Antibody–Drug Conjugates Targeting MUC1

According to statistics, over 140 ADC-targeted drugs are currently in clinical trials worldwide. The most popular targets include HER2, TROP2, EGFR, CLDN18.2, c-Met, CD19, PSMA, BCMA, and PDL1, all of which have been validated both clinically and commercially. However, these targets are highly competitive.

Recently, "unpopular targets" have been gaining more attention. These targets have traditionally faced significant challenges, including high difficulty in drug development, past clinical setbacks, and limited potential for indications. As a result, pharmaceutical companies have often deprioritized them in research and development investments. However, advancements in ADC drug development technology are expanding the range of viable targets. With the current focus on differentiation, pioneering pharmaceutical companies are now willing to take risks on these previously overlooked targets.

MUC1, part of the mucin family, was first isolated from human breast milk by Shimizu in 1982. It plays a crucial role in diagnosing and predicting the prognosis of various tumors, making it a promising "unpopular target" for pharmaceutical companies. This article offers an overview of the current research status on MUC1 for researchers.

Structure of MUC1

MUC1, also known as EMA (tumor-associated epithelial membrane antigen) or CD227, is a large, glycosylated protein with expected molecular weights ranging from 120 to 500 kDa, depending on the glycosylation status. MUC1 is highly expressed in epithelial-derived tumor tissues, such as lung cancer, pancreatic cancer, prostate cancer, epithelial ovarian cancer, and breast cancer. Its expression is associated with tumor metastasis and recurrence.

The MUC1 gene is located on chromosome 1q21 and was first cloned from complementary DNA (cDNA) libraries constructed from cell lines such as breast cancer. It contains seven exons, with the second exon containing a variable number of tandem repeat (VNTR) sequences. The MUC1 protein has two subunits: the amino-terminal region (MUC1-N) and the carboxyl-terminal region (MUC1-C). MUC1-N contains 20 to 120 VNTRs, which form the core component of MUC1. Each VNTR includes 20 amino acids that can be highly O-glycosylated.

MUC1

Figure 1. Structure of MUC1

In normal cells, MUC1 is expressed in the apical surface of epithelial cells. MUC1-N functions as a lubricant, moisturizer, and protective barrier, shielding epithelial cells from environmental pollutants, microbes, and other external threats. Additionally, external signals can cause epithelial cells to temporarily lose their polarity, resulting in widespread MUC1 expression across the cell surface. MUC1 then interacts with factors on the basolateral side of the cell, engaging in downstream signaling pathways that support cell repair and survival.

In tumor cells, MUC1 exhibits distinct biochemical characteristics and cellular distribution compared to normal cells. Typically, MUC1 expression in malignant tumors surpasses that in normal cells by over tenfold, correlating with the severity of the malignancy. Structurally, incomplete glycosylation and fewer branches expose new glycan and peptide epitopes on MUC1, making it susceptible to cleavage by extracellular proteases, thereby releasing MUC1-N. The release of MUC1-N triggers a conformational change in MUC1-C, altering its ligand state and activating downstream cellular signaling pathways such as MAPK, PI3K/Akt, and Wnt pathways [8]. Furthermore, reduced glycosylation levels of MUC1 diminish tumor cell adhesion strength, promoting conditions conducive to tumor metastasis. Moreover, the loss of polarity distribution of MUC1 results in its expression across the entire glandular epithelial surface and within the cytoplasm. These MUC1 variants typically interact with growth factors and their receptors, with MUC1-C playing a role in tumor cell invasion, metastasis, and angiogenesis through intracellular signaling and modulation of related biomolecules.

MUC1-experession

Figure 2. MUC1 is overexpressed on the surface of cancer cells.

Clinical Advances in MUC1

Given its role in tumor progression and its expression patterns in tumor tissues, MUC1 is increasingly seen as a promising target for developing drugs against solid tumors. Several companies have developed a variety of immunotherapies targeting MUC1, with some advancing to Phase 2 clinical trials. These therapies encompass a wide range of drug types, including antibody-drug conjugates (ADCs), monoclonal antibodies, bispecific antibodies, vaccines, chimeric antigen receptor T-cell (CAR-T) therapies, and chimeric antigen receptor natural killer cell (CAR-NK) therapies. Here, we offer you an overview of MUCI-ADCs in clinical trials.

Currently, there are a total of 15 ADCs targeting MUC1 worldwide. Only three of these MUC1-targeted ADCs have entered clinical stages: DS-3939 (Daiichi Sankyo/ Glycotope), M1231 (Merck/Sutro Biopharma), and DXC005 ( DAC Biotech).

