Grant R. Wilkinson

A physiological approach to hepatic drug clearance

A physiological approach has been developed recognizing that hepatic blood flow, the activity of the overall elimination process (intrinsic clearance), drug binding in the blood, and the anatomical arrangement of the hepatic circulation are the major biological determinants of hepatic drug clearance. This approach permits quantitative prediction of both the unbound and total drug concentration/time relationships in the blood after intravenous and oral administration, and any changes that may occur as a result of alterations in the above biological parameters. These considerations have led to a classification of drug metabolism based on the hepatic extraction ratio. The proposed classification allows prediction and interpretation of the effects of individual variations in drug‐metabolizing activity, route of administration, pharmacokinetic interactions, and disease states on hepatic drug elimination.

The drug transporter P-glycoprotein limits oral absorption and brain entry of HIV-1 protease inhibitors.

Currently available HIV-1 protease inhibitors are potent agents in the therapy of HIV-1 infection. However, limited oral absorption and variable tissue distribution, both of which are largely unexplained, complicate their use. We tested the hypothesis that P-glycoprotein is an important transporter for these agents. We studied the vectorial transport characteristics of indinavir, nelfinavir, and saquinavir in vitro using the model P-glycoprotein expressing cell lines L-MDR1 and Caco-2 cells, and in vivo after intravenous and oral administration of these agents to mice with a disrupted mdr1a gene. All three compounds were found to be transported by P-glycoprotein in vitro. After oral administration, plasma concentrations were elevated 2-5-fold in mdr1a (-/-) mice and with intravenous administration, brain concentrations were elevated 7-36-fold. These data demonstrate that P-glycoprotein limits the oral bioavailability and penetration of these agents into the brain. This raises the possibility that higher HIV-1 protease inhibitor concentrations may be obtained by targeted pharmacologic inhibition of P-glycoprotein transport activity.

Identification of functionally variant MDR1 alleles among European Americans and African Americans

MDR1 (P‐glycoprotein) is an important factor in the disposition of many drugs, and the involved processes often exhibit considerable interindividual variability that may be genetically determined. Single‐strand conformational polymorphism analysis and direct sequencing of exonic MDR1 deoxyribonucleic acid from 37 healthy European American and 23 healthy African American subjects identified 10 single nucleotide polymorphisms (SNPs), including 6 nonsynonymous variants, occurring in various allelic combinations. Population frequencies of the 15 identified alleles varied according to racial background. Two synonymous SNPs (C1236T in exon 12 and C3435T in exon 26) and a nonsynonymous SNP (G2677T, Ala893Ser) in exon 21 were found to be linked (MDR1ast;2) and occurred in 62% of European Americans and 13% of African Americans. In vitro expression of MDR1 encoding Ala893 (MDR1ast;1) or a site‐directed Ser893 mutation (MDR1ast;2) indicated enhanced efflux of digoxin by cells expressing the MDR1‐Ser893 variant. In vivo functional relevance of this SNP was assessed with the known P‐glycoprotein drug substrate fexofenadine as a probe of the transporters activity. In humans, MDR1ast;1 and MDR1ast;2 variants were associated with differences in fexofenadine levels, consistent with the in vitro data, with the area under the plasma level–time curve being almost 40% greater in the *1/*1 genotype compared with the *2/*2 and the *1/*2 heterozygotes having an intermediate value, suggesting enhanced in vivo P‐glycoprotein activity among subjects with the MDR1ast;2 allele. Thus allelic variation in MDR1 is more common than previously recognized and involves multiple SNPs whose allelic frequencies vary between populations, and some of these SNPs are associated with altered P‐glycoprotein function.

Lorazepam Is an Independent Risk Factor for Transitioning to Delirium in Intensive Care Unit Patients

Background:Delirium has recently been shown as a predictor of death, increased cost, and longer duration of stay in ventilated patients. Sedative and analgesic medications relieve anxiety and pain but may contribute to patients’ transitioning into delirium. Methods:In this cohort study, the authors designed a priori an investigation to determine whether sedative and analgesic medications independently increased the probability of daily transition to delirium. Markov regression modeling (adjusting for 11 covariates) was used in the evaluation of 198 mechanically ventilated patients to determine the probability of daily transition to delirium as a function of sedative and analgesic dose administration during the previous 24 h. Results:Lorazepam was an independent risk factor for daily transition to delirium (odds ratio, 1.2 [95% confidence interval, 1.1–1.4]; P = 0.003), whereas fentanyl, morphine, and propofol were associated with higher but not statistically significant odds ratios. Increasing age and Acute Physiology and Chronic Health Evaluation II scores were also independent predictors of transitioning to delirium (multivariable P values < 0.05). Conclusions:Lorazepam administration is an important and potentially modifiable risk factor for transitioning into delirium even after adjusting for relevant covariates.

