The human epidermal growth factor receptor (HER) proteins, consisting of four highly homologous members, epidermal growth factor receptor (EGFR, HER1), HER2, HER3, and HER4, play a central role in regulating cell proliferation, survival, differentiation, and migration. The overexpression of the HER family is considered to be one of the most common cellular dysregulations associated with multiple tumor types.
Antibody-drug conjugates (ADCs) represent a new and promising class of anticancer therapeutics that combine the cancer specificity of antibodies with cytotoxicity of chemotherapeutic drugs. Herein, we review the approved and investigational ADCs that target HER family receptors, including EGFR, HER2 and HER3.
ADCs Targeting EGFR
EGFR plays a key role in the signaling pathway of human epithelial cells and is involved in cell proliferation, invasion, apoptosis and angiogenesis. However, when mutations occur in EGFR, it leads to sustained activation of the EGFR signaling pathway, which results in abnormal cell proliferation.
Currently, several drugs targeting EGFR have been approved for marketing, including small molecule inhibitors, monoclonal antibodies, ADCs, and bispecific antibodies. Only one ADC targeting EGFR is approved for cancer treatment in Japan, Akalux® IV Infusion 250mg (cetuximab sarotalocan sodium). In addition, three ADCs targeting EGFR are in clinical studies, including depatuxizumabmafodotin (ABT-414), MRG003 and M1231.
Figure 1. Summary of anti-EGFR ADCs in clinical investigation. Source: Reference 
Rakuten Medical Inc. announced that its wholly owned subsidiary Rakuten Medical Japan K.K. has received marketing approval in Japan from the Ministry of Health, Labour and Welfare for Akalux® IV Infusion 250mg (cetuximab sarotalocan sodium) to treat unresectable locally advanced or recurrent head and neck cancer. It is currently the first new head and neck cancer treatment drug approved in the world, according to NHK. In addition, the BioBlade laser system, a medical device used in combination with Akalux, was also approved on September 2nd, 2020. Outside of Japan, Rakuten Medical does not have approval of its investigational therapies and is currently running a global Phase 3 trial.
ASP-1929 (i.e. Akalux) is an ADC consisting of cetuximab and IRDye700DX that targets EGFR (expressed on the surface of a variety of solid tumors including head and neck, esophageal, lung, colon, and pancreatic cancers). Due to the addition of a light-responsive substance, ASP-1929 can be locally activated by a red laser released by a light-guided fiber after binding to the tumor target, leading to the death of tumor cells. It is unique in that it uses antibody-mediated targeted delivery to achieve high tumor specificity, while using laser-activated biophysical mechanisms to precisely induce rapid cancer cell death without harming surrounding normal tissues.
Current clinical trials of ASP-1929 are focused on head and neck cancer. In a phase IIa clinical study of 30 patients with recurrent head and neck cancer that could not be treated with surgery, radiation or platinum-based chemotherapy, clinical data showed an objective response rate (ORR) of 28% (8/28) and a complete response rate (CR) of 14% (4/28). The median progression-free survival (mPFS) for the 28 evaluable patients was 173 days (5.7 months), and the median overall survival time (mOS) for all 30 patients was 278 days (9.1 months). The results of the study showed that treatment with ASP-1929 was safe and well tolerated.
ABT-414 consists of an EGFR-specific humanized antibody (ABT-806), a non-cleavable maleimidocaproyl (mc) linker and monomethyl auristatin F (MMAF), with an average number of approximately four MMAFs conjugated to each mAb. ABT-806 binds to a cryptic epitope in a cysteine-rich domain (CR1) of EGFR. Compared to other EGFR-targeting antibodies, ABT-806 has the lowest binding activity to EGFR in normal tissues, and ABT-414 retains the excellent binding and functional properties of ABT-806.
ABT-414 was explored for safety, pharmacokinetics and antitumor efficacy as monotherapy or in combination with temozolomide in patients with glioblastoma in a Phase I clinical trial (NCT01800695). ABT-414 demonstrated a manageable safety profile and an acceptable pharmacokinetic profile in the Phase I trial. However, the primary endpoint of overall survival was not met in the Phase II clinical study (NCT02343406). A phase II/III study (NCT02573324) is currently being conducted on participants with newly diagnosed glioblastoma with EGFR amplification. However, the trial was terminated in 2019 due to a lack of survival benefits.
