Clinical pharmacokinetic significance of the renal tubular secretion of digoxin

Is the human model RPTEC/TERT1 a relevant model for assessing renal drug efflux?

Active tubular secretion plays a major role in renal excretion of drugs thanks to the presence of many membrane transporters such as ABC transporters. These proteins facilitate drug transfer into the urine and could be a source of pharmacokinetic variabilities. Up to now, several human in vitro models of proximal tubule have been proposed but few of them have been characterized for predicting drugs renal efflux. The aim of this study was to determine whether the human model RPTEC/TERT1 meets all the criteria expected of a good model to assess renal drug transport. First, in vitro barrier properties were investigated. Then, the expression of several ABC transporters was assessed by immunofluorescence and relative quantification by liquid chromatography-high-resolution mass spectrometry (LC-HRMS) in comparison to the MDCK model. Finally, bidirectional transport studies were performed to evaluate the functionality of transporters and the abilities of model to discriminate several drugs. The RPTEC/TERT1 model formed a tight structure (192 Ω.cm² ) that was confirmed by paracellular permeability assays. Proteomic analysis and immunofluorescence staining showed the expression of several ABC transporters. Then, only the functionality of P-gp was confirmed by the active efflux of apixaban in this study. In addition, the RPTEC/TERT1 model presents the key criteria of a renal barrier and expresses several ABC transporters. Nevertheless, the BCRP and MRP's functionality was not confirmed and further investigations are required to valid this model as in vitro model for assessing renal drug efflux.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Effect of Lasofoxifene on the Pharmacokinetics of Digoxin in Healthy Postmenopausal Women

Lasofoxifene is in late-stage development for the prevention and treatment of osteoporosis. Digoxin is commonly prescribed for arrhythmias and congestive heart failure, has a narrow therapeutic index, and may be coadministered with lasofoxifene. This study was conducted to determine the effect of lasofoxifene (4-mg loading dose on day 11 followed by 0.5 mg/d on days 12-20) on the steady-state pharmacokinetics of digoxin (0.25 mg/d on days 1-20) in 12 healthy postmenopausal women. On days 10 and 20, blood and urine samples were collected for 24 hours to determine digoxin concentrations. The 90% confidence interval (CI) of least squares mean ratio for maximum concentration (C(max)) and area under the plasma concentration-time curve (AUC) was calculated. Lasofoxifene had no effect on digoxin plasma pharmacokinetics with a ratio (90% CI) of 95.4% (84.6%-107%) and 103% (97.7%-108%) for C(max) and AUC(0-24), respectively. The ratio of the percentage of dose eliminated unchanged in urine in 24 hours was 127% (116% to 142%). Coadministration of lasofoxifene had no effect on the steady state pharmacokinetics of digoxin.

Ribosomal protein L3 mediated the transport of digoxin in Xenopus laevis oocyte

Ribosomal protein L3 (RPL3) is known to be an indispensable and essential component for the peptidyltransferase center. In the present study, we found a novel function of RPL3 using a Xenopus laevis oocyte expression system. When expressed in X. oocytes, RPL3 mediated the high affinity transport of [(3)H]digoxin (K(m) = 213.3 ± 46.8 nM) in a time-, concentration-, and sodium-dependent manners. The maximum velocity of the transport of [(3)H]digoxin via RPL3 produced at physiological pH. However, we did not observe RPL3-mediated transport of several organic solutes such as [(14)C]androstenedione, [(3)H]dexamethasone, [(3)H]dehydroepiandrosterone sulfate, [(3)H]L-tryptophan, [(14)C]L-ascorbic acid, [(14)C]α-ketoglutarate, [(14)C]glutarate, [(3)H]methotrexate, [(3)H]bumetanide, [(3)H]probenecid, [(14)C]salicylic acid, [(14)C]theophylline and [(3)H]valproate. Our results suggest that RPL3 functions as a drug carrier protein and may be involved in the digoxin toxicity in the human body.

