DRAFT for review - do not cite or quote 1 2 3 4 5 6 7 IPCS EVALUATION OF ANTIDOTES 8 9 IN POISONING BY METALS AND METALLOIDS 10 11 12 13 14 15 16 17 18 Unithiol 19 (2,3-Dimercapto-1-propanesulphonic acid, DMPS) 20 21 22 23 24 25 26 27 28 April 2009 29 30 31 32 33 1. Introduction 34 35 Unithiol (2,3-dimercapto-1-propanesulphonic acid, DMPS) was developed and first 36 used in Russia in the 1950s (Petrunkin, 1956; Klimova, 1958), and later used in China 37 (He et al., 1984). It only became more widely used in America and Western Europe 38 since the mid-1970s (Hruby & Donner, 1987), and particularly since the late 1970s 39 when the Heyl Company in Germany began production (Aposhian, 1982; Aposhian et 40 al., 1984). 41 42 Both unithiol and succimer (2,3-dimercaptosuccinic acid, DMSA) are derivatives of 43 dimercaprol (2,3-dimercapto-1-propanol, British Anti-Lewisite, BAL), and they are 44 replacing dimercaprol as the main antidote used in the management of heavy metal 45 poisoning (Hruby & Donner, 1987; Aposhian et al., 1995; Andersen, 1999). These 46 derivatives have several advantages over dimercaprol including lower toxicity, 47 increased solubility in water and lower lipid solubility. It is due to these properties that 48 they are effective by oral administration (Hruby & Donner, 1987). Succimer is less 49 toxic than unithiol and where these two drugs appear to have similar efficacy as an 50 antidote for a particular metal, succimer is generally preferred. 51 52 Unithiol has been used in the management of acute and chronic poisoning with a 53 number of different metals and metalloids, and is particularly useful for arsenic, 54 bismuth and mercury. Unithiol can be given parenterally or orally depending on the 55 clinical situation and severity of poisoning. It is well tolerated and adverse effects are 56 relatively rare. Most common adverse effects are skin reactions such as rashes, 57 pruritis and blistering which are allergic in origin. Most resolve within a few days and 58 generally no treatment is required, but antihistamines and/or corticosteroids may be 59 given if necessary. 60 61 62 2. Name and chemical formula 63 64 International non-proprietary name: Unithiol 65 66 Synonyms: DMPS, sodium (DL)-2,3-dimercaptopropane-1-sulphonate, sodium 2,3- 67 dimercaptopropanesulphonate 68 69 IUPAC name: Sodium D,L-2,3-dimercapto-1-propanesulphonic acid 70 71 CAS No.: 4076-02-2 72 73 Chemical formula: H2C(SH)-HC(SH)-H2CSO-3Na.H2O 74 75 76 HS SO3H HS 77 2 78 Relative molecular mass: 228.28 (monohydrate) 79 80 Commercial Names: Dimaval® 81 82 Conversion: 1 g = 4.4 mmol 83 1 mmol = 228.3 mg 84 1 g/L = 4.4 mmol/L 85 1 mmol/L = 0.228 g/L 86 87 88 3. Physico-chemical properties 89 90 Physical condition: White crystalline powder 91 0 92 Melting point: 235 C (decomposes) 93 94 Boiling point: Not applicable 95 96 Solubility: Readily soluble in water (350 mg/ml); not readily soluble in 97 ethanol; not soluble in apolar solvents 98 99 Optical properties: Not applicable as the racemate is used 100 101 Acidity: pH 4.5-5.5 of an 1% aqueous solution 102 103 pKa: Not known 104 105 Stability in light: No specific advice with respect to storage is necessary 106 107 Thermal stability: Stable (e.g., aqueous solution may be sterilized and the 108 substance may also be heated for drying) 109 110 Refractive index and 111 specific gravity: Not applicable 112 0 113 Loss of weight on drying: 6-8% when dried to constant weight at 100 C 114 115 116 4. Pharmaceutical formulation and synthesis 117 118 4.1 Routes of Synthesis 119 120 Procedures for the synthesis of unithiol were first described in the 1950s (Johary & 121 Owen, 1955; Petrunkin, 1956). A short description of possible ways of synthesis for 122 unithiol is also given in Hopkins (1981): sodium 2-propanesulphonate is brominated in 123 acetic acid with bromine which gives sodium 2,3-dibromopropanesulphonate. The 124 latter may either be treated with sodium hydrosulphide to give unithiol or may be 125 treated with acetylthiopropane sulphonate which is then hydrolysed with hot aqueous 126 acetic acid thus leading to unithiol. 127 3 128 4.2 Manufacturing Process 129 130 The Heyl company uses a patented manufacturing process which involves 131 precipitation of unithiol as the lead salt, after which unithiol is released by addition of 132 hydrogen sulphide. The unithiol is subsequently recrystallised from alcohol (Ruprecht, 133 1997). 134 135 4.2.1 Parenteral Solution 136 137 The crystalline unithiol is diluted in freshly distilled water suited for injection. The 138 sterile filtered solution is then filled into ampoules. All steps have to be performed in 139 an atmosphere of sterile nitrogen in order to protect the sensitive compound against 140 oxidation. For the same reason only non-metallic working materials should be used. 141 A complex-forming agent such as sodium edetate (1% of the amount of unithiol) may 142 be added to the solution (water for injection) in order to bind ions eventually released 143 from working materials. The ampoules are then sterilized. 