Pharmacokinetics of acetazolimide after intravenous and oral administration in horses

OBJECTIVE

To determine the pharmacokinetics of acetazolamide administered IV and orally to horses.

ANIMALS

6 clinically normal adult horses.

PROCEDURE

Horses received 2 doses of acetazolamide (4 mg/kg of body weight, IV; 8 mg/kg, PO), and blood samples were collected at regular intervals before and after administration. Samples were assayed for acetazolamide concentration by high-performance liquid chromatography, and concentration-time data were analyzed.

RESULTS

After IV administration of acetazolamide, data analysis revealed a median mean residence time of 1.71 +/- 0.90 hours and median total body clearance of 263 +/- 38 ml/kg/h. Median steady-state volume of distribution was 433 +/- 218 ml/kg. After oral administration, mean peak plasma concentration was 1.90 +/- 1.09 microg/ml. Mean time to peak plasma concentration was 1.61 +/- 1.24 hours. Median oral bioavailability was 25 +/- 6%.

CONCLUSIONS AND CLINICAL RELEVANCE

Oral pharmacokinetic disposition of acetazolamide in horses was characterized by rapid absorption, low bioavailability, and slower elimination than observed initially after IV administration. Pharmacokinetic data generated by this study should facilitate estimation of appropriate dosages for acetazolamide use in horses with hyperkalemic periodic paralysis.

Relationship between acetazolamide blood concentration and its side effects in glaucomatous patients

Although acetazolamide, a carbonic anhydrase inhibitor, has an effect of lowering intraocular pressure, a number of side effects have been reported. Therefore, we investigated the relationship between the concentration of acetazolamide and its side effects, including plasma electrolyte imbalance. This study was conducted on 23 glaucomatous patients who received repeated doses of oral acetazolamide for one week or more. The concentrations of total and unbound plasma acetazolamide, as well as in the whole blood from the patients, were measured by high-performance liquid chromatography. The serum creatinine concentration, electrolyte concentrations, and adverse reactions were monitored. We found that plasma concentrations of chloride ion after repeated doses became higher than the normal range. This chloride ion concentration significantly correlated with the acetazolamide concentration in the erythrocytes, but not with the plasma concentration. The patients with erythrocyte acetazolamide concentration more than 20 microg/ml had higher incidences of the side effects. Periodical monitoring of erythrocyte acetazolamide concentration and plasma chloride ion can be easily and safely applied to elderly glaucomatous patients treated with acetazolamide for long periods to prevent overdosage and side effects.

Quantitation of acetazolamide in rat plasma, brain tissue and cerebrospinal fluid by high-performance liquid chromatography

A simple and sensitive high-performance liquid chromatographic method for the analysis of acetazolamide (AZ) in rat blood (plasma/serum, whole blood and serum ultrafiltrate), brain tissue and cerebrospinal fluid (CSF) was described. Quantitative extraction of AZ with ethyl acetate from both buffered plasma and brain tissue homogenate (pH 8.0) was achieved. Each extract was evaporated to dryness and the residue was chromatographed on a reversed-phase column. CSF was directly analysed without extraction step. The limits of detection were 0.05 microgram ml-1 for plasma, 0.02 microgram g-1 for brain tissue and 0.004 microgram ml-1 for CSF. Calibration curves were linear over the working ranges of 0.1-100 micrograms ml-1 for plasma, 0.05-50 micrograms g-1 for brain tissue and 0.025-50 micrograms ml-1 for CSF. The reproducibility of AZ assay in the rat biologic media indicated very low relative standard deviations (RSDs). The recoveries of AZ added to plasma and brain tissue were more than 96% with an RSD of less than 5%. The present method was applied to studies of plasma concentration profiles of the drug after administration and its distribution into central nervous system.

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.

Acute hemorrhagic gastritis associated with acetazolamide intoxication in a patient with chronic renal failure

Acetazolamide (Diamox) is a carbonic anhydrase inhibitor commonly used in patients with glaucoma in order to reduce intraocular pressure. Acetazolamide (AZ) is mostly excreted in the urine, therefore, the blood levels of AZ often tend to increase in patients with chronic renal failure. We experienced a case of chronic renal failure in a patient suffering from acute hemorrhagic gastritis associated with AZ intoxication. A 66-year-old female with chronic renal failure was referred to our hospital because of drowsiness and an acute deterioration of renal function. She had been treated with AZ, 500 mg per every day for eleven days for the treatment of glaucoma. Laboratory studies showed leukocyturia, thrombocytopenia, severe anemia, and tarry stools. The serum concentration of AZ was elevated to a maximum of 76.5 mg/ml. She was thus diagnosed as having AZ intoxication. On further examination, acute extensive hemorrhagic gastritis was also found by gastroscopy. Despite of the administration of intensive therapies, she died of disseminated intravascular coagulation (DIC) and septic shock due to bone marrow depression 6 days after admission. It is generally known that excessive blood levels of AZ inhibit not only the gastric juices but also prostaglandin levels and HCO3- excretion in the gastric mucosal barrier. We thus concluded that an excessive dose of AZ had probably destroyed the gastric mucosal barrier or thrombocytopenia due to bone marrow disorder and thus eventually led to the development of hemorrhagic gastritis. As far as we know, this is the first case report of acute hemorrhagic gastritis associated with AZ intoxication. Even though AZ tends to strongly bind to plasma protein and its clearance is generally poor by hemodialysis (HD), in our patient, HD was observed to be rather effective since the clearance of AZ was 45.8 ml/min on HD and 66 ml/min on direct hemoperfusion (DHP). DHP often reduces the number of platelets, also DHP needs a lot of heparin, therefore, we should have performed HD alone instead of DHP. In patients with an impaired renal function, AZ should therefore be administered very carefully in order to avoid an accumulation of the drug. In addition, HD alone should be used to remove any excessive amounts of AZ from the blood.

