Please comments with reference to Levofloxacin (a zwitterion at physiologic pH).
It is shown in literature that levofloxacin is 100 % bioavailable by oral absorption. Please let me know the site of absorption, means whether absorbed in stomach or intestine.
Chemically it is easy to think that.
But, I think that the drug is more absorbed in the small intestine because small intestine has a larger surface area than stomach.
Most drugs are salts of either weak acids or weak bases. They are absorbed in the undissociated, lipid soluble, form. The dissociation of weak acids is suppressed by low (acid) pH. Thus, theoretically, one would expect a weak acid to be absorbed, primarily in the stomach which has a low pH. However, the amount absorbed in the stomach, depends on how long the drug remains in the stomach. If it passes into the small intestine, even though the pH is higher, it will be absorbed there, particularly as the transit time is much longer, and there is a much greater absorbing area.
Absorbtion mainly occurs in intestine due to its large surface. A weak acid drug molecule is prone to be non-ionised in stomach pH. Therefore, drug molecules that come to intestine to be absorbed are increased. Because, non-ionised form of drugs is well-absorbed (amount and rate) in intestine.
Levofloxacin is a weak acid, has a pKa of 5.4, this would mean that for every dissocited molecule in the stomach of pH 1 there would be 5.4 x 100000 molecules undissociated. Therefore one would expect Levofloxain to be readily absorbed in the stomach. The bioavailability is about 98% and therefore oral adminstartion is almost as effective as i.v.
Dear Anthony
How can we justify absorption of levofloxacin in intestine?
I agree with Anthony Milton. Drug absorption is not only determined by the percentage of unionized form. Drug absorption is influenced by surface area of GIT. Intestines has larger surface area compared to that of stomach. Thus, most drugs have larger absorption in intestines
The absorption governed by multiple ways out of which the mostly commonly understood are active transport, passive transport, carrier mediated transport, Phagocytosis and pinocytosis. so it is very hard to say by which form the drug gets absorb in stomach or which form of drug gets absorb fastly or slowly. All the phenomenon of absorption works in accordance with each other. Surface area, Solubalization, pH of GI track, peristaltic moment, Food effect and many more. So, one has to do the extensive study to know which drugs absorbs at which stage.
Most of the drugs are better absorbed in their unionized form to have more bioavailability. It is a moderately weak acid which is better absorbed from the stomach. It is almost 99.99% unionized so it is better absorbed from the stomach.
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