Plasma clearance and tissue distribution of partially thiolated polycytidylic acid and its degradation products in rodents.

Radioactive (35S-labeled) partially thiolated polycytidylic acid (MPC) was administered i.v. to male Sprague-Dawley rats. Blood samples were taken at various intervals, and the radioactivity in plasma was determined. The concentration of total radioactivity in plasma decreased rapidly postinjection, independently of the dose, and could not be readily resolved into a series of exponential terms with a high degree of confidence. Coadministration with polyinosinic acid in a 1:1 ratio significantly decreased the clearance of radioactive compounds from the plasma; moreover, the clearance of radioactivity decreased with increasing dose. Complexing with polyinosinic acid also decreased the rate of degradation of [35S]MPC as evidenced by an increase of the trichloroacetic acid-precipitable fraction (i.e., oligonucleotides larger than five to ten nucleotide units), from 0.45 to 0.92 of the total radioactivity in plasma 60 min postinjection. The plasma clearance and organ distribution of radioactivity following injection of [35S]MPC were determined in normal and leukemic RFM/Un mice. About 90% of the 35S radioactivity was removed from the plasma in 5 and 10 min, respectively, in these two groups of mice, and the residual plasma levels of radioactivity at any given time were twice as high in the leukemic group throughout an observation period of 1 hr. Organ distribution studies demonstrated significantly greater (per mg tissue) accumulation of radioactivity in the livers and spleens of the leukemic versus normal mice at all time points, while the corresponding data for the kidneys were similar for the two groups. Another study, comparing the radioactivity in suspended and washed spleen cells harvested 60 min postinjection, indicated that 4 to 10 times more MPC and/or 35S-labeled oligonucleotides were localized and bound intracellularly in the spleens of the leukemic mice. These studies of the pharmacokinetic properties and metabolic degradation of [35S]MPC suggest that this polynucleotide may be protected from degradation by complexing with polyinosinic acid and that preferential accumulation of [35S]MPC occurs in organs infiltrated by leukemic cells.

Influence of posture on hepatic perfusion and the presystemic biotransformation of propranolol: simulation of the food effect.

Several research groups have reported that the oral administration of propranolol with protein-rich food leads to a marked increase (mean + 60%) in the area under the drug plasma concentration-time curve (AUC oral) of this highly metabolized and well-absorbed drug. It has been postulated that this "food effect" is caused at least in part by a transient increase in hepatic blood flow (QH) with its associated decrease in first-pass metabolism (hepatic extraction is a monotonic decreasing function of QH). A randomized crossover study using postural manipulations to produce changes in QH of the magnitude observed after food consumption (20% to 50%) was performed in an attempt to isolate the contribution of transient changes in QH to the food effect phenomenon. A solution of 80 mg propranolol HCl was taken orally and subjects were randomly assigned to postural manipulation protocols that should change QH such that AUC oral would be minimized (phase 1) or maximized (phase 2). Estimated QH (indocyanine green total body clearance from blood) was determined before and at three time points during each phase. It was observed that indocyanine green total body clearance during periods of standing was 15% to 40% below that observed during periods of seating (significant at p less than 0.05 for many of the appropriate comparisons). However, AUC oral for propranolol was not affected (mean +/- 1 SD; AUC phase 2/AUC phase 1+= 0.98 +/- 0.28) by these changes in QH, which are comparable to those encountered after food consumption.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of the oral antiarrhythmic lidocaine congener, tocainide.

Tocainide, a primary amine analogue of lidocaine, is effective against some experimental and clinical arrhythmias. Its pharmacokinetic behavior was studied in 6 healthy male subjects. Peak blood levels (CB max) and area under the blood concentration-time curve (AUC) were linearly related to dose with slopes of 0.0067 mcg/ml and 6 min mcg/ml per milligram of dose, respectively. Renal clearance of tocainide averaged 59 ml/min when urinary pH was uncontrolled or acidified, while it was reduced to 13 ml/min during intense sodium bicarbonate loading. Blood levels following intravenous infusion were well described by a 2-compartment open model with a volume of the central compartment of 0.92 L/kg. The t 1/2 beta was 11 hr and total body clearance was 166 ml/min. Loo-Riegelman analysis of the absorption rate did not allow unequivocal assignment of an "order" to the absorption process. Bioavailability approached 100%. Administration of drug 5 min after a test meal suppressed CB max 40% but minimally affected AUC. Approximately 50% of the drug was found to be plasma protein bound at clinically effective concentrations.

