Zinc

Wilson’s disease (hepatolenticular degeneration) is an autosomal recessive metabolic defect in hepatic excretion of copper in the bile. Copper accumulation in the liver leads to hepatocellular injury and eventual cirrhosis. When the liver capacity of storing copper is exceeded copper is released into the blood and is taken up in extra hepatic sites, such as the brain, resulting in motor disorders and psychiatric manifestations. Patients may present clinically with predominantly hepatic, neurologic, or psychiatric symptoms.

The active moiety in zinc acetate dihydrate is zinc cation, which blocks the intestinal absorption of copper from the diet and the reabsorption of endogenously secreted copper. Zinc induces the production of metallothionein in the enterocyte, a protein that binds copper thereby preventing its transfer into the blood. The bound copper is then eliminated in the stool following desquamation of the intestinal cells.

Pharmacodynamic investigations of copper metabolism in patients with Wilson’s disease included determinations of net copper balance and radiolabelled copper uptake. A daily regimen of 150 mg of zinc in three administrations was shown to be effective in significantly reducing copper absorption and inducing a negative copper balance.

Since the mechanism of action of zinc is an effect on copper uptake at the level of the intestinal cell, pharmacokinetic evaluations based on blood levels of zinc do not provide useful information on zinc bioavailability at the site of action.

Zinc is absorbed in the small intestine and its absorption kinetics suggest a tendency to saturation at increasing doses. Fractional zinc absorption is negatively correlated with zinc intake. It ranges from 30 to 60% with usual dietary intake (7-15 mg/d) and decreases to 7% with pharmacological doses of 100 mg/d.

In the blood, about 80% of absorbed zinc is distributed to erythrocytes, with most of the remainder being bound to albumin and other plasma proteins. The liver is the main storage for zinc and hepatic zinc levels are increased during maintenance therapy with zinc.

The plasma elimination half-life of zinc in healthy subjects is around 1 hour after a dose of 45 mg. The elimination of zinc results primarily from faecal excretion with relatively little from urine and sweat. The faecal excretion is in the greatest part due to the passage of unabsorbed zinc but it is also due to endogenous intestinal secretion.

Preclinical studies have been conducted with zinc acetate and with other zinc salts. Pharmacological and toxicological data available showed large similarities between zinc salts and among animal species.

The oral LD₅₀ is approximately 300 mg zinc/kg body weight (about 100 to 150 times the human therapeutic dose). Repeat-dose toxicity studies have established that the NOEL (No Observed Effect Level) is about 95 mg zinc/kg body weight (about 48 times the human therapeutic dose).

The weight of evidence, from in vitro and in vivo tests, suggests that zinc has no clinically relevant genotoxic activity.

Reproduction toxicology studies performed with different zinc salts showed no clinically relevant evidence of embryotoxicity, foetotoxicity or teratogenicity.

No conventional carcinogenicity study has been conducted with zinc acetate dihydrate.