Myocardial and skeletal muscle concentrations of digoxin in patients on long-term therapy

Inflammasome inhibition blocks cardiac glycoside cell toxicity

Chronic heart failure and cardiac arrhythmias have high morbidity and mortality, and drugs for the prevention and management of these diseases are a large part of the pharmaceutical market. Among these drugs are plant-derived cardiac glycosides, which have been used by various cultures over millennia as both medicines and poisons. We report that digoxin and related compounds activate the NLRP3 inflammasome in macrophages and cardiomyocytes at concentrations achievable during clinical use. Inflammasome activation initiates the maturation and release of the inflammatory cytokine IL-1β and the programmed cell death pathway pyroptosis in a caspase-1–dependent manner. Notably, the same fluxes of potassium and calcium cations that affect heart contraction also induce inflammasome activation in human but not murine cells. Pharmaceuticals that antagonize these fluxes, including glyburide and verapamil, also inhibit inflammasome activation by cardiac glycosides. Cardiac glycoside–induced cellular cytotoxicity and IL-1β signaling are likewise antagonized by inhibitors of the NLRP3 inflammasome or the IL-1 receptor–targeting biological agent anakinra. Our results inform on the molecular mechanism by which the inflammasome integrates the diverse signals that activate it through secondary signals like cation flux. Furthermore, this mechanism suggests a contribution of the inflammasome to the toxicity and adverse events associated with cardiac glycosides use in humans and that targeted anti-inflammatories could provide an additional adjunct therapeutic countermeasure.

Plasma vs heart tissue concentration in humans - literature data analysis of drugs distribution

Little is known about the uptake of drugs into the human heart, although it is of great importance nowadays, when science desires to predict tissue level behavior rather than to measure it. Although the drug concentration in cardiac tissue seems a better predictor for physiological and electrophysiological changes than its level in plasma, knowledge of this value is very limited. Tissue to plasma partition coefficients (Kp) come to rescue since they characterize the distribution of a drug among tissues as being one of the input parameters in physiologically based pharmacokinetic (PBPK) models. The article reviews cardiac surgery and forensic medical studies to provide a reference for drug concentrations in human cardiac tissue. Firstly, the focus is on whether a drug penetrates into heart tissue at a therapeutic level; the provided values refer to antibiotics, antifungals and anticancer drugs. Drugs that directly affect cardiomyocyte electrophysiology are another group of interest. Measured levels of amiodarone, digoxin, perhexiline and verapamil in different sites in human cardiac tissue where the compounds might meet ion channels, gives an insight into how these more lipophilic drugs penetrate the heart. Much data are derived from postmortem studies and they provide insight to the cardiac distribution of more than 200 drugs. The analysis depicts potential problems in defining the active concentration location, what may indirectly suggest multiple mechanisms involved in the drug distribution within the heart. Copyright © 2015 John Wiley & Sons, Ltd.

Application of permeability-limited physiologically-based pharmacokinetic models: part I-digoxin pharmacokinetics incorporating P-glycoprotein-mediated efflux

A prerequisite for the prediction of the magnitude of P-glycoprotein (P-gp)-mediated drug-drug interactions between digoxin and P-gp inhibitors (e.g. verapamil and its metabolite norverapamil) or P-gp inducers (e.g. rifampicin) is a predictive pharmacokinetic model for digoxin itself. Thus, relevant in vitro metabolic, transporter and inhibitory data incorporated into permeability-limited models, such as the "advanced dissolution, absorption and metabolism" (ADAM) module and the permeability-limited liver (PerL) module, integrated with a mechanistic physiologically-based pharmacokinetic (PBPK) model such as that of the Simcyp Simulator (version 12.2) are necessary. Simulated concentration-time profiles of digoxin generated using the developed model were consistent with observed data across 31 independent studies [13 intravenous single dose (SD), 12 per oral SD and six multiple dose studies]. The fact that predicted tmax (time of maximum plasma concentration observed) and Cmax (maximum plasma concentration observed) of oral digoxin were similar to observed values indicated that the relative contributions of permeation and P-gp-mediated efflux in the model were appropriate. There was no indication of departure from dose proportionality over the dose range studied (0.25-1.5 mg). All dose normalised area under the plasma concentration-time curve profiles (AUCs) for the 0.25, 0.5, 0.75 and 1 mg doses resembled each other. Thus, PBPK modelling in conjunction with mechanistic absorption and distribution models and reliable in vitro transporter data can be used to assess the impact of dose on P-gp-mediated efflux (or otherwise).

Inhibition of peripheral blood mononuclear cell proliferation by cardiac glycosides

INTRODUCTION

Prior studies have shown that ouabain, a cardiac glycoside that inhibits the sodium, potassium adenosine triphosphatase (Na+,K+ ATPase) enzyme, downregulates phytohemagglutinin (PHA)-induced peripheral blood mononuclear cell (PBMNC) proliferation.

OBJECTIVE

This study examined and compared the effects of both ouabain and digoxin, a cardiac glycoside used therapeutically in humans, on PBMNC proliferation.

