The Potential of Antibody-Drug Conjugates in the Treatment of Breast Cancer

Mateusz Opyrchal, MD, PhD

ABSTRACT

Targeted therapy through the use of antibody—drug conjugates holds great promise for the treatment of many malignancies, in particular, breast cancer. The potential of this class of drugs has been demonstrated by the success of ado-trastuzumab emtansine (T-DM1) in the treatment of breast cancer patients with amplified HER2 breast cancer who became resistant to first-line treatment. In this review, we summarize the data from the most recent trials of T-DM1 with point of emphasis for future clinical application. Novel agents with particular interest in patients with breast cancer are also discussed.

Introduction

Breast cancer is one of the most common malignancies and the second-leading cause of cancer-related mortality in women in the United States. Despite many advances in the treatment of this disease, a significant portion of women have disease progression and recurrence. Metastatic disease continues to be incurable, leading to increased morbidity and death. Most women are diagnosed with metastatic disease due to failure of current standard treatments and inherent resistance of their breast cancer to systemic therapies. Therefore, there continues to be an urgent need for development of novel therapeutics with both increased efficacy and decreased toxicity.

Targeted therapy with antibodies has demonstrated great success in the treatment of HER2-amplified breast cancers with the introduction of trastuzumab.¹Antibodies and their derivatives exert their antitumor effect via several mechanisms, including signal blockade, antibody dependent cellular cytotoxicity (ADCC), phagocytosis, and activation of the complement system.²Unfortunately, despite the success of anti-HER2 antibody therapy, other antibodies against the multitude of breast cancer-specific antigens have not shown the same efficacy.

Antibody—drug conjugages (ADCs) are composed of a targeted antibody, linker, and potent cytotoxic agent. In order to deliver its toxic payload, the antibody must be targeted to a cancer antigen that is readily internalized. In contrast to antibodies, ADCs are postulated to exert their antitumor effect solely through delivery of the toxic molecule directly to the cancer cells. This novel mechanism of action has led to renewed interest in many breast cancer antigens as potential therapeutic targets.^3,4

Antigen expression in normal tissues and nonspecific antibody binding are a major source of toxicities seen with ADCs. Therefore, selection of an antigen and antibody are critical to the development of an ADC. The linker binds the drug to the antibody. Linker instability and release of the drug systemically is another source of toxicity. Continued improvements in linker biochemistry have improved their systemic stability, resulting in decreased systemic release of the toxic drug and subsequently decreased systemic toxicity.^5,6

There are 2 classes of linkers: cleavable, which become cleaved in the presence of low pH in the endosome, leading to release of the cytotoxic drug, and noncleavable, where the whole antibody is degraded before the drug is released and activated.⁷The process of attaching the drug to the antibody through a linker her undergone great improvements, with a more predictable number of drug molecules bound to the antibody moiety, resulting in enhanced efficacy and predictable toxicity.

Early ADCs concentrated on improved delivery of chemotherapy agents commonly used in the treatment of various malignancies. Frequently, these drugs did not achieve high enough concentrations in tumor tissues to allow for a meaningful cytotoxic effect.⁷ADCs are able to increase the therapeutic window for cytotoxic drugs. The drug moieties in use today are mostly highly potent cytotoxic agents that could not be used alone due to their toxicities. The most common drugs in use with ADCs work through either inhibition of tubulin polymerization (auristatins and maytansinoids) or through DNA binding (calicheamicins).

In this review, we will describe current uses and potential future ADC treatments with applications in breast cancer.

