ADC Drug Research Report

01Characteristics of ADC drugs

❖ Target selection

ADC drugs are currently mainly used in the field of tumors, so antigen targets are required to be highly expressed in tumor cells, but low or not expressed in normal tissues, or only expressed in specific tissue types. This standard is strict enough to make it difficult to To find a good target, other factors need to be considered when selecting a target, including the cell type expressing the antigen, the cycle state of the cell (such as division or quiescence), the expression level of the target, and the target point accessibility, etc.; secondly, the target antigen should be present on the cell surface so that circulating mAbs can enter. At the same time, the antigen target should have a certain endocytosis ability to trigger the transport of the ADC-antigen complex into the cell. However, the number of tumor cell surface antigens is usually limited, and the internalization process of antigen-antibody complexes is usually inefficient. Therefore, target selection is challenging.

Among the ADC drugs currently on the market, the indications of five targets including CD22, CD30, CD33, CD79b and BCMA are hematological tumors; the three targets of HER2, Nectin-4 and Trop-2 are indicated for solid tumors. In terms of target accessibility, solid tumors are more obstructive than hematological tumors. The microenvironment of solid tumors and other factors make it difficult for drugs to penetrate. In this regard, the accessibility of hematological tumors is better, which is also the key factor why ADC drugs will make breakthroughs in the field of hematological tumors first (approved by the FDA in 2011). Adcetris for Hodgkin lymphoma and anaplastic large cell lymphoma). In addition to the above listed products, ADC drugs targeting CD19, Mesothelin, PSMA, EGFR, Nectin-4, CD56, CD138, CD74, etc. are also progressing rapidly or becoming more popular.

ADC antibody target

HER2 positivity, as a high incidence type in breast cancer, also has a high incidence in bladder cancer and gastric cancer. HER2 has been developed for many years as an important target in breast cancer treatment. Related monoclonal antibodies, small molecules and ADC drugs are all on the market. Currently, two ADC drugs for breast cancer have been approved globally: Kadcyla and Enhertu.

As a second-generation ADC drug, Kadcyla showed excellent OS and PFS in the EMILA study compared with lapatinib + capecitabine. Chemotherapy in patients with HER2-positive metastatic breast cancer (second-line therapy); in 2019, it was approved for residual invasion after preoperative neoadjuvant therapy because it was shown in the KATHERINE study to significantly reduce the risk of postoperative recurrence by 50% Postoperative adjuvant therapy in patients with HER2-positive early breast cancer with sexually transmitted diseases. According to the current registered clinical trials, Kadcyla is still further expanding the use of breast cancer.

Enhertu was originally developed by Daiichi Sankyo, and later transferred the development rights outside Japan to AstraZeneca. Enhertu uses Daiichi Sankyo's proprietary DXd technology to improve the hydrophobicity so that it can load 7-8 effector molecules. It has been approved 2-4 times that of ADC, its effector molecule is an innovative DNA topoisomerase inhibitor Dxd, and its activity is 10 times that of irinotecan. In addition, Enhertu also has stable and specific cleavable linkers and strong Because of its cell membrane permeability, Enhertu has received a lot of attention from the industry and has achieved good therapeutic effects. It has obtained multiple breakthrough drug qualifications, especially in breast cancer and other HER2-positive/mutated cancers after multi-line therapy. . In December 2019, Enhertu received accelerated FDA approval for patients with HER2-positive metastatic breast cancer who have received 2 or more anti-HER2 drugs in metastatic disease. Domestic Rongchang Bio's drug RC-48 has also achieved excellent efficacy in urothelial cancer and advanced gastric cancer other than breast cancer. RC48 has obtained a new human with stronger affinity and better endocytosis effect through CDR transplantation. Geneized antibody has a stronger binding affinity to cell surface HER2 than trastuzumab, and the naked antibody of RC48 also has the ability of transplanted tumor growth and is stronger than trastuzumab. In terms of urothelial cancer and gastric cancer, RC48 has already submitted an NDA for gastric cancer in China, and urothelial cancer will also apply for an NDA next year. Overseas, the indication for urothelial cancer has been approved by the FDA as a breakthrough therapy, and directly Phase 2 clinical trials in the United States have also been granted orphan drug designation by the FDA for gastric cancer indications.

