Yun K. Tam

Effects of intravenous lipid as a source of energy in parenteral nutrition associated hepatic dysfunction and lidocaine elimination: a study using isolated rat liver perfusion.

The effects on liver function and hepatic lidocaine elimination using 20% Intralipid® as a source of non‐protein calories (30%) in parenteral nutrition were studied using an isolated rat liver perfusion procedure. Rats were randomly assigned to one of the three treatment groups: PNL group (n =6), consisting of 16·94% dextrose, 2·46% Intralipid®, and 5·2% amino acids; PN group (n =5), consisting of 24·2% dextrose and 5·2% amino acids; and CF group (n =6), chow fed (rat chow and water). The rate of lidocaine metabolism was significantly reduced after 7 d in the two PN treated groups when compared to CF. Steatosis was observed in five out of six PNL treated animals and two out of five PN treated animals. Intrinsic clearance was reduced by 80% in the PNL group and by 60% in the PN animals (p <0·05). Molar metabolite to drug ratios revealed significant reductions in N‐dealkylation, m ‐hydroxylation, and aryl methyl hydroxylation in groups PNL and PN; these values amounted to 67–92% (p <0·05). These findings suggest that a dextrose–amino acid solution induced steatosis and reduced the rate of lidocaine metabolism. The incorporation of Intralipid® caused further deterioration.

Models of Hepatic Drug Elimination

The liver is, by nature, heterogeneous. It contains a complex vascular network for blood flow and a stationary phase consisting of enzymes within parenchymal cells. Several physiological processes, therefore, may combine to give observed ranges in drug elimination. Net changes in concentration are a consequence of a series of steps: uptake of substrate into liver cells, enzymatic reactions within the cells, release of metabolites and unconverted substrate from the cells into the sinusoids, and the net flow of the perfusing medium in the vasculature. In addition, substrate binding to proteins in the blood and in the liver can influence hepatic elimination. An understanding of each of these processes is necessary to fully comprehend the overall process of drug elimination, and these processes must be accounted for, either individually or by grouping and approximation, if a model for drug elimination is to be developed. Existing models of hepatic elimination may be classified according to their treatment of mixing within the vasculature and whether or not the model explicitly accounts for mass transfer between the heterogeneous phases of the liver. Four major classes may be defined: 1. Nonparametric homogeneous models, which assume that either complete mixing or no mixing occurs within the vasculature of the organ. 2. Homogeneous mixing models, which allow for a range of mixing phenomena. 3. Heterogeneous micromixing models, which allow for mass transport between the cells and vasculature and describe mixing within the vasculature on a microscopic level. 4. Heterogeneous compartmental models, which also describe interphase mass transfer but assume complete mixing on a microscopic level, and therefore use a time and spatially averaged approach to model mixing. The utility of these models of hepatic elimination will be critically assessed based upon (1) their ability to account for the influence of the aforementioned physiological processes upon elimination; (2) the data requirements of the model, in addition to its mathematical complexity and ease of use; and (3) the range of compounds and metabolites which may be described using the model.

Effect of the lipase inhibitor orlistat on the pharmacokinetics of four different antihypertensive drugs in healthy volunteers

Objective: To study the influence of the lipase inhibitor orlistat on the pharmacokinetics of the antihypertensive drugs atenolol, furosemide, captopril and nifedipine.Methods:Four open-label, crossover studies were performed on six to eight healthy male volunteers. Orlistat was given in doses of 50 mg 3 times daily mid-meal for 7 (nifedipine and captopril) or 8 days (atenolol and furosemide). The four antihypertensive drugs (atenolol 100-mg tablet, furosemide 40-mg tablet, captopril 50-mg tablet and nifedipine 20-mg slow-release tablet) were administered in single doses twice, once before and once together, with orlistat at the end of the orlistat treatment period.Results:The plasma concentration time profiles and the pharmacokinetic parameters estimated for these drugs were in the expected range, except for furosemide, whose bioavailability was lower than reported in the literature. This was probably due to the fact that furosemide was given during a meal. There were minor, but statistically significant, differences in one of the pharmacokinetic parameters of furosemide and nifedipine (no difference for captopril and atenolol) when these drugs were given alone and in combination with orlistat: the half-life of furosemide was slightly longer, the time to peak plasma concentrations of nifedipine was slightly longer. None of these are considered to be clinically significant changes.Conclusions:The lipase inhibitor orlistat given 50 mg 3 times daily does not alter the pharmacokinetics of atenolol, furosemide, nifedipine and captopril to a clinically significant extent.

