The coadministration of food delays the rate of galantamine absorption, but does not affect the extent of absorption.Plasma protein binding of galantamine is about 18%, which is relatively low. Metabolism
Approximately 75% of a dose of galantamine is metabolised in the liver. In vitro studies have shown that hepatic CYP2D6 and CYP3A4 are involved in galantamine metabolism. Within 24 hours of intravenous or oral administration approximately 20% of a dose of galantamine will be excreted unreacted in the urine.In humans, several metabolic pathways for galantamine exist. These pathways lead to the formation of a number of different metabolites. One of the metabolites that may result can be formed through the glucuronidation of galantamine. Additionally, galantamine may undergo oxidation or demethylation at its nitrogen atom, forming two other possible metabolites. Galantamine can undergo demethylation at its oxygen atom, forming an intermediate which can then undergo glucuronidation or sulfate conjugation. Lastly, galantamine may be oxidized and then reduced before finally undergoing demethylation or oxidation at its nitrogen atom, or demethylation and subsequent glucuronidation at its oxygen atom. For Razadyne ER, the once-a-day formulation, CYP2D6 poor metabolizers had drug exposures that were approximately 50% higher than for extensive metabolizers. About 7% of the population has this genetic mutation; however, because the drug is individually titrated to tolerability, no specific dosage adjustment is necessary for this population. Drug interactions
Since galantamine is metabolized by CYP2D6 and CYP3A4, inhibiting either of these isoenzymes will increase the cholinergic effects of galantamine. Inhibiting these enzymes may lead to adverse effects.
By binding to the allosteric site of the nAChRs, a conformational change occurs which increases the receptors response to acetylcholine. This modulation of the nicotinic cholinergic receptors on cholinergic neurons in turn causes an increase in the amount of acetylcholine released. However, recent studies suggest that Galantamine does not functionally act at human nAChRs α4β2 or α7 as a positive allosteric modulator.Galantamine also works as a weak competitive and reversible cholinesterase inhibitor in all areas of the body. By inhibiting acetylcholinesterase, it increases the concentration and thereby action of acetylcholine in certain parts of the brain. Galantamines effects on nAChRs and complementary acetylcholinesterase inhibition make up a dual mechanism of action. It is hypothesized that this action might relieve some of the symptoms of Alzheimers. Galantamine in its pure form is a white powder. The atomic resolution 3D structure of the complex of galantamine and its target, acetylcholinesterase, was determined by X-ray crystallography in 1999 (PDB code: 1DX6; see complex). There is no evidence that galantamine alters the course of the underlying dementing process. Pharmacokinetics
Absorption of galantamine is rapid and complete and shows linear pharmacokinetics. It is well absorbed with absolute oral bioavailability between 80 and 100%. It has a terminal elimination half-life of seven hours. Peak effect of inhibiting acetylcholinesterase was achieved about one hour after a single oral dose of 8 mg in some healthy volunteers.