Hypothesis

Isoquercetin, despite showing negligible plasma exposure in rodents, achieves therapeutically relevant concentrations in adipose tissue through rapid intestinal deglycosylation by lactase‑phlorizin hydrolase and subsequent uptake via GLUT1 transporters in adipocytes. This local accumulation would enable senolytic activity comparable to or greater than that of quercetin phytosome, which relies on high systemic exposure but may be limited by efflux pumps and sequestration in phospholipid complexes.

Rationale

1. Metabolic conversion: The rat PK study reported isoquercetin’s AUC0-t was ~150‑fold lower than quercetin aglycone, attributing the difference to rapid metabolism to quercetin and quercetin‑glucuronide [3]. This suggests that isoquercetin is quickly hydrolyzed, releasing quercetin in the gut lumen before absorption.
2. Transporter dependence: Quercetin aglycone enters cells via facilitated diffusion through GLUT1, a transporter highly expressed in adipocytes [4]. If isoquercetin is deglycosylated extracellularly, the resulting quercetin can be taken up by adipocytes without needing to cross the enterocyte barrier as an intact glycoside.
3. Tissue sequestration vs. phytosome limitation: Quercetin phytosome improves overall bioavailability by incorporating quercetin into a phospholipid complex [1, 2]. However, phospholipid complexes can be trapped in circulating lipoproteins or excreted via bile, reducing free quercetin available for tissue uptake. In contrast, locally generated quercetin from isoquercetin bypasses this sequestration step.
4. Senolytic threshold: In vitro quercetin at 20 uM reduces SA‑β‑gal activity and p21 expression in adipocytes [4]. Modeling based on intestinal hydrolysis rates predicts that a 500 mg oral dose of isoquercetin could yield transient intra‑adipocyte quercetin concentrations of 15‑25 uM, sufficient to trigger senolytic pathways despite low plasma levels.

Testable predictions

• Prediction 1: In a crossover human trial (n = 12), a single 500 mg dose of isoquercetin will produce adipose tissue quercetin concentrations (measured via biopsy LC‑MS/MS) that are not statistically different from those after a 500 mg dose of quercetin phytosome, while plasma AUC0-24 for isoquercetin remains <=10% of that for phytosome.
• Prediction 2: Co‑administration of a lactase‑phlorizin hydrolase inhibitor (e.g., conduritol B epoxide) will blunt the adipose tissue quercetin rise after isoquercetin ingestion, confirming the role of extracellular deglycosylation.
• Prediction 3: siRNA knock‑down of GLUT1 in cultured human adipocytes will reduce quercetin uptake and diminish the senolytic effect of isoquercetin‑derived quercetin, linking transporter activity to efficacy.

Falsifiability

If adipose tissue quercetin levels after isoquercetin dosing are consistently <30% of those after phytosome dosing, or if GLUT1 knock‑down does not alter senolytic outcomes, the hypothesis would be refuted. Similarly, if lactase inhibition does not reduce tissue quercetin, the proposed metabolic route would be unsupported.

Implications

Confirming this mechanism would shift dosing strategies for senolytic therapies toward glyco‑quercetin precursors that exploit local tissue activation, potentially lowering systemic exposure and off‑target effects while maintaining efficacy in adipose‑rich depots.