Serum phenobarbital concentration predictions by a personal computer software system

The ability of a personal computer software system to predict actual serum phenobarbital concentrations (SPC) in outpatients taking phenobarbital chronically was assessed by comparing actual with predicted SPC for accuracy, bias, and precision. Data for a four-year period were collected on patients at an outpatient clinic's pharmacokinetic consultation service. The study group included 50 adults and children with at least one SPC taken at a known time after dose administration. Input variables were weight, sex, height, age, concomitant drugs and diseases, phenobarbital dosage regimen, and the time and reported value of all SPC. Initially, SIMKIN (SIMulated KINetics) simulated dosing regimens on the basis of literature estimates of pharmacokinetic parameters; SPC were then estimated for these regimens and compared with actual values. One or two additional SPC were added to the input data and analyzed, and the predicted SPC compared with actual values. Although SIMKIN's accuracy and bias as measured by regression analysis and mean prediction error, respectively, were within clinically acceptable limits, the precision was not. However, these results are limited by the population studied. Patient compliance, concomitant phenytoin therapy, changes in phenobarbital pharmacokinetic parameters with chronic dosing, and disease interactions may significantly affect predictive ability. The clinical effects of these factors need to be evaluated to further improve predictions.

Pharmacokinetics of loprazolam and its principal metabolite in young subjects and elderly hospital patients

1. The pharmacokinetics of loprazolam and its principal pharmacologically active metabolite, the piperazine N-oxide, were compared in young subjects (aged 21-25 years) and elderly patients (aged 63-86 years) following single oral evening doses (0.5 mg and 1 mg). 2. Plasma loprazolam was assayed by a specific h.p.l.c./g.c. method. The N-oxide metabolite was assayed by gas chromography. 3. Mean times to peak plasma concentration of loprazolam did not differ significantly between young and elderly subjects and ranged from 1.6-2.7 h. There was, however, a longer mean time to peak concentration of the N-oxide metabolite in the elderly but this was only statistically different after the 0.5 mg dose (4.5 mg young, 6.4 h elderly). 4. Mean peak plasma concentrations of loprazolam did not differ significantly between young and elderly nor did plasma concentrations of the N-oxide metabolite. 5. Although the mean elimination half-life of loprazolam was not statistically significantly different between young and elderly subjects (range 10.9-16.0 h) there was a trend towards somewhat longer half-lives in the elderly. Furthermore, there was a small but significant increase in the half-life of the N-oxide metabolite in the elderly after the 1 mg dose from 11.7 h to 16.7 h. 6. The areas under the plasma concentration time curves for both loprazolam and its N-oxide were greater in the elderly being some 50-68% (mean 132.0 and 111.5 ng/ml h) above those found in young subjects (mean 89.8 and 66.0 ng/ml h).(ABSTRACT TRUNCATED AT 250 WORDS)

Modified sheath introducer for reduced arterial damage

An introducer-sheath dilator is described, which was modified to protect the leading lip of the sheath during insertion, thus reducing arterial damage. The new sheath and a standard sheath were inserted into the femoral arteries of six dogs. Subsequent histologic examination of the arteries proved the new sheath to be both safe and effective.

Pharmacokinetic monitoring in the community health-care setting

Ambulatory care pharmacokinetic profiling has been utilized with increasing frequency over the last decade. The demonstration of need is the first step in initiating outpatient pharmacokinetic service. This article identifies methods used in demonstrating a need and discusses the normal daily activities of an ambulatory pharmacokinetic service located in a university-based family medicine clinic. The reader will also learn of trends that ambulatory care pharmacokinetic programs will become involved with in the near future.

