Cross-Reactivity Profiles of Synthetic Incretin Peptides with Related Class B GPCRs

 

Synthetic incretin peptides designed for metabolic disease therapy must navigate a delicate pharmacological balance: achieving sufficient potency at their intended receptors while minimising unintended activation of structurally related class B1 G protein-coupled receptors (GPCRs). The secretin-like class B1 family comprises fifteen members, including the glucagon receptor (GCGR), glucose-dependent insulinotropic polypeptide receptor (GIPR), glucagon-like peptide-1 receptor (GLP-1R), and GLP-2 receptor (GLP-2R), all sharing substantial sequence homology and structural architecture. This homology creates an intrinsic cross-reactivity liability, as synthetic peptides engineered for one receptor may inadvertently engage others. Understanding these cross-reactivity profiles is essential for rational therapeutic design.

Structural Basis for Receptor Selectivity and Cross-Reactivity

Class B1 GPCRs possess a characteristic two-domain architecture comprising an extracellular domain (ECD) and a transmembrane domain (TMD), with the orthosteric binding pocket located at their interface. The endogenous incretin peptides GIP and GLP-1 share approximately 70% sequence similarity in their N-terminal regions, yet they exhibit remarkable selectivity for their cognate receptors: GIP does not bind GLP-1R, and vice versa. Structural studies have identified that residues at positions 1, 7, and 15 are critical for this discrimination. Substituting these positions in GIP with corresponding GLP-1 residues enables GIP to activate both receptors, demonstrating that selectivity is encoded in a small number of key residues.

The molecular basis for cross-reactivity in synthetic peptides often involves disruption of these selectivity determinants. Multi-receptor agonists are deliberately engineered by incorporating residues from multiple native hormones, thereby exploiting the shared sequence homology to achieve promiscuous activation. The dual agonist tirzepatide exemplifies this approach, with sequence elements drawn from both GIP and GLP-1 that enable balanced activation of both receptors.

Cross-Reactivity of Native and Synthetic Incretins at Related Receptors

Systematic screening of incretin peptides across the class B1 family reveals a spectrum of cross-reactivity. Native GLP-1 activates GLP-1R with high potency (EC₅₀ ~18 pM) but shows negligible activity at GIPR (EC₅₀ > 1 μM) and GCGR (EC₅₀ > 2 μM). Similarly, GIP is highly selective for GIPR (EC₅₀ ~7 pM) with no detectable activity at GLP-1R or GCGR. Glucagon, by contrast, exhibits moderate cross-reactivity at GLP-1R (EC₅₀ ~1.9 nM) while retaining selectivity for GCGR (EC₅₀ ~9 pM).

Synthetic multi-agonists intentionally blur these selectivity boundaries. The dual GLP-1R/GIPR agonist tirzepatide shows approximately 20-fold greater cAMP accumulation at GIPR than GLP-1R, reflecting an imbalanced mechanism of action. This imbalance is pharmacologically significant: at clinically relevant doses, tirzepatide achieves greater GIPR occupancy than GLP-1R occupancy, potentially accounting for its enhanced metabolic efficacy. Beyond its intended targets, tirzepatide exhibits biased agonism at GLP-1R, favouring cAMP generation over β-arrestin recruitment, which may reduce receptor desensitisation and enhance insulin secretion.

Cross-Reactivity at GLP-2R and Other Family Members

The GLP-2 receptor, activated by the intestinotropic hormone GLP-2, represents a particularly relevant cross-reactivity concern given its close phylogenetic relationship to GLP-1R. Native GLP-1 does not activate GLP-2R, and GLP-2 is selective for its cognate receptor. However, synthetic modifications can introduce unintended GLP-2R activity. N-terminal alkylation strategies designed to enhance DPP-4 resistance and tune receptor balance have been shown to alter cross-reactivity profiles in ways that are not always predictable from sequence considerations alone.

The broader secretin receptor family, including the secretin receptor (SCTR), vasoactive intestinal peptide receptors (VIP1R, VIP2R), and pituitary adenylate cyclase-activating polypeptide receptor (PAC1R), shares structural features with the incretin receptors. While systematic cross-reactivity data for synthetic incretins at these receptors are limited, the conserved architecture of the class B1 peptide-binding pocket suggests that highly engineered multi-agonists could potentially engage these related receptors under certain conditions.

Engineering Strategies to Modulate Cross-Reactivity

Recent medicinal chemistry efforts have focused on developing strategies to precisely tune receptor cross-reactivity. N-terminal chemical modifications, including alkylation and fluoroalkylation, have emerged as powerful tools for redirecting receptor engagement without wholesale scaffold redesign. These modifications exploit the fact that the N-terminus of class B1 peptide ligands reaches deep into the transmembrane activation pocket, where subtle changes in size, shape, polarity, and charge can redirect receptor activation trajectories.

For example, phenolic N-terminal modifications preferentially enhance GIPR activation while attenuating GLP-1R potency, whereas methylimidazole chemistry supports GLP-1R potency. These findings demonstrate that cross-reactivity profiles are not fixed properties of peptide scaffolds but can be systematically modulated through targeted chemical modifications. However, the same studies reveal that some scaffolds, such as tirzepatide, are relatively intolerant of N-terminal alkylation, with modifications diminishing activity at both intended receptors.

Clinical and Therapeutic Implications

The cross-reactivity profiles of synthetic incretins have direct clinical consequences. Treatment-emergent anti-drug antibodies in patients receiving tirzepatide show cross-reactivity to both GIP (40.3%) and GLP-1 (16.5%), with neutralising antibodies against GIPR (2.8%) and GLP-1R (2.7%) detected at low frequencies. While these immunogenicity findings do not directly reflect receptor pharmacology, they point out the necessity of understanding how synthetic peptides interact with the broader class B GPCR network.

The ongoing evolution toward higher-order multi-agonists—including tetra-receptor agonists incorporating GLP-1R, GIPR, GCGR, and Y2 receptor activities—further amplifies the challenge of managing cross-reactivity. These compounds must achieve sufficient potency at four distinct receptors while avoiding excessive activation of untargeted family members.

In conclusion, the cross-reactivity profiles of synthetic incretin peptides with related class B GPCRs reflect a complex interplay between conserved structural architecture and sequence-encoded selectivity determinants. Native incretins exhibit remarkable receptor discrimination, while synthetic multi-agonists deliberately exploit shared homology to achieve polypharmacology. The development of chemical strategies to precisely tune receptor engagement offers a promising path toward more selective and effective therapeutics.