METHODS

Peripheral blood mononuclear cells were isolated from healthy human subjects, incubated for 72 hours with and without PHA (2%) in the presence and absence of ouabain (10(-12) M to 10(-4) M) or digoxin (10(-9) M to 10(-6) M), and pulsed with 3H thymidine.

RESULTS

For PHA-stimulated PBMNCs in the ouabain-treated group (n = 10 subjects), the mean (+/-STD) % uptake (% 3H thymidine uptake in absence of ouabain) was 80.5 +/- 6.0 at 10(-12) M ouabain, 73.1 +/- 8.4 at 10(-10) M, 47.89 +/- 13.1 at 10(-8) M, 6.9 +/- 3.2 at 10(-6) M, and 3.4 +/- 1.6 at 10(-4) M. For PHA-stimulated cells in the digoxin-treated group (n = 9 subjects), the mean (+/-STD) % uptake (% 3H thymidine uptake in absence of digoxin) was 89.8 +/- 9.8 at 10(-9) M digoxin, 92.6 +/- 8.2 at 10(-8) M, 54.3 +/- 19.8 at 10(-7) M, and 1.0 +/- 2.4 at 10(-6) M. Repeated measures ANOVA demonstrated a significant effect of concentration of both glycosides on PBMNC proliferation (P < .01). The inhibitory effect was reversible, but was largely abbrogated if ouabain was added after 48 hours of incubation with PHA. Further, the inhibitory effect extended to PBMNCs stimulated with recall antigen (tetanus) and to fractionated PBMNCs (CD4+, CD8+ and CD19+) stimulated with mitogens. Additionally, dose-response inhibitory effects of glycosides on PBMNC Na+,K+ ATPase enzyme activity and interleukin-2 (IL-2) secretion by PHA-stimulated PBMNC were also noted. Neither glycoside had an effect on spontaneous PBMNC proliferation (no PHA) or trypan blue exclusion.

CONCLUSIONS

These studies demonstrate that both cardiac glycosides inhibited PHA-induced PBMNC proliferation, possibly via Na+,K+ ATPase inhibition, but not via cell toxicity. The concentration range over which inhibition was observed was similar for both glycosides. The results raise the possibility that therapeutic or toxic doses of digoxin could have an effect on cell-mediated immunity in vivo.

No adaptation to digitalization as evaluated by digitalis receptor (Na,K-ATPase) quantification in explanted hearts from donors without heart disease and from digitalized recipients with end-stage heart failure

Speculations about development of tolerance to the inotropic effect of digitalis have been engendered since studies in various in vitro systems and tissues not representative of the heart have shown up-regulation of sodium potassium adenosine triphosphatase (Na,K-ATPase) when exposed to digitalis. Moreover the digitalis receptor (i.e., Na,K-ATPase) concentration in the normal, vital human left ventricle has not been previously determined. On this basis, digitalis receptor concentration was quantified in the left ventricle of explanted hearts from subjects without heart disease and from patients with end-stage heart failure who had received digitalis therapy. This was performed using vanadate-facilitated 3H-ouabain binding to intact tissue samples giving values of 728 +/- 58 (n = 5) and 467 +/- 55 pmol/g wet weight (n = 6) (mean +/- SEM) (p < 0.005), respectively. However, some of the digitalis receptors may have retained digoxin before 3H-ouabain binding and thus may have escaped detection. To eliminate this effect of retained digoxin, samples were exposed to prolonged washing in buffer containing excess digoxin antibody, a method recently shown to clear digoxin from receptors and allow subsequent complete digitalis receptor quantification by 3H-ouabain binding. After washing in digoxin specific antibody, specific digitalis receptor concentration was 760 +/- 58 pmol/g (n = 5) and 614 +/- 47 pmol/g (n = 6) wet weight in samples of the normal and failing hearts, respectively (p < 0.08). Thus, digitalization was associated with occupancy of digitalis receptors in the failing human heart of 24% (p < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute myocardial uptake of digoxin in humans: correlation with hemodynamic and electrocardiographic effects

Acute myocardial uptake of digoxin was measured at a constant paced heart rate (75 beats/min) for 30 min after an intravenous bolus injection of 500 micrograms of digoxin in 14 patients with ischemic heart disease. Myocardial digoxin content, determined by serial measurement of aortocoronary sinus digoxin concentration gradients and coronary sinus blood flow, was expressed relative to coronary sinus blood flow at rest and correlated with simultaneous hemodynamic and electrocardiographic changes. Myocardial digoxin uptake was extensive (4.1 +/- 0.7% of total injected dose at 30 min) and prolonged, with rapid initial uptake (75.3 +/- 6.6% of maximum at 3 min), followed by a variable phase of slower accumulation. Peak left ventricular positive first derivative of left ventricular pressure (dP/dt) increased progressively (p less than 0.01), with a similar time course to that of myocardial digoxin accumulation; maximal change was 18.5 +/- 4.7% at 27 min. The ratio of inotropic effect to myocardial digoxin content did not vary significantly over the period of the experiment. However, peak inotropic effects in individual patients were not significantly related to peak myocardial digoxin content. The spontaneous PR interval increased transiently, with a peak increase of 5.9 +/- 1.8% (p less than 0.05) 12 min after digoxin administration. It is concluded that after intravenous bolus administration, 1) peak effects of digoxin on atrioventricular (AV) conduction occur early, whereas positive inotropic effects increase progressively for greater than or equal to 27 min; and 2) digoxin accumulation in the human myocardium is prolonged and is a determinant of inotropic effects, but not of prolongation of AV node conduction.

