Ivan Kopitovic
Tirzepatide (TZP) is a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist that shows full agonism at the GIP receptor (GIPR) and reduced potency with G-protein-biased partial agonism at the GLP-1 receptor (GLP-1R) in vitro. Weight-loss response may attenuate during prolonged therapy, and some patients remain on lower doses after dose reduction for tolerability. Whether clinically relevant GLP-1R signalling capacity remains available for intermittent additional stimulation under these conditions is unknown. It is proposed that adding intermittent low-dose liraglutide to a stable low-dose TZP background, termed Receptor-Primed Episodic Agonism (RPEA), could be investigated as a way to re-engage GLP-1R signalling without increasing the TZP dose. Liraglutide has an elimination half-life of approximately 13 h, but residual systemic exposure may remain at 48 h and repeated q48h administration may accumulate; the interval therefore requires formal PK characterisation rather than being assumed to provide complete washout. RPEA is defined generally as intermittent short-acting full GLP-1R agonism on a stable TZP background; liraglutide, 0.6 mg, and q48-72h are illustrative candidates rather than defining features. An incremental response alone would not demonstrate priming; the defining test is a background-by-challenge interaction exceeding a prespecified additive null and not explained by a pharmacokinetic interaction. The concept rests on four explicitly unproven propositions: (i) residual GLP-1R signalling capacity during low-dose TZP; (ii) an incremental pharmacodynamic effect from episodic full GLP-1R agonism; (iii) less cumulative receptor adaptation with intermittent than continuous full-agonist exposure; and (iv) pharmacodynamic effects that may not track plasma concentration exactly. RPEA generates falsifiable preclinical and clinical questions but is not a recommendation for clinical use. No safety data are available for this off-label two-agent combination, and current prescribing information for both products does not recommend coadministration with another GLP-1 receptor agonist. The combination must not be used outside an ethics-approved clinical trial.