Direct observation (NMR) of the efficacy of glucagon receptor antagonists in murine liver expressing the human glucagon receptor.
The demonstration of pharmacodynamic efficacy of novel chemical entities represents a formidable challenge in the early exploration of synthetic lead classes. Here, we demonstrate a technique to validate the biological efficacy of novel antagonists of the human glucagon receptor (hGCGR) in the surgically removed perfused liver prior to the optimization of the pharmacokinetic properties of the compounds. The technique involves the direct observation by (13)C NMR of the biosynthesis of [(13)C]glycogen from [(13)C]pyruvate via the gluconeogenic pathway. The rapid breakdown of [(13)C]glycogen (glycogenolysis) following the addition of 50 pM exogenous glucagon is then monitored in real time in the perfused liver by (13)C NMR. The concentration-dependent inhibition of glucagon-mediated glycogenolysis is demonstrated for both the peptidyl glucagon receptor antagonist 1 and structurally diverse synthetic antagonists 2-7. Perfused livers were obtained from a transgenic mouse strain that exclusively expresses the functional human glucagon receptor, conferring human relevance to the activity observed with glucagon receptor antagonists. This technique does not provide adequate quantitative precision for the comparative ranking of active compounds, but does afford physiological evidence of efficacy in the early development of a chemical series of antagonists.