Efficient oral chelation therapy for mercury toxicity and other heavy metal poisoning is now available with DMSA (Meso-2, 3-dimercaptosuccinic acid) and DMPS (2, 3-dimercapto propane-1-sulfonic acid). These oral chelators are water soluble and can be administered in the tablet form. The sulf-hydryl structures present in their structure is responsible for the property of chelation. These structures bind mercury in the body, leading to its excretion.

Oral chelators were derived from dimercaprol, also known as British anti-Lewisite, (BAL). BAL has been used for chelation therapy in heavy metal poisoning since the 1940s. In 1975, Friedheim demonstrated that DMSA was a better choice than BAL for chelation therapy in mercury toxicity. These oral chelators also have a milder toxicity profile compared to BAL and D-penicillamine. Ever since then, DMSA has been the first choice for oral chelation therapy for mercury poisoning.

In acute mercury toxicity, DMSA is given at a dose of 10 mg for every kilogram body weight divided over three times a day for five days. For example, a 60 kg person would be prescribed 200mg of DMSA, three times a day, for 5 days. Then the frequency of administration is reduced to twice a day for the next fourteen days. Thereafter, oral chelation therapy is guided by blood and 24-hour urine mercury levels. Chelation should be continued until the mercury blood level and the 24-hour urine mercury level falls below 20 microgram per liter.

Comparison of Oral Chelation Therapy with BAL in Mercury Toxicity:

Convenience of administration: Oral chelators are taken in the tablet form, whereas BAL needs to be given as painful injections into the muscles. Oral chelators are especially useful in chronic mercury toxicity (apart from acute toxicity) where long-term chelator therapy is required.

Oral chelators are stable at room temperature for long periods. They retain their chemical structure and function despite exposure to the environment whereas BAL is unstable and very susceptible to oxidation.

Chelation therapy using oral chelators like DMSA does not have any toxic effects on the brain. Some injectable chelators, like BAL for example, redistribute organic mercury from the system to the brain and may worsen the neural function in organic mercury toxicity. On the other hand, an oral chelator like DMSA removes mercury from the brain. Studies done on animals also highlights that oral chelation with DMSA is the most effective chelation therapy, reducing mercury levels up to 66.6% from the brain in organic mercury toxicity.

High Safety Margin: The dose required to produce toxicity using oral chelators is very high compared to the dose required for therapy. This room for error allows oral chelation therapy to be used safety without close monitoring by a physician. On the other hand, chelation therapy with BAL has to be monitored very strictly.

The safety and efficacy of oral chelators has been proven in multiple studies on animals and human beings. Treatment with DMSA results in the greatest urinary excretion of mercury compared to other heavy metal chelators. DMSA is highly effective in removing mercury from the blood, liver, brain, spleen, lungs, large intestine, skeletal muscles and bones.

Adverse effects:

Oral chelators generally do not produce any major side-effects other than stomach upsets, skin rashes. Very rarely chelation therapy may result in a reduction in blood cells and elevate the liver enzymes. However, they can cause deficiency of copper, zinc manganese and molybdenum. These minerals must be supplemented when oral chelation therapy is prescribed. The oral chelator, DMPS may cause asthmatic attacks and a reduction in blood pressure in some patients. Oral chelators remove mercury by excretion in urine. Therefore, they can only be used in people with normal kidney function.

Reference:

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3. Miller AL (1998) Dimercaptosuccinic acid (DMSA), a non-toxic, water-soluble treatment for heavy metal toxicity. Alternative Medicine Review 3, 199-207.

4. Clarkson TW, Magos L, Myers GJ (2003) The Toxicology of Mercury Current Exposures and Clinical Manifestations. New England Journal of Medicine 349, 1731-37.