Quantitation of interference in digoxin immunoassay in renal, hepatic, and diabetic disease.

A comparison of the results of a newly developed fluorescence-derivatization/HPLC method and a commercial immunoassay method (ACA, Dupont) for the measurement of serum digoxin concentrations in patients indicates that (1) the results from the ACA method agree well with those from the HPLC method in patients with cardiovascular disease but without renal, diabetic, and liver disease, (2) serum digoxin concentrations determined by the ACA method are overestimated in patients with renal, diabetic, or liver disease, and (3) the steady-state serum concentrations of hydrolyzed and reduced metabolites are relatively insignificant in patients receiving digoxin therapy, including patients with renal failure. The excellent reproducibility of the HPLC and immunoassay methods (coefficient of variation < 9.0%), together with the demonstrated specificity of the HPLC method with respect to potential interference from digoxin metabolites, endogenous digoxin-like immunoactive substances, and coadministered drugs and their metabolites, allows quantitation of the degree of interference in digoxin immunoassays under actual therapeutic drug monitoring conditions. Clinically significant interferences (0.3 to 1.1 ng/ml) with immunoassay determination were found in the majority of patients in all three diseases studied.

Extension of the serum digoxin concentration--response relationship to patient management.

The purposes of this investigation were to demonstrate how computer simulations may be employed to extrapolate data obtained from a single intravenous digoxin dose to multiple oral dosing patterns and how these simulations may apply to clinical situations. The intravenous data were obtained from a previous study of the pharmacokinetics of serum digoxin and its inotropic response (derived from systolic intervals) in 12 normal male volunteers. The simulations were applied to various clinical situations including variations in oral dosing, alternate loading doses, no loading versus loading dose, and intravenous versus oral dosing. A nonlinear relationship was found between response and the post-distribution serum digoxin concentration in the therapeutic range. Thus, the increase in inotropic response is less than proportional to the increase in digoxin concentration in serum. This nonlinear relationship has several important clinical implications for loading and maintenance dosing protocols. Such concepts may be important relative to more rational clinical use of digoxin and to decreasing digoxin toxicity.

Digoxin disposition in elderly humans with hypochlorhydria.

Digoxin (D3) metabolism is partially mediated by the gastrointestinal tract via acid hydrolysis of digitoxose sugar moieties and bacterial reduction of the lactone. The hypothesis that hypochlorhydria influences digoxin disposition was tested in six normochlorhydric (NC) and four hypochlorhydric (HC) subjects. D3 tablets were administered daily for 19 to 28 days, and quantitative urine and fecal samples were collected over the last 3 days (steady state). Samples were analyzed for D3 and its extractable metabolites by fluorescence-derivatization HPLC. Excretion of D3 in urine increased from 37% of the dose in NC to 46% in HC, whereas excretion of D3 in feces decreased from 29 to 14%. These changes were statistically significant (P < .05) and consistent with decreased hydrolysis of D3 by stomach acid and increased intestinal metabolism in HC. In each subject, D3 was added to anaerobic cultures of both feces and jejunal fluid. Digoxin was reduced in all but two of the fecal incubates, and was not reduced in any jejunal fluid incubates. Because dihydrodigoxin (DHD3) was found in only two hypochlorhydric subjects, in vitro measures of bacterial reduction of D3 were not predictive of in vivo excretion of reduced metabolites. Sugar-hydrolyzed, reduced metabolites were not found in any subjects. It is concluded that D3 disposition is altered by hypochlorhydria, and that an understanding of the metabolic mechanisms requires further study.

Disposition of tablet and capsule formulations of digoxin in the elderly.

