Hypothesis

We hypothesize that quercetin phytosome, when formulated with a phospholipid lipid carrier that promotes lymphatic transport, achieves plasma concentrations of free quercetin exceeding the senolytic threshold (5–20 µM) in humans, whereas isoquercetin fails to reach this range because its rapid intestinal deglycosylation and first‑pass glucuronidation shunt the molecule into metabolites that lack senolytic activity.

Rationale

Rat pharmacokinetic data show isoquercetin yields a low AUC (17.2 mg/L·min) and Cmax (0.35 µg/mL) and is quickly converted to quercetin aglycone and glucuronide metabolites [1]; quercetin aglycone attains ~150‑fold higher AUC in the same model. Human data for a lecithin‑micelle phytosome formulation indicate a 3–15‑fold bioavailability increase over standard quercetin but provide no absolute AUC or senolytic threshold correlation [2]. Topical phytosome studies lack plasma measurements [3], and a food‑product chew trial reported a Cmax of 1051.9 µg/L (~3.5 µM), well below the senolytic range [4]. These observations suggest that current oral formulations either do not deliver enough free quercetin or that conjugated forms are rapidly cleared, leaving a gap between reported plasma levels and the concentrations needed to clear senescent cells in vitro.

We propose two mechanistic reasons for the phytosome advantage: (1) incorporation into phospholipid micelles promotes uptake via intestinal lymphatics, bypassing hepatic first‑pass metabolism and preserving a larger fraction of quercetin as free aglycone; (2) the phospholipid carrier may transiently inhibit UDP‑glucuronosyltransferase (UGT) activity in enterocytes, slowing conjugation and extending the window of free quercetin availability.

Experimental Design

A randomized, double‑blind, three‑period crossover trial will enroll 12 healthy volunteers. Each participant will receive, in random order, a single oral dose of (A) quercetin phytosome (standard phospholipid formulation), (B) isoquercetin (glycoside), and (C) quercetin aglycone as a positive control, each separated by a 7‑day washout. In a separate arm, the same three formulations will be co‑administered with a low dose of the UGT inhibitor probenecid (500 mg) to test the contribution of conjugation blockade. Serial blood samples will be collected pre‑dose and at 0.25, 0.5, 1, 2, 4, 6, 8, 12, and 24 h post‑dose. Plasma will be extracted and quantified for free quercetin, quercetin glucuronide, and quercetin sulfate using validated HPLC‑MS/MS. The primary pharmacokinetic endpoint is the AUC0‑24 of free quercetin; the pharmacodynamic endpoint is the ex vivo senolytic activity of participant plasma, measured by adding plasma to cultured human senescent fibroblasts and assessing SA‑β‑gal positivity after 24 h.

Predicted Outcomes

If the hypothesis is correct, phytosome alone will produce a free quercetin AUC0‑24 that surpasses the threshold associated with senolytic activity (≥5 µM·h, corresponding to a Cmax ≥ 5 µM), and the phytosome + probenecid arm will show further elevation. Isoquercetin, with or without probenecid, will remain below the threshold because rapid deglycosylation leads to extensive glucuronidation that is not fully blocked by the inhibitor. The aglycone control is expected to exceed the threshold but may display a rapid decline due to hepatic clearance. A failure of phytosome to reach the senolytic range, or a demonstration that isoquercetin achieves comparable free quercetin levels when UGT is inhibited, would falsify the hypothesis.

This design directly links formulation‑dependent plasma quercetin exposure to a functional senolytic read‑out, addressing the current lack of pharmacodynamic evidence that underpins clinical senolytic trials.