Plasma Digoxin Research Articles

Digoxin concentration in saliva and plasma in infants, children, and adolescents with heart disease

Background Because of its narrow therapeutic index, therapeutic monitoring of digoxin is important in the management of infants and children receiving the drug for cardiac failure or arrhythmias, or following accidental ingestion. Whether saliva can replace plasma in the therapeutic monitoring of digoxin therapy in children is unclear. Objective This study assessed the value of determining saliva digoxin concentration in infants, children, and adolescents. Methods Infants, children, and adolescents receiving digoxin for various indications, whose digoxin dosage had remained unchanged for ≥10 days, and whose compliance was good according to the parents were enrolled. Digoxin concentration was measured in paired specimens of citric acid–stimulated mixed saliva and plasma obtained simultaneously. Results Eighteen children (10 boys, 8 girls; mean [SD] age, 42.3 [53.1] months [range, 2 months–14 years]) were included in the study. Digoxin therapy was administered for cardiac failure due to dilated cardiomyopathy in 9 patients (50.0%), ventricular septal defect in 4 (22.2%), supraventricular tachycardia in 3 (16.7%), and after cardiac surgery in 2 (11.1%). Digoxin concentration in the 20 paired specimens obtained varied from 0.0 to 0.92 ng/mL (mean [SD], 0.25 [0.26] ng/mL) in saliva and from 0.27 to 1.54 ng/mL (mean [SD], 0.77 [0.40] ng/mL) in plasma. The mean plasma/saliva digoxin concentration ratio was 2.8. Conclusions This study of infants, children, and adolescents receiving digoxin for a variety of indications and whose dose was unchanged for ≥10 days showed that marked individual variability in the saliva/plasma concentration ratio precludes the use of saliva in predicting the plasma digoxin concentration. The value of saliva digoxin (as opposed to plasma digoxin) measurements in the assessment of the cardiac effects of the drug in children remains to be determined.

A rapid and sensitive LC/MS/MS assay for quantitative determination of digoxin in rat plasma

Digoxin is a cardiac glycoside that is widely used for the treatment of congestive heart failure. To evaluate pharmacokinetics of digoxin in rats, a sensitive LC/MS/MS assay was developed and validated for the determination of digoxin concentration in rat plasma. For detection, a Sciex API3000 LC/MS/MS with atmospheric pressure ionization (API) mass spectrometry turbo ion spray inlet in the positive ion-multiple reaction monitoring mode was used to monitor precursor→product ions of m/z 798.6→651.6 for digoxin and m/z 577.6→433.3 for oleandrin, the internal standard (IS). The standard curve was linear ( r 2≥0.999) over the digoxin concentration range of 0.1–100 ng/ml in plasma for digoxin. The mean predicted concentrations of the quality control samples deviated by <5.8% from the corresponding nominal values; the intra-assay and inter-assay precision of the assay were within 8.6% relative standard deviation. At the lower limit of quantitation (LLQ) of 0.1 ng/ml, the mean deviation of predicted concentrations from the nominal value was within 3.7%. The extraction recoveries of digoxin and internal standard were 82.7±3.9 and 105.9±2.3%, respectively. The present method was successfully applied to characterization of pharmacokinetic profiles of digoxin in rats after oral administration.

Concentraciones plasmáticas de digoxina en cuatro esquemas terapéuticos de uso común en la práctica clínica

The different therapeutic schedules used for the prescription of digoxin have little theoretical support. To measure digoxin plasma levels in patients using four different prescription schedules. Four groups of patients were studied. Group I corresponded to 56 patients taking digoxin 0.25 mg/day, from Monday to Friday. Group II corresponded to 30 patients taking digoxin 0.25 mg/day, from Monday to Saturday. Group III corresponded to 53 patients taking digoxin 0.25 mg/day continuously. Group IV corresponded to 36 patients taking digoxin 0.125 mg/day continuously. Plasma digoxin levels were measured in two consecutive Mondays before taking the daily dose of the drug. Serum creatinine was also measured and creatinine clearance was calculated. The therapeutic plasma concentration range was set between 0.5 and 2 ng/ml. Mean plasma digoxin levels were 1.15 +/- 0.8 ng/ml in group I, 1.4 +/- 0.55 ng/ml in group II, 1.68 +/- 0.7 ng/ml in group III and 1.14 +/- 0.43 ng/ml in group IV. 93% of patients in group I, 80% of patients in group II, 75% of patients in group III and 94% of patients in group IV had therapeutic digoxin levels. A low creatinine clearance, an age over 65 and interactions with other drugs were risk factors associated with supratherapeutic levels, mostly seen in group II and group III with 20% and 24% respectively. Most patients using digoxin with different therapeutic schedules had plasma drug levels within the therapeutic range.