It can be seen from the above drug studies that HER2-ADC drugs have made continuous breakthroughs in the treatment of HER2-expressing related cancers, such as breast cancer, gastric cancer, bladder cancer, and colorectal cancer, highlighting the fact that HER2 target research has become mature at the moment , ADC drugs have further broadened the application scope of targeted therapy, and the potential of mature targets has been further tapped, especially in the later-line treatment and low-expression tumors, which will show better efficacy performance.

CD antigen has become an important target antigen for hematological tumor treatment, accounting for a large proportion of the currently marketed ADC drugs. From the point of view of targets, currently the targets of CD30, CD22, CD33 and CD79b have been approved for marketing, and they are mostly concentrated in hematological tumors, such as leukemia, lymphoma and non-Holy lymphoma. Among the targets under research, there are targets such as CD19, CD56, CD138, and CD37, all of which have drugs in clinical development. Among them, CD19 is an ideal target for the treatment of various B-cell malignant tumors. In the past two years, it has surpassed pd-1 and pd- l1 has become the most popular research and development target in the field of tumor immunotherapy, and it is also the target of CAR-T therapy. At present, there is no ADC drug targeting CD19 on the market, but ADCT-402 (loncastuximab tesirine) has been applied for FDA listing. For the treatment of relapsed/refractory diffuse large B-cell lymphoma (r/r DLBCL), the drugability of CD19 in the field of ADC has been basically verified, and it is expected to see the same targeted drugs on the market in the future.

In terms of indications, ADC drugs targeting CD series are still concentrated in the development of hematological tumors, and no substantial progress has been observed in solid tumors. Excellent, but shelved due to potential patient infection issues, so solid tumors are a promising direction for future CD series.

Trop2 is a cell surface protein encoded and expressed by the TACSTD2 gene. Its high expression plays a key role in tumor growth. It is also associated with more aggressive diseases and poor prognosis. The characteristics of its high expression and differential expression are very good ADC drug target selection. In 2020, the first Trop2-targeted ADC, Trodelvy, received fast-track approval from the FDA for adult patients with metastatic triple-negative breast cancer (mT NBC) who have received at least two prior therapies for metastatic disease. Trodelvy uses a humanized IgG1 mAb conjugated to the topozyme inhibitor SN-38, targeting the cell surface glycoprotein Trop2 expressed in more than 90% of triple-negative breast cancers.

Since TROP2 is highly expressed in a variety of cancer types and can be used to treat a variety of different cancers, it is expected that ADCs targeting Trop2 will play a role in a variety of cancer types in the future. According to the trial data, ADC drugs targeting Trop2 have shown good efficacy in non-small cell lung cancer, urothelial cancer and other subtypes of breast cancer. Many biomedical and technology companies in China have begun to develop targeting Antibody-drug conjugates of TROP2.

BCMA, also known as TNFRSF17, is a member of the TNF-receptor family and plays an important role in autoimmune diseases and humoral immunity, and is closely related to Hodgkin lymphoma, multiple myeloma and other diseases. GSK's Blenrep is the world's first approved ADC drug targeting BCMA. The approval of Blenrep not only marks the success of BCMA targets in the ADC field, but also shows that ADC drugs have the potential to treat relapsed or refractory disease.

C-Met is a membrane surface receptor protein with tyrosine kinase activity. Compared with normal tissues, C-Met has a significantly higher expression in tumor tissues. Excessive activation of C-Met signaling pathway is associated with the occurrence of various malignant tumors. Close relationship, the ADC targeting C-Met is still in a relatively early stage. Among the drugs under development, the fastest progress is ABBV-399 developed by AbbVie. Non-small cells have entered clinical phase 2. At present, many domestic companies are following the layout. Such as Hengrui, Rongchang, etc., fully illustrate the potential of C-Met.