The Influence of Cardiovascular Physiology on Dose/Pharmacokinetic and Pharmacokinetic/Pharmacodynamic Relationships

Inter- and intraindividual variability in the relationship between dose and clinical--or pharmacodynamic--response of a drug can be analysed in two steps: firstly, by considering the plasma pharmacokinetic response to a given dose and, secondly, by the connection between both pharmacokinetic and pharmacodynamic responses. As the cardiovascular system is the means of transport of endogenous and exogenous substances, blood flow fraction destined to each organ determines the relative mass of solute in plasma, which is constantly in contact with the tissue. Hence, not only the rate but also the extent of drug transfer would be increased when tissues are irrigated by a higher fraction of cardiac output. Aging and circadian rhythms present similar cardiac output distribution patterns when moving from young to aged adult and from nocturnal to diurnal hours. These two changes lead to an increased blood flow delivery to the extra-splanchnic-renal region in the elderly and in the morning, but with a decreased cardiac output in aged individuals and an increased one during the day. This scenario allows us to forecast substance concentrations outside the blood vessels, which are responsible for the extent of drug elimination and the intensity of drug effect. So available data on disposition and pharmacodynamics of drugs might be explained from another point of view that challenges current knowledge. Furthermore, the administration of cardiovascular active drugs might reverse the chronological sequence between pharmacokinetic and pharmacodynamic responses, since they could modify blood flow distribution.

Absence of a clinically relevant interaction between etanercept and digoxin

Etanercept, a soluble recombinant human tumor necrosis factor receptor (TNFr), is effective and well tolerated in the treatment of rheumatoid arthritis, juvenile rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, and psoriasis. The primary objective of this study was to investigate the potential pharmacokinetic and pharmacodynamic interaction between digoxin and etanercept at steady state. In a crossover, open-label, nonrandomized, 3-period study, 12 healthy male subjects received loading oral doses of digoxin 0.5 mg every 12 hours on day 1 and 0.25 mg every 12 hours on day 2, followed by a daily maintenance dose of 0.25 mg for a total of 27 days. Etanercept was administered as a twice-weekly 25-mg subcutaneous dose beginning on day 9 and continuing up to day 37 for a total of 9 doses. All ratios of maximum plasma concentration (C(max)) and area under the plasma concentration versus time curve (AUC) for pharmacokinetics of digoxin fell within the confidence interval of 0.8 to 1.25. Although not considered clinically relevant, the mean C(max) and AUC of etanercept were 4.2% and 12.5% lower, respectively, when etanercept was given with digoxin than when administered alone. There were no clinically relevant changes in the electrocardiogram (ECG) parameters, and adverse events did not increase when both drugs were combined. In conclusion, there is no clinically relevant interaction between etanercept and digoxin, and both drugs can be safely coadministered without the need for a dosage adjustment.

Isolation and characterization of a digoxin transporter and its rat homologue expressed in the kidney

Digoxin, which is one of the most commonly prescribed drugs for the treatment of heart failure, is mainly eliminated from the circulation by the kidney. P-glycoprotein is well characterized as a digoxin pump at the apical membrane of the nephron. However, little is known about the transport mechanism at the basolateral membrane. We have isolated an organic anion transporter (OATP4C1) from human kidney. Human OATP4C1 is the first member of the organic anion transporting polypeptide (OATP) family expressed in human kidney. The isolated cDNA encodes a polypeptide of 724 aa with 12 transmembrane domains. The genomic organization consists of 13 exons located on chromosome 5q21. Its rat counterpart, Oatp4c1, is also isolated from rat kidney. Human OATP4C1 transports cardiac glycosides (digoxin, K(m) = 7.8 microM and ouabain, K(m) = 0.38 microM), thyroid hormone (triiodothyronine, K(m) = 5.9 microM and thyroxine), cAMP, and methotrexate in a sodium-independent manner. Rat Oatp4c1 also transports digoxin (K(m) = 8.0 microM) and triiodothyronine (K(m) = 1.9 microM). Immunohistochemical analysis reveals that rat Oatp4c1 protein is localized at the basolateral membrane of the proximal tubule cell in the kidney. These data suggest that human OATP4C1/rat Oatp4c1 might be a first step of the transport pathway of digoxin and various compounds into urine in the kidney.

Lack of interaction of digoxin and P-glycoprotein inhibitors, quinidine and verapamil in human placenta in vitro

OBJECTIVE

To determine the effect of quinidine and verapamil, known antiarrhythmic agents and P-glycoprotein (Pgp) inhibitors, on digoxin transport from the maternal to the fetal compartment in the isolated perfused human placenta.

