INTRODUCTION:

Thimerosal is an organomercury compound that has been used as an antimicrobial preservative in biological and pharmaceutical preparations since the 1930s¹. It is used as an alternative to benzalkonium chloride and other phenylmercuric preservatives, and has both bacteriostatic and fungistatic activity. “Eli Lilly” a pharmaceutical company gave thiomersal the trade name “Merthiolate” and it has been used as a preservative in vaccines, immunoglobulin preparations, skin test antigens, antivenins, ophthalmic and nasal products, and tattoo inks (Table 1) in different concentrations. Its use as a vaccine preservative is controversial, and it is being phased out from routine childhood vaccines in the United States, Europe and few other countries². Increasing concerns over its safety have, however, led to questions regarding its continued use in formulations.

Table 1: Preservative role of Thimerosal

S. No.	Pharmaceutical Preparation	Concentration Use
1.	IM, IV, SC injection	0.01%
2.	Ophthalmic Solutions	0.001-0.15%
3.	Ophthalmic Suspensions	0.001-0.004%
4.	Otic preparation	0.001-0.01%
5.	Topical Preparations	0.01%

Thiomersal features mercury (II) with a coordination number 2, i.e. two ligands is attached to Hg, the thiolate and the ethyl group (Fig. 1). The carboxylate group is not coordinated, but confers solubility in water. Like other two-coordinate Hg(II) compounds, the coordination geometry is linear, with a 180° S-Hg-C angle³.

Fig. 1: Chemical structure of Thimerosal

PHOTO SENSITIVITY OF THIMEROSAL

Thimerosal is stable at normal temperatures and pressures (NTP) and resulted in discoloration on exposure to light. Aqueous solutions may be sterilized by autoclaving but are sensitive to light. The presence of trace amount of metal like copper increases the rate of oxidation in solutions. Edetic acid or edetates may be used to stabilize solutions but have been reported to reduce the antimicrobial efficacy of Thimerosal solutions. The solid material should be stored in a well-closed container, protected from light, in a cool, dry place.

INCOMPATIBILITIES

Thimerosal is incompatible with aluminum and other metals, strong oxidizing agents, strong acids and bases, sodium chloride solutions⁴, lecithin, phenylmercuric compounds, quaternary ammonium compounds, thioglycolate and proteins. The presence of sodium metabisulfite, edetic acid, and edetates in solutions can reduce the preservative efficacy of thimerosal⁵. In solution form, thimerosal is absorbed by plastic packaging materials, particularly polyethylene. It is strongly adsorbed by treated or untreated rubber caps that are in contact with solutions^{6, 7}. In combination with cyclodextrin, the effectiveness of thimerosal was reduced; however, this was related to the lipid nature of the other ingredients in the preparation⁸.

SAFETY ASPECTS OF THIMEROSAL

Thimerosal is widely used as an antimicrobial preservative in parenteral and topical pharmaceutical formulations. However, the use of thimerosal in pharmaceuticals has increased as a result of a greater awareness of the toxicity of mercury and other associated mercury compounds^9-10. The increasing numbers of reports of adverse reactions, particularly hypersensitivity^11–13 to thimerosal have led to suggestions that it should not be used as a preservative in eye drops¹⁴ or vaccines^15–17. More recent studies assessing the safety of thimerosal in vaccines have, however, suggested that while the risk of hypersensitivity reactions is present, the relative risk of neurological harm in infants is negligible given the quantities of thimerosal present in vaccines^18–20. Regulatory bodies in Europe and the USA have therefore updated their advice on the use of thimerosal in vaccines by stating that while it would be desirable for thimerosal not to be included in vaccines and other formulations the benefits of vaccines far outweigh any risks of adverse effects associated with their use^21-22.

The most frequently reported adverse reaction to thimerosal, particularly in vaccines^23-25 is hypersensitivity, usually with erythema and papular or vesicular eruptions. Although not all thimerosal-sensitive patients develop adverse reactions to vaccines containing thimerosal, there is potential risk. Patch testing in humans and animal experiments has suggested that 0.1% w/v thimerosal can sensitize children²⁶.

