Effect of Ritonavir on (99m)Technetium-Mebrofenin Disposition in Humans: A Semi-PBPK Modeling and In Vitro Approach to Predict Transporter-Mediated DDIs.

A semiphysiologically based pharmacokinetic (semi-PBPK) model was developed to describe a unique blood, liver, and bile clinical data set for the hepatobiliary imaging agent (99m)Technetium-mebrofenin ((99m)Tc-mebrofenin), and to simulate sites/mechanisms of a (99m)Tc-mebrofenin-ritonavir drug-drug interaction (DDI). The transport inhibitor ritonavir (multiple-dose: 2 × 300 mg) significantly increased systemic (99m)Tc-mebrofenin exposure as compared with control (4,464 ± 1,861 vs. 1,970 ± 311 nCi min/ml; mean ± SD), without affecting overall hepatic exposure or biliary recovery. A novel extrahepatic distribution compartment was required to characterize (99m)Tc-mebrofenin disposition. Ritonavir inhibited (99m)Tc-mebrofenin accumulation in human sandwich-cultured hepatocytes (SCH) (half maximal inhibitory concentration (IC50) = 3.46 ± 1.53 µmol/l). Despite ritonavir accumulation in hepatocytes, intracellular binding was extensive (97. 6%), which limited interactions with multidrug resistance protein 2 (MRP2)-mediated biliary excretion. These in vitro data supported conclusions from modeling/simulation that ritonavir inhibited (99m)Tc-mebrofenin hepatic uptake, but not biliary excretion, at clinically relevant concentrations. This integrated approach, utilizing modeling, clinical, and in vitro data, emphasizes the importance of hepatic and extrahepatic distribution, assessment of inhibitory potential in relevant in vitro systems, and intracellular unbound concentrations to assess transporter-mediated hepatic DDIs.CPT: Pharmacometrics & Systems Pharmacology (2013) 2, e20; doi:10.1038/psp.2012.21; advance online publication 2 January 2013.

In vitro-in vivo correlation of hepatobiliary drug clearance in humans.

The biliary clearance (Cl(biliary)) of three compounds was estimated using sandwich-cultured human hepatocytes (SCHH) and compared with Cl(biliary) values measured in vivo. Tc-99m sestamibi (MIBI) Cl(biliary) was determined in seven healthy volunteers using an oroenteric catheter to aspirate duodenal secretions, and gamma scintigraphy to determine gallbladder contraction; this technique was used previously to determine Tc-99m mebrofenin (MEB) and piperacillin (PIP) in vivo Cl(biliary). In vitro Cl(biliary) of MEB, MIBI, and PIP was quantified in SCHH as the ratio of mass excreted into bile canaliculi and area under the blood concentration-time curve (AUC) in medium. MIBI Cl(biliary) in vivo was 5.5+/-1.2 mL/min/kg (mean+/-SD). The rank order of Cl(biliary) predicted from SCHH corresponded well with the in vivo Cl(biliary) values in mL/min/kg for MEB (7.44 vs 16.1), MIBI (1.20 vs 5.51), and PIP (0.028 vs 0.032). In conclusion, the methods developed allowed for reproducible quantification of Cl(biliary) of drugs in healthy humans and prediction of Cl(biliary) from in vitro data.

Comparison of zafirlukast (Accolate) absorption after oral and colonic administration in humans.

PURPOSE: This study characterized the gastrointestinal (GI) absorption of zafirlukast after oral and colonic administration in humans. METHODS: Five healthy subjects received zafirlukast solution (40 mg) orally and via an oroenteric tube into the colon in a randomized, crossover fashion. Two additional subjects were dosed into the distal ileum. Serial blood samples were obtained and plasma concentrations were quantitated by HPLC. RESULTS: Mean +/- SD pharmacokinetic parameters after oral vs. colonic administration were: AUC infinity of 2076 +/- 548 vs. 602 +/- 373 ng x h/mL, respectively, and Cmax of 697 +/- 314 vs. 194 +/- 316 ng/mL, respectively. Mean colon:oral AUCalpha and Cmax were 0.29 and 0.30, respectively. Median tmax values were 2.0 and 1.35 hr after oral and colonic administration. First-order absorption rate constants (Ka and Kac) were estimated from a two-compartment model with first-order elimination. Kac:Ka was <0.5 in 4 of the 5 subjects dosed in the colon. CONCLUSIONS: Zafirlukast was absorbed at multiple sites in the GI tract. The rate and extent of zafirlukast absorption was less after colonic than oral administration. Zafirlukast was significantly absorbed in the distal ileum. This study demonstrated that gamma scintigraphy, digital radiography, and fluoroscopy can be used to track the movement and confirm the location of the oroenteric tube in the GI tract.

