Quinidine-induced changes in serum and skeletal muscle digoxin concentration; evidence of saturable binding of digoxin to skeletal muscle

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

Cardiovascular drug therapy in the elderly: theoretical and practical considerations

The elderly population is expanding rapidly throughout the world. Hypertension, heart disease and other cardiovascular disorders are prevalent conditions among this age group. Consequently, clinicians will spend a large proportion of their practices managing older adults with cardiovascular disorders. A large proportion of this time will be devoted to using pharmacotherapeutic strategies for the long-term management of chronic conditions. The physiological changes that accompany aging affect cardiovascular function, and the pharmacokinetics and pharmacodynamics of many cardiovascular medications are altered by these physiological changes. The interactions of these changes can have a profound effect on the agents used to treat cardiovascular disorders and may alter their therapeutic outcomes. Several classes of medications are used to treat chronic cardiovascular disorders in older adults. These include the ACE inhibitors and angiotensin II receptor antagonists, calcium channel antagonists, beta-adrenoceptor antagonists (beta-blockers), oral antiarrhythmic agents and warfarin. Drugs such as beta-blockers may aggravate decreased cardiac output and increase peripheral resistance, but are valuable adjuncts in many patients with congestive heart failure. Agents that reduce angiotensin II activity may have several benefits for treating heart failure and hypertension. Successful treatment of cardiovascular disorders in older adults requires the choice of the most appropriate agent, taking into consideration the complex interactions of pharmacokinetics, pharmacodynamics and disease effects.

Quinidine administration increases steady state serum digoxin concentration in horses

The aim of this study was to determine if quinidine administration increases steady state serum digoxin concentration in horses. Digoxin (0.01 mg/kg q. 12 h per os) was administered to 6 horses for 7 days. Steady state was confirmed by identifying statistically indistinguishable peak and trough serum digoxin concentrations on Days 4, 5, and 6. On Day 6, serum digoxin concentration was measured at baseline and 0.25, 0.5, 1, 2, 4, 6, 8 and 12 h after digoxin administration. On Day 7, quinidine (20 g at baseline and 10 g at 2, 4 and 6 h) was administered per os and serum digoxin concentration was measured at the same time intervals. Creatinine and renal digoxin clearances were measured on Days 6 and 7. Results indicated that there was approximately a doubling of serum digoxin concentration (from mean +/- s.d. 2.57 +/- 0.96 ng/ml at baseline to 4.28 +/- 1.31 at 15 min and 5.98 +/- 1.21 ng/ml at 30 min) after starting the administration of quinidine. This elevation persisted for the 12 h after starting quinidine administration. Renal digoxin and endogenous creatinine clearances decreased but the decrease in digoxin clearance was greater. Serum quinidine concentration achieved the therapeutic range (2-6 micrograms/ml) in 5 of the 6 horses. In summary, similar to findings in other species, quinidine administration increases steady state serum digoxin concentration in horses and this occurs, at least in part, due to a decrease in renal digoxin clearance. Some of the decrease in renal clearance is due to decreased glomerular filtration which is dissimilar to findings in other species.

The effect of digoxin dosage on the digoxin-quinidine interaction in the bile duct-cannulated rat

Pretreating anaesthetized bile duct-cannulated rats with 9 mg kg-1 quinidine significantly decreased the cumulative biliary excretion of digoxin and its metabolites after 10 or 100 micrograms kg-1 [3H]digoxin, although the effect was more marked in animals receiving the high dose of digoxin. In contrast, however, although quinidine pretreatment raised plasma radioactivity levels by 50-80% in animals given the higher dose of digoxin, no significant effect on circulating plasma levels was observed in rats receiving 10 micrograms kg-1 digoxin. Generally, quinidine had no statistically significant effect on other aspects of digoxin disposition, although with both digoxin doses there were trends towards a reduction in the direct intestinal secretion and urinary excretion of digoxin-derived radioactivity with an increase in tissue levels of radioactivity (apart from the small intestine wall where concentrations were reduced). The radioactivity in the bile after 100 or 10 micrograms kg-1 digoxin comprised about 25 and 33% of digoxin and digoxigenin bis-digitoxoside, respectively, as well as appreciable amounts of the monodigitoxoside and a highly polar component. This metabolite profile was unaffected by quinidine. The influence of cardiac glycoside dosage shown by the present work indicates that the digoxin-quinidine interaction and possibly analogous interactions involving other cardiac glycosides, may not always be readily detectable from plasma concentration data.

Skeletal muscle binding and renal excretion of digoxin in man

Ten healthy subjects were treated with three or four different doses of digoxin, 0.25 to 0.88 mg daily, in random order. Digoxin concentrations in serum (SDC) and skeletal muscle (SMDC) were determined as well as its renal clearance after 2 weeks of treatment with each dose. The mean ratio SMDC/SDC decreased non-significantly by about 20% from 33 +/- 15 at the lowest SDC interval (0.5-0.9 nmol/l) to 28 +/- 7 at the highest concentration interval (2.0-2.4 nmol/l). This is in accordance with findings from previous studies. No significant change was observed in the renal clearance of digoxin with increasing digoxin concentration, supporting the concept of first-order renal elimination of digoxin.