Absolute Bioavailability and PK/PD of Quercetin in Normoglycemic and Alloxan-Induced Diabetic Rats

This study aimed to determine the absolute bioavailability of quercetin and quantitatively evaluate its pharmacokinetic–pharmacodynamic (PK-PD) relationship regarding acute glucose-lowering effects in normoglycemic and alloxan-induced diabetic rats, addressing whether its in vivo efficacy is driven by the free aglycone or its biotransformed intermediates. Healthy and diabetic rats received single doses of quercetin either orally (75 mg/kg) or intravenously (38 mg/kg). Plasma concentrations of free quercetin were quantified using a validated HPLC-DAD method, and temporal PK-PD relationships between systemic exposure and the percentage variation of glycemia were mathematically evaluated employing Pearson correlation analysis. The absolute bioavailability of free quercetin was significantly impaired by the pathophysiological state, dropping from 59.7% in healthy rats to 40.9% in diabetic subjects. Despite this diminished systemic exposure, oral administration elicited significant hypoglycemic responses. Crucially, the Pearson correlation analysis revealed a pronounced temporal dissociation: the onset of glycemic reduction occurred independently of the maximal circulating concentration of free quercetin. Furthermore, intravenous delivery bypassed first-pass barriers and induced a markedly faster and deeper hypoglycemic effect (up to −47% in diabetic rats). Finally, the diminished bioavailability under diabetic conditions and the stark PK-PD temporal dissociation strongly suggest that quercetin’s acute antihyperglycemic effect is driven by rapid hepatic Phase II biotransformation, implicating conjugated metabolites (rather than the free aglycone) as the principal pharmacological effectors.