Targets

The optimal targets for ADCs are antigens expressed on the surface of cancer cells in high density, without expression on normal tissue.^7,8The antigen also has to be readily internalized after forming a complex with the ADC to allow for release of the cytotoxic payload. There has been an enormous amount of research to identify possible targets on cancer cells. Unfortunately, even the most attractive cancer antigens have potential downsides: either they are not expressed at high enough density on the surface of cancer cells or only small percentages of tumor cells express it. Most of them are also expressed to some degree in normal tissue, which elicits concern for toxicity.⁹

The majority of nonconjugated antibodies against breast cancer antigens have failed to show a clinical benefit. Their lack of efficacy could be due to incomplete signal blockade, insuficient ADCC or complement response. The successful antibodies became the most attractive for initial testing of ADCs. Because the ADCs’ mechanism of tumor killing is different from nonconjugated antibodies, it is believed that a broader range of tumor antigens can be targeted.

Identification of HER2-amplified tumors and the development of trastuzumab has been one of the major success stories in the treatment of breast cancer. Trastuzumab is a very attractive antibody to use as a scaffold for the delivery of a cytotoxic payload. It is linked to the maytansinoid DM1 through a non-cleavable linker.

T-DM1 Clinical Trial Findings

The efficacy and safety of ado-trastuzumab emtansine (T-DM1) in patients with metastatic breast cancer who were previously treated with trastuzumab has been evaluated in 2 phase III trials. The EMILIA study was a randomized, open-label, international trial of patients with HER2-amplified, unresectable, locally advanced or metastatic breast cancer who had previously received trastuzumab and a taxane. Patients were randomized 1:1 to receive T-DM1 or lapatinib (1250 mg daily) plus capecitabine (1000 mg/m2 of body-surface area every 12 hours). Primary endpoints included progression-free survival (PFS) by independent review, overall survival (OS), and safety. Secondary endpoints included PFS by investigator review, objective response rate (ORR), duration of response, and time to symptom progression.

A total of 495 patients were randomized to receive T-DM1, and 496 patients received lapatinib plus capecitabine. Baseline demographics were similar between groups. The median age was 53 years (range, 24-84 years). The majority of patients had visceral involvement of disease (68%) and many (61%) had received 0 to 1 prior chemotherapy regimen for locally advanced or metastatic disease.

PFS by independent review was prolonged in the T-DM1 group compared with those receiving lapatinib plus capecitabine, 9.6 months versus 6.4 months, respectively (P < .001).10 The bene t of T-DM1 therapy was most pronounced in younger patients with visceral disease. OS at the second interim analysis was a median of 30.9 months versus 25.1 months, favoring T-DM1; (hazard ratio [HR] 0.68; 95% CI, 0.55-0.85; P <.001). The estimated survival rates at 1 and 2 years were 85.2% and 64.7%, respectively, in the T-DM1 arm and 78.4% and 51.8%, respectively, in the lapatinib plus capecitabine arm.

For the secondary endpoints, investigator assessment revealed a similar benefit with T-DM1 as seen with the independent review, with a PFS of 9.4 months for T-DM1 and 5.8 months for lapatinib plus capecitabine (HR 0.66; 95% CI, 0.56-0.77; P <.001). The objective response rate was 43.6% for T-DM1 and 30.8% for lapatinib plus capecitabine, with a median duration of response of 12.6 months versus 6.5 months, respectively. Time to symptom progression, based on prespecified endpoints, was 7.1 months with T-DM1 compared with 4.6 months with lapatinib plus capecitabine.

In the T-DM1 group, 5.9% of patients discontinued T-DM1 due to adverse events (AEs). Serious AEs were reported less frequently in the T-DM1 group versus the lapatinib plus capecitabine group at 18% versus 15.5%, respectively. Grade 3 or greater AEs also occurred less frequently in the T-DM1 group, 40.8% versus 57%, respectively.