Alpha1-acid glycoprotein concentration and protein binding in trauma.

Alpha 1-Acid glycoprotein (AAG) concentrations were measured every 2 to 3 days in eight trauma patients and seven healthy subjects for approximately 3 wk. Mean AAG concentrations in the trauma patients rose from 100 mg/dl to a peak value of 243 mg/dl at 10 to 14 days. AAG levels averaged more than 200 mg/dl at 15 to 21 days. Mean AAG concentration in healthy subjects was 70 mg/dl with little inter- or intraindividual variability. Lidocaine was added to all serum samples from four of the patients and to selected samples from all of the healthy subjects and protein binding was determined. The binding ratio (bound concentration/free concentration) correlated strongly with AAG concentration in the trauma patients (r = 0.92), in the healthy subjects (r = 0.84), and in both groups combined (r = 0.96). AAG concentration and binding ratio for each of the four patients individually also correlated (P less than 0.05 in all cases). The change in free fraction associated with this increase in AAG was approximately doubled in each patient. Similar findings with drugs commonly used in trauma patients would be expected to alter serum concentration-response relationships significantly.

Effect of carbohydrates on estimated hepatic blood flow.

Recent experiments suggest that propranolol taken orally with a carbohydrate-rich meal increases its apparent bioavailability by reducing first-pass metabolism. It has been postulated that this increase in bioavailability may be secondary to a transient increase in hepatic blood flow (QH). To examine this hypothesis, we examined the effect of one of the carbohydrate meals (potato) tested in other propranolol studies on QH by measuring blood clearance (ClB) of indocyanine green (ICG). Ten minutes after eating 200 gm cooked potato, mean ICG blood clearance (ClB) in six subjects rose by 12% (range -13% to +41%). There also was a 10% mean increase (range -13% to +23%) in ICG ClB 60 min after the meal. It was then postulated that a larger carbohydrate meal might induce a more consistent and substantial increase in ICG ClB; therefore, five of the subjects were restudied after 400 gm potato. The increase in ICG ClB was of the order of that after 200 gm. Changes in QH of this magnitude would be expected to make a negligible contribution to the mean 50% increase in propranolol bioavailability reported by several investigators. It thus appears that factors other than change in QH play a dominant role in the reduced first-pass metabolism of propranolol after a meal rich in carbohydrates.

Effects of hydralazine, nitroglycerin, and food on estimated hepatic blood flow.

Several recent studies have shown that hydralazine and nitroglycerin may increase the apparent oral bioavailability of high-clearance drugs. It has been postulated that the mechanism responsible may be a vasodilator-induced transient increase in hepatic blood flow with an associated reduction in first-pass metabolism. To test this hypothesis, we examined the effect of hydralazine (25 mg) and sublingual nitroglycerin (2 doses of 0.6 mg separated by 30 minutes) on indocyanine green (ICG) blood clearance (ClB). Forty minutes after the start of nitroglycerin therapy, ICG ClB fell from a baseline of 648 +/- 98 to 607 +/- 151 ml/min, and was further decreased to 578 +/- 98 ml/min 80 minutes after dosing. Hydralazine induced no consistent effect on ICG ClB. ICG ClB was 744 +/- 376, 721 +/- 218, and 763 +/- 195 ml/min at baseline, 40 minutes, and 80 minutes after dosing. As a positive control, ICG ClB was assessed after a high-protein meal. After this meal, ICG ClB increased from 656 +/- 107 to 811 +/- 141 and 801 +/- 132 ml/min at 40 and 80 minutes after dosing. These data suggest that one or more mechanism(s) other than changes in hepatic blood flow are involved in the vasodilator-induced increase in the apparent oral bioavailability of high-clearance drugs.

