Antibody-drug conjugates, often referred to as ADCs, are a type of dual-targeted therapeutic. They consist of three key components: an antibody, a payload, and a linker. The antibody determines the target disease, the payload dictates the treatment mechanism, and the linker determines how the payload is connected to the antibody and released.
The concept of ADC drugs was first introduced by Paul Ehrlich in 1913. After 87 years of research and development, the first ADC drug, Mylotarg, was approved in 2000. In recent years, there have been significant breakthroughs in ADC technology, particularly in payload and linker research. Around 2020, there was an explosion in the approval of ADC products, with 11 different ADC varieties receiving regulatory approval. Among them, Enhertu and Trodelvy have broken through traditional ADC concepts and technical limitations, significantly improving the effectiveness of ADC therapy. They have shown promise in treating solid tumors with low antigen expression and resistance to traditional treatments, potentially becoming first-line therapies and leading the direction of ADC development.
As of January 13, 2023, there have been 15 ADC drugs approved for marketing worldwide, with 13 of them utilizing traditional small molecule payloads and linkers. Among these 13 products, six use MMAE/MMAF-like payloads, two use auristatins, two use calicheamicin, and two use maytansine, while one uses PBD. Regarding linkers, ten utilize cleavable linkers, and three use non-cleavable linkers. In terms of conjugation sites, nine use cysteine conjugation, and four use lysine conjugation.
First-generation ADCs
First-generation ADCs used mouse-derived antibodies, highly active payloads, unstable linkers, and random conjugation strategies. They had high immunogenicity, mixed components with different drug-to-antibody ratios (DAR), poor stability, and a narrow therapeutic window. Mylotarg is a representative example of the first-generation ADCs.
Second-generation ADCs feature humanized antibodies, highly active payloads, stable linkers, and random conjugation strategies. They have lower immunogenicity, improved metabolic stability, and better therapeutic indices. Examples of second-generation ADCs include Kadcyla and Adcetris.
Third-generation ADCs employ site-specific conjugation techniques with low DAR values and highly active payloads. They exhibit better stability and homogeneity. Prominent examples are ARX788 and A166.
Fourth-generation ADC technology uses high DAR values (4–8) and medium to low activity payloads. These ADCs have a bystander effect, and they are effective in tumors with both low and high antigen expression. Representative examples are Enhertu, Trodelvy, and SKB264.
First-generation ADCs were in the early exploration stage, with limited knowledge about payload activity, linker properties, and payload release mechanisms. Mylotarg, approved in 2000 for treating acute myeloid leukemia, had limited survival benefits when combined with chemotherapy, and it was eventually withdrawn from the market. The payload used in Mylotarg was calicheamicin, and its linker had poor metabolic stability, leading to premature payload release and hepatotoxicity.
Second-generation ADCs
Second-generation ADCs introduced a variety of linker types, including peptidic linkers cleaved by tissue proteases and non-cleavable linkers. Kadcyla, approved in 2013, was the first ADC targeting HER2 and solid tumors. Adcetris, developed by Seagen, used MMAE as the payload and a VC linker. This generation also saw the development of vedotin, which is widely used as a payload linker.
Compared to first-generation ADCs, second-generation ADCs demonstrated better clinical efficacy and safety. However, these ADCs shared common features in their payload linkers, including hydrophobicity, differential stability in the system, and challenging CMC control.
Third-generation ADCs
Third-generation ADCs learned from the earlier generation’s experiences, focusing on stability and avoiding heterogeneity. They used site-specific conjugation and low DAR strategies. ARX788, developed by Ambrx, and A166, developed by Sichuan Kelun, are examples of this generation. They use MMAF-like payloads with non-cleavable linkers or VC linkers.
The payload linkers in third-generation ADCs use advanced site-specific conjugation techniques, such as engineered cysteines, non-natural amino acids, enzymatic conjugation, glycan conjugation, and special conjugation handles. These ADCs tend to use low DAR values and highly active payloads like PBD and tubulysin.
In analyzing existing products, it becomes evident that site-specific conjugation is not the key factor influencing ADC toxicity. Instead, the key is whether products produced via site-specific conjugation exhibit improved stability.
Fourth-generation ADCs
Fourth-generation ADCs, represented by Enhertu, Trodelvy, and SKB264, have shown significant efficacy and have promising potential. Enhertu has redefined the treatment of breast cancer and is challenging first-line therapy. Trodelvy is the first ADC for triple-negative breast cancer, and achieved substantial revenue. SKB264 shows clinical promise and has been involved in significant business transactions.
In the fourth generation, payload mechanisms are novel, and there have been substantial updates in linker technology. Payloads in these ADCs primarily use topoisomerase I inhibitors of the camptothecin class, and they exhibit low target DNA concentrations, low toxicity, high plasma clearance, and a bystander effect, making them effective against tumors resistant to microtubule inhibitors.
Enhertu utilizes a unique tetrapeptide (GGFG), self-cleaving segment, and maleimide linker. GGFG can be cleaved by various tissue proteases (cathepsins). The self-cleaving segment, a first for Daiichi Sankyo, enhances the ADC’s stability. The conventional maleimide linker is used for ADC conjugation but can prematurely release in the bloodstream, causing systemic toxicity.
Trodelvy employs a novel “self-cleaving” linker that can release the payload in pure water, blood, and the tumor microenvironment, eliminating the need for intracellular uptake for anticancer effects. However, its payload has relatively lower activity and limited stability in the bloodstream, leaving room for further improvement in efficacy.
SKB264 uses an innovative pyrimidine linker, which significantly enhances metabolic stability compared to the maleimide linker. The payload is a highly potent new maytansine derivative. SKB264 has improved chemical stability, a longer half-life in pure water and plasma, and significantly better preclinical efficacy compared to Trodelvy. SKB264 is currently in phase III clinical trials and has the potential to be applied to various cancer treatments.
In conclusion, ADCs have evolved through four generations, each with improvements in stability, safety, and efficacy. The fourth generation holds great promise, with ADCs like Enhertu, Trodelvy, and SKB264 redefining cancer therapy and opening new possibilities for targeted treatment.