STUDY DESIGN

Isolated placental cotyledons from normal human placentae (n=20) were dually perfused with M199 medium enriched with albumin (0.3%) and glucose (0.1%). The maternal and the fetal circulation flow rates were 12 and 6 ml/min, respectively. Closed circulations were used to evaluate steady state transplacental gradient formation. In six placentae quinindine was added to the maternal circuit; after 45 min of perfusion, digoxin was added to the maternal circulation. The effect of verapamil on digoxin transfer from the maternal to the fetal compartments was explored in five placentae. In six additional placentae the transfer of digoxin was studied in the absence of quinidine. Transplacental passage of digoxin was calculated from repeated fetal and maternal perfusate samples. Digoxin levels were determined in perfusate samples by fluorescence polarization immunoassay. Antipyrine was added to the maternal reservoir of all placentae as reference substance.

RESULTS

The transfer of digoxin (alone) and in the presence of quinidine or verapamil was 10.93+/-3.71, 9.00+/-5.2 and 12.94+/-4.86%, respectively. The levels of digoxin in the fetal compartment, 0.62+/-0.20, 0.48+/-0.29 and 0.60+/-0.26 ng/ml, respectively, were not significantly affected by quinidine and verapamil. These Pgp modulators, also did not influence significantly the steady state levels of digoxin in the maternal compartment.

CONCLUSION

Neither quinidine nor verapamil affected the transplacental transfer of digoxin in vitro in normal human placentae. In contrast to the other tissues, they do not inhibit Pgp activity in term human placentae.

MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs

The multidrug transporter, MDR1-mediated interaction of digoxin with antiarrhythmic or antianginal drugs was examined in vitro by using the MDR1-overexpressing LLC-GA5-COL150 cells, which were established by transfection with human MDR1 cDNA into porcine kidney epithelial LLC-PK(1) cells. Amiodarone, its active metabolite monodesethyl-amiodarone (DEA), and quinidine markedly inhibited the basal-to-apical transport (renal secretion) of [(3)H]digoxin and increased the apical-to-basal transport (reabsorption), but cibenzoline and lidocaine showed slight inhibition of the transport, and disopyramide and mexiletin had no such effects. The IC(50) values for amiodarone, DEA and quinidine on [(3)H]digoxin transport in LLC-GA5-COL150 cells were 5.48 microM, 1.27 microM and 9.52 microM, respectively. These were comparable to, or only several times the achievable concentration in clinical use, suggesting that MDR1 could be responsible for the drug interaction between digoxin and amiodarone found in clinical reports and that DEA contributes the elevation of digoxin serum concentration. Similarly, dipyridamole altered the transport, but isosorbide showed only slight modification of the transport. The IC(50) value for dipyridamole was 40.0 microM, also only several times the achievable concentration in clinical use, indicating a risk of interaction.

Predictive performance of serum digoxin concentration in patients with congestive heart failure by a hyperbolic model based on creatinine clearance

OBJECTIVES

To formulate a simple equation for determining the daily dose requirements of digoxin by inclusion of creatinine clearance (Ccr) values as an explanatory variable.

METHODS

We included 235 routine monitoring and clinical laboratory test data (steady-state serum digoxin concentration and Ccr values), obtained from hospitalized patients receiving digoxin for treatment of congestive heart failure. The 107 data sets were fitted to a hyperbolic model to account for the relation between the ratio of serum digoxin level to the daily dose (L/D) and the Ccr values determined by six methods. Their correlation coefficients (r) were computed by non-linear regression analysis. To evaluate the validity of the best-fitting model, the predictive performance of the L/D ratios was compared with those given by seven reference models previously published, using another 128 data sets.

RESULTS

The hyperbolic model involving the Ccr values estimated by Cockcroft and Gault's equation showed the closest correlation (r = 0.81) between the actual and estimated Ccr values. Mean prediction error (ME), a measure of bias, of the L/D ratio (0.018 ng/mL) was almost negligible when other data were fitted to the proposed model, and this ME value proved to be much smaller than those calculated from the previously published prediction models. Mean absolute prediction error, a measure of precision, by the proposed model was also satisfactory for prediction.