Carbonic anhydrase-immobilized precolumn for selective on-line sample pretreatment in high-performance liquid chromatographic determination of certain sulphonamide drugs

Carbonic anhydrase (CA)-immobilized aminopropyl silica precolumn was developed for direct injection determination of certain sulphonamide drugs in biological fluids by column-switching (CS) high-performance liquid chromatography. Under the optimized conditions, only the sulphonamide drugs with an unsubstituted sulphonamide group were retained on the CA precolumn and separated on a reversed-phase analytical column. Of these, the retention of hydrochlorothiazide (HCT), chlorothiazide, acetazolamide, furosemide (FS) and chlorthalidone was almost quantitative. The peak area of HCT was proportional to the concentration in the range of 1-100 nmol/mL with relative standard deviations of 3.7% (5 nmol/mL) and 0.7% (100 nmol/mL). This CS system was applied to urine and plasma samples spiked with HCT and FS. Endogenous components of these were effectively removed, and HCT and FS were selectively retained on the CA precolumn. Almost quantitative recoveries and reproducibility were obtained.

Joint determination of todralazine and acetazolamide in human serum by differential pulse polarography

Differential pulse polarography (DPP) is proposed as a direct method for the quantitation of todralazine and acetazolamide in human serum. The method was applied to the determination of these drugs in human serum, after a liquid-liquid extraction process. This extraction process together with the use of the standard additions method is essential for the elimination of the matrix effect. The proposed method enables detection limits of 0.107 microgram ml-1 for acetazolamide and 0.111 microgram ml-1 for todralazine to be achieved at reduction potentials of -0.59 and -0.86 V, respectively, using Britton-Robinson buffer (pH 1.65) as the supporting electrolyte.

Acetazolamide in hemodialysis patients: a rational use after ocular surgery

Acetazolamide is a weak diuretic used to decrease production of aqueous humor in the eye. Hemodialysis patients undergoing ocular surgery may benefit from acetazolamide; however, no pharmacokinetic data are available for this group of patients. We report a patient who received acetazolamide 250 mg every 6 hours after ophthalmic surgery and developed reversible neurological side effects associated with very high plasma concentrations. Using pharmacokinetic analysis, we suggest an alternate administration of acetazolamide for end-stage renal patients.

Elucidating the relationship between acetazolamide plasma protein binding and renal clearance using an albumin infusion

The effect of plasma protein binding changes on drug clearance is an important concept in clinical pharmacology. In a hypoalbuminemic patient receiving acetazolamide, albumin infusion (50 g) increased acetazolamide plasma protein binding towards normal as the serum albumin concentration rose (r = 0.91, P < .001). The ratio of acetazolamide renal plasma clearance to creatinine clearance decreased as serum albumin levels increased (r = 0.78, P < .05) and the unbound drug fraction fell (r = 0.88, P < .01), but clearance ratios based on unbound plasma acetazolamide levels did not change. Albumin infusion resulted in a nonparallel decline over time between plasma and unbound plasma acetazolamide concentrations. These data demonstrate that, over the range of observed serum albumin concentrations, acetazolamide renal plasma clearance is sensitive to changes in plasma protein binding. Furthermore, our findings emphasize the importance of measuring unbound drug levels when protein binding changes occur during the course of drug disposition studies. Finally, this methodology allows for the fascile assessment of the effects of plasma protein binding changes on renal drug clearance.

Determination of acetazolamide in human serum by enzymatic assay

Carbonic anhydrase (CA) inhibitors, such as acetazolamide (AZ), formerly used as diuretics, still play a role in the treatment of glaucoma, epilepsy, and altitude sickness. There is now hard evidence from both in vitro and in vivo studies in animals that carbonic anhydrase plays a vital function in bone loss. Acetazolamide blocks bone resorption in these experimental models. We have postulated that acetazolamide has potential for the treatment of human conditions associated with bone loss. In preparation for a clinical trial of acetazolamide's effectiveness in this regard, we developed an enzymatic method for determining the total concentration of acetazolamide in human serum. Acetazolamide is stripped from binding to serum proteins by adding 10(-6) M salicylic acid and adjusting the pH to 2.5, followed by ultrafiltration through a membrane (10 kD cutoff). The latter permits the free acetazolamide to enter the filtrate but retains any carbonic anhydrase (31 kD) which may contaminate the serum from hemolysis. The carbonic anhydrase inhibitory activity in the filtrate, representing the acetazolamide, is determined in a carbonic anhydrase assay using acetazolamide as the standard. Recoveries of acetazolamide added to human serum ranged from 83% to 94% depending on the concentration. Precision, as judged by the coefficient of variation, was 10.5%.