The most common grade 3 or 4 AEs with T-DM1 were thrombocytopenia (12.9%), elevated aspartate aminotransferase (4.3%), and alanine aminotransferase (2.9%). The first occurrence of grade 3 or 4 thrombocytopenia was most commonly reported during the first 2 cycles of T-DM1. Subsequently, a higher incidence of bleeding events was also reported with T-DM1 (29.8% vs 15.8%).¹¹

TH3RESA was a phase III, randomized, multicenter, open-label trial in patients with HER2- amplified, unresectable locally advanced or recurrent breast cancer or metastatic breast cancer who had previously received both trastuzumab and lapatinib in the advanced setting and a taxane in any setting with progression after treatment, with a minimum of 2 HER2-directed regimens for advanced breast cancer.¹²Patients were randomized 2:1 to receive T-DM1 or treatment of physician&rsquo;s choice (TPC). TPC was limited to chemotherapy (any single agent), hormonal therapy for hormone receptor (HR)-positive disease (single-agent or dual therapy), or HER2-directed therapy (single-agent, dual HER2- targeted therapy, or combination with either single-agent chemotherapy or single-agent hormonal therapy). Best supportive care alone was not allowed.

Primary endpoints included investigator-assessed PFS and OS. Secondary endpoints included investigator-assessed objective response, duration of objective response, 6-month and 1-year survival, safety, general health status or quality of life (QOL) and health- related QOL, symptom severity and interference, and pain ratings. A total of 404 patients were randomized to receive T-DM1 and 198 patients to receive TPC. The majority of patients were younger than 65 years of age, with a median age of about 53 years, with metastatic disease and visceral disease involvement.

The median number of previous regimens for advanced breast cancer was 4 (range, 1-19), excluding single-agent hormonal therapy. Among the group who received TPC, the majority (83%) received combination therapy with a HER2-directed agent (83%), which most commonly included trastuzumab plus chemotherapy (68%) or trastuzumab plus lapatinib (10%). The remaining 17% received single-agent chemotherapy, which most commonly included vinorelbine (32%), gemcitabine (16%), or other (17%).¹²The primary endpoint of PFS was 6.2 months with T-DM1 compared with 3.3 months with trastuzumab plus lapatinib. PFS continued to be significantly improved in the T-DM1 group when compared with the subgroup of patients who received a trastuzumab-containing regimen as the TPC; however, the benefit with T-DM1 was consistent across all subgroups.

Overall survival (OS) at the first interim analysis was not statistically significant. Of those in the T-DM1 group, 15% had died versus 22% in the TPC group (HR 0.552; 95% CI, 0.369-0.826; P = .0034). However, estimated 6-month and 1-year survival rates were higher in the T-DM1 group. Regarding the secondary endpoints, the objective response was 31% with T-DM1 versus 9% with TPC (P <.0001). The median duration of response was 9.7 months with T-DM1, but had not yet been reached in the TPC group at the time of data cutoff.

AEs of any grade occurred in 94% of patients receiving T-DM1 versus 89% of those receiving trastuzumab plus lapatinib. Similar to the EMILIA trial, a higher incidence of grade 3 or greater thrombocytopenia was seen with T-DM1. The most common AEs with T-DM1, occurring in 10% or more of patients, included fatigue, asthenia, thrombocytopenia, dyspnea, and diarrhea. Regarding patient-reported out- comes, 57.8% of patients in the T-DM1 arm compared with 47.1% in the TPC arm experienced a clinically meaningful improvement in global health status based on prespecified criteria. The most bothersome symptoms in the T-DM1 group were fatigue and pain, while diarrhea and nausea/vomiting were reportedly more tolerable. Time to pain progression was not significantly different between groups.¹³

These 2 pivotal phase III trials have demonstrated that T-DM1 is active in HER2-amplified tumors that have progressed on treatment with trastuzumab. T-DM1 is generally well tolerated, with the most common AEs including thrombocytopenia, elevated alanine and aspartate aminotransferases, fatigue, diarrhea, and nausea/ vomiting.

