Inhibitors of Na+/K+-ATPase: Cardiac glycosides - Chemical structure and pharmacokinetics
Digoxin and digitoxin are the two principal cardiac glycosides.
Digoxin is made up of a molecule of digoxigenin and three molecules of digitoxose. Digitoxin is made up of a molecule of digitoxigenin and three molecules of digitoxose.
Chemical structure of digoxin and digitoxin differ only by one OH group: digoxin has one OH group more than digitoxin but their pharmacokinetic characteristics are very different.
The bioavailability by oral route of digitalin reaches nearly 100% that of the digoxin is approximately 75%.
Binding to plasma proteins is of 95% for digitoxin, and 25% for digoxin.
Digitoxin is more metabolized by the liver than digoxin which is eliminated primarily by the kidney without biotransformation. The dosage of digoxin must be reduced in patients with renal insufficiency.
The plasma half-life of digitoxin is from 4 to 6 days, that of digoxin approximately 40 hours. With constant dosage, i.e. without giving a loadind dose in the initial stage of therapy, it takes a long time to reach the steady-state concentration.
Digitalin and digoxin cross blood-brain barrier, which explains the possibility of neuropsychiatric adverse effects, particularly in case of overdose.
They cross the placental barrier and are sometimes given to the mother for treating disorders of the cardiac rhythm of the fetus in utero. A low fraction is eliminated in breast milk.
In adults, the effective therapeutic concentration of digoxin is about 1 microgram per liter, that of digitoxin between 13 and 25 micrograms per liter, sampling being made 8 to 24 hours after the last intake.
Ouabain could be synthesized by the adrenocortical gland and be found in low concentration in the blood of non treated persons. Its role remains to be specified.