The effects of age and liver disease on the disposition and elimination of diazepam in adult man.

This study investigates the separate effects of age and hepatocellular liver disease on the disposition and elimination of diazepam (Valium) in man. The drug was given either by rapid intravenous injection (0.1 mg/kg) or orally (10 mg) to 33 normal volunteers rnaging in age from 15 to 82 yr as well as to 9 individuals with alcoholic cirrhosis, 8 with acute viral hepatitis, and 4 with chronic active hepatitis. In the normal individuals, the terminal plasma half-life of diazepam, (t 1/2 (B)) exhibited a striking age-dependence; at 20 yr the t 1/2 (beta) was about 20 h, but it increased linearly with age to about 90 h at 80 yr. The plasma clearance of diazepam in the majority of the normal subjects was between 20 and 32 ml/min and showed no significant age-dependence. Cigarette smoking did not affect the half-life or the clearance. Additionally, neither the plasma binding (97.4 plus or minus 1.2%, mean plus or minus SD) nor the blood/plasma concentration ratio (0.58 plus or minus 0.16) of diazepam showed any age-related changes (P greater than 0.05). By contrast, analysis of the intravenous data according to a two-compartment open model indicated that both the initial distribution space (V1) and the volume of distribution at steady state [Vd(ss)] of diazepam increased linearly with age (P less than 0.005). The increase in Vd(ss) was secondary to the change in V1. It appears then that the prolongation of t 1/2 (beta) of diazepam with age is primarily dependent on an increase in the initial distribution volume of the drug. The plasma concentration/time course of the metabolite, desmethyldiazepam, was also affected by age. In older individuals, the initial presence and the peak values of desmethyldiazepam were observed later and the metabolite was present in lower concentrations. Despite the profound prolongation of t 1/2 (theta) with age, the constancy of diazepam clearance indicates that drug plasma concentrations will not accumulate any more in the old than the young, and chronic dosage more in the old than the young, and chronic dosage modifications based on pharmacokinetic considerations are unnecessary. Data obtained in patients with liver disease were compared with those found in age-matched control groups. Patients with cirrhosis showed a more than twofold prolongation in the half-life of diazepam (105.6 plus or minus 15.2 vs. 46.6 plus or minus 14.2 h, P less than 0.001).

Oral first‐pass elimination of midazolam involves both gastrointestinal and hepatic CYP3A‐mediated metabolism

To determine in humans the relative roles of intestinal and hepatic metabolism in the oral first‐pass elimination of a CYP3A substrate using midazolam as a model compound.

Interrelationship between substrates and inhibitors of human CYP3A and P-glycoprotein.

AbstractPurpose. CYP3A and P-gp both function to reduce the intracellular concentration of drug substrates, one by metabolism and the other by transmembrane efflux. Moreover, it has been serendipitously noted that the two proteins have many common substrates and inhibitors. In order to test this notion more fully, systematic studies were undertaken to determine the P-gp-mediated transport and inhibitory characteristics of prototypical CYP substrates. Methods. L-MDR1, LLC-PK1, and Caco-2 cells were used to evaluate established CYP substrates as potential P-gp substrates and inhibitors in vitro, and mdrla deficient mice were used to assess the in vivo relevance of P-gp-mediated transport. Results. Some (terfenadine, erythromycin and lovastatin) but not all (nifedipine and midazolam) CYP3A substrates were found to be P-gp substrates. Except for debrisoquine, none of the prototypical substrates of other common human CYP isoforms were transported by P-gp. Studies in mdrla disrupted mice confirmed that erythromycin was a P-gp substrate but the CYP3A inhibitor ketoconazole was not. In addition, CYP3A substrates and inhibitors varied widely in their ability to inhibit the P-gp-mediated transport of digoxin. Conclusions. These results indicate that the overlap in substrate specificities of CYP3A and P-gp appears to be fortuitous rather than indicative of a more fundamental relationship.