Adverse reactions with thimerosal used to preserve contact lens solutions have also been reported. Reactions include ocular redness, irritation, reduced lens tolerance and conjunctivitis^27-29. One estimate suggests that approximately 10% of contact lens wearers may be sensitive to Thimerosal³⁰. Thimerosal has also been associated with false positive reactions to old tuberculin³¹, ototoxicity³² and unusual reaction to aluminum³³ in which a patient suffered a burn at the site of an aluminum foil diathermy electrode after preoperative preparation of the skin with a 0.1% w/v thimerosal solution in ethanol (50%). Research showed that considerable heat was generated (exothermic) when such a solution came into contact with aluminum.

An interaction between orally administered tetracycline and thimerosal, which resulted in varying extents of ocular irritation, has been reported in patients using a contact lens solution preserved with Thimerosal³⁴. Serious adverse effects have been reported after the parenteral and topical use of products containing thimerosal. Five fatal poisonings resulted from the use of 1000 times the normal concentration of thimerosal in a chloramphenicol preparation for intramuscular injection³⁵. Ten out of 13 children died as a result of treatment of umbilical hernia (omphaloceles) with a topical tincture of Thimerosal³⁶. It has therefore been recommended that organic mercurial disinfectants should be restricted or withdrawn from use in hospital since absorption occurs readily through intact membranes. In a case of attempted suicide, a 44-year-old man drank 83 mg/kg of a thimerosal-containing solution. Despite spontaneously vomiting after 15 minutes, gastric lavage and administration of chelating agents on hospital admission, serious symptoms ultimately ended in coma. The patient survived and after 5 months treatment made a full recovery except for sensory defects in two toes³⁷.

CONTROVERSIAL PROSPECT OF THIMEROSAL

Several studies report that there is an association between mercury exposure and an increased risk of heart disease^38-40. In 2001, the Institute of Medicine (IOM) of the US National Academy of Sciences concluded that the hypothesis that exposure to thimerosal-containing vaccines could be associated with neurodevelopment disorders is not established and rests on indirect and incomplete information, primarily from analogies with methylmercury and levels of maximum mercury exposure from vaccines given in children. They concluded that the hypothesis is biologically possible, but the possible relationship between Thimerosal from vaccines and neurodevelopment disorders of autism, attention deficit hyperactivity disorder (ADHD), and speech or language delay remained seriously suspect⁴¹. Since the publication of the IOM report Geier⁴² published the first epidemiological evidence showing a direct association between thimerosal-containing childhood vaccines and neurodevelopment disorders in children. They showed that there was about 2 to 6-fold increased incidence of neurodevelopment disorders following an additional 75-100µg dosage of mercury from thimerosal-containing childhood vaccines in comparison to thimerosal-free childhood vaccines.