The TH3RESA and EMILIA studies also included a biomarker analysis. A greater relative risk reduction for PFS was seen in those patients expressing greater than median HER2 mRNA levels. The benefit was seen in low- and high-HER3 mRNA expressing tumors. In EMILIA, EGFR and PTEN expression had no influence on OS and PFS benefit for the T-DM1 treatment arm. Abnormalities in the PI3K-Akt pathway have been associated with resistance to trastuzumab therapy. PIK3CA mutations are the most common alteration in this pathway in breast cancer and have been found to confer resistance to anti-HER2 therapy. In the TH3RESA trial, PIK3CA mutations were not associated with a decreased PFS in the T-DM1— treated arm. In the EMILIA analysis, the PIK3CA mutations were associated with a shorter PFS in the control-treated arm, although this was not seen in the TH3RESA analysis.

While the biomarkers need to be con rmed in a prospective study, these data present early evidence that T-DM1 can overcome resistance to anti-HER2 therapy due to PIK3CA mutations. This is most likely due to the difference in mechanism of action of T-DM1 as it does not require interruption of HER2 signaling in order to cause tumor cell death.^11,14

TDM4874g (NCT01196052) was a phase II trial that investigated the safety and feasibility of T-DM1 following anthracycline-based chemotherapy in the adjuvant or neoadjuvant setting for patients with HER2-amplified early-stage breast cancer (EBC). Patients received 4 cycles of adjuvant or neoadjuvant T-DM1 after doxorubicin plus cyclophosphamide (every 2 or 3 weeks for 4 cycles) or 5- uorouracil plus epirubicin plus cyclophosphamide (every 3 weeks for 3 to 4 cycles). Patients could continue T-DM1 for up to 17 cycles of HER2-directed therapy. After the first 4 cycles of T-DM1, radiotherapy and 3 to 4 cycles of docetaxel with or without trastuzumab were optional before continuing T-DM1.

The primary endpoints were safety and rate of prespecified cardiac events occurring within the first 12 weeks of treatment with T-DM1. Of the 153 patients included, 99 received treatment in the adjuvant setting. The majority of patients (73.9%) were between 41 and 64 years of age. Almost all patients (96.7%) received T-DM1, for a median of 14 cycles, and 82% completed the planned approximate year of HER2- directed therapy.¹⁵

Of the 50 patients treated with neoadjuvant therapy and undergoing surgery, the pathologic complete response (pCR) rate was 56%. The pCR rate was 51.7% for patients with HR-positive disease and 61.9% for patients with HR-negative disease. No protocol prespeci ed cardiac events or congestive heart failures were reported in a median follow-up time of 24.6 months. Approximately 3% of patients had asymptomatic LVEF declines leading to the discontinuation of T-DM1.¹⁶The most common AEs included nausea, headache, epistaxis, asthenia, pyrexia, fatigue, arthralgia, thrombocytopenia, and myalgia.¹⁷

This study concluded that T-DM1 appears feasible and safe with concurrent radiotherapy or hormonal therapy and should be further investigated in EBC. These results compared favorably with the pCR rates achieved in the TRYPHENA trial (approximately 50%), which evaluated double anti-HER2 therapy with pertuzumab and trastuzumab.¹⁸Neoadjuvant T-DM1 was also evaluated as part of the ongoing I-SPY protocol (NCT01042379) where 83 women with invasive breast cancers 2.5 cm or greater (any HR status) were randomly assigned to either T-DM1 plus pertuzumab or trastuzumab plus paclitaxel for 12 weeks, then 4 weeks of doxorubicin plus cyclophosphamide, followed by surgery. More patients reached a pCR with the T-DM1 combination (46% to 64% depending on the biomarker signature) compared with the trastuzumab combination (17% to 33%). In addition to better pCR, this combination has better tolerability and fewer adverse effects than the paclitaxel combination with trastuzumab.

