Toxicity & Efficacy in ADCs: How to Get it Right

Allan Jordan, PhD — Senior Vice President, Oncology Drug Discovery, Research Management  Joshua Greally, PhD — ADC Discovery Consultant

Catch up on our ADC development series on YouTube to hear more expert insights.

ADC toxicity arises when the cytotoxic payload acts on healthy tissue alongside the tumor, often through premature release of the payload in the systemic circulation.

Antibody-Drug Conjugates (ADCs) still face significant challenges related to patient tolerability and therapeutic window, and ADCs often fail to significantly reduce toxicity compared to traditional cytotoxic agents, highlighting the ongoing struggle to improve specificity and safety.

Selective Targeting and Payload Design (‘Selectivity Squared’) 

Traditional ADC payloads typically fall into three main classes, each designed to disrupt cancer cell survival through distinct mechanisms. Microtubule inhibitors, such as MMAE and DM1, interfere with spindle formation in mitosis. Auristatins such as MMAE bind to the side of forming spindles causing kinks which results in improper alignment, while maytansinoids bind to the spindle terminus directly inhibiting its growth. Topoisomerase I inhibitors, including DXd and SN-38, inhibit the topo 1 enzyme which helps to regulate DNA under- and over-winding to remove knots and tangles from the genetic material. Topo1 functions by creating tightly controlled single-stranded breaks in DNA, which are key to cell regulation and DNA replication. Meanwhile, DNA-damaging/chelating agents—often highly potent—directly compromise DNA by binding into the minor groove and covalently bonding. 

A common trait with all the mechanisms employed by conventional ADC cytotoxic payload is that each is present in both healthy and disease state cells, thus limiting selectivity and substantially increasing systemic, payload-mediated ADC toxicity if the ADC is broken down outside the target cell.

Increasing the therapeutic window may involve enhancing both the selectivity of the antibody and the specificity of the payload. Modifying linker chemistry and optimizing payload conjugation are crucial, as even minor changes can drastically affect the balance between efficacy and toxicity. This is where the concept of a biologic/chemical bispecific molecule comes into play. The combination of a highly specific antibody with a targeted small molecule therapeutic can maximize targeted action, an approach we informally refer to as “selectivity squared”. Rather than relying solely on the antibody for specificity, we’re now integrating selectivity at multiple levels—the antibody, the small molecule payload, and even the linker. 

Multidisciplinary Approach to ADC Development 

Achieving this selectivity requires a multidisciplinary approach, bringing together chemistry, bioscience, DMPK, and both in vivo efficacy and ex vivo PK to understand how subtle changes can have a significant impact on the overall therapeutic profile. We’re used to developing small molecules and we’re used to small changes having significant effects on PK and toxicity and overall therapeutic profile. 

It’s no different in this emerging ADC paradigm. Every single atom change, linker adjustment, or payload modification can dramatically shift an ADC from a highly selective therapeutic to an ineffective or less selective compound. By embracing a holistic perspective—one that sees ADCs as hybrid molecules rather than just targeted delivery vehicles—we can unlock new potential and rethink how selectivity is built from the ground up. 

Learn more about Sygnature’s ADC capabilities here.

Catch up on the full ADC development blog series:

Part 1: Exploring new paths in ADC development

Part 2: What’s the Evolving Role of Medicinal Chemistry in ADC Development?

Part 3: Linker Technologies in ADCs: How They Impact Efficacy & Stability

Part 5: 3 Transformative Changes in ADC Development Over the Next Decade

 About the Authors

FAQs

What is the therapeutic window of an ADC?

The therapeutic window is the range between the dose at which an ADC delivers meaningful efficacy and the dose at which toxicity becomes unacceptable. A wider window means more room to dose effectively. Antibody selectivity, payload specificity, linker stability and conjugation method all shape it.

What are the main classes of ADC payload?

Conventional ADC payloads fall into three main classes: microtubule inhibitors (e.g. MMAE, DM1), topoisomerase I inhibitors (e.g. DXd, SN-38) and DNA-damaging agents. All three act on mechanisms present in healthy as well as diseased cells, which is why systemic payload toxicity remains a challenge.

What does “selectivity squared” mean in ADC design?

“Selectivity squared” is our informal term for building selectivity into more than one component of an ADC: pairing a highly specific antibody with a payload that is, in itself, mechanistically targeted, and a linker designed for controlled release. Rather than relying on the antibody alone, selectivity is layered at the antibody, payload and linker level.

What causes ADC toxicity if the antibody is targeted?

Targeting is never perfect. ADC payloads can be released prematurely in circulation, the antigen may be expressed at low levels on healthy tissue, and conventional cytotoxic payloads act on mechanisms every cell relies on. Reducing toxicity therefore means optimizing the payload, linker and conjugation holistically, not just the antibody.

How can I stay up to date with Sygnature’s ADC development insights?

Follow our five-part ADC development blog series, or catch up with Allan Jordan and Joshua Greally on our YouTube channel. To discuss your ADC project, get in touch with the team.