Common arterial trunk in a cat: a high-resolution morphological analysis with micro-computed tomography.

A 6-month-old female cat presented with respiratory distress. Physical examination showed a grade 5/6 holosystolic murmur with prominent precordial impulse over the left cranial chest wall. Echocardiography revealed bilateral hypertrophy of the ventricular walls, a dilated ascending aorta overriding the interventricular septum, a membranous ventricular septal defect and no obvious pulmonary trunk or pulmonary artery branches. Turbulent blood flow was detected around the ventricular septal defect and ascending aorta. Follow-up assessment, 12 months later, revealed marked and progressive biatrial dilation and biventricular hypertrophy. Four months after that, the cat died of severe congestive heart failure. To make a definitive postmortem diagnosis, we performed contrast enhanced micro-computed tomography (CT) on the ex vivo heart with micron-scale spatial resolution imaging and three-dimensional reconstruction. Micro-computed tomography analysis confirmed a common arterial trunk that bifurcated into the left pulmonary artery and aorta 5-mm distally from the truncal valve. The pulmonary trunk was absent. Slightly distal to the first branching, the common arterial trunk further branched into the right pulmonary artery and ascending aorta, indicating the aortic dominant form. Although CT angiography would be a preferred imaging modality for living animals, micro-computed tomography is a valuable tool for the ex vivo diagnosis of complex cardiac anomaly, such as presented in this cat.

Clinical pharmacology education and training at the National Institutes of Health.

In 1965, the National Institute of General Medical Sciences (NIGMS) established the intramural Pharmacology Research Associate Training (PRAT) program with the primary goals of providing postdoctoral training in pharmacology for individuals with or without previous pharmacology graduate training, and allowing individuals with doctoral degrees in pharmacology to obtain advanced training in other areas of science at the National Institutes of Health (NIH). The program utilized research preceptors drawn from laboratories that were conducting pharmacology-related research at the NIH campus. Although primary emphasis was placed on training laboratory scientists, a number of PRAT fellows obtained training that enabled them to pursue successful careers in clinical pharmacology. A partial listing of these individuals is shown in Table 1. Eventually, a clinical pharmacology training option was formalized within the PRAT program by the appointment of a Clinical Pharmacology Program Director, but this was subsequently suspended when this individual left NIH for a position in the pharmaceutical industry.

The effect of diphenylhydantoin on thyroxine metabolism in man.

The effect of 5,5'-diphenylhydantoin on thyroxine metabolism was examined in five normal volunteers. Intravenous injection of radiothyroxine was followed by a 10-12 day control and subsequent 9-14 day treatment periods. During oral administration of diphenylhydantoin, plasma thyroxine concentration decreased to about 80% of its pretreatment level and the plasma radiothyroxine disappearance rate increased a maximum of 20% over control estimates. These changes were a result of increases in both urinary and fecal excretion of radioisotope.A minimum plasma thyroxine was apparent after 10-12 days of diphenylhydantoin administration. In two of the subjects, treatment was sufficiently prolonged to achieve this new steady state. In these subjects, the decrease in total body thyroxine was balanced by the increase in the fractional turnover rate. As a result, absolute thyroxine degradation during diphenylhydantoin administration was unchanged from the pretreatment values. Plasma ultrafiltration was used to estimate the free thyroxine fraction at regular intervals during the control and treatment periods. During diphenylhydantoin treatment, there was little or no change in this fraction and therefore, absolute free thyroxine decreased. Thyroxine-binding globulin and thyroxine-binding prealbumin capacities remained constant. These results indicate that thyroxine degradation can proceed at a normal rate in subjects receiving diphenylhydantoin despite decreases in plasma free thyroxine concentration. If free thyroxine is the only portion of the hormone available for cellular utilization, then free thyroxine clearance must be increased in these subjects. This increase in clearance could represent either a direct stimulation of peripheral thyroxine metabolism by diphenylhydantoin, or it could reflect the response of intrinsic regulatory systems to a diphenylhydantoin-mediated displacement of thyroxine from thyroxine-binding globulin. Whatever the mechanism for this effect, a decreased free thyroxine value in patients receiving diphenylhydantoin may not imply hypothyroidism.

