Pharmacokinetic profile of recombinant human N-acetylgalactosamine 4-sulphatase enzyme replacement therapy in patients with mucopolysaccharidosis VI (Maroteaux-Lamy syndrome): a phase I/II study

AIM

Mucopolysaccharidosis VI (Maroteaux-Lamy syndrome) is a lysosomal storage disease caused by a deficiency of the enzyme-N-acetylgalactosamine 4-sulphatase (ASB). Enzyme replacement therapy with recombinant human ASB (rhASB) has been studied in a randomized, double-blind, two-dose (0.2 and 1.0 mg/kg/week) phase I/II study (n = 7) followed by an open-label single dose (1.0 mg/kg/week) extension study. We report the pharmacokinetic profile of rhASB and the impact of antibody development.

METHODS

Pharmacokinetic analysis was performed at weeks 1, 2, 12, 24, 83, 84 and 96. Infusions were administered over 4 hours using a ramp-up protocol. Plasma ASB and rhASB antibody concentrations and urine glycosaminoglycan (GAG) concentrations were determined.

RESULTS

The area under the plasma concentration-time curve (AUC(0-t)) for the high-dose group increased from week 1 to week 2, but remained unchanged at weeks 12 and 24. A large difference in mean AUC(0-t) was observed between the low- and high-dose groups. Pharmacokinetic results at weeks 83, 84 and 96 were similar to those at week 24. Six patients developed antibodies to rhASB. One patient developed high antibody levels in combination with a high ASB concentration, while a second patient also developed high antibody levels with undetectable ASB concentrations. Antibodies from the second patient blocked detection of ASB. By week 72, antibody levels had decreased in all patients. The high-dose rhASB produced a more rapid and greater percentage reduction in urinary GAG concentrations than the lower dose (70% versus 55% at 24 weeks). Antibody levels did not appear to influence urinary GAG concentrations.

CONCLUSION

Pharmacokinetic parameters appear to be independent of the duration of treatment and are not linear between the 0.2 and 1.0 mg/kg/week doses. Antibodies to rhASB develop in most patients, but their concentration decreases over time. Antibody formation may influence pharmacokinetic parameters during the early phases of treatment, although it appears to have limited impact on biochemical efficacy.

Median effect analysis of efficacy versus adverse effects of immunosuppressants

BACKGROUND

A rigorous model to describe concentration-effect relations-the median effect analysis-was applied to quantitate immunosuppressive versus adverse effects in human renal transplantation.

METHODS

The median effect equation was used to analyze data collected from three clinical studies, including the two phase III blinded, placebo-controlled trials (n = 1295 patients) of sirolimus versus azathioprine or placebo treatment added to a cyclosporine (INN, ciclosporin)/prednisone regimen and a sirolimus/azathioprine/prednisone (in the absence of cyclosporine) phase II cohort (n = 41 patients).

RESULTS

The clinical effects correlated with drug concentrations as expressed by the median effect equation. Sirolimus or cyclosporine alone permitted drug concentrations that were 5-fold and 2.2-fold lower, respectively, to render 90% of patients rejection-free, suggesting a synergistic interaction between the two drugs. Further, the sirolimus concentrations to render 50% of patients rejection-free were about 200-fold and 60-fold less, respectively, than the concentration that caused 50% of patients to experience thrombocytopenia or hypertriglyceridemia. The correlation coefficient of the median effect analysis for the occurrence of hypercholesterolemia was more robust for sirolimus than for cyclosporine. Although the concentrations for 50% of patients rendered rejection-free versus 50% affected by hypercholesterolemia were similar, a 7-fold difference was calculated between the concentrations at which 90% of patients were free of rejection versus patients who were affected by hypercholesterolemia.

CONCLUSION

The median effect analysis proffers a useful tool to assess both drug interactions and the windows between therapeutic versus toxic effects of immunosuppressive agents. The current analysis suggests a synergistic interaction between sirolimus and cyclosporine.

Pharmacokinetics of torsemide in patients with decompensated and compensated congestive heart failure

Plasma pharmacokinetics of oral furosemide have been shown to be influenced by degree of decompensation in patients with congestive heart failure (CHF). This open-label, sequential comparison trial was conducted to determine whether CHF decompensation also alters the pharmacokinetics and pharmacodynamics of torsemide. Twelve patients with CHF, defined by either hemodynamic parameters or clinical signs and symptoms, were enrolled. On admission for treatment of their CHF, the patients were given 100 mg oral torsemide (phase A). A second dose of oral torsemide 100 mg was administered after hemodynamic parameters and clinical signs and symptoms of decompensated CHF resolved (phase B). Plasma and urine samples were collected over a 24-hour period for determination of torsemide concentrations and urine sodium. Hemodynamic measurements and physical signs and symptoms also were evaluated. During phase A, patients had significantly greater urine output and fractional sodium excretion compared with phase B. A significant increase in the area under the plasma concentration-time curve (AUC) was observed during phase B compared with phase A. However, no significant differences in maximal excretion rate of torsemide were noted between phase A and phase B. Heart failure status slightly affects the plasma pharmacokinetics of torsemide; however, this does not significantly alter the maximal urinary excretion rate of torsemide.

The effects of diuresis on the pharmacokinetics of the loop diuretics furosemide and torsemide in patients with heart failure

PURPOSE

To evaluate the pharmacokinetics of furosemide and torsemide before and after diuresis in patients presenting with marked fluid overload.

SUBJECTS AND METHODS

We studied 44 patients with New York Heart Association class III or IV heart failure, ejection fraction < or =40%, and an estimated excess fluid body weight > or =6.8 kg. Oral furosemide or torsemide was administered before and after diuresis. Pharmacokinetic parameters were assessed before and after diuresis.

RESULTS

Following diuresis, maximum plasma concentration increased from 11.0+/-5.0 microg/mL to 13.9+/-6.8 with torsemide (P <0.05) and from 3.1< or =1.5 to 3.9+/-1.9 with furosemide (P=0.16). Maximum concentration increased by more than 30% in only one third of the patients. Total absorption (by area under the curve method) increased 6% among patients on torsemide (P=0.38) and 7% among patients on furosemide (P=0.63) and increased >30% in only 1 torsemide and 2 furosemide patients. The time to maximum concentration decreased from 1.40+/-.82 h to 0.81+/-0.36 with torsemide (P <0.01). There were no differences between furosemide and torsemide in the effects of edema on absorption.

