N-Acetylprocainamide pharmacokinetics in functionally anephric patients before and after perturbation by hemodialysis

Hemodialysis Clearance of Enarodustat (JTZ-951), an Oral Erythropoiesis Stimulating Agent, in Patients with End-Stage Renal Disease

The dialysis clearance of enarodustat (JTZ-951) was determined in patients (N = 6) with end-stage renal disease on hemodialysis. Enarodustat (5 mg PO) was administered before (day 1) and after hemodialysis (day 8) with pharmacokinetic assessments on the 2 occasions. Dialysis clearance was based on plasma and dialysate enarodustat concentrations. Fraction of administered dose recovered in dialysate, total predialyzer and postdialyzer plasma enarodustat concentrations, and total and unbound venous plasma concentrations were determined. Hemodialysis did not significantly affect overall total concentrations with similar mean area under the plasma concentration-time curve from time 0 to infinity (coefficient of variation) of 3350 (26.4%) and 3640 (20.9%) ng · h/mL on days 1 and 8, respectively, and mean terminal half-life was 9.35 (11.9%) and 9.96 (18.7%) hours on the 2 occasions. Mean maximum concentration was somewhat lower on day 1 compared to day 8 (404 vs 559 ng/mL); the difference did not significantly affect total exposure (area under the plasma concentration-time curve from time 0 to infinity). Plasma protein binding was high (>99%) with similar binding on the 2 occasions, and total pre- and postdialyzer enarodustat concentrations were similar. Plasma unbound enarodustat concentrations decreased during dialysis, with a postdialysis rebound presumably due to re-equilibration with peripheral tissues. Mean unbound area under the plasma concentration-time curve from time 0 to infinity was marginally lower (∼22%) on day 1 compared to day 8. Dialysis clearance (0.415 L/h) was insignificant relative to dialyzer plasma flow (∼20 L/h), and the fraction of administered dose recovered in dialysate was small (6.74% of dose) with low intersubject variability (coefficient of variation, 14.7%). Thus, enarodustat can be administered regardless of dialysis schedule, and dose supplementation is not required in patients with end-stage renal disease on hemodialysis.

Elucidation of the pathophysiology of intradialytic muscle cramps: pharmacokinetics applied to translational research

In the conventional concept of translational research, investigations flow from the laboratory bench to the bedside. However, clinical research can also serve as the starting point for subsequent laboratory investigations that then lead back to the bedside. This article chronicles the evolution of a series of studies in which a detailed analysis of pharmacokinetics in hemodialysis patients revealed new physiological insight that, through a systems approach incorporating kinetic, physicochemical, physiologic, and clinical trial results, led to an elucidation of the pathophysiology of intradialytic skeletal muscle cramps. Based on this understanding, a therapeutic path forward is proposed.

Combined Recirculatory-compartmental Population Pharmacokinetic Modeling of Arterial and Venous Plasma S(+) and R(–) Ketamine Concentrations

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

The pharmacokinetics of infused drugs have been modeled without regard for recirculatory or mixing kinetics. We used a unique ketamine dataset with simultaneous arterial and venous blood sampling, during and after separate S(+) and R(–) ketamine infusions, to develop a simplified recirculatory model of arterial and venous plasma drug concentrations.

Methods

S(+) or R(–) ketamine was infused over 30 min on two occasions to 10 healthy male volunteers. Frequent, simultaneous arterial and forearm venous blood samples were obtained for up to 11 h. A multicompartmental pharmacokinetic model with front-end arterial mixing and venous blood components was developed using nonlinear mixed effects analyses.

Results

A three-compartment base pharmacokinetic model with additional arterial mixing and arm venous compartments and with shared S(+)/R(–) distribution kinetics proved superior to standard compartmental modeling approaches. Total pharmacokinetic flow was estimated to be 7.59 ± 0.36 l/min (mean ± standard error of the estimate), and S(+) and R(–) elimination clearances were 1.23 ± 0.04 and 1.06 ± 0.03 l/min, respectively. The arm-tissue link rate constant was 0.18 ± 0.01 min–1, and the fraction of arm blood flow estimated to exchange with arm tissue was 0.04 ± 0.01.