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.

Torsade de pointes induced by N-acetylprocainamide.

N-Acetylprocainamide (NAPA), a class III antiarrhythmic drug, caused torsade de pointes in a 72 year old woman who had this arrhythmia on two previous occasions while being treated with quinidine and disopyramide. Initial evaluation with an intravenous infusion of NAPA indicated a favorable antiarrhythmic response. The QTC interval was prolonged, but the 2.4 ms/microgram per ml incremental QTC interval lengthening caused by NAPA was not greater than usual. During subsequent oral therapy with NAPA, torsade de pointes developed at plasma levels of this drug that appeared to be well tolerated during the initial evaluation.

Measuring Biomarker Progress.

A biomarker has been defined as "a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic process, or pharmacologic responses to a therapeutic intervention." This comprehensive definition of biomarkers arose from the April 1999 US Food and Drug Administration (FDA)/National Institutes of Health consensus conference on "Biomarkers and Surrogate Endpoints: Advancing Clinical Research and Applications," and emphasized that biomarkers are medical measurements, including physiological measurements, blood tests, molecular analyses of biopsies, genetic or metabolic data, and measurements from images. Research on biomarkers-organized and propelled by this definition-has skyrocketed, with over 200,000 PubMed citations in the last five years.

Antiarrhythmic potency of N-acetylprocainamide.

Compared to procainamide in an animal arrhythmic model, the antiarrhythmic potency of the N-acetylated metabolite of procainamide (NAPA) was 92% with respect to dose and 70% with respect to plasma level. The antiarrhythmic effects of combinations of the drugs were additive. Measurements of procainamide and NAPA plasma levels needed to suppress ventricular extrasystoles suggested that both compounds are nearly equipotent in patients as well. The average plasma level required for arrhythmia control in these patients was equivalent to 5.1 mcg/ml procainamide. Since patients on long-term procainamide therapy have plasma concentrations of NAPA that are usually comparable to, and occasionally greater than, their procainamide levels, dose regiments based on procainamide levels alone need revision to include consideration of the levels of this metabolite.

A concurrent audit of high digoxin plasma levels.

A concurrent audit was made of 92 patients with plasma or serum digoxin levels of 3.0 ng/ml or more. Evidence of digoxin toxicity was present in 44 of these patients, and premature blood sampling accounted for the high levels in 30 nontoxic patients. Another 14 patients tolerated high digoxin levels without apparent adverse effects. Impaired renal function appeared to increase the risk of digoxin toxicity, even though digoxin levels were similar in patients with and without toxicity. Pharmacokinetic predictions based on patient weight and creatinine clearance often deviated considerably from measured digoxin levels even when these were drawn appropriately.

Hemodialysis for severe procainamide toxicity: clinical and pharmacokinetic observations.

A 67-yr-old woman who ingested approximately 7 gm procainamide developed severe hypotension, renal insufficiency, and life-threatening cardiac toxicity. Hemodialysis doubled the rate of procainamide elimination and increased fourfold the clearance of NAPA, the N-acetylated metabolite of procainamide. Observations of procainamide and N-acetylprocainamide (NAPA) plasma levels during the patient's recovery suggest that lethargy and profound hypotension can be expected when these levels total 60 mug/ml and that severe cardiac toxicity should be anticipated with levels totaling 42 mug/ml or more. Hemodialysis also permitted investigation of the effects of hypotension on the pharmacokinetics of these compounds. The apparent volume of procainamide distribution was reduced from a normal value of 2 L/kg to 0.76 L/kg, and that of NAPA from 1.4 L/kg to 0.63 L/kg. The elimination + 1/2 of procainamide was prolonged from the normal of 3 hr to 10.5 hr, and that of NAPA from 6 to 35.9 hr. Procainamide absorption was also slowed in this clinical setting, causing procainamide plasma levels to continue rising for some time after toxicity was first recognized.