[Effect of acetazolamide on Menière's disease]

The effect of acetazolamide was assessed in 25 patients with Menière's disease. During the test session hearing threshold and plasmatic osmolality were monitored along with fluctuations in hearing loss, fullness, tinnitus and balance. A single, 250 mg dose of acetazolamide was administered to all patients via os early in the morning on an empty stomach. Hearing was tested prior to administration and every hour for five hours thereafter. Plasmatic osmolality was also assessed during the same session. In 52% of this group an improvement in the threshold was seen. The greatest shift was observed two hours after administration of acetazolamide at 250 Hz, whereas the smallest threshold shift corresponded to 2000 Hz. In all cases, plasmatic osmolality remained constant throughout. Of the patients 44% presented an improvement of all or one of the symptoms: hearing loss, tinnitus, fullness, balance. The data were compared with data obtained for a control group (9 patients) which received a placebo while following the same testing criteria. The results of this study suggest that acetazolamide can have a positive effect on endolymphatic hydrops. It should be stressed, therefore, that acetazolamide could be introduced in the diagnostic and therapeutic strategies applied in Menière's disease.

[Changes in (a-ET) PCO2 produced by acetazolamide in red cells, not in plasma]

The changes in (a-ET) PCO2, concentration of acetazolamide and inhibition rate of carbonic anhydrase in blood were measured for 3 hours after administration of acetazolamide to anesthetized dogs, in order to find whether the changes in (a-ET) PCO2 was in response to the concentration of acetazolamide in red cells or plasma. 1. The increase in (a-ET) PCO2 was stable for 3 hours after administration of acetazolamide (5-30 mg/kg). 2. According to the concentration of acetazolamide measured by HPLC, the concentration in erythrocytes increased quickly but decreased more slowly than in plasma. The concentration of acetazolamide in erythrocytes became higher than in plasma after 3 hours. 3. The change in the inhibition rate of CA activity in red cells and in plasma paralleled to the concentration of acetazolamide in red cells and in plasma. 4. (a-ET) PCO2 levels could be raised by low CA activity inhibited by acetazolamide in red cells, not in plasma. 5. Remaining in higher concentration of acetazolamide in erythrocytes might be occurred by the reason that acetazolamide combines with protein, especially CA, in erythrocytes to be unable to pass through the erythrocytes membrane smoothly.

Differential effects of flurbiprofen and aspirin on acetazolamide disposition in humans

The plasma concentration-time profile of acetazolamide (AZ) following an intravenous bolus dose (5 mg kg-1) was determined during control, aspirin and flurbiprofen (FLU) treatment periods. The unbound fraction of AZ in plasma increased three-fold in the presence of salicylate (SA) while, in contrast, FLU produced consistent, but statistically insignificant, increases in binding. SA caused a two-fold decrease in both unbound AZ renal clearance and apparent volume of distribution at steady-state, while FLU produced a small, but significant, increase only in the latter. The area under the concentration-time curve for AZ in erythrocytes was increased by about 40% during SA treatment while FLU had no effect. Our results suggest that on a pharmacokinetic basis FLU may be a safer nonsteroidal anti-inflammatory drug (NSAID) to co-administer with AZ.

Acetazolamide blood concentrations are excessive in the elderly: propensity for acidosis and relationship to renal function

Elderly glaucoma patients are often treated with acetazolamide, a carbonic anhydrase inhibitor with clearance dependent on renal function. A high incidence of metabolic acidosis and other adverse effects have been noted among these patients but the reasons for this have not been explained. We hypothesized that commonly used doses of acetazolamide among the elderly result in excessive blood concentrations and that these concentrations are related to acid-base disturbances. We measured steady-state acetazolamide levels in plasma, plasma ultrafiltrate (unbound), and erythrocytes among 12 elderly subjects (79.2 +/- 7.6 years old). Mean plasma (18.9 +/- 10.9 micrograms/mL) and ultrafiltrate concentrations (1.0 +/- 0.7 microgram/mL) exceeded the therapeutic range (plasma 5-10 micrograms/mL; ultrafiltrate 0.25-0.50 microgram/mL) for glaucoma control by two fold and were elevated in 75% of subjects. Plasma and ultrafiltrate acetazolamide levels significantly correlated with the dose adjusted for creatinine clearance (r = 0.91, P less than 0.001; r = 0.89, P less than 0.001, respectively). Acidotic subjects (serum total carbon dioxide less than or equal to 22 mEq/L) tended to have higher plasma, ultrafiltrate, and erythrocyte acetazolamide levels compared with nonacidotic subjects. Serum total carbon dioxide levels were significantly correlated with erythrocyte acetazolamide concentrations (r = -0.75, P = 0.03). The ratio of erythrocyte acetazolamide concentration to creatinine clearance separated acidotic from nonacidotic subjects (P less than 0.01). These findings suggest that some of the adverse effects of acetazolamide can be avoided by reducing the dose to compensate for age-related reductions in renal drug clearance.

