Clinical Potential of HER2 and HER3 Imaging in Breast Cancer

SYNOPSIS

Increased expression of the HER protein family, especially HER2 and HER3, are biomarkers in breast cancer that have become significant targets for molecular imaging and therapy. Imaging modalities that target HER2 and HER3 have the ability to diagnose breast cancer with a specific, biologically relevant target in a non-invasive manner. Repeat tumor biopsies do not address the issue of heterogeneity intratumorally or between primary disease and metastasis. HER2- and HER3- targeted PET is an important tool to diagnose disease state in breast cancer and evaluate response to therapies that target these receptors. PET is a highly sensitive and quantitative imaging technique with subnanomolar detection. PET and SPECT with radiolabeled biomolecules can be used for noninvasive detection and quantification of specific targets, allowing physicians and scientists alike to better understand the behavior and effectiveness of treatment across patient cohorts.

INTRODUCTION

Breast cancer is the most common cancer in women worldwide, with approximately 12% of women in the United States developing invasive breast cancer in her lifetime.¹ Although breast cancer has come a long way in regards to treatment options and overall survival, there are still challenges that occur mostly from intratumoral heterogeneity² and metastatic disease.³ While significant research has emerged to identify biological processes leading to breast cancer,⁴ selecting patients that would benefit from targeted therapies is still a major hurdle in the field. Non-invasive tools to stratify patients and facilitate precision medicine are needed to address this issue.^5–7

Although breast cancer is typically diagnosed and staged via three main biomarkers, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2),⁸ there are other biomarkers of interest that are relevant and may be beneficial to the clinical outcome of breast cancer. Reports of clinical benefit from HER2-targeted therapy in patients with a primary HER2-negative tumor have facilitated to spearhead the concept behind HER2-targeted molecular imaging for unsuspecting metastasis in breast cancer.^9–10 An additional biomarker of interest includes human epidermal growth factor receptor 3, or HER3.¹¹ HER3 overexpression has been linked with poor prognosis in multiple cancer subtypes including breast, which has driven interest in HER3 as potential target for both imaging and therapy. Additionally, emerging resistance to HER2-targeted therapy has driven the need to explore additional targets for both imaging and therapy.¹¹ The HER family in general has been extensively studied in breast cancer,¹² and there are many HER2-targeted agents for therapy and molecular imaging,^13–15 and strategies to target HER3 are also on the rise with clinical trials.^16–17

Molecular imaging, particularly with tracers used in positron emission tomography (PET) and single-photon emission computed tomography (SPECT) have the ability to non-invasively detect heterogeneity between primary tumors and metastases in breast cancer.^{6, 18} PET and SPECT-based measurement of the HER2 and HER3 expression in breast cancer offers several advantages over repeated biopsies in patient cohorts. Several groups have developed antibody, affibody, and nanoparticle-based radioligands for PET imaging preclinically and clinically,^19–22 many of which include radiotracers used to image HER2 and HER3 in breast cancer.^{13, 23} Pairing these targeting moieties with longer-lived radioisotopes such as zirconium-89 (⁸⁹Zr, t_1/2 = 78 h) for PET, along with lutetium-177 (¹⁷⁷Lu, t_1/2 = 6.7 days) and indium-111 (¹¹¹In, t_1/2 = 67 h) for SPECT can be used to match the biological half-life of antibodies in vivo (t_1/2 ~72 h for typical full-length antibodies).²⁴ Isotopes like copper-64 (⁶⁴Cu, t_1/2 = 12 h) or technium-99m (^99mTc, t_1/2 = 6 h) can be used in PET and SPECT, respectively, for both full length and antibody fragments, as well as affibodies.^25–27 Antibody fragments and affibody molecules are often labeled with shorter-lived isotopes such as gallium-68 (⁶⁸Ga, t_1/2 = 68 min) or fluorine-18 (¹⁸F, t_1/2 = 110 min) to pair appropriately with shorter biological half-lives in vivo.^28–31

Many of the PET and SPECT imaging tracers that have made it to the clinic (in Phase I/II trials) to detect HER2-positive breast cancer and metastases, are summarized in our previous review.¹⁵ This current review seeks to communicate an update about the current clinical trials for both HER2 and HER3-targeted imaging, which are outlined in Table 1. The current clinical radiotracers used to image HER3⁺ and HER2⁺ breast cancers are review within.

