Antibody drug conjugates (ADCs) are undoubtedly one of the hottest research areas in recent years. To date, there are 15 approved ADCs in the world, of which 13 have been approved by the FDA, one in China (Disitamab Vedotin) and one in Japan (Akalux).
Figure 1. FDA Approved ADCs 
Global sales of marketed ADCs exceeded $7 billion in 2022, and have already exceeded $5 billion in the first half of 2023, with the full year expected to surpass $10 billion.
Table. Sales of Approved ADCs from 2020 ~ 2023H1
In the first half of 2023, two ADC drugs reached $1 billion in sales, Kadcyla and Enhertu.
Based on Enhertu's growth rate over the last two years (3,450% year-on-year in 2021 and 153.52% year-on-year in 2022), Enhertu is expected to surpass Kadcyla in sales this year.
Besides approved ADCs, as of January 2023, close to 300 ADC drugs have entered the clinic, of which 92 have been discontinued due to toxicity or lack of efficacy, and 164 are in clinical drug development. As for the drugs in clinical development, 93 are in clinical phase I, 38 are in clinical phase I/II, 17 are in clinical phase II, and 15 are already in clinical phase II/III or III. 
Antigens targeted by clinically tested ADCs
The choice of the target antigen and selection of the monoclonal antibody are important parameters in determining the efficacy, therapeutic window, and toxicity profile of ADCs. Target antigens should be highly expressed in tumors, but low or even not in normal tissues, or at least limited to a given tissue type. Currently, popular targets include CD family (CD33, CD30, CD22, CD79β, CD19), HER2, Nectin-4, TROP2, BCMA, EGFR, etc.
Figure 2. Antigens targeted by clinically tested ADCs 
Linkers utilized by clinically tested ADCs
Linker design plays a critical role in modulating ADC stability in the systemic circulation and payload release efficiency in the tumors, which thus affects ADC pharmacokinetic (PK), efficacy and toxicity profiles.
Broadly speaking, linkers can be divided into two categories: cleavable or non-cleavable linkers. Of the clinical ADCs, 54% use cleavable linkers, which represent the most utilized linker class. 
Cleavable linkers depend on physiological conditions in the cell to cleave the linker and can be further subdivided into acid sensitive, protease sensitive or glutathione sensitive. Non-cleavable linkers form irreducible bonds with amino acid residues of the mAb, and therefore should be more stable in the bloodstream, have longer half-lives, and lower off-target toxicity. Non-cleavable linkers require ADC cleavage by the lysosome to release the payload.
Figure 3. Linkers utilized by clinically tested ADCs 
Payloads utilized by clinically tested ADCs
The payloads utilized in ADCs are highly potent cytotoxic drugs, exerting their effects on critical cellular processes required for survival. Payloads fall into four major classes: 1) microtubule inhibitors (maytansine derivatives (DM1/DM4) or auristatins (MMAE/MMAF)), 2) DNA-damaging agents (SN-38 or DXd, 3) topoisomerase I inhibitors, and 4) targeted small molecules (SM).
Figure 4. Payloads utilized by clinically tested ADCs 
ADCs In Clinical Phase 3
Below, a selection of ADCs in clinical phase 3 are listed.
|Trastuzumab duocarmazine (SYD985)||Byondis||HER2||HER2-positive breast cancer||FDA declined|
|Patritumab deruxtecan (HER3-DXd)||Daiichi Sankyo||HER3||NSCLC, breast cancer||Phase 3|
|Datopotamab deruxtecan (Dato-DXd)||Daiichi Sankyo||Trop-2||PD-L1+ TNBC, NSCLC, breast cancer||Phase 3|
|Telisotuzumab Vedotin (ABBV-399)||AbbVie, Inc.||c-Met||NSCLC||Phase 3|
|Depatuxizumab mafodotin (ABT-414)||AbbVie, Inc.||EGFR||Gliosarcoma||Phase 3|
|Tusamitamab Ravtansine （SAR408701）||ImmunoGen, Inc./Sanofi||CEACAM5||NSCLC||Phase 3|
|FS-1502||LegoChem Biosciences/复兴||HER2||HER2-positive breast cancer||Phase 3|
|ARX788||Ambrx, Inc./Zhejiang Medicine Co.||HER3||Brest cancer||Phase 3|
|BAT8001||Bio-Thera Solutions||HER2||HER2-positive breast cancer||Phase 3|
|A166||Kelun-Biotech||HER2||HER2-positive breast cancer,Cholangiocarcinoma,bladder cancer||Phase 3|
|SHR-A1811||Hengrui Pharmaceutical||HER2||Brest cancer||Phase 3|
|DP303c||CSPC ZhongQi Pharmaceutical||HER2||HER2-positive breast cancer, Ovarian cancer, Gastric Cancer||Phase 3|
|MRG002||Miracogen||HER2||HER2-positive uroepithelial carcinoma, Gastroesophageal junction cancer, HER2-positive breast cancer||Phase 3|
SYD985 (trastuzumab duocarmazine) is a next-generation HER2 ADC developed by Byondis utilizing its proprietary duocarmazine linker-drug (LD) technology ByonZine®. It is comprised of the anti-HER2 monoclonal antibody trastuzumab and a cleavable linker-drug called valine-citrulline-seco-DUocarmycin-hydroxyBenzamide-Azaindole (vc-seco-DUBA). 
