A Case Study Comparing Heterogeneous Lysine- and Site-Specific Cysteine-Conjugated Maytansinoid Antibody-Drug Conjugates (ADCs) Illustrates the Benefits of Lysine Conjugation

Antibody-drug conjugates in gastric cancer: from molecular landscape to clinical strategies

Antibody-drug conjugates (ADCs) represent a crucial component of targeted therapies in gastric cancer, potentially altering traditional treatment paradigms. Many ADCs have entered rigorous clinical trials based on biological theories and preclinical experiments. Modality trials have also been conducted in combination with monoclonal antibody therapies, chemotherapies, immunotherapies, and other treatments to enhance the efficacy of drug coordination effects. However, ADCs exhibit limitations in treating gastric cancer, including resistance triggered by their structure or other factors. Ongoing intensive researches and preclinical experiments are yielding improvements, while enhancements in drug development processes and concomitant diagnostics during the therapeutic period actively boost ADC efficacy. The optimal treatment strategy for gastric cancer patients is continually evolving. This review summarizes the clinical progress of ADCs in treating gastric cancer, analyzes the mechanisms of ADC combination therapies, discusses resistance patterns, and offers a promising outlook for future applications in ADC drug development and companion diagnostics.

Progress in the study of antibody-drug conjugates for the treatment of cervical cancer

Cervical cancer is the second most prevalent malignancy affecting women's health globally, and the number of morbidity and mortality from cervical cancer continues to rise worldwide. The 5-year survival rate of patients with recurrent or metastatic cervical cancer is significantly reduced, and existing treatment modalities have low efficacy and high adverse effects, so there is a strong need for new, effective, and well-tolerated therapies. Antibody-drug conjugates (ADCs) are a new targeted therapeutic modality that can efficiently kill tumor cells. This review aims to summarize the composition, research, and development history and mechanism of action of ADCs, to review the research progress of ADCs in the treatment of cervical cancer, and to summarize and prospect the application of ADCs.

Antibody-Drug Conjugate Overview: a State-of-the-art Manufacturing Process and Control Strategy

Antibody-drug conjugates (ADCs) comprise an antibody, linker, and drug, which direct their highly potent small molecule drugs to target tumor cells via specific binding between the antibody and surface antigens. The antibody, linker, and drug should be properly designed or selected to achieve the desired efficacy while minimizing off-target toxicity. With a unique and complex structure, there is inherent heterogeneity introduced by product-related variations and the manufacturing process. Here this review primarily covers recent key advances in ADC history, clinical development status, molecule design, manufacturing processes, and quality control. The manufacturing process, especially the conjugation process, should be carefully developed, characterized, validated, and controlled throughout its lifecycle. Quality control is another key element to ensure product quality and patient safety. A patient-centric strategy has been well recognized and adopted by the pharmaceutical industry for therapeutic proteins, and has been successfully implemented for ADCs as well, to ensure that ADC products maintain their quality until the end of their shelf life. Deep product understanding and process knowledge defines attribute testing strategies (ATS). Quality by design (QbD) is a powerful approach for process and product development, and for defining an overall control strategy. Finally, we summarize the current challenges on ADC development and provide some perspectives that may help to give related directions and trigger more cross-functional research to surmount those challenges.

Site-selective template-directed synthesis of antibody Fc conjugates with concomitant ligand release

Template-directed methods are emerging as some of the most effective means to conjugate payloads at selective sites of monoclonal antibodies (mAbs). We have previously reported a method based on an engineered Fc-III reactive peptide to conjugate a radionuclide chelator to K317 of antibodies with the concomitant release of the Fc-III peptide ligand. Here, our method was redesigned to target two lysines proximal to the Fc-III binding site, K248 and K439. Using energy minimization predictions and a semi-combinatorial synthesis approach, we sampled multiple Fc-III amino acid substituents of A3, H5, L6 and E8, which were then converted into Fc-III reactive conjugates. Middle-down MS/MS subunit analysis of the resulting trastuzumab conjugates revealed that K248 and K439 can be selectively targeted using the Fc-III reactive variants L6Dap, L6Orn, L6Y and A3K or A3hK, respectively. Across all variants tested, L6Orn-carbonate appeared to be the best candidate, yielding a degree and yield of conjugation of almost 2 and 100% for a broad array of payloads including radionuclide chelators, fluorescent dyes, click-chemistry reagents, pre-targeted imaging reagents, and some cytotoxic small molecules. Furthermore, L6Orn carbonate appeared to yield similar conjugation results across multiple IgG subtypes. In vivo proof of concept was achieved by conjugation of NODAGA to the PD1/PD-L1 immune checkpoint inhibitor antibody atezolizumab, followed by PET imaging of PD-L1 expression in mice bearing PD-L1 expressing tumor xenograft using radiolabeled [64Cu]Cu-atezolizumab.