Table 1.

Summary of current clinical trials with PET and SPECT radiotracers targeting HER2 and HER3-positive breast cancer in 2017. BC: breast cancer. MBC: metastatic breast cancer. IMPACT: IMaging PAtients for Cancer Drug selection.

64Cu-DOTA-Patritumab 89Zr-DFO-Lumretuzumab	RO, PK, and Treatment Response Treatment Response	NCT01479023 NCT01482377	Washington University School of Medicine Hoffmann-La Roche	Terminated Completed
89Zr-DFO-Trastuzumab	HER2+ BC HER2+ MBC IMPACT-MBC	NCT02065609 NCT02286843 NCT01957332	Washington University School of Medicine Memorial Sloan Kettering Cancer Center University Medical Center Groningen	Recruiting Recruiting Recruiting
89Zr-DFO-Pertuzumab	HER2+ MBC HER2+ MBC	NCT03109977 NCT02286843	Memorial Sloan Kettering Cancer Center Memorial Sloan Kettering Cancer Center	Completed Recruiting
64Cu-DOTA-Trastuzumab	Treatment Response in HER2+ BC HER2+ BC	NCT02827877 UMIN000017446	City of Hope Medical Center P-DIRECT	Recruiting Completed
177Lu-DOTA-Trastuzumab	HER2+ BC Healthy Patients and HER2+ BC HER2+ MBC	Bhusari et al. NCT02683083 NCT01304797	PGIMER Camel-IDS NV Merrimack Pharmaceuticals	Completed Recruiting Ongoing, Not Recruiting
[131I]-SGMIB Anti-HER2 VHH1
64Cu-MM-302

PET AND SPECT IMAGING WITH MONOCLONAL ANTIBODIES IN HER2-POSITIVE BREAST CANCER

An extensively studied target for imaging in breast cancer is HER2, as clinical trials utilizing this target are happening at cancer centers all over the world.^{9, 32–33} HER2 imaging can address the issue of tumor heterogeneity and reliably determine both the quantity and the functional status of tumor HER2 in individual lesions in a noninvasive manner. This technique is of critical importance to identify patients who would truly benefit from HER2-targeted therapy, and to monitor the change in HER2 status during immunotherapy.^34–35 We have previously reviewed HER2 targeted imaging,¹⁵ and provide an update of more recent work here.

⁸⁹Zr-Trastuzumab

⁸⁹Zr-trastuzumab has been vastly studied across patient cohorts and clinical trials for both primary and metastatic breast cancer.^{9, 36–40} Ulaner and coworkers have expanded the field with ⁸⁹Zr-trastuzumab to detect unsuspecting metastasis in both HER2⁺ and HER2⁻ primary breast tumors.^{9, 41} Multiple studies are being carried out at Memorial Sloan Kettering Cancer Center to image this HER2 expression discordance and guide biopsies.^41–42 Another current clinical trial utilizing ⁸⁹Zr-Trastuzumab is led by Laforest and coworkers at Washington University in St. Louis.⁴³ Twelve women were enrolled in the study; six with primary breast cancer and six with metastatic breast cancer. Eleven of these patients underwent ⁸⁹Zr-trastuzumab PET/CT during neoadjuvant or adjuvant therapy, and the remaining patient was imaged before neoadjuvant therapy. Laforest et al. found that increasing the dose to 62 ± 2 MBq (vs. the average dose of 37 MBq, which was used in previous studies done by Dijkers et al.^44–45 the ZEPHIR trial)³³ allowed for optimal images at later time points (Figures 1 and 2). The dose-limiting organ was found to be the liver, with a dose of 1.63 mSv/MBq. The biodistribution of ⁸⁹Zr-trastuzumab did not significantly change during the 7 days of imaging. Slow blood clearance was observed with a biological half-life of 113 h and an initial level of 58% ID. Overall, this study showed that imaging ⁸⁹Zr-trastuzumab imaging is safe, has acceptable dosimetry, and provides a noninvasive means of assessing HER2 status of individual lesions in patients with breast cancer.

FIGURE 1.