MRG003 consists of a fully human EGFR-specific IgG1 antibody, a protease-cleavable valine-citrulline (vc) linker and a monomethyl auristatin E (MMAE). MRG003 has been in a phase I clinical trial (CTR20180310 , NCT04868344) to evaluate the safety, pharmacokinetics and efficacy of MRG003 as a single agent in the treatment of patients with relapsed or refractory solid tumors. Encouragingly, MRG003 showed an acceptable safety profile and potential antitumor activity. Multiple Phase II studies of MRG003 (NCT05126719, NCT04868162, NCT04838964 and NCT04838548) in patients with recurrent or metastatic nasopharyngeal carcinoma, head and neck squamous cell carcinoma, advanced metastatic biliary tract cancer and advanced non-small cell lung cancer are currently underway.
M1231 is an exploratory ADC with a hemiasterlin-related payload bound to a bispecific antibody targeting both MUC1 and EGFR. hemiasterlin is a tripeptide that exerts its cytotoxicity by binding to microtubule proteins, thereby disrupting normal microtubule dynamics. the detailed structural information and preclinical data for M1231 have not been published. A phase I clinical study (NCT04695847) is currently underway as a monotherapy for patients with metastatic solid tumors, esophageal cancer and NSCLC.
ADCs Targeting HER2
HER2 is another important target for HER family cancer therapy. HER2 is a proto-oncogene that causes cancer cell division and proliferation, so HER2-positive patients (HER2 overexpression or amplification) need targeted therapy against HER2. HER2-positive patients generally have higher tumor grade, larger tumors, and more metastatic lymph nodes. HER2 gene abnormalities are found in a variety of cancers including breast, bladder, cervical, colorectal, gastrointestinal, and lung cancers, with HER2 amplification or overexpression present in approximately 15-20% of breast cancers, 22% of gastric cancers, and 28% of uroepithelial cancers.
Figure 2. HER2 expressing cancers, source: reference 
To date, there are three HER2-targeted ADCs on the market. T-DM1 (Kadcyla) was the first Her2-targeted ADC drug approved by the FDA for the treatment of advanced HER2+ breast cancer and was also approved for early high-risk patients with residual disease after neoadjuvant therapy. In 2019, T-DXd (DS-8201; Enhertu) became the second approved novel Her2-targeted ADC drug, which showed significant antitumor activity in patients with treatment-refractory Her2+ metastatic breast cancer. In addition, RC48 from RemeGen was approved in China in 2021 for the treatment of patients with Her2 overexpressing locally advanced or metastatic gastric cancer who have received at least 2 types of systemic chemotherapy.
Figure 3. Structure of approved anti-HER2 targeting ADCs, reference 
Many next-generation HER2-targeted ADCs are currently being investigated in clinical trials. These novel agents are designed using different payload and linker technologies to further improve their efficacy and tolerability. These studies demonstrate future directions for the treatment of patients with HER2-positive solid tumors.
Figure 4. Anti-HER2 ADCs in clinical trials, source: reference 
Ado-trastuzumab emtansine (T-DM1) is a novel ADC that consists of the monoclonal antibody trastuzumab conjugated to DM1 (a derivative of maytansine) through a thioether linker. DM1 can effectively inhibit microtubule protein polymerization and block cell mitosis, which in turn leads to apoptosis. In the phase III EMILIA study, the investigators compared the efficacy of T-DM1 and lapatinib plus capecitabine. Results showed that patients treated with T-DM1 had a median progression-free survival (PFS) of 9.6 months compared to 6.4 months in patients treated with lapatinib plus capecitabine. The median overall survival (mOS) was 30.9 months in the T-DM1 group and 25.1 months in the lapatinib plus capecitabine group.
Enhertu (DS-8201) is an ADC composed of a mAb targeting HER2 conjugated to a topoisomerase I inhibitor (DXd) at a drug-to-antibody ratio (DAR) of 7-8, breaking the traditional ADC drug DAR limit of 2-4. The anti-tumor activity of DXd is about 1000 times higher than that of conventional chemotherapeutic drugs, and it can effectively avoid cross-resistance. In addition, DXd has good cell membrane permeability and is able to penetrate cell membranes into adjacent tumor cells to exert a potent bystander effect. Bystander effect means that ADC can kill not only the target tumor cells, but also the surrounding heterogeneous tumor cells and metastatic tumor cells to enhance the efficacy.