Pharmacokinetic interactions with digoxin

Numerous pharmacological agents have been shown to produce clinically significant pharmacokinetic interactions with digoxin. Drugs which reduce digoxin absorption include the antacids aluminium hydroxide, magnesium hydroxide and magnesium trisilicate, the antidiarrhoeals kaolin and pectin, the hypocholesterolaemic agent cholestyramine and the chemotoxins cyclophosphamide, vincristine and bleomycin. Certain antibiotics including sulphasalazine, neomycin and aminosalicylic acid reduce digoxin absorption while others, including erythromycin and tetracycline, increase the bioavailability of digoxin in some patients. Capsule preparations of digoxin in solution are less subject to several of the interactions which affect the absorption and bioavailability of digoxin tablets. Various drugs induce alterations in the volume of distribution and clearance of digoxin. Cardiac patients receiving digoxin therapy are particularly prone to interactions with commonly co-administered medications such as the antiarrhythmics quinidine and amiodarone, the calcium channel blockers verapamil and nifedipine, and possibly some vasodilating agents. Studies of digoxin interactions have yielded discrepant results, indicating the need for careful analysis of investigational design before arriving at clinical conclusions.

Digoxin 1785-1985. I. Two hundred years of digitalis

This year we are celebrating the bicentenary of the publication, by William Withering, of An Account of the Foxglove and Some of its Medicinal Uses with Practical Remarks on Dropsy and Other Diseases (1). During these two hundred years digitalis has constantly been to the fore of medical thinking and it is appropriate that we should look back and examine the contributions which studies of this drug have made to medicine as we know it today. Some of the studies have been at the centre of fierce controversy and others have been of seminal importance in the development of new concepts.

3H-Ouabain binding to human mononuclear leucocytes

Specific binding of cardiac glycosides to intact human blood cells may be a suitable model for physiological or disease-induced changes in cardiac glycoside binding to human heart muscle. Since the erythrocyte contains no nucleus and has relatively few binding sites compared with heart muscle, intact mononuclear leucocytes were investigated in the present study. Using leucocyte suspensions from 34 normal subjects, 133 measurements of 3H-ouabain binding-were obtained. 3H-Ouabain bound to one type of binding site with an affinity (KD) of 2.8 +/- 1.2 X 10(-9) M, similar to that of human heart muscle. Association and dissociation were slow processes (k+1, 3.9 X 10(4) M-1 sec-1; k-1, 8.1 X 10(-5) sec-1, n = 2). The number of ouabain binding sites/leucocyte varied from 18,000 to 60,000 (mean +/- SD, 34,600 +/- 9,700), with no correlation with the proportion of monocytes present or with the serum K+-level of the donors. Large inter- and intra-individual differences in binding site number were measured which are probably a result of the heterogeneity of the cell suspension used. Thus, the ouabain binding site on human heart muscle and intact mononuclear leucocytes is probably identical. However, the number of binding sites in mixtures of mononuclear leucocytes shows large and inconsistent intraindividual variations, making these studies unsuitable for quantifying drug- or disease-induced changes in ouabain binding site number.

[Distribution of digoxin, digitoxin and their cardioactive metabolites in human heart and kidney tissue. A postmortem study]

A method was developed for the specific determination of digoxin and digitoxin, as well as their semisynthetic derivatives and dependent cardioactive metabolites, in autopsy samples of heart and kidney. A collective of six patients on long-term treatment with therapeutic doses of beta-acetyldigoxin had a mean myocardial digoxin content of 46.1 +/- 25.0 ng/g (SD); kidney: 50.3 +/- 30.3 ng/g. Digoxigenin bisdigitoxoside represented the second most important metabolite in heart and kidney; digoxigenin monodigitoxoside and digoxigenin follow, respectively. In a collective of seven patients on maintenance treatment with digitoxin, the mean tissue levels were higher but the metabolic pattern was similar (myocardial digitoxin content: 78.9 +/- 38.4 ng/g, renal content: 104.1 +/- 44.1 ng/g). The amount of digoxin formed by hydroxylation under long-term treatment with digitoxin in heart and kidney were approximately 10 ng/g. A case of digoxin intoxication differed both in the tissue content and in the metabolic distribution.