Conclusions

Arterial drug concentrations measured during drug infusion have two kinetically distinct components: partially or lung-mixed drug and fully mixed-recirculated drug. Front-end kinetics suggest the partially mixed concentration is proportional to the ratio of infusion rate and total pharmacokinetic flow. This simplified modeling approach could lead to more generalizable models for target-controlled infusions and improved methods for analyzing pharmacokinetic-pharmacodynamic data.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Combined high-efficiency hemodialysis and charcoal hemoperfusion in severe N-acetylprocainamide intoxication

Several extracorporeal techniques have been used to remove N-acetylprocainamide (NAPA), the major metabolite of procainamide, in patients intoxicated with this substance. We report a patient with life-threatening NAPA intoxication who was rapidly and successfully treated with combined high-efficiency hemodialysis and charcoal hemoperfusion. The hemodialyzer and hemoperfusion cartridge were placed in series such that the patient's blood was dialyzed before reaching the cartridge. Overall clearance of NAPA was 153 mL/min, with clearance due to hemodialysis averaging 102 mL/min and that due to hemoperfusion averaging 88 mL/min. Thus, addition of the hemoperfusion cartridge into the extracorporeal circuit resulted in a 50% increase in clearance over that obtainable by high-efficiency hemodialysis alone. In comparison to other modalities, this technique is more effective than either hemodialysis or charcoal hemoperfusion alone and can achieve a more rapid reduction of serum NAPA levels than that observed with slow continuous hemofiltration or hemodiafiltration.

Distribution characteristics of mitoxantrone in a patient undergoing hemodialysis

The pharmacokinetic profile of mitoxantrone in a patient undergoing hemodialysis is described. Significant characteristics of our patient included lymphoma with liver involvement, tumor lysis syndrome, renal and hepatic failure. Combination chemotherapy consisted of mitoxantrone, vincristine, and cyclophosphamide. Mitoxantrone plasma samples were obtained prior to dosing and at 0, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.5, 7.0, and 12 h after the intravenous infusion of a 17-mg dose over 20 min. Serum concentrations were determined by high-performance liquid chromatography. The serum concentration versus time curve was consistent with a three-compartment model. However, rebounds in serum drug concentrations were detected during the last portion of dialysis and after its completion. The gamma elimination half-life could not be determined due to the continued detection of rebounds in drug concentrations throughout the postdialysis sampling period. The alpha and beta distribution phases did not appear to be affected by hemodialysis. The peak mitoxantrone concentration fell within the reported range. Mitoxantrone does not appear to be eliminated by hemodialysis, and dose adjustments are not needed in patients undergoing this procedure.

Physiological basis of multicompartmental models of drug distribution

Although most pharmacokinetic studies are conducted in normal subjects, their clinical utility depends on the reliability with which the results can be extrapolated to patients. This reliability can be improved by increased understanding of how drug absorption and disposition mechanisms are affected by physiological changes or by disease. In recent years, important insight has been gained regarding the effects of altered renal function on drug elimination by the kidneys. There has also been considerable progress in defining the interaction of hemodynamic and metabolic factors that affect the hepatic elimination of drugs. Although comparatively little progress has been made in elucidating the underlying basis of changes in the rate and extent of drug distribution, Arthur Atkinson and colleagues analyse methods of compartmental pharmacokinetic analysis that may provide physiological insight into the factors affecting drug distribution.

Pharmacokinetics of ampicillin (2.0 grams) and sulbactam (1.0 gram) coadministered to subjects with normal and abnormal renal function and with end-stage renal disease on hemodialysis

The single-dose pharmacokinetics of intravenously administered ampicillin (2.0 g) and sulbactam (1.0 g) were studied in normal subjects and in patients with various degrees of creatinine clearance (CLCR). Six normal subjects (CLCR, greater than 60 ml/min), six patients with mild renal failure (CLCR, 31 to 60 ml/min), four patients with severe renal failure (CLCR, 7 to 30 ml/min), and four patients requiring maintenance hemodialysis (CLCR, less than 7 ml/min) were studied. The terminal half-lives for ampicillin and sulbactam more than doubled in patients with severe renal failure compared with subjects with normal renal function and mild renal insufficiency. CLCR significantly correlated with ampicillin (r = 0.88) and sulbactam (r = 0.54) total body clearance. Mean steady-state volume of distribution and nonrenal clearance for ampicillin and sulbactam were not affected by renal function. Hemodialysis approximately doubled the ampicillin and sulbactam total body clearance. Mean totals of 34.8 +/- 4.0% of the ampicillin dose and 44.7 +/- 3.2% of the sulbactam dose were removed during a 4-h hemodialysis treatment. A slight rebound in concentrations in serum after hemodialysis was observed for both drugs in all four subjects. In hemodialysis patients, the ampicillin half-life was 17.4 +/- 8.0 h and the sulbactam half-life was 13.4 +/- 7.4 h. The ampicillin and sulbactam half-lives were appreciably altered during the hemodialysis period (means of 2.2 and 2.3 h, respectively). The nearly parallel decrease in total body clearance, with volume of distribution and nonrenal clearance remaining relatively constant, suggests that the same ratio of ampicillin to sulbactam is appropriate regardless of renal function. An adjustment of the ampicillin (2.0 g) and sulbactam (1.0 g) dose to twice daily would be appropriate in patients with a CLCR between 7 and 30 ml/min. Doses should be given every 24 h for those undergoing maintenance hemodialysis. On hemodialysis days, doses should be given after hemodialysis.

