Recent Advances in ADCs

Once the payload is released from the lysosome, the type of payload used will determine which cell death program is triggered. Figure 4 (A-H) highlights mechanisms of different payload types. Figure 4 – (A) DNA inhibitors; (B) Splicing inhibitors; (C) RNA polymerase inhibitor; (D) Protein Degrader; (E) Microtubule inhibitor; (F) Bcl-xL inhibitor; (G) Topoisomerase inhibitor; (H) NAMPT inhibitor

Strategies for Enhanced Efficacy and Safety in ADCs

Over the past few years, extensive research has focused on the five critical elements of ADC design.⁷ Numerous failures and successes in preclinical and clinical programs have led to the development of innovative strategies aimed at improving and advancing the chemistry of these elements to create next-generation ADCs.

Target Antigen

Recent advancements in ADC research are focused on identifying cell surface antigens that are either overexpressed or uniquely expressed on cancer cells. This approach aims to enhance treatment specificity and minimize toxicity concerns. Additionally, researchers are exploring targeting components of the tumor microenvironment, such as stromal cells or vasculature, to disrupt tumor growth and metastasis. Novel antibody formats enable simultaneous targeting of multiple antigens, enhancing the efficacy of ADCs.⁷ Tumor-associated glycoproteins, such as mucins²³ and glycosylated proteins,²⁴ are also gaining attention due to their specific expression patterns in tumors. Next-generation ADCs are being developed to target a wide array of cell-membrane receptors and proteins. The most explored target antigens are highlighted in Table 2.

Targeting moiety

Antibodies used are exclusively of the IgG class (subclass IgG1, IgG2, IgG3, and IgG4) which have a unique profile with respect to the length of hinge region, the number of inter-chain disulfide bonds, and Fc-effector functions.

Table 3. Comparison chart of IgG subtypes ^3,26,27

Hematologic indications account for 6 out of 15 approved ADCs, while 6 of the remaining 9 ADCs targeting solid tumors work on treating breast cancer. However, inadequate penetration of tumor cells remains a concern limiting the drug’s efficacy for solid tumors. Research on alternative delivery systems like smaller targeting moieties can address the issue of poor penetration associated with large ADCs. Novel small-format drug conjugates are emerging at the preclinical stage, ranging from antibody-fragment drug conjugates (e.g., Fab, diabody, and scFv) to small protein scaffold-drug conjugates (e.g., Affibody, etc.) and are summarized in Table 4

Table 4. Small-format Drug Conjugates ^28,29,30

Small -format drug conjugate	Mass	Structure	Advantage(s)	Drawback(s)
	150 kDa	Disulfide linked, two heavy + two light chains	Longer half-life and better drug accumulation at tumor site Ability to carry a higher load of drugs molecules Triggers immune-mediated effector functions (ADCC, CDC)	Drug resistance, off-target toxicity, low penetration
	~ 75-80 kDa	Two fragments linked at hinge region	Better penetration than mAbs due to smaller size	Shorter half-life
	~50 kDa	Disulfide linked, one heavy + one light chain	Block targets (receptors & signaling pathways) without cross-linking	Not very potent in vivo; modest reduction in tumor growth
	~50 kDa	Two Fv domains connected by peptide linker	Rapid tumor penetration and accumulation due to smaller size	Higher dose needed to match potency of ADC
	~25-27 kDa	Variable region of heavy and light chain joined by peptide linker	Successful for applications where time critical elimination was necessary	Low delivery rate
	12.5 - 25 kDa	Lacks a light chain and heavy chain CH1 domain	Highly soluble and more stable than conventional antibodies	Slower and lesser internalization
	10-25 kDa	Monomeric proteins with stable tertiary structures	Small size, high affinity, excellent specificity and stability	Requires use of protein engineering to ensure continuous exposure after dosing
	~ 3.5-5 kDa	Protein motif containing 3 disulfide bridges (formed from pairs of cysteine residues)	3 disulfide bonds provide remarkable stability against chemical denaturation, and proteolysis	Less potent than free payload
	<5 kDa	Short chain of amino acids	Enhanced binding, diversity & capability to cross the cell membrane	Poor intrinsic pharmacokinetic properties of peptide; shorter half-life
	~ 1.5 - 2 kDa	Intramolecular disulfide bond & bicyclization of the peptides	High affinity binding and specificity to protein target, greater conformational rigidity and metabolic stability	Poor physicochemical properties

