Grapefruit juice increases felodipine oral availability in humans by decreasing intestinal CYP3A protein expression.

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 [14C 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 = 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 Cmax when felodipine was taken with either the 1st or the 16th glass of grapefruit juice relative to water (r = 0. 67, P = 0.043, and r = 0.71, P = 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.

Use of zinc tolerance test and 24-hour urinary zinc content to assess oral zinc absorption.

OBJECTIVES: The sensitivity of one plasma and two urinary methods to assess zinc absorption after oral dosing were compared over the dose range of 10 to 100 mg. METHODS: Eleven healthy subjects participated in this four-way crossover design study. After an overnight fast, the subjects received a single oral dose of zinc acetate corresponding to 10, 25, 50, or 100 mg of elemental zinc. Plasma zinc concentrations were measured at baseline (pre-zinc administration) and hourly intervals post-zinc administration for 9 hours. Urine was collected for 24 hours prior to and for 24 hours after zinc administration. During this 48-hour period, subjects consumed an isocaloric, caffeine-free diet containing 18 mg of elemental zinc per day. RESULTS: The area under the plasma zinc concentration versus time curve (PZAUC) increased linearly with doses between 10 and 50 mg, then flattened out. By contrast, urinary zinc excretion was approximately linear with doses in the 25 to 100 mg range, but no differences were observed in urinary zinc excretion after doses of 10 and 25 mg. CONCLUSIONS: 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.

Effect of intragastric pH on the absorption of oral zinc acetate and zinc oxide in young healthy volunteers.

BACKGROUND: Zinc is an important nutrient and is necessary to maintain a multitude of physiologic processes. Mineral supplements that provide physiologic doses of zinc may be used when dietary zinc is inadequate. Zinc is also used in pharmacologic doses to treat zinc deficiency and diseases such as Wilson's disease and acrodermatitis enteropathica. Although there are several zinc salts available, they are not equal in solubility, which is thought to be a key factor in zinc absorption. Moreover, the solubility of the salts is affected by pH, which may vary between pH 1 and 7 under various physiologic conditions in the stomach. The objectives of this 2-way 4-phase crossover study were to evaluate the effect of high (> or = 5) and low (< or = 3) intragastric pH on the absorption of zinc from the acetate and oxide salt in young healthy volunteers. METHODS: After a 9-hour fast, 10 healthy subjects (5 males and 5 females) were given a single oral dose of 50 mg of elemental zinc as the acetate or the oxide salt and under either high or low intragastric pH conditions. In all phases, a Heidelberg capsule pH detector-transmitter was used to continuously monitor intragastric pH. During the high pH phases, single oral doses of famotidine 40 mg oral suspension were administered before the zinc to raise the intragastric pH above 5. Intragastric pH < or = 3 was maintained in the low pH phases. RESULTS: The mean plasma zinc area under the curve for zinc acetate at low pH (AL), zinc acetate at high pH (AH), zinc oxide at low pH (OL), and zinc oxide at high pH (OH) were 524, 378, 364, and 66 micrograms x h/dL, respectively. The highest zinc plasma concentrations occurred with the acetate salt at a low intragastric pH, while the lowest plasma concentrations occurred with the oxide salt at a high intragastric pH. The importance of pH to the dissolution of these salts was verified by in vitro tests. Twenty-four-hour urinary zinc excretion was the highest for the AL phase and lowest for the OH phase. CONCLUSION: 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.

High viscosity hydroxypropylmethylcellulose reduces postprandial blood glucose concentrations in NIDDM patients.

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.

High-molecular-weight hydroxypropylmethylcellulose. A cholesterol-lowering agent.

BACKGROUND: We assessed the efficacy of a high-molecular-weight hydroxypropylmethylcellulose (K8515) as a cholesterol-lowering agent, the dose-response profile of its action, and the ability of adult subjects to tolerate its ingestion at effective doses. METHODS: These studies were conducted at the Clinical Research Center of The University of Michigan Hospitals, Ann Arbor. Efficacy was assessed in 10 normal and 12 mildly hyperlipidemic subjects in double-blind, randomized crossover trials of 1 and 2 weeks' duration, respectively. The dose-response profile was studied in 12 mildly hypercholesterolemic subjects in a nonrandomized control trial with doses given in escalating order. Tolerance was assessed by a questionnaire of adverse effects and bowel movement habits in all subjects. RESULTS: We found that 10 g of K8515 ingested in a prehydrated form three times a day with meals lowered total cholesterol levels by an average of 1.45 mmol/L (56 mg/dL) (32%) in normal subjects within 1 week. In two studies in subjects with mildly elevated cholesterol levels (with entry levels ranging from 5.35 mmol/L [207 mg/dL] to 6.70 mmol/L [260 mg/dL]), average reductions of 1.00 mmol/L (39 mg/dL) (18%) and 1.15 mmol/L (45 mg/dL) (20%) were observed within the same period. The effect was primarily due to a reduction in low-density lipoprotein cholesterol levels. Low-density lipoprotein levels in normal subjects were an average of 1.10 mmol/L (42 mg/dL) (38%) lower after a week of 10 g of K8515 three times a day with meals, and in the two studies in subjects with mild hyperlipidemia, the reductions in low-density lipoprotein levels after 1 week were 0.95 mmol/L (37 mg/dL) (23%) and 1.05 mmol/L (40 mg/dL) (25%). Although there was a tendency for high-density lipoprotein cholesterol levels to decrease, this was significant only in normal subjects. Decreases in cholesterol levels were not accompanied by any rise in triglyceride levels. Dose-response studies in those with mildly elevated cholesterol levels indicated that it is possible to achieve a 15% decrease in low-density lipoprotein cholesterol levels within 1 week at a dose of 6.7 g three times a day, with minimal adverse effects. CONCLUSION: These results suggest a role for high-molecular-weight hydroxypropylmethylcellulose in the clinical treatment of mild hypercholesterolemia.