Simultaneous genotyping of seven single-nucleotide polymorphisms in the MDR1 gene by single-tube multiplex minisequencing.

Responses to different drugs can vary widely among different individuals as a result of genetic variations in drug-metabolizing enzymes, transporters, receptors, and/or other cofactors. The multidrug resistance 1 (MDR1) transporter, a well-characterized member of the ATP-binding cassette superfamily, was shown to efflux a wide variety of structurally and functionally unrelated drugs, including anticancer, antiarrhythmic, antidepressant, antipsychotic, and antiviral agents. The pharmacogenetics of the MDR1 multidrug transporter have recently received much scientific attention. Several single-nucleotide polymorphisms (SNPs) have been identified in the MDR1 gene; some occur only in specific ethnic groups, whereas others occur in all ethnic groups but at significantly different allele frequencies among the different races [see Ref. (1) and references therein]. Nonetheless, the functional significance of these SNPs remains unclear. Various functional associations, some paradoxical, have been observed between the synonymous SNP (exon 26 3435C→T) and MDR1 protein expression and plasma drug concentrations (2)(3)(4)(5), drug-induced side effects (6), and drug response (7). The SNP exon 26 3435T allele has been associated with lower MDR1 expression in the duodenum (2), leukocytes (5), and placental tissues (8), leading to lower rhodamine efflux (5) and increased plasma digoxin concentrations (2). In addition, early-onset Parkinson patients have higher frequency of the SNP exon 26 3435T allele compared with late-onset patients or unaffected controls (9). However, although this same allele has been associated with lower MDR1 expression in peripheral blood mononuclear cells and better response to HIV-1 drugs, it has also been associated with lower plasma concentrations of nelfinavir (7). Additionally, the SNP exon 26 3435T allele has been associated with an increased risk of nortriptyline-induced postural hypertension, although blood concentrations of nortriptyline in these individuals were not significantly different from those in individuals carrying the C allele (6). Furthermore, no association has been demonstrated between the …

Digoxin pharmacokinetics and MDR1 genetic polymorphisms.

The effect of MDR1 C3435T single nucleotide polymorphism (SNP) in exon 26 on digoxin pharmacokinetics has recently been challenged. OBJECTIVE. To clarify the relationships between MDR1 genetic polymorphisms in exon 26 (C3435T) and 21 (G2677T/A) and digoxin pharmacokinetics. MDR1 genotypes for C3435T and G2677T/A SNPs were determined in 32 healthy subjects whose single oral dose digoxin pharmacokinetics had been measured over 48 h. A significant relationship was observed between C3435T SNP and digoxin AUCs ( p<0,05). Homozygous TT subjects had 20% higher digoxin plasma concentrations than CT and CC subjects and a trend for higher 48 h digoxin urinary recoveries (TT>CT>CC). Similar results, although not statistically significant, were observed from the MDR1 G2677T/A SNP. Our results confirm that the MDR1 C3435T single nucleotide polymorphism (SNP) significantly affects digoxin disposition kinetics, with homozygous TT subjects presenting the highest plasma concentrations.

