Solid-phase extraction and determination of ranitidine in human plasma by a high-performance liquid chromatographic method utilizing midbore chromatography

An improved high-performance liquid chromatographic (HPLC) method utilizing solid-phase extraction (SPE) and midbore chromatography was developed for the determination of ranitidine in human plasma. A mobile phase of 20 mM K2HPO4-acetonitrile-triethylamine (87.9:12.0:0.1, v/v) pH 6.0 was used with a phenyl analytical column and ultraviolet detection (UV). The method demonstrated linearity from 25 to 1000 ng/ml in 500 microliters of plasma with a detection limit of 10 ng/ml. The method was utilized in a pharmacokinetic study evaluating the effects of pancreatico-biliary secretions on ranitidine absorption.

Vancomycin serum concentrations in patients with renal dysfunction: a comparison of fluorescence polarization immunoassay and the enzyme-multiplied immunoassay technique

A study was conducted to determine whether assay-specific quantitative differences exist in the determination of vancomycin serum concentrations obtained from patients with renal dysfunction. Vancomycin serum concentrations were obtained during the first week of therapy for each of three time intervals: 48-96 h, 96-144 h, and 144-192 h after administration of the first dose of vancomycin. Vancomycin serum concentrations were measured using the enzyme-multiplied immunoassay technique (EMIT) and fluorescence polarization immunoassay (FPIA). Twenty patients with an estimated creatinine clearance < 40 ml/min who were receiving intravenous vancomycin were evaluated. Hemodialysis was required in 16 of 20 patients. Fifty samples were included in the data analysis. The mean (+/-SD) serum concentrations obtained with EMIT and FPIA were 10.9 mg/L (+/-5.3) and 12.6 mg/L (+/-5.7), respectively (p = 0.13), and were not statistically different. A linear relationship was observed between EMIT and FPIA (EMIT = 0.89 x FPIA - 0.24; r2 = 0.93). No statistically significant differences were observed in the calculated pharmacokinetic parameters between methods. FPIA and EMIT are comparable methods in determining vancomycin serum concentrations within the first week of vancomycin therapy in patients with moderate to severe renal dysfunction.

Evidence for reversible sequestration of morphine in rat liver

The residence of morphine in the systemic circulation is prolonged despite a high systemic clearance, suggestive of significant extravascular sequestration. The present study was conducted to test the hypothesis that morphine binds significantly in tissues, and that the liver plays an important role in morphine binding. [14C]Morphine was administered to male Sprague-Dawley rats 55 min before unlabeled morphine or saline. Blood 14C increased immediately after injection of unlabeled morphine; the area under the blood concentration-time curve (AUC) for 14C increased approximately 2-fold after morphine compared with saline injection. Residual radioactivity in the liver was lower in morphine-treated rats than in controls, suggesting that unlabeled drug displaced [14C]morphine (or a metabolite) from binding sites. To examine this phenomenon more directly, a recirculating isolated perfused liver system was employed. [14C]Morphine was added to the perfusate reservoir 15 min before unlabeled morphine or saline; perfusate and bile samples were collected for 120 min. Upon termination of perfusion, the liver was fractionated to identify the hepatic subcellular fraction(s) in which morphine was sequestered. The perfusate AUC for [14C]morphine was increased approximately 2-fold in response to unlabeled drug, consistent with the in vivo experiment. Morphine was associated preferentially with the cytosolic fraction, and [14C]morphine in all relevant fractions was reduced after administration of unlabeled morphine. In contrast, unlabeled drug had no influence on derived [14C]morphine-3-beta,D-glucuronide. These data are consistent with significant, reversible binding of morphine in hepatic tissue.

