Inhibition of N-acetylation of procainamide and renal clearance of N-acetylprocainamide by para-aminobenzoic acid in humans

Drug Metabolism Considerations in Patients With Chronic Kidney Disease

Chronic kidney disease (CKD) is a progressive process leading to end stage renal disease and either dialysis or transplantation. Patients with CKD often have numerous comorbid conditions such as diabetes, hypertension, and acid-base and electrolyte disorders that can lead to alterations in homeostasis. Changes in drug disposition including hepatic metabolism via phase 1 (ie, cytochrome P-450 enzymes) and phase 2 (ie, conjugation) pathways have been reported. Biotransformation of drugs and endogenous substances within the kidney itself may also be compromised in the presence of CKD. Reduced hepatic and renal clearance leads to systemic accumulation of the parent drug as well as active and toxic metabolites. Characterization of specific hepatic cytochrome (CYP) enzyme pathways in patients with CKD is an area of current research and will lead to an understanding of phenotypic and genotypic expression patterns of several key drug-metabolizing enzymes. The evolving knowledge of CYP enzymes and the alterations that can occur in CKD should allow clinicians to predict adverse consequences of drug therapy and thus prevent these events from occurring. The pharmacy practitioner can also provide important pharmacotherapy interventions in this special patient population, including dose individualization, therapeutic drug monitoring, and evaluation of therapeutic outcomes.

A review of the common properties of drugs with idiosyncratic hepatotoxicity and the "multiple determinant hypothesis" for the manifestation of idiosyncratic drug toxicity

Idiosyncratic drug toxicity is generally believed to be a phenomenon that cannot be readily evaluated experimentally. Reasons for this difficulty include the following: 1. It is a rare event (<1/5,000) and therefore impossible to be studied in clinical trials; 2. It is a human-specific event not detectable in experimental animals. To aid the understanding of idiosyncratic toxicity and to develop an experimental strategy for this phenomenon, a hypothesis is proposed. The hypothesis states that the low frequency of idiosyncratic drug toxicity is due to the requirements for the occurrence of multiple critical and discrete events, with the probability for the occurrence of idiosyncratic drug toxicity as a product of the probabilities of each event. The key determinants of these critical events are proposed to be: 1. Chemical properties; 2. exposure; 3. environmental factors; and 4. genetic factors. Based on this hypothesis, idiosyncratic drug toxicity can be evaluated experimentally via studying these key determinants. The chemical properties critical to idiosyncratic drug toxicity are identified via a review of the common properties of drugs that cause idiosyncratic liver toxicity. These properties include: 1. Formation of reactive metabolites. 2. Metabolism by P450 isoforms. 3. Preponderance of P450 inducers, and 4. Occurrence of clinically significant pharmacokinetic interactions with co-administered drugs. Based on this review, it is proposed that these common properties may be useful experimental endpoints for the prediction and therefore avoidance of the selection of drug candidates with idiosyncratic drug toxicity for further development.

Arylamine N-acetyltransferases and drug response

Arylamine N-acetyltransferases (NATs) play an important role in the interaction of competing metabolic pathways determining the fate of and response to xenobiotics as therapeutic drugs, occupational chemicals and carcinogenic substances. Individual susceptibility for drug response and possible adverse drug reactions are modulated by the genetic predisposition (manifested for example, by polymorphisms) and the phenotype of these enzymes. For all drugs metabolized by NATs, the impact of different in vivo enzyme activities is reviewed with regard to therapeutic use, prevention of side effects and possible indications for risk assessment by phenotyping and/or genotyping. As genes of NATs are susceptibility genes for multifactorial adverse effects and xenobiotic-related diseases, risk prediction can only be made possible by taking the complexity of events into consideration.