ジゴキシンとジヒドロピリジン系カルシウムきっ抗薬の薬物相互作用（第２報）―腎機能の変動による影響―

The effects of four dihydropyridine calcium antagonists (manidipine, benidipine, amlodipine and nitrendipine) on the plasma digoxin level and the renal hemodynamics were studied in both patients and conscious normotensive beagle dogs.In the clinical study, the combined administration of digoxin with calcium antagonists elevated, plasma digoxin levels compared with digoxin monotherapy, however, the difference was not significant. In dogs, the combined administration of digoxin with benidipine or nitrendipine resulted in a significantly higher plasma digoxin level after 8 and 24 hrs, respectively, but it did not affect the mean area under the plasma digoxin concentration time curve.In a renal hemodynamic study in dogs, the glomerular filtration rate was observed to decrease significantly following the combined administration with manidipine. Furthermore, the filtration fraction decreased significantly after the administration of manidipine and benidipine, but it did not affect the renal plasma flow. In addition, amlodipine slightly decreased the glomerular filtration rate. On the other hand, nitrendipine did not alter the renal hemodynamics.Since dihydropyridine calcium antagonists demonstrate different effects on renal hemodynamics during the combined administration of digoxin, administering the appropriate dosage of digoxin during combined therapy with calcium antagonists is therefore necessary to minimize the risk of digoxin toxicity.

HYPOTHALAMIC DIGOXIN, CEREBRAL CHEMICAL DOMINANCE, AND REGULATION OF GASTROINTESTINAL/HEPATIC FUNCTION

The role of the isoprenoid pathway in gastrointestinal and hepatic diseases, and its relation to hemispheric dominance, was assessed in this study. The following parameters were measured in patients with (i) acid peptic disease, (ii) ulcerative colitis, (iii) gallstones, (iv) cryptogenic cirrhosis liver, (v) Reye's syndrome, (vi) mesenteric artery occlusion, (vii) irritable bowel syndrome, and (viii) in individuals with right hemispheric, left hemispheric, and bihemispheric dominance: 1. plasma HMG CoA reductase, digoxin, dolichol, ubiquinone, and magnesium levels; 2. tryptophan/tyrosine catabolic patterns; 3. free radical metabolism; 4. glycoconjugate metabolism; and 5. membrane composition. In patients with gastrointestinal and hepatic disease there were elevated digoxin synthesis, increased dolichol, and glycoconjugate levels, and low ubiquinone and elevated free radical levels. The RBC membrane Na+-K+ ATPase activity and serum magnesium were decreased. There was also an increase in tryptophan catabolites and a reduction in tyrosine catabolites in the serum. There was an increase in cholesterol:phospholipid ratio and a reduction in the glycoconjugate level of RBC membrane in these groups of patients. The same biochemical patterns were obtained in individuals with right hemispheric dominance. An upregulated isoprenoid pathway and hyperdigoxinemia is characteristic of gastrointestinal and hepatic disease and in right hemispheric chemical dominance. Right hemispheric chemical dominance is important in deciding the predisposition to gastrointestinal and hepatic disease.

HYPOTHALAMIC DIGOXIN, HEMISPHERIC CHEMICAL DOMINANCE, AND MESENTERIC ARTERY OCCLUSION

The role of the isoprenoid pathway in vascular thrombosis, especially mesenteric artery occlusion and its relation to hemispheric dominance, was assessed in this study. The following parameters were measured in patients with mesenteric artery occlusion and individuals with right hemispheric, left hemispheric, and bihemispheric dominance: (1) plasma HMG CoA reductase, digoxin, dolichol, ubiquinone, and magnesium levels; (2) tryptophan/tyrosine catabolic patterns; (3) free radical metabolism; (4) glycoconjugate metabolism; and (5) membrane composition. In patients with mesenteric artery occlusion there was elevated digoxin synthesis, increased dolichol and glycoconjugate levels, low ubiquinone, and elevated free radical levels. The RBC membrane Na+-K+ ATPase activity and serum magnesium were decreased. There was also an increase in tryptophan catabolites and reduction in tyrosine catabolites in the serum. There was an increase in cholesterol:phospholipid ratio and a reduction in glycoconjugate level of RBC membrane in these patients. The biochemical patterns obtained in mesenteric artery occlusion is similar to those obtained in left-handed/right hemispheric dominant individuals by the dichotic listening test. But all the patients with mesenteric artery occlusion were right-handed/left hemispheric dominant by the dichotic listening test. Hemispheric chemical dominance has no correlation with handedness or the dichotic listening test. Mesenteric artery occlusion occurs in right hemispheric chemically dominant individuals and is a reflection of altered brain function. Hemispheric chemical dominance may thus control the risk for developing vascular thrombosis in individuals