Maternal-fetal pharmacokinetics of methanol

We undertook the present project to elucidate the physiologic factors that govern methanol delivery to the developing conceptus after maternal methanol exposure, and to develop a physiologically based toxicokinetic model to describe methanol disposition in pregnancy. A multi-experimental approach addressed the goals of this project. Initial experiments characterized the systemic disposition of methanol after intravenous or oral administration to nonpregnant female rats. Methanol absorption from the gastrointestinal tract was rapid (peak concentrations appeared within 1 to 2 hours after administration) and essentially complete (systemic bioavailabilities ranged from approximately 0.6 to 1.0). As anticipated for short-chain aliphatic alcohols, methanol elimination from the systemic circulation was nonlinear due to saturation of the metabolic route or routes responsible for converting methanol to formaldehyde and, ultimately, formic acid. However, a significant parallel linear route of methanol elimination was observed, which accounted for an increasingly significant fraction of total elimination as methanol doses (or systemic concentrations) increased. The disposition of methanol after oral or intravenous administration was similar in pregnant and nonpregnant female rats, regardless of the gestational stage (day 7, 14, or 20 after conception) at which the toxicokinetics of methanol were examined. This observation indicated that data from nonpregnant subjects could be used in the development of the maternal portion of a comprehensive physiologic model for methanol disposition. Parallel experiments in female mice indicated that methanol elimination was approximately twice as rapid in mice as in rats due to a significantly higher maximal velocity for methanol metabolism in the smaller rodent species. As was the case in the rat, relatively small changes in methanol elimination were observed during the course of gestation in pregnant mice. In both species, the rate of methanol metabolism by fetal liver in vitro was less than 10% that of the metabolic rate in adult liver. The kinetics of methanol delivery into the fetal environment were examined by determining amniotic fluid concentrations of methanol after intravenous administration to pregnant rats. The net rate of methanol translocation from maternal blood to amniotic fluid decreased as methanol concentration increased. Although the mechanism of this anomalous result is unknown, it possibly is due to a methanol-induced decrease in blood flow to the fetus.

Physiologic pharmacokinetic modeling of gastrointestinal blood flow as a rate-limiting step in the oral absorption of digoxin: implications for patients with congestive heart failure receiving epoprostenol

A previously validated physiologically based pharmacokinetic model was used to examine whether epoprostenol-induced increases in gastrointestinal blood flow (Qg) could alter digoxin systemic bioavailability to a clinically significant extent in severe congestive heart failure (CHF) patients. A series of simulations was conducted in which the influences of apparent gut tissue-to-plasma partition coefficient (Kg) and Qg on digoxin bioavailability were evaluated. Since epoprostenol also increases blood flow to the liver and kidneys, the effect of concurrent increases in regional blood flow to these organs on digoxin bioavailability also was evaluated. A range of Qg was studied from 25 L/h (assumed mesenteric arterial flow in CHF) to 65 L/h (portal venous flow in normal adults), and the area under the simulated digoxin concentration-time curve was used to calculate absolute digoxin bioavailability in each case. Simulations were conducted at a range of Kg from 1 to 50 (physiologically relevant range 5-25). At low values of Kg, the influence of changes in Qg on digoxin bioavailability was minimal. However, as apparent distribution into gut tissue increased (consistent with visceral congestion), the effect of changes in Qg was more substantial. In the physiologically relevant range of Kg, 40-160% increases in Qg were associated with approximately 6-40% increases in digoxin bioavailability. Therefore, the decrease in digoxin oral clearance previously observed in CHF patients receiving epoprostenol may be ascribed to increases in digoxin bioavailability, secondary to epoprostenol-induced increases in Qg.

Hepatobiliary disposition of valproic acid and valproate glucuronide: use of a pharmacokinetic model to examine the rate-limiting steps and potential sites of drug interactions

Previous work in this laboratory has suggested that the nonlinear disposition of valproic acid (VPA) in the rat may be due to nonlinear distribution of VPA into the liver. The present study was undertaken to elucidate further the hepatobiliary disposition of VPA. VPA (0.1-2 mmol/L) was incubated with isolated rat hepatocytes in vitro. Uptake of [(3)H]-VPA was linear from 10 to 50 seconds, with minimal (<7 percent) biotransformation. The initial velocity of VPA uptake varied in proportion with the extracellular concentration and was temperature independent, suggesting that VPA traverses the hepatocyte membrane predominantly by passive diffusion. In separate studies, the hepatobiliary disposition of VPA (20mg) was examined in the isolated perfused rat liver (IPL). A pharmacokinetic model was developed to describe the influence of phenobarbital on the hepatobiliary disposition of VPA and valproate glucuronide (V-G) in the IPL; all processes governing VPA and V-G disposition appeared to be linear. Acute administration of phenobarbital to the liver (1.12 mg) decreased the rate constant for canalicular egress of V-G (0.0489 +/- 0.0266 vs. 0.164 +/- 0.075 min(-1)). In vivo pretreatment with phenobarbital (75 mg/kg/d x 5 d) before liver isolation decreased the biliary excretion of both VPA (1.06E-04 +/- 0.27E-04 vs. 2.76E-04 +/- 0.45E-04 min(-1)) and V-G (5.63E- 03 +/- 1.98E-03 vs. 1.74E-02 +/- 0.5E-02 min(-1)), and increased the apparent volume of distribution of VPA (84.6 +/- 2.2 vs. 72.3 +/- 2.1 mL). In vivo phenobarbital pretreatment a changed V-G excretion from a formation to an elimination rate-limited process. These results are consistent with phenobarbital-associated impairment of canalicular egress of some organic anions. This work further supports the utility of pharmacokinetic modeling in: (1) determining the rate-limiting steps in hepatobiliary drug disposition and (2) identifying sites of drug interactions within the hepatobiliary system that may not be evident based on conventional mass-balance analysis.

