Peculiar reduction of graphene oxide into graphene after diffusion in exponentially growing polyelectrolyte multilayers.

In the present work, in situ reduction of graphene oxide (GO) into graphene was preformed, after diffusion in exponentially growing polyelectrolyte multilayers, using sodium citrate as the reducing agent. First, the graphene oxide was obtained by treating a commercial grade of Expanded Graphite (EG). Based on XRD and Raman spectroscopy results, a complete exfoliation of graphene nanopellets down to one layer was achieved during the oxidation process. Secondly, the diffusion of GO was carried out in an exponentially growing polyelectrolyte multilayer film made from poly(diallyldimethylammonium chloride) as the polycation and from poly(acrylic acid) as the polyanion. Electrical conductivity of the GO based films was measured during the reduction process as a function of time. The conductivity reached values of the order of 10(-4) S cm(-1), whereas the pristine polyelectrolyte multilayer was highly insulating (∼10(-8) S cm(-1)). The conductivity also reached a maximal value after about 24 h of reduction and decreased for longer reduction duration. Some tentative explanations for this peculiar finding will be given.

Concentration-dependent tubular secretion of acetazolamide and its inhibition by salicylic acid in the isolated perfused rat kidney.

An isolated perfused rat kidney (IPK) technique was used to study the effect of salicylic acid (SA) on the excretion of acetazolamide (AZ). Initial experiments were conducted in the absence of interactants at three nominal AZ concentrations (50, 100, and 250 micrograms/ml). Over the concentration range studied, AZ demonstrated net tubular secretion in the IPK. Significant decreases in excretion ratio (4.97 +/- 0.79-2.66 +/- 1.1) and secretory clearance (0.809 +/- 0.23-0.541 +/- 0.28) were observed with increasing AZ concentration, consistent with saturation of tubular secretion. Using a facilitated model for renal secretion, values of tubular transport parameters were obtained from a plot of excretion ratio vs. unbound AZ concentration: tmax = 118 +/- 29.4 micrograms/min, KM = 53.4 +/- 22.4 micrograms/ml, and tmax(A) = 6.31 +/- 2.82 micrograms/min. In the presence of SA (200 micrograms/ml), renal secretion of AZ was inhibited, as demonstrated by significant decreases in renal clearance (0.731 +/- 0.21-0.147 +/- 0.03) and excretion ratio (3.77 +/- 0.82-0.378 +/- 0.07). Although these findings were indicative of a reabsorption component to AZ excretion in the IPK that had not been previously proposed, the results were consistent with a previous investigation of concomitant administration of AZ and SA in humans (Br. J. Clin. Pharmacol. 27, 866, 1989), thereby endorsing utilization of the IPK as a screening tool for renal clearance mechanisms in humans.

Lymphopenic effect of carbamazepine in a patient with chronic lymphocytic leukemia.

OBJECTIVE: To report a dramatic and reproducible suppressive effect of carbamazepine on circulating lymphocytes in an elderly woman with chronic lymphocytic leukemia. CASE SUMMARY: An elderly woman taking phenytoin for a stroke-associated seizure disorder had lymphocyte count of 28,800 x 10(6) cells/L. Speculating an unusual lymphadenopathic effect of the phenytoin therapy, carbamazepine therapy was substituted. After 15 weeks of carbamazepine treatment, the lymphocyte count declined to 3200 x 10(6) cells/L. Because of severe diarrhea, carbamazepine therapy was stopped and phenytoin therapy was reinstituted. At the end of 4 months of phenytoin treatment, the lymphocyte count had increased to 23,200 x 10(6) cells/L. Phenytoin therapy was discontinued and carbamazepine therapy was begun. The lymphocyte count decreased to 10,700 x 10(6) cells/L. Severe diarrhea recurred and phenytoin treatment was reinstituted. Over 12 days the lymphocyte count increased to 28,900 x 10(6) cells/L. Phenytoin therapy was stopped and valproic acid therapy was started. The lymphocyte count continued to increase during valproic acid therapy, reaching a peak of 114,300 x 10(6) cells/L. DISCUSSION: In this patient with chronic lymphocytic leukemia, carbamazepine therapy had a significant and reproducible lymphopenic effect that was readily reversible upon discontinuation of the drug. Unfortunately, this effect was associated with severe diarrhea, preventing further attempts at exploiting this potentially beneficial action. CONCLUSIONS: Carbamazepine had a reproducible suppressive effect on lymphocyte counts in an elderly patient with chronic lymphocytic leukemia. This unique observation raises the possibility that carbamazepine therapy may have a useful effect in patients with chronic lymphocytic leukemia.

Gastric retention of enteric-coated magnesium chloride tablets.