CONCLUSION

Marked diuresis altered the pharmacokinetics of both furosemide and torsemide in only a small percentage of patients. The use of adequate doses of oral diuretics in edematous patients may be successful, thereby permitting home treatment with oral diuretics and avoiding the cost of hospitalizations or home intravenous administration services.

Pharmacodynamics of torsemide administered as an intravenous injection and as a continuous infusion to patients with congestive heart failure

The natriuretic and diuretic effects of a 100-mg dose of torsemide administered as a continuous infusion of torsemide and as a single bolus were compared in a group of patients with stable mild-to-moderate congestive heart failure (CHF). Patients received in random order 100 mg of torsemide as an intravenous bolus and as a 75-mg infusion over 24 hours started simultaneously with a 25-mg loading bolus. Administration of torsemide to patients with CHF as a continuous infusion was an effective dosing regimen, resulting in 24-hour diuresis and natriuresis that was numerically but not statistically greater than that observed with bolus administration. The response with continuous infusion occurred with less torsemide in the urine, resulting in a significantly greater efficiency of torsemide with this regimen. The effectiveness of torsemide as a continuous infusion does not mean that this mode of administration should be used in all patients. The response to 100 mg of torsemide in patients with mild-to-moderate CHF is the same whether administered as an intravenous bolus, a continuous intravenous infusion, or by mouth. This is consistent with the high bioavailability demonstrated in previous studies. The mode of therapy used should be dictated by each individual patient's needs. This study shows that continuous infusion is a viable option for administration of torsemide, and dosing guidelines for use of such a strategy are presented.

Bioavailability, pharmacokinetics, and pharmacodynamics of torsemide and furosemide in patients with congestive heart failure

The bioavailability, pharmacokinetics, and pharmacodynamics of torsemide (10 mg orally and intravenously) and furosemide (40 mg orally and 20 mg intravenously) were determined in a randomized crossover clinical trial in 16 patients with compensated congestive heart failure. Torsemide (time to reach maximum concentration [tmax], 1.1 +/- 0.9 hour) was more rapidly absorbed than furosemide (tmax, 2.4 +/- 2.5 hours), the absorption of which was delayed compared with that in healthy volunteers. Bioavailability of torsemide was also greater and less variable than that of furosemide. All four treatments yielded comparable changes from baseline in 24-hour electrolyte excretion. Based on the relationships between sodium excretion rate and fractional sodium and urinary drug excretion rate, response to both diuretic agents at the level of the nephron was decreased compared with previous studies with healthy subjects. Assessment of the clinical relevance, if any, of the difference in the variability of absorption warrants further study.

Absorption of isosorbide-5-mononitrate at specific sites in the gastrointestinal tract

As part of a development program for controlled- and extended-release formulations of isosorbide-5-mononitrate (ISMN), the rate and extent of drug absorption was evaluated after site-specific delivery in the gastrointestinal (GI) tract. Seven healthy male subjects received, on separate occasions, 20 mg of ISMN solution orally and via nasogastric tube to the jejunum, terminal ileum, and ascending colon. Compared with oral administration (AUC 2,963 hr x ng/mL, Cmax 442 ng/mL, Tmax 0.81 hr), placement of drug directly into the jejunum did not change the extent (AUC 2,844 hr x ng/mL) but increased the rate of absorption (Cmax 630 ng/mL, Tmax 0.28 hr) due to direct placement of drug into the intestine. Administration to the terminal ileum resulted in a rate of absorption comparable to that from the jejunum (Tmax 0.28 hr) but a reduction in extent (mean AUC 2,377 hr x ng/mL). Delivery to the ascending colon resulted in a further decrease in the extent (AUC 2,017 hr x ng/mL) and a "slowing" of the rate of absorption compared with the two intestinal sites (Cmax 392 ng/mL, Tmax 0.68 hr). Overall, bioavailability throughout the GI tract was sufficient to support development of controlled- and extended-release formulations.

The pharmacodynamics of intravenous and oral torsemide in patients with chronic renal insufficiency

The pharmacodynamics of intravenous and oral torsemide were determined in two randomized cross-over clinical trials in patients with chronic renal insufficiency. There was no significant difference in the rate or magnitude of the diuretic response between oral and intravenous administration. As has been shown with other loop diuretics, patients with chronic renal insufficiency have a reduced diuretic response compared with healthy subjects. This diuretic resistance is primarily related to a diminished delivery of drug to the urinary site of action. The response of torsemide at the tubular level is not different from that seen in subjects with normal renal function. Metabolites of torsemide do not appear to contribute to the diuretic response. A dose of 50 to 100 mg dependent on renal function is required to obtain a maximal response. A ceiling dose of approximately 100 mg in patients with chronic renal insufficiency is therefore recommended.

The pharmacodynamics of torsemide in patients with congestive heart failure

The pharmacodynamics of torsemide (a new loop diuretic of the pyridine sulfonylurea class) was studied in 16 subjects who had compensated congestive heart failure and had been receiving stable diuretic therapy. Oral doses of 50, 100, and 200 mg were studied by use of a randomized crossover design. The results of this study show that the pharmacokinetics of torsemide is linear up to at least a dose of 200 mg in patients with congestive heart failure. Approximately 20% of each of the three doses was excreted unchanged, consistent with previous findings in healthy volunteers. A hyperbolic relationship between diuretic effect and drug excretion rate was defined. The maximum urinary sodium excretion rate attained was about 0.6 mEq/min, which is about 20% of that in healthy subjects, indicating diuretic resistance in these patients. Although there was no saturation of the urinary excretory pathway with doses as high as 200 mg, the upper plateau of the dose-response curve was reached with doses of 50 mg, indicating that this dose represents a ceiling dose in patients with New York Heart Association class II and III congestive heart failure.