HYPOTHALAMIC DIGOXIN, CEREBRAL CHEMICAL DOMINANCE, AND PATHOGENESIS OF PULMONARY DISEASES

The isoprenoid pathway is a key regulatory pathway in the cell. It synthesizes digoxin, an endogenous membrane Na+-K+ ATPase inhibitor and modulator of synaptic transmission. The role of the isoprenoid pathway in lung diseases and its relation to hemispheric dominance was assessed in this study. The following parameters were measured in patients with (i) bronchial asthma, (ii) chronic bronchitis emphysemia, (iii) idiopathic pulmonary fibrosis, (iv) sarcoidosis, and (v) in individuals with right hemispheric, left hemispheric and bihemispheric dominance: 1. plasma HMG CoA reductase, digoxin, dolichol, ubiquinone, and magnesium levels, 2. tryptophan, tyrosine catabolic patterns, 3. free radical metabolism, 4. glycoconjugate metabolism, and 5. membrane composition. In patients with lung disease there was elevated digoxin synthesis, increased dolichol and glycoconjugate levels, and low ubiquinone and elevated free radical levels. The RBC membrane Na+-K+ ATPase activity and serum magnesium were decreased. There was also an increase in tryptophan catabolites and reduction in tyrosine catabolites in the serum. There was an increase in cholesterol:phospholipid ratio and a reduction in glycoconjugate level of RBC membrane in these patients. The same biochemical patterns were obtained in individuals with right hemispheric chemical dominance. An upregulated isoprenoid pathway and hyperdigoxinemia are characteristic of lung disease and right hemispheric chemical dominance. Right hemispheric chemical dominance is important in deciding the predisposition to lung disease.

HYPOTHALAMIC DIGOXIN, HEMISPHERIC CHEMICAL DOMINANCE, AND INFLAMMATORY BOWEL DISEASE

The isoprenoid pathway produces three key metabolites--endogenous digoxin, dolichol, and ubiquinone. It was considered pertinent to assess the pathway in inflammatory bowel disease (ulcerative colitis and regional ileitis). Since endogenous digoxin can regulate neurotransmitter transport, the pathway and the related cascade were also assessed in individuals with differing hemispheric dominance to find out the role of hemispheric dominance in its pathogenesis. All the patients with inflammatory bowel disease were right-handed/left hemispheric dominant by the dichotic listening test. The following parameters were measured in patients with inflammatory bowel disease and in individuals with differing hemispheric dominance: (1) plasma HMG CoA reductase, digoxin, dolichol, ubiquinone, and magnesium levels; (2) tryptophan/tyrosine catabolic patterns; (3) free-radical metabolism; (4) glycoconjugate metabolism; and (5) membrane composition and RBC membrane Na+-K+ ATPase activity. Statistical analysis was done by ANOVA. In patients with inflammatory bowel disease there was elevated digoxin synthesis, increased dolichol and glycoconjugate levels, and low ubiquinone and elevated free radical levels. There was also an increase in tryptophan catabolites and a reduction in tyrosine catabolites. There was an increase in cholesterol:phospholipid ratio and a reduction in glycoconjugate level of RBC membrane in these groups of patients. Inflammatory bowel disease is associated with an upregulated isoprenoid pathway and elevated digoxin secretion from the hypothalamus. This can contribute to immune activation, defective glycoprotein bowel antigen presentation, and autoimmunity and a schizophreniform psychosis important in its pathogenesis. The biochemical patterns obtained in inflammatory bowel disease is similar to those obtained in left-handed/right hemispheric dominant individuals by the dichotic listening test. But all the patients with peptic ulcer disease were right-handed/left hemispheric dominant by the dichotic listening test. Hemispheric chemical dominance has no correlation with handedness or the dichotic listening test. Inflammatory bowel disease occurs in right hemispheric chemically dominant individuals and is a reflection of altered brain function.­­

Pharmacokinetic study of the digoxin-acenocoumarol interaction in rabbits.