Buffer, pH, and ionic strength effects on the (Na+ + K+)-ATPase

A dog kidney (Na+ + K+)-ATPase preparation also catalyzes K+-independent and K+-activated phosphatase reactions with p-nitrophenyl phosphate as substrate. K+-independent activity increases with declining pH over the range 7.5 to 5.8, whereas the other two activities decrease. The increased K+-independent activity is similar with imidazole, histidine, and several Good buffers, and is thus attributable to free H+, probably by affecting enzyme conformations rather than by changing affinity for Mg2+ or substrate or by H+ occupying specific K+-sites. The decrease in K+-phosphatase and (Na+ + K+)-ATPase activities with pH also occurs similarly with those buffers, and is not due to changes in apparent affinity for substrate or for cation activators. However, the Good buffers Pipes and ADA inhibit the K+-independent phosphatase reaction strongly, the K+-activated reaction moderately, and the (Na+ + K+)-ATPase reaction little; both contain two acidic groups, unlike the other buffers tested. Inhibition of the phosphatase reaction by Pipes is associated with a decreased apparent affinity for K+ and an increased sensitivity to inhibition by Na+ and ADP, consistent with Pipes hindering conformational transitions to the E2 enzyme forms required for phosphatase hydrolytic activity.

New heavy-duty exchange guide wire

A major drawback of currently used exchange guide wires is insufficient stiffness, causing them to buckle or dislodge when used during catheter exchanges. A new heavy-duty exchange guide wire is described that facilitates catheter exchanges around sharp corners, such as those encountered during various cardiac interventional procedures. The wire was successfully used in experimental valvuloplasty and experimental dilation of the ductus arteriosus.

Effect of thioridazine on phenytoin serum concentration: a retrospective study

Serum phenytoin concentrations before and after the addition of thioridazine were retrospectively compared in 27 adults to determine if an interaction occurred between these two drugs. A change in the serum phenytoin concentration of +/- 4 micrograms/ml was considered clinically significant; by this definition four patients (14.8 percent) had an increase, two (7.4 percent) had a decrease, and most (77.8 percent) demonstrated no change. The mean difference was 0.8 microgram/ml +/- 3.7 micrograms/ml and was not found to be statistically significant (p less than 0.1). Clinically important alterations in phenytoin serum concentration as caused by thioridazine appear to be infrequent.

Fluoride and beryllium interact with the (Na + K)-dependent ATPase as analogs of phosphate

Fluoride irreversibly inhibits the (Na + K)-ATPase, and this inactivation requires divalent cations (Mg2+, Mn2+, or Ca2+), is augmented by K+, but is diminished by Na+ and by ATP. Prior incubation with the aluminum chelator deferoxamine markedly slows inactivation, whereas adding 1 microM AlCl3 speeds it, consistent with AlF-4 being the active species. Prior incubation of the enzyme with vanadate also blocks inactivation by fluoride added subsequently. Fluoride stimulates ouabain binding to the enzyme, and thus the analogy between AlF-4 and both orthophosphate and orthovanadate is reflected not only in the similar dependence on specific ligands for their enzyme interactions and their apparent competition for the same sites, but also in their common ability to promote ouabain binding. Beryllium also irreversibly inhibits the enzyme, and this inactivation again requires divalent cations, is augmented by K+, but is diminished by Na+ and by ATP. Similarly, prior incubation of the enzyme with vanadate blocks inactivation by beryllium added subsequently. Inactivation by beryllium, however, does not require a halide, and, unlike inactivation by fluoride, increases at basic pHs. These observations suggest that beryllium, as beryllium hydroxide complexes, acts as a phosphate analog, similar to AlF-4 and vanadate.

Reaction sequences for (Na+ + K+)-dependent ATPase hydrolytic activities: new quantitative kinetic models