Acute myeloblastic leukemia following prolonged treatment of Crohn's disease with 6-mercaptopurine.

A 65-year-old man with Crohn's disease died of acute myeloblastic leukemia after treatment for 11.8 years with 6-mercaptopurine, 1.5 mg/kg/day (100 mg/day). On cytogenetic analysis, most of the malignant bone marrow cells had deletion of chromosome 7, the most frequently reported cytogenetic abnormality in chemotherapy-related acute leukemia. This finding, together with previous reports of acute leukemia and other malignancies following prolonged treatment with azathioprine or 6-mercaptopurine for nonmalignant conditions including inflammatory bowel disease, indicates that long-term use of these drugs for inflammatory bowel disease may increase the risk of malignancy. However, the magnitude of the risk is unknown.

Effect of pancreatico-biliary secretions and GI transit time on the absorption and pharmacokinetic profile of ranitidine in humans.

PURPOSE: Ranitidine plasma concentration vs. time profiles and the extent of ranitidine absorption were examined in the presence and absence of pancreatico-biliary secretions in order to elucidate factors which may contribute to secondary peaks after oral ranitidine administration. METHODS: Ranitidine solution (300 mg) was administered to 4 fasting healthy subjects via an indwelling small-bore oroenteric tube located approximately 16 cm distal to the pylorus On 3 consecutive days, subjects randomly received ranitidine alone (control), ranitidine 10 min after 0.04 micrograms/kg IV cholecystokinin (CCK) sufficient to cause gall bladder emptying into the duodenum, and ranitidine 30 min after inflation of an occlusive duodenal balloon located approximately 10 cm distal to the pylorus to prevent pancreatico-biliary secretions from reaching the dosing port or beyond. Small bowel transit time (SBTT; min) was measured by breath H2. Serial blood samples, obtained over 12 hours in each treatment, were analyzed by HPLC to determine ranitidine AUC0-12 (ng*h/mL), as well as Cmax (ng/mL) and Tmax (min) of the first and subsequent peaks, if subsequent peaks were observed. RESULTS: Ranitidine AUC0-12 and Cmax were not altered significantly by treatments; treatment effects on SBTT varied. Secondary peaks were observed in subjects #1 and #3 during the control treatment and subjects #2 and #4 during the CCk treatment. No secondary peaks were observed in any subject during the balloon treatment, and Tmax1 was delayed. CONCLUSIONS: Results support the hypothesis that pancreatico-biliary secretions (present in the intestinal lumen during control or CCK treatment) and gastrointestinal transit time may influence the occurrence of secondary peaks in ranitidine concentration vs. time profiles.

Nutritional issues in inflammatory bowel disease.

Malnutrition is a very common problem in patients with chronic inflammatory bowel diseases. This article discusses the incidence, causes, and clinical consequences of malnutrition in these patient groups. The role of nutritional support administered enterally or parenterally either as primary or adjunctive therapy is highlighted, based on past and more recent controlled studies. Additional attention is given to the roles of glutamine, short-chain fatty acids, fish oil, and alternative nutritional therapy.

Use of the InteliSite capsule to study ranitidine absorption from various sites within the human intestinal tract.