Linker Chemistry and Synthesis

More than 80% of the clinically approved ADCs (13 out of 15) and >50% of those in clinical development employ cleavable linkers.³ Cleavable linkers come in a variety of motifs and release mechanisms, but all have a bond that is custom designed to be broken at either a lower pH, with an enzyme or in the presence of thiols. The cleavable linkers will release a metabolite that can have membrane permeability. Membrane permeability usually will mean that the cytotoxin will have bystander activity, which is the ability to diffuse in neighboring cells and have an effect.

On the other hand, a non-cleavable linker is a linker that does not contain any biologically or chemically labile bond and an active catabolite is released by the complete degradation of the antibody. The released catabolite will contain the amino acid from the antibody to which the linker-payload was conjugated. Catabolites from non-cleavable linkers do not cross membranes passively and thus do not distribute within a tissue. The two most common non-cleavable linkers that are used are maleimidocaproyl (Mc) and succinimidyl trans-4-(maleimidylmethyl)cyclohexane-1-carboxylate (SMCC). In general, a non-cleavable linker is better tolerated but less efficacious.³¹

The most popular linker is the protease cleavable linker that contains a valine-citrulline-para-aminobenzyl-carbamate moiety (vc-PABC).³² This is a traceless linker that allows the release of amine containing cytotoxins. Many di-, tri-, and tetra-peptide sequences are cleaved by proteases and changing the sequence can facilitate the synthesis of the linker-payload and improve the properties of the ADC construct. There are also traceless cleavable linkers that get cleaved by glucuronidases that offer higher water solubility.³³ The advantages of cleavable linkers are that they have good plasma stability and robust activity in a variety of cell lines and preclinical models.³⁴

Another means of releasing cytotoxin is to use the acidic environment found in the tumor microenvironment and in the lysosome. Hydrazones and carbonates are two commonly used motifs for pH-sensitive linkers. These acid labile linkers shed cytotoxins in circulation but are still powerful linker motifs.¹¹ Finally, the last type of cleavable linker discussed will be the one containing reducible bonds. These are usually identified by their disulfide bond which breaks in half in the presence of cysteine or glutathione. The advantages of disulfide linkers are that the kinetics of release can be controlled by steric bulk.³⁵

Non-specific payload release is a common problem encountered with the usage of existing linkers. The resulting off-target toxicity becomes a prime cause for limiting the therapeutic window.¹⁷ ADC stability and safety is primarily affected by factors such as the linker length, hydrophilicity, site of conjugation, conjugation method and steric hindrance around the site of the linker.³⁶ While designing an ADC it could be considered that if, for example, the tumor has consistent expression throughout the tissue then one could consider non-cleavable linkers to lower toxicity. If there is heterogeneous expression, then a cleavable linker generating a membrane permeable catabolite (with bystander activity) would be advantageous. But if there is low expression, then a potent cytotoxin will be required. With this information in hand, one can start testing the linker-payload that has the best chance of success and begin refining the ADC from the data generated.

Conjugation Technologies

DAR is a critical quality parameter which influences the overall efficacy, pharmacokinetics, and safety profile of an ADC molecule. This ratio is typically determined by the conjugation method employed during the development stage. The DAR is measured by a variety of analytical techniques, which should give similar results but almost never the same number.³⁷ Hence the usage of two different methods to determine the DAR is highly recommended. As the field evolves, finding orthogonal methods for DAR determination is becoming more routine.³⁸ The distribution of stochastic conjugations is usually referred to as heterogeneous, that is a mixture of populations (DAR 0 – 8) is obtained to give an average DAR (e.g., DAR 4), or homogeneous when the population is mostly one species (e.g., mostly DAR 2).³⁹

IgG molecules generally possess multiple intrinsic sites that can be engineered by introducing electrophilic handles to create reactive sites for bioconjugation. . Non-specific or stochastic conjugation methods utilize natural nucleophilic amino acids found on the antibody, particularly lysine and cysteine residues and do not require antibody engineering.