OBJECTIVE: To describe a patient with gastric retention of enteric-coated magnesium chloride tablets. Potential drug and disease etiologies accounting for failure to empty this dosage form are discussed. DESIGN: Single case report. CASE SUMMARY: A seriously ill patient with metastatic small-cell lung cancer accumulated 21 enteric-coated magnesium chloride tablets in his stomach during a four-day administration period. The patient had gastroscopic evidence of mild pylorospasm and suspected gastric motor dysfunction. The latter may have been the result of several factors including concurrent use of oxycodone, vagal dysfunction from chronic alcoholism and cisplatin-based chemotherapy, and possibly a paraneoplastic neuromuscular syndrome involving the gastrointestinal tract. CONCLUSIONS: Enteric-coated tablets are indigestible solids, often of considerable size. Strong antral contractions, associated with phase 3 of the interdigestive migrating myoelectric complex, are usually required to carry such dosage forms through a normal pyloric channel and into the duodenum. Seriously ill patients who may have gastric hypomotility or pyloric channel narrowing are probably not good candidates for therapy with large enteric-coated dosage forms.

Unmasking the significant enzyme-inducing effects of phenytoin on serum carbamazepine concentrations during phenytoin withdrawal.

OBJECTIVE: We report on two patients who appeared to exhibit profound induction of carbamazepine metabolism during cotherapy with phenytoin. Gradual withdrawal of phenytoin confirmed this impression. DESIGN: Two case studies. RESULTS: Two patients receiving carbamazepine and phenytoin as combination anticonvulsant therapy were admitted for comprehensive rehabilitation. A 23-year-old man had therapeutic serum phenytoin concentrations, but his serum carbamazepine concentrations were so low that they were nonquantifiable. Doubling the daily carbamazepine dosage did not yield quantifiable serum concentrations. When the daily phenytoin dosage was tapered from 500 to 200 mg, the carbamazepine concentration rose to 10.0 micrograms/mL. No further changes in serum carbamazepine concentrations were observed when the phenytoin was discontinued. A 49-year-old man was receiving large daily dosage of phenytoin (600 mg) and carbamazepine (2300 mg). In the process of tapering and discontinuing phenytoin, the patient became lethargic and confused. These signs and symptoms suggested carbamazepine toxicity. The patient was eventually stabilized on a carbamazepine dosage of 1200 mg/d, which produced a serum concentration of 8.4 micrograms/mL. When this patient had been receiving concurrent phenytoin therapy, approximately twice as much carbamazepine (2300 mg) was required to maintain a similar serum concentration. CONCLUSIONS: Phenytoin is a potent inducer of carbamazepine metabolism. Whenever phenytoin dosages are tapered and discontinued in patients receiving these medications concomitantly, frequent serum carbamazepine monitoring is recommended during the ensuing deinduction phase.

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.

Pharmacokinetics and dynamics of furosemide in the newborn piglet.

Furosemide was administered as either an i.v. bolus (6 mg/kg) or primed continuous infusion (4 mg/kg/hr) with quantitative fluid replacement to 10 3-day-old and 9 18-day old piglets. Total and unbound plasma as well as urinary furosemide concentrations were measured for up to 6 hr and drug disposition and renal sodium excretory dynamics were compared at the two ages. The plasma clearance of furosemide was concentration-independent over the range studied (0.1-10 mg/l). Steady-state volume of distribution and unbound fraction of furosemide in plasma were both considerably higher in the younger piglets (618 +/- 320 vs. 201 +/- 71 ml/kg, p less than .01 and 0.22 +/- 0.08 vs. 0.06 +/- 0.02 ml/kg, p less than .001, respectively) while unbound secretory clearance was several-fold lower in this age group (49.2 +/- 23 vs. 107 +/- 55 ml/min/kg, P less than .01). A log-logistic equation was fitted to sigmoidal plots of sodium excretion rate vs. log furosemide excretion rate. While basal response and slope parameters did not differ significantly, maximal response and stimulus required for half-maximal response were both reduced in the younger piglets (0.70 +/- 0.24 vs. 1.18 +/- 0.30 mmol/min and 0.06 +/- 0.04 vs. 0.14 +/- 0.06 mumol/min, respectively, P less than 0.05). Thus, younger piglets were more sensitive to the natriuretic effects of the drug. While term piglets were useful for studying the maturation of protein binding and renal drug excretory processes for furosemide, drug disposition was not comparable to that in human premature infants because of the higher secretory capability of the piglet.

Renal response to furosemide in very low birth weight infants during chronic administration.