To delineate better the reaction sequence of the (Na+ + K+)-ATPase and illuminate properties of the active site, kinetic data were fitted to specific quantitative models. For the (Na+ + K+)-ATPase reaction, double-reciprocal plots of velocity against ATP (in the millimolar range), with a series of fixed KCl concentrations, are nearly parallel, in accord with the ping pong kinetics of ATP binding at the low-affinity sites only after Pi release. However, contrary to requirements of usual formulations, Pi is not a competitor toward ATP. A new steady-state kinetic model accommodates these data quantitatively, requiring that under usual assay conditions most of the enzyme activity follows a sequence in which ATP adds after Pi release, but also requiring a minor alternative pathway with ATP adding after K+ binds but before Pi release. The fit to the data also reveals that Pi binds nearly as rapidly to E2 X K X ATP as to E2 X K, whereas ATP binds quite slowly to E2 X P X K: the site resembles a cul-de-sac with distal ATP and proximal Pi sites. For the K+-nitrophenyl phosphatase reaction also catalyzed by this enzyme, the apparent affinities for both substrate and Pi (as inhibitor) decrease with higher KCl concentrations, and both Pi and TNP-ATP appear to be competitive inhibitors toward substrate with 10 mM KCl but noncompetitive inhibitors with 1 mM KCl. These data are accommodated quantitatively by a steady-state model allowing cyclic hydrolytic activity without obligatory release of K+, and with exclusive binding of substrate vs. either Pi or TNP-ATP. The greater sensitivity of the phosphatase reaction to both Pi and arsenate is attributable to the weaker binding by the occluded-K+ enzyme form occurring in the (Na+ + K+)-ATPase reaction sequence. The steady-state models are consistent with cyclical interconversion of high- and low-affinity substrate sites accompanying E1/E2 transitions, with distortion to low-affinity sites altering not only affinity and route of access but also separating the adenine- and phosphate-binding regions, the latter serving in the E2 conformation as the active site for the phosphatase reaction.

Cation sites, spermine, and the reaction sequence of the (Na+ + K+)-dependent ATPase

Spermine, at 0.3 mM, inhibits the K+-nitrophenyl phosphatase activity of a dog kidney (Na+ + K+)-ATPase preparation, increasing the K0.5 for K+, reducing the Km for substrate, and affecting little the inhibition by Na+. These actions can be attributed, in a model of the phosphatase reaction, to parallel decreases in affinity for K+ and Na+ at their cytoplasmically accessible sites. In the (Na+ + K+)-ATPase reaction, spermine increases the K0.5 for Na+ and, to a lesser degree, the K0.5 for K+ as activators. With spermine, the double-reciprocal plots of velocity vs. ATP concentration (in the range 0.3-3 mM), at fixed levels of K+ (from 1 to 10 mM), remain parallel but are rotated clockwise and spread somewhat, reflecting stimulation at low ATP concentrations and inhibition at high ATP but low KCl concentrations. These actions can be attributed, in a steady-state ping-pong model of the ATPase reaction, solely to decreased rates of binding of Na+ and K+ to their sites, with major effects at the cytoplasmically accessible sites for Na+ (acceptance) and K+ (discharge), and with a lesser effect at the extracellularly accessible sites for K+ (acceptance). On these grounds, spermine is a highly specific and potentially valuable reagent for studying the reaction. Furthermore, the model for K+-ATP interactions not only supports a specific reaction sequence (K+ addition, Pi release, ATP addition, K+ release) but also argues against the availability of low-affinity substrate sites except during sharply restricted segments of the reaction sequence, thereby favoring proposals that the low-affinity substrate sites are transformed into high-affinity substrate sites with the E2 to E1 conformational change.

The pharmacokinetics of glibenclamide: a single dose comparison of four preparations in human volunteers

The pharmacokinetic properties of two new HB 420 glibenclamide preparations, Semi-Euglucon N 1.75 mg and Daonil N 1.75 mg, and of two older HB 419 glibenclamide preparations, Semi-Euglucon 2.5 mg and Daonil 2.5 mg, were compared in a randomized cross-over study in eight healthy volunteers. The HB 420 glibenclamide preparations induced peak plasma glibenclamide concentrations up to about 90 ng/ml at 1.3 to 1.4 hours after ingestion. Administration of the HB 419 preparations induced significantly lower peak plasma glibenclamide concentrations at 1.8 to 2.3 hours after ingestion. Glibenclamide was absorbed and eliminated more rapidly after administration of both HB 420 preparations than after administration of the HB 419 preparations. The mean elimination half-life of glibenclamide was 1.3 +/- 0.1 hours in the case of both HB 420 preparations and 2.0 to 2.5 +/- 0.2 hours in the case of HB 419 preparations. Although the HB 420 preparations contained lower doses of glibenclamide than the HB 419 preparations, the AUC values after administration of the former were similar to the AUC value obtained after administration of one of the HB 419 preparations, indicating improved absorption of glibenclamide from the newly developed preparations. However, the other HB 419 preparation was associated with the greatest AUC value of all, suggesting that glibenclamide was absorbed from this preparation almost as completely as from the HB 420 preparations.