Age-related changes in valproic acid binding to rat serum proteins in vitro

The effect of age on the in vitro binding of valproic acid (VPA) to serum proteins was investigated in rats ranging in age from 14 days (preweaning) to 24 months (senescent). The influence of free fatty acid (FFA) and total protein (TP) concentrations on age-related changes in binding was examined. The protein binding of VPA was altered during development and aging. The VPA fraction unbound (fu) at low VPA concentrations was significantly higher in older age groups (12 and 24 months old; fu = 0.26-0.30) than in younger animals (14, 20, and 40 days old; fu = 0.16-0.18). Binding was best described by a model incorporating a saturable and a nonsaturable binding site. Binding affinity at the saturable binding site was lowest at the extremes of age. Changes in binding at either the saturable or the nonsaturable site were not predicted by changes in TP or FFA with age. Changes in nonsaturable binding were marginally associated with age (p = 0.0952). A 3-fold increase in FFA concentrations was necessary to produce a 1.5-fold increase in VPA fu. There was less than a 2-fold difference in FFA concentrations between the age groups (range 0.219-0.379 mmol/L). Thus, the difference in FFA concentrations between the age groups may not have been large enough to cause measurable differences in displacement of VPA from binding sites. Changes in protein binding may contribute to age-related changes in disposition of VPA observed in the rat. Changes in the serum concentrations of specific FFA or proteins may play a role in the altered VPA binding with age, but changes in total FFA or protein concentrations do not account for the age-related differences observed. Further investigation is required to identify the mechanism(s) responsible for age-related changes in binding of VPA to serum proteins measured in vitro.

Pharmacokinetics and bioavailability of zidovudine and its glucuronidated metabolite in patients with human immunodeficiency virus infection and hepatic disease (AIDS Clinical Trials Group protocol 062)

The pharmacokinetics of zidovudine (ZDV) are established in patients with various stages of human immunodeficiency virus (HIV) disease. This study was conducted to determine the pharmacokinetic parameters of ZDV in patients with asymptomatic HIV infection and liver disease. HIV-infected volunteers with normal renal function were stratified according to the severity of liver disease (seven of eight were classified as mild). Each subject received a single intravenous dose of ZDV (120 mg) on the first day, followed by a single oral dose of ZDV (200 mg) on the second day. Blood samples were obtained over a 8-h collection interval, and concentrations of ZDV and its glucuronidated metabolite (GZDV) were determined by high-performance liquid chromatography. The following pharmacokinetic parameters were obtained after oral administration of ZDV to HIV-infected patients with mild hepatic disease; these values were compared with previously reported data in healthy volunteers. The area under the curve (AUC) (1,670 +/- 192 ng.h/ml), maximum concentration of drug in serum (1,751 +/- 180 ng/ml), and half-life (2.04 +/- 0.38 h) of ZDV were increased, while the apparent oral clearance (1.57 +/- 0.31 liter/h/kg of body weight) was decreased; AUC (7,685 +/- 1,222 ng.h/ml) and maximum concentration of drug in serum (5,220 +/- 1,350 ng/ml) of GZDV and the AUC ratio of GZDV to ZDV (2.79 +/- 0.43) after oral administration were decreased. ZDV absolute bioavailability was 0.75 +/- 0.15 in HIV-infected patients with hepatic disease. Although the ZDV apparent oral clearance was not impaired as significantly as in patients with biopsy-proven cirrhosis, our results suggest that ZDV, could accumulate in HIV-infected patients with mild hepatic disease because of impaired formation of GZDV. Patients with mild hepatic disease may require dosage adjustment to avoid accumulation of ZDV after extended therapy.