STUDY OBJECTIVE: To compare digoxin tablets and liquid-filled capsules with respect to excretion of the drug and its metabolites in urine and feces at steady state. DESIGN: A randomized, crossover trial, each period lasting 3 weeks, with no washout period. SETTING: A university hospital. PATIENTS: Six patients, five of whom were elderly, with histories of gastrointestinal disorders, such as hypochlorhydria, intestinal bacterial overgrowth, and inflammatory bowel disease. INTERVENTIONS: The patients received digoxin once/day in either tablet or capsule form for 3 weeks, and then were switched to the other formulation. Total urinary and fecal excretion from the last 3 days of each regimen were analyzed for the drug and metabolites. MEASUREMENTS AND MAIN RESULTS: No statistically significant differences were found between tablets and capsules in recovery of digoxin or its metabolites in urine or feces (p = 0.05). One subject had a 4-fold increase in urinary drug excretion and 50% decrease in fecal excretion after taking the capsules compared with tablets. Intersubject variability in extent and type of metabolite excretion was greater than intrasubject variability. CONCLUSIONS: Fecal analyses may be an accurate way to classify patients as formers of digoxin reduction products.

Spectral analysis of the configuration and solution conformation of dihydrodigoxigenin epimers.

The C20 configuration and solution conformation of each epimer of dihydrodigoxigenin has been studied by circular dichroism (CD) and NMR spectroscopy. Results from the CD spectra indicate that the two epimers have opposite orientations of the beta-carbon in the lactone ring. This finding, together with X-ray crystallographic data from a separate study on the minor epimer, establishes the C20 configuration of the minor epimer as S and of the major epimer as R. NMR evidence indicates that the average lactone rotamer for the minor epimer has the C22 position located on the C12 side of the steroid nucleus, whereas the average lactone rotamer for the major epimer has the C21 position located on the C12 side of the steroid nucleus. Molecular models indicate that these are the least-hindered positions for the respective rotamers. Physical data characterizing the two epimers are provided.

Tritiated naltrexone binding in plasma from several species and tissue distribution in mice.

The binding of 15,16,-3H-naltrexone in human, monkey, dog, guinea pig, rat, and mouse plasma was investigated over a range of concentrations, including predicted therapeutic levels. Studies using equilibrium dialysis at 37 degrees indicate that the extent of binding is independent of naltrexone concentration over the concentration range of 1-500 ng/ml for dog plasma and of 0.1-500 ng/ml for human, monkey, guinea pig, rat, and mouse plasma. The extent of naltrexone binding in plasma is similar in the six species studied, the range being from 20% bound in rat plasma to 26% in plasma from beagle and mongrel dogs. This relatively low extent of naltrexone binding in plasma is consistent with previous findings of a large apparent volume of distribution of this drug in the dog. To investigate further the distribution of tritiated naltrexone, the tissue levels of radioactivity in mice at 1, 5, and 15 min after intravenous administration of 8-3H-naltrexone were determined. Naltrexone was rapidly distributed from plasma to tissues, with less than 4% of the dose being present in plasma at 1 min after injection.

Plasma naltrexone kinetics after intravenous bolus administration in dogs and monkeys.

This investigation generated data characterize a specific electron-capture GLC assay reported previously for naltrexone and applied the method to a determination of naltrexone pharmacokinetics. Extraction efficiencies are reported for the assay, and mass spectral evidence indicates that naltrexone forms a triester when derivatized for electron-capture GLC with pentafluoropropionic anhydride and a base catalyst. Plasma level-time data for intravenous naltrexone at two dose levels in monkeys yielded no evidence of dose-dependent kinetics. A two-compartment open pharmacokinetic model was fitted to plasma level-time data for naltrexone in two dogs and yielded a total body clearance of 51-55 ml/min/kg. Urine collected for 0-24 hr contained 36% of the dose as naltrexone conjugates with less than 1% as unchanged naltrexone. Plasma level-time data for intravenous naltrexone in six monkeys yielded an average terminal half-life of 7.8 hr and a total body clearance of 64 ml/min/kg. The total body clearance for naltrexone was greater than the hepatic plasma or blood flow in both dogs and monkeys. This finding, together with the extremely low renal excretion of naltrexone, suggests the existence of elimination mechanisms besides liver metabolism and renal excretion.