There are 80 lysine residues in an IgG molecule, among which 20 are located in highly solvent-accessible positions.⁴⁰ Lysine-based conjugations are carried out by mixing the mAb with an activated ester. This generally results in a DAR distribution between 0 and 9 with an average DAR of 3.5. In contrast, cysteine residues are less abundant but are uniformly distributed throughout the antibody structure thus reducing heterogeneity as compared to lysine conjugation. There are 16 pairs of cysteines, which comprise of 12 intrachain disulfide bonds and 4 interchain bonds. Cysteine conjugation generally involves partial reduction of these 4 interchain bonds to form reactive cysteine thiol groups. These thiols are then available to react with thiol-specific maleimide linkers. The DAR is typically controlled by the amount of reducing agent used at the reduction stage. The standard conjugation using cysteine aims for a DAR around 4 with a DAR distribution between 0-8 for IgG1 antibodies. Currently, most ADCs employ cysteine conjugation for the attachment of the linker-payloads.^39,41

This stochastic approach often leads to the generation of a heterogeneous population of different species with a widely distributed DAR. A low DAR could present significant challenges like reduced efficacy, higher dosage requirements and suboptimal tumor penetration. On the other hand, an ADC with a higher DAR is more susceptible to aggregation, rapid systemic clearance and toxicity concerns.⁴² Since each of these generated subpopulations have different PK and efficacy profiles it could be hard to predict the effect of all the different species during clinical trials. A small population of such species could be generating most of the toxicity observed, due to extensive antibody modification, which may also cause structural changes affecting the biological function.⁴³

To address these challenges, several site-specific conjugation technologies have been developed. These strategies can create a more homogeneous ADC population with improved plasma stability, heightened tumor uptake, and enhanced binding efficiency.⁴³ The case of homogeneous ADCs into the clinic involved the modification of each interchain disulfide bonds (DAR8), enabling precise and site-specific conjugation, as demonstrated by trastuzumab deruxtecan (Enhertu®). This advancement allowed for more consistent DARs and improved therapeutic outcomes. In the past few years, numerous site-specific conjugation technologies have been developed. Table 5 provides an overview of commonly employed strategies in developing ADC constructs with improved homogeneity and therapeutic index.

Table 5. List of ADC Conjugation Technologies

Novel Payload Strategies

To mitigate the issues of toxicity and reduced half-life it would be beneficial if we could keep the DAR below 4 while maintaining ADC efficacy. For this reason, selecting an appropriate payload for a specific disease indication and using it at the optimal concentration based on its IC50 value is crucial. Cytotoxic compounds have different permeabilities, hydrophobicities, potencies and mechanisms of action. Payloads have diversified over years of research and novel strategies like topoisomerase II inhibitors, transcription inhibitors, protein synthesis inhibitors, PROTACs, and immunomodulators have been developed.A diverse array of payloads including small molecules, protein toxins, radionuclides, cytolytic immunomodulatory proteins, biologically active peptides and enzymes are under preclinical and clinical investigation.²⁰ Table 6 summarizes both payloads that are in use or under investigation in ADC development.

Table 6: Conventional and novel ADC Payloads³

Novel ADCs

i.            Immunostimulatory Antibody Conjugates (ISACs)⁴⁶ :

These conjugates are basically engineered mAbs linked with payloads that act as pattern recognition receptor (PRR) agonists. They work by enhancing the immune responses within the tumor microenvironment (TME) and further stimulate the adaptive immune system. Through the induction of pro-inflammatory cytokines and chemokines, they appear to be promising as monotherapies or in combination therapies and are currently in preclinical stages and early-phase clinical trials. .

ii.            Degrader Antibody Conjugates (DACs):

Degrader-Antibody Conjugates (DACs) combine the catalytic activity of proteolysis targeting chimera (PROTAC) payloads with the specificity of antibodies,⁴⁷ while overcoming the limitations of both ADCs and TPDs. DACs can recognize specific antigens and deliver degrader molecules to target tumors and tissues. These molecules have the potential to reduce toxicity associated with cytotoxic payloads while increasing the specificity of traditional degraders by linking them to highly specific mAbs.⁴⁸

Several DACs are currently being studied and have demonstrated superior in vitro and in vivo activity against target tumor cells. Using the BT474 xenograft model, the effect of ORM-5029 has reported superior single-dose activity as compared to Kadcyla® and similar to that of Enhertu®. ORM-5029 and ORM-6151, developed to selectively deliver catalytic GSPT1 protein degraders to HER-2 and SIGLEC3-expressing tumor cells respectively, are currently in Phase I clinical trials.

