Chelating Agents in Medicine

Abstract

In recent years there has been a growing awareness of the importance of trace metals in the environment, in their dichotomous role as essential nutritional factors on the one hand, and as toxic agents on the other. The apparent ambivalent nature of metal ions is shown as a continuum in a wide range dose-response curve. The response on the curve, being a function of dosage, covers survival of the organism through a deficiency state to normal health, toxicity, and death. This particular review deals with the toxic effects of this dose-response curve and the use of chelating agents to remove execessive quantities of metal ions which are producing toxic effects. The metals discussed include: (1) Heavy metal ions-lead, mercury, and cadmium; (2) essential trace metals in toxic excessive quantities-chromium, cobalt, copper, iron, manganese, molybdenum, selenium, and zinc; (3) less common toxic metal ions-aluminum, antimony, arsenic, beryllium, nickel, tellurium, thallium, and tin. Radioactive metals are also discussed. Chelation therapy is a form of treatment aimed at removing a metal ion from a molecular site at which it is producing a bio chemical lesion. The in vivo stability of a metal chelate is affected by its stability constant, hydrogen ion concentration (pH), competition for and by other metal ions and ligands, tendency of the metal to form insoluble hydroxides, distribution and metabolism of the chelate, and competition of endogenous biochemicals for complexing the metal ion. Speciation studies greatly assist this assessment. Effective chelation therapy depends upon the selection of an appropriate chelating agent for the controlled removal of an undesirable metal ion. The ligands most commonly used in metal ion detoxification include dimercaprol or BAL, unithiol, penicillamine, polyaminocarboxylic acids (EDTA series), desferrioxamine, and dithizone. Toxicity is generally a function of either the metal chelate or of the ligand removing essential metal ions. Chelation therapy is a safe procedure when properly administered. Toxic reactions are most commonly encountered when the clinician uses excessive dosages too rapidly. Chelation therapy should be administered slowly and the treatments spaced with sufficient time lapsing to permit the body to adjust, and the trace element status of the patient monitored throughout.