PURPOSE: The purpose of this study was to evaluate the extent of ranitidine absorption from an externally activated drug-delivery system in two distinct regions of the intestine (jejunum and ileum) in healthy human volunteers. This investigation also was designed to evaluate the utility of the InteliSite capsule for studying regional intestinal drug absorption in humans. METHODS: The intestinal absorption of ranitidine from the jejunum and ileum was compared in eight, healthy volunteers in this open-label, two-way crossover study. In two of the eight volunteers, absorption from the colon also was studied. Subjects swallowed the capsule containing ranitidine solution (121 mg) and 100 microCi of 99mTc-DTPA. The endcap of the capsule contained 20 microCi of (111)In-DTPA. At the desired intestinal site, the capsule was activated by the application of an external RF magnetic signal (6.78 MHz operating frequency) and the ranitidine solution was released. Blood samples were collected from a forearm vein for 12 hours after capsule activation. RESULTS: The capsule released the ranitidine solution when activated in the jejunum, ileum and colon (visualized by the gamma camera). There was no difference in the extent of ranitidine absorption or ranitidine pharmacokinetics when the capsule was activated in the jejunum or ileum. CONCLUSIONS: This study demonstrates the utility of a novel, externally activated drug-delivery system to assess site-specific intestinal drug absorption in humans. Results indicate that use of the InteliSite capsule method to evaluate site-specific intestinal ranitidine absorption in humans yields data similar to that obtained previously by means of oral intubation studies.

Intestinal effects of sulfate in drinking water on normal human subjects.

Uncontrolled observations implicate sulfate in drinking water at concentrations exceeding 500-700 mg/liter as a cause of diarrhea, but controlled studies have not been reported. We conducted a controlled study in normal adults to determine the effect of various drinking water sodium sulfate concentrations on bowel function. Ten healthy subjects were given a constant diet and fluid intake. Fluid consisted of 36 ml/kg/day of drinking water of various known sulfate concentrations and 500 ml of other fluid. In a dose-ranging study, four subjects received drinking water with sulfate concentrations of 0, 400, 600, 800, 1000, and 1200 mg/liters for six consecutive two-day periods. In a single-dose study, six other subjects received water with sulfate concentrations of 0 and 1200 mg/liter for two consecutive six-day periods. Stool mass, frequency, and consistency and mouth-to-anus appearance time of colored markers were measured. In the dose-ranging study, the only significant linear trend was decreasing mouth-to-anus appearance time with increasing sulfate concentrations. In the single-dose study, 1200 mg/liter sulfate caused a significant but clinically mild increase in mean stool mass per six-day pool from 621 g to 922 g (P = 0.03). When all 10 subjects were used to compare effects of 0 mg/liter and 1200 mg/liter sulfate, significant differences in stool consistency (P = 0.02) and transit time (P = 0.03) were observed. None of the subjects reported diarrhea or passed more than three stools per day. In 10 normal adult subjects, sulfate in drinking water at a concentration of 1200 mg/liter, which is higher than reported to occur in US municipal water sources, caused a measurable but clinically insignificant increase in stool mass and decrease in stool consistency and appearance time, but no change in stool frequency and no complaint of diarrhea.

Sumatriptan absorption from different regions of the human gastrointestinal tract.

Sumatriptan exhibits low oral bioavailability partly due to presystemic metabolism, which may vary with regional differences in metabolic activity throughout the gastrointestinal tract. This study evaluated sumatriptan absorption in humans after administration orally and by oroenteric tube into the jejunum and cecum. Because the site of cecal administration varied, pharmacokinetic parameters for sumatriptan and its major metabolite were compared statistically only after oral and jejunal administration. One-half of the oral dose was recovered in the urine as parent (3%) and metabolite (46%). Sumatriptan was absorbed throughout the gastrointestinal tract; absorption was similar after oral and jejunal administration, and less after cecal administration. The metabolite AUC and the AUC ratio (metabolite/parent) were significantly lower after jejunal compared to oral administration; the AUC ratio was two-fold lower after cecal administration. Results suggest that presystemic metabolism of sumatriptan varies throughout the gastrointestinal tract and/or regional differences exist in the absorption of metabolite formed within the gastrointestinal tract.

Marked resistance of normal subjects to tube-feeding-induced diarrhea: the role of magnesium.

