Tigecycline pharmacokinetics in subjects with various degrees of renal function

The pharmacokinetic parameters of tigecycline were assessed in subjects with severe renal impairment (creatinine clearance <30 mL/min, n = 6), subjects receiving hemodialysis (4 received tigecycline before and 4 received tigecycline after hemodialysis), and subjects with age-adjusted, normal renal function (n = 6) after administration of single 100-mg doses. Serial serum and urine samples were collected and assayed using validated liquid chromatography with tandem mass spectrometer (LC/MS/MS) methods. Concentration-time data were then analyzed using noncompartmental pharmacokinetic methods. Tigecycline renal clearance in subjects with normal renal function represented approximately 20% of total systemic clearance. Tigecycline clearance was reduced by approximately 20%, and area under the tigecycline concentration-time curve increased by approximately 30% in subjects with severe renal impairment. Tigecycline was not efficiently removed by dialysis; thus, it can be administered without regard to timing of hemodialysis. Based on these pharmacokinetic data, tigecycline requires no dosage adjustment in patients with renal impairment.

Absence of an Interaction Between Tigecycline and Digoxin in Healthy Men

STUDY OBJECTIVE

To evaluate a potential interaction between tigecycline and digoxin using pharmacokinetic and pharmacodynamic assessments.

DESIGN

Open-label, three-period, one-sequence crossover study.

SETTING

Hospital-affiliated, inpatient clinical pharmacology unit.

SUBJECTS

Twenty healthy men.

INTERVENTION

Tigecycline 100 mg was administered intravenously as a single dose on day 1 (period 1). Digoxin was administered as a 0.5-mg oral loading dose on day 7, followed by 0.25 mg/day on days 8-14 (period 2). Digoxin 0.25 mg/day was continued on days 15-19; in addition, on day 15, a loading dose of tigecycline 100 mg was administered intravenously, followed by 50 mg every 12 hours starting on the evening of day 15 through the morning of day 19 (period 3).

MEASUREMENTS AND MAIN RESULTS

Pharmacokinetic assessments were performed on days 1 and 19 for tigecycline and on days 14 and 19 for digoxin. Electrocardiographic parameters were measured at baseline and on days 1, 14, and 19 to assess digoxin pharmacodynamics. Serum tigecycline concentrations were determined by liquid chromatography with tandem mass spectrometry detection, and plasma and urine digoxin concentrations were determined by radioimmunoassay. Tigecycline area under the concentration-time curve (AUC), AUC from 0-12 hours (AUC(0-12)), weight-normalized clearance, and mean resistance time were not affected by concomitant multiple-dose digoxin administration, but tigecycline half-life was decreased during period 1, apparently due to fewer detectable terminal concentrations in some subjects. Digoxin steady-state AUC(0-24), weight-normalized oral dose clearance, cumulative amount of drug excreted in urine over 24 hours, renal clearance, and QTc (change from baseline) were not affected by multiple-dose tigecycline administration.

CONCLUSION

No significant effects of tigecycline on digoxin pharmacokinetics and pharmacodynamics were noted, but a small effect of digoxin on tigecycline pharmacokinetics cannot be ruled out due to design issues with period 1 of the study.

Effects of age and sex on single-dose pharmacokinetics of tigecycline in healthy subjects

The pharmacokinetics of tigecycline was evaluated in 46 healthy young and elderly men and women. Except for the volumes of distribution at steady state (approximately 350 liters in women versus 500 liters in men), there were no significant differences in tigecycline pharmacokinetic parameters. Based on pharmacokinetics, no dosage adjustment is warranted based on age or sex.