Computer prediction of serum theophylline concentrations in ambulatory patients

This study assessed the ability of a computer program Simulated Kinetics (SIMKIN) to predict serum theophylline concentrations in ambulatory patients receiving oral theophylline. Data were collected by retrospective review of prospectively obtained data. A total of only 20 measured serum theophylline concentrations could be included in the study, although records of 195 patients were reviewed. An estimated patient compliance of 90-110% was required and was computed using prescription refill information. Predicted serum theophylline concentrations were generated for each patient by entering into the SIMKIN program the characteristics pertinent to theophylline disposition and the patient's theophylline dosing regimen. Actual and SIMKIN-predicted theophylline concentrations were compared by using simple linear regression and by constructing a 95% confidence interval around the mean prediction error and root mean squared error. The ability of SIMKIN to predict therapeutic category, i.e., subtherapeutic, therapeutic, or toxic, was assessed using Fisher's exact test. SIMKIN predictions of individual theophylline concentration were insufficiently accurate to replace confirmatory followup monitoring of actual levels. However, SIMKIN was able to predict the therapeutic category with 70% accuracy. We conclude that SIMKIN may be useful for categorizing a dose regimen of theophylline as therapeutic, but that it is of little use in predicting individual concentrations in outpatients when literature-averaged pharmacokinetic parameters are the sole criteria for prediction, and compliance cannot be accurately assessed.

Comparison of eight phenytoin dosing methods in institutionalized patients

Until now, no evaluation of phenytoin dosing methods has been undertaken in a large group of patients, to our knowledge. The goal of this study was to determine which of eight different dosing methods most accurately predicts a phenytoin steady-state concentration. Seventy-six patients were chosen, retrospectively, from a state-funded institution for the mentally retarded. Eligibility criteria included two or more different doses of phenytoin and corresponding plasma concentrations. Relative predictive performance was determined by comparing results of simple linear regression. Also, relative bias and precision were determined by comparing mean prediction errors, root mean squared errors, and respective 95% confidence intervals. Of the methods requiring one dose-concentration pair, Rambeck's nomogram was the best predictor of phenytoin concentrations. The methods requiring two known doses and plasma concentrations were more accurate. Their predictive performance was equivalent, although use of the Tozer equation might be preferred for its convenience. None of the methods tested were sufficiently precise to substitute for confirmatory serum phenytoin concentrations.

Pharmacists' views on mandatory patient counseling

A questionnaire was sent to a randomly selected sample of Florida pharmacists to obtain information on the acceptability of mandatory patient counseling. The results of the questionnaire demonstrated that most Florida pharmacists who responded to the survey do not favor a patient counseling regulation. Factors that correlated with a desire for mandatory counseling include: an institutional practice site; assistant director and staff pharmacist position; and a perceived higher income for the patients served by pharmacists. These data indicated that if the mandatory method is selected to stimulate pharmacist-patient interaction, the interrelated issues of reimbursement, facility changes, technician support, patient profiles, continuing education programs, and pharmacists' attitudes toward regulated practice will need to be addressed.