The tolerance of healthy subjects to increasing rates of tube feeding was studied to better understand the etiology of diarrhea among tube-fed patients. Five volunteers were fed Osmolite HN by continuous duodenal infusion beginning at 314 kJ (75 kcal).kg body wt-1 x d-1 and progressing each 24 h until no longer tolerated. The five subjects were able to tolerate maximum 24-h infusions of 331-511 kJ.kg-1 x d-1 (198-340 mL/h). Diarrhea developed in only three subjects. Compared with nondiarrheal stools, the high-speed supernatant of the diarrheal stools had significantly higher concentrations of magnesium (192 +/- 22 mmol/L vs 139 +/- 17 mmol/L, P = 0.005), lower concentrations of potassium and phosphorus, and similar concentrations of calcium. The mean carbohydrate, fat, and nitrogen contents were not significantly different. We conclude that normal adult males are remarkably tolerant to duodenal infusion of this typical, isotonic tube-feeding product. The diarrhea that occurred in three of the volunteers at very high infusion rates appeared to be osmotic and attributable predominantly to magnesium.

Absorption of digoxin from tablets and capsules in subjects with malabsorption syndromes.

The relative steady-state bioavailability of two oral digoxin dosage forms was studied in 17 subjects with malabsorption syndromes. Male subjects received the following treatments in randomized crossover fashion for 14 days: three 0.125-mg digoxin tablets or three 0.1-mg digoxin capsules once daily. Female subjects received digoxin on the same schedule but at two-thirds the dose. Serum and urine samples were collected and analyzed for digoxin by radioimmunoassay, and treatments were compared by evaluating pharmacokinetic parameters. The mean area under the serum concentration versus time curve for tablets (28.1 h.nmol/L [21.9 h.ng/mL]) was smaller (p less than 0.03) than that for capsules (31.1 h.nmol/L [24.3 h.ng/mL]), and the mean maximum serum digoxin concentration for tablets (2.9 nmol/L [2.3 ng/mL]) was lower (p less than 0.02) than that for capsules (4.0 nmol/L [3.1 ng/mL]). There was no difference in cumulative urinary excretion of digoxin between the two treatments. In contrast to previous reports, we observed that digoxin from Lanoxin Tablets appears to be well absorbed in subjects with malabsorption. Nevertheless, these subjects absorbed digoxin from capsules better than from tablets, with the greatest differences occurring in subjects without a colon and in those subjects with the lowest serum carotene concentrations.

Effect of altering small bowel transit time on sustained release theophylline absorption.

The relationship between variations in small bowel transit time (SBTT) and the absorption of theophylline from a sustained-release product was evaluated in a three-way, randomized, crossover study in 12 healthy male nonsmokers. Subjects received sustained-release theophylline (600 mg) with loperamide (8 mg every 6 hour x 8 doses). metoclopramide (15 mg every 6 hour x 8 doses) or placebo (every 6 hour x 8 doses). Theophylline solution (400 mg) was used as a reference standard. Serum samples were collected periodically for 72 hours for theophylline concentration determinations. SBTT was measured by the lactulose hydrogen breath test. Compared with placebo (98 +/- 53 min), SBTT was increased with loperamide (211 +/- 87 min; P less than 0.001) and decreased with metoclopramide (55 +/- 18 min; P less than 0.001). Loperamide decreased the rate, but not the extent of theophylline absorption from this product. This was evident from the reduced Cmax, the prolonged Tmax, and the decreased fraction of the dose absorbed at 24 hours, while the area under the curves remained the same. In contrast, metoclopramide had no effect either on rate or extent of absorption. The data suggest that the effect of loperamide on these absorption parameters was due to an increase in the dissolution time of this sustained-release product.

Efficacy of tube feeding in supplying energy requirements of hospitalized patients.

During a 6-week period, all adult patients in a university hospital receiving ready-to-feed nasoenteric tube feeding formula were prospectively studied. The study objective was to determine each patient's caloric intake from tube feeding relative to their energy needs and to identify factors causing decreased feeding intake. Each of 35 patients was visited at least once daily to determine their volumetric intake of tube feeding formula. Daily review of patient care records and nursing interviews were used to identify interruptions in therapy. Patient's basal energy expenditures (BEE) were calculated using the Harris-Benedict equation. Calorie goals were set by members of the Nutrition Support Service or clinical dietitians. Intakes averaged 1095 +/- 41 Kcal (SEM) per day or 61% of their mean calorie goal of 1791 +/- 41 Kcal. Mean daily calorie intake was statistically different (p less than 0.05) from mean energy goal on patient study days 1 through 5, 7, and 8. Only 16 of the 35 patients achieved an intake of 100% of their energy goal on any day of therapy. Calorie goals averaged 1.4 times BEE. Mean daily calorie intake did not exceed BEE until study day 10. Eighteen % of potential feeding time was lost due to temporary feeding interruptions; primarily inadvertent extubation (4.6%), gastrointestinal intolerance (4.7%), medical procedures requiring discontinuation of feeding (2.8%), and feeding tube positioning difficulties (1.5%). In addition, physicians ordered only 75% of calculated energy goals. These data indicate that tube feeding therapy, when provided under usual hospital conditions, does not meet patient's energy requirements.