Highly efficient site-specific protein modification using tyrosinase from Streptomyces avermitilis: Structural insight

Tyrosinase-mediated protein conjugation has recently drawn attention as a site-specific protein modification tool under mild conditions. However, the tyrosinases reported to date act only on extremely exposed tyrosine residues, which limits where the target tyrosine can be located. Herein, we report a tyrosinase from Streptomyces avermitilis (SaTYR), that exhibits a much higher activity against tyrosine residues on the protein surface than other tyrosinases. We determined the crystal structure of SaTYR and revealed that the enzyme has a relatively flat and shallow substrate-binding pocket to accommodate a protein substrate. We demonstrated SaTYR-mediated fluorescence dye tagging and PEGylation of a surface tyrosine residue that was unreacted by other tyrosinases with an approximately 95.2 % conjugation yield in 1 h. We also present a structural rationale that considers the steric hindrance from adjacent residues and surrounding structures along with the extent of solvent exposure of residues, as necessary when determining the optimal positions for introducing target tyrosine residues in SaTYR-mediated protein modification. The study demonstrated that the novel tyrosinase, SaTYR, extends the scope of tyrosinase-mediated protein modification, and we propose that site-specific tyrosine conjugation using SaTYR is a promising strategy for protein bioconjugation in various applications.

Predictive Nature of High-Throughput Assays in ADC Formulation Screening

Utilization of high-throughput biophysical screening techniques during early screening studies is warranted due to the limited amount of material and large number of samples. But the predictability of the data to longer-term storage stability is critical as the high-throughput methods assist in defining the design space for the longer-term studies. In this study, the biophysical properties of two ADCs in 16 formulation conditions were evaluated using high-throughput techniques. Conformational stability and colloidal stability were evaluated by determining Tm values, kD, B22, and Tagg. In addition, the samples were placed on stability and the extent of aggregate formation over the 8-week interval was determined. The rank order of the 16 different formulations in the high-throughput assays was compared to the rank order observed during the stability studies to assess the predictive capabilities of the screening methods. It was demonstrated that similar rank orders can be expected between high-throughput physical stability indicating assays such as Tagg and B22 and traditional aggregation by SEC data, whereas conformational stability read-outs (Tm) are less predictive. In addition, the high-throughput assays appropriately identified the poor performing formulation conditions, which is ultimately what is desired of screening assays.

Added Value of Internal Fragments for Top-Down Mass Spectrometry of Intact Monoclonal Antibodies and Antibody-Drug Conjugates

Monoclonal antibodies (mAbs) and antibody-drug conjugates (ADCs) are two of the most important therapeutic drug classes that require extensive characterization, whereas their large size and structural complexity make them challenging to characterize and demand the use of advanced analytical methods. Top-down mass spectrometry (TD-MS) is an emerging technique that minimizes sample preparation and preserves endogenous post-translational modifications (PTMs); however, TD-MS of large proteins suffers from low fragmentation efficiency, limiting the sequence and structure information that can be obtained. Here, we show that including the assignment of internal fragments in native TD-MS of an intact mAb and an ADC can improve their molecular characterization. For the NIST mAb, internal fragments can access the sequence region constrained by disulfide bonds to increase the TD-MS sequence coverage to over 75%. Important PTM information, including intrachain disulfide connectivity and N-glycosylation sites, can be revealed after including internal fragments. For a heterogeneous lysine-linked ADC, we show that assigning internal fragments improves the identification of drug conjugation sites to achieve a coverage of 58% of all putative conjugation sites. This proof-of-principle study demonstrates the potential value of including internal fragments in native TD-MS of intact mAbs and ADCs, and this analytical strategy can be extended to bottom-up and middle-down MS approaches to achieve even more comprehensive characterization of important therapeutic molecules.

