Advances in Immunotherapy for Breast Cancer

Introduction

Breast cancer is the most commonly diagnosed malignancy in women, with more than 2.3 million new cases in 2022.¹ Historically, breast cancer was treated with cytotoxic chemotherapy and was initially thought to be less responsive to immunologic agents due to factors creating a poorly immunogenic tumor environment, such as lower tumor mutational burden (TMB) and fewer tumor-infiltrating lymphocytes (TILs).² However, as cancer treatment has evolved, there has been increased interest in incorporating immunotherapy into breast cancer regimens due to success in the treatment of melanoma, small and non–small cell lung cancer, and other advanced malignancies.^3-5

Breast cancer is an extremely heterogeneous disease, further classified based on the expression of estrogen receptors (ERs), progesterone receptors, and amplification of HER2.⁶ Hormone receptor–positive (HR+) and HER2-positive (HER2+) tumors are both amenable to targeted treatments based on the presence of cell surface markers.⁷ Triple-negative breast cancer (TNBC), which expresses none of these markers, has been historically difficult to target. In addition, TNBC is frequently diagnosed at a more advanced stage and higher grade, and it is characteristically more aggressive than other breast tumor types.⁸ PD-L1 expression has been shown to be elevated in TNBC, making PD-1/PD-L1 blockade with immunologic agents a valuable treatment option.⁹ TMB and immune infiltration are variable across tumor types but have been shown to be higher and more predictive for response to immunotherapy in patients with TNBC.¹⁰ Given these factors, the first immunotherapy trials in breast cancer were done in TNBC.^11,12 Although most extensively studied in this tumor type, there is emerging evidence that immunotherapy can be efficacious in HR+ and HER2+ cancers.¹³ Ongoing research is investigating new immunotherapy agents, combinations with various chemotherapies, and optimal timing of regimens. Emerging technologies utilizing tumor antigens in vaccine trials, bispecific antibodies, and antibody-drug conjugates (ADCs) are further changing the immunotherapy landscape in breast malignancies.

This review summarizes advances in immunotherapy for early and metastatic breast cancer. We highlight emerging developments in immunotherapy for this disease, such as biomarkers to assist with patient selection for immunotherapy, escalation and de-escalation of current regimens, and emerging technologies such as novel combinations of ADCs with immunotherapy. Finally, we present an overview of the current landscape of clinical trials for immunotherapy in breast cancer.

Immunotherapy for TNBC

Advanced-Stage TNBC

The first trials investigating immunotherapy in breast cancer were done in patients with heavily pretreated, metastatic TNBC and utilized pembrolizumab, a PD-1–blocking antibody. The phase 1 KEYNOTE-012 trial (NCT01848834) evaluated pembrolizumab monotherapy in advanced PD-L1-positive tumors, with an overall response rate (ORR) of 18.5% in the TNBC cohort and multiple patients with lasting responses.¹¹ However, in the phase 3 KEYNOTE-119 trial (NCT02555657), pembrolizumab did not improve overall survival (OS) or progression-free survival (PFS) compared with chemotherapy.¹⁴ The IMpassion trials investigated a PD-L1 blocking agent, atezolizumab, which received accelerated FDA approval after the phase 3 IMpassion130 trial (NCT02425891) demonstrated an increase in OS of 10.5 months in the atezolizumab plus chemotherapy arm.¹² Ultimately, FDA approval was withdrawn after follow-up data showed median OS in the intention-to-treat population was not statistically significant.¹⁵ The phase 3 IMpassion131 trial (NCT03125902) utilized a similar design with a different chemotherapy backbone. It was a negative trial with a median PFS of 6 months with atezolizumab vs 5.7 months with placebo.¹⁶ The phase 3 IMpassion132 trial (NCT03371017) was ultimately negative, as well, showing no OS benefit with the addition of atezolizumab to chemotherapy in early-relapsing metastatic disease, providing additional insight into the use of atezolizumab in TNBC.¹⁷