Table 7: List of Preclinical and Clinical Degrader-Antibody Conjugates³

iii.            Dual-Drug ADCs⁴⁹:
Dual-Drug or Dual-Payload ADCs (dpADCs) incorporate two distinct payloads with complementary mechanisms of action, aiming to deliver a more potent cytotoxic response to cancer cells. These drugs are currently in preclinical development as summarized in Table 8.

Toxicity concerns and mechanisms

Despite being designed to enhance the targeted delivery of cytotoxic payloads to specific cancer cells, ADCs often encounter challenges related to toxicity. First-generation ADC data revealed that only a small fraction (~0.003 – 0.08%) of the injected dose reached the intended tumor cell leading to the distribution of a significant amount of cytotoxin to non-target tissues and subsequent adverse events.

The toxicity associated with ADCs can be broadly classified into on-target and off-target toxicity. On-target toxicity occurs when the ADC binds to the intended cell surface protein on healthy cells, leading to adverse effects (Figure 12A). Off-target toxicity involves non-specific endocytosis, binding of the ADC to Fc/C-type lectin receptors, or internalization of free payload by passive diffusion across the cell membrane (Figure 12B and 12C)⁵². The bystander toxicity effect is observed when cleavage of the payload from ADCs leads to the diffusion of free payload into neighboring healthy cells.⁵³

While ADCs with the same class of payload-linkers share similar toxicity profiles, using the same ADC to treat different cancers may result in varied toxicities. Common adverse events associated in late-stage clinical and marketed ADCs outlined in Table 9, can vary in severity, with some reaching grade 3 or higher. Among commonly used payload classes, peripheral neutropenia or thrombocytopenia is reported as the most common toxicity, followed by peripheral neuropathy, hepatotoxicity, skin rash, ocular toxicity, and toxicity of the gastrointestinal tract.⁵⁴

Various strategies have been explored recently to address these challenges and reduce ADC-associated toxicities. These strategies include modifying conjugation technology or payload-linker chemistry, making antibody modifications, adjusting dosage regimens, and implementing inverse targeting strategies.⁵²

Table 9: Common adverse effects associated with different payload classes

Clinical Aspects of Antibody-Drug Conjugates

Approved Antibody-drug conjugates

The first ADC to receive market approval, gemtuzumab ozagamicin, was introduced in 2001 by Pfizer, withdrawn in 2010, and re-introduced in 2017 along with inotuzumab ozogamicin. Brentuximab vedotin, the second ADC launched in 2011 by SeaGen Inc. (formerly Seattle Genetics, Inc. and now Pfizer) and Millenium Pharmaceuticals (now Takeda), achieved $1.089 billion in sales in 2024.⁵⁵ Trastuzumab deruxtecan, launched by Daiichi Sankyo and trastuzumab emtansine, launched by Roche, continue to enjoy blockbuster status with $3.754 billion⁵⁶ and $2.192 billion⁵⁷ respectively in sales, in 2024. Between 2019 and 2025, additional ADCs have received approval, bringing the total number of approved ADCs to 15, as summarized in Table 10.

Table 10 : Approved Antibody-drug conjugates

Combination Trials

ADCs are being explored in combination with chemotherapy, molecularly targeted agents, radiotherapy, immunotherapy and endocrine therapy, in preclinical and clinical studies. Combination therapies generally reduce the likelihood of drug resistance and two agents with different mechanisms of action can be used to achieve favorable treatment outcomes.⁵⁹ ADC monotherapies may be insufficient to treat certain tumor types, hence there is growing interest in exploring combination therapies. These trials appear to be primarily focused on advancing their use into earlier lines of therapy or earlier stages of disease. As per recent data, around half of the ADC trials initiated each year are combination trials (i.e. 197 out of 386 ADC trials in 2024).³ Currently, four ADCs have had combination therapies approved by global regulatory authorities. Table 11 summarizes these approved ADC combination therapies.