CONCLUSION

The newly developed model provided good predictive performance of serum digoxin level. Taking simplicity in practical use into account, the clinical application of the proposed model will allow for accurate and rapid determination of the initial maintenance dosing regimen of digoxin based on the individual Ccr value, without actual measurement of its serum concentration.

Digoxin up-regulates MDR1 in human colon carcinoma Caco-2 cells

Because MDR1 (P-glycoprotein) plays an important role in pharmacokinetics such as absorption and excretion of xenobiotics and multidrug resistance, an understanding of the factors regulating its function and expression is important. Here, the effects of digoxin on cell sensitivity to an anticancer drug, MDR1 function, and expression were examined by assessing the growth inhibition by paclitaxel, the transport characteristics of the MDR1 substrate Rhodamine123, and the level of MDR1 mRNA, respectively, using human colon carcinoma Caco-2 cells, which are widely used as a model of intestinal epithelial cells. The sensitivity to paclitaxel, an MDR1 substrate, in Caco-2 cells pretreated with digoxin was lower than that in non-treated cells. The accumulation of Rhodamine123 was reduced by pretreatment with digoxin and its efflux was enhanced. The level of MDR1 mRNA in Caco-2 cells was increased in a digoxin concentration-dependent manner. These results taken together suggested that digoxin up-regulates MDR1 in Caco-2 cells.

MDR1 genotype-related pharmacokinetics of digoxin after single oral administration in healthy Japanese subjects

PURPOSE

To evaluate the MDR1 genotype frequency in the Japanese population and to study the relationship between the MDR1 genotype and the pharmacokinetics of digoxin after single oral administration in healthy subjects.

METHODS

The MDR1 genotype at exon 26 was determined in 114 healthy volunteers by polymerase chain reaction-restriction fragment length polymorphism. The serum concentration-time profile of digoxin was examined after single oral administration at a dose of 0.25 mg.

RESULTS

It was found that 35.1 % (40/114) of subjects were homozygous for the wild-type allele (C/C). 52.6% (60/114) were compound heterozygotes with a mutant T-allele (C3435T) (C/T), and 12.3% (14/114) were homozygous for the mutant allele (T/T). There was no effect of gender or age on the distribution. The serum concentration of digoxin after a single oral administration increased rapidly, attaining a steady state in all subjects; however, it was lower in the subjects harboring the T-allele. AUC0-4 h values (+/-SD) were 4.11 +/- 0.57, 3.20 +/- 0.49. and 3.27 +/- 0.58 ng h/ml, respectively, with a significant difference between C/C and C/T or T/T.

CONCLUSIONS

The serum concentration of digoxin after single oral administration was lower in the subjects harboring a mutant allele (C3435T) at exon 26 of the MDR1 gene.

Factors influencing the prediction of steady state concentrations of digoxin

The prediction error in the Bayesian analysis program for digoxin was evaluated in Japanese patients, and factors influencing the accuracy were investigated. Serum concentrations of digoxin were monitored two times and were compared with the predicted values obtained by using the Bayesian analysis program. The prediction error at the first time was 43.1%. Although this estimation error was reasonably restored at the second time of monitoring, the prediction error remained at 26.6%. These data suggested that unknown factors not included in the program affected the serum concentration of digoxin. Retrospective research of the digoxin serum concentrations in the patients suggested the coadministration of the drugs, which were the P-glycoprotein modulators, as well as the unexpected alteration of the serum creatinine, were the important factors influencing the prediction of the drug serum concentrations. We next examined the inhibitory effect of quinidine, verapamil and spironolactone on the transcellular transport of digoxin by using human P-glycoprotein overexpressing LLC-GA5-COL150 cells. Quinidine, verapamil and spironolactone could inhibit the transcellular transport of digoxin by 50%. In addition, the reduction of the renal clearance by 50%, which could possibly be caused by this inhibition, led to the increase of 36% in the steady state through concentrations of digoxin in the physiological pharmacokinetic model. In conclusion, the prediction of long-term serum concentration-time profiles of digoxin, based on the Bayesian analysis, will be disturbed by the coadministration of the P-glycoprotein modulators and the unexpected alteration of the serum creatinine.