ROR1, a transmembrane tyrosine kinase receptor, is significantly elevated in a variety of hematological cancers and solid tumors, including B-cell chronic lymphocytic leukemia (CLL), acute lymphoblastic leukemia (ALL), non-Hodge Gold lymphoma (NHL) and myeloid hematological cancers, solid tumors including colon cancer, lung cancer, pancreatic cancer, ovarian cancer and other cancers, have become tumor-specific targets for remarks.

On November 5, 2020, Merck spent $2.75 billion to acquire VelosBios and obtained its flagship product under development, VLS-101, which is an ADC drug targeting ROR1. In addition, this year CStone also announced a partnership with LegoChem for a ROR1-targeting drug. The ADC drug LCB71 shows the potential of the ROR1 target in the application of ADC drugs. Taking VLS-101 in the phase 1 clinical treatment of hematological tumors as an example, the ORR of MCL patients treated with multiple other anticancer therapies reached 47%, and diffuse large B The ORR of patients with cellular lymphoma reaches 80%, and the ROR1 target may generate a new generation of ADC star drugs in the future.

With the maturity of ADC design technology, many biopharmaceutical companies around the world have begun to explore the use of ADCs to treat indications other than oncology, including ophthalmology, immunology, anti-infection, endocrine/metabolism and other disease areas. Most of these ADCs under research for non-cancer indications are still in the early development stage, but the proportion is gradually increasing, and the proportion in the early development stage is significantly higher than that in the clinical stage. In the later stage, non-cancer indications may become the next "blue ocean" for ADC drug development. According to incomplete statistics, most of the ADC drugs used in the treatment of non-cancer indications are non-cytotoxic drugs, accounting for 90% of the R&D projects. These non-cytotoxic drug loads include immunomodulators, enzymes, and antisense oligonucleotides. Innovative treatment models such as nucleotides and siRNA are expected to usher in innovative therapies for non-cancer indications in the future.

❖ Antibody selection

High specificity and high affinity are the main characteristics that should be possessed by antibodies in ADC drugs. In addition, the antibody should also have low immunogenicity, low cross-reactivity, and appropriate linkage and binding properties. All current ADC antibodies are IgG molecules due to their high affinity for the target antigen and long half-life in blood. Compared with other IgG molecules, IgG1 has better binding activity and easier production, and is the first choice for ADC drug development.

Early ADCs mostly used mouse-derived or chimeric antibodies, which easily caused human anti-mouse antibody reactions. Currently, ADC development uses humanized antibodies or fully humanized antibodies. The screening of antibodies needs to consider the following factors: specific binding, the endocytosis efficiency of the ADC drug mediated by the antigen-antibody complex, and the accurate localization in cells after endocytosis (such as lysosomes). In addition, modification of antibodies, such as glycosylation modification of antibody constant regions or modification of attachment sites, can help to improve the properties and efficacy of drugs.

Antibody selection of ADC drugs

❖ Toxin molecule (Payload)

The toxin molecule is a key factor determining the lethality of ADC. In addition to extremely high toxicity, it also needs to have sufficient water solubility and stability in serum. The toxin drugs currently in clinical use can be divided into two categories according to their mechanism of action: (1) Microtubule inhibitors, typically auristatins (MMAE, MMAF, MMAD), maytansine and maytansine derivatives Derivatives (DM1, DM4), block the cell cycle by combining with microtubules to prevent the polymerization of microtubules; (2) DNA damage agents, represented by Calichemicin, duocarmycins, doxorubicin, calicheamicin, by combining with The minor groove of DNA binds and promotes DNA strand alkylation, breakage or cross-linking. Compared with traditional chemotherapeutic drugs, these highly active cytotoxic chemical drugs have stronger killing power to cancer cells, and usually an average dose of 4-6 molecules can kill targeted cancer cells. The development of drugs loaded with different mechanisms of action of cancer cells will help improve the effectiveness and safety of new-generation antibody-drug conjugates on the one hand, and help avoid patents on the other hand, and will also improve the innovation and competitiveness of drugs.

