Nanocarrier With Built-In Efflux Blocker Lifts Luteolin Absorption 29-Fold

Luteolin is one of those plant compounds that looks spectacular on paper and then quietly disappoints in the bloodstream. Found in celery, parsley, chamomile and dozens of other foods, this flavonoid carries a polyhydroxy flavone backbone that gives it genuine anti-inflammatory, antioxidant and anticancer credentials in laboratory settings. Yet when a person swallows it, the body treats it almost as a nuisance. A rat study cited by the research team reported an absolute oral bioavailability of just 17.5 percent, because luteolin dissolves poorly in gut fluids, is rapidly conjugated by phase II metabolic enzymes, and is then actively pumped back out of intestinal cells by P-glycoprotein, one of the body’s most efficient chemical gatekeepers. A new study published in the Journal of Advanced Research argues that all three barriers must be attacked at once, and it demonstrates a delivery system that does exactly that, with striking results.

The team, led by Yimei Zheng, Boyu Chen and colleagues at Guangdong Ocean University, built a self-microemulsifying drug delivery system, or SME, around luteolin. These systems are concentrated mixtures of oil, surfactant and co-surfactant that remain as clear liquids in a capsule but spontaneously bloom into nanoscale droplets, typically 20 to 100 nanometers across, the moment they meet gastrointestinal fluid. The sudden explosion of surface area dissolves lipophilic molecules that would otherwise sit undissolved in the gut lumen, protects them from oxidation and pH swings, and presents them to the intestinal epithelium in a form cells readily accept. What distinguishes this formulation from most previous nanocarriers is the deliberate choice of emulsifier: D-α-tocopheryl polyethylene glycol 1000 succinate, better known as TPGS, a Food and Drug Administration-approved amphiphilic polymer that happens to be a potent inhibitor of P-glycoprotein itself.

Formulation development began with pseudo-ternary phase diagrams, a classical pharmaceutical tool for mapping where oil, surfactant and water coexist as a stable microemulsion. The researchers combined TPGS as surfactant, polyethylene glycol 400 as co-surfactant and isopropyl myristate as the oil phase, titrating water into 25 different compositions and watching for turbidity and phase separation. The sweet spot emerged at a TPGS-to-PEG 400 mass ratio of 2:1, which produced the largest microemulsion region. Below that ratio, insufficient TPGS left interfacial tension too high; above it, the relative shortage of PEG 400 compromised co-solubilization. Oil content mattered just as much: higher proportions of isopropyl myristate destabilized the system by diluting the emulsifiers, echoing earlier reports that oil fractions above roughly 40 percent tend to yield unstable emulsions.

Three candidate formulations, designated TP12-I2, TP11-I2 and TP21-I3, were loaded with luteolin at 50 milligrams per gram of carrier and put through a demanding characterization gauntlet. Dynamic light scattering showed narrow, monomodal size distributions with mean droplet diameters below 50 nanometers. The formulations survived alternating 4 and 45 degree Celsius cycles, freeze-thaw swings between minus 20 and 25 degrees, and centrifugal stress in a LUMiSizer analyzer without significant changes in droplet size or visible phase separation. Transmission electron microscopy confirmed uniform, spherical particles with no aggregation. Fourier-transform infrared spectroscopy revealed subtle shifts in luteolin’s phenolic C–O stretch at 1265 wavenumbers and its carbonyl band moving from 1656 to 1653 wavenumbers, fingerprints of hydrogen bonding between the drug and its carrier, while ultraviolet-visible spectra retained luteolin’s characteristic 255 and 350 nanometer peaks, proving the molecule emerged chemically intact.

Release and antioxidant performance both improved markedly. In dialysis experiments, free luteolin plateaued after about 12 hours with only 9.23 percent cumulative release, whereas the encapsulated drug kept diffusing past the 24-hour mark, reaching 39.43 percent, a 4.3-fold enhancement driven by the enormous oil-water interfacial area of the nanodroplets. In DPPH and ABTS radical-scavenging assays, the SME formulations outperformed free luteolin at equivalent concentrations, an effect the authors attribute partly to improved solubility and partly to TPGS itself, which contributed measurable radical-scavenging activity of its own. Dye dispersion and conductivity experiments traced how the formulation behaves in the gut: below 40 percent water it forms water-in-oil structures, transitions through a bicontinuous phase between 40 and 70 percent, and completes inversion to a gut-friendly oil-in-water microemulsion above 70 percent hydration.