Renal response to furosemide following initial and chronic doses was investigated in premature infants with bronchopulmonary dysplasia. Seven infants (mean birth weight = 890 +/- 216 g, mean gestational age at birth = 27.7 +/- 2.6 weeks, mean postnatal age at the start of diuretic therapy = 2.7 +/- 0.9 weeks) were studied. Twelve-hour urine collections were performed after the initial dose, and following chronic doses after 1 week and 3 weeks of therapy. Volume of each urine sample was measured and concentrations of furosemide, sodium and creatinine determined. Linear dose-response relationships were found between the logarithm of the urinary furosemide excretion rate and diuretic/natriuretic response (urine output and urinary sodium excretion rate). The furosemide excretion rate required to achieve midrange diuretic and natriuretic responses was significantly greater during chronic dosing than following initial doses, indicating a decrease in renal responsiveness to drug with sustained use. Increasing postconceptional age was associated with a decrease in initial responsiveness to furosemide. These data demonstrate that in premature infants renal sensitivity to furosemide decreases with chronic use as well as with increasing postconceptional age at the start of therapy. The decrease in renal sensitivity to drug with chronic use is of much greater magnitude, and appears to represent renal compensation for drug-induced diuresis and natriuresis.

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.

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.

Furosemide pharmacokinetics in very low birth weight infants.

The pharmacokinetics of furosemide were studied longitudinally during long-term administration in 10 very low birth weight infants with bronchopulmonary dysplasia. Mean birth weight of the infants was 829 +/- 217 g, mean gestational age at birth was 26.6 +/- 2.9 weeks, and mean postnatal age at the start of therapy was 2.4 +/- 1.0 weeks. Serial determinations of furosemide pharmacokinetic parameters were performed during 2 weeks to 3 months of long-term therapy. Plasma half-life was prolonged in infants less than 31 weeks postconceptional age (gestational + postnatal age), frequently exceeding 24 hours. All infants less than 29 weeks postconceptional age whose dosing schedule was once every 12 hours accumulated furosemide to potentially ototoxic levels. Furosemide renal clearance increased and plasma half-life decreased in association with increasing postconceptional age. Furosemide secretory clearance was very low in patients less than 31 weeks postconceptional age, resulting in a reliance on glomerular filtration to deliver drug to its main site of action within the lumen of the loop of Henle. Thus elevated plasma levels may be required to ensure adequate luminal delivery and adequate diuresis in these infants with low secretory clearance. Nevertheless, the current dosing schedule (once every 12 hours) of furosemide should be modified to once every 24 hours in infants of low postconceptional age to avoid possible toxic effects.

Toxic interaction between acetazolamide and salicylate: case reports and a pharmacokinetic explanation.

Two elderly patients, who were chronically receiving aspirin, developed lethargy, incontinence, and confusion after dosing with acetazolamide. Unbound plasma acetazolamide concentrations were elevated and plasma protein binding was reduced, suggesting an interaction with aspirin. In vitro studies demonstrated a concentration-dependent effect of salicylate on acetazolamide binding to serum proteins. At a therapeutic serum acetazolamide level of 8.0 micrograms/ml, the unbound percentage of acetazolamide in serum was 3.3% and increased to 11.0% and 30.0%, with serum salicylate levels of 200 and 386 micrograms/ml, respectively. Furthermore, the apparent association constant of acetazolamide for binding to serum proteins was decreased by 58% and 86% of its control value at these respective salicylate concentrations. The maximal binding capacity of serum for acetazolamide was not affected by salicylate. Pharmacokinetic studies in four volunteers showed that the plasma protein binding and renal clearance of acetazolamide were significantly reduced during chronic salicylate dosing. Salicylate appears to competitively inhibit the plasma protein binding of acetazolamide and simultaneously to inhibit acetazolamide renal tubular secretion. Caution is advised when acetazolamide and salicylate are used concurrently.

Lack of effect of influenza vaccine on theophylline levels and warfarin anticoagulation in the elderly.

Steady state theophylline concentrations were prospectively measured in 13 elderly institutionalized individuals following influenza vaccination. Vaccine-mediated increase in concentrations could not be identified. In seven individuals receiving warfarin, no change in prothrombin or partial thromboplastin times occurred following vaccination. No adverse drug reactions were observed. The following year 14 individuals receiving theophylline and 12 receiving warfarin were vaccinated without adverse drug reactions noticed. These data fail to support previous reports of influenza vaccine inhibition of theophylline metabolism or vaccine-enhanced anticoagulation and are consistent with recent reports on younger subjects.

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.

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.

Observations on lithium disposition in the elderly.

The disposition kinetics of lithium were studied in six elderly women who had been maintained on this medication for several years. Lithium administration was stopped abruptly. Plasma, whole blood, and erythrocyte half-lives were then measured, as well as renal plasma clearance and distribution volume. Half-life values in whole blood and in its components were found to be similar. The plasma lithium half-life approximated that in younger schizophrenics, but appeared prolonged when compared with half-lives reported for young normal volunteers. Renal plasma clearance and distribution volume of lithium appeared substantially reduced when compared with published values. The data are consistent with altered lithium disposition in the aged.