1.29; 95% CI 0.87-1.91), although patients undergoing PBI reported fewer acute adverse events  compared with patients undergoing WBI (incidence rate ratio [IRR] 0.53; 95% CI 0.31-0.92) and acute grade ≥2 adverse events (IRR 0.21; 95% CI 0.07-0.62). These findings support using PBI as the adjuvant radiotherapy modality for select patients with favorable-risk early-stage breast cancer.

Another meta-analysis looked at assessing the survival benefit of adding CDK4/6 inhibitors to standard ET in older patients with advanced breast cancer. The study included 10 trials with 1985 older patients with advanced ER+ breast cancer who received ET with or without CDK4/6 inhibitors. The findings showed that adding CDK4/6 inhibitors to ET (letrozole or fulvestrant) significantly reduced the mortality risk by 21% (HR 0.79; 95% CI 0.69-0.91) and progression risk by 41% (HR 0.59; 95% CI 0.51-0.69) in older patients (age ≥ 65 years) with advanced breast cancer. Grade 3-4 neutropenia and diarrhea were similar in older patients. This study supports the use of CDK4/6 inhibitors as a reasonable treatment modality for older patients. More studies dedicated to the geriatric population are needed to help elaborate on the efficacy and tolerability of such agents in this population.

The phase 3 National Surgical Adjuvant Breast and Bowel Project B-42 (NSABP B-42) trial evaluated the role of extended letrozole therapy in postmenopausal breast cancer patients who were disease-free after 5 years of aromatase inhibitor–based therapy. The study included 3966 postmenopausal women with stage I-IIIA HR+ breast cancer who were randomly assigned to receive letrozole or placebo for 5 more years. After a median follow-up of 10.3 years, letrozole significantly improved disease-free survival (10-year absolute benefit 3.4%; HR 0.85; P = .01) compared with placebo, although there were no differences noted in overall survival between the groups (HR 0.97, P = .74). Furthermore, letrozole significantly reduced the breast cancer–free interval (HR 0.75, ,P = .003) and distant recurrence (HR 0.72, P = .01). There were no notable differences in toxicity, particularly rates of osteoporotic fractures and arterial thrombotic events, between the groups. Extended therapy with aromatase inhibitors beyond 5 years can be considered for select patients with early-stage breast cancer. Careful consideration of risks and benefits is needed to make these recommendations. 

After a median follow-up of 21.6 mo, the dalpiciclib group demonstrated a significantly longer median progression-free survival (PFS) compared with the placebo group (30.6 mo vs 18.2 mo; stratified hazard ratio [HR] 0.51; 95% CI 0.38-0.69; P < .0001). Overall, the dalpiciclib group demonstrated a manageable safety profile, although a higher percentage of grade 3/4 adverse events was noted with dalpiciclib than with placebo (90% vs 12%), as expected. Overall survival data for this CDK4/6 inhibitor are yet to come. These results suggest that dalpiciclib in combination with endocrine therapy is an alternative treatment for this group of patients, especially in countries where the traditionally approved CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) are not available.

The optimal sequencing of endocrine therapy (ET) after progression on CDK4/6 inhibitor–based therapy remains a challenge. In the phase 2 MAINTAIN trial, 119 patients (all of whom had HR+/HER2- metastatic breast cancer and who progressed on ET and CDK4/6 inhibitors) were randomly assigned to receive a different ET (fulvestrant or exemestane) from the previous ET they had received plus either the CDK4/6 inhibitor ribociclib or placebo. In the study by Kalinksky and colleagues, at a median follow-up of 18.2 mo, a significant improvement in PFS was observed in the switched ET-plus-ribociclib group compared with the switched ET-plus-placebo group (HR 0.57; P = .006). The phase 2 MAINTAIN trial is the first randomized trial to show the benefit of a CDK4/6 inhibitor after progression on another CDK4/6 inhibitor. It is important to note that the majority of patients in the MAINTAIN study previously received palbociclib in the first-line setting, which in recent studies has been demonstrated to be inferior to other CDK4/6 inhibitors. Therefore, it is important to confirm whether this will hold true upon progression from ribociclib or abemaciclib in the first-line setting. In addition, more data are needed to compare this approach with other ET treatment options, such as phosphoinositide 3-kinases inhibitors and oral selective estrogen receptor degraders.