Intravenous epoprostenol sodium does not increase hepatic microsomal enzyme activity in rats

Previous studies have indicated that epoprostenol may increase hepatic microsomal enzyme activity both in animals and humans. However, interpretation of the results of these studies may be confounded by the route of epoprostenol administration or small sample sizes. The primary objective of the present investigation was to evaluate the effects of epoprostenol (given as a continuous intravenous infusion) on hepatic microsomal enzyme activity in rats. Male Sprague Dawley rats (220-290 g) received infusions of either vehicle (glycine buffer, 1 mL/hr) or 0.2 microgram/kg/min epoprostenol through a jugular vein cannula for 24 hr or 7 days. At the end of the infusion, a 25 mg/kg i.v. bolus of antipyrine was administered and blood samples were collected over 6 hr. Serum antipyrine concentrations were determined by HPLC. Twenty-four hr post-infusion, hepatic microsomes were prepared, and cytochrome P-450 content was determined by difference spectroscopy. Cytochrome P-450 content and antipyrine clearance values determined from serum antipyrine concentration-time profiles were not significantly different between treatment groups. Antipyrine clearance [mean (SD)] in the 24-hr vehicle-treated group was 3.68 (0.49) mL/min/kg versus 4.35 (1.1)mL/min/kg in the epoprostenol-treated group. In the 7-day vehicle-treated rats, antipyrine clearance was 5.43 (1.0) mL/min/kg compared to 4.68 (0.61) mL/min/kg in epoprostenol-treated rats. A statistically significant effect of infusion duration was observed in the control group, i.e., antipyrine clearance in rats treated with vehicle for 7 days was significantly greater than that observed in rats treated with vehicle for 24 hr. However, the increase was less than 50%. These data suggest that when epoprostenol is administered as an intravenous infusion to rats, no significant alterations in hepatic microsomal enzyme activity occur. Based on these data, long term changes in hepatic metabolism in response to chronic epoprostenol administration are not expected.

Gastrointestinal transit and distribution of ranitidine in the rat

PURPOSE

Ranitidine gastrointestinal distribution was examined in the rat small intestine after oral administration to determine whether intestinal transit or secretion (exsorption) may influence the appearance of secondary peaks in ranitidine serum concentration-time profiles.

METHODS

Male Sprague-Dawley rats received ranitidine (50 mg/kg) by oral gavage, and the mass of ranitidine recovered in all small intestinal segments (approximately 12 cm each) was determined 30, 60, 90, or 120 min after administration. In a separate group of anesthetized rats, the small intestine was divided into two segments of equal length that were perfused with normal saline in a single-pass manner. Rats received an escalating, zero-order IV infusion of ranitidine for 30 min, and venous blood and intestinal effluent were collected over 90 min to quantitate ranitidine exsorption.

RESULTS

Thirty min after oral administration, > 50% of the recovered ranitidine mass resided in the lower half of the small intestine in all rats. Ranitidine mass in 5 of 16 rats displayed a bimodal distribution with significant amounts of ranitidine recovered from the stomach 60 to 90 min after dosing. Ranitidine exsorption was more efficient from the lower jejunum and ileum than from the duodenum and upper jejunum. However, intestinal secretion of ranitidine was minor (5% of the IV dose).

CONCLUSIONS

Ranitidine absorption from the lower ileum contributes significantly to systemic ranitidine concentrations before and during the time of the first concentration maximum. Separation of the drug mass into multiple boluses may contribute to secondary peaks in ranitidine concentration-time profiles. Exsorption did not contribute significantly to ranitidine distribution in the gastrointestinal tract.

Regional gastrointestinal absorption of ranitidine in the rat

PURPOSE

Ranitidine absorption from isolated segments of rat small intestine (duodenum, midgut, and terminal ileum) was investigated to examine the influence of pH and 50% bile, and to determine if ranitidine is absorbed preferentially from a specific region.