Anterior and posterior reprojection PET images of a patient with HER2-positive metastatic breast cancer on a day 3 and b day 6 following ⁸⁹Zr-trastuzumab administration demonstrate greater tracer uptake in the right femoral metastatic lesion without other significant change in tracer biodistribution on day 6 compared to day 3. The area of intense activity in the right chest is related to administration of ⁸⁹Zr-trastuzumab via a port catheter.

From Laforest R, Lapi SE, Oyama R, Bose R, et al. [89Zr]Trastuzumab: Evaluation of Radiation Dosimetry, Safety, and Optimal Imaging Parameters in Women with HER2-Positive Breast Cancer. Mol Imaging Biol 2016;18(6): 952-959; with permission.

FIGURE 2.

Axial PET (top) and fused PET/CT (bottom) images of a patient with HER2-positive metastatic breast cancer at the chest level on a day 2 and b day 4 following ⁸⁹Zr-trastuzumab administration demonstrate greater tracer uptake in the right humeral metastatic lesion on day 4 compared to day 2.

From Laforest R, Lapi SE, Oyama R, Bose R, et al. [89Zr]Trastuzumab: Evaluation of Radiation Dosimetry, Safety, and Optimal Imaging Parameters in Women with HER2-Positive Breast Cancer. Mol Imaging Biol 2016;18(6): 952-959; with permission.

⁸⁹Zr-Pertuzumab

As ⁸⁹Zr-trastuzumab has been associated with avidity on PET/CT that does not allow correlate with HER2 positivity on pathology, additional HER2 targeted tracers have been investigated. For example, pertuzumab is a newer humanized monoclonal antibody that binds to the HER2 receptor at a site distinct from trastuzumab and appears to be more efficient than trastuzumab.⁴⁶ A first-in-human trial of ⁸⁹Zr-pertuzumab imaging in patients with HER2-positive breast cancer has demonstrated safety and dosimetry.⁴⁷ A potential clinical application of ⁸⁹Zr-pertuzumab and other HER2-targeting agents was demonstrated in patients with two primary breast malignancies, one HER2+ and the other HER2-, who developed brain metastases. HER2-targeted imaging allowed determination of the HER2-status of the brain metastases, and could assist in the choice of HER2-targeted therapy. In one patient, resection of a symptomatic brain metastasis confirmed a HER2-positive metastasis, as imaged on ⁸⁹Zr-pertuzumab PET/CT (Figure 3).

FIGURE 3.

46-year-old woman with both HER2-positive and HER2-negative primary breast malignancies and recently diagnosed brain metastases. Sequential maximum-intensity projection (MIP) images (A) 1 day, (B) 2 days, (C) 6 days, and (D) 8 days following administration of ⁸⁹Zr-pertuzumab. Blood pool and liver background clears on sequential images. Excreted bowel activity is seen on days 1 and 2. Bilateral kidney activity is visualized on all days. Increasing activity in foci overlying the skull is seen as time progresses (arrows). Decreasing activity is seen in the blood pool of the superior sagittal sinus (arrowheads). (E) Gadolinium-enhanced T1 weighed MR of the brain demonstrates enhancing brain metastases (arrows) and the superior sagittal sinus (arrowhead). (F) Axial fused PET/CT, CT, and PET images 8 days following ⁸⁹Zr-pertuzumab administration demonstrate avidity in the brain metastases (arrows) and minimal residual avidity in the superior sagittal sinus (arrowhead).

From Ulaner GA, Lyashchenko SK, Riedl C, et al. First-in-human HER2-targeted imaging using 89Zr-pertuzumab PET/CT: Dosimetry and clinical application in patients with breast cancer. J Nucl Med. In Press; with permission.

