Lactulose

Lactulose is a synthetic disaccharide which is metabolised by gastro-intestinal bacterial flora to low molecular weight acids (chiefly lactic and acetic acids). There is no endogenous metabolising enzyme in the human gut.

Its mode of action in constipation is as an osmotic agent producing soft stools.

In the colon lactulose is broken down by colonic bacteria into low molecular organic acids. These acids lead to a lowering of pH in the colonic lumen and via an osmotic effect to an increase of the volume of colonic contents. These effects stimulate peristalsis of the colon and return the consistency of the stool. The constipation is cleared and the physiological rhythm of the colon is reinstated.

In hepatic encephalopathy (HE) the effect has been attributed to suppression of proteolytic bacteria by an increase of acidophilic bacteria (e.g. lactobacillus), trapping of ammonia in the ionic form by acidification of the colonic contents, catharsis due to the low pH in the colon as well as an osmotic effect, and alteration of the bacterial nitrogen metabolism by stimulating the bacteria to utilize ammonia for bacterial protein synthesis.

Lactulose is poorly absorbed from the gastro-intestinal tract and it reaches the colon unchanged.

There are no human lactulose disaccharide enzymes; metabolism of lactulose to lactic acid occurs via gastro-intestinal microbial flora only. Due to its poor bioavailability, plasma lactulose concentrations are negligible.

Metabolism is complete at doses up to 25-50 g or 40-75 ml; at higher dosages, a proportion may be excreted unchanged.

There are no known changes in kinetic properties in patients with organic diseases which may alter drug disposition

The results of acute, sub-chronic and chronic toxicity studies in various species indicate that the compound has very low toxicity. The effects observed, appear to be more related to the effect of bulk in the gastrointestinal tract than to a more specific toxic activity. In reproduction and teratology experiments in rabbits, rats or mice, no adverse effects were found.