Effect of food on lidocaine kinetics: mechanism of food-related alteration in high intrinsic clearance drug elimination.

The effect of high-protein meal on the hepatic clearance (ClH) of intravenous lidocaine, because of its conceptual importance in understanding first-pass metabolic phenomena, was evaluated in nine healthy males. Our randomized crossover study demonstrated that mean ClH rose from 1245 to 1477 ml/min (P less than 0.03) as a result of the meal (i.e., mean area under the blood concentration-time curve decreased 20%). The magnitude of the change in clearance correlated weakly with fasting ClH (r = 0.54; slope = -0.037% per ml/min; intercept = 67.2%; P less than 0.15). In a separate study, it was observed that the meal did not influence lidocaine serum protein binding; the free fraction of lidocaine in samples drawn from the subjects in the fasting state averaged 0.305 +/- 0.027 while that from subjects who had eaten was 0.321 +/- 0.042. These data suggest that the mean clearance of lidocaine is increased by stimulation of hepatic blood flow rate. Furthermore, the magnitude of this increase is consistent with expectations based on a simple physiologic model. Thus, these data provide experimental support for the hypothesis that transient increases in splanchnic blood flow rate observed after a high-protein meal may explain apparent improvement of the oral bioavailability of model high intrinsic clearance drugs.

Mechanisms of nonlinear disposition kinetics of sulfamethazine.

Five healthy male subjects received oral doses of 10 and 40 mg/kg of sulfamethazine (SMZ) approximately 14 days apart in a nonrandomized crossover study. Blood and urine samples were collected for at least 24 and 72 hr, respectively. All samples were assayed by the Bratton-Marshall procedure for SMZ and apparent N-acetylsulfamethazine (NSMZ). Recovery of total drug (SMZ + NSMZ) in urine was 88.9% following the low and 79.5% following the high dose. The low and high dose plasma concentration time curves were not readily superimposable (i.e., nonlinear kinetic behavior was observed). The data suggest that several mechanisms contribute to the nonlinearity. Specifically, a dose-dependent decrease in absorption rate displaced the plasma concentration-time curve to the right in some subjects, whereas apparent metabolic clearance (Clm) decreased with increasing dose (estimated assuming dose = amount of SMZ + NSMZ in urine to 72 hr) in all subjects (0.35 ml/min/kg for the low and 0.23 for the high dose). Still greater dose-dependent effects were found when apparent Clm of unbound drug was determined, since free fraction rose from 0.11 to 0.30 over the observed plasma concentration range. Renal clearance (ClR) of Smz appeared to be a complex function of time. In the low dose study it ranged from an average of 0.071 ml/min/kg at 2 hr to 0.146 ml/min/kg at 6 hr after drug. After the high dose comparable values were 0.083 and 0.128. Interindividual variability and pronounced nonlinear kinetics of SMZ after 40 mg/kg suggest that this dose is probably a poor choice for the determination of acetylator phenotype.

Effect of food on hepatic blood flow: implications in the "food effect" phenomenon.

It has been suggested that alteration in the apparent oral bioavailability of propranolol taken with food may be due to a transient increase in QH. To investigate this hypothesis more closely, the time course of effect of a high-protein meal on QH was examined with the model compound ICG. Forty minutes postprandial, the mean increase in estimated QH was 69% above the control. QH was still elevated a mean of 36% at 100 min but by 280 min had decreased to a value that did not differ from control. Computer simulations were performed to predict the magnitude of change in the apparent oral bioavailability of propranolol that would be expected based on the observed QH changes. These simulations suggest that simple changes in QH alone cannot account for the increase in apparent oral bioavailability when propranolol is taken with food.

Tocainide kinetics and metabolism: effects of phenobarbital and substrates of glucuronyl transferase.

