Antibody-drug conjugates (ADCs) as precision targeted cancer therapies have started to deliver on their early promise as an innovative novel drug modality as demonstrated by the recent increase in market approvals. For decades, ADCs promised much in terms of targeted delivery of a potent cytotoxin, but for various reasons that included a lack of efficacy, fast clearance, dose limiting toxicities and an overall narrow therapeutic index (TI), their ability to deliver patient benefits were lacking.
As a greater array of therapeutic targets were mined as suitable for ADC candidates, the ADC development landscape therefore increased to encompass a huge number of targets, indications and combinations of different conjugation and linker optionalities (1).
Of all of these ADC targets, HER2 has been fertile ground for ADC drug hunters, spearheaded by the approval of Kadcyla® and Enhertu®. Interestingly, both of these ADCs incorporate trastuzumab (Herceptin® as the monoclonal antibody, and the influence of this targeting agent does not stop there, with [vic-]trastuzumab duocarmazine (SYD985) containing trastuzumab in Phase III, as well as other trastuzumab-ADC Phase I candidates and several engineered variants in different phases of clinical development (see figure 1) (2). This is supplemented by the Indian approved Kadcyla® biosimilar, with several others following this route in different countries.
A number of additional HER2 targeting ADC programs do expand further than trastuzumab, with the Chinese approved ADC Aidexi® utilising hertuzumab, and other ADCs in early stages of clinical development using novel antibodies. However, perhaps reasons for the exploitation of trastuzumab and the heavy focus of HER2 as a target of choice for demonstrating ADC developability is the depth and breadth of preclinical translational models, the ready availability of Herceptin® and the approved benchmarking controls.
The journey from Kadcyla® to Enhertu® is an interesting one in that several key architectural design features were improved upon. These included moving from a non-cleavable to a cleavable linker, shifting the cytotoxin mechanism of action from the anti-mitotic maytansinoid DM1 to the topoisomerase I inhibitor DXd and increasing the drug-antibody ratio (DAR) from a heterogeneous distribution averaged at 3.5 to a homogeneous DAR of 8. This higher DAR aimed at widening the TI by exploiting a lower potent cytotoxin.
The concept of higher DAR species was primarily associated with faster clearance, higher toxicity and poor PK profiles, ultimately resulting in lower efficacy (3,4). Early approvals therefore settled around the sweet-spot of a DAR of 4, setting the paradigm of suitable TI. Development of Daiichi Sankyo’s HER2- targeting Enhertu® and Immunomedics’ TROP-2 targeting Todelvy® in this higher DAR bracket both possessed DARs of 8 and were clearly an excursion outside of this sweet spot. The success of these higher DAR ADCs was made possible by delivering payloads that were lower in potency and modulated hydrophobicity compared to the microtubulin inhibitor auristatins and maytansines, and the DNA alkylating anthracyclines and PBD dimers. Ultimately this allowed a reduction in aggregation and improvements in PK profile.
This combination of increased hydrophilic linkers and bystander active payloads in both Enhertu® and Trodelvy® and the increased release of the payload in the tumour microenvironment (TME) for Trodelvy® has rendered these higher DAR ADCs relevant to improved patient treatments through improvements in TI.
As previously noted, Enhertu® uses the same antibody trastuzumab as the previously approved ADC Kadcyla® and a direct comparison of Enhertu® versus Kadcyla® in the Phase III metastatic HER2-positive breast cancer trial DESTINY clearly demonstrated Enhertu®’s superiority over Kadcyla®. The 12-month progression free survival (PFS) with Enhertu® treatment was 75.8% compared to Kadcyla®’s 34.1%, with the inevitability that Enhertu® will replace Kadcyla® in second-line therapy (5).
Enhertu® differs from Kadcyla® in its payload mechanism of action and the improvement in clinical efficacy in low expressing or hetoerogeneous expressing HER2 tumours is driven both by the higher DAR and the stability profile of the drug.