Antiarrhythmic efficacy of N-acetylprocainamide in patients with premature ventricular contractions.

Oral administration of a 1.5-gm dose of N-acetylprocainamide (NAPA) to 9 patients with premature ventricular contractions (PVCs) confirmed previous indirect evidence that this metabolite of procainamide has antiarrhythmic efficacy and potency comparable to those of procainamide. Although the mechanism by which NAPA acts as an antiarrhythmic drug is not known, it was found that the 6 patients with coupled PVCs responded to NAPA therapy and that the 3 patients without coupled PVCs failed to respond. Coupling interval prolongation also occurred during NAPA therapy in 4 of the 6 responding patients. These observations suggest that NAPA may terminate coupled PVCs by slowing and then interrupting conduction of re-entrant impulses, as has been proposed for procainamide. NAPA plasma concentrations of 7.4-17.2 mug/ml were well tolerated by the patients and produced an average fall of 3 mm Hg in mean arterial pressure and a 7.6% mean increase in corrected QT interval.

Absolute bioavailability in man of N-acetylprocainamide determined by a novel stable isotope method.

Absorption of a single oral dose of N-acetylprocainamide (NAPA) was studied in 3 normal subjects. Approximately 85% of the oral dose was absorbed and peak plasma NAPA concentrations were reached in 45 to 90 min. In 2 subjects, NAPA was absorbed at a fast initial rate, then more slowly, prolonging the apparent elimination phase half-life. Absolute bioavailability was determined by a new stable isotope method that entailed intravenous injection of NAPA 13C at the same time that an unlabeled NAPA capsule was given orally. Plasma levels and urine excretion of both compounds were determined by mass fragmentography. Bioavailability was assessed by deconvoluting the plasma level vs time curves resulting from intravenous and oral drug administration, and also by comparing the relative percentage of NAPA and NAPA-13C excreted unchanged in the 24 hr after simultaneous administration.

Characterization of theophylline binding to serum proteins in pregnant and nonpregnant women.

Sera from 10 subjects in the third trimester of pregnancy and from 10 nonpregnant women were studied to elucidate the mechanism underlying decreased theophylline protein binding during pregnancy. Consistent with the physiologic hypoalbuminemia of pregnancy, serum albumin concentrations averaged only 3.2 +/- 0.3 gm/dl (+/- SD) in pregnant subjects, compared with 4.4 +/- 0.3 gm/dl in control subjects (p less than 1 x 10(-6], and this was the main cause of decreased theophylline binding. Saturation binding studies indicated a single class of theophylline binding sites. Theophylline binding capacity (N) was greater in pregnant (N = 4.3 +/- 1.0) than in nonpregnant (N = 3.3 +/- 0.4) subjects, but binding affinity (ka) averaged only 227 +/- 69 (mol/L)-1 in pregnant subjects, compared with 303 +/- 44 (mol/L)-1 in control subjects (F2,17 = 4.26; p = 0.032). At a theophylline plasma concentration of 10 micrograms/ml, the combined effects of hypoalbuminemia and lowered ka would reduce theophylline binding to 31% +/- 3% in pregnant women, compared to 39% +/- 3% in nonpregnant control subjects (p less than 1 x 10(-5]. Nonesterified fatty acid concentrations were similar in both subject groups and did not contribute to the pregnancy-associated decrease in theophylline binding.

Efficacy and safety of N-acetylprocainamide in long-term treatment of ventricular arrhythmias.