The phase 3 KEYNOTE-355 trial (NCT02819518) was the first trial to evaluate pembrolizumab in the first line for metastatic TNBC, combining pembrolizumab with chemotherapy (paclitaxel, nab-paclitaxel, or gemcitabine plus carboplatin) vs placebo plus chemotherapy.¹⁸ This landmark trial showed that for patients with a PD-L1 combined positive score (CPS) greater than 10, the median PFS was 9.7 months in the pembrolizumab group vs 5.6 months in the placebo group. These results led to the FDA approval of the KEYNOTE-355 regimen, the current standard of care for first-line treatment of patients with metastatic TNBC if their CPS is greater than 10.

Early-Stage TNBC

Given the success in the metastatic setting, immunotherapy was then investigated in early-stage TNBC. Using this therapy at earlier disease stages allows for treatment of a lower tumor burden, as well as a more immunologically active tumor microenvironment (TME).¹⁹ The phase 3 IMpassion031 trial (NCT03197935) combined atezolizumab with neoadjuvant chemotherapy (NACT), along with adjuvant atezolizumab, and demonstrated a 17% higher pathologic complete response (pCR) rate in the atezolizumab arm.²⁰ The phase 3 KEYNOTE-522 trial (NCT03036488) evaluated pembrolizumab with NACT followed by adjuvant pembrolizumab in patients with untreated stage II or III TNBC.²¹ The pembrolizumab arm showed a statistically significant increase in pCR (64.8% vs 51.2%) and led to FDA approval of this regimen for early-stage TNBC. At 60 months, OS was significantly higher in the pembrolizumab arm (86.6% vs 81.7%).²² Updated results from KEYNOTE-522 included exploratory biomarker analysis, showing the association of TMB, a T-cell–inflamed 18-gene expression profile (TcellinfGEP), and non-TcellinfGEP consensus signatures with pCR and event-free survival (EFS).²³ These biomarkers are thought to reflect characteristics of the TME, such as the presence of TILs. TNBC tumors with robust immune infiltrate and a favorable TME, characterized by diffuse TIL distribution and high TIL density, have been associated with increased rates of pCR and improved relapse-free survival.²⁴ Evidence from patients treated with the KEYNOTE-522 regimen showed that higher TILs are associated with higher pCR rates.²⁵ Pembrolizumab’s long-term benefit was also influenced by residual cancer burden (RCB), and as seen in exploratory analysis, the higher the RCB class, the higher the risk of recurrence.²⁶ However, pembrolizumab improved disease-free survival (DFS) across all RCB classes, suggesting benefit even without pCR. As a result, the KEYNOTE-522 regimen is now the standard of care for patients with newly diagnosed stage II or III TNBC.

Several other studies have investigated immunotherapy in early-stage TNBC. The phase 3 NeoTRIP trial (NCT02620280) tested atezolizumab with NACT, followed by surgery and adjuvant anthracycline-based chemotherapy in patients with high-risk TNBC. The pCR rate for atezolizumab was 48.6% compared with 44.4% for the control, which was not statistically significant.²⁷ The phase 3 ALEXANDRA/IMpassion030 study (NCT03498716) investigated adjuvant atezolizumab combined with chemotherapy vs standard chemotherapy in early-stage TNBC. Accrual was stopped in 2022, after an interim futility analysis showed 127 invasive disease–free survival events in the immunotherapy arm vs 112 in the control, with a hazard ratio greater than 1.

The reason for discrepancies among NeoTRIP, ALEXANDRA/IMpassion030, and positive trials such as KEYNOTE-522 in early-stage TNBC is a matter of active debate. One significant difference in the trials is the type of immunotherapy: Atezolizumab is an anti–PD-L1 monoclonal antibody, whereas pembrolizumab is an anti–PD-1 monoclonal antibody. Results from the phase 3 GeparDouze/NSABP B-59 trial (NCT03281954) recently provided additional insight into this area after investigating neoadjuvant atezolizumab in high-risk TNBC with the same treatment schema as KEYNOTE-522.²⁸ This trial did not show statistically significant improvement in EFS (85.2% vs 81.9%, respectively).²⁹ These findings, along with the negative results from NeoTRIP and ALEXANDRA/IMpassion030, suggest decreased efficacy with atezolizumab. Chemotherapy backbones also differed between these trials. NeoTrip incorporated adjuvant-only anthracycline chemotherapy without concurrent immunotherapy, whereas KEYNOTE-522 combined these therapies in the neoadjuvant setting.Combining anthracyclines and immunotherapy may enhance T-cell response in TNBC, which may affect outcomes.³⁰