Figure 5. Structure of SYD985 
On 16 May, 2023, The FDA has issued a complete response letter for a biologics license application (BLA) for SYD985 as a treatment for patients with HER2-positive, unresectable, locally advanced or metastatic breast cancer. In the CRL, the regulatory agency requested additional information that will require additional time and resources that extend beyond the current evaluation period.
Patritumab deruxtecan (HER3-DXd, U3-1402) is the world's first HER3 ADC developed by Daiichi Sankyo and is currently being explored for indications such as breast cancer and non-small cell lung cancer.
HER3-DXd is derived by conjugating patritumab (U3-1287), a monoclonal antibody to the extracellular structural domain of HER3, and deruxtecan, a cytotoxic topoisomerase I inhibitor, by a maleimide-GGFG linker via a cysteine-sited coupling, with a DAR value of 8 (Figure 6).
Figure 6. Structure of HER3-DXd
At ASCO 2023, Daiichi Sankyo further announced Part A results from a Phase 2 study of HER3-Dxd in patients with HER2-negative metastatic breast cancer (MBC). The results showed an overall response rate (ORR) of 35%, a clinical benefit rate (CBR) of 48%, a median duration of response (DOR) of 10.0 months, and a 6-month PFS rate of 60% in the overall population (Figure 7) .
Figure 7. Phase 2 study of HER3-DXd 
Recently, Daiichi Sankyo at the 2023 WCLC Congress presented the primary results of the pivotal phase 2 study HERTHENA-Lung01 of HER3-DXd for the treatment of patients with EGFR-mutated non-small cell lung cancer. The results showed that a confirmed ORR of 29.8% was observed with HER3-DXd (5.6 mg/kg) in 225 patients with EGFR-mutated NSCLC, as assessed by blinded independent center review (BICR), with 1 complete response(CR), 66 partial response(PR), and 99 stable disease (SD) with a DOR of 6.4 months and a disease Control Rate (DCR) of 73.8%.
As of May 18, 2023, the median progression-free survival (PFS) was 5.5 months and the median overall survival (OS) was 11.9 months (Figure 8) .
Figure 8. HER3-DXd- HERTHENA-Lung01 results 
Datopotamab deruxtecan(Dato-DXd, DS-1062)
Dato-DXd, a targeted Trop-2 ADC co-developed by AstraZeneca and Daiichi Sankyo, couples a recombinant humanized anti-Trop-2 IgG1 antibody with a topoisomerase I inhibitor (DXd) via a tetrapeptide-based linker. The tetrapeptide-based linker releases DXd upon proteolytic processing by lysosomal enzymes such as histone proteases.On average, the number of targets of the drug linker for an antibody molecule is 4 (Figure 9) .
Figure 9. Structure of Dato-DXd 
Unlike Trodelvy, the first approved Trop-2 ADC for the treatment of breast cancer, AstraZeneca and Daiichi Sankyo plan to apply Dato-DXd for the treatment of advanced non-small cell lung cancer.
At the 2023 ASCO Annual Meeting, AstraZeneca and Daiichi Sankyo presented results from a (TROPION-Lung02) study of Dato-DXd for advanced NSCLC. The results showed that Dato-DXd in combination with pembrolizumab or Dato-DXd in combination with pembrolizumab and platinum-based chemotherapy had an ORR of 38% and 49%, a DCR of 84% and 87%, and a PFS of 8.3 months and 7.3 months, respectively, in the overall population of advanced NSCLC. In the first-line treatment population, the ORR of Dato-DXd in combination with pembrolizumab or Dato-DXd in combination with pembrolizumab and platinum-based chemotherapy was 50% and 57%, respectively, with a DCR of 91% in both cases, and the median DORs have not yet been reached in either case .
However, Dato-DXd is currently experiencing safety issues, and the results of the TROPION-Lung01 trial, while meeting the clinical primary endpoint, showed serious safety issues, with not only the discovery of interstitial pneumonitis, but also a grade 5 adverse event in which the patient died due to an adverse reaction to the drug.
Huateng Pharma is dedicated to being your most reliable partner to provide chemical synthesis and high-quality PEG linkers for ADC drugs. We are committed to promoting the progress of your ADC discovery and development projects.
 Maecker H, Jonnalagadda V, Bhakta S, Jammalamadaka V, Junutula JR. Exploration of the antibody-drug conjugate clinical landscape. MAbs. 2023;15(1):2229101. doi:10.1080/19420862.2023.2229101
 ASCO Highlights 2023, DAIICHI SANKYO CO., LTD.
 Daisuke Okajima et.al, Datopotamab Deruxtecan, a Novel TROP2-directed Antibody–drug Conjugate, Demonstrates Potent Antitumor Activity by Efficient Drug Delivery to Tumor Cells, Mol Cancer Ther 2021;20:2329–40
 ASCO-IR-deck-June-2023.pdf (astrazeneca.com)