The MARIANNE trial is the first phase III study that evaluated the combination of a targeted antibody and an ADC for first-line metastatic breast cancer. Patients were randomized 1:1:1 to 1 of 3 arms: trastuzumab plus docetaxel, T-DM1 plus pertuzumab, or T-DM1 plus placebo. The primary endpoint is PFS by independent review and secondary endpoints include safety, overall response rate, OS, duration of response, and QOL. The clinical trial results showed no increase of PFS in the T-DM1—containing treatment arms. Median PFS was 15.2 months and 14.1 months in the T-DM1 plus pertuzumab arm and the T-DM1 alone arm, respectively, compared with 13.7 months with trastuzumab plus a taxane. ORRs were comparable between the groups as well. ORRs were 64.2%, 59.7%, and 67.9% for the T-DM1 plus pertuzumab arm, T-DM1 alone, and trastuzumab plus a taxane arm, respectively. The results of MARIANNE concluded that trastuzumab, pertuzumab, and a taxane remain first-line therapy, but T-DM1 is a preferred second-line option.¹⁷

T-DM1 is being evaluated alone and in combination with chemotherapy and other targeted agents in multiple trials (Table 1). Over the next several years, we will be learning more about its full scope of activity, modes of resistance, and possibly better methods of choosing patients for therapy. We will continue to learn more about this class of drugs and the best possible partners for combination therapies.

T-DM1 in Brain Metastases

Brain metastases pose a specific challenge in treating patients with metastatic breast cancer since there are limited options for systemic treatments that can cross the blood—brain barrier. Many factors contribute to this, most notably is the size of the drug molecules and the high expression of P-glycoprotein that actively mediates the efflux of chemotherapy agents.¹⁹Prospective studies have demonstrated the efficacy of single—agent lapatinib or lapatinib plus capecitabine in HER2-positive central nervous system (CNS) metastases.²⁰Some retrospective studies suggest that trastuzumab increases the time to development of brain metastases and improves survival after brain metastases development.^21,22Krop et al in a retrospective exploratory analysis in EMILIA found significant improvement in OS in the T-DM1 treated arm compared with the lapatinib plus capecitabine treated arm among metastatic breast cancer patients with CNS metastases at baseline, 26.8 months versus 12.9 months, respectively. PFS was similar in both treatment arms, 5.9 months versus 5.7 months, respectively.²⁰This finding is very intriguing especially in this difficult-to-treat patient population. Further studies are needed to confirm this finding.

Early Clinical Trials

Additional ADCs are currently being tested in early clinical trials (Table 2). SYD985 is a new HER2-targeting ADC that is linked to a duocarmycin payload (vc-seco-DUBA) conjugated to trastuzumab. Preclinical trials have demonstrated the efficacy of SYD985 in patients with breast cancer who have low expressions of HER2 superior to T-DM1.²³There is currently a phase I study recruiting patients for the first in-human study to evaluate safety and efficacy in cancer patients (NCT02277717). The primary outcome will be the incidence of dose-limiting toxicities and secondary outcome measures are: number of patients with AEs, area under the plasma concentration versus time curve of SYD985, peak plasma concentration, change in baseline of hematology and blood chemistry parameters, number of patients who develop antibodies against the ADC, and ORR.

MEDI4276

Another drug that has shown potential in preclinical development in T-DM1—resistant cells is MEDI4276. It is a bivalent, biparatopic antibody; thus, it binds to 2 distinct nonoverlapping epitopes on HER2, which results in antibody-receptor clustering and promotes internalization, lysosomal traf cking, and degradation. Potent cell killing activity was observed in both in vitro and in vivo xenograft mouse models.²⁴This drug has demonstrated tumor regression in HER2-positive tumor models that have either developed resistance to T-DM1 or in models with lower HER2 expression refractory to T-DM1.24 MEDI4276, is currently being tested in a phase I clinical trial (NCT02576548).

XMT-1522

XMT-1522 is another novel ADC that targets HER2. However, it incorporates HT-19, a human anti-HER2 antibody that is noncompetitive for HER2 binding with trastuzumab and pertuzumab to allow for potential combination therapies.²⁵The combination of XMT-1522 with trastuzumab and/or pertuzumab did demonstrate more rapid internalization, more complete HER2 degradation, and greater antitumor activity in the NCI-N87 gastric cancer xenograft model compared with XMT-1522 alone or the combination of pertuzumab and trastuzumab.²⁵This drug has shown anticancer activity in low HER2-expressing tumors.²⁶There is currently a phase Ib dose-escalation and expansion in patients with advanced breast cancer and other advanced tumors expressing HER2 scheduled to be open for recruitment (NCT02952729).