A study was carried out to evaluate the potential pharmacokinetic interaction between digoxin and acenocoumarol. The binding of digoxin to rabbit cardiac tissue homogenates was assessed in vitro, using the equilibrium dialysis technique. An increase in the first-order constant (p<0.05) and a reduction in the partition coefficient in the equilibrium situation (p<0.001) of digoxin were observed when the cardiac homogenates were previously treated with acenocoumarol. In the in vivo study, the kinetics of digoxin administered in single and multiple dosage regimens were compared in control rabbits and in rabbits treated simultaneously with acenocoumarol. Kinetic analysis of the results was performed using Non-linear Mixed Effects Modeling (NONMEM). In the presence of acenocoumarol, the population distribution volume (Vd) of digoxin was increased by 40-60%, no differences being found as regards the elimination clearance. Also, joint administration of both drugs led to a reduction in digoxin concentrations in the heart (p<0.01) at the end of the dosage regimen. Both sets of results point to the hypothesis of a hitherto unreported possible pharmacokinetic interaction between the two drugs affecting the distribution process. This interaction could lead to lower plasma digoxin levels, in view of the increased Vd, and a possible reduction in the therapeutic effect, owing to the decrease in affinity and in concentration in heart tissue.

Mejor resultado del ajuste de la dosis de digoxina en ancianos si se tiene en cuenta que tanto la eliminación como el volumen de distribución del fármaco disminuyen cuando se reduce la función renal

To evaluate three methods for digoxin dose adjustment in aged patients. We determined the plasma digoxin levels that would be attained in 87 old patients with doses adjusted to the kidney function by means of three separate procedures. Age: 79.0 "6.3 years of age; creatinin clearance (Clc): 0.70" 0.23 ml/Kg of lean body weight and minute. Only the methods that adjust both the digoxin clearance and the volume of distribution to the Clc achieve the independence between the digoxinemia and the kidney function. The best of them, by calculating the elimination constant (K) and the volume of distribution (V) as linear functions of the Clc, so that K ranges between 0.173 and 0.462 days-1 and V between 4 and 10 l/Kg of lean body weight when the Clc varies from 0 to 110 ml/minute, achieve digoxinemia figures between 0.8 y 2.0 ng/ml and above 2.0 ng/ml in the 81.6% and 0.0% of the patients (95% confidence intervals (95% CI): 72.2% to 88.4 and 0.0% to 4.6%), respectively; with a precision and a bias of 0.43 and -0.06 ng/ml (95% CI: 0.38 to 0.48 and -0.16 to 0.03 ng/ml), respectively. The described method would lead to good results if digoxin has not been prescribed in order to control the cardiac frequency in the setting of auricular fibrilation.

Hypothalamic digoxin-mediated model for subacute sclerosing panencephalitis.

The isoprenoid pathway including endogenous digoxin was assessed in subacute sclerosing panencephalitis (SSPE). This was also studied for comparison in patients with right hemispheric and left hemispheric dominance. The following parameters were measured in patients with SSPE and in individuals with right hemispheric, left hemispheric and bihemispheric dominance-(a) plasma HMG CoA reductase, digoxin, dolichol, ubiquinone, and magnesium levels; (b) tryptophan/tyrosine catabolic patterns; (c) free-radical metabolism; (d) glycoconjugate metabolism; and (e) membrane composition and RBC membrane Na(+)-K(+) ATPase activity. The isoprenoid pathway was upregulated with increased digoxin synthesis in patients with SSPE and in those with right hemispheric dominance. In this group of patients: (a) the tryptophan catabolites were increased and the tyrosine catabolites reduced; (b) the dolichol and glycoconjugate levels were elevated; (c) lysosomal stability was reduced; (d) ubiquinone levels were low and free-radical levels increased; and (e) the membrane cholesterol:phospholipid ratios were increased and membrane glycoconjugates reduced. On the other hand, in patients with left hemispheric dominance the reverse patterns were obtained. The upregulated isoprenoid pathway and hypothalamic digoxin are involved in the pathogenesis of SSPE. SSPE occurs in right hemispheric chemically dominant individuals and a pathogenetic model for SSPE implicating hypothalamic digoxin is proposed.