❖ Linker

Linkers are the basis for ADCs to effectively deliver cytotoxic drugs. Linkers must remain stable in blood circulation, otherwise they will produce unexpected toxicity, and when entering tumor cells, they can rapidly release effective cytotoxic drugs to kill cancer cells.

At present, it is mainly divided into cleavable linker and non-cleavable linker. Cleavable linkers mainly include chemically unstable linkers, such as acid-labile linkers (hydrazones, which can be effectively cleaved in acidic environments such as lysosomes) and disulfide linkers (which can be used in a reducing intracellular environment. Selectively cleaved by glutathione), and enzymatically cleaved linkers (peptide linkers, which can be cleaved by proteases), mainly using their environmental differences in the blood system and tumor cells, low pH (acid sensitivity), proteases Hydrolytic (protease-sensitive) and reducing environments (glutathione-sensitive), are rapidly broken down in tumor cells, and Adcetris uses such "resectable" linkers; however, "unresectable" linkers are used throughout the drug bloodstream. During the transportation process, the integrity of the coupling of antibodies and chemical drugs is always ensured, and the ADC antibody components are completely degraded by cytoplasmic and lysosomal proteases. The common thioether linker has good stability in plasma and a long half-life. , Kadcyla uses such "uncut" linkers.

The cytotoxic drug released by the cleavable linker after the ADC drug is cleaved can penetrate the cell membrane and kill the surrounding tumor cells, known as the bystander effect; in contrast, for the non-degradable linker , Even if the antibody part is degraded by proteases, there are still amino acid residues connected to linkers and cytotoxic drugs. This charged metabolite cannot effectively pass through the cell membrane, so it usually does not have a bystander effect.

02ADC drug mechanism

❖ Specific binding

Antibody-drug conjugates specifically bind to the epitope of cancer cells through the antibody part with "guidance" effect, and then enter the cancer cell through antigen-mediated endocytosis, and pass through special environment (such as lysosome) inside the cell. or low pH environment) to release "warhead" highly active cytotoxic drugs, and finally achieve specific killing of cancer cells. The mechanism of action is shown in the figure. Antibody-drug conjugates are also considered to be a more advanced drug delivery system, and their anti-cancer mechanism of action is completely different from that of antibody drugs.

ADC drug Mechanism

❖ ADCC effect

Antibody-drug conjugates improve antibody-dependent cell-mediated cyto-toxicity (ADCC), and the antibody Fab fragments of such drugs bind to antigenic epitopes of virus-infected cells or tumor cells, The Fc segment of its antibody binds to FcR on the surface of killer cells (NK cells, macrophages, etc.), thereby mediating the killer cells (NK cells, macrophages, etc.) to directly kill cancer cells.

❖ Inhibit downstream signaling pathways

The antibody part of the antibody-drug-coupled drug specifically binds to the epitope antigen target of cancer cells and inhibits the downstream signaling of the antigen receptor. For example, the antibody part of Roche's antibody-drug-coupled drug Kadcyla can bind to the HER2 receptor of cancer cells and inhibit HER2 and HER1. , HER3 or HER4 form heterodimers and inhibit cell growth signaling pathways. At the same time, HER2 can activate a variety of downstream signaling pathways including PI3K, MAPK, etc. Kadcyla's antibody hinders the normal conduction of these signaling pathways, blocks cancer cells at the set point, and induces cancer cell apoptosis.

❖ Bystander effect

"Bystander": The drug (or linker-drug composition) released by the antibody-drug conjugate within the cancer cell is permeable or transmembrane, and the released drug can kill adjacent cancer cells. This phenomenon is called the "bystander effect". The expression of antigens in solid tumor cells is usually heterogeneous, so antibody-drug conjugates may not be able to directly and effectively kill adjacent antigen-negative cancer cells. When the antibody-drug conjugates release cytotoxins outside cancer cells or in target cells, they can The developed small molecule drugs can not only kill antigen-positive cancer cells, but also other nearby cancer cells through the bystander effect. At the same time, the bystander effect of such drugs also destroys the environment for tumor growth, such as tumor stromal cells and tumor blood vessels, thereby further enhancing the effect of killing cancer cells.