How to establish a pharmacokinetics consulting service for ambulatory patients

Guidelines on establishing a pharmacokinetics consultation service are presented with emphasis on outpatient services. Need for the service must first be demonstrated, using the literature and local data on frequency of serum drug concentration monitoring for particular drugs and patient subgroups. Physician interest in the program must be determined. The pharmacist intervention may be restricted to technical interpretation of serum drug concentration data, or it can include advice based on the patient's disease process, drug effects, and the interaction between the two. In the service described, a patient interview, a chart review, drug analysis, and a written consultation were provided. A flow chart showing movement of the patient through the service was used, and the pharmacist documented the consultation in the patient's medical record, including recommendations for dosage change and follow-up. Cost items to consider in establishing the service include equipment and supplies, space, and personnel. Staff members who are responsible for serum drug analyses must meet state-specific requirements. In estimating revenues, the current number of requests for serum drug concentrations should be doubled and the institution's estimated rate of collection should be used. After the service is established, the cost of a computerized system for pharmacokinetic predictions may be justified by the increased efficiency of such a system.

Accuracy of serum gentamicin concentration predictions generated by a personal-computer software system

Predictions of serum gentamicin concentration and half-life, using a personal-computer software system (SIMKIN [simulated kinetics]), were compared for accuracy as increasing amounts of patient-specific data were supplied to the computer. Data for a two-year period were collected for patients of a hospital's pharmacokinetic consultation service; the study group included adults who had at least one serum concentration for which time of last gentamicin dose was recorded. Input variables were age, weight, height, sex, serum creatinine concentration, concomitant drugs and diseases, gentamicin dosage, time of infusion, dosing interval, number of doses on each regimen, and time and reported value of all serum gentamicin concentrations. Individualized dosing regimens were calculated on the basis of literature estimates, and half-life and serum concentrations were then estimated for these regimens and compared with actual values. One or two measured serum concentrations were then added to the input data. The computer-estimated half-lives (obtained from single-point or two-point analysis in different dosage intervals) were compared with the half-lives determined from actual serum concentration data. Gentamicin serum concentrations were similarly compared. The computer's ability to predict subsequent serum concentrations improved in sequence for literature-averaged prediction and single-point and multipoint analysis. Accuracy of predicting whether peak concentrations were therapeutic or subtherapeutic and whether trough concentrations were toxic also improved as more patient-specific data were input. SIMKIN appropriately evaluated demographic and laboratory data and adequately predicted gentamicin half-lives and serum concentrations.

The oral contraceptive PPI: its effect on patient knowledge, feelings, and behavior

The distribution of the PPI by pharmacists is at best controversial. Although most health professionals agree that the patient has a right to receive information about drugs, they disagree on the best way to provide that information. Since the oral contraceptive PPI has been in routine use for approximately 10 years, a large data base exists that can be used to determine not only patient acceptance of the PPI, but also knowledge, feelings, and behavior secondary to the PPI. In this study, 50 women of childbearing age completed a questionnaire, and it was learned that 84 percent had taken or were currently taking oral contraceptives, and 90 percent of those received a PPI; however, only 61 percent of these women read all of it. The women performed poorly on the knowledge exam (mean +/- SD, 44.5 +/- 21.2, range, 0-83 percent); those who read all of the PPI or who were white had higher scores (p less than 0.02 and p less than 0.001, respectively). Thirty-eight percent of the women thought that the PPI information was inadequate, suggesting that it needs to be rewritten and/or supplemented with information from pharmacists. After reading the PPI, 12 percent contacted their pharmacist for additional information. Pharmacists are in a unique position to provide oral contraceptive information that enables women to make informed judgments regarding benefit:risk ratios on a personal basis.

Effects of pyridoxal phosphate treatment on the (Na + K)-ATPase

Reaction of a dog kidney (Na + K)-ATPase with pyridoxal phosphate, followed by borohydride reduction, reduced the catalytic activity when measured subsequently. The time course of inactivation did not follow a first-order process, and certain characteristics of the residual enzymatic activity were modified. Moreover, various catalytic activities were diminished differently: Na-ATPase activity was largely spared, K-phosphatase activity was diminished only by half that of the (Na + K)-ATPase, whereas (Na + K)-CTPase and Na-CTPase activities were diminished more. ATP, ADP, CTP, nitrophenyl phosphate, and Pi all protected against inactivation. Increasing salt concentrations increased inactivation, but KCl slowed and NaCl hastened inactivation when compared with choline chloride. Occupancy of certain substrate or cation sites seemed more crucial than selection of conformational states. For the residual (Na + K)-ATPase activity the K0.5 for K+ was lower and the K0.5 for Na+ higher, while the sensitivities to ouabain, oligomycin, and dimethylsulfoxide were diminished; for the residual K-phosphatase activity the K0.5 for K+ was unchanged, the sensitivity to ouabain and oligomycin diminished, but the stimulation by dimethylsulfoxide increased. These properties cannot be wholly accommodated by assuming merely shifts toward either of the two major enzyme conformations.