There are several options for adjuvant radiation therapy for early-stage breast cancer. A meta-analysis of 14 randomized controlled trials and six comparative observational studies assessed the efficacy of whole breast irradiation (WBI) compared with partial breast irradiation (PBI) in 17,234 adults with early-stage breast cancer. Results of this meta-analysis showed that PBI was not significantly different from WBI, with similar rates of ipsilateral breast recurrence  at 5 years (relative risk [RR] 1.34; 95% CI 0.83-2.18) and 10 years (RR 1.29; 95% CI 0.87-1.91), although patients undergoing PBI reported fewer acute adverse events  compared with patients undergoing WBI (incidence rate ratio [IRR] 0.53; 95% CI 0.31-0.92) and acute grade ≥2 adverse events (IRR 0.21; 95% CI 0.07-0.62). These findings support using PBI as the adjuvant radiotherapy modality for select patients with favorable-risk early-stage breast cancer.

Another meta-analysis looked at assessing the survival benefit of adding CDK4/6 inhibitors to standard ET in older patients with advanced breast cancer. The study included 10 trials with 1985 older patients with advanced ER+ breast cancer who received ET with or without CDK4/6 inhibitors. The findings showed that adding CDK4/6 inhibitors to ET (letrozole or fulvestrant) significantly reduced the mortality risk by 21% (HR 0.79; 95% CI 0.69-0.91) and progression risk by 41% (HR 0.59; 95% CI 0.51-0.69) in older patients (age ≥ 65 years) with advanced breast cancer. Grade 3-4 neutropenia and diarrhea were similar in older patients. This study supports the use of CDK4/6 inhibitors as a reasonable treatment modality for older patients. More studies dedicated to the geriatric population are needed to help elaborate on the efficacy and tolerability of such agents in this population.

The phase 3 National Surgical Adjuvant Breast and Bowel Project B-42 (NSABP B-42) trial evaluated the role of extended letrozole therapy in postmenopausal breast cancer patients who were disease-free after 5 years of aromatase inhibitor–based therapy. The study included 3966 postmenopausal women with stage I-IIIA HR+ breast cancer who were randomly assigned to receive letrozole or placebo for 5 more years. After a median follow-up of 10.3 years, letrozole significantly improved disease-free survival (10-year absolute benefit 3.4%; HR 0.85; P = .01) compared with placebo, although there were no differences noted in overall survival between the groups (HR 0.97, P = .74). Furthermore, letrozole significantly reduced the breast cancer–free interval (HR 0.75, ,P = .003) and distant recurrence (HR 0.72, P = .01). There were no notable differences in toxicity, particularly rates of osteoporotic fractures and arterial thrombotic events, between the groups. Extended therapy with aromatase inhibitors beyond 5 years can be considered for select patients with early-stage breast cancer. Careful consideration of risks and benefits is needed to make these recommendations. 

After a median follow-up of 21.6 mo, the dalpiciclib group demonstrated a significantly longer median progression-free survival (PFS) compared with the placebo group (30.6 mo vs 18.2 mo; stratified hazard ratio [HR] 0.51; 95% CI 0.38-0.69; P < .0001). Overall, the dalpiciclib group demonstrated a manageable safety profile, although a higher percentage of grade 3/4 adverse events was noted with dalpiciclib than with placebo (90% vs 12%), as expected. Overall survival data for this CDK4/6 inhibitor are yet to come. These results suggest that dalpiciclib in combination with endocrine therapy is an alternative treatment for this group of patients, especially in countries where the traditionally approved CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) are not available.