Timing of immunotherapy may be another critical factor to consider. ALEXANDRA/IMpassion030 used adjuvant-only immunotherapy, a key difference when compared with NeoTRIP and KEYNOTE-522, which included neoadjuvant administration. The phase 3 A-BRAVE trial (NCT02926196) further explored adjuvant immunotherapy, investigating the anti–PD-L1 avelumab vs observation.³¹ A-BRAVE did not meet its primary DFS end point, although there was improvement in distant DFS and OS. Overall, the lack of positive results with the adjuvant-only design suggests decreased benefit of immunotherapy in this setting. This is further supported by data in other tumor types, such as melanoma and small cell lung cancer, suggesting that the presence of an intact tumor at the time of immunotherapy administration may be crucial for treatment response.^32,33 Resection of the primary tumor prior to immunotherapy administration may reduce T-cell activation and diversity due to the removal of TME.^19,34 The S1418 SWOG trial (NCT02954874), assessing adjuvant pembrolizumab for early TNBC with residual disease post NACT, is pending and will provide additional information. Table 1 summarizes the similarities and differences of notable neoadjuvant immunotherapy trials in early-stage TNBC.

De-escalating existing treatments for TNBC is an area of active research beyond evaluating new immunotherapy agents and regimens. The phase 3 SCARLET trial (NCT05929768) is investigating the de-escalation of the chemotherapy backbone investigated in KEYNOTE-522, removing the doxorubicin/cyclophosphamide component for early-stage TNBC.³⁵ This trial is powered for EFS as the primary end point and will provide important data on the necessity of anthracyclines in the regimen. The need for adjuvant immunotherapy is also under investigation. As shown in negative adjuvant-only trials such as ALEXANDRA/IMpassion030 and A-BRAVE, adjuvant immunotherapy may not be necessary. Investigators in the phase 3 OptimICE-pCR trial (NCT05812807) are studying de-escalation of treatment in early-stage TNBC with pCR after completion of neoadjuvant chemotherapy and immunotherapy.³⁶ Patients with pCR were randomly assigned to receive adjuvant pembrolizumab or observation, with the primary end point of recurrence-free survival. This trial is ongoing, and results will help inform the necessity of adjuvant therapy in patients with TNBC who achieve pCR.

In addition to de-escalating in patients with pCR, escalating treatment in patients who do not have an optimal response is an important area of research. Patients with residual disease after neoadjuvant treatment have a high risk of recurrence and worse overall survival.³⁷ For residual disease after NACT, investigators in the phase 3 OptimICE-RD trial (NCT05633654) are studying the addition of the Trop-2-directed ADC sacituzumab govitecan (SG) to pembrolizumab.³⁸ SG is currently approved for patients with metastatic TNBC after at least 2 lines of therapy per the phase 3 ASCENT trial (NCT02574455).³⁹ OptimICE-RD will provide additional evidence on the utility of ADCs in TNBC and further clarify whether SG has a synergistic effect with immunotherapy. Another trial investigating escalation therapy and ADCs in TNBC is the phase 3 TROPION-Breast03 trial (NCT05629585).⁴⁰ This trial is utilizing another Trop-2-directed ADC, datopotamab deruxtecan (Dato-DXd), combined with durvalumab (an anti–PD-L1) vs Dato-DXd alone or chemotherapy in patients with residual disease after neoadjuvant treatment. This trial is ongoing, with invasive DFS as the primary end point, and will shed light on the implementation of ADCs with immunotherapy in this patient population. Table 2 summarizes the ongoing adjuvant trials in early-stage TNBC.