⁶⁴Cu-Trastuzumab

⁶⁴Cu-DOTA-trastuzumab has also been used in multiple clinical trials to date.^25–27 The first study to use this radiotracer was to determine the safety, distribution, internal dosimetry, and initial HER2–positive tumor images of ⁶⁴Cu-DOTA-trastuzumab in humans by Tamura et al.²⁶ Since this study, this tracer has been used for a number of studies to detect both HER2⁺ primary and metastatic disease and assess therapy response. Tamura and coworkers have expanded upon this work to assess intratumoral heterogeneity in HER2⁺ breast cancer.⁴⁸ In a separate study, SUVmax values were correlated with HER2-immunohistochemistry (IHC) scores and assessment for HER2 status by HER2-PET imaging strongly correlated with histologic HER2 expression status. Tamura and coworkers have also previously demonstrated HER2-specific ⁶⁴Cu-trastuzumab accumulation in a specimen of removed brain metastasis using IHC and autoradiography.²⁵ An image of the month study highlighted the first report describing the visualization of HER2-specific intratumoral heterogeneity (IHC 1⁺ and 2⁺) using HER2 PET imaging (Figure 4).⁴⁸ This interesting finding suggests that HER2 PET imaging could facilitate decision making for clinical treatment strategies. The relationship between HER2 PET imaging and the effects of anti-HER2 therapy, as well as the use of a high-resolution dedicated breast PET scanner, remain to be evaluated.

FIGURE 4.

Visualization of HER2-specific intratumoral heterogeneity (IHC 1⁺ and 2⁺) with ⁶⁴Cu-trastuzumab.

From Sasada S, Kurihara H, Kinoshita T, et al. Visualization of HER2-specific breast cancer intratumoral heterogeneity using 64Cu-DOTA-trastuzumab PET. Eur J Nucl Med Mol Imaging 2017; doi: 10.1007/s00259-017-3781-6; with permission.

Mortimer and coworkers are also using the ⁶⁴Cu-trastuzumab to assess metastatic breast cancer.⁴⁹ Women with biopsy-confirmed metastatic breast cancer and not given trastuzumab for 2 months underwent complete staging, including ¹⁸F-FDG PET/CT. Although ¹⁸F-FDG has been shown to be avid for some cancers, it does not fully address the heterogeneity in breast cancer.^5,50–52 Patients were classified as HER2-positive (HER2⁺) or negative (HER2⁻) based on fluorescence in situ hybridization (FISH) and IHC of biopsied tumor tissue (Figure 5). Eighteen patients underwent ⁶⁴Cu-trastuzumab injection, preceded in 16 cases by trastuzumab infusion (45 mg). PET/CT was performed on 1 and 2 days after h post-injection with ⁶⁴Cu-trastuzumab (Figure 6).

FIGURE 5.

Tumor uptake (SUVmax) of ⁶⁴Cu-DOTA-trastuzumab versus patient IHC/FISH score from biopsied tumor. Data are from ⁶⁴Cu-DOTA-trastuzumab PET/CT scans acquired 1 day (A, B) and 2 days (C, D) post injection. In A and C, the data for individual tumors (black dots) are grouped by patient and IHC/FISH score. Intra-patient means are represented by red horizontal lines. In B and D, SUVmax values for individual lesions (open circles) are combined across patients and grouped by IHC/FISH score (n = number of tumors per group). Intra-group medians are represented as amplitudes of rectangular overlays; errors bars denote 1st and 3rd quartiles. In both analyses, tumor uptake is generally higher for the HER2⁺ subgroups (3+ or 2+/FISH+) (P < 0.005). Relative variability of uptake was greater for the HER2⁺ than the HER2⁻ group, both among (P < 0.001) and within (P < 0.05 on Day 2) patients.

From Mortimer JE, Bading JR, Park JM, et al. Tumor Uptake of 64Cu-DOTA-Trastuzumab in Patients with Metastatic Breast Cancer. J Nucl Med 2017; doi: 10.2967/jnumed.117.193888; with permission.

FIGURE 6.

Examples of increased tumor uptake and tumor-to-non-tumor contrast between 1 and 2 days after injection of ⁶⁴Cu-DOTA-trastuzumab. White and red arrows respectively denote tumors and blood pool as seen in transaxial PET/CT fusion images. The images on the left are from a HER2⁺ (IHC2⁺/FISH⁺) patient. Panels A (Day 1) and C (Day 2) show a large metastasis in a pre-vascular lymph node for which uptake was concentrated at the tumor surface. The upper intensity threshold (white color) corresponds to SUV = 22 g/ml. At the times of the 2 scans (24 and 48 h), measured SUVmax for the tumor was 17 and 27 g/ml, respectively, while the SUV for blood was 15 and 11 g/ml. The right-hand column depicts a metastatic mass in the right breast of a HER2-negative patient (IHC1⁺). The upper intensity threshold for the images (white color) was set at SUV = 10 g/ml. The SUVmax for the tumor increased from 2.6 to 5.0 g/mL between (B) the Day 1 (25 h post-injection) and the (D) Day 2 (49 h post-injection) scan, while the SUV for blood declined from 12 to 8.8 g/mL.