Interethnic differences in genetic polymorphism of debrisoquin and mephenytoin hydroxylation between Japanese and Caucasian populations.

Interethnic differences in debrisoquin and mephenytoin hydroxylation have been compared between normal white (n = 183) and Japanese (n = 100) subjects with the 8‐hour urinary metaboUc ratio of debrisoquin and the urinary S/R enantiomeric ratio of mephenytoin to identify extensive (EM) and poor (PM) metabolizers. In white subjects the frequency of PMs was 8.7% and 2.7% for debrisoquin and mephenytoin, respectively. In contrast, in Japanese subjects no PMs of debrisoquin were identified, while the incidence of PMs of mephenytoin was 18%. These substantial differences (P < 0.001) in polymorphic distributions of oxidative drug metabolizing ability have implications for interethnic efficacy and toxicity of drugs and other xenobiotics that are metabolized by the involved cytochrome P‐450 isozymes.

Inhibition of P-glycoprotein-mediated drug transport : A unifying mechanism to explain the interaction between digoxin and quinidine

BACKGROUND Although quinidine is known to elevate plasma digoxin concentrations, the mechanism underlying this interaction is not fully understood. Digoxin is not extensively metabolized, but it is known to be transported by the drug efflux pump P-glycoprotein, which is expressed in excretory tissues (kidney, liver, intestine) and at the blood-brain barrier. Accordingly, we tested the hypothesis that inhibition of P-glycoprotein-mediated digoxin transport by quinidine contributes to the digoxin-quinidine interaction. METHODS AND RESULTS First, we demonstrated active transcellular transport of both digoxin and quinidine in cultured cell lines that express P-glycoprotein in a polarized fashion. In addition, 5 micromol/L quinidine inhibited P-glycoprotein-mediated digoxin transport by 57%. Second, the effect of quinidine on digoxin disposition was studied in wild-type and in mdr1a(-/-) mice, in which the gene expressing the major digoxin-transporting P-glycoprotein has been disrupted. Because the in vitro data showed that quinidine itself is a P-glycoprotein substrate, quinidine doses were reduced in mdr1a(-/-) mice to produce plasma concentrations similar to those in wild-type control animals. Quinidine increased plasma digoxin concentrations by 73.0% (P=0.05) in wild-type animals, compared with 19.5% (P=NS) in mdr1a(-/-) mice. Moreover, quinidine increased digoxin brain concentrations by 73.2% (P=0.05) in wild-type animals; by contrast, quinidine did not increase digoxin brain concentrations in mdr1a(-/-) mice but rather decreased them (-30.7%, P<0.01). CONCLUSIONS Quinidine and digoxin are both substrates for P-glycoprotein, and quinidine is a potent inhibitor of digoxin transport in vitro. The in vivo data strongly support the hypothesis that inhibition of P-glycoprotein-mediated digoxin elimination plays an important role in the increase of plasma digoxin concentration occurring with quinidine coadministration in wild-type mice and thus support a similar mechanism in humans.

Reduction of Liver Blood Flow and Propranolol Metabolism by Cimetidine

We studied the influence of cimetidine on liver blood flow in eight normal subjects. Cimetidine acutely reduced liver blood flow during fasting by almost 25 per cent, as measured by indocyanine green clearance. Chronic cimetidine therapy (300 mg four times daily for seven days) reduced the flow by 33 per cent, as measured over eight hours by calculating the relative disposition of oral and intravenous propranolol. In addition to reducing the clearance of intravenous propranolol by decreasing live blood flow, cimetidine also inhibited the metabolism of oral propranolol and thereby further reduced elimination. The reduction in clearance of oral propranolol correlated positively (r = 0.87, P less than 0.05) with the average steady-state concentration of plasma cimetidine, suggesting that the inhibition of drug metabolism by cimetidine is dose related. Pulse rates at rest were markedly lower after propranolol plus cimetidine than after propranolol alone. The reduction in liver blood flow produced by cimetidine has important therapeutic implications for patients with alterations in liver and gastrointestinal blood flow and when drugs are used whose hepatic elimination depends on liver blood flow.