Immunotherapy for HR+ Breast Cancer

Advanced-Stage HR+ Breast Cancer

HR+ disease is the most common type of breast cancer and is heterogeneous, comprising luminal A and luminal B subtypes.⁴¹ To date, monotherapy trials of immunotherapy in metastatic HR+ breast cancer have shown its limited efficacy, likely related to low TMB, PD-L1 expression, and TILs infiltration as compared with TNBC, all of which are associated with a decreased response to immunotherapy.^42-44 However, due to the higher percentage of HR+ breast cancers and the success of immunotherapy in TNBC, there continues to be interest in utilizing this treatment in HR+ disease.

The phase 1 KEYNOTE-028 trial (NCT02054806) investigated pembrolizumab monotherapy in patients with heavily pretreated, PD-L1–positive metastatic HR+ breast cancer and found minimal benefit, with only partial response in 3 of 25 patients.⁴⁵ In a separate trial investigating eribulin chemotherapy with or without pembrolizumab, no added benefit was found in the immunotherapy arm in the metastatic HR+ population.⁴⁶ Investigators in the phase 2 SACI-IO HR+ trial (NCT04448886) randomly assigned patients with metastatic HR+ disease to SG plus pembrolizumab vs pembrolizumab alone, which showed a nonstatistically significant trend toward improved PFS in the combination arm.⁴⁷These trials’ results suggest that there may be a benefit to adding an ADC such as SG to immunotherapy, but additional data are needed.

Early-Stage HR+ Breast Cancer

HR+ breast cancer has a more favorable prognosis than other subtypes. However, there is a higher-risk group of early-stage HR+ disease, including tumors with higher-grade positive lymph nodes, high Ki-67, and low ER expression.⁴⁸ Standard treatment includes neoadjuvant chemotherapy followed by adjuvant endocrine therapy, but there is interest in the use of immunotherapy. The phase 2 I-SPY 2 trial (NCT01042379) is an ongoing, adaptively randomized, neoadjuvant study focused on identifying patients who could benefit from immunotherapy, beyond their identified biomarkers, PD-1/PD-L1 status, mismatch repair deficient/high microsatellite instability, and TMB. Patients with early-stage, high-risk, HER2-negative disease were randomly assigned in one trial arm to receive NACT with or without pembrolizumab. The pCR rates were 30% in the pembrolizumab arm compared with 13% in the control arm for HR+ disease. Although pembrolizumab significantly increased the pCR rates of the HR+ group, the effect was more pronounced in the TNBC cohort.¹³ Using pretreatment biopsy data from patients who received neoadjuvant pembrolizumab, the I-SPY 2 trial also identified a signature of 53 genes known as ImPrint, which demonstrated greater than 90% sensitivity and greater than 80% specificity for predicting pCR in these patients.⁴⁹ Importantly, the data showed an association between ImPrint scores and pCR rates in patients with HR+ disease, potentially developing a way to identify patients in this subset who are more likely to respond to immunotherapy.

Various trials are investigating the addition of immunotherapy to traditional chemotherapy regimens in HR+ disease. The ongoing phase 3 KEYNOTE-756 trial (NCT03725059) is evaluating this approach in patients with untreated, high-grade, early-stage HR+ breast cancer.⁵⁰ Pembrolizumab combined with neoadjuvant chemotherapy, followed by surgery and adjuvant pembrolizumab, has led to a statistically significant increase in pCR (24.3% vs 15.6%), regardless of PD-L1 status. As CPS increased, pCR rates also increased in the pembrolizumab arm. Patients with a CPS greater than 1 benefited regardless of ER percentage, although there was an increased benefit if it was less than 10%. More patients were downgraded to lower RCB 0-1 in the pembrolizumab group. The study is powered to test EFS as a primary end point.

