Pharmacokinetics of prednisolone in man during acute and chronic exposure to high altitude

INTRODUCTION

The exposure to high altitude (H) produces several physiologic alterations which may induce changes in the pharmacokinetics of drugs. This hypothesis has been confirmed in previous studies which suggest that drugs which are highly bound to plasma proteins are most likely to exhibit altered pharmacokinetics.

OBJECTIVES

To further elucidate the influence of H on pharmacokinetics, prednisolone was selected, since it is highly bound to plasma proteins, renally excreted and poorly bound to red blood cells.

SUBJECTS, MATERIALS AND METHODS

Prednisolone (80 mg) was given orally to three groups of young healthy volunteers. One group was residing at sea level (L): the same volunteers were studied again after 15 hours of exposure to high altitude (3600 m, HA group), and volunteers living at H for at least six months (group HC).

RESULTS

There were no statistically significant differences in the pharmacokinetic parameters calculated from plasma data in the three situations studied. When calculated from whole blood data, however, AUC and Cmax were increased and both volume of distribution and clearance diminished after exposure to H, either acute or chronically. Binding to proteins increased significantly after H exposure from 57% in group L to 75% and 94% in group H and HC, respectively. Binding to erythrocytes also increased with H exposure from 43.7% in group L to 50.6% and 61.6% in group HA and HC, respectively. The prednisolone/prednisone ratio in urine was 11.1 in group L, 7.3 in group HA and 45.6 in group HC.

CONCLUSION

Since prednisone has very little intrinsic glucocorticoid activity and has to be converted to prednisolone for therapeutic effect, the alteration of the prednisolone/prednisone ratio, as a result of high altitude exposure could be clinically relevant. Additional experiments are desirable to further evaluate this observation.

Effects of high altitude exposure on the pharmacokinetics of furosemide in healthy volunteers

INTRODUCTION

A cascade of pathophysiological events occurs with the ascension to high altitude (H). We have performed studies on the effects of exposure to H on the pharmacokinetics of drugs. The hypothesis behind these studies has been that the exposure to H, which produces marked physiological changes in the body, may alter pharmacokinetics, and consequently, pharmacodynamics. Our previous studies suggest that drugs highly bound to plasma proteins are most likely to exhibit altered disposition.

OBJECTIVE

In continuation of our research, we selected furosemide which is about 98% bound to plasma proteins, renally excreted and has low binding to red blood cells.

SUBJECTS, MATERIALS AND METHODS

Furosemide (40 mg) was administered orally to 3 groups of young healthy volunteers. One group who had been residing at sea level (group L), the same group after 15 hours of exposure to high altitude (3,600 m, group HA) and a group of volunteers living at H for at least 6 months (group HC).

RESULTS

Our results are in accordance with the most recent pharmacokinetic studies on furosemide in which a terminal half-life of approximately 20-30 h was reported. Total proteins were 9.3% and 12.7% higher in groups HA and HC, respectively, than in group L. Albumin in group HC was 8.2% higher than group L. Bilirubin increased 17.7% and 41.2% in groups HA and HC, respectively, in comparison with group L. A rapid disposition rate constant in groups HA and HC was the only pharmacokinetic parameter that was significantly different from those in group L. Concentration of furosemide in plasma water increased significantly after H exposure, thus, the binding diminished from 97.2% in group L to 95.1% and 91.1% in groups HA and HC, respectively.

CONCLUSION

Exposure to H produces an increase in the free fraction of furosemide in humans, which could be of therapeutic importance.

Pharmacokinetics of lithium in healthy volunteers after exposure to high altitude

INTRODUCTION

Exposure of the human body to high altitude causes a number of physiological changes. In previous studies, we observed that these changes may alter the pharmacokinetics of drugs. The number of erythrocytes/mm3 increases both, after acute exposure to high altitude (HA), i.e. within 12 - 24 h after reaching high altitude (H), as well as in chronic exposure (HC) (> 10 months) to H. Also binding of drugs to biologic material may change with exposure to HA and/or HC.