METHODS

Ranitidine (50 mg/kg) was administered into each segment in pH 5 or pH 7 buffer, or in 50% bile. Venous blood was collected at various times for 40 min from the right jugular vein.

RESULTS

When ranitidine was administered in pH 7 buffer or in 50% bile, Cmax and AUC0-40 were significantly greater after administration into the terminal ileum compared to the duodenum and midgut. AUC0-40 was significantly greater when ranitidine was administered in pH 5 buffer or in 50% bile into the duodenum compared to the midgut. Cmax was significantly different between administration into the duodenum and midgut only when ranitidine was administered in 50% bile. Ranitidine administration in pH 5 buffer significantly decreased AUC0-40 and Cmax after administration into the midgut, and AUC0-40 after administration into the terminal ileum compared to administration with pH 7 buffer or in 50% bile. Bile had no significant effect on AUC0-40 after ranitidine administration into the duodenum and midgut compared to administration in pH 7 buffer. However, bile significantly increased AUC0-40 and Cmax after ranitidine administration into the terminal ileum compared to administration with pH 7 and pH 5 buffer.

CONCLUSIONS

Results suggest that ranitidine is absorbed from the entire small intestine. However, the terminal ileum is the optimal site of gastrointestinal absorption. Furthermore, bile enhances ranitidine absorption from the terminal ileum.

High-performance liquid chromatographic evaluation of the effect of heat treatment on trimethoprim and sulfamethoxazole stability in serum

Heat treatment of patient samples is utilized as a method to decrease the risk of accidental transmission of human immunodeficiency virus (HIV). Heat treatment has been reported to affect the outcome of drug analysis. In this study, the effects of heat treatment (56 degrees C for 5 h) and storage for 2 months at -20 degrees C on the stability of trimethoprim (TMP) and sulfamethoxazole (SMX) at three different concentrations in serum (10/100, 1/20, and 0.1/5 microgram/mL) each were evaluated. Simultaneous determination of TMP, SMX, and sulfamethazine (SMeth), the internal standard, in serum was performed by a reversed-phase high-performance liquid chromatographic (HPLC) procedure with isocratic elution and ultraviolet detection. The peak/height ratios (PHRs) for each sample from untreated and heat-treated groups were compared. No statistically significant differences were found between untreated and heat-treated groups for TMP. Heat treatment decreased the PHR for SMX at 100 micrograms/ml concentration (p = 0.042) and increased the PHR for SMX at 20 micrograms/ml concentration (p = 0.049). These marginal differences are unlikely to be significant. Storage of samples for 2 months at -20 degrees C had no statistically significant effect on sample PHRs. Thus, heat treatment of serum does not alter clinical interpretation of TMP and SMX at clinically relevant concentrations and may protect laboratory workers from accidental HIV exposure.

Radiolabeling of methylphosphonate and phosphorothioate oligonucleotides and evaluation of their transport in everted rat jejunum sacs

PURPOSE

The therapeutic use of antisense oligonucleotides will likely involve their administration over protracted periods of time. The oral route of drug dosing offers many advantages over other possible routes when chronic drug administration is necessary. However, little is known about the potential for oligonucleotide uptake from the gastrointestinal tract. This issue is addressed in the current work.

METHODS

We have developed a simple procedure for radiolabeling oligonucleotides by reductive alkylation with 14C-formaldehyde. We have utilized this approach, as well as 5' addition of fluorophores, to prepare labeled methylphosphonate and phosphorothioate oligonucleotides for use in intestinal transport studies. An everted rat gut sac model was employed to compare the transport of oligonucleotides to that of model compounds whose permeation properties are better understood.

RESULTS

We demonstrate that both methylphosphonate and phosphorothioate oligonucleotides are passively transported across the intestinal epithelium, probably by a paracellular route. The rates of transport for both types of oligonucleotides were similar, and were significantly greater than that of the very high MW polymer blue dextran, but were lower than the transport rate of valproic acid, a low MW compound known to have high oral availability.

CONCLUSIONS

A significant degree of permeation of oligonucleotides across the gastrointestinal epithelium does occur, but it is still unclear whether this is sufficient to permit effective oral administration of oligonucleotides as drugs.