Mechanisms of Resistance to Antibody-Drug Conjugates

The treatment of cancer patients has dramatically changed over the past decades with the advent of monoclonal antibodies, immune-checkpoint inhibitors, bispecific antibodies, and innovative T-cell therapy. Antibody-drug conjugates (ADCs) have also revolutionized the treatment of cancer. Several ADCs have already been approved in hematology and clinical oncology, such as trastuzumab emtansine (T-DM1), trastuzumab deruxtecan (T-DXd), and sacituzumab govitecan (SG) for the treatment of metastatic breast cancer, and enfortumab vedotin (EV) for the treatment of urothelial carcinoma. The efficacy of ADCs is limited by the emergence of resistance due to different mechanisms, such as antigen-related resistance, failure of internalization, impaired lysosomal function, and other mechanisms. In this review, we summarize the clinical data that contributed to the approval of T-DM1, T-DXd, SG, and EV. We also discuss the different mechanisms of resistance to ADCs, as well as the ways to overcome this resistance, such as bispecific ADCs and the combination of ADCs with immune-checkpoint inhibitors or tyrosine-kinase inhibitors.

Antibody drug conjugates, targeting cancer-expressed EGFR, exhibit potent and specific antitumor activity

The monoclonal antibody '40H3' binds to EGFRvIII and to full-length EGFR when it is overexpressed on cancer cells. To generate candidate cytotoxic antibody-drug conjugates (ADCs), 40H3 was modified by the addition of small molecular weight payloads that included two tubulin-modifying agents, two topoisomerase inhibitors and a pyrrolobenzodiazepine (PBD) dimer. Conjugates retained antigen binding activity comparable to the unmodified 40H3 antibody. The cytotoxicity of five distinct ADCs was evaluated on a variety of EGFR-expressing cells including three triple negative breast cancer (TNBC) lines. Generally, the 40H3 conjugate with the PBD dimer (40H3-Tesirine) was the most active killing agent. The killing of EGFR-positive cells by 40H3-Tesirine correlated with the number of surface binding sites for 40H3. However, bystander killing was also evident in experiments with antigen-negative cells. In vivo tumor xenograft experiments were conducted on two TNBC tumor lines. Three treatments with the 40H3-Tesirine ADC at 1 mg/kg were sufficient to achieve complete remissions without evidence of mouse toxicity. Data support the development of ADCs derived from the 40H3 antibody for the treatment of cancers that express EGFRvIII or high levels of EGFR.

Synthesis and Evaluation of Small Molecule Drug Conjugates Harnessing Thioester-Linked Maytansinoids

Ligand-targeting drug conjugates are a class of clinically validated biopharmaceutical drugs constructed by conjugating cytotoxic drugs with specific disease antigen targeting ligands through appropriate linkers. The integrated linker-drug motif embedded within such a system can prevent the premature release during systemic circulation, thereby allowing the targeting ligand to engage with the disease antigen and selective accumulation. We have designed and synthesized new thioester-linked maytansinoid conjugates. By performing in vitro cytotoxicity, targeting ligand binding assay, and in vivo pharmacokinetic studies, we investigated the utility of this new linker-drug moiety in the small molecule drug conjugate (SMDC) system. In particular, we conjugated the thioester-linked maytansinoids to the phosphatidylserine-targeting small molecule zinc dipicolylamine and showed that Zn8_DM1 induced tumor regression in the HCC1806 triple-negative breast cancer xenograft model. Moreover, in a spontaneous sorafenib-resistant liver cancer model, Zn8_DM1 exhibited potent antitumor growth efficacy. From quantitative mRNA analysis of Zn8_DM1 treated-tumor tissues, we observed the elevation of gene expressions associated with a “hot inflamed tumor” state. With the identification and validation of a plethora of cancer-associated antigens in the “omics” era, this work provided the insight that antibody- or small molecule-based targeting ligands can be conjugated similarly to generate new ligand-targeting drug conjugates.

Recent developments in chemical conjugation strategies targeting native amino acids in proteins and their applications in antibody-drug conjugates

Many fields in chemical biology and synthetic biology require effective bioconjugation methods to achieve their desired functions and activities. Among such biomolecule conjugates, antibody-drug conjugates (ADCs) need a linker that provides a stable linkage between cytotoxic drugs and antibodies, whilst conjugating in a biologically benign, fast and selective fashion. This review focuses on how the development of novel organic synthesis can solve the problems of traditional linker technology. The review shall introduce and analyse the current developments in the modification of native amino acids on peptides or proteins and their applicability to ADC linker. Thereafter, the review shall discuss in detail each endogenous amino acid's intrinsic reactivity and selectivity aspects, and address the research effort to construct an ADC using each conjugation method.