From Mortimer JE, Bading JR, Park JM, et al. Tumor Uptake of 64Cu-DOTA-Trastuzumab in Patients with Metastatic Breast Cancer. J Nucl Med 2017; doi: 10.2967/jnumed.117.193888; with permission.

The observations made by Mortimer and coworkers show that most tumors are well visualized with PET and uptake is indicative of binding to HER2 within 1 d post injection, even in patients classified as HER2⁻, which indicates that trastuzumab uptake in metastatic breast cancer is sufficiently rapid, and does not require longer lived isotopes or lag times. This is significant for PET imaging of trastuzumab with respect to both patient radiation dose and clinical applicability.

¹⁷⁷Lu-Trastuzumab

Bhusari and coworkers developed a ¹⁷⁷Lu-trastuzumab tracer for radioimmunotherapy of HER2 expressing breast cancer and also explored its potential as an imaging agent.⁵³ This study demonstrated that the development, characterization and radiolabeling of ¹⁷⁷Lu-trastuzumab can be efficiently achieved in-house and that this radiotracer is safe to be administered to the patients. The HER2 positive primary and metastatic breast lesions specifically take up ¹⁷⁷Lu-trastuzumab, with two patient examples shown in Figures 7 (Day 1 and Day 7 time points) and Figure 8 (Day 5). This strategy can be considered as an agent for palliative treatment in combination to other conventional treatments for treatment of HER2 metastatic breast cancer.

FIGURE 7.

(a) Whole body image of a 60-year-old breast cancer patient HER2 (2⁺) at Day 1 and Day 7 post administration of 370 MBq ¹⁷⁷Lu-trastuzumab (5 mg, co administered with 20 mg cold trastuzumab). Tracer uptake can be observed in primary breast tumor (black arrow head) with T/N = 2.9 and 3.2 at Day 1 and Day 7, respectively. The bone metastasis in the acetabulum region was visualized at Day 1 (T/N = 1.2) and Day 7 (T/N = 2.5) (blue arrow heads). (b) SPECT/CT, CT, and SPECT images showing lymph node metastases (T/N = 2.4) which could not be localized on whole body scan (white arrow heads).

From Bhusari P, Vatsa R, Singh G, Parmar M, Bal A, Dhawan DK, Mittal BR, Shukla J. Development of Lu-177-trastuzumab for radioimmunotherapy of HER2 expressing breast cancer and its feasibility assessment in breast cancer patients. Int J Cancer 2017;140(4):938-947; with permission.

FIGURE 8.

(a) Whole Body images of a 30-year-old woman patient with HER2 (3⁺) breast cancer showing ¹⁷⁷Lu-trastuzumab (445 MBq) uptake in primary breast tumor (black arrow head; T/N = 3.1). (b) SPECT/CT and CT sagittal sections of the primary breast tumor showing localization of ¹⁷⁷Lu-trastuzumab (T/N = 3.6) (white arrow head). (c) SPECT/CT and CT images showing ¹⁷⁷Lu-trastuzumab localization at the metastatic sites: lymph node; T/N = 3.4 and lesion in the sternum; T/N = 1.3 (white arrow heads) and lung metastases (T/N = 1.5) at Day 5 post administration of 444 MBq ¹⁷⁷Lu-trastuzumab.

From Bhusari P, Vatsa R, Singh G, Parmar M, Bal A, Dhawan DK, Mittal BR, Shukla J. Development of Lu-177-trastuzumab for radioimmunotherapy of HER2 expressing breast cancer and its feasibility assessment in breast cancer patients. Int J Cancer 2017;140(4):938-947; with permission.