Age-dependent intestinal hydrolysis of valproate glucuronide in rat

1. Age-dependent differences in the intestinal hydrolysis of the glucuronide conjugate of valproic acid were evaluated in the Fisher-344 rat at 14 and 40 days, and 24 months of age. 2. Hydrolysis occurred more quickly when incubations were conducted under anaerobic as compared with aerobic conditions. 3. The rate of hydrolysis of valproate glucuronide was most rapid in the contents of the large intestine (caecum and colon); no difference in rate was noted between age groups during incubations with large intestinal contents. 4. Hydrolysis in the tissues of the large and small intestines, and the contents of the small intestine, was more rapid in the 14-day-old rat than in the older age groups. Differences in the rates and sites of hydrolysis in the 14-day-old animal may be due to regional differences in the number and types of microorganisms or mammalian beta-glucuronidase present in the gastrointestinal tract. 5. Differences in intestinal hydrolysis of valproate glucuronide may account in part for age-related changes in enterohepatic recirculation of valproate in young animals; other mechanisms apparently are responsible for altered valproate disposition in senescent animals.

Age-dependent intestinal absorption of valproic acid in the rat

The absorption of valproic acid (VPA) across isolated perfused segments of jejunum, ileum and colon was examined in situ in 14-day- to 24-month-old Fischer-344 rats. Within each age group, the intrinsic absorptive clearance (Cla) of VPA at a perfusate concentration of 1 mg/ml was highest in the jejunum, lowest in the colon, and intermediate in the ileum. When intestinal Cla was normalized for the dry weight of the segment, within-group variability decreased. In all segments, VPA Cla normalized by dry weight decreased during development (< or = 20 to 90 days) and remained relatively constant during aging (90 days to 24 months). The mechanism of valproate absorption (active vs. passive) was examined across age in everted intestinal sacs prepared from each of the three segments. Data were consistent with active transport of VPA in the jejunum and ileum of rats of all ages, and in the colon of pre-weanling animals. Colonic absorption of VPA appeared to occur by passive diffusion in adult rats. In contrast, colonic absorption of d-glucose occurred, only by passive diffusion in all age groups. These data indicate that, during development, significant alterations in the rate of VPA absorption occur throughout the rat intestine. Furthermore, while active transport of VPA by the small intestine was present throughout the age range investigated, active transport by the colon became negligible by the time of weaning.

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.

Effects of probenecid on the pharmacokinetics and pharmacodynamics of adinazolam in humans

The effects of probenecid (2 gm) on the pharmacokinetics, pharmacodynamics, and uricosuric effects of adinazolam and N-desmethyladinazolam were assessed after single dose administration of adinazolam mesylate sustained-release tablets (60 mg) in a randomized, four-way crossover, double-blind study involving 16 healthy male volunteers. Probenecid decreased adinazolam oral clearance, renal N-desmethyladinazolam clearance, and the amount of N-desmethyladinazolam excreted in the urine. Probenecid increased the N-desmethyladinazolam/adinazolam AUC ratio, adinazolam maximum concentration (Cmax), N-desmethyladinazolam Cmax, and N-desmethyladinazolam time to reach Cmax. Uric acid renal clearance was increased significantly by adinazolam or probenecid administration compared with placebo; however, coadministration of adinazolam plus probenecid had no additive effect on uric acid clearance. Psychomotor performance was decreased in the adinazolam plus probenecid treatment compared with the adinazolam treatment. Probenecid potentiated the psychomotor effects of adinazolam after coadministration of the compounds, predominantly because of alterations in N-desmethyladinazolam pharmacokinetics. Therefore the adinazolam dose may need to be reduced when coadministered with probenecid.

Bile flow but not enterohepatic recirculation influences the pharmacokinetics of ranitidine in the rat