Antibody drug conjugates in gastrointestinal cancer: From lab to clinical development

The antibody-drug conjugates (ADCs) are one the fastest growing biotherapeutics in oncology and are still in their infancy in gastrointestinal (GI) cancer for clinical applications to improve patient survival. The ADC based approach is developed with tumor specific antigen, antibody carrying cytotoxic agents to precisely target and deliver chemotherapeutics at the tumor site. To date, 11 ADCs have been approved by US-FDA, and more than 80 are in the clinical development phase for different oncological indications. However, The ADCs based therapies in GI cancers are still far from having high-efficient clinical outcomes. The limited success of these ADCs and lessons learned from the past are now being used to develop a newer generation of ADC against GI cancers. In this review, we did a comprehensive assessment of the key components of ADCs, including tumor marker, antibody, cytotoxic payload, and linkage strategy, with a focus on technical improvement and some future trends in the pipeline for clinical translation. The various preclinical and clinical ADCs used in gastrointestinal malignancies, their target, composition and bioconjugation, along with preclinical and clinical outcomes, are discussed. The emphasis is also given to new generation ADCs employing novel mAb, payload, linker, and bioconjugation methods are also included.

Site-selective lysine conjugation methods and applications towards antibody-drug conjugates

Site-selective protein modification is of significant interest in chemical biology research, with lysine residues representing a particularly challenging target. Whilst lysines are popular for bioconjugation, due to their nucleophilicity, solvent accessibility and the stability of the resultant conjugates, their high abundance means site-selectivity is very difficult to achieve. Antibody-drug conjugates (ADCs) present a powerful therapeutic application of protein modification, and have often relied extensively upon lysine bioconjugation for their synthesis. Here we discuss advances in methodologies for achieving site-selective lysine modification, particularly within the context of antibody conjugate construction, including the cysteine-to-lysine transfer (CLT) protocol which we have recently reported.

Key metrics to expanding the pipeline of successful antibody-drug conjugates

Although the recent FDA approval of six new antibody-drug conjugates (ADCs) is promising, attrition of ADCs during clinical development remains high. The inherent complexity of ADCs is a double-edged sword that provides opportunities for perfecting therapeutic action while also increasing confounding factors in therapeutic failures. ADC design drives their pharmacokinetics and pharmacodynamics, and requires deeper analysis than the commonly used C_max and area under the curve (AUC) metrics to scale dosing to the clinic. Common features of current FDA-approved ADCs targeting solid tumors include humanized IgG1 antibody domains, highly expressed tumor receptors, and large antibody doses. The potential consequences of these shared features for clinical pharmacokinetics and mechanism of action are discussed, and key design aspects for successful solid tumor ADCs are highlighted.

Importance and Considerations of Antibody Engineering in Antibody-Drug Conjugates Development from a Clinical Pharmacologist's Perspective

Antibody-drug conjugates (ADCs) appear to be in a developmental boom, with five FDA approvals in the last two years and a projected market value of over $4 billion by 2024. Major advancements in the engineering of these novel cytotoxic drug carriers have provided a few early success stories. Although the use of these immunoconjugate agents are still in their infancy, valuable lessons in the engineering of these agents have been learned from both preclinical and clinical failures. It is essential to appreciate how the various mechanisms used to engineer changes in ADCs can alter the complex pharmacology of these agents and allow the ADCs to navigate the modern-day therapeutic challenges within oncology. This review provides a global overview of ADC characteristics which can be engineered to alter the interaction with the immune system, pharmacokinetic and pharmacodynamic profiles, and therapeutic index of ADCs. In addition, this review will highlight some of the engineering approaches being explored in the creation of the next generation of ADCs.

Antibody-Drug Conjugates-A Tutorial Review

Antibody-drug conjugates (ADCs) are a family of targeted therapeutic agents for the treatment of cancer. ADC development is a rapidly expanding field of research, with over 80 ADCs currently in clinical development and eleven ADCs (nine containing small-molecule payloads and two with biological toxins) approved for use by the FDA. Compared to traditional small-molecule approaches, ADCs offer enhanced targeting of cancer cells along with reduced toxic side effects, making them an attractive prospect in the field of oncology. To this end, this tutorial review aims to serve as a reference material for ADCs and give readers a comprehensive understanding of ADCs; it explores and explains each ADC component (monoclonal antibody, linker moiety and cytotoxic payload) individually, highlights several EMA- and FDA-approved ADCs by way of case studies and offers a brief future perspective on the field of ADC research.

