Introduction to HER and ADCs
Members of the human epidermal growth factor receptor (HER) family, including HER1 (EGFR), HER2, HER3, and HER4, play a core role in regulating cell proliferation, survival, differentiation, and migration. The overexpression of the EGFR family is considered one of the most common cell dysfunctions associated with various types of tumors.
Antibody-drug conjugates (ADCs) are a new and promising class of cancer treatment drugs that combine the cancer specificity of antibodies with the cytotoxicity of chemotherapy drugs. The US Food and Drug Administration (FDA) approved two HER2-directed ADCs, trastuzumab-emtansine (T-DM1) and trastuzumab-deruxtecan (DS-8201a), for the treatment of HER2-positive metastatic breast cancer in 2013 and 2019, respectively. Currently, multiple ADCs targeting HER family receptors (EGFR, HER2, or HER3) are undergoing clinical trials. The success of these drugs has changed the treatment of HER-positive cancer and injected new vitality into the ADC development field.
Targeted drugs against HER
Due to the critical role of the HER family in the carcinogenic process, two main targeted therapies have been developed in the past 20 years to block the HER-driven pathway, including small molecule compounds that inhibit the intracellular tyrosine kinase activity and monoclonal antibodies targeting the extracellular domain (ECD) of the receptor.
Eight tyrosine kinase inhibitors (TKIs) have been approved for clinical use, tightly binding to the kinase domain of the HER family. The first-generation TKIs of the HER family include Erlotinib, Gefitinib, and Lapatinib. Erlotinib and Gefitinib selectively bind to the ATP binding site of EGFR, the only two single target TKIs available so far, while Lapatinib is the first TKI approved for breast cancer, showing equal activity against EGFR and HER2. The next-generation TKIs, including Afatinib, Dacomitinib, and Neratinib, are irreversible pan-HER2 inhibitors (EGFR, HER2, and HER4), with Afatinib and Dacomitinib approved for non-small cell lung cancer (NSCLC) and Neratinib approved for breast cancer. Among these six TKIs, Erlotinib, Gefitinib, and Afatinib are still first-line treatments for NSCLC. Finally, Osimertinib is a third-generation EGFR TKI, showing significant effectiveness in NSCLC patients with EGFR activating and T790M mutations.
Unlike TKIs, monoclonal antibodies bind to the extracellular domain of receptors, thereby preventing the binding of receptors to ligands or their dimerization. So far, multiple monoclonal antibodies targeting the HER receptor have been approved for clinical use. Currently, there are two HER2-targeted monoclonal antibodies in the market, including trastuzumab (Herceptin®) and pertuzumab (Perjeta®). Trastuzumab, which was first approved in 1998, has been widely used to treat HER2-positive breast cancer and stomach cancer. Pertuzumab, which was approved in 2012, has been used since 2012 to treat HER2-positive breast cancer.
Dual blockade of HER2 with trastuzumab and pertuzumab has become a first-line treatment for patients with metastatic breast cancer. So far, five EGFR-targeting monoclonal antibodies have been approved for clinical use, namely cetuximab (Erbitux®), panitumumab (Vectibix®), nimotuzumab (BIOMAB-EGFR®), necitumumab (Portrazza®), and amivantamab (Rybrevant®). Cetuximab and panitumumab are approved for metastatic colorectal cancer (CRC), while cetuximab is also approved for treating KARS wild-type CRC and advanced squamous cell carcinoma. Nimotuzumab is approved for treating head and neck squamous cell carcinoma, and necitumumab is approved for treating refractory metastatic squamous non-small cell lung cancer. Unlike the aforementioned anti-EGFR monoclonal antibodies, amivantamab is a bispecific antibody that can simultaneously bind to the extracellular domains of EGFR and MET, and it has recently been approved for treating adult patients with advanced or metastatic NSCLC with EGFR exon 20 insertion mutations.
ADCs targeting EGFR
EGFR is amplified or overexpressed in various types of tumors and has been proven to be an important tumor target. However, no ADCs targeting EGFR are currently approved for the treatment of cancer. At present, three EGFR-targeting ADCs are undergoing clinical trials, including depatuxizumabmafodotin (ABT-414), MRG003, and M1231. ABT-414 is the most advanced EGFR-targeting ADC to date and has entered Phase III clinical trials.