Secondary peaks in oral concentration-time profiles following ranitidine administration may be due to discontinuous absorption along the gastrointestinal tract, postabsorptive storage and release, and/or enterohepatic recirculation (ER). The suitability of the rat as an animal model for studying mechanisms of the double peaks, and the relationship between ER and the occurrence of secondary peaks in ranitidine concentration-time profiles, were examined in the present investigation. Male Sprague-Dawley rats received ranitidine by oral gavage (50 mg/kg), and blood was collected at various times for 6 hr after dosing. Eight rats with chronic bile duct and jugular vein cannulae received ranitidine with bile flow intact or interrupted in a randomized complete cross-over design. Bile duct-cannulated (BDC) rats were divided into two groups: four rats received ranitidine immediately after bile flow interruption, and four rats received ranitidine 3 hr after bile flow interruption. Blood and bile were analyzed for ranitidine by HPLC. The area under the ranitidine concentration-time profile, the maximum serum ranitidine concentration, the time of maximum concentration, the fraction of ranitidine absorbed at each blood sample, biliary clearance, and the percentage of the dose recovered in bile as ranitidine were determined. Results indicated that the rat is an appropriate model for studying mechanisms responsible for the double peaking phenomenon. Multiple peaks or plateaus were observed in the ranitidine concentration-time profiles of all rats after oral administration with bile flow intact. Secondary peaks were evident in only two concentration-time profiles of BDC rats when bile flow was interrupted. Less than 3% of the dose was recovered in the bile as ranitidine or metabolites.(ABSTRACT TRUNCATED AT 250 WORDS)

Venous irritation related to intravenous administration of phenytoin versus fosphenytoin

STUDY OBJECTIVE

To compare the frequency, severity, and time course of venous irritation after administration of a single intravenous dose of phenytoin with an equimolar dose of fosphenytoin, a water-soluble phenytoin prodrug.

DESIGN

Randomized, double-blind, two-period, crossover study.

SETTING

University hospital clinical research unit.

PATIENTS

Twelve healthy volunteers within 15% of ideal body weight and with no clinically significant abnormalities on physical examination, medical history, or laboratory assessment.

INTERVENTIONS

Volunteers randomly received a 30-minute infusion of phenytoin sodium 250 mg (250 mg/5 ml) or an equimolar dose of fosphenytoin 375 mg (375 mg/5 ml). Subjects returned for the crossover treatment 14-21 days later.

MEASUREMENTS AND MAIN RESULTS

Subjects assessed venous irritation (pain, burning, itching), and investigators evaluated phlebitis (erythema, swelling, tenderness), induration, exudation, and cording. Phenytoin was associated with a significantly higher degree of pain at the infusion site in all subjects and a significant degree of phlebitis in eight subjects (p < 0.05); cording occurred in six subjects. The time course of phenytoin-induced phlebitis was bimodal. Erythema and tenderness were prominent at the end of the infusion and again at 24 hours. Cording was first noted between 24 hours and 1 week after infusion. In contrast, fosphenytoin was associated with mild pain in two subjects, one incident of phlebitis, and no erythema or cording.

CONCLUSIONS

Fosphenytoin administration resulted in significantly less venous irritation and phlebitis compared with an equimolar dose of phenytoin. The clinical use of this water-soluble phenytoin prodrug should minimize the frequency and severity of infusion-site reactions and should allow convenient, rapid, intravenous administration of drug, undiluted or admixed with intravenous solutions.

Site-dependent intestinal hydrolysis of valproate and morphine glucuronide in the developing rat

A previous pharmacokinetic study in developing rats suggested that enterohepatic recirculation of valproic acid was absent prior to weaning. One explanation for this observation is that the rate, extent, and/or primary site of glucuronide hydrolysis in the gastrointestinal tract changes during postnatal development. To test this hypothesis, the hydrolysis of two model glucuronide conjugates, valproate glucuronide and morphine-3-beta,D-glucuronide, was examined in vitro in homogenates of small and large intestine obtained from rats at 5-60 days postpartum. Analysis of initial hydrolysis rates indicated that the principal hydrolytic site for both glucuronide conjugates shifted from the upper to lower intestine as the animals developed. The initial hydrolysis rate (nmol/min/g) for valproate glucuronide decreased from 38.1 +/- 10.2 to 8.25 +/- 2.42 in the small intestine, and increased from 14.2 +/- 2.3 to 105 +/- 22 in the large intestine, as rats developed from 5 to 60 days postpartum, respectively. Likewise, the intestinal hydrolysis rate for morphine-3-beta,D-glucuronide decreased from 3.70 +/- 0.46 to 0.646 +/- 0.165 in the small intestine, and increased from 3.50 +/- 0.48 to 115 +/- 30 in the large intestine, as rats developed from 5 to 60 days postpartum, respectively. If hydrolysis occurs immediately after excretion of conjugate into the intestine in neonatal rats, minimal temporal delay between excretion of conjugate and reabsorption of liberated parent may occur, therefore concealing the secondary increase in serum drug concentrations associated with enterohepatic recirculation. In contrast, the time required for conjugates to reach the primary hydrolytic site in adult animals is sufficient for appearance of secondary peaks in the serum drug concentration-time profile.