Pharmacokinetics of N-Acetylprocainamide

Shortly after Dreyfus and his colleagues demonstrated that procainamide was metabolized by acetylation to N-acetylprocainamide (NAPA), Drayer, Reidenberg and Sevy reported that NAPA had antiarrhythmic activity in an animal model. We confirmed these findings and found that plasma levels of NAPA were high enough to warrant consideration in managing patients requiring procainamide therapy. However, the actual impetus for developing NAPA as an antiarrhythmic drug in its own right was provided by the initial studies of NAPA pharmacokinetics in normal subjects. In these studies, we showed that NAPA has an elimination-phase half-life that is more than twice as long as procainamide and suggested that patient compliance and arrhythmia suppression might be improved if NAPA were used to circumvent the inconvenience of the frequent dosing schedule that has been recommended for procainamide.

From the standpoint of managing individual patients with NAPA, the pharmacokinetics of this drug continue to provide the scientific basis for designing dose regimens that will have maximal antiarrhythmic efficacy and minimal toxicity. This review summarizes the salient features of NAPA pharmacokinetics and outlines an approach for individualizing therapy with this drug.

Procainamide toxicity in a patient with acute renal failure

A patient developed acute renal failure while receiving oral procainamide (PA). This lead to severe PA and N-acetyl procainamide (NAPA) toxicity. Rebound of NAPA plasma levels postdialysis prolonged the toxicity, which was treated with hemodialysis, hemoperfusion, and combined hemodialysis-hemoperfusion. Because of the potential for PA and NAPA toxicity in patients with renal insufficiency, especially in patients with changing renal function due to acute renal failure, it is recommended that the use of PA be curtailed in this population and that another substitute antiarrhythmic agent be used.

Torsades de pointes due to n-acetylprocainamide

A 66-year-old female with chronic renal failure received five doses of procainamide and developed marked QT interval prolongation and recurrent episodes of torsades de pointes, which were temporally related to high serum n-acetylprocainamide (NAPA) levels and not to procainamide levels. Repeated hemodialysis was effective in lowering NAPA levels. Torsades de pointes is a potential hazard of NAPA accumulation during procainamide administration to patients with renal insufficiency.

Pharmacokinetics of cibenzoline in patients with renal impairment

Pharmacokinetic values of cibenzoline, a new, investigational, antiarrhythmic drug, were determined in 13 patients with varying degree of renal impairment, creatinine clearance range between 5 and 53 mL/min. Cibenzoline plasma levels were measured after direct intravenous injection of one single 1 mg/kg dose. The apparent volume of distribution of the drug (276 1) was similar to that reported in healthy subjects. Total body clearance decreased with creatinine clearance, and there was a close correlation between cibenzoline renal clearance and creatinine clearance (r = 0.956; P less than 0.001). Plasma elimination half-life was prolonged, with values ranging from 7:4 to 23.6 hours. This study showed that cibenzoline total body clearance correlated with the degree of renal impairment, and it is suggested that in patients with chronic renal failure dosage should be adjusted according to creatinine clearance values.

N-acetylprocainamide kinetics during intravenous infusions and subsequent oral doses in patients with coronary artery disease and ventricular arrhythmias