Glembatumumab Vedotin

Glembatumumab vedotin (CDX-011) is an ADC composed of CR011, a fully human IgG2 monoclonal antibody against glycoprotein nonmetastatic gene B (GPNMB), conjugated via a valine-citrulline link to the potent microtubule inhibitor monomethyl auristatin E (MMAE). GPNMB has been found to be expressed in a multitude of cancers^27-29and overexpressed in triple-negative breast cancer cell lines.³⁰GPNMB expression has also been associated with high endothelial cell density and was shown to induce endothelial cell migration in vitro. It has also been found to upregulate metalloproteases.^31,32Its exact function in breast cancer is not known, although it may play a role in invasion and metastasis. GPNMB is also expressed in normal tissue, with its mRNA detected in bones, adipose, thymus, skin, placenta, heart, kidneys, pancreas, lungs, liver, and skeletal muscle, which increases the potential for toxicity with any anti-GPNMB therapy.^33-35

It appears that normally functioning GPNMB is localized in the intracellular component, while its expression in tumor cells is mostly on the cytoplasmic membrane.^36-38This characteristic makes it a very attractive target for ADCs and minimizes the potential for toxicities from this therapy. In pre-clinical studies, a single dose of CDX-011 led to the regression of the GPNMB-expressing breast cancer cell line MDA-MB-468 in an animal model.³⁸CDX- 011 is currently being investigated in patients with melanoma and breast cancer.

A phase I/II study assessed the safety and activity of glembatumumab in patients with locally advanced and metastatic breast cancer and examined the relationship between GPNMB expression and response. Patients included those with unresectable or metastatic breast cancer who had undergone at least 2 prior chemotherapy regimens. Use of a taxane, anthracycline, and capecitabine (as well as trastuzumab if HER2 amplified) was required. A 3+3 dose escalation design was used. A total of 42 heavily pretreated patients were enrolled, with a median of 7 prior regimens. Three patients who were treated at low dose and 10 patients at the maximum tolerated dose (MTD) had triple-negative breast cancer (TNBC). Eighty-four percent of tumors tested were positive for GPNMB. Worsening neuropathy was identified as the dose-limiting toxicity.

The primary endpoint was reached in the phase II study at a dose of 1.88 mg/kg, with 33% of the 27 evaluable patients achieving PFS at 12 weeks. At the phase II dose, median PFS was 9.1 weeks for all patients, 17.9 weeks for those with triple-negative disease, and 18 weeks for GPNMB-expressing tumors. Non—dose-limiting hematologic toxicity and mild-to-moderate rash were observed. The investigators concluded that glembatumumab has an acceptable safety profile as well as antitumor activity in heavily pretreated pa- tients with metastatic breast cancer.³⁹Although the response rates in nonselected patient populations were low, the results in the GPNMB-expressing TNBC cohort were encouraging. These results speak to the targeted nature of this therapy and might result in first targeted therapy for patients with TNBC. Further studies evaluating glembatumumab are ongoing in GPNMB-expressing tumors (NCT01997333).