An Efficient Conjugation Approach for Coupling Drugs to Native Antibodies via the PtII Linker Lx for Improved Manufacturability of Antibody-Drug Conjugates

The PtII linker [ethylenediamineplatinum(II)]2+ , coined Lx, has emerged as a novel non-conventional approach to antibody-drug conjugates (ADCs) and has shown its potential in preclinical in vitro and in vivo benchmark studies. A crucial improvement of the Lx conjugation reaction from initially <15 % to ca. 75-90 % conjugation efficiency is described, resulting from a systematic screening of all relevant reaction parameters. NaI, a strikingly simple inorganic salt additive, greatly improves the conjugation efficiency as well as the conjugation selectivity simply by exchanging the leaving chloride ligand on Cl-Lx-drug complexes (which are direct precursors for Lx-ADCs) for iodide, thus generating I-Lx-drug complexes as more reactive species. Using this iodide effect, we developed a general and highly practical conjugation procedure that is scalable: our lead Lx-ADC was produced on a 5 g scale with an outstanding conjugation efficiency of 89 %.

Antibody Co-Administration Can Improve Systemic and Local Distribution of Antibody-Drug Conjugates to Increase In Vivo Efficacy

Several antibody-drug conjugates (ADC) showing strong clinical responses in solid tumors target high expression antigens (HER2, TROP2, Nectin-4, and folate receptor alpha/FRα). Highly expressed tumor antigens often have significant low-level expression in normal tissues, resulting in the potential for target-mediated drug disposition (TMDD) and increased clearance. However, ADCs often do not cross-react with normal tissue in animal models used to test efficacy (typically mice), and the impact of ADC binding to normal tissue antigens on tumor response remains unclear. An antibody that cross-reacts with human and murine FRα was generated and tested in an animal model where the antibody/ADC bind both human tumor FRα and mouse FRα in normal tissue. Previous work has demonstrated that a "carrier" dose of unconjugated antibody can improve the tumor penetration of ADCs with high expression target-antigens. A carrier dose was employed to study the impact on cross-reactive ADC clearance, distribution, and efficacy. Co-administration of unconjugated anti-FRα antibody with the ADC-improved efficacy, even in low expression models where co-administration normally lowers efficacy. By reducing target-antigen-mediated clearance in normal tissue, the co-administered antibody increased systemic exposure, improved tumor tissue penetration, reduced target-antigen-mediated uptake in normal tissue, and increased ADC efficacy. However, payload potency and tumor antigen saturation are also critical to efficacy, as shown with reduced efficacy using too high of a carrier dose. The judicious use of higher antibody doses, either through lower DAR or carrier doses, can improve the therapeutic window by increasing efficacy while lowering target-mediated toxicity in normal tissue.

Antibody-Drug Conjugates: The New Frontier of Chemotherapy

In recent years, antibody-drug conjugates (ADCs) have become promising antitumor agents to be used as one of the tools in personalized cancer medicine. ADCs are comprised of a drug with cytotoxic activity cross-linked to a monoclonal antibody, targeting antigens expressed at higher levels on tumor cells than on normal cells. By providing a selective targeting mechanism for cytotoxic drugs, ADCs improve the therapeutic index in clinical practice. In this review, the chemistry of ADC linker conjugation together with strategies adopted to improve antibody tolerability (by reducing antigenicity) are examined, with particular attention to ADCs approved by the regulatory agencies (the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA)) for treating cancer patients. Recent developments in engineering Immunoglobulin (Ig) genes and antibody humanization have greatly reduced some of the problems of the first generation of ADCs, beset by problems, such as random coupling of the payload and immunogenicity of the antibody. ADC development and clinical use is a fast, evolving area, and will likely prove an important modality for the treatment of cancer in the near future.