Use of acetazolamide as an adjunct to carbamazepine in refractory partial seizures

Acetazolamide (Diamox) (AZM) was evaluated as an adjunct to carbamazepine (CBZ) monotherapy in 48 refractory partial seizure patients at a tertiary care referral center. Patient ages ranged from 6 to 64 years (average 28 years). Seizure frequencies for the pre-AZM baseline period (CBZ monotherapy) were compared with the seizure frequencies at different daily doses of AZM. Patients with a 50% decrease in seizure frequency were considered responders. Twenty-one patients were responders (44%) and three became completely seizure-free. Effective doses ranged from 3.8 to 22.0 mg/kg/day. Effective plasma concentrations ranged from 1 to 22 micrograms/ml in selected patients. Durations of response time to AZM ranged from 3 to 30 months (average 12.9 months). Three patients lost response, one temporarily. Side effects were seen in 10 patients, requiring discontinuation in three.

Enhancement of the ocular hypotensive effect of acetazolamide by diflunisal

We studied the effect of diflunisal on intraocular pressure in patients with glaucoma who were receiving maximally tolerated therapy. Diflunisal therapy, 500 mg twice daily, was started in 48 patients for one week. No changes were made in their regular antiglaucoma medications. Intraocular pressure was reduced an additional 3.8 +/- 3.1 mm Hg (+/- S.D.) in the acetazolamide-treated patients (P less than .0001) and 1.6 +/- 1.5 mm Hg in methazolamide-treated patients (P less than .02), while no significant reduction in intraocular pressure was found in patients receiving topical medications alone. In 15 acetazolamide-treated patients, total plasma concentrations of acetazolamide after diflunisal therapy were significantly higher than the prediflunisal levels, suggesting a modest decrease in renal excretion. In seven acetazolamide-treated patients, free plasma concentrations of acetazolamide were found to increase 5.6-fold after diflunisal therapy. We concluded that diflunisal potentiated the ocular hypotensive effect of acetazolamide by increasing its free plasma level.

Effect of salicylate on serum protein binding and red blood cell uptake of acetazolamide in vitro

The diffusion of acetazolamide from buffered saline and buffered albumin solutions into human erythrocytes has been characterized. A model was developed for describing the effects of both intra- and extracellular binding on the approach to distributional equilibrium. Unbound acetazolamide entered the cells via an apparent first-order process at a rate that was unaffected by salicylate at a therapeutic concentration of 200 micrograms/mL. Salicylate concentrations ranging from approximately 100 to 400 micrograms/mL, were, however, extremely effective in displacing acetazolamide from its serum protein binding sites. Free fractions of acetazolamide in human serum were found to increase by an order of magnitude as salicylate concentrations approached 400 micrograms/mL, thereby greatly increasing the concentration of unbound drug available for passive diffusion into cells. The results indicate that while competitive binding effects, which may alter unbound drug concentration-time profiles and potentially impact on toxicity, do occur, alterations in red cell membrane permeability, which could adversely affect carbon dioxide transport, are not of significance.

Solid-phase extraction of acetazolamide from biological fluids and subsequent analysis by high-performance liquid chromatography

A sensitive, relatively fast and simple to operate high-performance liquid chromatographic method for the determination of acetazolamide in plasma and saliva is described. Quantitative extraction of the drug from both plasma and saliva was achieved using commercially available reversed-phase octadecylsilane-bonded silica column (Bond-Elut C18, 2.8 ml capacity). Acetazolamide and the internal standard are retained on the Bond-Elut C18 column and reproducibly recovered by elution with methanol. Liquid-liquid partition chromatography, carried out on a 30-cm mu Porasil column (10-microns porous silica) using a mobile phase consisting of dichloromethane-ethanol-water-glacial acetic acid (500:65:65:1), provided adequate separation with acceptable retention times. Acetazolamide levels in the region 50-100 ng/ml can be determined in 100 microliters of plasma or 200 microliters of saliva employing ultraviolet detection at 254 nm with a sensitivity of 0.005 absorbance units full scale. Although the method is primarily used to determine steady-state drug levels in paediatric patients, its general applicability is illustrated by the 24-h plasma and saliva concentration profiles obtained from a male volunteer following oral administration of acetazolamide.