ADCs in Clinical Development: Future scope

Among 1876 ADCs identified by March 2025, approximately 23% i.e. 424 ADCs are in different stages of clinical trials. Notably, a significant proportion (69%), of these clinical candidates are actively undergoing investigation in trials.

Antibody-drug conjugates for non-oncological applications

While the majority of ADCs in development remain focused on oncology, the landscape is gradually expanding. Unlike in oncology, these ADCs utilize non-toxic payloads to modulate biological functions without affecting cell viability.⁶² The identification of highly specific targets and development of non-toxic payloads (anti-inflammatory, anti-infective or neuroprotective) specific to the disease indication is essential to minimize off-target effects. Additionally, their usage in chronic non-oncological indications may increase the risk of immunogenicity, requiring careful design. While its research for non-cancer indications is still in its early stages, ongoing preclinical and early-phase clinical trials hold promise. Applications in myotonic dystrophy, scleroderma, Duchenne muscular dystrophy, rheumatoid arthritis, amyloidosis, obesity and bacterial infections are currently being explored as summarized in Table 13.

Table 13: A list of ADCs that have been tested for indications other than oncology³

Understanding the Challenges of Discontinued ADC Programs

Although the field of ADCs has had many successes and new approvals in the last few years, there are still many discontinued programs that provide important information. The most common reasons for discontinuation of ADCs include lack of sufficient efficacy at maximum tolerated dose (MTD), low tolerability, and safety reasons. Few others have also been discontinued due to pipeline reprioritization or due to the competitive landscape. Amongst the discontinued ADCs, most have used auristatin- and maytansinoid-based payloads, and in many cases, there may have been the wrong selection of drug-linker for the indication. Some antibodies targeting HER2+ cancers have not progressed in the clinic or did not show meaningful improvements to trastuzumab emtansine, but careful selection of new drug-linkers led to trastuzumab deruxtecan, a very promising new ADC. Table 14 displays targets and ADCs that have entered the clinic and have not proceeded. This knowledge along with careful analysis of the failures has the potential to inspire a new generation of ADCs.

Business Landscape of Antibody-Drug Conjugates

A flurry of deal-making activity is occurring in the ADC space, with these modalities re-emerging as a leading field of interest. With blockbuster deals from Abbvie’s $10.1 billion acquisition of ImmunoGen to Pfizer’s acquisition of Seagen for $43 billion in 2023, and multiple licensing agreements and strategic collaborations, ADC deals range from early to late-stage to marketed products and include a variety of oncology targets and indications. The integration of early pioneering ADC companies with large pharmaceutical firms highlights the increasing therapeutic potential ADC programs for various disease indications.

The tables below summarize recent partnership deals, venture capital funding events, and successful IPOs in the ADC space. Table 15 lists key licensing deals and mergers and acquisitions (M&As) in the ADC space from January 2020. Table 16 lists key venture capital funding events and IPOs in the ADC space since January 2020.

Table 15: Antibody-Drug Conjugate Licensing and Merger and Acquisition (M&A) Deals

Table 16: List of Antibody-Drug Conjugate Venture Capital Funding Events and IPOs

Conclusions

In summary, ADCs have emerged as an important therapeutic class of agents that can lead to new opportunities in the treatment of various cancers. Recent significant scientific and clinical advances in the field of ADCs have highlighted it as an important space for continued research and investment.

Acquiring insights into the novel strategies, understanding the efficacy and safety profiles of ADCs, identifying patterns of success and failure, and optimizing R&D strategies are crucial steps to streamline the development process and increase the likelihood of regulatory approval. Antibody engineering and site-specific conjugation technologies have also shown potential to enhance the therapeutic index in preclinical studies. An integrated approach, combining careful target selection with optimization of the antibody, linker, and payload components of the ADC tailored to specific disease indications, holds promise for future ADC approvals.

Major pharmaceutical companies, biotechnology firms, and academic institutions are actively engaged in the development of ADCs through strategic partnerships and collaborations. This collaborative approach not only accelerates the pace of innovation but also mitigates the inherent risks associated with drug development, making ADCs an attractive investment opportunity.

Overall, ongoing advancements in ADC technology, alongside refinements in clinical processes and combination therapies, offer scope of enhanced treatment outcomes.