OBJECTIVE

Since lithium is transported into and out of erythrocytes and binds strongly to erythrocytes, but is not plasma protein-bound, we selected this drug as candidate for the present study.

SUBJECTS, MATERIAL AND METHODS

Lithium carbonate 300 mg were administered orally to young healthy volunteers. One group residing at low altitude (Santiago, Chile, 600 m, group L), these same volunteers after 15 hours of exposure to high altitude (4,360 m, group HA), and volunteers living at high altitude for at least 10 months (group HC).

RESULTS

We found a significant increase of both hematocrit and red blood cell count (RBC) after exposure to H, both, acute or chronic. Elimination half-life increased 64.1% in group HA and 111.4% in group HC in comparison to group L. We also found an increase in volume of distribution: + 18.9% in group HA, and + 35.8% in group HC when measured in plasma, and + 16.9% in group HA and + 18.8% in group HC when measured in whole blood. Lithium uptake by the erythrocytes increases: the value of 36.7 +/- 22.7% in Group L rose to 54.8 +/- 21.1% and to 54.6 +/- 24.2% in groups HA and HC, respectively. Total clearance decreases at high altitude, though the differences were significant only in group HC (37%).

CONCLUSION

Results indicate that exposure to H produces alterations in the pharmacokinetics of lithium and that these variations may be clinically relevant.

Pharmacokinetics of acetazolamide in healthy volunteers after short- and long-term exposure to high altitude

Exposure to high altitude results in significant physiologic changes and may precipitate mountain sickness, ranging from mild symptoms above 2,500 m to severe symptoms above 4,000 m. In a previous study, changes in the pharmacokinetics of meperidine were observed after exposure to high altitude. This study was conducted to investigate whether similar changes occur for acetazolamide, which is prescribed for prophylaxis of acute mountain sickness. Acetazolamide 250 mg was administered orally to young, healthy male volunteers in groups of 12 each: those residing at sea level (group L), these same volunteers on the day after arrival at high altitude (4,360 m, group HA), and volunteers living at high altitude for 10 months or longer (group HC). Serial blood samples were collected for 24 hours and acetazolamide concentrations were measured in whole blood, plasma, and plasma water. The elimination rate constant (lambda z) was significantly increased in group HA compared with group L. Clearance uncorrected for bioavailability (Cl/F) increased significantly in group HA compared with group L, and further increased in group HC. Apparent volume of distribution (Vz/F) was decreased by 17% in group HA compared with group L, and increased by 37% in group HC compared with group HA. Mean residence time (MRT) was significantly decreased in group HA compared with groups L and HC. Erythrocyte (RBC) uptake increased significantly after a significant increase in RBC count in group HC compared with group L. The extent of protein binding (EPB), however, was significantly decreased in group HA compared with groups L and HC. Free acetazolamide concentrations were significantly lower in group HC than in group L 12 hours after administration. Based on these observations, it is suggested that patients travelling to high altitude, especially altitudes above 4,000 m, should be closely monitored and acetazolamide dosage adjusted as necessary.

Urinary excretion of acetazolamide in healthy volunteers after short- and long-term exposure to high altitude

Acetazolamide is recommended for the prophylaxis of acute mountain sickness symptoms which sets in on climbing to high altitudes (H) above 2,500 m. It is primarily excreted unchanged in urine. In a previous study, we reported on the changes in urinary excretion of meperidine and its metabolite normeperidine on exposure to high altitude. In this study, we investigated the effect on urinary excretion of acetazolamide. The study was carried out in three groups of 12 healthy male volunteers each: at sea level (group L), these same volunteers the day after arrival at high altitude of 4,360 m (group HA), and subjects residing for approximately 10 months at high altitude (group HC). Urine was collected for the periods of 0-2, 2-4, 4-8, 8-12, 12-24 and 24-36 h after peroral administration of a single 250 mg dose. Urinary pH was measured and the concentrations of acetazolamide were determined. There were no significant changes observed in the amount of acetazolamide excreted in urine over 36 h. The urinary pH ranged from 4.5 to 7.8 for L, from 4.2 to 6.9 for HA and from 3.1 to 6.7 for HC. The Fel (fraction eliminated unchanged in urine) was calculated from the amount excreted in 36 h in urine and dose, assuming a bioavailability of 1 based on literature data. No significant changes in Fel were seen.