Lack of pharmacokinetic interaction between oral pantoprazole and cisapride in healthy adults

Pantoprazole, an irreversible proton pump inhibitor, may be administered with cisapride, a prokinetic agent. As increased cisapride concentrations may result in longer electrocardiogram (ECG) QTc intervals, a crossover study was conducted in healthy subjects to evaluate the oral pharmacokinetic interaction between cisapride (20 mg) and pantoprazole (40 mg). After dosing, serial blood samples and 12-lead ECGs were collected, and cisapride plasma concentrations were quantitated. For cisapride alone, mean parameter values were the following: peak concentration (Cmax), 56 ng/mL; time to Cmax (tmax), 1.7 hours; area under the concentration-time curve (AUC), 426 ng x h/mL; and terminal half-life (t1/2), 5.8 hours. Pantoprazole coadministration did not alter cisapride AUC or other pharmacokinetic parameters except for a slight 17% decrease in Cmax' resulting in 90% confidence limits of 79% to 88%, which were marginally outside strict bioequivalence limits. In addition, cisapride did not affect ECG QTc intervals, with or without pantoprazole. Therefore, no dosage adjustment is needed when pantoprazole and cisapride are coadministered.

Bromfenac disposition in patients with impaired kidney function

OBJECTIVES

To compare the pharmacokinetics of bromfenac among normal subjects and renally compromised patients and patients with end-stage renal disease.

METHODS

Bromfenac pharmacokinetics were examined after a single 50 mg oral dose in 18 subjects with normal kidney function, 12 subjects with decreased kidney function, and 10 dialysis-dependent subjects. Protein binding was assessed by equilibrium dialysis.

RESULTS

Mean peak concentrations and areas under the concentration versus time curve ranged from 3.3 to 3.9 micrograms/ml and 5.1 to 6.9 micrograms.hr/ml, respectively. The mean unbound fraction in the subjects receiving dialysis (0.29%) was nearly twice that in the subjects with normal kidney function (0.17%) and in the subjects with impaired kidney function (0.16%), but no differences were detected in clearance, volume of distribution, or their free fraction-corrected counterparts. Bromfenac half-life nearly doubled in the impaired and dialysis groups but was shorter than the anticipated 8-hour dose interval. Eight subjects had a total of 11 study events; none were serious and all were self-limited.

CONCLUSIONS

These findings suggest that no dosage adjustment is necessary in patients with impaired kidney function, but clinical monitoring appropriate for their individual condition is recommended.

The pharmacokinetics of venlafaxine when given in a twice-daily regimen

The comparative bioavailability of the novel antidepressant venlafaxine and its pharmacologically active metabolite O-desmethylvenlafaxine was assessed when venlafaxine was given orally twice daily (75 mg bid) or 3 times daily (50 mg tid). Eighteen healthy subjects participated in an open-label, randomized, two-period, crossover study lasting 12 days. Each subject was randomly assigned to take venlafaxine according to a bid or a tid regimen through day 8 and was crossed over to the other regimen on days 9 to 12. The daily dose was titrated up to 150 mg/d and was held constant on days 5 to 12. Plasma samples for quantitation of venlafaxine and O-desmethylvenlafaxine were obtained during a 24-hour steady-state interval on days 8 and 12. Analysis of variance showed no significant differences between the two venlafaxine regimens for peak concentration (Cmax), area under the curve during 24 hours (AUC0-24), trough concentration, or fluctuation ratio for venlafaxine or O-desmethylvenlafaxine in plasma. The bioequivalence ratios for Cmax and AUC0-24 of both compounds were calculated to compare the bid regimen and the tid regimen. The mean value for each of the 4 ratios was between 96 and 100%, and the 90% confidence limits around each ratio were within 90 to 110%. These results indicate that dividing a daily 150-mg venlafaxine dose into 2 or 3 doses provides equivalent total exposure and peak plasma concentrations of venlafaxine and O-desmethylvenlafaxine, its active metabolite. Therefore, based on pharmacokinetic considerations, it appears that the same daily dose of venlafaxine can be given in either two or three divided doses without compromising efficacy.