The cellular story is where the design philosophy pays off. Using Caco-2 monolayers, the gold-standard model of the intestinal barrier, the team showed that luteolin-SME accumulated inside cells at significantly higher levels than free luteolin within 15 minutes, with uptake rising linearly for an hour before slowing. Cooling cells to 4 degrees Celsius slashed uptake, proving the process is energy-dependent, and a panel of endocytosis inhibitors pinpointed the routes: chlorpromazine and nystatin, which block clathrin-mediated and caveolae-mediated pathways respectively, both suppressed uptake, while amiloride, a macropinocytosis blocker, had no significant effect. The nanodroplets, in short, enter intestinal cells through the same receptor-driven machinery cells use to internalize nutrients.

The P-glycoprotein evidence is the study’s most layered contribution. Verapamil and cyclosporine A, two canonical efflux inhibitors, each boosted luteolin uptake, confirming the flavonoid is a genuine P-gp substrate. TPGS alone also increased intracellular luteolin, and a rhodamine 123 assay showed that luteolin-SME reduced P-gp efflux activity in a concentration-dependent manner. Western blotting then revealed something subtler: the formulation downregulated total P-gp protein expression, with the effect scaling with TPGS content, suggesting both immediate functional blockade and a longer-term suppression of the pump itself. Molecular docking simulations tied the mechanism together at the atomic level, showing TPGS and luteolin bind the same substrate pocket of P-gp with binding energies of minus 8.0 and minus 7.2 kilocalories per mole, both hydrogen-bonding to the GLN-191 and THR-195 residues. TPGS, in other words, acts as a competitive antagonist that physically occupies the pump’s recognition site.

Permeability and pharmacokinetics translated the cellular mechanisms into whole-body numbers. Across validated Caco-2 monolayers with transepithelial electrical resistance near 900 ohms per square centimeter, the apparent permeability coefficients of the SME formulations ran 2.8 to 3.5 times higher than free luteolin, aided by a transient, fully reversible 25 to 35 percent loosening of tight junctions that recovered within 24 hours. In mice given 200 milligrams per kilogram orally, the area under the plasma concentration-time curve soared from 8.628 to 247.729 milligrams per liter times hours, a 29-fold increase, while peak plasma concentration jumped 16-fold from 1.705 to 27.546 milligrams per liter. Mean residence time lengthened from 3.889 to 6.57 hours, the half-life extended, and clearance dropped dramatically. Fluorescent biodistribution imaging with the dye DiR showed the formulation lingering in the intestine at 10 hours, long after the free-probe signal had vanished, and concentrating more strongly in liver and kidney.

Therapeutically, the payoff appeared in inflammation models. In lipopolysaccharide-stimulated RAW264.7 macrophages, luteolin-SME cut nitric oxide, TNF-α and IL-6 by roughly 77, 49 and 48 percent relative to the inflamed group, restored catalase and superoxide dismutase activities, suppressed the lipid peroxidation marker MDA, and reduced reactive oxygen species far more effectively than free luteolin, while nudging macrophage morphology from a pro-inflammatory M1 state toward an anti-inflammatory M2-like phenotype and protecting mitochondrial membrane potential. In an LPS-induced sepsis model in mice, the formulation lowered serum TNF-α, IL-6 and IL-1β by about 37, 26 and 39 percent, reduced the liver enzymes ALT and AST by roughly 65 and 64 percent, and preserved near-normal hepatic architecture under the microscope. Biosafety testing showed no cytotoxicity across intestinal, liver and macrophage cell lines and hemolysis rates below 5 percent. The authors present the platform as a general solution for poorly soluble, efflux-limited natural products, a class that includes many of the flavonoids now being studied for chronic inflammatory disease, and the 29-fold bioavailability leap suggests the strategy deserves close attention from anyone trying to turn promising plant chemistry into medicine that actually reaches the bloodstream.

Subject of Research: A TPGS-functionalized self-microemulsifying drug delivery system that enhances the oral bioavailability and anti-inflammatory efficacy of the flavonoid luteolin by inhibiting P-glycoprotein efflux.

Article Title: Boosting luteolin bioavailability via P-glycoprotein efflux inhibition: a self-microemulsifying drug delivery systems

Article References: Zheng, Y., Chen, B., Huang, X., Ai, C., Teng, H., & Chen, L. (2026). Boosting luteolin bioavailability via P-glycoprotein efflux inhibition: a self-microemulsifying drug delivery systems. Journal of Advanced Research, 88, 1095-1117. https://doi.org/10.1016/j.jare.2026.01.030

Image Credits: AI Generated

DOI: 10.1016/j.jare.2026.01.030

Keywords: luteolin, self-microemulsifying drug delivery system, TPGS, P-glycoprotein, oral bioavailability, flavonoids, Caco-2 cells, pharmacokinetics, anti-inflammatory, antioxidant, nanocarriers, sepsis