The pharmacokinetics of intravenous and oral torsemide in patients with chronic renal insufficiency

Torsemide is a diuretic that acts in the thick ascending limb of the loop of Henle. Unlike furosemide, it undergoes substantial hepatic elimination and should not accumulate in patients with renal insufficiency. Therefore the pharmacokinetics of intravenous and oral torsemide and its metabolites were investigated in patients with chronic renal insufficiency. Two groups of 24 patients stratified by creatinine clearance (30 to 60 ml/min and < 30 ml/min) were studied in two separate randomized dose escalating crossover studies, one using intravenous torsemide and the other using oral torsemide. The pharmacokinetics of both intravenous and oral torsemide were linear over the dosage range studied. Absolute bioavailability was essentially 100%. Renal clearance was greatly diminished and correlated with renal function. Total plasma clearance and half-life were not related to renal function and were found to be similar to those of healthy subjects. The substantial nonrenal clearance of torsemide prevents accumulation in patients with chronic renal insufficiency.

Pharmacokinetics and pharmacodynamics of torasemide in congestive heart failure

The pharmacokinetic profile and pharmacodynamic activities of torasemide, a new pyridine sulfonylurea acting on the loop of Henle, are described. Absorption of the drug was unchanged in patients with congestive heart failure, though maximum concentrations occurred at 1.7 h compared with 0.9 h in healthy subjects. The volume of distribution after oral administration was also unchanged in patients with heart failure, but oral clearance was reduced by 50%, consistent with the increase in elimination half-life; renal clearance and maximum urinary torasemide excretion rate were also reduced by 50%. Thus, the primary pharmacokinetic alteration in patients with heart failure compared with healthy subjects was a reduction in the rate of delivery of torasemide to its site of action in the loop of Henle. Fractional sodium excretion and urinary torasemide excretion rate were similar in patients with heart failure and healthy subjects, though the relationship between the excretion rates of sodium and torasemide was depressed in the patients with heart failure. Thus, the primary pharmacodynamic alteration in patients with heart failure compared with healthy subjects was less total sodium excretion per molecule of torasemide reaching the renal tubule. Torasemide was effective in inducing loss of body weight and increased sodium excretion in patients with congestive heart failure. Single intravenous and oral doses of 20 mg both produced similar significant increases in total sodium excretion. Torasemide, 5-20 mg once daily for up to 6 weeks, produced significant loss of body weight and increased total sodium excretion confirming the diuretic effectiveness of torasemide in patients with congestive heart failure.

Bioavailability, pharmacokinetics, and pharmacodynamics of torsemide in patients with cirrhosis

The bioavailability, pharmacokinetics, and pharmacodynamics of torsemide (10 mg orally and intravenously) were determined in a randomized crossover clinical trial with 12 patients with ascites caused by cirrhosis. Torsemide was rapidly absorbed with a bioavailability of 96.3% (confidence interval, 84% to 109%). Compared with healthy subjects, patients with cirrhosis exhibit a decrease in nonrenal clearance and increases in bioavailability, volume of distribution, renal clearance, elimination half-life, and percentage of the dose excreted into the urine. A greater proportion of the dose is delivered to the site of action over a more prolonged period of time. In spite of a shift of the pharmacodynamic curve to the right in patients with cirrhosis, there was no significant difference in natriuresis. Pharmacokinetic changes of torsemide in cirrhosis therefore compensate for the pharmacodynamic abnormality.

Carpal tunnel syndrome secondary to wrist and finger flexor spasticity

Ten patients with spastic wrist flexion deformities secondary to traumatic brain injury were evaluated for carpal tunnel syndrome. The angle of wrist flexion deformity averaged 75 degrees (range, 58 to 115 degrees). Nerve conduction studies demonstrated prolonged median motor and/or sensory latencies in all patients. Preoperative wick catheter measurements of carpal tunnel pressures in eight patients averaged 11 mm Hg in the resting position, 21 mm Hg in maximal wrist flexion, and 15 mm Hg in maximal extension. Each patient had carpal tunnel release with simultaneous wrist and finger flexor tendon releases or lengthenings. At surgery nine of the median nerves were constricted at the proximal edge of the transverse carpal ligament. The presence of normal carpal tunnel pressures and impingement of the median nerve at the proximal edge of the transverse carpal ligament indicates that the chronically flexed posture of the wrist resulted in median nerve compression, and this condition may be aggravated by underlying pressure from the spastic finger flexors.

Aminoglycoside dosing regimen and pharmacokinetic parameters in the guinea pig

To complement a study on aminoglycoside dosing regimen and ototoxicity in the guinea pig, we designed an experiment to examine: (1) the effect of dosing regimen on guinea pig pharmacokinetic parameters, and (2) possible differential accumulation after repeated intramuscular administrations of netilmicin and amikacin (150 mg/kg/day) for 7 days by 1 or 3 daily injections. The area under the curve (AUC infinity) and the maximum plasma concentration (Cmax) were dose-dependent. Within each regimen, no significant difference was observed between days 1 and 7. Little or no accumulation was observed after 21 days of treatment. The results show a good dose-dependence of AUC infinity and Cmax and are in accordance with data from human studies. Moreover, the fact that no accumulation occurred in the guinea pig suggests that it is a suitable animal model to evaluate the relation between aminoglycoside ototoxicity and dosing regimen.

Pharmacokinetics and pharmacodynamics of dilevalol

The pharmacokinetics and pharmacodynamics of dilevalol, the R,R stereoisomer of labetalol, were evaluated in nine subjects. Dilevalol was given as a single 50 mg intravenous dose and as a 400 mg daily oral dose for 7 days. To study the effects of hepatic enzyme inhibition, each subject received dilevalol in the presence of and absence of cimetidine. Cardiac beta-blockade was assessed by use of standardized treadmill tests for 48 hours after oral dilevalol. The three-compartment model analysis showed that systemic clearance (29.8 +/- 5.7 ml/min/kg), volume of distribution (16.6 +/- 4.1 L/kg), and terminal half-life (11.7 +/- 2.7 hours) were not altered by cimetidine. However, there was a 20% increase in the area under the curve (p less than 0.05) and an 11% increase in systemic bioavailability (p less than 0.05) after oral administration. Dilevalol caused significant cardiac beta-blockade for more than 24 hours, but these effects were not altered by cimetidine. The pharmacokinetic changes are consistent with a decrease in first-pass extraction of a high clearance drug.