• "Bystander" effects of ADC drugs

03Improvement direction of ADC drugs

❖ Find new targets and expand the scope of indications

❖ Find new toxic small molecules, which are safer and more effective

Due to the limited number of tumor cell surface antigens, the number of small molecule drugs linked to antibodies is also limited, and the efficiency of macromolecules to pass through the cell membrane is low, so the number of toxic small molecules that can enter the interior of tumor cells is limited, which is very important for small molecules to kill tumor cells. higher efficiency requirements. Due to the requirement of high toxicity, the current trials of traditional cytotoxic anti-tumor drugs combined with ADC have mostly ended in failure. The mainstream technical route is to use derivatives of the auristatins and maytansinoids family to destroy DNA or microtubules after entering tumor cells to inhibit their proliferation.

❖Optimize Linker design

The ideal ADC drug should remain stable in the circulatory system, reduce the shedding of toxic small molecules as much as possible to avoid excessive adverse reactions, and at the same time, after entering the tumor, it should effectively release toxic small molecules to complete the function of killing cancer cells. The balance of these goals has high requirements on the design of the linker.

❖Homogenized DAR

In addition to finding new targets, new toxic small molecules and optimizing linker design, there are other improvement ideas for ADC drug development, such as improving the uniformity of drug/antibody ratio (DAR). If the number of small molecule drugs bound to each antibody fluctuates widely, the naked antibody will compete with ADC for binding sites, thereby weakening the therapeutic effect; optimizing the binding of antibody molecules to toxic small molecules and improving the uniformity of DAR is conducive to strengthening Antitumor effect.

ADC drug research and development enthusiasm is high, and it is ushering in an outbreak period. As a combination of monoclonal antibodies and small-molecule toxic drugs, ADC drugs have both antibody drug targeting and chemotherapeutic tumor killing properties. With the continuous iteration and maturity of technology, ADC drugs have undergone three generations of technological changes. The application of the third-generation fixed-point coupling technology has made ADC drugs more uniform, stable and effective, and has achieved better and better results in treatment. During the two years from 2019 to 2020, a total of 6 ADC drugs have been approved by the FDA for marketing. Among them, Adcetris and Kadcyla, which were launched earlier, achieved sales of US$1.081 billion and US$1.572 billion respectively in 2019, and have become "blockbuster" products. In addition, Polivy, Enhertu and Trodelvy also have the potential to become blockbusters. Currently, the pharmaceutical industry is enthusiastic about ADC research and development, and it is expected to see more ADC drugs approved for marketing in the future.

ADC drug patent technology will become the core competitiveness of ADC pharmaceutical companies. An efficient ADC drug needs to comprehensively consider antibodies (target and antibody screening optimization), small molecule drugs, linkers and linking technologies and their effective combinations. The upstream of the ADC drug industry chain is mainly composed of R&D companies that master these three core technologies. In the future, the patent value of the technology platform will continue to be highlighted, and mastering more patented technologies will become the core competitiveness of ADC pharmaceutical companies.

Domestic ADCs started late and still focus on popular targets. Most of the domestic companies engaged in ADC drug development are transformed from the biopharmaceutical business such as monoclonal antibodies. In the past two years, pharmaceutical companies focusing on the ADC technology field have gradually emerged in the market, such as Nuo Ling Bio, Qi De Pharmaceutical, Duo Xi Bio, etc. At present, domestic ADC The research targets mainly focus on the HER2 and CD families. Overall, the development of domestic ADC drugs has room for further improvement compared with foreign countries in terms of antibody targets, indications and ADC structure design. With the application of the first domestic ADC drug Rongchang Bio RC48 in 2020, it marks the Chinese ADC drug. With the arrival of the first year of commercialization, from the perspective of investment value in the future, it is necessary to focus on the sustainable value of the technology platform of ADC pharmaceutical companies, new breakthroughs in product indications, and outsourcing CMO/CDMO industry opportunities created by the complex process of ADC drugs.