An unusual electrocardiographic pattern of cardiac glycoside poisoning

The electrocardiogram (ECG) presented in Fig. 1 probably linked to the development of late cardiac was registered about 24 h after the deliberate ingespotentials, are also evident. The association of tion of a vague dose of digoxin (probably 5–10 mg) bradycardia and delayed afterdepolarizations due to in a 46-year-old female affected by human immunodigoxin-induced intracellular calcium overload, is deficiency virus infection. probably responsible of this pattern of repolarization. No signs or symptoms related to cardiac inThe more pronounced abnormalities recorded in the volvement were documented before the event; the index beat (*) compared with the following beat, patient declared drug ingestion during the daily were probably due to a longer R–R preceding medical interview. By this time plasma digoxin level interval. was .4 ng/ml (normal range 0.8–2 ng/ml). She was This patient without cardiac symptoms confirm asymptomatic. No significant plasma electrolyte abthat, in subjects with excessive serum digitalis connormalities were found. The patient was treated with centrations, the likelihood of having clinical signs of digoxin-specific FAB antibody fragments. A control intoxication is lesser if the electrocardiogram records ECG obtained 24 h later, with a digoxin plasma level a slow rhythm compared with signs of increased of 1.85 ng/ml, is illustrated in Fig. 2. The remaining automaticity [1]. hospital stay was uneventful. The initial ECG (Fig. 1) shows some peculiar and References unusual abnormalities of ventricular repolarization characterized by an apparent QT-interval prolonga[1] Abdad-Santos F, Carcas AJ, Ibanez C, Frias J. Digoxin level and tion (not confirmed by the calculation of the QTc) clinical manifestations as determinants in the diagnosis of digoxin and a QT-interval dispersion. Prominent U-waves, toxicity. Ther Drug Monit 2000;22:163–8.

The C3435T mutation in the human MDR1 gene is associated with altered efflux of the P-glycoprotein substrate rhodamine 123 from CD56+ natural killer cells.

P-glycoprotein (PGP) is a membrane protein which determines drug disposition in humans (e.g. digoxin). It is also expressed in various leukocyte lineages with highest expression in CD56+ natural killer cells. Recently, a polymorphism in exon 26 (C3435T) of this gene was shown to correlate with intestinal PGP expression and function in humans. Carriers homozygous for this polymorphism (TT) showed more than two-fold lower PGP expression and higher digoxin plasma concentrations compared to the CC group. However, it is not known whether this mutation in the MDR1 gene is also associated with altered PGP function in peripheral blood cells. We therefore assessed efflux of the PGP-substrate rhodamine 123 from CD56+ natural killer cells. Leukocytes were isolated from whole blood of 10 CC, 10 CT and 11 TT healthy Caucasian individuals. Using flow cytometry, rhodamine fluorescence was determined in CD56+ cells. Moreover, MDRI mRNA was quantified in leukocytes by real-time polymerase chain reaction. Subjects with CC genotype revealed a significantly lower rhodamine fluorescence (i.e. higher PGP function) compared to individuals with TT genotype (51.1 +/- 11.4% versus 67.5 +/- 9.5%, p < 0.01). Heterozygous individuals had an intermediate rhodamine fluorescence (61.4 +/- 6.3%). MDR1 mRNA normalized for cyclophilin was lowest in the TT population (1.29 +/- 1.01), intermediate in heterozygous subjects (1.60 +/- 0.76) and highest in the CC group (1.91 +/- 0.94; not significant). In summary, subjects being homozygous for C in position 3435 of the MDR1 gene have a more pronounced efflux of rhodamine from CD56+ natural killer cells and a higher MDR1 mRNA expression in leukocytes than subjects with the TT genotype. Measurement of rhodamine efflux using flow-cytometry from peripheral blood cells allows assessment of genetically determined differences in P-glycoprotein function.

P-glycoprotein-mediated transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin.