The kinetics of N-acetylprocainamide (NAPA) were studied in 5 patients (all men, mean age = 62) with coronary artery disease and ventricular arrhythmias during loading infusions of 0.22-0.45 mg/kg/min, prolonged (19-48 hrs) intravenous infusions 2.5-5.2 mg/min, and in 4 of the patients, during subsequent oral doses 1.5-3 g every 8 hrs. Serum, concentrations of NAPA were determined by high-performance liquid chromatography. The individual concentration-time profiles could, with one exception, be described by a two-compartment, open, kinetic model with apparent first-order elimination. The kinetic variables were: initial distribution volume (Vc) 0.20 +/- 0.11 l/kg (mean +/- SD); steady-state distribution volume (Vss) 1.58 +/- 0.55 l/kg; distributional clearance (Cle) 133 +/- 23 ml/(kg X hr); absorption rate constant (Ka) 0.354 +/- 0.173 hr-1; and fraction of dose reaching systemic circulation (F) 1.00 +/- 0.14. The data for one patient who had received increasing oral dosages of 1.5, 2, 2.5 and 3 g every 8 hours resulted in systematic underprediction of observed concentrations at the two highest oral dosing rates. This suggests the possibility of some degree of nonlinearity or time-dependent change in the kinetic behavior of NAPA. Only low concentrations of procainamide, less than 1 mg/L, were found at the end of the infusions.

Problems in designing hemodialysis drug studies

A clear understanding of the pharmacokinetics of a drug and of the proper methods for calculating dialyzer clearance is essential in designing hemodialysis studies. Hemodialysis should not begin until drug distribution is complete. Institution of dialysis prior to distribution equilibrium will result in increased removal of drug compared to what would be found in the clinical setting. All methods of calculating dialyzer clearance should be compared to that using total amount of drug recovered in the bath divided by the area under the drug concentration versus time curve during dialysis. To adequately probe the effect of the artificial kidney on drug concentrations sufficient plasma samples must be drawn postdialysis to define the rebound phenomena.

Metabolites of cardiac antiarrhythmic drugs: their clinical role

Most antiarrhythmic drugs are extensively metabolized, and the accumulation of the metabolites of several of these drugs has been documented. In some cases, the steady-state plasma concentrations of metabolites are considerably greater than is the concentration of the parent drug. Several of these metabolites have been evaluated in animal models for antiarrhythmic activity and their potencies have been defined relative to the activity of their parent compound. Evaluations of activity are generally conducted in animal arrhythmia models, and very few metabolites of antiarrhythmic drugs have been evaluated directly in patients. However, from knowledge of antiarrhythmic activity in animals and the degree to which a metabolite accumulates in the plasma of patients, one can make qualitative judgments about its therapeutic role. Such judgments, however, need to be recognized as tenuous. Quantitative judgments require further information regarding the relationship between the parent drug and metabolite when present simultaneously in the myocardium. One must consider whether the effects of the parent drug and metabolite are additive, synergistic, or even antagonistic. The latter case is most possible with drug-metabolite pairs where the metabolite accumulates substantially, but does not have significant antiarrhythmic potency. Other considerations include noncardiac effects of the metabolites. As in the case of the mono-desethyl metabolite of lidocaine, the significance of its accumulation relates more to central nervous system side effects than to direct cardiac actions. The role of active metabolites also much be considered in regard to differences in the disposition kinetics between the parent drug and metabolite. The most obvious situation where this is important is in designing clinical drug evaluation protocols. As illustrated by the metabolites of encainide and lorcainide, the time course of accumulation and disappearance of the metabolites may be much longer than that of the parent drug. Clinical evaluations at steady state must take into account the time required to achieve steady-state concentrations of the metabolites as well. Similarly, after discontinuation of drug administration, the time required before washout is complete may be totally dependent on the kinetics of the metabolite, and not the parent drug. Variability in metabolic activity also needs to be considered. It has been shown with procainamide and encainide that genetic factors can influence the rate of production of active metabolites and consequently influence the clinical efficacy of these drugs. Another consideration that deserves attention is the question of drug interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Complexation versus hemodialysis to reduce elevated aminoglycoside serum concentrations

Seven patients with acutely elevated aminoglycoside serum concentrations were studied comparing the effect of hemodialysis (n = 3) with removal by complexation using ticarcillin or carbenicillin (n = 4). Aminoglycoside serum half-life before intervention averaged 96 hours for the dialysis group and 67 hours for the complexation group. Ticarcillin was used for a minimum of 48 hours, while hemodialysis removal was estimated over 48 hours, which included two 4-hour dialysis periods. Aminoglycoside serum half-life was reduced to an average of 11 hours with hemodialysis, while with complexation using ticarcillin, it was reduced to 12 hours. During the 48-hour comparison period, complexation removed approximately 50% more aminoglycoside than did hemodialysis, primarily because the improved removal technique was sustained over the entire time. Complexation appears to be as effective as continuous hemodialysis in lowering excessive aminoglycoside serum concentrations. Complexation with ticarcillin can be more rapidly initiated, is less expensive and there is a low risk of adverse reactions. This method provides continued treatment of infections in patients with elevated serum concentrations and/or nephrotoxicity who require cessation of aminoglycoside therapy.

