To evaluate whether alterations in the multidrug-resistance (MDR)-1 gene correlate with intestinal MDR-1 expression and uptake of orally administered P-glycoprotein (PGP) substrates, we analyzed the MDR-1 sequence in 21 volunteers whose PGP expression and function in the duodenum had been determined by Western blots and quantitative immunohistology ( n = 21) or by plasma concentrations after orally administered digoxin ( n = 8 + 14). We observed a significant correlation of a polymorphism in exon 26 (C3435T) of MDR-1 with expression levels and function of MDR-1. Individuals homozygous for this polymorphism had significantly lower duodenal MDR-1 expression and the highest digoxin plasma levels. Homozygosity for this variant was observed in 24% of our sample population ( n = 188). This polymorphism is expected to affect the absorption and tissue concentrations of numerous other substrates of MDR-1.

As with grapefruit juice, a food product, physicians should also be aware of potential drug-herb interactions. The interaction of St John's wort extract with digoxin kinetics was time dependent. The mechanism involved may be induction of the P-glycoprotein drug transporter.

Extracts of St. John's wort (Hypericum perforatum) are used in treatment of depression. They contain various substances with the naphthodianthrones hypericin and pseudohypericin as characteristic ingredients. These compounds were shown to cause phototoxicity in cell culture and in animals. A placebo-controlled randomized clinical trial with monitoring of hypericin and pseudohypericin plasma concentration was performed to evaluate the increase in dermal photosensitivity in humans after application of high dose hypericum extracts. The study was divided into a single dose and a multiple dose part. In the single dose period, each of 13 volunteers received in a double blind fourfold complete crossover design, either placebo, or 900, 1800 or 3600 mg of a standardized hypericum extract (LI 160) containing zero, 2.81, 5.62 and 11.25 mg of total hypericin (total hypericin is the sum of hypericin and pseudohypericin). Maximum total hypericin plasma concentrations were observed about 4 h after dosage and were 0, 0.028, 0.061 and 0.159 mg/L, respectively. Before and 4 h after drug intake, the subjects were exposed at small areas of their back to increasing doses of solar simulated irradiation (SSI, with combined ultraviolet A, UV-A, and UV-B light) and another part was exposed to selective UV-A light irradiation. Minimal erythema dose was determined 5, 20 and 68 h after irradiation. Comparison of SSI sensitivity without and with hypericum extract did not show and difference and there was no dose-related trend in light sensitivity. Sensitivity to selective UV-A light was increased only after the highest dose from a minimal tanning dose of 10.8 J/ cm2 (mean) after placebo to 8.7 J/cm2 after 3600 mg extract with marginal statistical significance (p = 0.03 by one sided paired t-test). There was no correlation between total hypericin plasma concentrations and photosensitivity. In the multiple dose part, 50 volunteers received 600 mg hypericum extract t.i.d. with a daily dose of 5.6 mg of total hypericin. Comparison of UV light sensitivity before dosing with day 15 of treatment showed a slightly increased SSI sensitivity expressed by decrease of the MED from 0.17 to 0.16 J/cm2 (p = 0.005 by Wilcoxon test), and similarly, sensitivity to UV-A light increased (the mean tanning dose decreased from 9.9 to 7.8 J/cm2, p < 0.0001). This increase in cutaneous light sensitivity could be compensated by reducing irradiation time by 21%. Doses used in this study were higher than typical doses in current commercial preparations. In spite of these high doses in the double blind single dose part, frequency of side effects was equal to placebo medication and UV light sensitivity was not or only marginally increased. The study does not, however, exclude phototoxic reactions with doses above 11.25 mg of total hypericin and plasma levels above 100 micrograms/L. Furthermore, phototoxicity may be different after application of pure hypericin, since some constituents in the plant extract may exhibit protective effects.