Binding of amiodarone by serum proteins and the effects of drugs, hormones and other interacting ligands

Amiodarone is chiefly bound to albumin (62.1%) and much of the remainder (33.5%) is carried on a high molecular weight protein, probably beta-lipoprotein. Analysis of data for amiodarone binding to albumin revealed a high affinity primary binding site (Ka 5.6 X 10(6) litre mol-1) with about four secondary sites (average Ka 1.9 X 10(5) litre mol-1). Studies of the binding of amiodarone in serum revealed one type of binding site only with an affinity constant (Ka 4.2 X 10(6) litre mol-1) similar to that of the primary site on albumin. The secondary albumin binding sites do not seem therefore to be utilized in whole serum and the affinity of the lipoprotein must be similar to that of the primary amiodarone binding site on albumin. The effects of a wide range of compounds on albumin binding of amiodarone were examined by equilibrium dialysis. Quinidine, amitriptyline, cephazolin and palmitate decreased albumin-bound [125I]amiodarone. Neither warfarin nor digoxin affected the binding of amiodarone by albumin, thus of the three drugs known to be potentiated by concomitant amiodarone administration, only potentiation of quinidine could be explained by displacement from serum albumin. Rifampicin, frusemide, phenytoin, (-)-adrenaline, bromocresol green, (-)-noradrenaline and bromocresol purple were found to increase binding of [125I]amiodarone by albumin.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacokinetics of digoxin in newborn and adult sheep

The pharmacokinetics of digoxin were determined in 12 ewes and 13 newborn sheep after bolus drug administration and under steady state drug conditions. After death, tissue distribution of digoxin was determined and normalized to plasma drug concentrations at steady state. Volume of distribution and total drug clearance were lower at steady state than the comparable variables calculated from bolus drug administration. No significant difference between ewes and newborns was shown for drug distribution half-life (0.72 vs. 0.76 hr), drug elimination half-life (15.2 vs. 13.7), or renal drug clearance (0.86 vs. 0.89 liters/kg/hr). Total drug clearance as well as the area derived and steady state volumes of distribution were higher in newborns than in ewes. Digoxin secretion into the urine was limited in newborns, as evidenced by a lower renal digoxin clearance to creatinine clearance ratio in newborns than in ewes (371 vs. 600%). The plasma concentration of digoxin at steady state correlated well with myocardial drug concentrations. Drug distribution was similar in both age groups; however, the tissue to plasma digoxin ratio in kidney was higher in newborns than in ewes (mean 469 vs. 263, respectively). Although age-related differences in drug clearance and distribution volume existed, intersubject variation was substantial, and the demonstrated variations were not large enough to account for the high doses of digoxin used to treat congestive heart failure in immature subjects.

Digoxin disposition in obesity: clinical pharmacokinetic investigation

Digoxin pharmacokinetics were studied in 16 obese (mean +/- SD weight, 100.2 +/- 36.8 kg) and 13 control (64.6 +/- 10.5 kg) subjects. All subjects had normal renal function and no other coexisting disease. After administration of 0.75 mg digoxin by intravenous infusion, multiple plasma samples obtained over the 96 hours following infusion were analyzed for digoxin concentration by radioimmunoassay. Pharmacokinetic parameters were determined by weighted iterative nonlinear least squares regression analysis. Elimination half-life (t 1/2) was not different between obese and control groups (35.6 +/- 10.5 vs 41.2 +/- 16.7 hours). Absolute volume of distribution (Vd) also was not different (981 +/- 301 vs 937 +/- 397 liters), nor was total clearance of digoxin (328 +/- 82 vs 278 +/- 87 ml/min). Elimination t 1/2 was significantly negatively correlated with clearance among all subjects (r = -0.46; p less than 0.01). Using percent ideal body weight (IBW) as a measure of obesity, no correlation was found between percent IBW and Vd (r = 0.03). Thus digoxin is similarly distributed into IBW in obese and normal weight subjects, and there is no significant distribution of digoxin into excess body weight over IBV. In addition, there is no difference in total metabolic clearance or elimination half-life between obese and control subjects. Digoxin loading and maintenance dosage should be calculated on the basis of IBW, which reflects lean body mass, rather than TBW, which reflects adipose tissue weight in addition to lean body mass.

The influence of heart rate on digoxin-induced inhibition of myocardial Na+-K+-ATPase activity in the dog

1. Groups of sedated dogs were studied at spontaneous heart rates (HR), 55-100/min, or at paced HR 200/min, with or without intravenous digoxin administration. After 60 min, active rubidium uptake (86 Rb+) of ventricular samples was determined in vitro. 2. Untreated fast and slow HR groups had similar uptakes. Following digoxin, 0.08 mg/kg, uptake was less at fast than slow HR (63.8, s.e.m. = 4.5 v. 87.5, s.e.m. = 5.0 pmol/mg LV/15 min, P less than 0.01). After 0.125 mg/kg, values were again lower in the fast HR group in which five of seven developed ventricular tachycardia. 3. Heart rate does not alter in vitro activity of myocardial Na+-K+-ATPase but does influence inhibition of the enzyme resulting from digoxin administration.

Age dependence of myocardial Na+-K+-ATPase activity and digitalis intoxication in the dog and guinea pig

