Phenytoin protein binding and dosage requirements during acute and convalescent phases following brain injury.

OBJECTIVE: To longitudinally evaluate unbound and total serum phenytoin concentrations during intravenous phenytoin maintenance dosage and to determine the relationship among phenytoin protein binding, serum albumin, and unbound fatty acid concentrations in patients with head injuries during intensive care unit (ICU) and convalescent care. DESIGN: Serum albumin and phenytoin unbound fraction were determined twice weekly during ICU and convalescent care in 10 patients receiving phenytoin following acute brain injury. Phenytoin protein binding was also determined in 10 healthy control subjects. MAIN OUTCOME MEASURES: Longitudinal serum phenytoin concentrations associated with dosage adjustments targeted to achieve unbound phenytoin serum concentrations between 1.0 and 2.0 mg/L were documented during ICU and convalescent care. Longitudinal phenytoin binding was correlated with serum albumin and unbound fatty acid concentrations in neurotrauma patients. RESULTS: ICU patients received intravenous therapy for a mean of 15.0 days. The mean +/- SD initial phenytoin intravenous dosage regimen of 6.0 +/- 0.7 mg/kg/d resulted in mean +/- SD total and unbound phenytoin concentrations of 3.2 +/- 2.3 and 0.3 +/- 0.2 mg/L. Two patients had seizures associated with low phenytoin concentrations. Four patients continued to receive oral phenytoin therapy during convalescent care; phenytoin dosage requirements decreased over time in these patients. During acute and convalescent care, the phenytoin unbound fraction ranged from 6.0% to 18.3% and correlated with albumin (r2 = 0.61, p < 0.0001) but did not correlate with unbound fatty acid concentrations. The mean phenytoin unbound fraction was 10.1% and 8.9% for the ICU and convalescent patients with brain injuries, respectively, and was 7.0% for the healthy volunteers. CONCLUSIONS: Phenytoin protein binding was significantly correlated with albumin and was more variable in ICU and convalescent patients with brain injuries than in healthy volunteers. The high dosage requirements and subtherapeutic unbound phenytoin concentrations observed during acute care are best explained by increased metabolism. Phenytoin dosage requirements decreased during convalescence.

Evaluation of the Technicon Immuno 1 free thyroxine assay.

The Technicon Immuno 1 free thyroxine (fT4) assay, a modified two-step procedure that is fully automated on a random access analyzer, was evaluated-at two clinical sites. The method had excellent precision and correlated well overall with three other estimates of free thyroxine: free thyroxine index (FTI) measured on the Immuno 1; the Abbott IMx fT4 assay (Abbott Park, IL); and the Clinical Assays two-step manual fT4 assay. Using a combination of thyrotropin and FTI assay results as a "gold standard" for defining thyroid status, the Immuno 1 fT4 method had a sensitivity of 100% and specificity of 98.3% for hyperthyroidism, versus 93.8% and 99.3%, respectively, for hypothyroidism. In conclusion, the Immuno 1 fT4 assay is useful in screening for thyroid disease.

Comparison of gentamicin pharmacokinetic parameters determined by fluorescence polarization immunoassay and latex agglutination methods.

Fluorescence polarization immunoassay (FPIA) (TDx, Abbott Laboratories Diagnostics, Irvin, TX, U.S.A.) is commonly utilized for quantitative determination of gentamicin serum concentrations. Recently, automated homogeneous latex agglutination (LA) (Technicon Immuno-1, Miles Diagnostics Division, Tarrytown, NY, U.S.A.) for the quantitative determination of gentamicin serum concentrations has been approved for commercial use. The purpose of this study was to determine whether gentamicin serum concentration-time data from the same patients assayed by FPIA and LA would produce the same estimates for half-life, elimination rate constant, distribution volume, drug clearance, dosage interval, and dose. A total of 70 pre- and postinfusion serum samples were obtained from 19 patients. Each sample was divided into two aliquots; one was assayed by FPIA and the other by LA. The correlation coefficient between the two assay methods was 0.99 (y = 1.03x - 0.05). The mean differences for half-life, volume of distribution, elimination rate constant, total body clearance, and gentamicin dosage were 0.13, 0.32, 0.66, -0.99, and 0.03%, respectively. No statistically significant differences were seen in calculated gentamicin pharmacokinetic parameters (p < 0.05). Pharmacokinetic parameters and dosage recommendations derived from FPIA and the LA assay using pre- and postinfusion serum concentration-time data appear interchangeable and do not result in differences between gentamicin dosing regimens.