On December 20, 2019, the FDA approved ENHERTU (DS-8201) for the treatment of adult patients with HER2-positive unresectable or metastatic breast cancer who have had two or more anti-HER2 therapies in the presence of metastasis;
On January 15, 2021, the FDA approved ENHERTU (DS-8201) for use in adult patients with locally advanced or metastatic HER2-positive gastric or gastroesophageal adenocarcinoma who have received a trastuzumab-based regimen;
On August 5, 2022, the FDA approved ENHERTU (DS-8201) for unresectable or metastatic and HER2 low-expressing breast cancer, rewriting the classification criteria for breast cancer;
On August 11, 2022, the FDA accelerated approval of ENHERTU (DS-8201) for non-small cell lung cancer with HER2 mutations, making it the first HER2-targeted therapy for lung cancer.
RC48 (Disitamab Vedotin)
RC48 (Disitamab Vedotin), developed by RemeGen, is an innovative anti-HER2 ADC consisting of humanized monoclonal antibody conjugated on MMAE via a cleavable valine-citrulline (VC) linker. The antibody Disitamab targets different epitopes of the HER2 receptor and has a better molecular affinity for HER2 targeting compared to trastuzumab. The VC linker is stable, and can only be cleaved by cathepsins when RC48 are endocytosed into lysosomes, thus releasing a payload to kill tumor cells. The toxin MMAE is a synthetic derivative with anti mitogenic effects.
Trastuzumab duocarmazine (SYD985)
SYD985 (Trastuzumab duocarmazine) is an HER2-targeted ADC developed by Byondis, which is composed of the anti-HER2 trastuzumab, a cleavable valine-citrulline peptide linker, and a duocarmycin derivative.
The antibody moiety of SYD985 is able to bind to HER2 targets on the surface of cancer cells, resulting in ADC being internalized by the cells. The payload of SYD985 is inactivated at the time of coupling to the antibody. After hydrolytic cleavage of the linker, the inactivated cytotoxic drug vc-seco-DUBA is activated, which induces DNA damage leading to tumor cell death.
In July 2022, the FDA accepted a BLA application for the drug, which met the primary endpoint of progression-free survival (PFS) in the TULIP trial and significantly improved PFS compared to physician's choice (PC) therapy in patients with HER2-positive unresectable locally advanced or metastatic breast cancer, with a PFS of 7.0 months in the SYD985 arm and a PFS of 4.9 months in PC’s choice. In addition, there was a trend toward improved overall survival (OS) in the group receiving SYD985.
Unfortunately, however, on May 15, 2023, Byondis announced that it received a Complete Response Letter (CRL) from the FDA regarding the Biologics License Application (BLA) for SYD985 for the treatment of HER2-positive breast cancer.
In the CRL, the FDA indicated that additional information is needed to support the approval decision which will require additional time to review. Byondis indicated that it will continue to move forward with the marketing of SYD985 in the EU and the UK.
ARX788 is a site-specific ADC collaboratively developed by Ambrx and NovoCodex Biopharmaceuticals. It consists of an anti-HER2 antibody conjugated to a potent microtubule protein inhibitor, AS269, using a unique nonnatural amino acid–enabled conjugation technology a noncleavable linker.
On Mar 2023, NovoCodex announced positive clinical data from the randomized Phase 3 trial of the anti-HER2 ADC ARX788, ACE-Breast-02, a large study conducted in China specifically in patients with HER2-positive locally advanced or metastatic breast cancer.
The study enrolled 441 patients with HER2-positive breast cancer who were refractory to prior treatment with paclitaxel and trastuzumab. Eligible patients were randomized 1:1 to receive either the new experimental drug ARX788 or the control drug lapatinib in combination with capecitabine.
The results showed that ARX788 significantly improved progression-free survival (PFS) in patients with HER2-positive locally advanced or metastatic breast cancer.
In October 2022, updated data published in CCR's ARX788 Phase 1 trial in China for HER2-positive breast cancer showed an objective response rate of 65.5%, a disease control rate of 100%, and a median progression-free survival of 17.02 months.
ADCs Targeting HER3
HER3 is overexpressed in multiple cancer types and is thought to predict poor prognosis. Despite the lack of significant kinase activity, HER3 exerts its function through HER3 homodimerization or HER2/HER3 heterodimerization, thereby activating downstream signaling pathways to promote cell survival and proliferation. Importantly, HER3 signaling has been shown to be associated with mechanisms of resistance to anti-EGFR/HER2 therapies and is emerging as a promising therapeutic target for EGFR-mutant NSCLC. Patritumab deruxtecan (U3-1402) is the only ADC currently undergoing clinical studies.