(1) Depatuxizumab-Mafodotin（ABT-414）
ABT-414 is composed of a humanized EGFR-specific antibody (ABT-806), an uncleavable maleimide-hexyl (mc) linker, and a methylated maytansine derivative (MMAF). The average number of MMAF molecules bound to each monoclonal antibody is approximately four. ABT-806 binds to a cryptic epitope in the cysteine-rich domain 1 (CR1) of EGFR, and compared with other EGFR-targeting antibodies, ABT-806 has the lowest binding activity to EGFR in normal tissues. ABT-414 retains the excellent binding and functional characteristics of ABT-806.
In the Phase I clinical trial (NCT01800695), the safety, pharmacokinetics, and anti-tumor efficacy of ABT-414 as a single therapy or in combination with temozolomide were explored in patients with glioblastoma. ABT-414 demonstrated manageable safety and acceptable pharmacokinetic features in the phase I trial. However, the primary endpoint of overall survival was not met in the phase II clinical study (NCT02343406). Currently, a phase II/III study (NCT02573324) is being conducted on participants with newly diagnosed glioblastoma with EGFR amplification. However, due to the lack of survival benefits, the trial has been terminated since 2019.
(2) MRG003
MRG003 is composed of a fully human EGFR-specific IgG1 antibody, a protease cleavable valine-citrulline (vc) linker, and a methyl auristatin E (MMAE). Phase I clinical trials (CTR20180310, NCT04868344) have been conducted to evaluate the safety, pharmacokinetics, and efficacy of MRG003 as a monotherapy for patients with relapsed or refractory solid tumors. Encouragingly, MRG003 exhibited acceptable safety and potential anti-tumor activity. Currently, multiple Phase II studies targeting relapsed or metastatic nasopharyngeal carcinoma, head and neck squamous cell carcinoma, advanced metastatic cholangiocarcinoma, and late-stage non-small cell lung cancer patients are ongoing (NCT05126719, NCT04868162, NCT04838964, and NCT04838548).
(3) M1231
M1231 is an exploratory ADC that combines a semi-synthetic payload with dual-specific antibodies targeting both MUC1 and EGFR. The semi-synthetic payload, a tripeptide, exerts its cytotoxicity by binding to microtubules and disrupting normal microtubule dynamics. Detailed structural information and pre-clinical data for M1231 have not yet been released. Currently, Phase I clinical trials (NCT04695847) are underway, investigating M1231 as a standalone therapy for patients with metastatic solid tumors, esophageal cancer, and NSCLC.
ADCs targeting HER2
HER2 is another important target for cancer therapy in the HER family. So far, there are three HER2-targeting ADCs on the market. T-DM1 (Kadcyla) is the first FDA-approved HER2-targeting ADC for the treatment of advanced HER2-positive breast cancer and is also approved for use in early high-risk patients with residual lesions after neoadjuvant therapy. In 2019, T-DXd (DS-8201; ENHERTU) became the second approved new HER2-targeting ADC drug, showing significant anti-tumor activity in the treatment of refractory HER2-positive metastatic breast cancer patients. In 2021, RC48 was approved for sale in China for the treatment of locally advanced or metastatic gastric cancer patients with HER2 overexpression who have received at least two systemic therapies.
Currently, many next-generation HER2-targeted ADCs are being studied in clinical trials. These new drugs are designed with different payloads and linking technologies to further improve their efficacy and tolerability. These studies demonstrate the future direction of treating patients with HER2-positive solid tumors.
In addition, seven HER2-targeted ADCs have been announced as having terminated clinical trials.
These data indicate that the clinical failure rate of HER2-targeted ADCs is relatively low (23.3%), indicating that HER2 is a very ideal target for ADCs.