DS-3939 (Daiichi Sankyo/ Glycotope)

In 2018, Daiichi Sankyo and Glycotope entered a global licensing agreement. Under this agreement, Daiichi Sankyo utilized Glycotope's research on the tumor-associated TA-MUC1 antibody GT-00A along with its proprietary DXd-ADC technology to develop DS-3939. DS-3939 is composed of a monoclonal antibody linked through a cleavable tetrapeptide linker to multiple topoisomerase I inhibitors as payloads (specifically a derivative of exatecan), with a drug-to-antibody ratio (DAR) of 8.

In September 2023, DS-3939, Daiichi Sankyo's sixth proprietary ADC drug, began Phase 1/2 clinical trials for locally advanced, metastatic, or unresectable solid tumors including non-small cell lung cancer, breast cancer, urothelial carcinoma, ovarian cancer, bile duct cancer, and pancreatic ductal adenocarcinoma. The first patient dosing has been completed, but the company has not yet disclosed any clinical data.

DS-3939

Figure 3. DS-3939

M1231 (Merck/Sutro Biopharma)

M1231 is a bispecific ADC co-developed by Sutro Biopharma and Merck, targeting both MUC1 and EGFR. It utilizes Sutro's non-natural amino acid site-specific conjugation technology, linking a cleavable Val-Cit SUTRO linker to connect the bispecific antibody with the cytotoxin Hemiasterlin. Hemiasterlin, a tripeptide, binds to tubulin proteins to exert its cytotoxicity. The bispecific antibody portion incorporates Merck's SEED bispecific antibody technology platform to prevent the mispairing of the two heavy chains. The antibody targeting MUC1 is in the single-chain variable fragment (scFv) format, while the antibody targeting EGFR is in Fab format.

M1231

Figure 4. M1231

Preclinical studies have demonstrated that M1231 exhibits robust in vitro and in vivo activity and stability. Additionally, compared to its corresponding monoclonal antibody ADC, M1231 shows enhanced internalization and anti-tumor efficacy, demonstrating potent anti-tumor activity in NSCLC and ESCC PDX models.

In January 2021, M1231 initiated Phase 1 clinical trials (NCT04695847) targeting indications including metastatic solid tumors, esophageal cancer, and non-small cell lung cancer. According to clinical trial data, the trial is currently completed, but results have not yet been disclosed.

DXC005 ( DAC Biotech)

DXC005 is an ADC drug developed by DAC Biotech, consisting of a recombinant humanized monoclonal antibody against MUC1 (DXA005) linked to a TubulysinB-like analog (Tub201) through a cysteine residue via an interchain disulfide bond.

Tubulysin is an inhibitor of tubulin proteins, disrupting microtubule polymerization and arresting cells in the G2/M phase of the cell cycle, leading to cell death. Tubulysin B-like analogs are potent cytotoxic molecules with hydrophilic and slow-release linkers, featuring a maleimide group at the linker's terminal for conjugation to the cysteine thiol group of the anti-MUC1 monoclonal antibody.

DXC005's anti-MUC1 monoclonal antibody typically links around 4.0-4.2 Tub201 molecules on average.

On July 15, 2022, DAC Biotech 's novel ADC drug DXC005 (project code: DXC005-001), developed independently, administered its first dose to a subject in the Department of Digestive Oncology at Beijing Cancer Hospital. Phase 1 trials are ongoing, with no clinical data disclosed yet.

Summary

MUC1 is a highly glycosylated type I transmembrane protein expressed on the apical surface of epithelial cells, serving a protective role. In various solid tumors, MUC1 is abnormally glycosylated and overexpressed, playing a critical role in tumor development, invasion, metastasis, and drug resistance. Therefore, MUC1 is considered a promising target for future cancer therapies.

Currently, there are numerous drug development projects targeting MUC1. Although many MUC1-targeting antibodies and ADCs have shown strong anti-tumor effects in preclinical studies, the targeted drugs that have entered clinical trials have yet to demonstrate outstanding efficacy. This indicates that the journey of MUC1-targeted drug development still faces significant challenges.

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References:
[1] Nath S, Mukherjee P. MUC1: a multifaceted oncoprotein with a key role in cancer progression[J]. Trends Mol Med,2014, 20(6): 332-342.
[2]Nabavinia M S, Gholoobi A, Charbgoo F, et al. Anti-MUC1 aptamer: A potential opportunity for cancertreatment[J]. Med Res Rev, 2017,37 (6):1518-1539.