Rebound following hemodialysis of cimetidine and its metabolites

The effect of hemodialysis on the pharmacokinetics of cimetidine and its metabolites was studied after the intravenous administration of a 300-mg dose of cimetidine. Serum concentrations were monitored before, during, and after hemodialysis. Cimetidine pharmacokinetics were similar to those in patients with end-stage renal failure, with a total body clearance of 3.3 +/- 1.0 mL/min/kg and a half-life of 4.1 hours. Dialysis caused an initial decrease in serum concentrations for cimetidine, hydroxymethyl cimetidine, and cimetidine sulfoxide, followed by a rebound in serum concentrations immediately after treatment. Dialysis clearances, as calculated from extraction ratios, were 83 +/- 15, 90 +/- 16, 93 +/- 24, and 126 +/- 23 mL/min for cimetidine, hydroxymethyl, sulfoxide, and creatinine, respectively. After four hours of dialysis, 10% +/- 3.2% of the cimetidine dose was recovered in the dialysate. The sulfoxide and hydroxymethyl metabolites demonstrated relatively constant serum concentrations for up to 24 hours postdosing, and therefore, estimates of their terminal half-lives were not possible. In one patient, accumulation of both metabolites occurred during multiple dosing. Hemodialysis is an ineffective means of decreasing the total body load of cimetidine and its metabolites. Presumably, sequestration of the drug in body tissues decreases the amount of drug present in the blood available for dialysis removal, with rebound occurring due to postdialysis re-equilibriation between blood and extravascular fluids.

Drug prescribing in renal failure: dosing guidelines for adults

The data base for rational guidelines to safe, efficacious drug prescribing in adults with renal insufficiency are presented in tabular form. Current medical literature was extensively surveyed to provide as much specific information as possible. When information is lacking, however, recommendations are based on pharmacokinetic variables in normal subjects. Nephrotoxicity, important adverse effects, and special considerations in renal patients are noted. Adjustments are suggested for hemodialysis and peritoneal dialysis when appropriate.

Principles of drug dose adjustment during hemodialysis

Drug removal during dialysis is influenced by physical properties of the drug, its pharmacokinetics, and by the choice of artificial kidney. The amount of drug removed during dialysis can be estimated in several ways. If changes in drug concentration produced by dialysis are used, one must be certain that postdialysis drug reequilibration is complete.

Charcoal hemoperfusion for treatment of ethchlorvynol overdose

Charcoal hemoperfusion is commonly employed to treat overdose of a number of drugs. There are varying reports of its efficacy in the treatment of ethchlorvynol overdose. Herein is reported a case of ethchlorvynol overdose successfully treated with charcoal hemoperfusion within 5 h of ingestion of 12.5 g of the drug. The patient was deeply comatose at that time but recovered consciousness at the end of 4 h of hemoperfusion. Ethchlorvynol clearance over the charcoal varied from 118 to 147 ml/min. There were no bleeding complications. Prompt charcoal hemoperfusion may be an effective mode of treatment in cases of ethchlorvynol intoxication.

Some consequences of drug choice and dosage regimen for patients with impaired kidney function

The object of this article is to discuss difficulties in extrapolating the performances of a drug for which the kinetic parameters are derived in healthy volunteers, to patients with severely impaired kidney function. The theoretical background of some actual or probable background is given, and a possible solution for these problems is offered, that is, choosing another drug from the same drug group. In patients without kidney function, metabolism is the only pathway of elimination. When the elimination of the metabolite formed occurs by means of renal excretion only, this metabolite accumulates in patients with impaired or absent kidney function. When a metabolic pathway of the parent drug is part of a metabolic equilibrium, the metabolic return reaction results in an "apparent parent compound," with a half-life identical to that of the accumulated metabolite. In this way, the concentration of the "apparent" parent compound increases and the half-life of the sum of parent and "apparent" parent drug will change. Examples of this drug behavior are given for sulfamethoxazole, sulfametrole, sulfamethizole, procainamide, and N4-acetylprocainamide.