Digoxin is a drug with a narrow therapeutic index, which is substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidneys. Whereas particle size, dissolution rate and lipophilic properties have been identified as determinants for absorption of digitalis glycosides, little is known about P-glycoprotein transport characteristics of digitalis glycosides such as digitoxin, alpha-methyldigoxin, beta-acetyldigoxin and ouabain. Using polarized P-glycoprotein-expressing cell lines we therefore studied whether these compounds are substrates of P-glycoprotein. Polarized transport of digitalis glycosides was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells (LLC-PK1 cells stably transfected with the human MDR1 P-glycoprotein). Inhibition of P-glycoprotein-mediated transport of these compounds in Caco-2 cells was determined using the cyclosporine analogue PSC-833 (valspodar) as inhibitor of P-glycoprotein. No polarized transport was observed for ouabain. However, basal-to-apical transport of digitoxin, alpha-methyldigoxin and beta-acetyldigoxin was greater than apical-to-basal transport in Caco-2 and L-MDR1 cells. In Caco-2 cells net transport rates of these compounds were similar to those of digoxin (digoxin: 16.0+/-4.4%, digitoxin: 15.0+/-3.3%, beta-acetyldigoxin: 16.2+/-1.6%, alpha-methyldigoxin: 13.5+/-4.8%). Furthermore, polarized transport of these compounds could be completely inhibited by 1 microM PSC-833. In summary, these data provide evidence that not only digoxin, but also digitoxin, alpha-methyldigoxin and beta-acetyldigoxin are substrates of P-glycoprotein.

The population pharmacokinetics of digoxin in dogs with heart disease.

Clinical monitoring of digoxin plasma concentration is recommended because of both a large inter-animal variation in digoxin pharmacokinetics and the small ratio of toxic/therapeutic concentrations. Digoxin concentration data from clinical therapeutic drug monitoring (TDM) are used to adapt dosage regimes. Hierarchical mixed effects modeling of TDM data can be used to identify factors that are covariables for pharmacokinetic model parameters. Knowledge of such pharmacokinetic model covariables may help to improve dose calculations and predictions and can also help to understand the pharmacodynamics of drug action and toxicity. Such models have been developed for digoxin in humans (Sheiner et al., 1977). The pharmacokinetics of digoxin in healthy dogs has been described previously (Button et al., 1980b) and dosage regimes have been examined in dogs with induced (Button et al., 1980a) or naturally occurring congestive heart failure (Tobias et al., 1989). These previous studies showed that dose adjustments made according to assumed covariables for digoxin pharmacokinetics did not accurately achieve the desired plasma concentrations in individual animals (Tobias et al., 1989). Individualized dosage regimes using TDM are necessary to achieve desired plasma digoxin concentration in dogs (Button et al., 1980a). We applied hierarchical mixed effects modeling to examine the pharmacokinetics of digoxin in 161 cases with naturally occurring heart disease in dogs. Of these, 32 were examined prospectively and 129 retrospectively. We aimed to describe the pharmacokinetics of digoxin in dogs with heart disease and to identify covariables, which could be used to predict an individual dog’s digoxin pharmacokinetics. Samples for digoxin analysis were taken at steady-state, after known but varying doses, dose intervals and dose-to-sample intervals, from routine clinical TDM cases. These samples were analyzed using commercial kit RIA or EMIT assays. Covariable data collected included serum creatinine, serum potassium, body weight (kg) x (x=0.75, 1), estimated body surface area, age and the formulation administered (Lanoxin tablets or pediatric elixir, Glaxo Wellcome). A one compartment open pharmacokinetic model for oral administration was applied to the data using proprietary software (PPharm v 1.51, Innaphase, France). Initial modeling identified two distinct populations of cases, one with a markedly slower absorption rate constant (ka) than the other (Fig. 1). Analysis for k-means clustering based on Euclidean differences identified two populations, one of 19 cases and one of 142 cases (Systat v 8.0, SPSS Inc. Chicago, IL, USA). The means of the two sample populations for ka differed statistically using Student’s t-test (PB0.01) (Table 1.). No correlation was identified between the 19 slow absorbing cases and any known covariable, including dose formulation (Fig. 1). All 19 slow absorbers were from the 32 cases (19/32=59%) which had been admitted to the hospital, a stressful event, within 2 h prior to dosing: in contrast, the remaining 129/142 (91%) cases had been dosed at home prior to sample collection. Acute stress has been associated with release of corticotrophin releasing factor (CRF) and intracerebroventricular infusion of CRF has been shown to delay total gastric emptying in dogs (Lee & Sarna, 1997). Therefore, we hypothesize that the slower absorption of orally administered digoxin was due to delayed gastric emptying in 59% cases admitted to hospital for dosing. Stress and pain have previously been shown to alter drug absorption and metabolism (Jamali & Kunz-Dober, 1999). Knowledge of a delay in absorption of drugs administered to patients by the oral route at the time of hospitalization, if confirmed, would influence selection of the route of drug administration for such patients. Multimodal populations violate the assumption of normal/ log-normal population distribution for parametric population pharmacokinetic analysis. Therefore, all 19 cases identified to have a low ka were deleted from further analyses. For the remaining 142 cases the number of observations per subject

Functional polymorphisms of the human multidrug-resistance gene: multiple sequence variations and correlation of one allele with P-glycoprotein expression and activity in vivo.