Lovastatin does not affect oral glucose tolerance in hypercholesterolemic patients

In 15 non-diabetic Type II hypercholesterolemic patients, the effect of 80 mg lovastatin daily on oral glucose tolerance was investigated. Using a randomized, double-blind, two-panel, parallel design, patients on a low cholesterol diet received lovastatin (n = 7) or placebo (n = 8) for 6 weeks. After 6 weeks of treatment, patients receiving lovastatin had a significant reduction in total cholesterol (30%), LDL-cholesterol (36%), and triglycerides (26%). Time courses of plasma glucose and serum insulin changes from baseline after the oral glucose tolerance test were evaluated by AUC. No statistically significant differences were observed in the AUC of changes from baseline between treatment groups or within either treatment group at prestudy, 6 weeks, and poststudy. No patient had a clinically important laboratory or clinical drug-related adverse effect during the study. This study demonstrated that short-term administration of 80 mg lovastatin daily effectively lowers cholesterol without having adverse effects on oral glucose tolerance.

The effect of acute hepatic injury on the disposition of vitamin A in the rat

The distribution of vitamin A was measured in various body fluids and tissues in rats with carbon tetrachloride induced acute liver injury compared to vehicle treated controls. All rats received 25,000u of retinol palmitate intraperitoneally 24 hr prior to study and 50,000u on the day of the study. Rats with liver injury had significant elevations of unesterified retinol in plasma and saliva, and significant elevations of retinol palmitate in plasma, urine, and kidney. Also, liver disease caused a significant decrease in the liver concentration of retinol palmitate, and a significant decrease in the bile and kidney levels of unesterified retinol. These results suggest that redistribution of vitamin A from liver to other areas occurs after acute liver injury in rats. Also, increased levels of vitamin A in urine, saliva or plasma may be a noninvasive marker for liver injury in man after vitamin A challenge.

Double-blind comparison of captopril and enalapril in mild to moderate hypertension

To compare the antihypertensive and humoral effects of the angiotensin-converting enzyme inhibitors captopril and enalapril, 20 patients with essential hypertension, not receiving treatment for 2 weeks and consuming a prescribed sodium ion intake, were randomly assigned to two parallel, double-blind treatment groups with stratification based on race and untreated seated diastolic blood pressure. These groups received a placebo (day -1) followed by either captopril, 200 mg every 12 hours (n = 9), or enalapril maleate, 20 mg every 12 hours (n = 11), alone (days 1 to 14) and then with hydrochlorothiazide, 25 mg every 12 hours (days 16 to 28). Captopril and enalapril were coadministered alone (day 15) and with hydrochlorothiazide (day 29) to assess whether further decreases in blood pressure would occur. Captopril and enalapril alone caused comparable decreases (p less than 0.05) in the mean 12 hour time-averaged seated diastolic blood pressure from values on day -1 (placebo), on day 1 (11 and 9 mm Hg, respectively) and day 14 (8 and 7 mm Hg, respectively). The addition of hydrochlorothiazide further decreased (p less than 0.05) blood pressure in each group (7 and 8 mm Hg, respectively) from values on day 14. Combined use of captopril and enalapril did not result in further reduction. Coupled with the comparable changes observed in each treatment group in serum angiotensin-converting enzyme activity, plasma renin activity and plasma aldosterone concentration, these data support the view that captopril and enalapril have similar antihypertensive effects and mechanisms.

Effect of renal failure or biliary stasis on the pharmacokinetics of amiodarone in the rat

The single dose intravenous pharmacokinetics of amiodarone (50 mg/kg) were examined in rats with 72 h of biliary stasis secondary to bile duct ligation compared with paired control animals; and in rats with uranyl nitrate induced acute renal failure compared with paired control animals. Plasma and tissue levels (liver, kidney, heart, and lung) of amiodarone (1) and its N-deethyl metabolite 2 were obtained at 4 and 24 h following drug administration. Pharmacokinetic parameters were derived from plasma samples obtained over a 24-h period. Compared with controls, biliary stasis caused a decrease in the total clearance of 1 (1.74 versus 0.35 L/h/kg) and in the volume of distribution at steady state (21.1 versus 5.0 L/kg); renal failure caused a decrease in total clearance (1.67 versus 0.9 L/h/kg) and an increase in apparent elimination half-life (13.7 versus 10.1 h). Both disease processes produced significantly higher plasma levels of 1 when compared with control animals at 4 and 24 h. However, only the cholestatic animals had consistently higher tissue levels of 1 in the face of elevated plasma levels. In normal rats, no 1 or 2 was detected in the urine after a 50 mg/kg intravenous dose of 1, and less than 0.5% of the total dose of amiodarone (1) was excreted into bile by 12 h.