Pharmacokinetics of dilevalol in normotensive and hypertensive volunteers

Dilevalol is a novel antihypertensive agent combining vasodilation due to selective beta 2-adrenergic receptor agonism with nonspecific antagonism of beta 1- and beta 2-adrenergic receptors. Studies of dilevalol's pharmacokinetics in normotensive and hypertensive volunteers have demonstrated that (1) it is rapidly and well absorbed; (2) because of extensive first-pass metabolism its absolute oral bioavailability is about 12%; (3) its mean elimination half-life is 8 to 12 hours after administration of single oral or intravenous doses to normal volunteers, a value consistent with once-daily dosing; and (4) food does not appear to alter its bioavailability or pharmacokinetics.

Antihypertensive effect of dilevalol is directly related to dose and plasma concentrations

Dilevalol is a novel antihypertensive agent combining vasodilation due to selective beta 2-adrenergic receptor agonism with nonspecific beta antagonism. To determine the relation of dilevalol dose and plasma concentration to antihypertensive effect, dilevalol (n = 15) or placebo (n = 3) was administered to 18 hypertensive subjects. The study was performed under blinded conditions during a 21-day hospitalization after a 3-week drug-free outpatient phase. In the 15 hypertensive patients receiving dilevalol orally in single morning doses of 200, 400 and 800 mg each for 5 days, the drug was shown to reduce blood pressure effectively for 24 hours at all doses. The antihypertensive effect was significantly related to dose administered and to the concentration of unchanged dilevalol measured in plasma. Dilevalol did not cause excessive changes in heart rate at rest and did not produce postural hypotension. The antihypertensive effectiveness of dilevalol was essentially the same after the first and fifth (steady state) doses at each dose level. Finally, no tendency toward rebound hypertension or tachycardia was observed after the abrupt discontinuation of dilevalol in these patients.

Pharmacokinetics and oral bioavailability of scopolamine in normal subjects

The pharmacokinetics and bioavailability of scopolamine were evaluated in six healthy male subjects receiving 0.4 mg of the drug by either oral or intravenous administration. Plasma and urine samples were analyzed using a radioreceptor binding assay. After iv administration, scopolamine concentrations in the plasma declined in a biexponential fashion, with a rapid distribution phase and a comparatively slow elimination phase. Mean and SE values for volume of distribution, systemic clearance, and renal clearance were 1.4 +/- 0.3 liters/kg, 65.3 +/- 5.2 liters/hr, and 4.2 +/- 1.4 liters/hr, respectively. Mean peak plasma concentrations were 2909.8 +/- 240.9 pg/ml following iv administration and 528.6 +/- 109.4 pg/ml following oral administration. Elimination half-life of the drug was 4.5 +/- 1.7 hr. Bioavailability of the oral dose was variable among subjects, ranging between 10.7 and 48.2%. The variability in absorption and poor bioavailability of oral scopolamine indicate that this route of administration may not be reliable and effective.

Extension of the serum digoxin concentration--response relationship to patient management

The purposes of this investigation were to demonstrate how computer simulations may be employed to extrapolate data obtained from a single intravenous digoxin dose to multiple oral dosing patterns and how these simulations may apply to clinical situations. The intravenous data were obtained from a previous study of the pharmacokinetics of serum digoxin and its inotropic response (derived from systolic intervals) in 12 normal male volunteers. The simulations were applied to various clinical situations including variations in oral dosing, alternate loading doses, no loading versus loading dose, and intravenous versus oral dosing. A nonlinear relationship was found between response and the post-distribution serum digoxin concentration in the therapeutic range. Thus, the increase in inotropic response is less than proportional to the increase in digoxin concentration in serum. This nonlinear relationship has several important clinical implications for loading and maintenance dosing protocols. Such concepts may be important relative to more rational clinical use of digoxin and to decreasing digoxin toxicity.

Day-night differences in steady-state theophylline pharmacokinetics in asthmatic children

Potential day-night differences of theophylline absorption and disposition were examined in day-active asthmatic children. Theophylline was given orally as TheoDur tablets and Somophyllin-CRT capsules (random crossover) every 12 hr (0700 and 1900), and patients were studied during two consecutive dosing intervals. In addition, patients were studied during the last 24 hr of a 48-hr continuous, intravenous aminophylline infusion. Serum theophylline concentrations were essentially constant during the intravenous infusion for the day and night periods. Thus, day and night clearances were nearly identical. Following oral administration of Somophyllin-CRT or TheoDur, areas under the serum concentration-time curves were greater during the day than the night, with Somophyllin-CRT yielding greater areas than TheoDur for both dosing intervals. Theophylline was absorbed more rapidly during the day than the night, as evidenced by a time to maximum concentration that occurred earlier in the daytime dosing interval. We conclude that theophylline clearance is not characterized by a circadian rhythm and that absorption of theophylline from Somophyllin-CRT and TheoDur is more rapid and complete during the day than the night.

Pharmacokinetics and bioavailability of dilevalol in normotensive volunteers

The bioavailability and pharmacokinetics of dilevalol following oral and intravenous administration were investigated in 12 healthy male volunteers. Dilevalol HCl was administered as a 200-mg oral tablet and a 50-mg intravenous infusion using a randomized cross-over design. Blood and urine samples were collected over 60 hours and analyzed for unchanged and total (unchanged plus Glusulase-released) dilevalol using a high performance liquid chromatography (HPLC) assay. After intravenous administration, total body clearance and volume of distribution of unchanged dilevalol were determined to be 23.2 mL/min/kg and 24.6 L/kg, respectively. After oral administration, a mean maximum concentration of 62 ng/mL was reached at an average peak time of 1.4 hours. Drug was eliminated with a half-life of 8.3 hours after oral administration and 12 hours after intravenous administration. Based on plasma levels and urinary excretion of total dilevalol, the drug was completely absorbed; however, due to first-pass metabolism, the absolute bioavailability of unchanged drug was 11 to 14%.