Tocainide, a lidocaine congener with low hepatic clearance, is eliminated predominantly by formation of a novel glucuronide conjugate. This suggested the possibility of metabolic interactions with enzyme inducers or competitive substrates for glucuronyl transferase. The time course of tocainide blood concentration as well as the urinary excretion-time profiles of drug and principal metabolite (a glucuronide of tocainide carbaminic acid, TOCG) were examined in six subjects before and after 15 days on phenobarbital (100 mg/day). In another study, the effect of salicylamide and clofibrate on the time courses of tocainide and TOCG urinary excretion were examined in four of the same six subjects. After 600 mg tocainide HCl by mouth, the area under the tocainide blood concentration-time curve was 48.2 +/- 11.9 hr micrograms/ml for the control dose and 49.6 +/- 4.2 hr micrograms/ml (mean = SD) after phenobarbital. Percent of dose excreted unchanged in urine (46.0 +/- 4.9 and 43.4 +/- 5.6) and percent of dose excreted as TOCG (30.6 +/0 3.3 and 27.7 +/- 7.2) were not affected by phenobarbital (data presented as control and after phenobarbital). Because salicylamide has been reported to be a potent inhibitor of the glucuronidation of some drugs and because clofibrate yields metabolites that may be competitive inhibitors of tocainide conjugation, the two were given together with tocainide. Average percent of dose recovered in urine as unchanged tocainide in 24 hr was 26.8%, 28.3%, and 29.7% in the control, salicylamide, and clofibrate studies. The urinary excretion of TOCG was also not affected. It is concluded that under the conditions of our investigation, the principal urinary metabolite of tocainide, a glucuronide of tocainide carbaminic acid, is formed by a mechanism not subject to induction by phenobarbital or competitive inhibition by salicylamide or clofibrate.

Individualization of phenytoin dosage regimens.

Two methods for arriving at optimum, individual phenytoin dosage regimens have been evaluated in 12 patients. (1) Individual Michaelis-Menten pharmacokinetic parameters for phenytoin were estimated from two reliable steady-state phenytoin serum concentrations resulting from different daily doses: The observed steady-state phenytoin serum levels obtained after 3 to 8 wk of compliance with dosage regimens calculated from the individual pharmacokinetic parameters agreed well with predicted levels (r = 0.824, p less than 0.02). The average deviation between observed and predicted levels was 0.04 mug/ml (range, +/- 3.2 mug/ml). (2) A previously published nomogram for making adjustments in phenytoin dosage regimens: The serum phenytoin concentration actually expected from the dose indicated by the nomogram was calculated using individual pharmacokinetic parameters. The daily dose for one patient would have exceeded his estimated maximal rate of metabolism. The correlation between calculated and predicted phenytoin serum levels in the other 11 patients was weak but significant (r= 0.360, p less than 0.05). The average deviation was --3 mug/ml (range, 3.9 to --11.3 mug/ml). It was concluded that the use of individual pharmacokinetic parameters is practical and is also superior to the nomogram.

Free drug concentration monitoring in clinical practice. Rationale and current status.

Recent advances in techniques to determine free drug concentrations have lead to a substantial increase in the monitoring of this parameter in clinical practice. The majority of drug binding to macromolecules in serum can be accounted for by association with albumin and alpha 1-acid glycoprotein. Albumin is the primary binding protein for acidic drugs, while binding to alpha 1-acid glycoprotein is more commonly observed with basic lipophilic agents. Alterations in the concentrations of either of these macromolecules can result in significant changes in free fraction. Diseases such as cirrhosis, nephrotic syndrome and malnourishment can result in hypoalbuminaemia. Burn injury, cancer, chronic pain syndrome, myocardial infarction, inflammatory diseases and trauma are all associated with elevations in the concentration of alpha 1-acid glycoprotein. Treatment with a number of drugs has also been shown to increase alpha 1-acid glycoprotein serum concentrations. A wide variety of biological fluids have been examined for their ability to provide an estimation of free drug concentration at receptor sites. The most useful fluid for estimating free drug concentrations appears to be plasma or serum, with subsequent treatment of the sample to separate free and bound drug by an appropriate technique. The two most widely used methods are equilibrium dialysis and ultrafiltration. Of these two, ultrafiltration has the greatest utility clinically because it is rapid and relatively simple. The major difficulty associated with this method involves the binding of drug to the ultrafilters, but significant progress has been made in solving this problem. Several authors have endorsed the routine use of free drug concentration monitoring. Data examining the clinical usefulness of free drug concentration monitoring for phenytoin, carbamazepine, valproic acid, disopyramide and lignocaine (lidocaine) are reviewed. While available evidence suggests that free concentrations may correlate with clinical effects better than total drug concentrations, there are insufficient data to justify the recommendation of the routine use of free drug concentration monitoring for any of these agents at present.