The effect of bosentan on the pharmacokinetics of digoxin in healthy male subjects

AIMS

To investigate the effect of multiple oral dose treatment with the endothelin receptor antagonist bosentan on the pharmacokinetics of digoxin in healthy subjects.

METHODS

This was an open-label, randomized, two-way crossover study in 18 evaluable young male subjects. They received, on two occasions which were separated by at least 2 weeks washout period, 0.375 mg digoxin once daily for 13 days following a loading dose of 0.375 mg given twice on the day before the once daily dosing regimen started. On one occasion treatment with 500 mg bosentan twice daily was started on the eighth day of digoxin treatment and continued for 1 week. Serum concentrations of digoxin were determined up to 24 h postdose on day 8 (first day of bosentan treatment) and day 14 (last day of bosentan treatment) of the digoxin treatment period. Plasma concentrations of bosentan were measured at two time points after the first bosentan dose and up to 12 h after the last morning dose of bosentan. Safety was assessed by adverse events, clinical laboratory tests, blood pressure and pulse rate measurements and ECG recordings.

RESULTS

Steady-state of digoxin was always achieved after 7 days of treatment. Serum concentrations of digoxin were within the usual therapeutic range. Average steady-state Cmax and Ctr were 2-2.1 microg l-1 and 0.65-0.69 microg l-1, respectively, when given alone. Bosentan did not lead to statistically significant changes in Cmax and Ctr of digoxin. AUC (0,24h) of digoxin, however, was slightly reduced after 1 week of treatment with bosentan. The reduction was 12% on average with a narrow 95% confidence interval of 0-23%. Bosentan pharmacokinetic parameters after 1 week of treatment were as expected with a mean Cmax of 3260 microg l-1 and a mean AUC (0, 12h) of 12 600 microg l-1 h.

CONCLUSIONS

Treatment with bosentan 500 mg twice daily for 1 week did not show clinically relevant effects on the pharmacokinetics of digoxin in healthy human subjects

A model for the prediction of digoxin-drug interactions at the renal tubular cell level

Digoxin-drug interactions are relatively common causes of digitalis toxicity. Recently, the clinical importance of the renal tubular secretion of digoxin has been proven by documenting drug interactions at this level. The authors describe a model using cultured renal tubular cell monolayers that can be used to predict drug interactions with the cardiac glycoside. This model accurately documents known clinical digoxin interactions such as those with verapamil and propafenone. The common feature of these interactions is that they involve P-glycoprotein substrates (e.g., digoxin, vincristine, vinblastine) or inhibitors (e.g., quinidine, cyclosporine). In the case of the newly described interaction of digoxin with itraconazole, the model preceded the emergence of clinical cases.

Lack of interaction between valaciclovir, the L-valyl ester of aciclovir, and digoxin

AIMS

Changes in both digoxin and aciclovir renal clearance following coadministration with some other renally eliminated drugs have been reported. The potential interaction of valaciclovir, with its antiherpetic metabolite aciclovir, and digoxin was investigated.

METHODS

Twelve healthy volunteers (seven males, five females) participated in an open, randomized, four-period crossover study. Valaciclovir, 1000 mg, was given alone on one occasion, and on another, after the second of two 0.75 mg digoxin doses administered 12 h apart. Blood samples and all urine were collected up to 12 h following the valaciclovir dose for aciclovir radioimmunoassay. On a third occasion, digoxin was given alone and on a fourth, with 1000 mg valaciclovir three times/day for 8 days starting 12 h before the first digoxin dose. Blood samples were taken up to 168 h and all urine collected up to 24 h following the second dose for digoxin radioimmunoassay.

RESULTS

There were no clinically significant differences in digoxin or aciclovir pharmacokinetic parameters when digoxin or valaciclovir was given alone or in combination.

CONCLUSIONS

No dosage adjustment is required when valaciclovir and digoxin are coadministered.