Effects of oligomycin and quercetin on the hydrolytic activities of the (Na+ +K+)-dependent ATPase

Quercetin inhibited a dog kidney (Na+ + K+)-ATPase preparation without affecting Km for ATP or K0.5 for cation activators, attributable to the slowly-reversible nature of its inhibition. Dimethyl sulfoxide, a selector of E2 enzyme conformations, blocked this inhibition, while the K+-phosphatase activity was at least as sensitive to quercetin as the (Na+ + K+)-ATPase activity, all consistent with quercetin favoring E1 conformations of the enzyme. Oligomycin, a rapidly-reversible inhibitor, decreased the Km for ATP and the K0.5 for cation activators, and its inhibition was also diminished by dimethyl sulfoxide. Although oligomycin did not inhibit the K+-phosphatase activity under standard assay conditions, a reaction presumably catalyzed by E2 conformations, its effects are nevertheless accommodated by a quantitative model for that reaction depicting oligomycin as favoring E1 conformations. The model also accounts quantitatively for effects of both dimethyl sulfoxide and oligomycin on Vmax, Km for substrate, and K0.5 for K+, as well as for stimulation of phosphatase activity by both these reagents at low K+ but high Na+ concentrations.

Comparison of standard and modified enzyme immunoassay of phenytoin

A modified method for the enzyme immunoassay (EMIT, Syva Company) of phenytoin is presented and compared with the standard method. Serum samples from 14 patients were analyzed for phenytoin content using both methods. All assays were performed by the same individual. Within-day and between-day variations of the modified method were determined. A carry-over study was done to determine if a sample with a high phenytoin concentration might contaminate subsequent samples with lower concentrations or whether samples with low concentrations could dilute subsequent samples with higher concentrations. Within-day and between-day variations of the modified method were 7.5% and 9.9%, respectively. These values are less than the 10% coefficient of variation limit claimed by the manufacturer of the standard method. The carry-over study revealed no significant carry-over with the modified method. An excellent correlation was observed between the values obtained from the two methods. The modified method can reduce assay costs by up to 40%. The modified method was found to provide accuracy and precision equivalent to the standard EMIT method at a substantial cost savings.

The absorption of intramuscular chlordiazepoxide (Librium) in patients with severe alcoholic liver disease

Intramuscular chlordiazepoxide (CDX) is commonly administered to alcoholic liver disease (ALD) patients requiring prompt management of alcohol agitation, anxiety, and delirium tremors. Sedative action is associated with plasma levels of approximately 2.0 mcg/ml, however, intramuscular CDX has been shown to produce peak concentrations consistently below 2.0 mcg/ml in normal subjects. The present study was designed to define the absorption characteristics of intramuscular 25 mg CDX in males with ALD. Five normal males had mean CDX absorption half-lives of 3.0 h and peaked at 0.8 mcg/ml in 7.2 h, while 11 males with ALD had mean absorption half-lives of 9.0 h, and peaked at 0.7 mcg/ml in 19.1 h when they received the drug dissolved in normal saline. Four other males with ALD who received CDX dissolved in the manufacturer's diluent had significantly slower mean absorption half-lives of 16.1 h which peaked at 0.3 mcg/ml in 35.2 h. Significant linear correlations were found with age (r = 0.60, p less than 0.01), body weight (r = 0.55, p less than 0.01), and serum albumin (r = 0.60, p less than 0.01). Because of the extremely slow intramuscular absorption of CDX dissolved in normal saline or the manufacturer's diluent in males with ALD, we do not recommend this route of administration in this population.