Effect of cimetidine on the pharmacokinetics and pharmacodynamics of quinidine.

The influence of cimetidine (1.2 g/day for 7 days) on the disposition and pharmacodynamic effects of a single oral dose of quinidine was studied in 6 normal volunteers. Cimetidine reduced the mean apparent oral clearance of quinidine (+/- standard error of the mean) from 25.5 +/- 2.7 to 16.2 +/- 1.4 liters/h (p less than 0.05). This was reflected in a 55% (range 30 to 109) increase in the mean half-life from 5.8 +/- 0.2 to 9.0 +/- 0.6 hours (p less than 0.05). Peak quinidine plasma concentrations and times to peak were also increased (p less than 0.05). Plasma protein binding and urinary excretion of quinidine were unchanged by cimetidine treatment. Alterations in the pharmacokinetic variables of quinidine were mirrored in simultaneously measured electrocardiographic parameters. Changes in Q-T, rate-corrected Q-T, QRS, and R-R intervals after a single oral dose of quinidine sulfate (400 mg) were significant. Treatment with cimetidine potentiated these pharmacodynamic changes, but failed to achieve significant differences from quinidine alone. Thus, cimetidine impairs the elimination of oral quinidine in normal volunteers. This interaction may lead to quinidine toxicity in patients in whom cimetidine is concomitantly administered.

Pharmacokinetic and metabolic aspects of the moclobemide-food interaction.

The effect of a protein-rich meal on the pharmacokinetics of moclobemide was studied after intravenous (75 mg) and oral (100 mg) administrations of this selective MAO-A inhibitor to eight healthy male volunteers. The meal chosen did not affect plasma concentration-time curves of the drug after oral administration apparently, because the influence of blood flow changes to the liver on hepatic first-pass metabolism (AUC increases) and on systemic clearance (AUC decreases) balance each other out.

Development of a bacteria-independent model of the multiple organ failure syndrome.

Criteria that allow definition of the multiple organ failure syndrome include pulmonary, hepatic, renal, and gut barrier dysfunction along with characteristic histopathologic changes. It has been difficult to study multiple organ failure due to lack of a satisfactory experimental model that would reproduce the pathophysiologic and histopathologic characteristics, would be stable enough to allow study over several days, and would be free of exogenous primary bacterial infection. We have studied pathophysiologic and histopathologic alterations in a potential model of multiple organ failure. Wistar rats received one of the following solutions by intraperitoneal injection: 4 mL of saline, 4 mL of mineral oil, or 1 mg per gram of body weight of zymosan A in 4 mL of mineral oil. Animals that received zymosan developed hypoxia, decreased creatinine clearance, and changes in hepatic microsomal cytochrome P450 content and aniline hydroxylase activity. Bacterial translocation occurred in the zymosan group. The lungs, liver, and kidneys of the animals that received zymosan exhibited histopathologic changes. We conclude that this model fulfills our criteria for a model of multiple organ failure.

The hepatic first-pass metabolism of problematic drugs.

The first-pass hepatic metabolism of a number of important therapeutic agents is inconsistent with traditional models that assume that the hepatic extraction ratio of a drug is constant in each individual (independent of the concentration of drug in the hepatic sinusoidal blood and also independent of the history of exposure to the drug). In this review, the authors examine the first-pass metabolism of five "problematic drugs" (propranolol, lidocaine, propafenone, verapamil, and nitroglycerin). Each of these compounds has unique facets to its hepatic clearance and pharmacokinetics as well as striking similarities. Selected aspects of first-pass metabolism are reviewed, and a theory that may explain some of the unusual behavior of the four lipophilic bases (propranolol, lidocaine, propafenone, and verapamil) is presented. Finally, the unusual and variable clearance of nitroglycerin is discussed.