The phase 3 CheckMate-7FL trial (NCT04109066) is also studying the addition of immunotherapy to NACT in patients with untreated, high-risk, HR+ breast cancer. Instead of pembrolizumab, CheckMate-7FL uses nivolumab, another anti–PD-1 agent.⁵¹ The addition of nivolumab to NACT has resulted in a statistically significant improvement in pCR compared with the control group (24.5% vs 13.8%). The benefit of nivolumab was greater in populations with PD-L1 greater than 1, with pCR rates of 44.3% vs 20.2%, respectively. Greater benefit was observed with increasing CPS scores and stromal TILs greater than 1%. As in KEYNOTE-756, the benefit of immunotherapy on pCR and RCB 0-1 was greater in ER-low (< 50%) tumors. Table 3 lists the similarities and differences between the CheckMate-7FL and KEYNOTE-756 trials. There are ongoing analyses to further elucidate which HR+ populations derive the greatest benefit from the addition of immunotherapy.

The data from the KEYNOTE-756 and CheckMate-7FL trials are promising for adding immune-targeted therapy to the HR+ population; however, there are no EFS data reported at this time. The CheckMate-7FL trial will not be powered for EFS due to early closure with the approval of CDK 4/6 inhibitors in high-risk HR+ breast cancer. CDK 4/6 inhibitors add a challenge for the implementation of immunotherapy in this population, as there are concerns for the toxicity of the combined drugs, including higher rates of pneumonitis and hepatitis.^52-54 Further studies are needed to understand the safety profile and sequence of CDK 4/6 inhibitors and immunotherapy. Regarding the underlying question of which patients who are HR+ are most appropriate for immunotherapy, there are promising indicators of immunotherapy response, such as the ImPrint data and biomarkers from the CheckMate-7FL trial. However, it is still unclear which unique identifiers are the most useful in predicting pCR and EFS. The toxicity of immunotherapy is an important factor to consider as well. CheckMate-7FL did have higher discontinuation rates due to treatment-related adverse effects during the neoadjuvant phase in the nivolumab group.⁵¹ Currently, no immunotherapy regimens are approved for breast cancer subtypes other than TNBC.

Emerging Immunotherapy Strategies in Advanced-Stage Breast Cancer

There is growing interest in bispecific antibodies, with promising new data for the treatment of breast cancer. The combination of new anti–PD-1/PD-L1 and anti-VEGF bispecific antibodies with taxane chemotherapy as first-line treatment in metastatic TNBC is being investigated in multiple trials. Ivonescimab, combined with taxane chemotherapy as first-line treatment for metastatic TNBC, has shown an 80% ORR and a median PFS of 9.4 months.⁵⁵ PM8002/BNT327, in combination with nab-paclitaxel, has also demonstrated a high overall response rate of 73.8% and median PFS of 13.5 months.⁵⁶ For both bispecifics, the positive ORR and PFS results appeared to be independent of tumor PD-L1 expression. This combination is promising, but questions remain regarding the ideal chemotherapy or ADC combination, the correct population to benefit from the drugs, and potential biomarkers to drive treatment. These bispecifics will need to be evaluated in larger phase 3 randomized controlled trials, and PM8002/BNT327 will be evaluated in I-SPY 2.

Emerging Immunotherapy Strategies in Early-Stage Breast Cancer

The I-SPY 2 trial continues to investigate novel experimental regimens in the neoadjuvant setting for early-stage disease. One arm evaluated a combination of anti–PD-1 and anti–LAG-3 treatment in patients with high-risk, early-stage, HER2-negative breast cancer who were receiving cemiplimab and fianlimab with paclitaxel (PCF). pCR rates in all patients who were HER2 negative were 44% compared with 21% in the control, 53% compared with 29% in TNBC, and 36% compared with 14% in HR+ disease.⁵⁷ The PCF combination was highly effective in patients with positive ImPrint status, with 50% of patients with a pCR having the Immune+ signature. However, endocrinopathies were significant in the PCF arm, with 34.2% of patients experiencing hypothyroidism and 22.3% experiencing adrenal insufficiency, toxicities that limited the utility of the regimen.