(1) Trastuzumab duocarmazine
Trastuzumab duocarmazine (SYD985) is a HER2-targeting ADC composed of the trastuzumab antibody, a cleavable linker, and a payload of duocarmycin. The payload is membrane-permeable, allowing it to potentially enter neighboring cells regardless of HER2 expression level. Phase 1 studies showed acceptable toxicity and anti-tumor activity in HER2-positive and HER2-low-expressing breast cancer. In an expanded study targeting HER2-positive MBC patients, 33% (16/48) achieved objective responses.
These results form the basis of the phase III TULIP trial (NCT03262935), which includes HER2-positive MBC patients previously treated with T-DM1 and evaluates PFS as the primary endpoint (results pending). SYD985 is also being studied in combination with paclitaxel (NCT04602117) and niraparib (NCT04235101). Its use in combination with doxorubicin and cyclophosphamide is being studied in the I-SPY trial (NCT01042379), a large adaptive neoadjuvant trial designed to evaluate pathological complete response (pCR) rates with different biologic agents and chemotherapy.
(2) A166
A166 is a conjugate of Trastuzumab and Duostatin-5 (a derivative of Auristatin). In 2020, the results of a Phase I trial (NCT03602079) on 27 evaluable patients showed a disease control rate (DCR) of 59%, with 7 patients (26%) achieving a partial response (PR) at doses of 3.6mg/kg and 4.8mg/kg. The study is currently ongoing.
(3) XMT-1522
XMT-1522 is an ADC composed of the HT-19 antibody skeleton. HT-19 is a human IgG1 monoclonal antibody that targets HER2, binding to domain IV of HER2 and forming an epitope different from the binding site of trastuzumab. The effective payload is an auristatin derivative (AF-HPA). Preclinical data showed that XMT-1522 is effective against HER2-positive breast cancer and gastric cancer cell lines, as well as xenograft models resistant to T-DM1. Preliminary results of a Phase I study (NCT02952729) showed that in the groups receiving doses of 16 and 21.3mg/m2, the overall DCR was 83% (5/6), with 1 PR. In patients receiving doses lower than 16 mg/m2, the DCR was 25% (3/12).
(4) ALT-P7（HM2/MMAE）
ALT-P7 is a new HER2-targeted ADC consisting of a trastuzumab variant conjugated with MMAE. The results of the first human study showed a disease control rate (DCR) of 77.3% (17/22) in measurable patients, with 2 cases of partial response (PR). In patients with a median prior treatment duration of 6 months, the median PFS was 6.2 months. Currently, the second phase of the study is under planning.
(5) ARX788
ARX788 is a site-specific ADC consisting of an anti-HER2 antibody and a highly potent microtubule inhibitor AS269, conjugated using a unique non-natural amino acid conjugation technology and an uncuttable linker. The results of the first stage of the trial showed antitumor activity in HER2-positive breast cancer, with an ORR of 56% in the 1.3mg/kg dose group and increased to 63% at the 1.5mg/kg dose. Thus, the FDA granted fast-track designation in January 2021. Currently, ARX788 is undergoing a two-part phase 1 dose escalation study in HER2-positive solid tumor patients (NCT03255070).
(6) PF-06804103
PF-06804103 is a derivative antibody of trastuzumab that is coupled with AUR-06380101, a novel and effective auristatin derivative, through a cleavable linker. PF-06804103 has shown efficacy in low HER2 expression breast, gastric, and lung tumor models. Preliminary results from a phase 1 dose-escalation study (NCT03284723) showed an objective response rate (ORR) of 52.4% (11/21) at a dose of ≥3 mg/kg. All patients had received HER2-targeted therapy with a median of six prior treatments.
(7) MRG002 and ZW49
MRG002 and ZW49 are two ADCs that use auristatin as the payload and are coupled with different HER2 monoclonal antibodies. The former uses a humanized anti-HER2 IgG1 monoclonal antibody, while the latter uses ZW25, a bispecific anti-HER2 antibody that recognizes distinct binding sites of trastuzumab and pertuzumab. Currently, both ADCs are undergoing phase 1 clinical trials (CTR20181778, NCT04492488, NCT04742153, NCT03821233).