Four patients with chronic ventricular arrhythmias, shown to respond over the short term to N-acetylprocainamide (NAPA), were treated for between 3 and 4 yr with NAPA, and 24-hr ambulatory ECGs were obtained monthly to monitor their responses. When the patients were ambulatory and receiving NAPA, the mean frequency of premature ventricular complexes averaged 70% (range 60% to 82%) below that recorded at 6-mo intervals when the patients were hospitalized and receiving placebo. Analysis of variance showed that NAPA exerted an antiarrhythmic effect in these patients and that tolerance to this effect did not develop with long-term therapy. Plasma NAPA concentrations required to achieve this level of response averaged 21 micrograms/ml (12 to 35 micrograms/ml) and were roughly twice as high as those which appeared to be maximally effective when the patients were hospitalized for their initial evaluation. NAPA therapy was associated with positive antibody titers in only one patient and seems less prone to cause drug-induced lupus erythematosus than procainamide, but NAPA shares the gastrointestinal and other side effects of procainamide.

Pharmacokinetics of prednisolone transfer to breast milk.

Prednisolone transfer to breast milk was studied in three nursing women who required oral steroid therapy for asthma. Each patient received a 50 mg intravenous dose of prednisolone phosphate, and blood and breast milk were sampled for 6 hours. Concentrations of prednisolone in milk declined more rapidly than in serum but were similar to expected unbound serum concentrations, suggesting that exchange between unbound prednisolone in serum and breast milk is relatively rapid and bidirectional. Because an average of 0.025% (range, 0.010% to 0.049%) of the prednisolone dose was recovered in milk, prednisolone transfer to breast milk does not appear to pose a clinically significant risk to nursing infants.

Procainamide and N-acetylprocainamide kinetics investigated simultaneously with stable isotope methodology.

The pharmacokinetics of procainamide (PA) and N-acetylprocainamide (NAPA) were compared in 3 normal subjects after simultaneous intraveous injection of PA and NAPA-13C. The distribution kinetics of both compounds were modeled with a 3-compartment mamillary system, and it was found that their steady-state distribution volumes were not significantly different, averaging 1.41 L/kg for PA and 1.46 L/kg for NAPA. However, the intercompartmental clearances of NAPA were slower than those of PA. In these normal subjects, the average elimination t1/2 and total elimination clearance for PA were 2.5 hr and 589.8 ml/min, and for NAPA were 6.2 hr and 233.7 ml/min. Mean renal clearances of PA (346.7 ml/min) and of NAPA (199.5 ml/min) exceeded the usual rate of glomerular filtration, which suggests that both compounds are eliminated in part by renal tubular secretion. All subjects were phenotypic rapid acetylators of isoniazid and converted approximately one fourth of the administered PA dose to NAPA-12C. The fate of 15.4% of the administered PA and 14.5% of the administered NAPA-13C was not determined.

Effect kinetics of N-acetylprocainamide-induced QT interval prolongation.

We attempted to correlate clinical response with the effects of N-acetylprocainamide (NAPA) on the QT interval in five patients with stable chronic ventricular arrhythmias. A 15 mg/kg dose of NAPA was administered and a pharmacokinetic-pharmacodynamic model was used to relate plasma NAPA concentrations to changes in corrected QT interval (QTc). NAPA volume of distribution, elimination clearance, and elimination half-life averaged 1.37 +/- 0.19 L/kg, 174 +/- 63 ml/min, and 8.2 +/- 1.4 hours, respectively (mean +/- SD), and NAPA renal clearance averaged 1.9 +/- 0.6 times creatinine clearance. QTc prolongation was characterized by a linear-effect model in the first four patients and averaged 2.4 msec for every microgram per milliliter NAPA in a hypothetic biophase. QTc prolongation in patient 5 was exaggerated and was analyzed with an Emax model. Nonetheless, NAPA did not control this patient's arrhythmia. Conversely, patient 1 subsequently developed torsade de pointes even though QTc prolongation in this patient was comparable to that in patients 2 through 4, who responded satisfactorily to NAPA. We conclude that QT interval changes during initial NAPA administration do not reliably predict subsequent clinical response.

Comparison of prednisolone kinetics in patients receiving daily or alternate-day prednisone for asthma.