Patritumab deruxtecan (HER3-Dxd; U3-1402) is composed of an anti-HER3 mAb (patritumab), a cleavable GGFG linker, and the topoisomerase I inhibitor DXd, with a DAR of 8. By binding to HER3 on the surface of cancer cells, U3-1402 is endocytosed into the cells to release deruxtecan, which in turn acts as a cytotoxic agent to kill cancer cells.
Figure 5. Structure of U3-1402, source: Daiichi Sankyo official website
Data from two early clinical trials of patritumab deruxtecan (HER3-DXd) (NSCLC U31402-A-U102, breast cancer U31402-A-J101) were presented during the 2023 annual meeting of the Japanese Society of Medical Oncology (JSMO).
U31402-A-U102 is a dose-escalation and dose-expansion phase I clinical trial. In the dose-escalation arm, patients with locally advanced or metastatic EGFR-mutated NSCLC with disease progression after prior EGFR-TKI treatment were enrolled.
A pooled analysis was performed on 102 patients with EGFR-mutated NSCLC receiving the recommended dose of HER3-DXd (5.6 mg/kg), 78 of whom had received prior triple-generation EGFR-TKI as well as platinum-containing chemotherapy.
The results showed that at a median follow-up of 23 months (11.8-36.0 months), patients in the patritumab deruxtecan (5.6 mg/kg) group had an objective response rate (ORR) of 40.2%, a disease control rate (DCR) of 78.4%, a duration of response (DOR) of 7.6 months, a progression-free survival (PFS) of 6.4 months and overall survival (OS) of 15.8 months.
In patients who had received prior triple-generation EGFR-TKI and platinum-containing chemotherapy, patritumab deruxtecan showed consistent efficacy with an ORR of 41%, median PFS of 6.4 months and median OS of 16.2 months. In addition, the ORR was 36.4% and 44.7% in patients with and without a history of central nervous system (CNS) metastases, respectively. However, no clear correlation was shown between treatment response status and HER3 expression levels.
U31402-A-J101 is an ongoing Phase I/II clinical trial enrolling patients with HER2-negative/HER3-positive locally advanced or metastatic breast cancer. The study is divided into three parts: dose escalation, dose exploration and dose extension, with a treatment dose of 4.8 mg/kg or 6.4 mg/kg in the dose extension arm, and includes patients with HR+/HER2- as well as triple-negative breast cancer (TNBC).
In patients with HR+/HER2-/HER3 positive breast cancer, the ORR in the HER2 low expression subgroup (n=58) was 36.2%, DOR was 7.2 months, PFS was 5.8 months, and OS was 13.7 months; in HER2(0) patients (n=39), the ORR was 28.2%, DOR was 7.0 months, PFS was 8.2 months, and OS was 14.6 months.
In HER3 positive TNBC patients, the ORR in the HER2 low expression subgroup of patients (n=29) was 20.7%, DOR was 4.1 months, PFS was 4.4 months, and OS was 12.7 months. In HER2(0) patients (n=19), the ORR was 26.3%, DOR was 8.4 months, PFS was 8.4 months, and OS was 16.6 months.
HER-directed ADCs are emerging as a highly promising therapeutic for patients with HER-positive cancers. With the success of T-DM1, T-DXd and RC48, there are a large number of novel HER-targeted ADCs with promising activity currently under investigation that will offer new hope for the treatment of HER+ cancers.
 Yu J, Fang T, Yun C, Liu X, Cai X. Antibody-Drug Conjugates Targeting the Human Epidermal Growth Factor Receptor Family in Cancers. Front Mol Biosci. 2022 Feb 28;9:847835. doi: 10.3389/fmolb.2022.847835. PMID: 35295841; PMCID: PMC8919033.
 Oh DY, Bang YJ. HER2-targeted therapies - a role beyond breast cancer. Nat Rev Clin Oncol. 2020;17(1):33-48. doi:10.1038/s41571-019-0268-3
 Patritumab Deruxtecan Continues to Show Encouraging Clinical Activity in Distinct Patient Populations with Metastatic Lung and Breast Cancer in Updated Results of Two Early Trials https://www.daiichisankyo.com/files/news/pressrelease/pdf/202303/20230320_E.pdf