The use of single sample clearance estimates to probe hepatic drug metabolism: handprinting the influence of cigarette smoking on human hepatic drug metabolism

Pharmacokinetics of drugs used in critically ill adults

Critically ill patients exhibit a range of organ dysfunctions and often require treatment with a variety of drugs including sedatives, analgesics, neuromuscular blockers, antimicrobials, inotropes and gastric acid suppressants. Understanding how organ dysfunction can alter the pharmacokinetics of drugs is a vital aspect of therapy in this patient group. Many drugs will need to be given intravenously because of gastrointestinal failure. For those occasions on which the oral route is possible, bioavailability may be altered by hypomotility, changes in gastrointestinal pH and enteral feeding. Hepatic and renal dysfunction are the primary determinants of drug clearance, and hence of steady-state drug concentrations, and of efficacy and toxicity in the individual patient. Oxidative metabolism is the main clearance mechanism for many drugs and there is increasing recognition of the importance of decreased activity of the hepatic cytochrome P450 system in critically ill patients. Renal failure is equally important with both filtration and secretion clearance mechanisms being required for the removal of parent drugs and their active metabolites. Changes in the steady-state volume of distribution are often secondary to renal failure and may lower the effective drug concentrations in the body. Failure of the central nervous system, muscle, the endothelial system and endocrine system may also affect the pharmacokinetics of specific drugs. Time-dependency of alterations in pharmacokinetic parameters is well documented for some drugs. Understanding the underlying pathophysiology in the critically ill and applying pharmacokinetic principles in selection of drug and dose regimen is, therefore, crucial to optimising the pharmacodynamic response and outcome.

Cigarette smoking enhances the elimination of quinine

The pharmacokinetics of a single dose (600 mg) of quinine sulphate were examined in a group of non-smokers (n = 10) and in heavy cigarette smokers (n = 10). The mean (+/- s.d.) oral clearance of quinine in smokers (0.189 +/- 0.075 1 h(-1) kg(-1)) was significantly greater than in non-smokers (0.107 +/- 0.045 1 h(-1) kg(-1) , P < 0.01). The unbound clearance of quinine which reflects activity of the drug-metabolizing enzyme, was considerably greater (1.5-fold) in the smokers than in the non-smoker subjects. The mean elimination half-life of quinine in smokers was 7.5 +/- 1.4 (s.d.) h, significantly shorter (P < 0.005) than the mean value in non-smokers (12.0 +/- 3.1 h). These results suggest that cigarette smoking enhances the elimination of quinine. The clinical significance of these findings is unknown but they indicate the need for caution in the administration of quinine to patients who are heavy cigarette smokers.

Use of single sample clearance estimates of cytochrome P450 substrates to characterize human hepatic CYP status in vivo

1. Single sample clearance estimates (CL/F) of orally administered ethosuximide were obtained in four groups of healthy adult subjects. One group was treated with phenobarbital to induce CYP2B/2C and CYP3A activity; one group was treated with rifampin to induce CYP3A activity; one group consisted of cigarette smokers (increased CYP1A activity), and one group was untreated (controls). Ethosuximide CL/F values were slightly, though not significantly, increased among cigarette smokers (12.5% increase) and the phenobarbital group (25% increase), but rifampin treatment resulted in a significant increase (65%). 2. The influence of rifampin treatment on the single sample oral clearances of antipyrine, theophylline, phenytoin, carbamazepine, ethosuximide, quinidine, valproic acid, and lorazepam was investigated to determine whether rifampin induces only CYP3A. Rifampin treatment significantly increased the oral clearance of each drug from 1.4-fold (valproic acid) to 3.4-fold (quinidine). These findings indicate that the inductive effect of rifampin extends well beyond CYP3A.

The effects of cobalt chloride, SKF-525A, and N-(3,5-dichlorophenyl)succinimide on in vivo hepatic mixed function oxidase activity as determined by single-sample plasma clearances

1. Four drugs--antipyrine, theophylline, quinidine, and ethosuximide--were used as probes of in vivo hepatic mixed function oxidase (MFO) activity. Functional MFO activity was evaluated by estimating probe clearances subsequent to pretreatment of rats with either cobalt chloride, SKF-525A, or N-(3,5-dichlorophenyl) succiminide (NDPS). 2. Clearances of each probe were estimated from single plasma concentration measurements. Each pretreatment altered the clearances of this panel of probes in a different way. NDPS pretreatment increased theophylline clearance while slowing quinidine and ethosuximide clearances. SKF-525A slowed all probe clearances except for ethosuximide. Cobalt chloride slowed all probe clearances except for theophylline. 3. The use of multiple probes as substrates for the hepatic cytochrome P-450 system can provide some insight into the functional consequences of xenobiotic exposures on that system. Moreover, xenobiotic-induced functional changes on hepatic MFO when assessed in vivo appear to be modest relative to changes in in vitro activity or hepatic cytochrome P-450 content. This minimally invasive multiprobe method may be useful for assessing xenobiotic influences on human hepatic MFO in vivo.

The use of single sample clearance estimates to probe hepatic drug metabolism: handprinting the influence of phenobarbitone on human hepatic drug metabolism

1. Single sample clearance estimates, CL, were calculated for seven drugs employed as probes of human hepatic drug-metabolizing enzymes. Clearance estimates were calculated in healthy young adult male volunteers either taking no pretreatment, or taking phenobarbitone (PB) 100 mg nightly for 3 nights. This intermittent regimen (3 nights on, followed by 4 nights off) was repeated for at least 3 consecutive weeks prior to challenge with an individual probe. 2. Valproic acid was selected as a probe of both peroxisomal and microsomal beta-oxidase activity; antipyrine, phenytoin, quinidine, and carbamazepine were selected as probes of hepatic mixed-function oxidases (MFO), and lorazepam as a probe for UDP-glucuronosyl transferase activity. 3. Clearances of all probes except lorazepam, theophylline and phenytoin were approximately 20-30% faster in PB-treated than in control subjects; however, only in the case of carbamazepine did the increased clearance approach statistical significance. Neither phenytoin nor theophylline clearances were increased by PB. 4. A clearance index (probe CL for PB-treated subjects divided by probe CL for untreated subjects) was calculated for each probe, and an ordinal transformation of the log of the resultant ratio was plotted for each probe giving rise to a 'handprint' of the effect of PB on drug-metabolizing activity.