Toxicokinetics of 1,2-dibromo-3-chloropropane (DBCP) in the rat

The objective of this study was to determine the kinetics of absorption, distribution, and elimination of DBCP after intravenous (iv) administration in plasma, and after oral administration in water or corn oil, to conscious, fed, male Fischer 344 rats. Rats were prepared with an external jugular vein cannula and were dosed with 0.1, 1, or 10 mg/kg DBCP into the penile sinus or orally as a solution in water or in corn oil (1 mg/kg only). Blood was sampled at various times up to 12 hr, concentrations of DBCP were determined by gas chromatography, and data were evaluated by classical pharmacokinetic techniques. After oral administration in water, absorption of DBCP was rapid, and the distribution and elimination phase was biexponential. There did not appear to be any saturation of DBCP absorption, distribution, or elimination at the high oral or iv dose. After oral administration of DBCP in a corn oil vehicle, absorption was prolonged, suggesting retention of DBCP in the stomach; this could contribute to the toxic effects of DBCP on the forestomach when chronically administered in corn oil. The areas under the blood concentration/time curve were similar regardless of vehicle, suggesting that systemic toxicity might be independent of the vehicle.

Pharmacokinetics of amphotericin B in infants and children

The pharmacokinetics of amphotericin B (AmB) have not previously been evaluated in children. Five very small, premature infants and five older children received 0.25-1.0 mg of AmB/kg per 24 hr for Candida infections. Serum concentrations of AmB, measured by bioassay, were used to determine various pharmacokinetic parameters of AmB. A one-compartment model of drug distribution was most consistent with the data. The volume of AmB distributed per kilogram of body weight was smaller and the elimination clearance more rapid than those previously reported for adults. Serum levels were approximately one-half those seen in adults given comparable doses. The mean concentrations of AmB after various doses were as follows: at 0.25 mg/kg, 0.08 microgram/ml; at 0.50 mg/kg, 0.20 microgram/ml; at 0.75 mg/kg, 0.42 microgram/ml; and at 1.0 mg/kg, 0.54 microgram/ml. Interpatient variability was, however, marked, especially among the premature infants. AmB pharmacokinetics are different in infants and children than in adults; these differences may have implications for determining optimal pediatric dosing regimens.

Analysis of pharmacokinetic profiles in 232 renal and 87 cardiac allograft recipients treated with cyclosporine

Analyses of cyclosporine (CsA)-related compounds (Cs) using the radioimmunoassay method were performed by 232 profiles in renal and 87 profiles in cardiac allograft recipients. Cardiac allograft recipients tended to have an increased, dose-corrected area under the concentration (AUC) v time curves after oral and, particularly, intravenous CsA administration, to display a slower drug clearance rate, and to have a slightly smaller volume of distribution. The impact of age, sex, hepatic impairment, nephrotoxicity, and concomitant corticosteroid dose could be discerned within the large number of studies in renal recipients. Only nephrotoxic cardiac transplant patients showed a significant alteration of increased AUC:dose and drug half-life compared with patients with normal renal function, a similar change to that observed in nephrotoxic renal transplant patients. Pediatric compared with adult renal recipients showed a threefold increased rate of mean drug clearance, namely 39.6 mL/min/kg in children v 12.3 mL/min/kg in adults, which may explain the almost 50% reduction in area under the plasma concentration curve v the time curve. There also appeared to be somewhat better oral absorption of CsA by children. Study of greater numbers of cardiac transplant recipients will be necessary to detect the subtle impact of demographic factors on pharmacokinetic parameters.

Simplified method for determination of plasma nifedipine by gas chromatography

The gas chromatographic assay for nifedipine was simplified by mixing 1 mL of plasma, 50 microL internal standard, and 950 microL of ethyl ether. After vortexing and centrifugation, the organic layer was transferred and evaporated, then reconstituted with 100 microL of ether. A 10-microL sample was injected into the GC. Several steps as well as the use of special lighting and columns are eliminated in the procedure. This method is determined to be linear, specific, sensitive, precise, and accurate.

Clinical significance of day-night differences in serum theophylline concentration with special reference to Theo-Dur

In two studies, 25 diurnally active patients with asthma (6 to 17 years of age) were evaluated at steady state for day-night differences in serum theophylline concentration (STC) by frequent blood sampling over two consecutive 12-hour dosing intervals while being given treatment with Theo-Dur. In both studies findings were similar; Cmax was greater and Tmax shorter after dosing at 7 A.M. or 8 A.M. versus 7 P.M. or 8 P.M., with Cmax-C min approximately 7 micrograms/ml. For the morning dosing, 22 of 25 patients exhibited Cmax within 4 hours; 23 of 25 exhibited C min 12 hours after dosing. For the evening ingestion, the situation was very different: C min occurred within the initial 4 hours in 22 of 25 patients, whereas Cmax occurred in 22 of 25 patients just before the next (morning) dose. The findings indicate that the most appropriate time to estimate Cmax in patients given Theo-Dur is within the 4 hours after morning ingestion. The best time to estimate C min is a few hours after the evening ingestion. Sampling at these times is likely to represent within 10% to 20% the actual Cmax or C min.

Pharmacodynamics and pharmacokinetics of orally administered bishydroxycoumarin in the goat

Eight goats, 2 nontreated controls and 6 treated, were used to study the pharmacodynamics and pharmacokinetics of bishydroxycoumarin. In 5 of the 6 treated goats, there was a significant relationship between prothrombin times and drug concentrations. Activated clotting times did not change with time in either the controls or the treated goats. Five of 6 treated goats reached a plateau of drug concentration after 24 to 36 hours. Lag times for onset of pharmacologic effect ranged from 12 to 24 hours. The one goat (No. 3) that did not respond in concert with the other 5 was extremely nervous and became anorectic during the period of indoor confinement.