In the I-SPY 2 trial, patients are stratified by tumor response-predictive subtypes by immunotherapy signature (Immune +/–), DNA repair deficiency (DRD +/–), luminal signatures, and HR/HER2 status. Patients identified as HR+/HER2–/Immune–/DRD–, HR–/HER2/Immune-/DRD–, HER2+/Immune+, and HER2–/Immune-/DRD+ were eligible for the Dato-DXd plus durvalumab arm. Of the 106 patients randomly assigned to this regimen, 36 early responders proceeded to surgery after initial therapy, and the patients with HER2–/Immune+ subtype showed a promising pCR rate of 72%.⁵⁸ Dato-DXd plus durvalumab is being evaluated in the ongoing phase 3 TROPION-Breast04 study (NCT06112379), which includes patients with early-stage TNBC and HR-low/HER2-negative breast cancer, with dual primary end points of pCR and EFS.⁵⁹ Building on the findings of I-SPY 2, the ongoing phase 3 SWOG S2206 trial (NCT06058377) is evaluating the addition of durvalumab to chemotherapy in patients with MammaPrint Ultrahigh early-stage HR+ breast cancer, a group shown to have an improved response to immunotherapy in prior analyses.⁶⁰

The Future of Immunotherapy

There are many questions still to be answered regarding the role of immunotherapy in breast cancer. In TNBC, the optimal chemotherapy backbone and role of adjuvant immunotherapy remain to be defined. Results of the ongoing OptimICE-pCR trial will help determine the need for adjuvant immunotherapy, at least in those patients who experience a pCR. The SCARLET trial is evaluating an anthracycline-sparing regimen in early-stage TNBC to determine whether the addition of immunotherapy can allow for the de-escalation of chemotherapy. Given the remarkable efficacy observed with ADCs in the advanced setting, numerous studies are investigating combinations of these targeted chemotherapy drugs with immunotherapy. OptimICE-RD and TROPION-Breast03 are ongoing and studying Trop-2–targeting ADCs in the adjuvant setting for patients with TNBC who do not achieve a pCR with neoadjuvant chemoimmunotherapy.

In the early-stage HR+ space, it remains to be seen whether the improvement in pCR with immunotherapy noted in the CheckMate-7FL and KEYNOTE-756 trials will translate to an EFS or OS benefit. Further, given the approval of CDK4/6 inhibitors in the adjuvant setting for high-risk HR+ disease, biomarkers predictive of immunotherapy benefit will be needed to identify patients who are most appropriate for immunotherapy, given toxicity concerns with the combination of CDK4/6 inhibitors and immunotherapy.^52-54 The development of the ImPrint signature from the I-SPY trial is an exciting investigational biomarker with the potential to identify patients with early-stage disease (both HR+ and TNBC) who respond favorably to immunotherapy.

Predictive biomarkers of immunotherapy benefit remain elusive. Immunotherapy does not come without risks; identifying patients who will benefit and those at risk for adverse effects is of paramount importance. Research into the role of the host—such as the gut microbiome, germline genetics, and HLA genotyping—in predicting immunotherapy benefit is ongoing.⁶¹ Although the incorporation of immunotherapy into the fabric of breast cancer treatment was delayed compared with other malignancies, we now know that there are many patients with breast cancer who can benefit from immunotherapy. There are numerous ongoing trials in both early- and advanced-stage breast cancer studying a variety of different immunotherapy agents aimed at improving outcomes from this common and deadly disease (Figure 1 and Figure 2).Overall, immunotherapy has revolutionized breast cancer treatment and continues to change the landscape of breast cancer care, with new advances on the horizon for all tumor subtypes.

Corresponding Author

Rita Nanda, MD

The University of Chicago

Department of Medicine

Section of Hematology/Oncology

900 E 57th St

Suite 8118, Chicago, IL 60637

RNANDA@bsd.uchicago.edu

Phone: +1-773-834-2756

Fax: +1-773-834-2484

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