NANOPARTICLES IN HER2-TARGETED IMAGING AND THERANOSTICS

Nanoparticles and other nanomaterials have also been embellished with the HER2 target in mind. Therapeutic nanoparticles are designed to deliver their drug payloads through enhanced permeability and retention (EPR) in solid tumors. The extent of EPR and its variability in human tumors is a controversial issue in the field, as it may be the reason for variable responses to therapeutic nanoparticles in clinical studies. This issue was addressed by Lee and coworkers in an imaging study in metastatic breast cancer with ⁶⁴Cu-MM-302, a radiolabeled HER2-targeted PEGylated liposomal doxorubicin.⁵⁴ Nineteen patients with HER2-positive metastatic breast cancer underwent 2 to 3 PET/CT scans post-administration of ⁶⁴Cu-MM-302 as part of a clinical trial of MM-302 plus trastuzumab with and without cyclophosphamide.^55–57 Tumor accumulation of ⁶⁴Cu-MM-302 at 24 to 48 hours varied from 0.52–18.5 %ID/kg (Figure 9), including deposition in bone and brain lesions, along with significant background uptake of ⁶⁴Cu-MM-302 in the liver and spleen. Peak liposome circulation was found to be between 24 and 48 h based on computational analysis, and high ⁶⁴Cu-MM-302 deposition was associated with more favorable treatment outcomes. These findings deliver key evidence and quantification of the EPR effect in HER2⁺ metastatic tumors, supporting the use of nanoparticle imaging as a tool to stratify patients that would benefit from such therapy.

FIGURE 9.

Biodistribution of ⁶⁴Cu-MM-302 in patients. Maximum intensity projection PET images of 2 patients with HER2-positive breast cancer injected with 30 mg/m2 of MM-302 and a tracer dose of ⁶⁴Cu-MM-302 (400 MBq). PET/CT Images were acquired at 0.6 and 19 hours post-injection in patient 02 (A), and 0.7, 24, and 47 hours post-injection in patient 06 (B). Immediately after administration, ⁶⁴Cu-MM-302 activity was primarily confined in the blood pool because of the extended circulation property of liposomes. On days 2 and 3, ⁶⁴Cu-MM-302 uptake was evident in normal spleen and liver, as well as in various tumor lesions.

Figure reproduced from H Lee et al., Clin Cancer Res 2017, with permission from the publisher.

Other nanoparticles on the preclinical end that show promise include studies with ^99mTc-nanosilica⁵⁸ and superparamagnetic nanoparticles⁵⁹ for detection of HER2-positive breast cancer. This emerging realm will bridge the gap between therapies and imaging in the field of nanomedicine, allowing for new subsets of targeted molecules.

PET AND SPECT IMAGING WITH MONOCLONAL ANTIBODIES IN HER3-POSITIVE BREAST CANCER

As there are emerging strategies with monoclonal antibodies to diagnosis disease, assess progression, and interpret specific biomarkers within breast cancer, an up and coming area of this strategy is developing with HER3-targeted antibodies. One of the latest treatments incorporated into clinical trials is patritumab, which binds the extracellular domain of HER3 and promotes receptor internalization, leading to the inhibition of basal and ligand-induced HER3 activation and downstream signaling. Patritumab was labeled with ⁶⁴Cu and imaging studies were done in patients with multiple tumor types (including breast) that were HER3⁺, in a recent publication from Lockhart and coworkers.¹⁶ Dosimetry, safety, and receptor occupancy (RO) were the main objectives of this clinical trial. The tumor SUVmax was 5.6 ± 4.5, 3.3 ± 1.7 and 3.0 ± 1.1 at 3, 24 and 48 h, respectively, in the dosimetry cohort. The liver was the dose-limiting organ, with a critical dose of 0.46 ± 0.086 mGy/MBq. In the cohort of patients undergoing RO, the tumor-to-blood ratio decreased from 1.00 ± 0.32 at baseline to 0.57 ± 0.17 after patritumab treatment, corresponding to a RO of 42.1 ± 3.9%. Future studies will be needed to improve the characterization of HER3 receptor occupancy by patritumab and its relationship with serum patritumab levels. Additional trials will likely incorporate reduced the time between ⁸⁹Zr-labeled and unlabeled antibody dosing to reduce liver uptake of the labeled product.