Acute phenobarbital administration alters the disposition of acetaminophen metabolites in the rat

The effects of acute phenobarbital (PB) administration on the disposition of acetaminophen (APAP), acetaminophen glucuronide (AG), and acetaminophen sulfate (AS) were examined in serum, bile, and urine of rats after a 100 mg/kg iv bolus dose of APAP. PB was administered intravenously as either an acute low (12 mg/kg) or high (60 mg/kg) dose to achieve PB serum concentrations equivalent to, or 5-fold higher than, PB concentrations in previous studies where impaired biliary excretion of AG and AS was noted after PB pretreatment for 5 days. Acute high-dose PB administration decreased the formation clearance of AG by 36% (from 3.14 +/- 0.64 to 2.00 +/- 0.70 ml/min/kg), resulting in a significant decrease in the percentage of the dose recovered in urine as AG. Decreased urinary recovery of AS after acute high-dose PB administration was due to an approximate 50% reduction in the renal clearance of AS (from 10.5 +/- 2.1 to 5.44 +/- 2.95 ml/min/kg). Although acute PB administration did not impair the biliary excretion of APAP or AS to a statistically significant extent, there was a trend toward decreased biliary excretion of AG. Large interanimal variability in AG biliary excretion was noted in rats receiving acute PB. These data indicate that serum PB concentrations are not related directly to impaired biliary excretion of AG or AS, and suggest that some other factor is responsible for the inhibition of AG and AS excretion at canalicular transport sites after PB pretreatment.

Hepatic disposition of acetaminophen and metabolites. Pharmacokinetic modeling, protein binding and subcellular distribution

Successful pharmacokinetic modeling often requires the ability of a simple model to describe a complex series of physiological processes. However, a simple model may be inappropriate. Physiologically-relevant modeling may offer a more appropriate description, but requires further support from in vitro/in vivo data. A well-stirred hepatic model with linear processes was proposed to describe in vivo disposition of acetaminophen and metabolites after a 100 mg/kg bolus of acetaminophen to vehicle- or phenobarbital-pretreated, renal-ligated rats. Model simulations underpredicted acetaminophen glucuronide (AG) concentrations at early time points in serum, and were inconsistent with AG biliary excretion-rate profiles. Intracellular binding of AG by ligandin was hypothesized, and a cytosolic compartment with reversible binding was incorporated into the model. In this second model, only AG bound in the cytosolic compartment was available for excretion into bile. Model 2 better described the AG biliary excretion rate-time profiles based on calculated Akaike's information criterion values. However, no apparent change was observed in the underprediction of AG serum concentrations. Parameter estimates derived from the two models also were different. The rate constants regulating AG formation and sinusoidal egress were increased significantly after phenobarbital pretreatment according to model 1, while the AG biliary excretion rate constant was decreased significantly. Parameter estimates based on model 2 suggested that phenobarbital pretreatment impaired the cytosolic binding of AG but increased significantly the AG biliary excretion rate constant. The physiologic relevance of model 2 was not supported by a subsequent investigation of the protein binding and subcellular distribution of acetaminophen and metabolites. Acetaminophen, AG and acetaminophen sulfate (AS) were not bound extensively in hepatic cytosol (mean +/- SD unbound fractions were 0.90 +/- 0.08, 0.97 +/- 0.08, and 0.88 +/- 0.06, respectively). Phenobarbital pretreatment did not alter significantly the unbound fractions of acetaminophen, AG or AS in hepatic cytosol. Acetaminophen was distributed to a greater extent in lysosomes than in the nuclear, mitochondrial, microsomal and cytosolic fractions. Distribution of AS predominated in cytosolic and lysosomal fractions. AG was detected only in cytosol. Phenobarbital pretreatment decreased the content of acetaminophen, AG and AS in all hepatic fractions. This study demonstrates the utility of pharmacokinetic modeling in exploring mechanistic hypotheses. However, these results underscore the importance of obtaining pivotal data from in vitro/in vivo studies to validate hypothesized mechanisms.