Significance of acetylator phenotype in pharmacokinetics and adverse effects of procainamide

The pharmacokinetics and development of antinuclear antibodies (ANAs) during procainamide (PA) therapy were studied in 35 patients with ventricular arrhythmias. Sixteen of the subjects were rapid and 19 were slow acetylators. Twenty-six of them (13 rapid and 13 slow acetylators) received PA therapy (2.4g sustained-release PA X HCl daily in three doses) for at least 16 weeks. On maintenance therapy, rapid acetylators had insignificantly lower serum PA concentrations and slightly higher N-acetylprocainamide (NAPA) concentrations than slow acetylators. The unchanged PA fraction (PA/PA + NAPA) in the rapid acetylators was somewhat lower than in the slow acetylators. Rapid acetylators excreted more NAPA in urine than did slow acetylators (p less than 0.05), whereas the difference in PA excretion was not significant. More than 80% of the given drug was excreted as PA and NAPA. Spontaneous or exercise-induced arrhythmias were recorded in 6 rapid and 8 slow acetylators. ANAs (titre at least 20) appeared in 6 rapid and 8 slow acetylators. The mean time until ANA development in rapid acetylators was only marginally longer than in slow acetylators. The results suggest that acetylation phenotyping is not of great significance in predicting the development of ANAs during PA therapy.

Clinical pharmacokinetics of N-acetylprocainamide

Since N-acetylprocainamide was identified in the urine of patients receiving procainamide, this compound has been studied both as a metabolite of procainamide and as a separate antiarrhythmic agent. N-acetylprocainamide absorption following oral administration is more than 8-% complete. 59 to 89% of N-acetylprocainamide is excreted unchanged in the urine in subjects with normal renal function. Deacetylation of N-acetylprocainamide to procainamide is a minor route of N-acetylprocainamide elimination. The half-life of N-acetylprocainamide in patients with normal renal function has been reported to vary between 4.3 and 15.1 hours. Total body clearance (mean +/- SD) of N-acetylprocainamide in patients with normal renal function has been reported to range from 2.08 +/- 0.36 ml/min/kg to 3.28 +/- 0.52 ml/min/kg. There is a linear relationship between N-acetylprocainamide clearance and creatinine clearance. The half-life of N-acetylprocainamide in functionally anephric patients may be as long as 42 hours; however, it can be effectively cleared from plasma by haemodialysis. N-acetylprocainamide is 10% protein-bound. There is an age-related decline in N-acetylprocainamide clearance, mostly due to the decrease in creatinine clearance that occurs with ageing. In the neonate, the half-life of acetylprocainamide is prolonged. Several therapeutic trials carried out to assess the effectiveness of N-acetylprocainamide in suppressing chronic ventricular premature beats have now been reported. If there is a therapeutic response to N-acetylprocainamide it will probably occur at a plasma concentration between 15 and 25 micrograms/ml. A high degree of overlap has been reported between the concentration range associated with arrhythmic suppression and the range of concentrations where intolerable side effects begin to occur. No severe cardiac toxicity has been reported with oral therapy despite plasma concentrations as high as 40 micrograms/ml. However, hypotension has been reported in association with a rapid intravenous bolus of N-acetylprocainamide. A maximum intravenous infusion rate of 50 mg/min has been recommended. N-acetylprocainamide in patients receiving procainamide; however, N-acetylprocainamide concentrations remain below the therapeutic range in patients with normal renal function. In patients with renal failure receiving procainamide, N-acetylprocainamide concentrations rise dramatically. The dose of N-acetylprocainamide must be adjusted in patients with renal insufficiency, and it should be used more cautiously in the very old and very young. N-acetylprocainamide plasma concentration monitoring would be valuable clinically in patients with renal insufficiency receiving either N-acetylprocainamide or procainamide, and in the very young and the aged.

Analysis of the contributions of permeability and flow of intercompartmental clearance

Recent pharmacokinetic studies indicate that both flow and permeability contribute to intercompartmental clearance. A previous analysis of flow and permeability components of transcapillary exchange has been adapted to a three-compartment model of PA and NAPA pharmacokinetics. Data from a study that simultaneously determined the pharmacokinetic parameters of these two compounds made it possible to estimate permeability coefficients for the fast equilibrating compartment averaging 3.32 liters/min for PA and 1.35 liters/min for NAPA, and for the slow equilibrating compartment averaging 2.05 liters/min for PA and 0.78 liters/min for NAPA. These results were then used to estimated flow-intercompartmental clearance relationships for PA and NAPA and to predict the extent of hemodynamic changes causing the slow intercompartmental clearance of NAPA to decrease by 77% during hemodialysis without an apparent alteration in fast intercompartmental clearance.