The signposting of improved therapeutic benefits was shown preclinically with the platelet decreases associated with thrombocytopenia as the dose-limiting toxicity (DLT) of Kadcyla®, and irreversible axonal degeneration correlated with peripheral neuropathy in humans not observed in Enhertu® (6). There was also preclinical data showing Enhertu®’s good activity in Kadcyla® resistant cell lines (7).
Whilst these improvements in TI observed over current state of the art technologies with Enhertu® and Trodelvy® are game-changers, their advances offer more of a glimpse of the potential upsides and gains that could be made.
The nature of the maleimide linker employed in Enhertu® and Trodelvy® means that the maximum DAR achievable is 8, and whilst these higher DAR ADCs have demonstrated the potential for increasing the drug loading, it could signpost the next-generation ADC disruptive technologies may overcome these limitations that can further push the boundaries of high DAR ADCs. Increases in DAR to achieve greater than 8 payloads should therefore be accompanied with the concomitant decreases in linker-payload hydrophobicity, and tuned with payload bystander effects if low antigen copies are anticipated.
It may be some time until HER2 target saturation for ADCs is reached, but this doesn’t mean a slowing of trastuzumab-containing drugs. Given the expanding clinical candidate landscape and continued development of trastuzumab biobetters as well as the potential for ADC biosimilars, and HER2 ADCs will continue to rise.
Highlights of Prescribing Information
Trastuzumab (Herceptin®; Genentech/Roche)(Prescribing Information)
Ado-trastuzumab emtansine (Kadcyla®; Genentech/Roche)(Prescribing information)
Fam-trastuzumab deruxtecan-nxki (Enhertu®; Daiichi Sankyo and AstraZeneca) (Prescribing Information)
Sacituzumab govitecan-hziy (Todelvy®; Gilead/Immunomedics) (Prescribing Information)
Disitamab vedotin (Aidexi®; Remegen) (Prescribing Info China)
 Coats S, Williams M, Kebble B, Dixit R, Tseng L, Yao NS, Tice DA, Soria JC. Antibody-Drug Conjugates: Future Directions in Clinical and Translational Strategies to Improve the Therapeutic Index. Clin Cancer Res. 2019 Sep 15;25(18):5441-5448.
 Nieto C, Vega MA, Martín del Valle MA. Trastuzumab: More than a Guide in HER2-Positive Cancer Nanomedicine. Nanomaterials 2020, 10, 1674-1695.
 Nakada T, Sugihara K, Jikoh T, Abe Y, Agatsuma T. The Latest Research and Development into the Antibody-Drug Conjugate, [fam-] Trastuzumab Deruxtecan (DS-8201a), for HER2 Cancer Therapy. Chem. Pharm. Bull. 2019, 67(3):173-185.
 Sukumaran S, Zhang C, Leipold DD, Saad OM, Xu K, Gadkar K, et al. Development and Translational Application of an Integrated, Mechanistic Model of Antibody-Drug Conjugate Pharmacokinetics. AAPS J. 2017;19(1):130–40.
 GlobalData Healthcare. ESMO 2021: positive data supports Enhertu use over Kadcyla in breast cancer treatment [Internet]. Clinical Trials Arena. 2021. Available from: https://www.clinicaltrialsarena.com/comment/esmo-2021-enhertu-breast-cancer-treatment/
 Ogitani Y, Aida T, Hagihara K, Yamaguchi J, Ishii C, Harada N, et al. DS-8201a, A Novel HER2-Targeting ADC with a Novel DNA Topoisomerase I Inhibitor, Demonstrates a Promising Antitumor Efficacy with Differentiation from T-DM1. Clin Cancer Res. 2016 Oct 15;22(20):5097–108.
 Takegawa N, Nonagase Y, Yonesaka K, Sakai K, Maenishi O, Ogitani Y, et al. DS-8201a, a new HER2-targeting antibody–drug conjugate incorporating a novel DNA topoisomerase I inhibitor, overcomes HER2-positive gastric cancer T-DM1 resistance. Int J Cancer. 2017 Oct 15;141(8):1682–9.