Pharmacokinetics of meperidine in healthy volunteers after short- and long-term exposure to high altitude

Increased numbers of erythrocytes have been shown ex vivo to increase meperidine uptake, and one of the major physiologic changes that occurs at high altitude is an increase in hematocrit and erythrocytes. A study was therefore conducted to evaluate the effects of high altitude on the pharmacokinetics of meperidine. Intramuscular doses (0.75 mg/kg) of meperidine were given to three groups of healthy volunteers (age range, 18-20 years): participants living at sea level (group L), those same participants the day after arrival at high altitude (4,360 m; group HA), and participants who had lived at high altitude for > or = 10 months (group HC). Blood samples were collected for 12 hours after drug administration. Meperidine was measured in whole blood, plasma, and plasma water. Elimination rate constant (lambda z) and clearance uncorrected for bioavailability (Cl/F) were significantly lower at high altitudes than at sea level in plasma (HA and HC) and in whole blood (HA only). Mean residence time (MRT) was significantly higher at high altitudes than at sea level in plasma (HA and HC) and in whole blood (HA only). Hematocrit was significantly increased at both time points at high altitude in comparison to values at sea level, and was also higher after a long-term stay at high altitude than after arrival at high altitude. Erythrocyte binding increased significantly from 41.3% at sea level to 43.8% at arrival at high altitude to 50.9% after a long-term stay at high altitude. The extent of protein binding tended to decrease with high altitude, but this decrease was not significant. Free concentrations of meperidine in plasma water measured 1, 2, and 4 hours after administration were significantly increased after 2 and 4 hours.

Urinary excretion of meperidine and normeperidine in man upon acute and chronic exposure to high altitude

The urinary excretion of unchanged meperidine (M) varies with change of pH and metabolism. Since exposure of man to high altitude (H) may cause significant physiologic changes, we investigated its effects on the urinary excretion of M. The study was carried out in 3 groups of healthy, male volunteers (ages 18-20 years): at sea level (L), at 4360 m the day after arrival at H (HA), and at 4360 m in subjects residing for > 10 months at H (HC). Urine was collected for the periods of 0-4, 4-8, 8-12 and 12-24 h. Urinary pH was measured and the concentrations of M and normeperidine (N) were determined. The 24 h excretion of M and N was significantly decreased for L vs. HA and L vs. HC. Significance was also seen for the periods 0-4, 4-8 and 8-12 h. The ratio of amount excreted M/N for the 24 h period was highly significant for L vs. HA and L vs. HC. The urinary pH ranged from 5.3-5.9 for L, 5.9-7.0 for HA, and 5.1-5.7 for HC. The Fel (fraction of M eliminated in the unchanged form in urine) significantly decreased from L to HA and HC.

Disposition of nifurtimox and metabolite activity against Trypanosoma cruzi using rat isolated perfused liver

Nifurtimox disposition was investigated using the rat isolated perfused-liver method after administration of 25 micrograms mL-1 nifurtimox, and its disappearance was monitored by analysing the perfusate sample at various times. Biliary excretion was also measured. The drug concentration profile underwent a biexponential decline over the 2-h study period, with a terminal half-life of 62.76 +/- 17.56 min. Nifurtimox is a high clearance compound (15.23 +/- 5.53 mL min-1). The extraction ratio was 0.621 +/- 0.159. Biliary excretion accounted for 0.05% of the dose, the remainder consisting of highly polar metabolites. By 2 h, a minimal fraction of unchanged nifurtimox was recovered from the perfusate. Nifurtimox activity against Trypanosoma cruzi (clone CA-1) during the perfusion was also determined. Epimastigotes isolated from continuous culture were exposed to the samples of perfusate at different perfusion times in a microtitre plate. After an incubation time of 72 h at 27 degrees C, the parasite number in each well was counted under a microscope. From 0 to 75 min after the perfusion, the anti-trypanosomal activity decreased, but an increase in activity was observed at the later times. These findings show that active metabolites are formed during the perfusion.