The effect of single doses of cimetidine on estimated hepatic blood flow

Controversy exists as to whether H2-receptor antagonists decrease hepatic blood flow. This study examined the effect of single doses of cimetidine 300 and 600 mg po on apparent hepatic blood flow as estimated by indocyanine green (ICG) clearance. A double-blind, repeated-measure study was performed in nine supine healthy men. Following an overnight fast, placebo or cimetidine was administered one hour prior to ICG 0.5 mg/kg iv. Plasma samples were obtained serially for a period of 20 minutes following dye administration and stored at -70 degrees until high performance liquid chromatographic analysis. Cimetidine had no apparent effect on mean +/- SD ICG clearance following placebo, cimetidine 300 mg, and cimetidine 600 mg (366 +/- 66 vs. 336 +/- 55 vs. 350 +/- 58 ml/min/m2, respectively; NS). Corresponding values for estimated hepatic blood flow were 632 +/- 109, 580 +/- 103, and 617 +/- 112 ml/min, respectively; NS. No statistically significant changes in ICG half-life or volume of distribution at steady state occurred as a function of treatment. Contrary to previous reports, single-dose cimetidine administration appeared to have no appreciable effect on hepatic blood flow. These results implicate cimetidine binding to the cytochrome P-450 system as the sole mechanism responsible for inhibition of the systemic clearance of co-administered drugs metabolized by the liver.

Effect of phenobarbitone on the pharmacokinetics and tissue levels of amiodarone in the rat

Phenobarbitone pretreatment has been shown to increase amiodarone total clearance and decrease amiodarone elimination half-life after a single intravenous amiodarone dose in the rat. Coadministration of phenobarbitone with amiodarone for 7 days resulted in decreased tissue amiodarone levels compared to controls. These results may have implications for patients undergoing therapy with amiodarone and concomitant potent enzyme inducing drugs.

Effect of captopril and hydrochlorothiazide on the response to pressor agents in hypertensives

The effect on arterial pressure of incremental doses of norepinephrine (2 to 10 micrograms/min) and angiotensin II (50 to 800 ng/min) administered over 10 min periods was studied in sodium-replete hypertensive patients after crossover oral treatments with placebo, captopril 50 mg in a single dose, captopril 50 mg three times daily for one week and hydrochlorothiazide 50 mg daily for a week. Neither captopril nor hydrochlorothiazide affected the dose response to infusions of angiotensin II. In comparison to placebo responses, however, both single and multiple-dose captopril therapy, and hydrochlorothiazide attenuated the pressor responses to infusions of norepinephrine. Captopril significantly depressed angiotensin converting enzyme activity from pre-dose levels and angiotensin II infusions significantly elevated plasma aldosterone concentrations. These results confirm findings reported for single dose captopril in normotensive volunteers and indicate that attenuation of the vascular responsiveness to sympathetic stimulation may contribute to the antihypertensive effects of captopril and hydrochlorothiazide therapy.

Pharmacokinetics of pentobarbital, quinidine, lidocaine, and theophylline in the thermally injured rat

Previous studies have shown that rats with 15% third-degree burns show a severe depression in various in vitro hepatic drug-metabolizing enzymes. This effect was assessed in vivo by measuring the disposition of four liver-metabolized drugs in 16% third-degree burned rats at 7 d postburn. Compared with pair-fed control rats, pentobarbital demonstrated a significantly prolonged clearance and elimination half-life without a change in volume of distribution. Quinidine demonstrated a significantly increased volume of distribution and a significantly decreased clearance without a change in elimination half-life. Lidocaine showed a significantly increased volume of distribution. Theophylline, which is only 50% metabolized in the rat, showed no changes in any pharmacokinetic parameters. The free drug fractions of quinidine and lidocaine were found to be significantly decreased at 1 d postburn and normal at 7 d postburn. These results warrant pharmacokinetic studies in human burn patients.