Standards of laboratory practice: cardiac drug monitoring

In this Standard of Laboratory Practice we recommend guidelines for therapeutic monitoring of cardiac drugs. Cardiac drugs are primarily used for treatment of angina, arrhythmias, and congestive heart failure. Digoxin, used in congestive heart failure, is widely prescribed and therapeutically monitored. Monitoring and use of antiarrhythmics such as disopyramide and lidocaine have been steadily declining. Immunoassay techniques are currently the most popular methods for measuring cardiac drugs. Several reasons make measurement of cardiac drugs in serum important: their narrow therapeutic index, similarity in clinical complications and presentation of under- and overmedicated patients, need for dosage adjustments, and confirmation of patient compliance. Monitoring may also be necessary in other circumstances, such as assessment of acetylator phenotypes. We present recommendations for measuring digoxin, quinidine, procainamide (and N-acetylprocainamide), lidocaine, and flecainide. We discuss guidelines for measuring unbound digoxin in the presence of an antidote (Fab fragments), for characterizing the impact of digoxin-like immunoreactive factor (DLIF) and other cross-reactants on immunoassays, and for monitoring the unbound (free fraction) of drugs that bind to α1-acid glycoprotein. We also discuss logistic, clinical, hospital, and laboratory practice guidelines needed for implementation of a successful therapeutic drug monitoring service for cardiac drugs.

Comparison of fluorescence polarization immunoassay with liquid chromatography for quantification of procainamide and N-acetylprocainamide in urine

The objective of this study was to compare the precision and accuracy of fluorescence polarization immunoassay (FPIA) with high-performance liquid chromatography (HPLC) for measurement of procainamide (PA) and N-acetylprocainamide (NAPA) concentrations in urine. To determine the correlation between FPIA and HPLC, urine PA and NAPA concentrations were assayed using both techniques in samples obtained from study patients receiving PA and in spiked samples. In samples from patients, FPIA-determined PA and NAPA concentrations were 19 +/- 9% lower and 28 +/- 31% higher, respectively, than those determined by HPLC. The slope of the FPIA-HPLC regression lines for PA and NAPA differed significantly from that of the line of unity (the slope that would result if FPIA and HPLC yielded identical concentrations). In spiked samples, FPIA-determined PA and NAPA concentrations were 15 +/- 2% and 11 +/- 2% lower than HPLC-determined concentrations, respectively, and the slopes of the FPIA-HPLC regression lines differed significantly from the line of unity. Therefore, FPIA cannot be recommended as a urine assay method when quantitative assessment of urine PA or NAPA excretion is needed for pharmacokinetic studies.

Disposition of procainamide in patients with chronic congestive heart failure receiving medical therapy

Dosage reduction of procainamide has been recommended in patients with congestive heart failure (CHF). However, these recommendations are based primarily on studies with unmatched control groups, suboptimal blood sampling, and in patients not receiving angiotensin-converting enzyme (ACE) inhibitors. These agents increase renal blood flow, which theoretically may offset alterations in drug disposition in patients with CHF. The pharmacokinetics of procainamide in patients with chronic CHF and in matched controls were compared. A single intravenous dose of 750 mg of procainamide was administered to 9 patients with chronic New York Heart Association (NYHA) class II or III CHF (mean +/- SD left ventricular ejection fraction 22 +/- 9%) receiving medical therapy and 7 control subjects matched for age and gender. Blood and urine samples were collected at intervals over a period of 48 and 72 hours, respectively. Patients with CHF and control subjects were demographically similar, with the exception of concomitant medications, including ACE inhibitors (8/9 versus 1/7, respectively). There were no significant differences between patients with CHF and control subjects in mean +/- SD peak serum concentrations (Cmax), area under the serum concentration-time curve (AUC0-infinity), total clearance, renal clearance, half-life (t1/2), or volume of distribution (Vd) of procainamide. Similarly, there were no significant differences between patients with CHF and control subjects in the mean +/- SD Cmax, AUC0-infinity, renal clearance, or t1/2 of N-acetylprocainamide (NAPA). Procainamide dosage reduction may not be necessary in patients with chronic stable CHF who are receiving medical therapy.