The disposition of nifurtimox in the rat isolated perfused liver: effect of dose size

The disposition of nifurtimox was studied in the rat isolated perfused liver using a recirculating system. The drug was administered as a bolus (5.0, 15.0 or 30.0 micrograms mL-1), and its disappearance was monitored by analysing perfusate samples. In all experiments perfusate disappearance was monoexponential, and no significant difference was found between the three doses for the elimination constant (0.016, 0.011 and 0.012 min-1, respectively), half-life (46.6, 65.8 and 66.8 min, respectively), extraction rate (0.128, 0.091 and 0.099, respectively) and distribution volume (41.1, 47.3 and 30.7 mL g-1, respectively). At 30 micrograms mL-1 the hepatic clearance was lower than the other concentrations of nifurtimox (0.66, 0.51 and 0.34 mL min-1 g-1, respectively). Relatively little parent drug was recovered from the liver at the end of the perfusions. In summary, nifurtimox is cleared slowly from the rat isolated perfused liver, is poorly extracted by hepatocyte cells and is completely metabolized from 2 to 4 h after perfusion.

The pharmacokinetics of nifurtimox in chronic renal failure

The pharmacokinetics of nifurtimox, a drug used in the treatment of Trypanosoma cruzi infections, has been studied in seven patients with chronic renal failure undergoing haemodialysis, and in seven healthy subjects. Each subject took nifurtimox 15 mg.kg-1 orally and blood samples were obtained for 10 h after administration. Nifurtimox in serum was analyzed by HPLC. The patients with chronic renal failure had a higher Cmax than the control subjects due to a change in systemic availability. An alternative explanation would be that both the distribution volume and the clearance had changed. The mean half-life in the patients with chronic renal failure was similar to that in the healthy subjects.

Polarographic study of nifurtimox

Nifurtimox is polarographically reducible over the whole pH range, the nitro group being reduced to the hydroxylamine group in a 4e process and subsequently the amine being formed in a 2e process at a pH value below 4. The C = N-N linkage is reduced by a mechanism involving reductive fission of the N-N bond. The differential pulse polarographic peaks for the reduction of the nitro group to the hydroxylamine group at pH 6 were used in developing a new polarographic method for the determination of nifurtimox in pharmaceutical forms.

Pharmacokinetics of a nitrofuran compound, nifurtimox, in healthy volunteers

Nifurtimox disposition was evaluated in 7 healthy volunteers. Each subject received an oral dose of 15 mg/kg of nifurtimox and blood sample was obtained 11 h after the drug administration. We used an analytical method previously assessed in the literature. The pharmacokinetic analysis was carried out according to a one-compartment model and the most important parameters established were elimination half-life, distribution volume, and clearance. Serum concentrations were low in relation to the high doses administered. The low serum concentration is probably the result of a marked first pass effect.

Ranitidine disposition in severe hepatic cirrhosis

The effect of severe liver disease on ranitidine disposition was evaluated by comparing its kinetics in 5 healthy subjects and 11 patients with alcoholic cirrhosis. Cirrhotic patients had severe liver disease as evidenced by the presence of ascites, hepatic encephalopathy, jaundice, muscle wasting, and low serum albumin, but creatinine clearance did not differ significantly between controls and cirrhosis. Following intravenous administration of ranitidine, systemic clearance was decreased in cirrhosis. These decrease may be associated with changes in renal function, and decrease in hepatic metabolism, usually present in patients with severe hepatic failure. The distribution volume of ranitidine was also decreased in cirrhotics, but the difference between patients and controls was not significant. Biological half-life was significantly longer in cirrhotic patients than volunteers. This difference may be due to decrease in total body clearance found in cirrhotic patients. It is concluded that patients with severe liver cirrhosis could have elevated plasma level of ranitidine and that a reduction of ranitidine dosage is warranted in these patients.