Demographic factors affecting the pharmacokinetics of cyclosporine estimated by radioimmunoassay

In order to assess the impact of demographic factors on serum levels of cyclosporine (CsA) estimated by radioimmunoassay (RIA) in renal allograft recipients, 493 pharmacokinetic studies were performed in 212 patients. Neither the presence of diabetes mellitus nor the CsA dosing frequency affected the measured pharmacokinetic parameters. Age over 45 years led to slower CsA clearance with resultant increase in maximum serum concentration (Cmax) per administered milligram, and increased volume of distribution. Female patients showed more rapid drug clearance, but greater volume of distribution. Concomitant hepatic impairment reduced drug clearance, increasing the area under the curve (AUC) per administered milligram of drug, and the Cmax. Patients treated with a rapid steroid taper showed a shorter half-life and lower Cmax than those receiving a slow steroid taper. Nephrotoxicity was associated with increased AUC per administered mg, while patients with acute tubular necrosis requiring dialysis showed poorer drug absorption, lower Cmax, and longer time to peak. The only effect of cimetidine administration was a slightly shortened time to peak. Serial analyses posttransplant in 17 patients suggested a tendency toward improved drug absorption with no effect on other parameters. These studies demonstrating the significant impact of demographic factors thus afford a basis on which to predict the trend of anticipated CsA levels as measured by RIA in renal allograft recipients.

Allopurinol absorption from different sites of the rat gastrointestinal tract

Allopurinol exhibits good bioavailability (78-90%) after administration of oral dosage forms to humans and rabbits; however, it is not absorbed rectally from any of the dosage forms to any significant extent. Oral administration of allopurinol in a polyethylene glycol suspension, to which allopurinol may be reversibly complexed, to rabbits has been shown to produce erratic and poor absorption of allopurinol. This suggests the possibility of differential absorption of allopurinol from various sites of the GI tract. The mechanism of allopurinol absorption was investigated in rats using the in situ Levine technique. The allopurinol absorption rate was 0.56 +/- 0.10 microgram/min/cm from the upper portion of the small intestine and was 0.48 +/- 0.12 microgram/min/cm from the midgut. The absorption from the lower portion of the small intestine was 0.33 +/- 0.14 microgram/min/cm and from the upper and lower large intestine segments was negligible (0.04 +/- 0.06 microgram/min/cm). The normalization of these absorption rates for surface area yielded flux values (normalized absorption rate as microgram/min/cm2) with significant differences in permeability between small and large intestine for allopurinol. The allopurinol absorption rate increased with increases in the dose, and there was a linear relationship between dose and absorption rate. Thus, allopurinol absorption, although specific to particular sites, is not dose dependent in the dose range from 0.25 to 2.5 mg/mL. Differences in the rates of absorption may be due to anatomical differences of the various parts of the GI tract or due to physicochemical properties of the drug itself.

Effect of surgery and cardiopulmonary bypass on indocyanine green pharmacokinetics

The pharmacokinetics of indocyanine green (ICG), a marker for hepatic blood flow (HBF), were studied at selected times during the initial 24-hour postoperative period in patients undergoing cardiac surgery with (n = 33) and without (n = 13) cardiopulmonary bypass. Cardiopulmonary bypass (CPB) caused a depression in ICG clearance and, by implication, HBF, reaching 48% of the pre-anesthetic and 60% of the pre-bypass values 2 hours post-CPB, returning to control values between 8 and 12 hours post-CPB. Similar decreases, although to a lesser extent, were seen in the non-CPB patients. Simulations with model compounds showed that these changes could produce clinically significant alterations in drug concentration-time profiles.

Plasma volume and estimated liver plasma flow during hyperbaric and hyperoxic exposures in awake dogs

Five different experiments were conducted to determine if estimated liver plasma flow and/or plasma volume were changed as a result of exposure to 2.8 atmospheres absolute (ATA) while breathing 100% oxygen or 6 ATA while breathing compressed air. The experiments were designed to separate the relative roles of the ambient pressure, the partial pressure of oxygen, the time of high oxygen exposure or some combination of these factors on any observed changes. We found that time was not a factor in the changes seen. Hyperbaria resulted in a decrease in estimated liver plasma flow at all pressures greater than 1 ATA. There was an apparent increase in plasma volume at 1.3 ATA and a return towards 1 ATA values at higher pressures. Hyperoxia resulted in a decrease in estimated liver plasma flow at 975 mm Hg but not at 912 mm Hg. The flow was then increased again at 2128 mm Hg. Plasma volume decreased significantly at 912 mm Hg returned to baseline (152 mm Hg) values at 975 mm Hg and then decreased again at 1054 and 2128 mm Hg PO2.

Contribution of the lung to total body clearance of meperidine in the dog

The potential role of the lung in the disposition of meperidine was examined in six conscious dogs instrumented for measurement of appropriate hemodynamic parameters. Following an intravenous bolus injection of 5 mg/kg, blood samples were collected simultaneously from cannulas placed in the left ventricle and the pulmonary artery, concurrent with the hemodynamic measurements. Pulmonary clearance, calculated from concentration differences between pulmonary arterial and left ventricular blood and pulmonary blood flow, averaged 12.1 +/- 3.70 mL/min/kg, accounting for 58 +/- 13% of the total clearance of the drug. These results help resolve differences between previously reported meperidine clearance in the dog and physiological blood flows and imply that the dog may be a poor model for this drug in humans where clearance is primarily hepatic.

Comparative bioavailability of intravenous and oral chloramphenicol in adults

The comparative bioavailability of chloramphenicol from intravenous succinate, oral palmitate, and oral base preparations was studied in a crossover manner in 12 adult patients. Chloramphenicol was administered at a dose of 1 Gm every 6 hours, and blood samples were collected at steady state. For the succinate study, total urine output was also collected. The bioavailability of active chloramphenicol from the succinate preparation averaged 85.8 +/- 42.3 and 78.8 +/- 50.1 per cent of the free base and palmitate forms, respectively. This lower availability appeared to be due to variable excretion of unchanged succinate in the urine, averaging 27 +/- 11 per cent of the dose. Regardless of dosage form or route of administration, plasma chloramphenicol concentrations remained in the therapeutic range (5 to 25 mg/liter) for the entire dosage interval, implying that no change needs to be made when changing dosage form or route of administration. The interpatient variability, however, supports the need for monitoring of plasma chloramphenicol concentrations, especially in newborn infants, persons with liver disease, or those receiving other medications that alter chloramphenicol metabolism.