High-performance liquid chromatography of lidocaine and nine of its metabolites in human plasma and urine.

A simple high-performance liquid chromatographic procedure for quantifying lidocaine and nine of its metabolites in human plasma and urine is described. This method involves aqueous acetylation of the metabolites with acetic anhydride under mild basic conditions (pH approximately 8.5). Lidocaine and the derivatized metabolites are extracted under basic conditions with ethyl acetate. Calibration curves constructed for these compounds are linear in the concentration range studied (0.2-500 micrograms/ml). The relevance of this method is shown by measuring the plasma and urine profiles of two patients with suspected myocardial infarction.

Endotoxin and cytokine released during parenteral nutrition

BACKGROUND Apart from its benefits, parenteral nutrition (PN)-related complications have been reported. Studies have shown that PN could alter cytochrome P450 (CYP) activity. One of the possible mechanisms is through cytokine and nitrite release, which is triggered by endotoxin. The purpose of this study is to investigate the potential release of endotoxin, cytokines, and nitrite during PN. METHODS Rats were randomly assigned into either (a) the PN group, which received continuous PN infusion only; or (b) the control group, which received normal chow with saline infusion. The infusions were administered continuously for 7 days, and then blood was collected and microsomes were prepared from the excised livers. RESULTS Endotoxin levels in the PN group were significantly higher in portal vein but not in inferior vena cava when compared with those of the controls. TNF-alpha and IL-6 levels were significantly higher in the PN group (p < .05). However, IL-1 beta levels were not significantly different in the 2 groups (p > .05). The nitrite levels, the end product of nitric oxide formation, were found to be almost 2 times higher after PN (p < .05). CONCLUSIONS It is confirmed that a 7-day infusion treatment of PN in rat may be linked to bacterial translocation, which leads to increased levels of endotoxin. This increase could trigger cytokine release, which could down regulate CYP activities.

Quantification of Three Lidocaine Metabolites and Their Conjugates

A method has been developed to quantify three lidocaine metabolites, N-ethylglycyl-2,6-xylidide (MEGX), glycyl-2,6-xylidide (GX), and 4-hydroxy-2,6-xylidine (4-OH-XY), and their conjugates in pooled human urine using enzymic hydrolysis. The commonly used enzymes, pure β-glucuronidase, sulfatase, and a mixture of the two, were tested for their efficiencies in hydrolyzing the conjugates. Initially, it was found that 4-OH-XY was highly unstable after it was released from conjugates by β-glucuronidase and the enzyme mixture. This problem was corrected by purging the sample with nitrogen prior to incubation. It has been determined that 4-OH-XY is present in human urine exclusively as its glucuronide. The percentage of MEGX in free and in conjugated forms (glucuronide, sulfate, and others) are 44.9 ± 6.8,16.6 ± 4.5, 6.6 ± 1.8, and 31.9 ± 4.4, respectively. GX was present mostly in the free form (90.6 ± 10.5%).

Hydralazine Pharmacokinetics and Interaction with Food: An Evaluation of the Dog as an Animal Model

The intravenous and oral dose-dependent pharmacokinetics of hydralazine and the effect of concurrent administration of food with hydralazine in dogs were evaluated for comparison with published human data. Four dogs were given intravenous and oral doses of hydralazine at 0.25, 1.0, 2.5, and 4.0 mg/kg. In addition, the oral 2.5 mg/kg dose was given with a meal. Blood samples were collected at appropriate intervals and analyzed for hydralazine. Pharmacokinetic analysis showed that AUCoral/ dose (5552 to 13218 mg-min/ml) and F (0.36 to 0.77) increased significantly with dose, indicating saturation of first-pass metabolism, as is seen in humans. Total-body clearance (70 ml/min/kg) and steady-state volume of distribution (9 L/kg) were similar to human values. The bioavailability of hydralazine in the dog was decreased by 63% when the dose was given with a meal, which is comparable to some human data. It was concluded that the dog may be a useful model in which to study mechanisms of the hydralazine-food interaction.