Infants and young animals tolerate higher doses of digitalis glycosides, relative to body weight, than adults. One possible explanation for this could be an age-dependent difference in the myocardial digitalis receptor, the Na+-K+-ATPase. Two functions of this enzyme were studied in adult, 1- and 6-week-old dogs and guinea pigs: in vitro myocardial uptake of rubidium (86Rb) and binding of ouabain. In guinea pigs, rubidium uptake (pmol Rb/mg LV per 15 min) was: 1 week old: 100.9 +/- 7.1 (mean +/- SE); 6 week: 79.8 +/- 6.7 adult: 55.2 +/- 7.9; (1 week: 6 week: P less than 0.025; 1 week: adult, P less than 0.001; 6 week: adult, P less than 0.025). Similarly in dogs, rubidium uptake was significantly greater at 1 week than at 6 weeks (208 +/- 13 vs. 144 +/- 9; P less than 0.001) and the latter greater than in adults (111 +/- 4) (P less than 0.005). Other groups of anesthetized adult and 6-week-old dogs were given digoxin, 0.3 mg/kg, iv. The young dogs took significantly longer to become cardiotoxic (17.3 +/- 3.4 min vs. 9.3 +/- 1.4 min; P less than 0.025), while their myocardial digoxin uptake was at least as great. Rubidium uptake showed an average decrease of 56% after digoxin but residual uptakes were not different in the two groups. Data for ouabain binding showed similar differences between the various groups of dogs studied. Increased myocardial Na+-K+-ATPase activity, reflected in greater active cation transport and specific enzyme binding, has been demonstrated in young animals and may be partly responsible for their greater tolerance to digitalis glycosides.

Digoxin concentration in right atrial myocardium, skeletal muscle and serum in man: influence of atrial rhythm

Serum, right atrial myocardium and skeletal muscle collected from 32 adult patients undergoing open heart surgery were analyzed for digoxin by radioimmunoassay. Preoperatively 20 patients were in sinus rhythm, but not in patients with atrial fibrillation, there was a highly significant correlation between digoxin concentration in serum and right atrial myocardium, in skeletal muscle and right atrial myocardium, and in serum and skeletal muscle. The means and variances of the ratios right atrial myocardium/serum and right atrial myocardium skeletal muscle were significantly higher in patients with atrial fibrillation than in those with sinus rhythm. This, plus the lack of difference in ratios skeletal muscle serum between these groups of patients, indicate increased right atrial digoxin binding in atrial fibrillation in man. This conclusion is further supported by the finding of similar or higher digoxin concentration in right atrial myocardium than in left ventricular myocardium in atrial fibrillation (6 patients), and a lower digoxin concentration in right atrial myocardium than in left ventricular myocardium in sinus rhythm (3 patients).

[Forensic aspects of digoxin-poisoning: toxicological and morphological findings (author's transl)]

Case report. A 82 year old woman died 80 min after accidental ingestion of 5 mg beta-methyl-digoxin. The autopsy and the histological examination revealed non-specific alterations due to shock and preexisting coronary heart disease. Digoxin levels in various fluids and tissues were estimated by radioimmunoassay: bloodplasma 20--25 ng/ml, liquor 10--13 ng/ml, liver 100--110 ng/g, kidney 130--145 ng/g; the gastric fluid contained 0,6 mg. Forensic aspects of glycosid-intoxication, especially of the varying concentrations in different tissues, are discussed.

Investigation of cardiac glycoside levels in human post mortem blood and tissues determined by a special radioimmunoassay procedure

Even after the introduction of radioimmunological methods the question of a cardiac glycoside causing or contributing to the death of a patient can not be answered satisfactorily. By means of a special radioimmunoassay procedure for digoxin as well as for the structurally related methyl- and acetylderivatives we measured the concentrations in human blood and post mortem tissues. We investigated the glycoside contents in the blood of intravenously digitalised (Novodigal) al) patients before and after death. At autopsy blood specimens were taken from the heart and the femoral vein. We found an increase of the glycoside level up to a highly toxic range (7--15 ng/ml) especially in the heart blood. Thus post mortem blood levels of digoxin and its derivatives are not suitable for a final decision in alleged cases of fatal poisonings. Measuring various concentrations in tussues and body fluids of the above cardiac glycosides mentioned revealed the kidney concentration to be of high value in confirming a digitalis poisoning. This organ and the heart show the highest tissue concentrations. Interpretations of fatal digitalis poisonings should be based on the additional knowlege of these concentrations. Individual cardiac glycosides may be analyzed by a combination of thin layer chromatography and radioimmunoassay.

Postmortem tissue and plasma concentrations of digoxin in newborns and infants

Postmortem tissue and plasma concentrations of digoxin were studied in 13 premature newborns, 6 mature newborns, and 5 older infants (age 1 to 14 months). The pertinent results of our study are as follows: The tissue digoxin concentrations tend to be higher in premature and mature newborns than in infants. This difference is statisitcally significant with respect to the concentration in myocardium and skeletal muscle. The renal digoxin concentration of premature newborns is significantly lower than that of mature newborns, the tissue concentrations in the other organs examined being essentially equal. In all age groups examined, skeletal muscle contains the greatest portion of digoxin, followed by the liver. The relation of myocardial to plasma digoxin concentration shows no significant difference between the various age groups. Within the groups, the variation is relatively large.

[Plasma digoxin concentration in different age groups (author's transl)]

Plasma digoxin concentration during maintenance therapy with digoxin was determined in premature and mature newborns, infants, children and adults. The plasma digoxin concentration of newborns was significantly higher than in adults; in addition, in the group of premature newborns it also was higher than in infants and children. These differences in plasma digoxin concentrations may be explained by differences in dose, volume of distribution and excretion rate of digoxin in the various age groups.