(8) BDC-1001
BDC-1001 is composed of a biosimilar of trastuzumab and TLR 7/8 agonists through an unbreakable linker. BDC-1001 can activate antigen-presenting cells while retaining antibody-mediated effector functions such as ADCC. Preclinical data show that BDC-1001 induces effective immune-mediated anti-tumor effects in xenograft models and its safety has been demonstrated in the first human study. The efficacy of BDC-1001 alone or in combination with anti-PD1 is still pending (NCT04278144).
Finally, a novel pertuzumab-based ADC design shows a decrease in HER2 affinity at acidic intracellular pH, which increases lysosomal delivery and cytotoxicity in xenograft models with low HER2 expression. It is expected to enter further clinical trials.
ADCs targeting HER3
HER3 is overexpressed in multiple types of cancer and is believed to be predictive of poor prognosis. Despite lacking 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 is associated with the resistance mechanism of anti-EGFR/HER2 therapy and is becoming a promising therapeutic target for EGFR mutant NSCLC. Patritumab deruxtecan (U3-1402) is currently the only ADC under clinical investigation.
U3-1402 consists of an anti-HER3 monoclonal antibody (Patritumab), a cleavable GGFG linker, and a topoisomerase I inhibitor Dxd. U3-1402 has a DAR of 8 and shows higher HER3-specific binding affinity in other human HER family receptors (including EGFR, HER2, and HER4) and strong anti-tumor activity in patient-derived xenograft models with acceptable safety profiles. Currently, the phase I/II study of U3-1402 for HER3-positive metastatic breast cancer (NCT02980341) is ongoing. Early reports of the trial have shown that U3-1402 has good anti-tumor activity and tolerable safety. Meanwhile, a phase I clinical trial (NCT03260491) is being conducted to explore the efficacy and safety in metastatic or unresectable non-small cell lung cancer.
Challenges and prospects of ADC development
As one of the fastest-growing cancer drugs, ADCs currently face three major challenges:
1) Improving cancer cell uptake of ADCs has always been the main challenge in ADC development. Currently, ADC relies on high-level target antigen expression on the surface of cancer cells to ensure the efficient release of cell toxicity payload during endocytosis. Studies have shown that effective cell killing by HER2-targeted ADCs is usually related to the level of HER2 expression on the cell surface, which typically requires a high level of HER2 surface expression. The level of target antigen expression on the tumor surface significantly limits the therapeutic efficacy of existing ADCs. Therefore, increasing the uptake of ADCs by cancer cells may meet more market demands, especially for patients with lower antigen expression levels.
2) Systemic toxicity is still one of the main factors that lead to the failure of ADC clinical trials. Toxic effects are related to multiple factors, including antibodies, payload drugs, linkers, and target antigens.
3) New resistance to ADC therapy is another obstacle that needs to be overcome. Research has shown that the ineffective internalization and lysosomal transport or degradation of ADCs may be the main mechanism of T-DM1 resistance. In addition, antibody-related resistance mechanisms may also lead to T-DM1 resistance, including reduced HER2 expression, expression of truncated HER2 forms, or ERBB2 gene mutations.
To overcome the above challenges, the future development directions for ADC may include:
1) Exploration of recombination antibody methods to improve the cancer cell delivery and lysosomal transport of ADC. Such as bispecific antibodies and antibodies that bind to cell-penetrating peptides, to improve ADC’s cancer targeting and lysosomal delivery.
2) There is still a great need for improvements in ADC design. In the next generation of ADC, it is necessary to develop new effective payload platforms, linker technology, and coupling strategies, to maximize the therapeutic effect and minimize the toxicity of ADC.
3) Clinical and translational methods will also play a critical role in improving the therapeutic window of ADC. Combination therapy is considered to have the ability to improve drug efficacy and reduce ADC resistance. Additionally, optimizing patient selection and monitoring response signals as clinical biomarkers can also improve ADC’s therapeutic index.
References
1. Antibody-Drug Conjugates Targeting the Human Epidermal Growth Factor Receptor Family in Cancers. Front Mol Biosci. 2022;9: 847835.
2. Implementing antibody-drug conjugates (ADCs) in HER2-positive breast cancer: state of the art and future directions. Breast Cancer Res. 2021; 23: 84.