H₁ Receptor Study Reveals Key to Designing More Effective Antihistamines OR GPCR Thermodynamics: New Insights for Drug Design & Antihistamine Development

G-protein-coupled receptors (GPCRs) are a vast family of cell surface proteins crucial for recognizing hormones, neurotransmitters, and drugs. These receptors orchestrate a wide range of physiological processes and are the targets of over 30% of currently available medications. The histamine H₁ receptor (H₁R) is a key member of this family, playing a central role in allergic reactions, inflammation, vascular permeability, airway constriction, wakefulness, and even cognitive functions. Despite the widespread use of antihistamines to target H₁R, their therapeutic effectiveness can be limited, prompting researchers to explore new approaches to ligand design.

Recent advances in drug design emphasize the importance of considering not only how strongly a compound binds to its target protein (affinity or binding energy) but also the thermodynamic properties of that interaction – specifically, enthalpy and entropy. The balance between these two factors, known as enthalpy–entropy compensation, is increasingly recognized as critical for understanding how selectively a drug binds to its target and distinguishes between similar molecules. However, directly measuring these thermodynamic parameters for cell surface proteins like GPCRs has proven challenging.

A research team led by Professor Mitsunori Shiroishi from the Department of Life System Engineering at Tokyo University of Science (TUS), Japan, has addressed this gap. Their systematic investigation of the binding thermodynamics of the H₁R was published online in ACS Medicinal Chemistry Letters on January 26, 2026. The team, including Mr. Hiroto Kaneko and Associate Professor Tadashi Ando, focused on doxepin, a tricyclic antidepressant also known for its potent antihistamine properties targeting H₁R.

“Doxepin is a compound that has been widely used as an H₁R inhibitor,” explains Prof. Shiroishi. “In this study, we successfully measured the thermodynamic signatures of doxepin geometric isomers (E– and Z-isomers) to the H₁R, prepared via a budding yeast expression system, using isothermal titration calorimetry and molecular dynamics simulations.”

Doxepin exists as a mixture of E– and Z-isomers, differing in their three-dimensional arrangement. Previous research from the same team revealed that the Z-isomer binds to H₁R with approximately five times the affinity of the E-isomer. They also identified a specific threonine residue (Thr112^3.37) as a key contributor to this difference in selectivity. The current study aimed to clarify the molecular basis of this selectivity through a detailed thermodynamic analysis.

To achieve this, the researchers created two versions of H₁R: a wild-type version (H₁R_WT) and a mutant version (T112^3.37V) where the threonine residue was replaced with valine. They then tested the interaction of both versions with doxepin as a mixture of isomers, and then with the individual E– and Z-isomers.

The results showed no difference in the overall binding energy between the wild-type and mutant receptors when interacting with doxepin. However, the contributions of enthalpy and entropy to that binding energy differed significantly. Binding to the wild-type receptor was primarily driven by enthalpy (energy released during binding), while binding to the mutant receptor showed a reduced enthalpic contribution and a relatively larger entropic contribution (related to the disorder or randomness of the system).

Specifically, the binding of the Z-isomer to the wild-type receptor was associated with a larger increase in enthalpy and a greater decrease in entropy compared to the E-isomer. These differences were not observed with the mutant receptor. Consistent with their previous findings, the Z-isomer exhibited higher binding energy to the wild-type receptor, while both isomers showed comparable binding energies to the mutant receptor. These observations highlight the role of Thr112^3.37 in balancing the enthalpic gains and entropic losses during ligand binding, with a more pronounced effect on the interaction with the Z-isomer.

To further investigate the molecular mechanisms underlying this selectivity, the researchers employed molecular dynamics simulations. These simulations revealed that the high-affinity binding of the Z-isomer is linked to conformational restrictions – a more rigid structure – which aligns with the observed high enthalpy and reduced entropy associated with its binding.

“These mechanistic insights into the enthalpy-entropy trade-off in GPCR-ligand interactions highlight the importance of considering conformational constraints and flexibility in designing ligands with optimized thermodynamic properties,” remarks Prof. Shiroishi. “This could lead to the development of drugs with improved selectivity, reduced side effects, and longer-lasting therapeutic effects. Our approach, combining thermodynamic analysis with molecular dynamics simulations, can be applied to other GPCRs and proteins, aiding rational drug design.”

The study underscores that even subtle differences in molecular conformation can significantly impact the enthalpy-entropy balance, and understanding these principles could be instrumental in designing more effective therapeutics while minimizing off-target effects and maximizing efficacy. The interplay between entropy and enthalpy isn’t limited to GPCR interactions; it’s a fundamental principle applicable to a wide range of protein-drug interactions.

this research provides valuable new insights into the thermodynamic principles governing GPCR–ligand interactions and offers a promising framework for the development of more targeted and effective medications.