To evaluate whether alterations in the multidrug-resistance (MDR)-1 gene correlate with intestinal MDR-1 expression and uptake of orally administered P-glycoprotein (PGP) substrates, we analyzed the MDR-1 sequence in 21 volunteers whose PGP expression and function in the duodenum had been determined by Western blots and quantitative immunohistology (n = 21) or by plasma concentrations after orally administered digoxin (n = 8 + 14). We observed a significant correlation of a polymorphism in exon 26 (C3435T) of MDR-1 with expression levels and function of MDR-1. Individuals homozygous for this polymorphism had significantly lower duodenal MDR-1 expression and the highest digoxin plasma levels. Homozygosity for this variant was observed in 24% of our sample population (n = 188). This polymorphism is expected to affect the absorption and tissue concentrations of numerous other substrates of MDR-1.

Functional polymorphisms of the human multidrug-resistance gene: Multiple sequence variations and correlation of one allele with P-glycoprotein expression and activity in vivo

To evaluate whether alterations in the multidrug-resistance (MDR)-1 gene correlate with intestinal MDR-1 expression and uptake of orally administered P-glycoprotein (PGP) substrates, we analyzed the MDR-1 sequence in 21 volunteers whose PGP expression and function in the duodenum had been determined by Western blots and quantitative immunohistology (n = 21) or by plasma concentrations after orally administered digoxin (n = 8 + 14). We observed a significant correlation of a polymorphism in exon 26 (C3435T) of MDR-1 with expression levels and function of MDR-1. Individuals homozygous for this polymorphism had significantly lower duodenal MDR-1 expression and the highest digoxin plasma levels. Homozygosity for this variant was observed in 24% of our sample population (n = 188). This polymorphism is expected to affect the absorption and tissue concentrations of numerous other substrates of MDR-1.

P-glycoprotein: a defense mechanism limiting oral bioavailability and CNS accumulation of drugs.

Transport by ATP-dependent efflux pumps such as P-glycoprotein is an increasingly recognized determinant of drug disposition. P-glycoprotein does not only contribute to multidrug resistance (MDR) in tumor cells, it is also expressed in normal tissues with excretory function such as liver, kidney and intestine. Apical expression of P-glycoprotein in such tissues results in reduced drug absorption from the gastrointestinal tract and enhanced drug elimination into bile and urine. Moreover, expression of P-glycoprotein in the endothelial cells of the blood-brain barrier prevents entry of certain drugs into the central nervous system. Human P-glycoprotein has been shown to transport a wide range of structurally unrelated drugs such as digoxin, quinidine, cyclosporine and HIV-1 protease inhibitors. Drug administration to P-glycoprotein knock-out and control mice provided data on the importance of P-glycoprotein for absorption after oral administration and penetration through the blood-brain barrier. Moreover, P-glycoprotein knock-out mice were used to identify inhibition of P-glycoprotein-mediated transport as a mechanism for drug interactions such as the digoxin-quinidine interaction. Studies in humans indicate a particular importance of intestinal P-glycoprotein for bioavailability of the immunosuppressant cyclosporine. Moreover, induction of intestinal P-glycoprotein by rifampin has now been identified as the major underlying mechanism of reduced digoxin plasma concentrations during concomitant rifampin therapy. In summary, P-glycoprotein functions as a defense mechanism, which determines bioavailability and CNS concentrations of drugs. Modification of P-glycoprotein function is an important underlying mechanism of drug interactions in humans. However, disposition of a drug and its metabolites frequently is not only determined by P-glycoprotein, but also by drug-metabolizing enzymes and possibly by drug transporters other than P-glycoprotein [e.g. members of the MRP family (MRP = multidrug resistance-associated proteins)].