Finding a target does not guarantee payload release
Antibody-drug conjugates (ADCs) and small molecule-drug conjugates (SMDCs) commonly depend on receptor-mediated endocytosis and lysosomal trafficking. The paper notes that only about 180 of more than 2,000 human membrane proteins support efficient internalization, leaving many otherwise selective surface targets outside the conventional conjugate-drug playbook.
Fibroblast activation protein (FAP) illustrates this constraint. It is expressed on cancer-associated fibroblasts across multiple tumour types but internalizes poorly. The case highlights a central delivery problem: target recognition is only the first step; what happens after binding can determine both efficacy and therapeutic window.
PhoPEx chemistry turns molecular proximity into cleavage
The researchers developed phosphorus(V)-phenol exchange (PhoPEx) chemistry and linkers designed to remain stable in circulation yet react with a proximal nucleophilic residue inside a target-binding pocket. For FAP, ligand binding positions the reactive centre near Tyr745, triggering linker cleavage and extracellular release of a membrane-permeable payload.
BTR therefore separates two functions: ligand-receptor recognition determines where the conjugate goes, while proximity chemistry determines when release occurs. The process does not require lysosomal trafficking or an externally administered trigger.
A layered evidence chain from chemistry to animal models
The study evaluated specificity, release efficiency, biodistribution and therapeutic window. FAP-BTR-SMDC showed more than 1,000-fold binding selectivity over homologous proteins. The same chemistry also supported an activatable FAP fluorescence probe evaluated in 31 patient-derived lymph nodes.
In one cell-derived xenograft model, a single administration produced complete responses in 8 of 8 mice within 30 days. Antitumour activity was also observed across several patient-derived xenograft models. The team extended BTR to PD-L1 and to an mRNA-display-derived FAP macrocyclic peptide, providing initial evidence that the framework can be explored across targets and ligand classes.
Industry implication: druggability may depend on binding-pocket chemistry, not only internalization
The advance is more than a new linker. It reframes conjugate design around additional questions: does the target offer an accessible binding pocket, is a suitable nucleophilic residue positioned nearby, and can the released payload reach the intended cells? This could bring structural biology, ligand discovery, linker chemistry and pharmacokinetics into a more integrated workflow.
BTR is presented as complementary to ITR, not a replacement. Efficiently internalizing targets such as HER2 and TROP2 remain well suited to established intracellular-release approaches. BTR offers a potential path for targets previously constrained by poor internalization and may also support theranostic applications.
A methodological signal for precision kidney therapeutics
The BTR study shows that candidate differentiation can arise not only from target selection, but also from the integration of delivery, release and validation. Podigy follows these transferable methods while advancing multiple therapeutic modalities and AI-assisted R&D for kidney diseases. Specific targets, molecular designs and mechanisms in Podigy's own programs are undisclosed; the reported research is independent of Podigy's pipeline.
The evidence is primarily from in vitro studies, ex vivo clinical specimens and mouse CDX/PDX tumour models and should not be interpreted as demonstrated human therapeutic benefit. Extracellular BTR release requires membrane-permeable payloads and warrants continued evaluation of off-target release, bystander effects and systemic toxicity. Translation to non-oncology diseases, soluble targets or kidney applications requires target-specific structural, pharmacokinetic and in vivo validation.