Mezlocillin pharmacokinetics in pediatric oncology patients

The pharmacokinetics of mezlocillin were studied in 31 children (age, 2 to 19 years) with malignancies and normal renal and hepatic functions. Mezlocillin was administered intravenously over 30 min every 4 h at doses ranging from 12.2 to 125 mg/kg. Blood samples were obtained over one dosage interval at steady state. For all patients, the mean clearance was 0.21 +/- 0.11 liter/h per kg, the mean distribution volume was 0.26 +/- 0.13 liter/kg, and the mean elimination half-life was 0.97 +/- 0.51 h. Trough concentrations were 23.0 +/- 29.9 mg/liter before the dose was administered and 20.4 +/- 27.5 mg/liter at the end of the dosing interval. Peak concentrations averaged 245 +/- 90.4 mg/liter, and average concentrations for the dosing interval were 83.7 +/- 40.4 mg/liter. There were no apparent effects of sex, malignancy, age, or dose on either the kinetic parameters or plasma concentrations. Overall, the disposition parameters for mezlocillin in this patient group were comparable to those reported in adults.

Papaverine disposition in cardiac surgery patients and the effect of cardiopulmonary bypass

Cardiac surgery involving cardiopulmonary bypass (CPB) causes substantial physiologic changes which may potentially alter the pharmacokinetic properties of drugs used during and after the procedure. Studies with fentanyl have implied a relationship between prolonged elimination half-lives following CPB and decreased liver perfusion during and after the procedure. To further test this hypothesis, the effects of CPB on the pharmacokinetics of papaverine, a coronary vasodilator currently being added to the cardioplegic solution to prevent vasospasm, were studied. The drug was given to two groups of patients, one (n = 6) undergoing surgery with and one (n = 5) without CPB, the latter serving as controls. Plasma papaverine concentrations declined biexponentially in the control patients with a mean elimination half-life of 1.30 +/- 0.25 h, total plasma clearance of 13.8 +/- 3.75 ml/min/kg, volume of distribution of 1.52 +/- 0.45 l/kg and volume of distribution, steady-state, of 0.992 +/- 0.530 l/kg. For the CPB group, only half-life was estimated, and averaged 2.77 +/- 0.28 h, significantly greater (p less than 0.01) than that in the controls. These results further confirm the increased half-lives seen with other hepatically cleared drugs following CPB and have implications in the clinical management of patients given drugs eliminated in this manner.

Antimicrobial therapy of febrile children with malignancies and possible sepsis

A prospective study of 100 pediatric patients (2 months to 17 years of age) who had malignancies and fever was conducted. Gentamicin or netilmicin and a beta-lactam antibiotic were administered as initial empiric treatment. Before therapy profound granulocytopenia (fewer than 500 polymorphonuclear leukocytes/microliter) was present in 66% of children and persisted to the end of therapy in 42% of children. Of the 40 children with microbiologically documented infections, 38 (95%) responded to therapy. The aminoglycoside dosing regimen of 2 mg/kg/dose intravenously over 60 minutes every 6 hours produced antibiotic concentrations in serum of 5.8 +/- 0.3 microgram/ml at the end of the infusion in the netilmicin group and 1.5 +/- 0.1 microgram/ml 6 hours after the infusion and of 6.2 +/- 0.2 and 0.9 +/- 0.1 microgram/ml for the two time periods in the gentamicin group. The serum half-lives, volumes of distribution and the total body clearance rates were comparable for netilmicin and gentamicin. No accumulation of netilmicin or gentamicin was noted. Seven patients had renal compromise, five before institution of antibiotic therapy and two while on therapy. Four episodes of ototoxicity were not related to antibiotic therapy. Superinfection occurred in five children. The combination of either gentamicin or netilmicin with a beta-lactam antibiotic produced excellent results for episodes of fever in neutropenic children with cancer. In children with severe underlying disease and/or granulocytopenia, antibiotic combinations have achieved an optimal efficacy. Future emphasis should be placed on prevention, immunoregulation and nonbacterial pathogens.

Pharmacokinetics of pentobarbital under hyperbaric and hyperbaric hyperoxic conditions in the dog

High hydrostatic pressure has been shown to reverse the anesthetic effects of barbiturates. However, attempts to distinguish between two possible causes of this reversal, changes in drug disposition or changes in drug-receptor interaction, have not been reported. This study examined the possible effects of hyperbaria and hyperbaric hyperoxia on the distribution and clearance of pentobarbital in the dog. The drug was administered to six mixed-breed dogs as a 30 mg/kg i.v. bolus at 1 ATA breathing air, 6 ATA breathing air, and 2.8 ATA breathing 100% oxygen, with serial blood sampling for 12 h. Pharmacokinetic and statistical analyses showed no significant effects of hyperbaria or hyperbaric hyperoxia on the total plasma clearance, volume of distribution or elimination half-life. If pressure reversal of barbiturate anesthesia occurs at these pressures, changes in the disposition of the drug are not the causative factors.

Salicylate pharmacokinetics in the dog at 6 ATA in air and at 2.8 ATA in 100% oxygen

Hyperbaric air and hyperbaric hyperoxia, which have been shown to decrease both liver plasma flow and plasma volume in dogs, may potentially affect the disposition of drugs whose distribution and/or elimination are dependent upon those actions. This study examined the effects of those conditions on the disposition of salicylic acid, using the dog as a model. The drug was administered to six mixed-breed dogs as a 10 mg sodium salicylate/kg i.v. bolus at 1 ATA breathing air (control), at 2.8 ATA breathing 100% O2, and at 6 ATA breathing air, followed by serial blood sampling for 8 h. Statistical analysis showed a significant increase (p less than 0.05) in salicylate clearance at 2.8 ATA compared to control with a subsequent, although not statistically significant, increase in elimination half-life. There were no significant differences between the values observed at 6 ATA and either control or 2.8 ATA. As 100% O2 at 2.8 ATA is used during hyperbaric oxygen medical therapy and during decompression, this change in disposition of this commonly used agent may have implications in man. Studies in man must be conducted, however, to determine if the same conclusions apply.