Influence of advanced age on the disposition of acetazolamide

The disposition kinetics of acetazolamide (AZ) has been studied in four young and four elderly healthy volunteers, each of whom received an intravenous bolus dose of 5 mg/kg. The concentration time profile of AZ was determined in plasma, plasma ultrafiltrate, erythrocytes and urine. While the mean area under unbound plasma concentration-time curves was 81% higher in elderly subjects, areas based on total drug concentrations were similar in both groups. The mean renal plasma clearance was similar in both groups. The mean renal plasma clearance was similar between young and old for total AZ, but was significantly lower in the elderly for unbound drug (8.88 ml min-1 kg-1 vs 15.7 ml min-1 kg-1). Renal clearance of unbound AZ correlated well with creatinine clearance (r = 0.846, P less than 0.01). Peak erythrocyte levels were 45% higher in the elderly group (37.2 micrograms/ml vs 25.3 micrograms/ml) and were paralleled by a 46% increase in the mean area under the erythrocyte concentration-time curve for this age group. The unbound fraction of AZ in plasma was significantly greater in elderly than younger subjects (6.9 vs 4.1%, P less than 0.05). Integrated AZ erythrocyte concentrations correlated positively with AZ free fraction in plasma and inversely with its unbound renal clearance. These observed differences in AZ disposition between elderly and young have served to clarify host factors which may importantly influence susceptibility to adverse effects.

Role of carbonic anhydrase in bone: plasma acetazolamide concentrations associated with inhibition of bone loss

Earlier reports from our laboratory have indicated that the carbonic anhydrase inhibitor acetazolamide blocks the hypercalcemic response to parathyroid hormone. In addition, we have reported that acetazolamide when administered by several routes partially prevents denervation-induced bone loss in a rat model of disuse osteoporosis. Continuous subcutaneous infusion required the least daily dose (8 mg/kg). The present study extends these earlier findings in several ways. It was found that in partially preventing denervation-induced bone loss: (1) incorporation of 1 M THAM [tris(hydroxymethyl)aminoethane] enhanced the potency of acetazolamide such that it was effective at daily doses of 0.6 mg/kg; (2) acetazolamide in the presence of 1 M THAM was effective at plasma concentrations as low as 50 ng/ml which are more than 500-fold less than peak plasma concentrations normally encountered in the human when acetazolamide is being used as a therapeutic agent; and (3) another inhibitor, benzolamide, was also effective by continuous subcutaneous infusion.

Local ocular hypotensive effect of topically applied acetazolamide

Acetazolamide's usefulness in the treatment of the glaucomas is limited by the systemic side effects that often accompany its oral administration, and topical administration was initially thought to have no effect upon the intraocular pressures of human and rabbit eyes. Recent studies, however, have shown the usefulness of water-loading tests for screening drugs with potential antiglaucomatous activity. We found evidence that topical acetazolamide has the ability to lessen the increase in intraocular pressure after water-loading in pigmented rabbits and correlated this observation with low levels (0.0 to 0.7 microgram/ml) of plasma acetazolamide. Further, a separate study showed that 10% topical acetazolamide can enhance the ocular hypotensive effects of systemically administered acetazolamide in normal pigmented rabbits, suggesting that topically applied acetazolamide can have a local effect on intraocular pressure.

Determination of acetazolamide in biological fluids by reverse-phase high-performance liquid chromatography

A high-performance liquid chromatographic method for the determination of acetazolamide in whole blood, plasma, and urine was developed. Samples of biological fluids containing various concentrations of acetazolamide were spiked with the internal standard, sulfadiazine. Samples were then mixed with a 50% ammonium sulfamate solution. Whole blood samples were heated for 25 s in boiling water. All samples were extracted with ethyl acetate; a phosphate buffer (pH 8.0) was used to wash the extracts. Acetazolamide was back-extracted into a glycine buffer (pH 10.0), which was then washed with ether. Separation of acetazolamide and internal standard from other biological constituents was achieved on a 10-micron C18 reverse-phase column using an acetonitrile-methanol-acetate buffer (pH 4.0). The eluant was monitored at 254 nm. All calibration curves were linear, and the results from reproducibility studies were excellent. Application of the method to human pharmacokinetic studies was demonstrated.

Chronic acetazolamide intoxication

Severe acidosis associated with acetazolamide therapy is rare. We report the first case in which plasma and whole blood acetazolamide concentrations were measured. A 61 year-old patient receiving oral acetazolamide for treatment of glaucoma presented with a 7 day history of declining mental status. The patient was lethargic and oriented only to name. The respiratory rate was 36 per minute in a Kussmaul pattern with arterial blood gases revealing a pH of 7.23, pO2 68 mmHg, paCO2 14 mmHg and bicarbonate 6 mEq/L. Serum creatinine was 3.1 mg%, Cl 126 mEq/L, and anion gap 15. Urine pH was 6.0. Infection and other causes of acidosis and bicarbonate loss were excluded, and he was discharged with normal mental status and improving acid-base balance 18 days after admission. Acetazolamide concentrations four days after the last dose were 26.38 mcg/ml and 38.84 mcg/ml in serum and whole blood, respectively. The serum half-life was 34 hours, compared to a range of 1.5 to 6 hours in subjects with normal renal function. Monitoring acetazolamide concentrations may be useful in adjusting dosage and preventing toxicity in patients with decreased renal function.