The optimal sequencing of endocrine therapy (ET) after progression on CDK4/6 inhibitor–based therapy remains a challenge. In the phase 2 MAINTAIN trial, 119 patients (all of whom had HR+/HER2- metastatic breast cancer and who progressed on ET and CDK4/6 inhibitors) were randomly assigned to receive a different ET (fulvestrant or exemestane) from the previous ET they had received plus either the CDK4/6 inhibitor ribociclib or placebo. In the study by Kalinksky and colleagues, at a median follow-up of 18.2 mo, a significant improvement in PFS was observed in the switched ET-plus-ribociclib group compared with the switched ET-plus-placebo group (HR 0.57; P = .006). The phase 2 MAINTAIN trial is the first randomized trial to show the benefit of a CDK4/6 inhibitor after progression on another CDK4/6 inhibitor. It is important to note that the majority of patients in the MAINTAIN study previously received palbociclib in the first-line setting, which in recent studies has been demonstrated to be inferior to other CDK4/6 inhibitors. Therefore, it is important to confirm whether this will hold true upon progression from ribociclib or abemaciclib in the first-line setting. In addition, more data are needed to compare this approach with other ET treatment options, such as phosphoinositide 3-kinases inhibitors and oral selective estrogen receptor degraders.

There are several options for adjuvant radiation therapy for early-stage breast cancer. A meta-analysis of 14 randomized controlled trials and six comparative observational studies assessed the efficacy of whole breast irradiation (WBI) compared with partial breast irradiation (PBI) in 17,234 adults with early-stage breast cancer. Results of this meta-analysis showed that PBI was not significantly different from WBI, with similar rates of ipsilateral breast recurrence  at 5 years (relative risk [RR] 1.34; 95% CI 0.83-2.18) and 10 years (RR 1.29; 95% CI 0.87-1.91), although patients undergoing PBI reported fewer acute adverse events  compared with patients undergoing WBI (incidence rate ratio [IRR] 0.53; 95% CI 0.31-0.92) and acute grade ≥2 adverse events (IRR 0.21; 95% CI 0.07-0.62). These findings support using PBI as the adjuvant radiotherapy modality for select patients with favorable-risk early-stage breast cancer.

Another meta-analysis looked at assessing the survival benefit of adding CDK4/6 inhibitors to standard ET in older patients with advanced breast cancer. The study included 10 trials with 1985 older patients with advanced ER+ breast cancer who received ET with or without CDK4/6 inhibitors. The findings showed that adding CDK4/6 inhibitors to ET (letrozole or fulvestrant) significantly reduced the mortality risk by 21% (HR 0.79; 95% CI 0.69-0.91) and progression risk by 41% (HR 0.59; 95% CI 0.51-0.69) in older patients (age ≥ 65 years) with advanced breast cancer. Grade 3-4 neutropenia and diarrhea were similar in older patients. This study supports the use of CDK4/6 inhibitors as a reasonable treatment modality for older patients. More studies dedicated to the geriatric population are needed to help elaborate on the efficacy and tolerability of such agents in this population.

The phase 3 National Surgical Adjuvant Breast and Bowel Project B-42 (NSABP B-42) trial evaluated the role of extended letrozole therapy in postmenopausal breast cancer patients who were disease-free after 5 years of aromatase inhibitor–based therapy. The study included 3966 postmenopausal women with stage I-IIIA HR+ breast cancer who were randomly assigned to receive letrozole or placebo for 5 more years. After a median follow-up of 10.3 years, letrozole significantly improved disease-free survival (10-year absolute benefit 3.4%; HR 0.85; P = .01) compared with placebo, although there were no differences noted in overall survival between the groups (HR 0.97, P = .74). Furthermore, letrozole significantly reduced the breast cancer–free interval (HR 0.75, ,P = .003) and distant recurrence (HR 0.72, P = .01). There were no notable differences in toxicity, particularly rates of osteoporotic fractures and arterial thrombotic events, between the groups. Extended therapy with aromatase inhibitors beyond 5 years can be considered for select patients with early-stage breast cancer. Careful consideration of risks and benefits is needed to make these recommendations.