Effect of cimetidine on the pharmacokinetics of quinidine and lidocaine in the rat

Because of previously reported drug interactions involving cimetidine and liver-metabolized drugs, the intravenous pharmacokinetics of quinidine (25 mg/kg) and lidocaine (15 mg/kg) were investigated in anesthetized rats pretreated with a single intraperitoneal dose of cimetidine (60 mg/kg) and compared with saline pretreated controls. Significant reductions of 35 and 23% in the respective total clearances of quinidine and lidocaine were observed in the presence of cimetidine. The quinidine volume of distribution was significantly decreased in the cimetidine-treated rats, while the lidocaine volume of distribution was not altered significantly. There was no significant change in the elimination half-life for either drug in the presence of cimetidine. These results suggest cautious use of quinidine or lidocaine when cimetidine is prescribed concurrently.

The effect of thermal injury on drug metabolism in the rat

Because of previously reported hepatic abnormalities in burns, the activity of the hepatic drug metabolizing system was assessed in burned Sprague-Dawley rats. In 16% burned male rats, pentobarbital sleeping times were significantly prolonged from day 1 to day 24 postburn, and trichloroethanol sleeping times were significantly prolonged from day 1 to day 10 postburn. The activity of p-nitroanisole O-demethylase was significantly depressed in male rats at 5, 10, and 17 days post-16% burn. In female rats, this enzyme was more depressed at 10 days post-16% burn than in male rats. A direct relationship was found between per cent burn and impairment of enzyme activity. The depression of drug metabolizing enzymes was inducible by phenobarbital pretreatment. Pretreatment with the immunosuppressive drug azathioprine prevented the enzyme depression at 5 days postburn, a result which possibly implicates a postburn hyperimmune response as the mechanism for the depressed enzyme levels.

Impaired drug-metabolizing ability in the burned rat

Research indicating hepatic dysfunction in burn injury led us to investigate the effect of thermal injury on the rat hepatic microsomal drug-metabolizing system. Heated water was used to produce burns corresponding to 15% of the rat's total body surface area. In vitro measurements of p-nitroanisole o-demethylase and aniline hydroxylase activities were significantly depressed on days 1 and 10 postburn. Likewise, in vivo sleeping and paralysis times with pentobarbital and zoxazolamine were significantly prolonged on these same postburn days. On day 10 postburn, the burned rats had significantly decreased levels of urinary D-glucaric acid, a metabolite that correlates with drug metabolizing activity. These results raise the possibility of impaired drug metabolism in human burn victims.

Urinary excretion of D-glucaric acid by severely burned patients

The 24-hr excretion of urinary D-glucaric acid (UGA) has been measured in 5 seriously burned adults and compared with 6 healthy adults. In the burn patients mean UGA was 14.4 +/- 5.4 (+/- SD) mumoles/day and 28.7 +/- 6.5 mumoles/day in controls (p less than 0.002). In a 6-year-old female, UGA was also found to be very low. In a seventh burn patient, an adult male taking 20 mg of fluphenazine until his injury, his UGA was still in the normal range (29 mumoles/day) on the day of admission but descended to 21 mumoles/day at 2 days, to 16 at 4 days, and to 13 at 6 days. Treatment with fluphenazine was then reinstituted and on the tenth day UGA was 28 mumoles/day, indicating that after thermal injury UGA can respond to drugs. Although the inference has not been proved that decreased UGA corresponds to a decreased activity of drug metabolism, there is evidence of a strong correlation between increased UGA and increased drug metabolism. A decrease of UGA in disorders that generally lower metabolic activity supports a possible correlation in severe burns. If drug metabolism activity is lowered in the seriously burned patient, drug overdose may well result from the usual clinical doses.