Site-Specific 89Zr- and 111In-Radiolabeling and In Vivo Evaluation of Glycan-free Antibodies by Azide-Alkyne Cycloaddition with a Non-natural Amino Acid

Antibody-drug conjugates (ADCs) are a class of targeted therapeutics consisting of a monoclonal antibody coupled to a cytotoxic payload. Various bioconjugation methods for producing site-specific ADCs have been reported recently, in efforts to improve immunoreactivity and pharmacokinetics and minimize batch variance-potential issues associated with first-generation ADCs prepared via stochastic peptide coupling of lysines or reduced cysteines. Recently, cell-free protein synthesis of antibodies incorporating para-azidomethyl phenylalanine (pAMF) at specific locations within the protein sequence has emerged as a means to generate antibody-drug conjugates with strictly defined drug-antibody-ratio, leading to ADCs with markedly improved stability, activity, and specificity. The incorporation of pAMF enables the conjugation of payloads functionalized for strain-promoted azide-alkyne cycloaddition. Here, we introduce two dibenzylcyclooctyne-functionalized bifunctional chelators that enable the incorporation of radioisotopes for positron emission tomography with ⁸⁹Zr (t_1/2 = 78.4 h, β⁺ = 395 keV (22%), γ = 897 keV) or single photon emission computed tomography with ¹¹¹In (t_1/2 = 67.3 h, γ = 171 keV (91%), 245 keV (94%)) under physiologically compatible conditions. We show that the corresponding radiolabeled conjugates with site-specifically functionalized antibodies targeting HER2 are amenable to targeted molecular imaging of HER2+ expressing tumor xenografts in mice and exhibit a favorable biodistribution profile in comparison with conventional, glycosylated antibody conjugates generated by stochastic bioconjugation.

Effect of Linker-Drug Properties and Conjugation Site on the Physical Stability of ADCs

The physical stability of antibody drug conjugates is dictated by the properties of the antibody, linker-drug, and conjugation site. Two linker-drugs were chosen that are different in terms of hydrophobicity and polar surface area to evaluate the effect of linker-drug properties on antibody-drug conjugate (ADC) behavior. Site-specific and non-site-specific conjugation was used to investigate the role of conjugation site in conformational and colloidal stability. Finally, 2 antibodies were selected to determine if the observed results were antibody-specific. The conformational stability is affected, with the highest degree of destabilization observed when conjugation results in the removal of interchain disulfide bonds. Although conformational destabilization occurred in the domain in which conjugation occurred and domains distinct from the conjugation site, no correlation could be drawn between linker-drug properties and conformational stability. Evaluation of aggregation by size exclusion HPLC confirmed a relationship between linker-drug hydrophobicity and aggregation propensity under thermal stress in all ADCs tested. The extent of aggregation was far greater in the conjugates generated with a more hydrophobic antibody, illustrating that the properties of both the antibody and linker-drug contribute to aggregation. These studies emphasize that the distinct properties of the molecule as a whole warrant a case-by-case evaluation of each ADC.

Automated linkage of proteins and payloads producing monodisperse conjugates

Controlled protein functionalization holds great promise for a wide variety of applications. However, despite intensive research, the stoichiometry of the functionalization reaction remains difficult to control due to the inherent stochasticity of the conjugation process. Classical approaches that exploit peculiar structural features of specific protein substrates, or introduce reactive handles via mutagenesis, are by essence limited in scope or require substantial protein reengineering. We herein present equimolar native chemical tagging (ENACT), which precisely controls the stoichiometry of inherently random conjugation reactions by combining iterative low-conversion chemical modification, process automation, and bioorthogonal trans-tagging. We discuss the broad applicability of this conjugation process to a variety of protein substrates and payloads.

Controlled protein functionalization holds great promise for a wide variety of applications.

Site-Specific Conjugation of the Indolinobenzodiazepine DGN549 to Antibodies Affords Antibody-Drug Conjugates with an Improved Therapeutic Index as Compared with Lysine Conjugation

Antibody-drug conjugates have elicited great interest recently as targeted chemotherapies for cancer. Recent preclinical and clinical data have continued to raise questions about optimizing the design of these complex therapeutics. Biochemical methods for site-specific antibody conjugation have been a design feature of recent clinical ADCs, and preclinical reports suggest that site-specifically conjugated ADCs generically offer improved therapeutic indices (i.e., the fold difference between efficacious and maximum tolerated doses). Here we present the results of a systematic preclinical comparison of ADCs embodying the DNA-alkylating linker-payload DGN549 generated with both heterogeneous lysine-directed and site-specific cysteine-directed conjugation chemistries. Importantly, the catabolites generated by each ADC are the same regardless of the conjugation format. In two different model systems evaluated, the site-specific ADC showed a therapeutic index benefit. However, the therapeutic index benefit is different in each case: both show evidence of improved tolerability, though with different magnitudes, and in one case significant efficacy improvement is also observed. These results support our contention that conjugation chemistry of ADCs is best evaluated in the context of a particular antibody, target, and linker-payload, and ideally across multiple disease models.