Preparation of 123l and 131l mlBG by ascorbate reduction: A faster, safer method

Full text: Meta-iodobenzylguanidine labelled with 123l or 131l has been prepared in a number of hospital radiopharmacies by the method of van Dormalen and Janssen (Method 1), which uses a copper catalysed exchange reaction accomplished by heating at 140 degree C in a heavy walled glass vial. The product is purified by passage through a small anion exchange column to remove unreacted iodine. More recently, the method of Rossouw (Method 2) has become available which utilizes ascorbic acid and copper (II) ions to effect the iodination at 100 deg C. Since the latter method has a number of potential advantages, the object of this work was to compare the results obtained from each method under conditions of routine preparation. Analysis of preparations using Method 1 gave the following results: 131l, yield 89.3 ± 10.4% (n = 54); 123l, yield 85.5 ± 10.6% (n = 5). The radiochemical purity was determined by both chromatography on Whatman No.1 / butanol, acetic acid, water (60:15:25) and cellulose acetate electrophoresis with the following results (mean values from both techniques): 131l-mlBG 99.8 ± 0.1%;131l- iodine 0.2 ± -0.1%. For 123l, 123l-mlBG 99.2 ± 0.2%; 123l-iodine 0.6 ± 0.2%. When prepared by Method 2, the results were: 131I, yield 86.0 ± 9.8% (n = 5); 123l, yield 97.0 ± 5.0% (n = 6). Radiochemical analyses were as follows: 131l-mlBG 98.9 ± 0.1%;131l-iodine 1.0 ± 0.2%. For 123l, 123l-mlBG 97.8 ± 0.7%; 123l-iodine 2.1 ± 0.6%. It is concluded that although Method 2 resulted in products of slightly lower purity, the small increase in free iodide for both radionuclides can be tolerated in view of the routine use of thyroid blockade with Lugol's solution. This method is much preferred as it removes the risk of explosion during the heating step. It is also considerably faster and reduces the chance of spillage as there is no need for passage through an anion exchange column. Both the 123l and 131l products show equivalent biodistribution in mice to Method 1 and are now used clinically on a routine basis