Relationship of serum and myocardial digoxin concentration to electrocardiographic estimation of digoxin intoxication

Serum and myocardial digoxin levels were studied in 18 patients who came to autopsy. An independent analysis of electrocardiograms prior to death was made to ascertain the relationship between serum and tissue levels of digoxin and clinical estimation of drug toxicity. Patients with arrhythmias of digoxin toxicity had higher mean serum and tissue digoxin levels than patients without arrhythmia. There was overlap in the patient groups, however, and the differences were not statistically significant. The tissue to serum ratio was lower in the toxic patients. The latter phenomenon is unexplained but may be related to decreased tissue binding.

Physiologically based pharmacokinetic model for digoxin disposition in dogs and its preliminary application to humans

A physiologically based pharmacokinetic model for digoxin disposition developed in the rat was modified to account for the interspecies differences in tissue-to-plasma digoxin concentration ratios and applied to the dog. The model provided a quantitative assessment of the time course of digoxin concentrations in dog plasma, various tissues, and urine. It also predicted the effect of renal failure on digoxin pharmacokinetics in the dog. An attempt to scale the dog model to humans by simply considering differences in organ volumes, organ flow rates, and digoxin clearances was partially successful. Good predictions of plasma digoxin concentration and urinary digoxin excretion after a single dose and of steady-state plasma, heart, and skeletal muscle digoxin concentrations were obtained. However, the model predicted considerably higher kidney digoxin concentrations than are actually found. Although the model adequately characterized the time course of digoxin concentrations in patients with moderate renal impairment, it provided a relatively poor fit to that observed in anuric patients.

Differential effects of digoxin at comparable concentrations in tissues of fetal and adult sheep

Electrocardiogram (ECG) electrodes and carotid arterial and superior vena caval (SVC) catheters were placed in eight nonpregnant ewes and 11 fetuses (109-129 days gestation) to measure heart rate, arterial presssure, P-R interval, left ventricular pre-ejection period (PEP), and left ventricular ejection time (LVET), before and after digoxin infusion into the SVC. After the ewes were killed, the steady state concentration of digoxin in plasma was related to the concentration in midbrain and left ventricular free wall. Although concentrations of digoxin in tissue differed between fetuses and ewes, tissue-plasma ratios were similar; the myocardial-plasma ratio was 87 for fetuses and 90 for ewes and the midbrain-plasma ratios were 6.4 and 5.3, respectively. In spite of these similarities, physiological and toxic effects differed at comparable plasma concentrations. Reduction in PEP/LVET ratio was greater in ewes than fetuses, and P-R interval prolongation was linearly related to digoxin concentration in fetuses but uncommon at plasma concentrations below 2 ng/ml in ewes. Arrhythmias occurred in six ewes, but in only one fetus, even though the mean steady state concentration of digoxin in plasma was 4.5 ng/ml in the fetuses and 2.3 ng/ml in the ewes. Atropine had little effect on digoxin-induced P-R interrval prolongation, and isoproterenol produced no tachyarrhythmias in the fetuses. Age-related differences in inotropic and arrhythmogenic effects of digoxin exist exist and are related to differences in drug response rather than drug kinetics; this provides experimental support for the different dosage responses.

Myocardial ouabain content and susceptibility to ouabain cardiotoxicity associated with circulatory volume overload in the dog

The influence of circulatory volume overload on the myocardial uptake of ouabain and on cardiotoxicity was studied in the unanaesthetised dog with aorto-caval fistula. One hour after tritiated ouabain (0-02 mg/kg IV) both ventricles and atria contained more ouabain than did those of normal dogs (left ventricle (LV), 166+/-23 (SD) ng/g vs. 97+/-19 ng/g, P less than 0-001) while concentrations in skeletal muscle, liver, kidney and plasma were not different in the two groups. In other experiments ouabain was infused to cardiotoxicity (7-5 microgram/kg followed by 3 microgram/kg/min). Cardiotoxicity occurred earlier in dogs with fistula than in normals (16-5+/-2-7 min vs. 24-1+/-2-4 min, P less than 0-001). Ouabain concentrations in myocardium were not different (LV, 434+/-58 ng/g, vs. 442+/-42 ng/g) while concentrations in liver and kidney were less in those with fistula (181+/-35 ng/g vs. 278+/-69 ng/g, P less than 0-001; 1422+/-189 ng/g vs. 2747+/-479 ng/g, P less than 0-001). Average content of skeletal muscle was also less, in proportion to administered dose. The increment in myicardial ouabain content associated with aorto-caval fistula appears to be physiologically active and hence is presumably specifically bound to the digitalis receptor. The observations in this model suggest the possibility of augmented cardiac glycoside uptake in some clinical cardiac diseases.

Interpretation of the serum digoxin concentration

Significant problems exist in the interpretation of serum digoxin concentration data. Failure to distinguish between results that do not require precise clinical correlation (proof of absorption, presence of drug, etc) and those which depend upon clinical correlation for their meaning ('toxicity' or 'effectiveness') can result in interpretive errors. Problems relating to the source of the serum digoxin concentration can also confound interpretation. Such difficulty may be controllable (obtaining the sample at the proper time, haemolysis, etc) or related to the laboratory technique (cross-reactivity with digoxin metabolites or other medications, technical errors, or lack of precision). Variation within the same patient over time or between patients related to disease (alterations in electrolytes, adrenergic or parasympathomimetic tone, or other medications) may prevent the direct attribution of an observed phenomenon to a particular digoxin concentration. Techniques for determining the effect of digoxin do exist and can be used to gather data for clinical correlations. Ways of improving the interpretaion of serum digoxin concentrations also exist and should be used to improve their value in patient management. The serum digoxin concentration seems to have an important future role. However, we need to know how better to interpret and exploit serum digoxin concentration data.