144 145 4.2.2 Capsules 146 147 The active compound is thoroughly mixed with the filling aid until homogeneity is 148 achieved. Thereafter the mixture is filled into commercially available hard gelatine 149 capsules. 150 151 4.3 Presentation and formulation 152 153 At an analytical grade unithiol is available from several manufacturers. The 154 pharmaceutical product is available from Heyl Chemisch-pharmazeutische Fabrik 155 GmbH & Co. KG, Berlin, Germany, both for oral and parenteral administration. The 156 sodium salt of a racemic mixture is used for medical purposes. 157 158 Unithiol is available in a pharmaceutical preparation as capsules and a parenteral 159 injection. The capsules contain 100 mg unithiol and the ampoules 250 mg as a 5% 160 solution. The injection can be administered either intravenously or intramuscularly. 161 162 163 5. Analytical methods 164 165 5.1 Quality control procedures for the antidote 166 167 The quality control procedures listed below are oriented towards national and 168 supranational pharmacopoeial standards. Quality control parameters for the antidote 169 include 170 • Identity 171 • Purity 172 • By-products (mainly disulphides) are assayed by high performance liquid 173 chromatography (HPLC) and should not amount to more than 5% of total 174 peak area. 175 • Bromide content: maximum 0.5% (as potassium bromide). 176 • Heavy metals: maximum 20 ppm (as lead). 177 • Loss of weight on drying: 6.0-8.0%. 4 178 • pH of an 1% aqueous solution: 4.5-5.5. 179 • The content may be assayed by iodometric titration. The assay by HPLC is 180 preferred however, because of its specificity. At least 95% is set for 181 requirements. The method has been validated formerly possessing a 182 variation coefficient of 1.5% and a recovery rate of 99.7%. 183 184 The pharmaceutical preparations are additionally controlled for 185 • Ampoules 186 • Sterility. 187 • Filling volume. 188 • Optical appearance (colour, clarity of the solution, particulate matter). 189 • Testing for leaks. 190 191 • Capsules 192 • Uniformity of filling weight. 193 • Disintegration time (maximum 30 minutes in water). 194 • Optical appearance. 195 196 5.2 Methods for identification of the antidote 197 198 Several methods are available to identify the antidote including HPLC, infrared 199 spectroscopy, colour reaction with sodium nitroprusside and flame spectroscopy 200 specifically for the sodium in the molecule. 201 202 5.3 Methods for identification of the antidote in biological samples 203 204 Methods for qualitative and quantitative determination of unithiol and its metabolites 205 are published by Maiorino et al. (1987; 1988; 1991) and Hurlbut et al. (1994). The 206 urine is treated with sodium borohydride and analysed by HPLC with fluorescence 207 detection. 208 209 5.4 Analysis of the toxic agent in biological samples 210 211 Heavy metals should be analysed in blood and urine before, during and after antidotal 212 therapy. Sensitive methods, such as atomic absorption spectroscopy (AAS) or 213 inductively coupled plasma-atomic emission spectroscopy (ICP-AES), can be used 214 (Berman, 1980; Bertram, 1983). 215 216 217 6. Shelf-life 218 219 The shelf life for the commercial available pharmaceutical preparations Dimaval® is 5 220 years for the capsules and 4 years for the ampoules. The expiry date is stated on 221 each package. Although no special advice for storage is given, it is recommended 222 that capsules are stored in a dry place. 223 224 225 7. General properties 226 5 227 Unithiol, like succimer and dimercaprol, owes it metal-binding properties to the 228 presence of two adjacent thiol groups. Unithiol is a water-soluble dithiol, a derivative 229 of dimercaprol and is capable of forming complexes with a number of metals and 230 metalloids. The advantages of unithiol over dimercaprol are: 231 • Lower local and systemic toxicity. 232 • Better solubility in water. 233 • Active by oral administration. 234 235 It should be noted that unithiol is not a true chelating agent; a chelator is a molecule 236 which binds a metal or metalloid ion by at least two functional groups to form a stable 237 ring complex known as a chelate. For mercury, it has been shown that unithiol (and 238 succimer) do not form a true chelate and as such both could be considered 239 suboptimal as metal antidotes (George et al., 2004). However, there are currently 240 no other substances available with the advantages of these two drugs (high water 241 solubility with relatively low toxicity).