Renal transport of drugs: specificity and molecular mechanisms

1. The kidney is principally an excretory organ for drugs and their metabolites and has developed high capacity transport systems to rapidly eliminate the large quantities of foreign compounds delivered to it. 2. There are specific and selective transporters for organic cations and organic anions at the contraluminal and luminal membranes of epithelial cells lining the proximal tubule. 3. For organic anions, the transporter (termed PAH-transporter) is located at the contraluminal cell membrane and uptake is a tertiary active transport process. Important physico-chemical properties for interaction with this transporter are hydrophobicity, ionic charge strength and electron-attracting side groups. 4. For organic cations, there is evidence for one common transport system at the contraluminal membrane and substrate requirements are the degree of hydrophobicity, ionic charge strength and hydrogen bond formation. At the luminal membrane, an electroneutral H+/organic cation exchanger in which the degree of substrate hydrophobicity is critical for interaction and a choline transport system have been described. 5. P-Glycoprotein present in renal brush border membranes transports organic cationic drugs but is dissimilar to the luminal electroneutral H+/organic cation exchanger. 6. Clinically, several drugs have been implicated in causing interactions via these transport systems. For organic anions, probenecid blocks the tubular secretion of other organic anions and this has been used to prolong the duration of action of penicillin-like compounds. For organic cations, cimetidine and trimethoprim are potent inhibitors of the secretion of a number of organic cations, particularly procainamide, which can result in significant clinical toxicity. 7. An understanding of the mechanisms of renal tubular secretion of drugs and their metabolites will allow for the prediction of drug interactions involving renal excretory mechanisms.

The effects of epoprostenol on drug disposition. I: A pilot study of the pharmacokinetics of digoxin with and without epoprostenol in patients with congestive heart failure

The influence of epoprostenol on the pharmacokinetics of drugs administered concurrently to patients with congestive heart failure (CHF) receiving epoprostenol was evaluated as a secondary objective of a Phase II pilot study. A total of 278 blood samples were collected from 30 patients with end-stage CHF receiving conventional therapy alone or conventional therapy plus epoprostenol. Estimates of oral clearance (Cl), volume of distribution, and absorption rate constant of digoxin were generated from plasma digoxin concentrations using nonlinear mixed effects modeling, and the effect of epoprostenol on Cl of digoxin was evaluated by univariate analysis. Additional factors that were evaluated by univariate analysis included age, obesity, time since study entry, cardiac output, concomitant use of angiotensin-converting enzyme (ACE) inhibitor, concomitant dobutamine, and estimated creatinine clearance. Backward elimination was used to arrive at a final model that included concomitant epoprostenol as a covariate. The final model revealed an approximate 15% decrease in Cl of digoxin in response to short-term administration of epoprostenol that was no longer apparent by the end of the 12-week treatment phase. Simulations revealed that this effect, although statistically significant, would not be clinically significant in most patients; however, the potential exists for short-term elevation of digoxin concentrations in response to concurrent administration of epoprostenol.

Pharmacokinetics of cardiovascular drugs in children. Inotropes and vasopressors

Infants and children with congenital or acquired heart disease and children with systemic disease often require pharmacological support of their failing circulation. Catecholamines may serve as inotropic (enhance myocardial contractility) or vasopressor (elevate systemic vascular resistance) agents. Noncatecholamine inotropic agents, such as the cardiac glycosides or the bipyridines, may be used in place of, or in addition to, catecholamines. Developmental changes in neonates, infants and children will affect the response to inotropic or pressor therapy. Maturation of the gastrointestinal tract, liver and kidneys alters absorption, metabolism and elimination of drugs, although there are few clear examples of this among the vasoactive drugs considered in this review. Changes in body composition affect the volume of distribution (Vd) and clearance (CL) of drugs. Developmentally based pharmacodynamic differences also affect the responses to both therapeutic and toxic effects of inotropes. These pharmacodynamic differences are based in part upon developmental changes in myocardial structure, cardiac innervation and adrenergic receptor function. For example, the immature myocardium has fewer contractile elements and therefore a decreased ability to increase contractility; it also responds poorly to standard techniques of manipulating preload. Available data suggest that dopamine and dobutamine pharmacokinetics are similar to those in adults. Wide interindividual variability has been noted. A consistent relationship between CL and age has not been demonstrated, although one investigator demonstrated an almost 2-fold increase in the CL of dopamine in children under the age of 2 years. The CL of dopamine appears to be reduced in children with renal and hepatic failure. Fewer data are available regarding the pharmacokinetics of epinephrine (adrenaline), norepinephrine (noradrenaline) and isoprenaline (isoproterenol). Digoxin pharmacokinetics have been extensively evaluated in infants and children. The Vd for digoxin is increased in infants and children. Children beyond the neonatal period display increased CL of digoxin, approaching adult values during puberty. Although it was previously thought that children both needed and tolerated higher serum concentrations of digoxin than adults, more recent studies indicate that adequate clinical response can be achieved with serum concentrations similar to those aimed for in adults, with decreased toxicity. Evaluation of studies of digoxin pharmacokinetics is complicated by the presence of an endogenous substance with digoxin-like activity on radioimmunoassay. Limited studies of amrinone pharmacokinetics in infants and children indicate a dramatically larger Vd, and a decreased elimination half-life in older infants and children, compared with values observed in adults.