Effect of metoclopramide, bethanechol, and loperamide on gastric residence time, gastric emptying, and mouth-to-cecum transit time.

The effects of metoclopramide, bethanechol, and loperamide on the gastric residence time (GRT), gastric emptying (GE), and mouth-to-cecum transit time (MCTT) of a solution were investigated in three separate studies of five healthy male volunteers each. Metoclopramide in doses of 5, 10, and 15 mg prolonged GRT by 33, 88, and 162%, respectively, almost reaching statistical significance (p 0.058). A relationship was observed between GRT prolongation, and metoclopramide area under the plasma-time curve (p 0.01) and metoclopramide observed time to maximum concentration (p 0.01). Metoclopramide had an inconsistent effect on MCTT. Bethanechol 50 mg prolonged GRT by 64% (p 0.031) and had no effect on MCTT. Loperamide at doses of 2 and 8 mg prolonged GRT by 18 and 115% (p 0.043) and MCTT by 30 and 130% (p 0.0001), respectively. None of these motility-altering agents affected GE.

The assay of glandular kallikrein and prekallikrein in human mixed saliva.

The measurement of glandular kallikrein in biological fluids most often utilizes a synthetic substrate, H-D-valylleucylarginine-p-nitroanilide (S-2266), which assesses amidase activity. Although this substrate has reasonable specificity for glandular kallikrein, other tryptic-like proteases found in mixed saliva may also cause hydrolysis. The primary purpose of this study was to assess the accuracy of the use of this substrate for the measurement of glandular kallikrein in human mixed saliva. An additional objective was to determine the presence of prekallikrein in mixed saliva. The addition of soybean trypsin inhibitor (SBTI), which inhibits other tryptic-like enzymes but not glandular kallikrein, resulted in an approx. 30 per cent decrease in the hydrolysis of S-2266 by centrifuged mixed human saliva. A correlation of 0.918 was obtained between the biological assays for kinin release and amidase activity in 19 subject samples. Amidase activity increased following treatment of saliva with trypsin, indicating the presence of prekallikrein in human mixed saliva. It is concluded that S-2266 is an accurate substrate for the assay of glandular kallikrein in human mixed saliva; that the inclusion of SBTI in the assay mixture is needed to inhibit non-kallikrein proteases that may also hydrolyse the synthetic substrate; and that prekallikrein is present in mixed saliva. Thus any future studies of changes in the level of kallikrein in saliva may wish to consider the presence of both active and total levels of glandular kallikrein.

Efficacy of metoclopramide as an adjunct to duodenal placement of small-bore feeding tubes: a randomized, placebo-controlled, double-blind study.

We examined whether metoclopramide would improve the success rate of transpyloric intubation of a weighted Corpak feeding tube when fluoroscopic guidance is not used. Seventy patients were randomized in a prospective, double-blind fashion to receive either placebo (n = 35) or metoclopramide, 10 mg (n = 35) parenterally, administered immediately after the feeding tube was inserted. Tube location was determined independently by two observers who examined radiographs obtained after barium was instilled via the tube. There was no significant increase in the success rate of duodenal intubation in the total group following metoclopramide, 60%, compared to placebo, 49%. However, analysis of subgroups among the placebo-treated patients revealed that diabetes mellitus, but not other medical conditions, decreased the success rate for duodenal intubation, 20 vs 60% (p less than 0.05). Among diabetic patients, metoclopramide resulted in a significant increase in duodenal placement compared to placebo (p less than 0.05). We conclude that parenteral metoclopramide significantly increases the frequency of transpyloric intubation with small feeding tubes without fluoroscopic guidance in diabetic patients but not in nondiabetic patients.