New Peptide Linker Design Makes Cancer-Killing Antibody Drugs More Effective at Destroying Nearby Tumor Cells | Research

The takeaway

A novel tripeptide-cleavable ADC linker design achieved improved bystander killing — the ability to kill neighboring cancer cells that don't express the drug's target — through optimized drug release and side chain modifications.

Improved bystander killing and in vivo efficacy
Optimized peptide linker design and extended maytansinoid side chains enhanced the ability of ADCs to kill nearby untargeted cancer cells, outperforming previous conjugate designs in mice

What the researchers found

Tripeptide linkers with all-L or single D-alanyl residues effectively released cytotoxic maytansinoid metabolites inside cells. D-alanyl residues didn't impair activity unless directly attached to the self-immolative group. Extending the maytansinoid side chain increased bystander killing without affecting direct cytotoxicity. The best-performing ADCs showed improved in vivo efficacy compared to previous maytansinoid conjugate designs.

Why it matters

Improved bystander killing means ADCs can kill not just the tumor cells they bind to but also nearby cancer cells that may not express the target antigen — addressing a key limitation of conventional ADC therapy in heterogeneous tumors.

The numbers in context

Tripeptide (trialanyl) linker; D vs L alanyl stereochemistry; variable methylene side chain length; improved in vivo efficacy in mice

How the study worked

Conjugates were synthesized with varying peptide linker stereochemistry and maytansinoid side chain lengths. In vitro testing assessed direct cytotoxicity and bystander killing in target-positive and target-negative cells. Best candidates were evaluated in vivo in mouse tumor models and compared to previously described maytansinoid ADC types.

What this study cannot tell us

Mouse xenograft models may not predict human therapeutic responses. The study focuses on one antibody and one payload class (maytansinoids). Long-term safety and manufacturing feasibility at scale were not addressed. Bystander killing may also affect nearby healthy tissue, potentially increasing toxicity.

How to read the evidence

Published in ACS Medicinal Chemistry Letters, this is a well-designed medicinal chemistry study with systematic structure-activity relationships and in vivo validation. It provides strong preclinical evidence for the linker optimization approach.

When this study was published

Published in 2019, this study informed ADC linker design approaches that continue to influence the rapidly advancing ADC field, which has seen multiple new approvals since.

The bigger picture

ADCs are one of the fastest-growing cancer drug classes (>15 approved), but tumor heterogeneity limits their effectiveness — not all cancer cells in a tumor express the antibody target. Bystander killing addresses this by allowing the released cytotoxic payload to kill neighboring target-negative cells. Optimizing the peptide linker that connects drug to antibody is critical for controlling when and how the drug is released. This study's systematic approach to linker design — testing stereochemistry and payload chemistry — advances the rational engineering of next-generation ADCs.

Questions still open

Does increased bystander killing translate to better clinical outcomes in patients, or does it also increase toxicity to healthy tissue?
Can this peptide linker design be combined with newer payloads beyond maytansinoids for even greater therapeutic effect?
How does the D-alanyl substitution affect ADC stability during long-term storage and in circulation?

Common questions

What is bystander killing in cancer treatment?

In antibody-drug conjugates, bystander killing refers to the ability of the released cytotoxic drug to diffuse out of the targeted cell and kill nearby cancer cells — even if those neighbors don't express the target protein. This is important because tumors are often heterogeneous: some cells express the target and some don't. Without bystander killing, the untargeted cells survive and can cause tumor regrowth.

What is a peptide linker in an ADC?

The peptide linker is the chemical bridge connecting the antibody to its toxic drug payload. It must be stable in the bloodstream (to avoid releasing the drug prematurely and causing side effects) but cleavable inside cancer cells (to release the drug where it's needed). In this study, a tripeptide made of alanine amino acids was used, which is cleaved by proteases inside the cell's recycling compartments, triggering a self-immolation cascade that releases the active drug.

Read the original research

Peptide-Cleavable Self-immolative Maytansinoid Antibody-Drug Conjugates Designed To Provide Improved Bystander Killing.

ACS medicinal chemistry letters, 10(10), 1393-1399

View on PubMed ↗Publisher’s paper ↗

Citation

Costoplus, Juliet A; Veale, Karen H; Qiu, Qifeng; Ponte, Jose F; Lanieri, Leanne; Setiady, Yulius; Dong, Ling; Skaletskaya, Anna; Bartle, Laura M; Salomon, Paulin; Wu, Rui; Maloney, Erin K; Kovtun, Yelena V; Ab, Olga; Lai, Kate; Chari, Ravi V J; Widdison, Wayne C. (2019). Peptide-Cleavable Self-immolative Maytansinoid Antibody-Drug Conjugates Designed To Provide Improved Bystander Killing.. ACS medicinal chemistry letters, 10(10), 1393-1399. https://doi.org/10.1021/acsmedchemlett.9b00310