Specific and sensitive determination of digoxin and metabolites in human serum by high performance liquid chromatography with cyclodextrin solid-phase extraction and precolumn fluorescence derivatization.

A precolumn fluorescence derivatization high performance liquid chromatographic method has been developed for the simultaneous determination of digoxin and its metabolites digoxigenin bisdigitoxoside, digoxigenin monodigitoxoside digoxigenin, and dihydrodigoxin (20-R and 20-S epimers) in human serum. Digoxin and its metabolites were extracted from serum samples (containing digitoxin as internal standard) with a cyclodextrin solid-phase extraction (SPE) column. Fluorescent derivatives were formed by reaction of the analytes with 1-naphthoyl chloride in the presence of 4-dimethylaminopyridine under a nitrogen atmosphere in a glove box with controlled relative humidity (26% r.h. or less). The derivatives were isolated using cyclodextrin and C1 SPE columns sequentially, and determined by HPLC using silica column separation and fluorescence detection. Calibration curves were linear over the concentration range from 0.25 to 4.0 ng ml-1. Recoveries of digoxin and its metabolites from serum ranged from 62 to 86%, and coefficients of variation from repetitive analyses ranged from 6.9 to 20.9% and from 5.8 to 12.2% at 0.5 ng ml-1 and 2.0 ng ml-1, respectively. This method has been shown capable of specifically determining digoxin and its major metabolites in serum, and has been successfully used in the determination of digoxin and its metabolites in serum samples collected from patients undergoing digoxin therapy. This method thus permits the investigation of digoxin metabolism and pharmacokinetics after the administration of commercial dosage forms.

Naltrexone metabolism and sustained release following administration of an insoluble complex to rhesus monkeys and guinea-pigs.

Both the release and the metabolism of naltrexone have been evaluated after intramuscular administration of a sustained release [15,16-3H2]naltrexone aluminium tannate complex in guinea-pigs and rhesus monkeys. In both species, measurable excretion of radioactivity was obtained for greater than 50 days and complete recovery of the dose was obtained in the guinea-pig. The radioactivity excretion rate-time profile differed in the two species with guinea-pig yielding a continuously declining rate and monkey yielding a peak at 5 days. In selected monkey urine samples (days 4, 17-20 and 49-52) subjected to t.l.c., evidence was obtained for the presence of naltrexone, beta-naltrexol and 2-hydroxy-3-O-methyl-beta-naltrexol, mostly as glucuronide and/or sulphate conjugates. The t.l.c. data also suggest that in monkey a naltrexone metabolite builds up relative to naltrexone over the 52 day release period.

A comparison of two surgical techniques for preparation of rats with chronic bile duct cannulae for the investigation of enterohepatic circulation.

Two surgical biliary cannulation procedures for study of enterohepatic circulation in chronically cannulated rats were compared in a randomized study. Control rats (group A) had only a jugular vein cannula and no laparotomy, whereas experimental group-B rats additionally had the bile duct cannulated at two locations, one for collecting bile from the liver and the other for returning bile into the duodenum. Experimental group-C rats had the jugular vein cannula as well as the bile duct cannula for collecting bile, but the duodenal wall was cannulated for returning bile to the intestine. Several physiologic and biochemical indicators were monitored daily after surgery, including body weight, bile flow rate, and plasma concentrations of bilirubin and creatinine, and activities of glutamate-pyruvate transaminase (GPT) and lipase. Overall duration of survival of group-B rats was shorter than that of group-A rats (P < 0.05), whereas no difference was found between groups A and C. Group-B rats had higher bilirubin concentration than did controls (P < 0.05), whereas group-C rats did not. Group-B rats had higher plasma lipase activity than did rats of the other two groups, and this analyte was more variable in group-B rats. Rats of groups B and C had high GPT activity after surgery (P < 0.05). A statistically significant loss of body weight was associated with group-B rats over 8 days after surgery and for group-C rats over 2 days after surgery, after which body weight stabilized in group-C rats.(ABSTRACT TRUNCATED AT 250 WORDS)