Bicarbonaturic effect of acetazolamide in the dog: the influence of graded volume expansion

Studies were performed in anesthetized dogs to characterize the effect of a progressive volume expansion on the acetazolamide-induced bicarbonaturia. A closed system with urine reinfusion was used in all these experiments. In normovolemic dogs, 24% of the filtered bicarbonate was excreted into the urine while this value reached 62% when a 10% expansion was superimposed on a continuous infusion of acetazolamide. When a single dose of acetazolamide was given, fractional bicarbonate excretion increased from 21% in normovolemic dogs to 46% during 10% expansion. Without acetazolamide administration, 13% of the filtered bicarbonate was excreted during a 10% expansion. The continuous infusion of acetazolamide in normovolemic dogs increased fractional bicarbonate excretion in a progressive fashion, from 25 to 40%. This study shows that an acute volume expansion potentiates markedly the bicarbonaturic effect of acetazolamide, fractional bicarbonate excretion exceeding by far the simple additive effect of acetazolamide and expansion. We speculate that volume expansion might prevent a compensatory rise in acetazolamide-insensitive bicarbonate reabsorption in sites other than the superficial proximal convoluted tubules.

Effectiveness of generic acetazolamide

Comparisons were made between the ocular hypotensive effects and blood levels achieved with the single-dose administration of either generic acetazolamide or brand-name acetazolamide (Diamox). The relative cost of the two products was surveyed. The effect of food on the absorption of acetazolamide was also evaluated. The generic and brand-name acetazolamide were equivalent in their effects on intraocular pressure. Comparable blood levels of acetazolamide were obtained with the two products. The cost of generic acetazolamide was 37% less than brand-name acetazolamide, when available. Food intake did not appear to influence the absorption of acetazolamide.

Monitoring acetazolamide treatment

Electron capture gas chromatography was used to determine plasma concentrations after various doses of acetazolamide. In 40 patients steady state plasma concentrations were determined for daily doses of 187.5, 375, 750, and 1000 mg. Mean plasma concentrations increased with increasing dosages but there were marked interindividual variations. Part of the interindividual variation was explained by a positive correlation between age and plasma concentration. In 10 patients with previously untreated glaucoma intraocular pressure (IOP) responses and plasma concentrations were determined for increasing doses of acetazolamide. Increasing IOP reductions were obtained up to a dose of 750 mg while a daily dose of 1000 mg acetazolamide had no further effect on IOP. The relationship between IOP reduction and plasma concentration showed great interindividual variations, from a pressure reduction of 11 mmHg at 6 micrograms/ml acetazolamide to a pressure reduction of 0 mmHg at 11 micrograms/ml. As a rule, the maximal effect on IOP was obtained at a plasma concentration between 5 and 10 micrograms/ml. In most patients a daily dose of 1000 mg resulted in plasma concentrations above 10 micrograms/ml.

Determination of acetazolamide in biological fluids by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) assay for acetazolamide is presented. A 100-microliter sample is mixed with an aliquot of the internal standard solution and the mixture, buffered at pH 4.5, is extracted with ethyl acetate. The extract is evaporated to dryness and the residue is analyzed by HPLC, using a reverse-phase octadecylsilane column. The wavelength of the detection is 254 nm. The coefficient of variation (CV) in the within-day analysis of replicate 10-microgram/ml acetazolamide samples in human blood plasma was 6.5%, while the between-day CV was 7.1%. The procedures was developed for the 1-25 microgram/ml acetazolamide concentration range. The internal standard used is similar in chemical structure to acetazolamide and can be readily prepared in one step from a commercially available precursor. In addition to blood serum or plasma, the assay can also use aqueous and vitreous humor samples. Theophylline and acetaminophen interfere in the assay. The technique was used to determine the concentration of acetazolamide in the blood serum of human volunteers after an oral dose of the drug, and in the aqueous and vitreous humors of rabbits after an intravenous dose of acetazolamide.

Acetazolamide dosage forms in the treatment of glaucoma

Patients with chronic glaucoma had a carefully scheduled series of intraocular pressure measurements before and after taking acetazolamide for one week at the following dosages: none, 500 mg of sustained-release capsules once a day, 500 mg of capsules twice a day, and 250 mg of tablets four times a day. A capsule taken once a day, which is better tolerated than one taken twice a day by some patients, offers a substantial pressure-lowering effect that lasts at least 23 hours, although the magnitude of the pressure lowering is less than with higher dosages. One capsule twice a day appears to be as effective in the regulation of IOP as one tablet four times a day. The 45% reduction in outflow pressure is achieved with an acetazolamide serum concentration in the range of 15 to 20 micrograms/mL.

