Overcoming the undruggable barrier: Structure-guided discovery of a potent small molecule CD28 antagonist with
Saurabh Upadhyay1, Hossam Nada1, Sungwoo Cho1
1Department of Radiology, Molecular Imaging Innovations Institute (MI3), Weill Cornell Medicine, New York, NY 10065, USA.
Abstract:
CD28 is the prototypical costimulatory receptor that integrates with TCR signaling to sustain T-cell activation, proliferation, and survival. While indispensable for adaptive immunity, persistent CD28 signaling drives autoimmunity, graft-versus-host disease, and inflammatory pathology. Despite its therapeutic relevance, CD28 has long been regarded as an undruggable target due to its flat, solvent-exposed dimer interface, restricting modulation to biologics. Previous reports of small molecule CD28 inhibitors have been limited by weak potency. Here, we report the first structure-guided optimization yielding a best-in-class small molecule CD28 antagonist that overcomes these limitations. Guided by biophysical and functional assays, optimization of the preliminary CD28 hits 8VS and 22VS yielded BPU11, a chemically tractable lead with markedly improved pharmacokinetic stability, aqueous solubility, and plasma persistence. BPU11 consistently disrupted CD28-B7 interactions across biochemical and cellular systems, and potently suppressed T-cell activation in both a tumor-PBMC co-culture and a human PBMC-mucosal tissue model, functionally mimicking the biologic antagonist FR104. Molecular docking and dynamics simulations revealed engagement of the lipophilic canyon of CD28 through stabilizing hydrogen-bonding and hydrophobic interactions. These findings redefine the druggability of CD28, expand the pharmacological space of immune checkpoint modulation beyond antibodies, and establish BPU11 as a foundation for next-generation small molecule immunotherapies.
Insights
Researchers developed a novel small molecule CD28 antagonist, BPU11, overcoming previous limitations in targeting this immune checkpoint. This breakthrough offers a new avenue for small molecule immunotherapies beyond biologics.
Area of Science:
- Immunology
- Pharmacology
- Structural Biology
Background:
- CD28 is a crucial costimulatory receptor for T-cell activation, essential for adaptive immunity.
- Dysregulated CD28 signaling contributes to autoimmune diseases and inflammation.
- CD28's structure was previously considered a barrier to small molecule inhibition, limiting therapeutic options.
Purpose of the Study:
- To develop a potent small molecule CD28 antagonist.
- To overcome the limitations of previous CD28 inhibitors.
- To establish a foundation for next-generation small molecule immunotherapies.
Main Methods:
- Structure-guided optimization of preliminary CD28 inhibitors (8VS, 22VS).
- Utilized biophysical and functional assays for optimization.
- Employed molecular docking and dynamics simulations to understand target engagement.
Main Results:
- Developed BPU11, a best-in-class small molecule CD28 antagonist with improved drug-like properties.
- BPU11 effectively disrupted CD28-B7 interactions in biochemical and cellular assays.
- BPU11 suppressed T-cell activation in preclinical models, mimicking biologic antagonist FR104.
Conclusions:
- Redefined the druggability of CD28, expanding therapeutic strategies beyond biologics.
- BPU11 represents a significant advancement in small molecule immune checkpoint modulation.
- BPU11 serves as a promising lead for developing novel small molecule immunotherapies.