Procainamide accumulation kinetics in the immediate postmyocardial infarction period.

The rate of change of plasma procainamide concentration during 36 hours of constant-rate intravenous infusion was examined in five acute myocardial infarction patients. It was observed that a steady-state plasma concentration was established in about 16 hours, which is consistent with simulations of plasma concentrations based on pharmacokinetic constants obtained from studies in young healthy volunteers. However, the steady-state level that was attained in these patients was markedly higher than that which the simulations predicted. Thus, on the average, acute myocardial infarction patients have lower total body clearances of procainamide than normal volunteers.

Factors influencing procainamide total body clearance in the immediate postmyocardial infarction period.

Fifteen acute myocardial infarction patients (only one of whom had evidence of significant renal dysfunction) received a constant-rate intravenous infusion of procainamide at one rate for a least 24 hours. Steady-state plasma levels achieved during these infusions were used to calculate total body clearance (C/B). Linear regression analysis of C/B versus a variety of clinical and laboratory patient characteristics yielded only body weight (or parameters derived from it) as a significant covariant (r = 0.713, P less than or equal to 0.005). Interestingly, the data from these 15 patients suggest that the presence of a significant degree of heart failure at the start of therapy did not result in a significant decrease in C/B (C/B = 5.9 ml/min/kg when class 0-I failure was present at the start of therapy and C/B = 5.5 ml/min/kg when class III-IV failure was present). If the data from five other patients who were studied previously are added to the group reported here, the conclusions reached would be the same. These data suggest that in patients with good renal and hepatic function, initial procainamide infusion rate could be selected on the basis of body weight and need not consider the initial presence of moderate heart failure. However, intense clinical monitoring for signs of impeding serious toxicity is strongly recommended since the observed regression line did not predict total body clearance accurately in 10-15 per cent of the patients studied.

Estimation of tris(2-butoxyethyl) phosphate in biological fluids: novel intersubject variability in recovery from human serum.

Tris(2-butoxyethyl) phosphate (I), a plasticizer commonly found in evacuated blood collection tubes, displaces many basic drugs from their binding sites on serum proteins and causes them to redistribute from serum into red blood cells (i.e., artificially lowering serum or plasma drug concentration). Thus, the ability to quantitate I in serum or plasma may be helpful in establishing the suitability of various lots of evacuated blood collection tubes for use in drug level monitoring and pharmacokinetic studies. In the process of establishing a minor modification of an assay which has been reported, remarkable and reproducible interindividual variability (n = 10) in the slope of standard curves was observed (range, 0.0143-0.0486). This variability appeared to be caused by differences in the recovery of I from the serum of these individuals. The source of this difference seemed to be related to serum lipoprotein concentration since the slope of standard curves was highly correlated with serum triglyceride concentration (r = -0.800) as well as with the sum of serum triglyceride and cholesterol concentrations (r = -0.881). These observations suggest that the examination of interindividual differences in the recovery of drugs and related compounds from serum should be a routine part of assay development.

Absorption kinetics of procainamide in humans.

Plasma procainamide concentrations following the administration of 500 mg of procainamide hydrochloride via intravenous infusion, conventional capsules, and sustained-release tablets were compared in 11 healthy male volunteers. Two-compartment open modeling of the plasma levels from the intravenous infusion experiments yielded mean Kel, k12, and k21 values of 0.0162, 0.0542, and 0.0233 min-1, respectively. The bioavailability of the oral preparations (versus intravenous) averaged 83% for the capsule and 79% for the sustained-release tablet. Calculations using a previously reported method suggested that absorption was a first-order process with mean ka's of 0.0336 and 0.0039 min-1 for the capsule and sustained-release tablet, respectively. The sustained-release formulation exhibited delayed release and adequate bioavailability.