External

• Eyewash: 0.2% berberine solution, 2 drops in each eye, three times daily (Khosla et al 1992)

Clinical note — Berberine absorption

Berberine is poorly absorbed, with up to 5% bioavailability (Pan et al 2002). In vitro data has clearly demonstrated that berberine is a potent antibacterial; however, in vivo data has established low bioavailability. Berberine has been shown to upregulate the expression and function of the drug transporter P-glycoprotein (Pgp) (Lin et al 1999). Pgp belongs to the super family of ATP-binding cassette transporters that are responsible for the removal of unwanted toxins and metabolites from the cell (Glastonbury 2003). It appears that Pgp in normal

intestinal epithelia greatly reduces the absorption of berberine in the gut. In vivo and in vitro methods have been used to determine the role of Pgp in berberine absorption by using the known Pgp inhibitor cyclosporin A (Pan et al 2002). Coadministration increased berberine absorption six-fold and clearly demonstrated the role of Pgp in absorption.

Increased expression of Pgp can lead to cells displaying multi-drug resistance (Glastonbury 2003). As previously reported a certain flavonolignan in many Berberis spp. has the ability to inhibit the expression of multi-drug resistant efflux pumps (Stermitz et al 2000a, b) allowing berberine and certain antibiotics to be more effective.

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