Another antibody-based radiotracer was developed by Bensch and Lamberts et al. with ⁸⁹Zr-lumretuzumab PET to assess therapy response with lumretuzumab in patients with solid tumors.¹⁷ Lumretuzumab is a humanized monoclonal antibody directed against the extracellular domain of HER3, displacing its ligand and inhibiting heterodimerization and downstream signaling. Furthermore, it has been shown that lumretuzumab can cause cell death through antibody-dependent cellular cytotoxicity (ADCC). A phase I study in patients with HER3-positive solid tumors showed that lumretuzumab monotherapy was well tolerated and clinically useful, leading to an imaging study with ⁸⁹Zr-lumretuzumab. This trial recruited 20 patients with histologically confirmed HER3-expressing tumors that underwent ⁸⁹Zr-lumretuzumab PET. In one arm of the trial, ⁸⁹Zr-lumretuzumab was given alongside escalating doses of unlabeled lumretuzumab and scans were performed 2, 4 and 7 days post-injection to determine optimal imaging conditions. In another arm of the trial, patients were scanned following tracer injection before (baseline) and after a pharmacodynamic (PD)-active lumretuzumab dose to determine saturation. HER3 expression was determined via immunohistochemical analysis of skin biopsies.

Optimal PET conditions were found to be 4 and 7 days after administration of ⁸⁹Zr-lumretuzumab with 100 mg unlabeled antibody. At baseline, post-administration of “cold” lumretuzumab (100 mg), the tumor SUVmax was 3.4 ± 1.9 at 4 days post-injection of ⁸⁹Zr-lumretuzumab. Tumor uptake decreased by 11.9% ± 8.2, 10.0% ± 16.5, and 24.6% ± 20.9 after lumretuzumab administration (escalating doses of 400, 800 and 1600 mg, respectively) when compared to baseline. HER3 expression was also downregulated in concurrent skin biopsies at the lowest dose (400 mg) lumretuzumab. Overall, this study with ⁸⁹Zr-lumretuzumab PET showed promising biodistribution and tumor specificity. Despite the correlative trend between increased doses of lumretuzumab and decreased ⁸⁹Zr-lumretuzumab uptake, the tumor uptake never plateaued, indicating a lack of tumor saturation. Therefore, it is possible that the dosing of the radiotracer will need further optimization to have optimal delineation.

Although there are a number of preclinical tracers to target HER3 that are on the rise, few have yet to reach the clinic to drive the relevance of this protein in breast cancer imaging. Some of the more recent antibody developments include those that are not only relevant for diagnosis,^60–62 but also assessing therapy response,^63–65 some of which were actually used in the clinical trials described above. Affibodies are also an up and coming strategy that has been more embellished upon with the HER2 target, but is also slowly on the rise with regards to HER3.^66–69 Radiotracers that are successful for both diagnostic purposes and predicting therapy are particularly useful from a breast cancer standpoint, as many chemotherapeutic and targeted therapy options can be toxic and have harmful side effects. The utilization of a specific, biologically relevant radiotracer to detect changes in HER3 expression and assess therapy response serves to improve breast cancer treatment.

CONCLUSIONS

HER receptor status, namely HER2 and HER3, encompass a subset of critical biomarkers in breast cancer patients, and can be essential to making treatment decisions and finalizing prognosis. The heterogeneous expression of HER proteins both intratumorally and between primary and metastatic disease limits the value of tumor biopsies and demonstrates a need for accurate, whole body assessment, which is where targeted molecular imaging arises as a valuable tool to solve this clinical issue. Antibodies, antibody fragments, affibodies, and nanoparticles targeted to HER2 and HER3 can be radiolabeled for PET and SPECT imaging to target HER2-and HER3 lesions and metastases and therapy response. These clinical tools are necessary to make the push towards precision medicine, and will serve to improve the outcomes for breast cancer patients everywhere.

KEY POINTS.

• The human epidermal growth factor receptor (HER) family members are of increasing interest to target by small molecules, affibody moieties, and monoclonal antibodies.

• Imaging can simultaneously assess HER expression of primary and metastatic sites, which may vary across lesions within any given patient. PET and SPECT imaging of HER2 and HER3 allows for non-invasive diagnosis in breast cancer, has the ability to detect unknown metastatic disease, and addresses the issue of tumor heterogeneity within breast cancer.

• Resistance against the epidermal growth factor receptor (EGFR) and HER2-targeting agents is a clinically relevant problem that requires optimization of targeting other members of the HER family, increasing relevance in HER3 from a therapy and imaging standpoint.

• HER3 is strongly involved in the development and maintenance of many tumor types, and is emerging to play a significant role in breast cancer.