Fluorescein and acidosis. Implications for flap perfusion studies.

The visual fluorescein technique underpredicts survival of a skin flap during the first 24 hours after raising the flap. This problem limits its use as a research and clinical tool. There is no compelling explanation for this observation, but two facts are known: fluorescein is a derivative of phthalein, a pH indicator, and ischemic tissues become acidotic, with the pH falling below 7.0. These observations lead to the hypothesis that acidosis quenches fluorescence in distal skin flaps. No data could be found regarding this effect. Therefore, the effect of acidosis on sodium fluorescein was studied in vitro. A spectrophotometer was used to measure the intensity of the fluorescence of sodium fluorescein in buffered solutions of different hydrogen ion concentrations. Two studies were performed at different concentrations of fluorescein. At a concentration of 10(-5) g/mL, there is a drop of 26% in fluorescence intensity from pH 7.5 to 7.0, and 51% between pH 7.5 and 6.5. At 10(-7) g/mL, there is a 43% decrease in fluorescence between pH 7.5 and 6.5. This study supports our hypothesis that acidosis quenches the fluorescence of fluorescein. This effect must be considered when interpretating basic studies of skin flap microcirculation.

Compatibility and pH variability of four injectable phenytoin sodium products.

The interlot variability in apparent pH of injectable phenytoin sodium products and the compatibility and stability of phenytoin sodium in admixtures of these products in 0.9% sodium chloride injection were studied. Dilantin (Parke-Davis) and three generic products (Elkins-Sinn, Lyphomed, Solopak) were used. Six lots of each product were diluted to 9.2 and 18.4 mg/mL concentrations. The apparent pH values of undiluted lots and diluted product admixtures were measured. Phenytoin concentrations in the admixtures were measured by turbidimetric immunoassay. Concentration and pH were measured immediately after admixture and at 0.5, 1, and 2 hours; samples were examined for crystallization at each time, and portions were filtered to determine differences in drug concentration between the filtered and unfiltered samples. The Dilantin lots had the lowest interlot variability and significantly higher mean +/- S.D. apparent pH (12.00 +/- 0.06) than the Solopak (11.38 +/- 0.33), Elkins-Sinn (11.39 +/- 0.21), and Lyphomed (11.68 +/- 0.36) products. The apparent admixture pH was significantly higher for Dilantin than for the other products. No crystallization occurred in the Dilantin admixtures; crystallization varied in the other products. Filtration did not significantly alter phenytoin concentrations. No significant differences were detected in phenytoin concentrations between products or sampling times. Interlot variability in pH was lowest for Dilantin, and apparent pH values of the undiluted products and in the admixtures at both drug concentrations were significantly higher for Dilantin than for the other products. Microscopic evidence of physical incompatibility was noted in some generic product lots with lower apparent pH values. Stability of phenytoin in the admixtures over two hours was demonstrated for all four phenytoin sodium injectable products studied.

Comparison of pharmacokinetic and safety profiles of amantadine 50- and 100-mg daily doses in elderly nursing home residents.

Pharmacokinetic and adverse event profiles of 50- and 100-mg amantadine doses administered daily for up to 21 days for influenza prophylaxis were compared in 82 elderly nursing home residents (mean age 85 yrs, 68% female). We sought to determine if a standard daily dose of 50 mg would achieve mean steady-state trough serum concentrations (CPSSt) of 300 ng/ml and be associated with a lower frequency of adverse events than 100-mg doses. Statistically significant relationships were found between CPSSt and dosage (in mg/kg/day) and serum creatinine. Adverse events were more common with the 100-mg dose (24% vs 14%); 94% occurred in women. Amantadine CPSSt and apparent clearance were not significantly different between sexes. Thirty-nine residents (89%) receiving 50 mg daily achieved CPSSt below 300 ng/ml compared to 42% receiving 100 mg. Standard daily amantadine doses of 50 mg may not achieve adequate CPSSt in elderly nursing home residents, but 100 mg may lead to excessive CPSSt and adverse events, especially in the presence of renal impairment.

Relative bioavailability of rectally administered phenobarbital sodium parenteral solution.