Hemodialyzability of acetazolamide

Because the dialyzability of acetazolamide is not known, we undertook a study to determine the in vivo dialysance of this drug and found it to average 22 ml per minute. The quantity of the drug removed by four hours of dialysis was approximately 30% of the dose administered intravenously one half hour before dialysis. Accordingly, considerable amounts of the drug are removed by dialysis, despite its high intraerythrocytic distribution and plasma protein binding properties. Therefore, hemodialysis may be effective in the management of acetazolamide overdose, particularly when complicated by the presence of renal failure. The dialysance can vary under different dialyzing conditions. An acetazolamide/urea nitrogen extraction ratio of 0.16 established in this study can be used to predict the acetazolamide dialysance in various dialyzing conditions.

Quantitation of acetazolamide in plasma by high-performance liquid chromatography

A method for estimating acetazolamide concentrations in human plasma is described. Buffered plasma (pH 4.8) containing chlorothiazide as an internal standard is extracted twice with ethyl acetate. The extract is evaporated, redissolved, and chromatographed on silica gel with hexane-chloroform-methanol-acetic acid (65:25:10:0.25) as the mobile phase. The extraction efficiencies were greater than 90%, the coefficients of variation at 1 and 30 micrograms/ml of plasma were 3.5 and 2.0%, respectively, and the calibration curves were linear and had an intercept of essentially zero. The suitability of the method for pharmacokinetic studies was verified in a normal volunteer dosed with 250-mg (solution) and 500-mg (sustained-release tablet) acetazolamide formulations.

Acidosis inhibits the hypocalcemic effect of acetazolamide

The effect of acetazolamide on calcium metabolism was examined using sham-operated, ureter-ligated and nephrectomized rats. Acetazolamide doses from 10 to 500 mg/kg produced significant hypocalcemic effects in ureter-ligated and nephrectomized rats. However, doses of acetazolamide up to 1000 mg/kg were devoid of hypocalcemic activity when administered to sham-operated rats. Sham-operated rats exhibited an acidotic response to acetazolamide while ureter-ligated rats did not. Attenuation of this drug-induced acidotic response with i.p. injections of tris(hydroxymethyl)amino-methane uncovered a hypocalcemic effect of acetazolamide in sham-operated rats. Also, the hypocalcemia associated with acetazolamide treatment of ureter-ligated rats was negated when an acidosis was induced by prior injection of NH4Cl. These data indicate that the administration of inhibitors of carbonic anhydrase produces a hypocalcemia when a metabolic acidosis is not present.

Acetazolamide binding to two carbonic anhydrase isoenzymes in human erythrocytes

Acetazolamide binding to high activity and low activity carbonic anhydrase isoenzymes in red blood cells was studied. Inhibitory constants of 0.041 and 2.72 microM and maximum binding capacities of 17.2 and 155 microM, respectively, were found.

Nonlinear model for acetazolamide

Intravenous bolus injections of 14C-labeled acetazolamide were made in rabbits. Plasma, urine, and washed red blood cell concentrations were measured, the latter indicating bound drug. AUTOAN and NONLIN were used to fit the plasma data to a linear two-compartment model. However, utilization of the urine and red blood cell data suggested that a nonlinear model was more appropriate. The developed nonlinear system uses a one-compartment model with two tissue-binding parameters. The system simultaneously fits three equations describing drug in the plasma, in the body, and bound to red blood cells, Six parameters were estimated. The initial plasma concentration and the maximum amount bound to tissue protein (minus red blood cell protein) correlated with dose. The dissociation constant from this protein fraction suggested that it is composed mainly of the enzyme, carbonic anhydrase. The dissociation constant for the red blood cell fraction suggested that the drug binds to other protein in addition to carbonic anhydrase. The elimination constants were quite similar, indicating little variation from one animal to another. Utilization of the concepts of site and mechanism of action in this model should be of considerable help in relating drug concentration to pharmacological resonse.

Extractive alkylation of sulphonamide diuretics and their determination by electron-capture gas chromatography

The extractive alkylation of 11 sulphonamide diuretics has been evaluated using tetrabutylammonium, tetrapentylammonium or tetrahexylammonium as counter ion at different pH values and methyl iodide in methylene chloride as the organic phase. The sulphonamides are methylated within 20 min with tetrahexylammonium as counter ion in 0.2 M sodium hydroxide at 50 degrees. The derivatives have been identified by mass spectral and nuclear magnetic resonance analysis. Relative retentions of the derivatives are given using 1% SE-30 as the stationary phase. A contaminant, dimethylsulphuric acid, occurs in methyl iodide and seriously disturbs the gas chromatographic analysis. The application of the extrative alkylation to biological samples is demonstrated by the direct analysis of acetazolamide in serum. 0.1 M tetrapentylammonium in 0.5 M sodium hydroxide is suitable as the aqueous phase with 1.6 M methyl iodide as alkylating reagent in methylene chloride. The trimethyl derivative of acetazolamide formed has been determined by electron-capture gas chromatography down to 0.5 microgram/ml in a 0.1-ml serum sample. The relative standard deviation at the 10 microgram/ml level is 6.6% (n = 10).