Drug disposition under hyperbaric and hyperbaric hyperoxic conditions: meperidine in the dog

Patients being treated for a variety of conditions with hyperbaria or hyperbaric hyperoxia, and ill or injured deep sea divers being decompressed, may require concomitant drug therapy. This study examined the possible effects of those conditions on the distribution and elimination of meperidine, using the dog as a model. The drug was administered to six mixed-breed dogs as a 1.4 mg/kg i.v. bolus at 1 ATA breathing air, at 2.8 ATA breathing 100% O2, and at 6 ATA breathing air, and followed by serial blood sampling for 3 h. Statistical analysis showed no effects of hyperbaria or hyperbaric hyperoxia on the elimination half-life, total plasma clearance, or volume of distribution. These studies demonstrated marked differences between man and the dog in the elimination of meperidine. This probably means these results cannot be extrapolated to man.

Theophylline pharmacokinetics during hyperbaria and hyperbaric hyperoxia in the dog

The effect of hyperbaria and hyperbaric hyperoxia on the disposition kinetics of theophylline were investigated in the dog. The drug was administered as a 5 mg/kg iv bolus at 1 atmospheres absolute (ATA), 2.8 ATA and 6 ATA. Serial blood samples were collected over an eight hour period and analyzed for theophylline concentration using a gas chromatographic method. From the resultant data, elimination half-life, volume of distribution and total body clearance were calculated. There were no apparent effects of hyperbaria or hyperbaric hyperoxia on any of the parameters describing theophylline disposition, implying that, in the animal model studied, distribution, elimination and effect should remain constant.

The effect of obesity on acetaminophen pharmacokinetics in man

This study examined the absorption and disposition of orally administered acetaminophen in morbidly obese patients as compared to subjects of normal weight, and possible changes in disposition as the patients underwent weight reduction through dietary modification. The overall disposition of acetaminophen was not affected by a weight loss of 8 to 30 kg; elimination half-life, time to reach the peak, and peak plasma concentration varied within each subject but not in a systematic way. The half-life was the same in the obese patients (2.6 +/- 0.85 hours) and normal subjects (2.6 +/- 0.12 hours). However, maximum plasma concentrations were reached at a significantly later time and were significantly lower in the obese patients as compared to the normals, implying an apparently lower absorption rate. The area under the plasma concentration-time curve for the obese patients when normalized to ideal body weight was more consistent with that in the normal subjects than when normalized to total body weight. Administration of a normal dose of acetaminophen to an obese patient should yield plasma levels in the same range as persons of normal weight. As total weight may exceed 200 per cent of the ideal weight in this patient group, dosing according to total rather than ideal weight could lead to toxic or lethal effects when using the 10 mg/kg dosing recommendation.

Bioavailability of allopurinol oral and rectal dosage forms

The bioavailability of allopurinol from orally administered tablets and rectally administered suppositories is reported. Two types of rectal suppositories (cocoa butter and polyethylene glycol) were compounded and contained 300 mg allopurinol (from oral tablets). Five healthy volunteers received 300 mg allopurinol orally from tablets or rectally from suppositories in a randomized, three-way crossover design. Serial blood samples were drawn for 72 hours following administration and were analyzed by high-pressure liquid chromatography for allopurinol and its metabolite, oxipurinol. The interaction between allopurinol and PEG was studied in vitro using a dialysis method. Serum allopurinol levels following oral administration of tablets peaked at 1.5 +/- 0.23 microgram/ml at 5.20 +/- 0.65 hours. Allopurinol was not detectable after administration of cocoa butter/allopurinol suppositories; oxipurinol peaked at 0.34 +/- 0.14 microgram/ml at 13 +/- 11 hours. The bioavailability of allopurinol from the cocoa butter suppository, relative to the tablet, was 5.77 +/- 2.5%. Neither allopurinol nor oxipurinol was detectable (less than 0.1 microgram/ml) in the sera of persons following administration of PEG suppositories. Dialysis studies showed decreased loss of allopurinol from the dialysis sac as PEG concentration increased. The rectal suppositories of allopurinol used in this study did not appear to be an efficient means of administering this drug.

Pharmacokinetics of high-dose meperidine in surgical patients

Intravenous meperidine is used during surgical procedures at doses up to 5 mg/kg. At the commonly used analgesic intravenous doses of 50 to 100 mg (0.7 to 1.4 mg/kg), the drug exhibits two-compartment open model pharmacokinetics with elimination primarily by hepatic metabolism (> 90% of a dose). Drugs eliminated to a large extent by metabolism may, however, be subject to dose-dependent kinetics. We therefore studied the pharmacokinetics of surgery involving the lower part of the abdominal aorta and the femoral and popliteal arteries. Serial blood samples were collected over 10 hours and plasma meperidine concentrations were determined by a gas chromatographic assay. Semilogarithmic plots of plasma meperidine concentration vs time were biphasic; however, the curves' irregularities during the time the arterial vessels were clamped suggest that the data should be analyzed in a model-independent manner. These irregularities suggest that the disposition of meperidine is altered during the clamping period and that these alterations are quickly reversed following removal of the clamp. Our findings agree with those reported in the literature following a total dose of 50 mg (half-life average 4.4 hours, total plasma clearance 10.4 ml/min/kg, and apparent volume of distribution 3.74 L/kg). The present results thus indicate that although meperidine is eliminated to a large extent by metabolism, the processes involved are apparently not saturated at doses 7 times greater than normal.

Effect of cardiopulmonary bypass on fentanyl distribution and elimination

Fentanyl kinetics was studied in two groups of six patients, one group undergoing surgery with and one without cardiopulmonary bypass; the latter served as the controls. Plasma fentanyl concentrations declined biexponentially in the control patients with an average half-life (t1/2 beta) of 3.3 +/- 1.1 hr, total plasma clearance of 11.2 +/- 3.4 ml/min/kg, and volume of distribution (Vd beta) of 3.2 +/- 1.5 l/kg. The plasma concentration/time curves were severely disrupted during cardiopulmonary bypass but appeared to regain a log-linear decay once bypass was complete. This elimination phase had a t1/2 of 5.2 +/- 2.7 hr, longer than that in the control patients. Since fentanyl is eliminated primarily by hepatic metabolism, decreased liver plasma flow observed during and after bypass, as evidenced by a 30% decrease in indocyanine green clearance, may contribute to the extended t1/2. The prolonged t1/2 had clinical importance because of potentially prolonged effects and their relation to other drugs and the clinical management of the patient.