Rectal administration of antiepileptic drugs may be a useful alternative route when oral administration is not possible due to illness, surgery, or status epilepticus. Although parenteral administration often replaces oral administration in these circumstances, there is not always a desirable intravenous line available or repeated intramuscular injections may not be practical. The purpose of this study was to determine the relative bioavailability and time course of absorption of the commercially available parenteral phenobarbital sodium solution administered rectally in comparison with the same preparation given intramuscularly. Seven healthy adult volunteers were given phenobarbital 5 mg/kg intramuscularly and rectally five weeks apart. Eighteen blood samples were drawn over 288 hours. Pharmacokinetic parameters following intramuscular versus rectal administration were the following: area under the curve 5916 vs. 5253 mumol.h/L; half-life 112 vs. 113 h; time to maximum concentration 2.1 vs. 4.4 h; and maximum serum concentration 36.2 vs. 31.4 mumol/L. Mean relative bioavailability for rectal phenobarbital was 90 percent. Therefore, the parenteral phenobarbital sodium solution given rectally is well absorbed and provides a useful alternative route of administration.

Circadian and seasonal variations in iodine excretion in children with and without endemic goiter.

The urinary iodine excretion was measured in 193 children 11 +/- 1.5 years of age living in the endemic goiter area of Dîmbovita, Romania. One hundred and thirty four of the children showed some degree of endemic goiter, 59 showed none. All children followed a diurnal activity pattern with rest during the night. They received their usual iodine supplement of 1 gm potassium iodide once a week during the school year (which included the time of all measurements made). Urine was collected in six 4-hour samples over a 24-hour span. The examinations were conducted during the months of March, June, September and December. Iodine was determined by an automated ceric ion arsenic acid method using a Technicon Autoanalyzer. Circadian and seasonal variations of urine volume and iodine excretion were statistically verified by the cosinor technique and the seasonal variations also by one way analysis of variance using the circadian means as input. A comparable circadian rhythm of iodine excretion was found in the children with and without endemic goiter, with an acrophase during the evening (20:16 with a 95% C.I., from 19:32 to 21:04). The circadian rhythm in iodine excretion has to be taken into account whenever an estimate of the 24-hour excretion is attempted from a sample covering less than the entire 24-hour span. There was a statistically significant seasonal variation of the 24-hour iodine excretion in the boys with and without endemic goiter and in the group as a whole. The 24-hour iodine excretion during March was 102 +/- 6 mcg, during June 81 +/- 4 mcg, during September 79 +/- 3 mcg and during December 102 +/- 7 mcg. The average 24-hour iodine excretion pooled over all seasons was 91 +/- 3 mcg/24 hrs in the children with and 91 +/- 5 mcg/24 hrs in the children without endemic goiter. During March and December the iodine excretion indicates an iodine intake not usually associated with a high prevalence of endemic goiter. However, during the months of June and September (and presumably even more during the months of July and August when during summer vacation no iodine supplementation was given in school) the 24-hour iodine excretion indicates some degree of iodine deficiency. The seasonal variation in urinary iodine excretion thus points to a time when increased iodine prophylaxis may be of value.

Characterization of beta-lactam antagonist in Supplement C.

In the course of using Mueller-Hinton agar with 1% Supplement C (SC) (Difco, Detroit, MI) as a susceptibility test medium for Haemophilus influenzae, one lot of SC was encountered whose use was associated with markedly increased ampicillin MICs. Acidimetric and chromogenic cephalosporin filter paper disc tests of SC failed to detect beta-lactamase activity. Macrobroth dilution MIC tests to determine substrate specificity showed SC to antagonize benzylpenicillin and ampicillin but not cephalothin, cefazolin, or cefaclor, with the antagonism being prevented by the addition of clavulanic acid. High pressure liquid chromatographic analysis of reference and reaction solutions of benzylpenicillin with SC showed almost complete degradation of benzylpenicillin to benzylpenicilloic acid after 24 hr at 37 degrees C. For two other lots of SC that had passed MIC quality control testing, similar high pressure liquid chromatographic studies demonstrated slow conversion of small amounts of benzylpenicillin to benzylpenicilloic acid. These findings indicate that the beta-lactam antagonism by SC was due to the presence of a contaminating beta-lactamase directed primarily toward the penicillins.

Screening biological samples for underivatized drugs using a splitless injection technique on fused silica capillary column gas chromatography.

A rapid screening technique is presented for the detection of underivatized drugs using a multi-level temperature program. This allowed for the identification of major clinically significant drugs extracted from serum, urine, and other body fluids. Acidic or basic extractions are injected into a gas chromatograph equipped with a flame ionization detector and a single wide-bore, thick film capillary using the splitless mode of injection. Identification is by retention time relative to that of a common internal standard. Deactivation of the fused silica liner is necessary to consistently detect low concentrations of specific drugs.