A clinical assessment of the potential for pharmacological interaction between nimesulide and digoxin in patients with heart failure

The potential interaction between nimesulide, a nonsteroidal anti-inflammatory drug, and digoxin was studied in 9 patients [6 males, 3 females; mean age 67 (range 57 to 70) years] with mild heart failure. All patients were receiving maintenance therapy with digoxin (0.25 mg/day, orally) and were treated with oral nimesulide 100mg twice daily for 7 days. Blood samples were collected at 8am and 6pm for 4 days before and throughout the nimesulide treatment period for determination of serum digoxin concentrations. Physical health, electrocardiographic recordings and blood and urine samples were also monitored. Mean serum digoxin concentrations remained within the normal therapeutic range throughout the study despite large interindividual variation. Furthermore, there were no significant differences between the morning and afternoon serum digoxin concentrations and there was no major change in the clinical condition of any patient. These results indicate that short term administration (7 days) of conventional therapeutic doses of nimesulide (100mg twice daily) does not modify the serum digoxin profile in patients with low class heart failure treated with a maintenance dose (0.25 mg/day) of this cardiac glycoside.

Pharmacokinetic drug interactions with rifampicin

Rifampicin, an antituberculosis drug, is usually administered for 4 to 12 months with other antituberculosis drugs or medications from other classes. A potential for drug interactions often exists because rifampicin is a potent inducer of hepatic drug metabolism, as evidenced by a proliferation of smooth endoplasmic reticulum and an increase in the cytochrome P450 content in the liver. The induction is a highly selective process and not every drug metabolised via oxidation is affected. Case reports and studies have demonstrated enhanced metabolism of several drugs; most of these interactions are clinically important. At the start of rifampicin treatment, and again at the end, clinicians must check the dosages of any accompanying medications with which rifampicin may potentially interact. Monitoring of clinical response and blood drug concentrations is essential to adjust the drug dosage during rifampicin therapy. Rifampicin also interacts with cholephils such as bilirubin and bromosulphthalein. Its pharmacokinetics are reported to be altered by ethambutol, p-aminosalicylic acid (through its excipient component), ketoconazole, cyclosporin, clofazimine, probenecid and phenobarbital through one or other of the following mechanisms--impaired absorption of rifampicin, competition between the drug and rifampicin for hepatic uptake and altered hepatic metabolism of rifampicin. Most interactions affecting rifampicin have been relatively minor or are not expected to alter its therapeutic efficacy.

Interaction between digoxin and indomethacin or ibuprofen

To study a potential interaction between digoxin and two non-steroid anti-inflammatory drugs, indomethacin (50 mg three times daily) and ibuprofen (600 mg three times daily) were given for 10 days to 10 and 8 patients, respectively, on chronic digoxin treatment. Serum digoxin measured by fluorescence polarisation immunoassay increased significantly (P less than 0.05) during treatment with indomethacin from pre-treatment values of 0.73 +/- 0.34 nmol l-1 (mean +/- s.d.) to a mean value of 1.02 +/- 0.43 nmol l-1, while administration of ibuprofen did not change the steady state serum concentration of digoxin. The result demonstrates that some non-steroidal anti-inflammatory drugs such as indomethacin increase serum digoxin to levels high in the therapeutic range. This should be taken into consideration when co-administering other drugs known to increase the serum concentration of digoxin such as several antiarrhythmics.