The formulations of Hepatitis B (HB) vaccine manufactured during the 1990s in US contained the preservative thimerosal (Th) and this HB vaccine contained 12.5μg ethyl mercury. However in 1999 Food and Drug Administration (FDA) recommendation for the reassessment of Th use in vaccines. By 2001 the majority of pediatric vaccines routinely recommended in the U.S. for children 6 years of age and under were produced without Th, with the exception of multi-dose inactivated influenza and meningococcal polysaccharide vaccines. Dorea et al⁴³. investigated that Th-containing vaccine, including the neonatal HB vaccine; continue to be used routinely in developing countries including India. In a U.S. study performed prior to the removal of Th from HB vaccines, blood mercury levels were significantly elevated in both pre-term and term infants post-HB vaccination⁴⁴. While blood mercury levels are a poor reflection of body-burden of mercury, it is notable that these levels were higher in the pre-term infants when compared with term infants⁴⁴. These findings suggest that newborns, especially pre-term infants, might have decreased ability to eliminate mercury since hepatic metallothionein and glutathione synthesis, both requirements for efficient mercury elimination, are not present in the neonate^45-46. Neurobehavioral tests to assess early neonatal behavioral functioning are therefore commonly used to detect effects of post-natal events or interventions⁴⁷, such as exposure to organomercurials⁴⁸. Macaques have also been used extensively in previous studies of methyl and ethyl mercury toxicokinetics and developmental neurotoxicity^49-54 making them a preferred model for addressing possible neurodevelopmental concerns regarding vaccine safety. There have been several animal studies looking at the effects of thimerosal-containing vaccines (TCVs) and/or Th on neurodevelopment, behavior, immune function, and toxicology^55-58. Burbacher et al.⁵⁰ examined the disposition and distribution of mercury in the brain of cynomolgus macaques administered methyl mercury or TCVs. In their study, cynomolgus macaques received TCVs and were sacrificed at various time points post-vaccination. While their data demonstrated that tissue distribution and clearance rates differed between methyl mercury and Th-exposed infants, the proportion of inorganic mercury in the brain was substantially higher for animals receiving TCVs⁵⁰. Once inorganic mercury has accessed the brain, its half-life is much longer than both ethyl and methyl mercury, and it has the potential to accumulate in cases of prolonged or repeated exposure⁵⁹. In a mouse model, exposure to mercury vapor resulted in a preferential accumulation of mercury in the brainstem, regardless of concentration used⁶⁰. Similarly, after intramuscular injection, inorganic mercury accumulated in brainstem motor nuclei of mice⁶¹. In clinical studies of mercury poisoning, exposure to organic mercury either pre- or post-natally resulted in brainstem defects in children^62-64. Since the acquisition of motor reflexes is controlled by the brainstem, it is possible that very early exposure to ethyl mercury may adversely affect emerging brainstem function⁶⁵. Brainstem injury may then disturb the development or functioning of higher structures^66-67.

CRITICAL FINDINGS

Japan 1955 (Minamata Disease)

Minamata disease affected Minamata Bay of Japan in 1955. Fishing community on bay dominated by Chisso chemical plant was the main center to be affected by Minamata disease. Firstly the Strange illness struck fish, birds and cats, then the human beings. This was eventually a methylmercury poisoning. Widely publicized in U.S. through documentary photography of W. Eugene Smith “Mercury poisoning” and became symbol of industrial pollution and corporate greed. Soon it became a concern in U.S. as studies revealed methylmercury concentrated up the food chain.

Iraq 1972

In this year a worst mercury poisoning incident in history occurred. A homemade bread consumption contaminated by methylmercury fungicide resulted in 6350 hospitalizations and 459 hospital deaths. After this an intensive study was carried out by FDA and specified the acceptable daily intake in adults: 0.4 mcg/kg/day.

Japan and Iraq 1980

In this year congenital methylmercury poisoning was described in Japan and Iraq. This study research concluded that fetus is more sensitive than mother. The syndrome such as cerebral palsy, mental retardation, deafness and blindness are observed in fetus.

Faroe Islands 1990

A prospective study was carried out on Hg Exposure. Pilot whale was used as an animal model and administered with 1-3 dinners/week, containing mercury. The outcome resulted in language and developmental delays in pilot whale.

RECENT FINDINGS

Thimerosal-preserved vaccines are major cause of current chronic-condition ‘autism’ epidemic (Table 2)

Table 2: Recent findings of Thimerosal-preserved vaccines

S. No.	Author	Recent Findings
1.	Kern et al.⁶⁸	A biomarker of mercury body-burden correlated with diagnostic domain specific clinical symptoms of autism spectrum disorder
2.	Majewska et al.⁶⁹	Age-dependent lower or higher levels of hair mercury in autistic children than in healthy controls.
3.	Geier et al.⁷⁰	Blood mercury levels in autism spectrum disorder: Is there a threshold level?
4.	Geier et al.⁷¹	The biological basis of autism spectrum disorders: Understanding causation and treatment by clinical geneticists.
5.	Hewitson et al.⁷²	Influence of pediatric vaccines on amygdala growth and opioid ligand binding in rhesus macaque infants.

CONCLUSION:

The past as well as the present research on the use of Thimerosal as a preservative concluded inadequate results. However, limiting its use in the vaccine as preservative will still remain controversial. Lot of prospects is still pending in the literature to be explored and categories this preservative among the banned drugs.

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Received on 23.09.2011 Accepted on 15.10.2011

Asian J. Pharm. Res. 1(4): Oct. - Dec. 2011; Page 87-90