[Fibre type and glycoside concentrations of human skeletal muscle (author's transl)]

In 2 patients digitalized with digoxin or betamethyldigoxin, postmortal glycoside concentrations were determined in 7 different skeletal muscle specimens by radioimmunoassay. In the same specimens, planimetric measurements of histochemical fibre types I and II were carried out. There were higher glycoside concentrations in predominantly type I fibre muscle biopsies.

Clinical pharmacokinetics of digoxin in infants

Based on clinical experience, infants with congestive heart failure are given larger doses of digoxin than adults, whether calculated on the basis of body weight or surface area. The reasons for this difference in dosage are not clear. The myocardium of the infants might be more resistant to the effects of digoxin than that of adults, and/or differences might exist between infants and adults concerning the absorption, distribution and elimination of the glycoside. Infants have been found to absorb digoxin in solution at the same rate and to the same extent as adults. The relative distribution of the glycoside to different tissues is also similar in the two age-groups. However, the binding of digoxin to several tissues seems to be more extensive in infants than in adults. In agreement with this, the apparent volume of distribution of the glycoside is larger in infants than in adults. As no enhanced urinary excretion has been found in infants there might be a non-renal elimination of the glycoside. With most prevailing dose schedules for digoxin, serum concentrations higher than those considered optimum for adults are often obtained in infants. It is known that infants tolerate higher serum digoxin concentrations than adults without developing signs of toxicity. However, it is not known whether such high concentrations are necessary for obtaining an adequate inotropic effect on the myocardium of the infants. If the relation between serum concentration and effect is the same in infants and adults, the loading (digitalising) dose generally given to infants is unnecessarily high.

[Determination of glycoside concentrations in human tissue by means of radioimmunoassay (author's transl)]

After extraction of myocardial and skeletal muscle biopsy and autopsy specimens tissue glycoside concentrations can be determined by radioimmunoassay. Total tissue extraction of digoxin and beta-methyl-digoxin varies between 87 and 95%, the variation coefficient for repeated determinations is 10.2%. Glycoside concentrations of left ventricular papillary muscle obtained after mitral valve replacement were 69.0 +/- 25.05 ng/g with a tissue to serum relation of 46.6 +/- 8.96:1 and a correlation coefficient of r = 0.8442. In autopsy left ventricular papillary muscle glycoside concentrations were 105.2 +/- 27.35 ng/g with an almost identical tissue to serum relation of 46.2 +/- 9.57:1 and a corresponding serum concentration of 2.3 +/- 0.63 ng/ml. In adults glycoside concentrations of autopsy specimens of the right ventricle were significantly lower by 28 to 30% than those of the left ventricle. Glycoside concentrations of skeletal muscle specimens (m. pectorialis major) were 14.7 +/- 10.35 ng/g with a tissue to serum relation of 9.7 +/- 3.00:1 (r = 0.8377), which corresponds to approximately 1/5 to 1/4 of the concentrations of the left ventricular myocardium.

Myocardial uptake of digoxin in chronically digitalized dogs

1 The time course of myocardial uptake of digoxin, increase in contractility and changes in myocardial potassium concentration was studied for 90 min following an intravenous digoxin dose to long-term digitalized dogs. 2 Nineteen dogs were investigated by the use of a biopsy technique which allowed sampling before and after administration of digoxin. 3 Ten minutes after administration of digoxin the myocardial concentration increased from 60 to 306 nmol/kg tissue, the myocardial concentration of digoxin was significantly lower (250 nmol/kg tissue) after 30 min and then increased again. 4 The transmural myocardial distribution of digoxin was uniform before and 90 min after administration of digoxin in long-term digitalized dogs but at 10 min after administration, both the subepicardial and the subendocardial concentration of digoxin were significantly lower than that of the mesocardial layer. 5 During the first 10 min the dp/dtmax increased to 135% of the control level. The increase remained unchanged during the rest of the study. 6 Myocardial potassium decreased throughout the study. 7 The M-configuration of the myocardial uptake curve and the non-uniformity of myocardial distribution of digoxin observed at 10 min after administrating digoxin to long-term digitalized dogs indicate that the distribution of myocardial blood flow may be changed during chronic digitalization.

