The increase in oral availability of felodipine and other commonly used medications when taken with grapefruit juice has been assumed to be due to inhibition of CYP3A4, a cytochrome P450 that is present in liver and intestine. To evaluate the effect of repeated grapefruit juice ingestion on CYP3A4 expression, 10 healthy men were given 8 oz of grapefruit juice three times a day for 6 d. Before and after receiving grapefruit juice, small bowel and colon mucosal biopsies were obtained endoscopically, oral felodipine kinetics were determined, and liver CYP3A4 activity was measured with the [ 14 C N -methyl] erythromycin breath test in each subject.Grapefruit juice did not alter liver CYP3A4 activity, colon levels of CYP3A5, or small bowel concentrations of P-glycoprotein, villin, CYP1A1, and CYP2D6. In contrast, the concentration of CYP3A4 in small bowel epithelia (enterocytes) fell 62% ( P ϭ 0.0006) with no corresponding change in CYP3A4 mRNA levels. In addition, enterocyte concentrations of CYP3A4 measured before grapefruit juice consumption correlated with the increase in C max when felodipine was taken with either the 1st or the 16th glass of grapefruit juice relative to water ( r ϭ 0.67, P ϭ 0.043, and r ϭ 0.71, P ϭ 0.022, respectively). We conclude that a mechanism for the effect of grapefruit juice on oral felodipine kinetics is its selective downregulation of CYP3A4 in the small intestine. ( J. Clin. Invest. 1997. 99:2545-2553.)
This study indicates that intragastric pH and salt solubility-dissolution are important in the oral absorption of zinc. Specifically, the oxide salt is not an appropriate zinc salt to use in those patients with elevated intragastric pH.
The ability of high viscosity hydroxypropylmethylcellulose (HPMC) to reduce postprandial glucose concentrations was assessed in patients with non-insulin-dependent diabetes (NIDDM) and healthy volunteers. The study design consisted of a two-way crossover, single-dose administration of 10 g prehydrated high viscosity HPMC, or placebo, with a standard carbohydrate-rich meal. In patients with NIDDM, HPMC reduced blood glucose concentrations at the 60-, 75-, 90-, 120- and 150-min sampling intervals, with an average reduction in the maximum postprandial blood glucose concentration, Cmax, of 24% (P < 0.05). The time at which the maximum concentration was reached, Tmax, remained unchanged. The area under the blood concentration versus time plot, AUC0-6h, was reduced by an average of 15% (P < 0.05). The blood concentration profile of insulin followed that of glucose. Concentrations were significantly lower than in the placebo phase only at the 120-min sampling time, while pharmacokinetic parameters (Cmax, Tmax and AUC0-6h) were unchanged. These results suggest that alterations in the blood glucose profile are mediated by luminal events rather than by changes in hormonal response. In contrast to the NIDDM patients, neither the pharmacokinetic parameters nor the blood glucose concentrations at specific sampling times were significantly affected by the co-administration of HPMC in healthy volunteers. Overall, the results of this study suggest that HPMC may be a useful adjunct in the management of NIDDM.
Plasma zinc concentration is a useful method of evaluating oral zinc absorption from doses of 10 to 50 mg. Urinary zinc excretion is an alternative method of assessing zinc absorption, particularly when doses of 50 to 100 mg of elemental zinc are administered.
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