The EMERGE trial is another phase II study investigating the activity of glembatumumab in advanced breast cancer positive for GPNMB expression. The primary endpoint was ORR. A GPNMB expression analysis found that expression was much higher in the TNBC samples compared with all other types. The primary endpoint ORR was 6% for the glembatumumab arm and 7% for the investigator&rsquo;s choice (IC) arm. However, malignant epithelial cells with a higher level of GPNMB expression had a statistically significant higher tumor response compared with all other types. For tumors with &ge;25% malignant cells overexpressing GPNMB, the ORR was 30% in the glembatumumab arm and 9% for the IC arm. Patients with TNBC had an 18% response rate with glembatumumab compared with no response with IC treatment. For patients with both high GPNMB expression and TNBC tumors, the response rate was 40% in the glembatumumab arm and 0% in the IC arm. This trial suggests that patients with GPNMB overexpression and TNBC may derive the greatest benefit from glembatumumab. The study also reported that the ADC was better tolerated than the investigator&rsquo;s choice of chemotherapy.⁴⁰METRIC, a phase II trial (NCT01997333), has been initiated to confirm the findings in this study; the primary analysis endpoint will be PFS. Secondary endpoints include ORR, duration of response, OS, QOL, and safety. Eligible patients will have TNBC with >25% expression of GPNMB.⁴¹

Trop-2

Trop-2 is another potential cancer antigen that is a subject of interest in breast cancer. Trop-2 is a calcium signal transducer that has been associated with stimulation of growth of malignant cells.⁴²Its full biological function has not yet been elucidated. Trop-2 overexpression has been found in a variety of cancers, including breast, ovarian, prostate, endometrial, lung, colon, and thyroid, but it has limited cytoplasmic membrane presence in normal tissue.^{43- 49}Its expression on the cytoplasmic membrane, as opposed to expression in the intracellular component, is associated with worse outcomes in patients with breast cancer. Therefore, Trop-2 is an attractive target, especially in more aggressive breast cancer phenotypes, such as the triple-negative subtype.^43-49

IMMU-132

IMMU-132 is an ADC consisting of hRS7, a readily internalized antibody against Trop-2, which is conjugated through a cleavable linker to SN-38, an active metabolite of irinotecan (CPT-11). Early data from an ongoing phase I/II clinical trial in epithelial cancers were reported. In a 3+3 design, neutropenia was found to be the dose-limiting toxicity, and the phase II component proceeded at doses of 8 mg/kg and 10 mg/kg in patients with colorectal, small-cell lung, and TNBCs. Of the 36 patients in the phase II component, 14 had an assessment of response and 8 were found to have at least stable disease. Interestingly, 5 patients were found to be homozygous UGT1A1 *28/*28, with 2 of them experiencing more severe hematologic and gastrointestinal toxicities. A phase II study (NCT01631552) evaluating IMMU-132 at 10 mg/kg in patients with metastatic TNBC has reported preliminary data. Sixty-nine patients have received treatment with ORR of 29% (19/66) with 2 confirmed complete responses and 17 confirmed partial responses. Median PFS is 5.6 months with median OS of 14.3 months. The treatment was very well tolerated with most grade 3/4 toxicities being hematologic.⁵⁰

BAY94-9343

BAY94-9343 is an ADC consisting of IgG1-directed against the cell surface glycoprotein mesothelin, which in turn is conjugated through a cleavable disulfide-containing linker to the maytansinoid DM4. Mesothelin has been found to be expressed in over 60% of TNBC.^51,52BAY94-9343 recently completed phase I clinical testing (NCT01439152). Eligible patients had solid tumors refractory to standard treatment; there were 45 patients with mesothelioma, pancreatic, breast, ovarian, or other cancers. There were a total of 147 individuals in the expansion cohort consisting of patients with mesothelioma and ovarian cancer were included but there were no data on response. The MTD was found to be 6.5 mg/kg given IV every 21 days. Patients with mesothelioma and ovarian cancer had partial responses of 31% and 9%, as well as stable disease in 50% and 44% of patients, respectively. Thirty-one percent of the patients with mesothelioma had a partial response of >600 days. Patients with mesothelioma using BAY94- 9343 as a second-line option had a 50% response rate. Durable response rates were found in patients with both ovarian cancer and mesothelioma.⁵³A phase II clinical trial is ongoing for use in metastatic pleural mesothelioma (NCT02610140). There are no plans for a breast cancer—specific trial at this time, but we will watch future developments closely.