Factors Responsible for the Variability of Saquinavir Absorption: Studies Using an Instrumented Dog Model

AbstractPurpose. To study the effect of dose and food on the bioavailability of saquinavir in dogs. Methods. A Youden Square block design was used for six female mongrel dogs (20-24 kg) who received six saquinavir treatments. The six randomized treatments were 1 mg/kg intravenous infusion over 30 min; 200, 400, 600, and 800 mg of saquinavir in the form of 200-mg capsules given orally with food; and 400 mg of saquinavir given orally after an overnight fast. A 200-mg 14C-saquinavir capsule was used to replace one of the 200-mg unlabeled saquinavir capsules in the 200- and 800-mg oral study. Results. Absorption of saquinavir from the gut was variable. (FA: 49-95%). The 14C-saquinavir study shows that the total radioactivity absorbed from the gut was insignificantly different from that of unlabeled saquinavir, suggesting first-pass gut metabolism was unimportant. The bioavailability of saquinavir under fasting condition was significantly lower (8.41 ± 4.7% vs. 20.3 ± 2.6%, p < 0.05). Saquinavir underwent significant first-pass liver metabolism because hepatic clearance values (22 to 30 ml min-1kg-1) approached that of liver blood flow. Conclusions. Incomplete gut absorption and extensive first-pass liver metabolism are the causes for low bioavailability of saquinavir in dogs. Absorption was further reduced under fasted conditions.

Hepatic blood flow measurements and indocyanine green kinetics in a chronic dog model.

The objective of this study was to compare hepatic blood flow measurements using ultrasonic flow probes and ICG in a conscious dog model and to evaluate whether ICG can be used to estimate relative change in hepatic blood flow. Seven mongrel dogs (3 M, 4 F, BW = 21 ± 1.8 kg, Hct = 0.39 ± 0.05) were used in the study. Catheters were surgically inserted into carotid artery and portal, hepatic and jugular vein. Transit-time ultrasonic flow probes were implanted around the portal vein and hepatic artery. After two weeks of recovery, a single i.v. bolus dose of ICG (0.5 mg/kg) was administered to each dog. The disposition profiles for ICG in the four catheters were measured for 15 minutes and the hepatic blood flow reading from the probes recorded. Jugular vein ICG blood clearance (Cl = 5.9 ± 1.1 ml/min/kg) was low compared to the electronically measured hepatic blood flow rate (Q = 27.8 ± 9.1 ml/min/kg). Extraction ratios (E = 0.15 ± 0.05) estimated using data from the inlet and the outlet of the liver were consistent with the clearance values, suggesting that ICG is not highly extracted by dog livers. Three dogs were used in experiments where liver blood flow was increased by food intake. Consistent with characteristics of low extraction ratio drugs, ICG was insensitive to blood flow changes while there was an overall increase in electronically measured liver blood flow of 30%. Therefore, ICG is a poor indicator of hepatic blood flow and the present dog model permits continuous and reliable measurements of hepatic blood flow and can be a useful tool in studying the effects of hepatic hemodynamics on pharmacokinetics.

Pharmacokinetics and presystemic gut metabolism of methyldopa in healthy human subjects

This study examined the pharmacokinetics and metabolism of methyldopa after giving single 250‐mg oral and intravenous doses to 16 healthy human volunteers. A 48‐hour washout period was allowed between oral and intravenous treatments. Blood and urine samples were collected; methyldopa was assayed in blood and urine, and its metabolites (methyldopa sulfate, α‐methyldopamine, and α‐methyldopamine sulfate) were assayed in urine. Pharmacokinetic parameters were recorded as follows: half‐life was 2.0 ± 0.7 hours; total body and renal clearance were 268 ± 72 and 107 ± 35 mL/min, respectively; and volume of distribution at steady‐state was 33 ± 11 L. The absolute bioavailability of the drug was 42 ± 16%. The measurable metabolites in urine after oral and intravenous administration accounted for 27% and 17% of the dose, respectively. Methyldopa sulfate was the most abundant metabolite recorded; its quantity was higher after oral than after intravenous administration, 20.1 ± 5.7% versus 6.7 ± 5.3% of the dose (P < .05), suggesting significant presystemic gut metabolism. First‐pass gut metabolism for methyldopa was estimated to be 17.6 ± 6.9% of the dose given.