Antidotes and treatment of poisonings

Drugs used to counteract drug overdosage are considered under the appropriate headings, e.g., physostigmine with atropine; naloxone with opioids; flu-mazenil with benzodiazepines; antibody (Fab fragments) with digitalis; and N-acetyl-cysteine with acetaminophen intoxication.

Chelating agents (A) serve as antidotes in poisoning with heavy metals. They act to complex and, thus, "inactivate" heavy metal ions. Chelates (from Greek: chele = claw [of crayfish]) represent complexes between a metal ion and molecules that carry several binding sites for the metal ion. Because of their high affinity, chelating agents "attract" metal ions present in the organism. The chelates are non-toxic, are excreted predominantly via the kidney, maintain a tight organometallic bond also in the concentrated, usually acidic, milieu of tubular urine and thus promote the elimination of metal ions.

Na2Ca-EDTA is used to treat lead poisoning. This antidote cannot penetrate cell membranes and must be given parenterally. Because of its high binding affinity, the lead ion displaces Ca2+ from its bond. The lead-containing chelate is eliminated renally. Nephrotoxicity predominates among the unwanted effects. Na3Ca-Pentetate is a complex of dieth-ylenetriaminopentaacetic acid (DPTA) and serves as antidote in lead and other metal intoxications.

Dimercaprol (BAL, British Anti-Lewisite) was developed in World War II as an antidote against vesicant organic arsenicals (B). It is able to chelate various metal ions. Dimercaprol forms a liquid, rapidly decomposing substance that is given intramuscularly in an oily vehicle. A related compound, both in terms of structure and activity, is di-mercaptopropanesulfonic acid, whose sodium salt is suitable for oral administration. Shivering, fever, and skin reactions are potential adverse effects.

Deferoxamine derives from the bacterium Streptomyces pilosus. The Lüllmann, Color Atlas of Pharmacology © 2000 Thieme All rights reserved. Usage subject to terms and conditions of license.

substance possesses a very high iron-binding capacity, but does not withdraw iron from hemoglobin or cytochromes. It is poorly absorbed enterally and must be given parenterally to cause increased excretion of iron. Oral administration is indicated only if enteral absorption of iron is to be curtailed. Unwanted effects include allergic reactions. It should be noted that blood letting is the most effective means of removing iron from the body; however, this method is unsuitable for treating conditions of iron overload associated with anemia.

D-penicillamine can promote the elimination of copper (e.g., in Wilson's disease) and of lead ions. It can be given orally. Two additional uses are cystinu-ria and rheumatoid arthritis. In the former, formation of cystine stones in the urinary tract is prevented because the drug can form a disulfide with cysteine that is readily soluble. In the latter, pen-icillamine can be used as a basal regimen (p. 320). The therapeutic effect may result in part from a reaction with aldehydes, whereby polymerization of collagen molecules into fibrils is inhibited. Unwanted effects are: cutaneous damage (diminished resistance to mechanical stress with a tendency to form blisters), nephrotoxicity, bone marrow depression, and taste disturbances.

EDTA: Ethylenediaminetetra-acetate

A. Chelation of lead ions by EDTA

Arsenic, mercury, gold ions

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