SGN-LIV1A

SGN-LIV1A is an ADC with a backbone of an antibody against the antisolute family 39 zinc transporter member 6 (LIV-1) conjugated with a cleavable linker to MMAE. It is currently being tested in a phase I clinical trial (NCT01969643). LIV-1 expression is induced by estrogen stimulation and has been shown to lead to epithelial-to-mesenchymal transition (EMT), progression of the disease, and metastasis.^54,55Expression of LIV-1 has been found in breast cancers that have progressed on endocrine therapies and, in a smaller percentage of TNBC cells. Preclinical results have shown very robust activity against cells expressing LIV-1.⁴⁴SGN-LIV1A has robust in vitro and in vivo activity both alone and in combination with other chemotherapy agents.⁵⁶A phase I study of SGN- LIV1A in patients with breast cancer is ongoing and recruiting patients (NCT01969643).⁵⁷

Novel areas of exploration

Novel areas of exploration with potential application in treatment of breast cancer are ADCs that target antigens in the tumor vasculature and stroma. A new target being studied for solid tumors is VEGFR2 (CD309). Xiangbao et al found that in cancer model anti-VEGFR-2 ScFv-As2O3-stealth nanoparticles could inhibit angiogenesis.⁵⁸Other targets in tumor vasculature and stroma that have shown potential in solid tumors are Fibronectin extra-domain B (ED-B)59, endothelin receptor ETB60, tenascin c61, collagen IV62, and periostin.⁶³Tumor microenvironment has been shown to be increasingly important in tumor proliferation, metastasis, resistance to therapies and immune avoidance.^64-66Targeting antigens in the tumor microenvironment has a potential to greatly expand possible targets in breast cancer and may lead to novel approaches and combination treatments.

Conclusions

ADCs offer enormous potential for developing targeted therapies against breast cancer, as well as other malignancies. The targeted nature of this therapy results in an improved therapeutic window for the cytotoxic payload, which allows for the use of highly toxic molecules that have a prohibitive toxicity profile if used alone. Currently, for the ADC to be functional, it needs to be internalized after binding to its intended antigen. Developing new methods of release of a toxic payload may lead to identification of new targets, as well as increased bystander effect with resulting enhanced antitumor effect.

Although antibodies have the advantage of long in vivo half-lives, their large size may inhibit penetration into solid tumors. ADCs encounter the same limitation currently with antibody targets and forms scaffolding for delivery of the toxic moiety. One area of research to overcome this limitation is development of conjugates to antibody fragments or other targeted delivery systems with much smaller size with improved tumor penetrance. Continuing research in combination therapies is ongoing with improved penetrance of ADCs due to the effect of partner drug.

ADCs are designed to decrease toxicities of their chemotherapy payload. As such they are potentially a good partner for combination therapies. As seen with development of T-DM1, as new ADCs are developed, there will be increased interest in combining them with conventional chemotherapy, targeted agents, and immunotherapy approaches.

Another promising field is investigating antigens in tumor stroma for ADC targeting. Developing novel drugs for the tumor microenvironment holds the promise of interfering with proliferation, angiogenesis, invasion, and metastasis. With the increasing evidence for potential of immunotherapies in breast cancer and TNBC in particular, special interest will be given to ADCs designed to potentiate immune response against cancer. The hope is that the exciting preclinical data will result in innovative combination trials.

Currently, there are more than 50 ADC compounds being investigated at various stages of development. It is quite conceivable that more compounds will show efficacy in the treatment of breast cancer as new antigens are discovered. Theoretically, as more of these compounds reach the clinic, they should be active against any cancer cell expressing high enough density of the given antigen, irrespective of tissue of origin. Molecular pathology will play an increasingly important role in identifying treatment options for our patients. Scientists and clinicians will have to decide which tumors have suf cient probability of expressing a given antigen to be tested. As the era of personalized medicine arrives, tumor antigens should be a part of the gamut of tests for choosing the best treatment options for patients.