Bioavailability of acetazolamide tablets

Plasma acetazolamide levels were measured by an enzymatic assay following single 250-mg oral tablet doses to 20 healthy volunteers; five different lots of acetazolamide tablets from a single manufacturer were used in a balanced incomplete block design. From the measured plasma levels, estimates of the bioavailability parameters (area under the plasma concentration versus time curve, time to peak plasma concentration, and peak plasm concentration) were obtained by least-squares digital computer fitting. No significant differences among the tablets were observed (alpha = 0.05) for the analysis of variance of the area under the curve or time to peak parameters. Two tablets, however, provided statistically higher peak plasma concentrations than the other three. Thus, lot-to-lot bioinequivalence of acetazolamide tablets was observed. Some in vitro tests employed showed general trends for correlation with the in vivo data. However, considerable refinement of these technique appears necessary for in vitro prediction of the observed lot-to-lot bioinequivalence.

Short-term dose response characteristics of acetazolamide in man

Nine patients with ocular hypertension each randomly received on separate days 0, 63, 125, 250, and 500 mg of acetazolamide (Diamox). In a double masked manner, acetazolamide plasma levels and intraocular pressure were monitored for seven hours following administration. Plasma levels increased linearly with dose, reaching 30 microgram/ml with the 500-mg dose. Maximum plasma levels occurred at one hour, and the minimum IOP was at two hours. The maximum IOP effect was a 30% to 35% fall. The IOP response was related to dose and plasma level, up to a 63-mg dose, which produced an average fall of 8.2 mm Hg. Little further average effect was documented at higher doses or plasma levels. The duration of response was slightly prolonged by 250 mg, but 500 mg showed no greater response. Thus, a 63-mg dose or a plasma level of 4 to 5 microgram/ml was as effective in lowering IOP as higher doses that produced plasma levels of 10 microgram/ml or more.

Dosage form index: an objective criterion for evaluation of controlled-release drug delivery systems

A dimensionless parameter, the dosage form index (DLtau) is proposed for evaluating the performance of drug delivery systems. The index is defined as the ratio of the maximum to minimum concentrations of the drug in plasma within each interdose interval (in hours), tau, during repetitive administration of the dosage form in the quasisteady state. Dosage form indexes can be averaged among subjects or within subjects at successive time periods to arrive at a mean value. As an example, two GI therapeutic systems--the 15- and 20-mg/hr acetazolamide systems that deliver drug at constant rates for 6 and 12 hr and contain 125 and 250 mg, respectively--were compared in normal subjects with a commercial sustained-release product containing 500 mg of acetazolamide. The dosage form index, DI24, was 4.9 for the sustained-release dosage form and 3.2 for the 20-mg/hr system; DI12 was 1.6 for the 15-mg/hr system.

Uptake of acetazolamide by human erythrocytes in vitro

The binding of acetazolamide to human erythrocytes was studied in vitro. Blood and plasma samples were analyzed by electron-capture GLC. At 37 degrees, drug equilibrated between plasma and erythrocytes in approximately 40 min. The effect of plasma concentration on the steady-state level of drug within the erythrocytes was examined. Erythrocyte accumulation of acetazolamide appeared to be a composite of two processes: a nonlinear, saturable process and a linear, diffusion-controlled process. By appropriate linear transformation of the data, estimates of the data, estimates of the erythrocyte binding capacity and the dissociation constant for the drug were obtained.

GLC analysis of acetazolamide in blood, plasma, and saliva following oral administration to normal subjects

An electron-capture GLC assay for acetazolamide in biological fluids was developed. Extraction efficiency was 69-104%. The minimum detectable amount of acetazolamide was 10 ng/sample. Concentrations of acetazolamide were determined by GLC in blood, plasma, plasma water, and saliva after oral administration of a single 250-mg dose to five volunteers. Erythrocyte levels were calculated from whole blood and plasma data. Concentration of free drug in the plasma was measured in samples of plasma water obtained by microultrafiltration. Peak plasma levels of 10-18 microng/ml were reached 1-3 hr after the dose. At least 1 hr later, erythrocyte levels reached peak concentrations of 13-29 microng/ml. Over 31 hr, plasma levels declined more rapidly than erythrocyte levels. Saliva concentrations averaged 1% of those in plasma and decreased at a rate equal to that of plasma. Saliva levels were proportional to, but not equal to, water concentration. Saliva to plasma ratios were consistent for any given individual and, therefore, offer a means of monitoring drug dosage without resorting to frequent blood sampling.

Assay for acetazolamide in plasma

A method for the analysis of acetazolamide, 5-acetamido-1,3,4-thiadiazole-2-sulfonamide, sensitive to 25 ng/ml in plasma, was developed. After extraction of acetazolamide and its propionyl analog, 5-propionamido-1,3,4-thiadiazole-2-sulfonamide, the internal standard, from plasma with ethyl acetate and removal of lipids from the residue of the ethyl acetate extract with methylene chloride, the sulfonamides were chromatographed on an octadecyl trichlorosilane bonded phase using high-pressure liquid chromatography. The method was developed to study plasma level profiles of different dosage forms of acetazolamide.