Pharmacokinetics of tritiated ouabain, digoxin and digitoxin in guinea-pigs

1. Elimination rates of tritiated ouabain, digoxin and digitoxin after single intravenous administrations were investigated in guinea-pigs, the total radioactivity in whole blood being traced for a period of up to 2 weeks. 2. In the initial rapid phase of elimination between 2 and 30 min following intravenous glycoside administration, the concentration decline of radioactivity in the blood was found to be identical for the three glycosides investigated, this part of the elimination curve displaying a hyperbolic shape. 3. During this early elimination phase, rapid metabolic degradation and excretion of digoxin had already taken place. The maximum concentration of radioactivity in the bile was reached 4 min following intravenous administration of 3H-digoxin. The positive inotropic response occurred in the cat heart-lung preparation 1.5 min after intravenous injection of a therapeutic dose of digoxin, indicating a quick occupation of binding sites in the tissues. 4. The biological half-lives of tritiated ouabain, digoxin and digitoxin averaged 11 h, 2.5 days and 4.1 days, respectively, as determined by the terminal exponential elimination phase, in guinea-pigs. This terminal phase was attained 6-12, 7-24, and 24-48 h after administration of ouabain, digoxin and digitoxin, respectively. 5. The findings reveal that in guinea-pig, as has been demonstrated in man, the elimination rates of the three glycosides increase according to their hydrophobic properties. 6. The biological half-lives of tritiated ouabain, digoxin and digitoxin obtained in the guinea-pig closely resemble those found in healthy man.

A study of serum and myocardial digitoxin concentrations in man during cardiac arrest

In 22 digitalized (of a total of 39) patients studied at random by radioimmunoassay during cardiac arrest, the mean serum digoxin concentration was 2.6 (+/- 1.86, range 0.6-8.2) ng/ml, significantly higher (P less than 0.001) than the "eudigitalized" concentration (1.3 +/- 0.52, range 0.5-2.3 ng/ml) determined under carefully standardized conditions in a non-toxic population. Half of the arrest patients had serum digoxin levels in the toxic range (2.4 ng/ml or above), mainly due to significant renal failure (mean serum creatinine concentration 2.9 +/- 2.66 v. 1 +/- 0.26 mg/dl for non-toxic subjects, P less than 0.001), partly due to a higher mean daily digoxin dose (0.40 v 0.31 mg/day, P less than 0.05) and frequently associated with potent diuretic therapy (73 v 54%). A smaller fraction of digitalized patients survived, both short- (27%) and long-term (14%), than did non-digitalized subjects (35% and 26%, respectively). The mean myocardial digoxin concentration was 150 (+/- 63.3, range 52-252) ng/g with an average myocardial/serum ratio of 62.5 (range 38-91). There were significant positive correlations between the serum digoxin and left-ventricular myocardial digoxin concentration (r=0.8107, P less than 0.01) or serum creatinine concentration (r=0.4637, P less than 0.001).

Post-mortem distribution and tissue concentrations of digoxin in infants and adults

By means of 86Rb uptake inhibition assay, the distribution and tissue concentrations of digoxin in various tissues during maintenance therapy were studied post mortem in 12 infants (aged 5 days to 8 months) and 17 adults (aged 49-91 years). The mean maintenance dose for infants was 0.014 mg/kg bw/24 h and for adults, 0.005 mg/kg bw/24 h. The same relative distribution of the glycoside found in infants and in adults was: choroid plexus greater than ventricular myocardium greater than kidney greater than liver greater than skeletal muscle. Between infants and adults, the mean digoxin concentrations in choroid plexus, kidney, liver, and skeletal muscle did not differ significantly; however, significant differences were found in the glycoside concentrations in ventricular and in atrial myocardium. Both infants and adults showed a difference in the content of the glycoside within the heart, the concentration in ventricular muscle being significantly higher than in atrial. There seemed to be no direct relation between the tissue concentrations of the glycoside (myocardium, skeletal muscle) and the daily maintenance dose/mg/kg bw/24 h). The results suggest that the myocardial binding of digoxin is higher in infants than in adults.

Correlation of serum concentrations with heart concentrations of digoxin in human subjects

Biopsies of cardiac tissue were taken from patients undergoing surgery for coronary artery or valvular disease. All subjects were on maintenance doses of digoxin, which were stopped 48 hours before surgery. The ratios, heart:serum, for digoxin content were remarkably similar and the variance was small.

Comparison of serum digoxin level measurement with acetyl strophanthidin tolerance testing

Serum digoxin levels (SDL) were compared with tolerance for the rapidly acting cardiac aglycone, acetyl strophanthidin (AS). AS titration tests were performed on 133 patients with diverse cardiac disorders. All were receiving maintenance digoxin. Both exquisite AS sensitivity and tolerance for a 1.0 mg AS were associated with a wide range of SDL values. Concordance and discordance between the two methods in assessing degree of digitalization were evaluated by considering SDL of 1.4 ng/ml to be the mean value for patients without glycoside-induced cardiac arrhythmia. An SDL of < 1.5 ng/ml with tolerance for 1.0 mg AS and an SDL of > 1.4 ng/ml with sensitivity to 1.0 mg AS or less constituted concordant responses. An SDL of < 1.5 ng/ml with intolerance for 1.0 mg or less AS and an SDL of > 1.4 ng/ml with tolerance for 1.0 mg AS comprised discordant responses. In 60 of 144 (42%) AS titrations discordant results were observed. Severe pulmonic, coronary, and aortic valvular heart disease, as well as old age, contributed to unusual AS sensitivity. Titration with AS clarified pharmacologic quantification of SDL by providing insight into optimum therapeutic glycoside dose.