DMPS (Unithiol)

Why this drug is interesting

DMPS is one of the two water-soluble analogues built to improve on dimercaprol: "DMPS (unithiol) and DMSA (succimer), dithiol water-soluble analogs of BAL, were developed in the Soviet Union and China in the late 1950s."¹ Where succimer became the oral lead chelator, DMPS became the agent most associated with mercury.

It also has something unusual behind it for a drug in this class — a published UK case with an objective before-and-after neurological result, from the West Midlands Poisons Unit.

The UK case

A 36-year-old jewellery producer presented with tremor, slurred speech, lethargy, headache and incoordination. On examination he had "a coarse tremor of the outstretched hands and protruded tongue, slurred speech, 'Hatter's shakes' handwriting, impaired heel-toe walking and heel-shin coordination, mild dysdiadochokinesis, and constricted visual fields to confrontation".²

He "received four 5-day courses of oral 2,3-dimercapto-1-propanesulfonate (30 mg/kg/day), which was associated with substantial objective clinical improvement" and the excretion of 99,406 microg mercury.² The authors' recommendation: DMPS "should be considered in symptomatic patients who have been exposed to mercury vapor and who have supporting analytical confirmation of the diagnosis".²

Pharmacology

Mechanism

Two adjacent thiol groups bind divalent mercury and trivalent arsenic, competing with the enzyme sulfhydryl groups these metals inactivate. The sulfonate group makes the molecule water-soluble, which is the whole design difference from dimercaprol and drives everything that follows: it can be given orally or intravenously rather than as a painful oil-based intramuscular injection, and the chelate is renally excreted.

Timing

Note the tension with the UK case, where chelation of an established, chronic vapour exposure still produced improvement.² Acute salt ingestion and chronic vapour inhalation are different problems with different time courses, and the urgency rule belongs to the former.

Dosing

Intravenous preparations exist and are generally preferred in severe acute poisoning where absorption is unreliable or the patient is vomiting. Course structure — treat, pause, reassess, repeat — reflects the pattern in the case report and the general principle that chelation is given in courses guided by excretion and clinical state rather than continuously to an arbitrary endpoint.

Practical use in the ED

1. Take the occupational history. Jewellery and gold work, dentistry, amalgam recovery, thermometer and barometer breakage, and some artisanal mining are the classic vapour exposures.
2. Distinguish the form of mercury. Elemental vapour (neurological), inorganic salts (corrosive gastrointestinal injury and acute kidney injury) and organic mercury (methylmercury, chronic neurological) behave differently and are not managed identically.
3. Call NPIS on 0344 892 0111 early — for the agent, the dose, and to locate stock of an unlicensed antidote.
4. Send confirmatory blood and urine mercury before or alongside starting treatment, per the authors' own criterion.²
5. Do not use dimercaprol in organic mercury poisoning, and prefer DMPS or DMSA generally, because they do not redistribute metal to the brain.¹
6. Deal with the source. A workshop that produced one poisoned jeweller has probably exposed others, and this becomes an occupational health and environmental problem as much as a clinical one.

Critical appraisal

DMPS sits on a narrow evidence base that is nonetheless the best available for its indication. Kosnett's summary for the whole dithiol class is measured: animal experiments and "in some instances" human data indicate enhanced arsenic and mercury excretion, and controlled animal experiments support a therapeutic role in prompt treatment of acute poisoning by arsenic and inorganic mercury salts.¹ There are no controlled human trials.

This is also the clearest available argument against the chelation offered privately for unmeasured "heavy metal toxicity". The evidence supports prompt chelation of confirmed acute poisoning, and a defensible trial in confirmed symptomatic chronic exposure with analytical support. It does not support chelating people who feel unwell and have not been shown to have a metal in them — and the class's own literature says the outcome benefit in chronic intoxication is unestablished.

References

1. 1
   Kosnett MJ. The role of chelation in the treatment of arsenic and mercury poisoning. Journal of Medical Toxicology 2013;9(4):347–354. PubMed 24178900. Source of the development of DMPS (unithiol) and DMSA as water-soluble BAL analogues in the Soviet Union and China in the late 1950s, the higher therapeutic index and absence of brain redistribution compared with BAL, the within-minutes-to-hours timing requirement, and the statement that therapeutic efficacy in chronic intoxication in terms of morbidity and mortality is largely unestablished. Abstract read 6 September 2026.
2. 2
   Bradberry SM, Sheehan TM, Barraclough CR, Vale JA. DMPS can reverse the features of severe mercury vapor-induced neurological damage. Clinical Toxicology (Philadelphia) 2009;47(9):894–898. PubMed 19852623. West Midlands Poisons Unit. Source of the 36-year-old jewellery producer case, the presenting and examination findings quoted, the regimen of four 5-day courses of oral DMPS 30 mg/kg/day, the 99,406 µg mercury excretion, the "substantial objective clinical improvement" wording and the authors' recommendation requiring analytical confirmation. A single case report — cited as such. Abstract read 6 September 2026.
3. 3
   electronic Medicines Compendium. medicines.org.uk/emc. Searched 6 September 2026 for unithiol, DMPS and the continental brand Dimaval: all three return a "No search results" page. No SmPC is published for DMPS in the UK; the continental European product's label is deliberately not reproduced here. The emc is an industry-hosted compendium of published SmPCs and PILs, not the MHRA register — this establishes that no SmPC is published, rather than that no marketing authorisation exists anywhere.
4. 4
   National Poisons Information Service. TOXBASE · NPIS 0344 892 0111. Required. DMPS is unlicensed in the UK; the indication, agent, route, dose and course structure are NPIS decisions, and NPIS is also the practical route to obtaining the drug.