Prednisolone pharmacokinetics were compared in seven patients with asthma managed by alternate-day prednisone therapy and in seven patients with asthma requiring daily doses of prednisone. Steroid requirements of these patients were carefully characterized and had been stable for at least 12 months. Prednisolone volume of distribution, elimination clearance, and elimination t1/2 averaged 0.606 +/- 0.061 and 0.553 +/- 0.162 L/kg, 2.28 +/- 0.43 and 1.93 +/- 0.54 ml/min/kg, and 204 +/- 44 and 214 +/- 19 minutes in patients receiving alternate-day or daily prednisone therapy, respectively. These results indicate that differences in these pharmacokinetic parameters do not account for the well-established clinical observation that some patients require daily prednisone doses and that their disease cannot be managed with alternate-day steroid therapy.

Theophylline pharmacokinetics in pregnancy.

Theophylline pharmacokinetics were studied serially in five women during and after pregnancy. Theophylline protein binding was reduced to 11.1% +/- 4.7% (P less than 0.01) and 13.0% +/- 5.9% (P less than 0.01) during the second and third trimesters of pregnancy, respectively, compared with 28.1% +/- 2.8% when the patients were more than 6 months postpartum. Similar comparisons indicate that theophylline distribution volume and elimination t1/2 were increased from 30.7 +/- 4.4 L and 262 +/- 57 minutes to 36.8 +/- 4.2 L (P less than 0.05) and 389 +/- 73 minutes (P less than 0.01) in the third trimester of pregnancy. In the second and third trimesters, intrinsic nonrenal clearance was reduced to 0.82 +/- 0.25 ml/min X kg (P less than 0.05) and 0.67 +/- 0.18 ml/min X kg (P less than 0.01) compared with a remote postpartum value of 1.25 +/- 0.37 ml/min X kg. However, these reductions were offset by increases in theophylline intrinsic renal clearance so that apparent reductions in the overall unbound clearance of this drug did not reach statistical significance either during pregnancy or in the early postpartum period.

Kinetics of epsilon-aminocaproic acid distribution, elimination, and antifibrinolytic effects in normal subjects.

The kinetics of epsilon-aminocaproic acid (EACA) distribution and elimination were studied in six normal subjects after a single 10-gm iv dose. Steady-state distribution volume averaged 30.01 or 0.39 l/kg. Mean elimination t 1/2 was 294 min and the elimination clearance was 0.19 l/min. Renal excretion of unchanged EACA accounted for 68% of its elimination and renal EACA clearance averaged 115% of creatinine clearance. EACA antifibrinolytic effect kinetics were also characterized in five of the subjects by the monitoring of clot lysis times in whole blood and platelet-rich plasma. Peak antifibrinolytic effects were observed 15 to 60 min after peak EACA plasma concentrations were attained. A model of maximal fibrinolysis inhibition (Emax) was used to estimate a half-maximal inhibition (IC50) of 63 +/- 19.7 microgram/ml. This agrees with the value of 0.55 mM or 72 microgram/ml that has been reported for the dissociation constant of the EACA-plasminogen complex and is consistent with the proposed biochemical mechanism of EACA action.

Acute tolerance to cocaine in humans.

There is controversy as to whether acute tolerance develops to the principal effects of cocaine in humans. The studies described here demonstrate the phenomenon of acute tolerance to cocaine chronotropic and subjective effects and the rate and extent of tolerance development. Stable plasma cocaine concentrations were produced and then maintained in volunteer cocaine users by administering an intravenous cocaine injection followed by a cocaine infusion designed to compensate for the plasma clearance of cocaine. The euphoric effect (high) intensified to a peak at about 1 hour and then declined toward baseline at 4 hours despite the presence of constant plasma cocaine levels. The chronotropic effect reached a peak within 10 minutes and then declined, with a half-life of 31 +/- 13 (mean +/- SD) minutes toward a plateau at 33% +/- 21% of its peak intensity. Tolerance development was quantified as an exponential process, with a rate constant (tolerance factor) accounting for the progressive alteration of the cocaine concentration-effect relationship.