9 1994 by Humana Press Inc. All rights of any nature whatsoever reserved. 0163-4992/94/24-25/001-007/$5.40 Linking Radiosiiver to Monoclonal Antibodies Reduced by Ascorbic Acio Comparison of Results with Stable Silver Using Gravimetric Technique and Silver llO-M Using Radiotracer Technique D. K. HAZRA,* V. L. LAHIRI, A. K. GUPTA, N. K. PAINULY,MANISH PATHAK, PANKAJ KtlANNA, R. K. GUPTA, P. KHANNA-HAZRA, AND SHABD SARAN Nuclear Medicine and Ria Unit, Postgraduate Department of Medicine, S.N. Medical College, Agra 282 002 India ABSTRACT Radiosilver-111 and Radiogold-199 were proposed by us (1) as suit- able isotopes for radioimmunotherapy in areas such as India by reason of their suitable half-lives and B-emissions (Ag-111 T,/2 = 7.45 d and Au-199 T1/2 = 3.15 d). Since silver is monovalent, it is difficult to link to conventional bifunctional chelates. We therefore explored the use of sulfur-based linkers (2). Encouraged by the Thakur and De Fulvio Technique (3) of linking technetium to disulfide groups in antibodies reduced by ascorbic acid that is eminently biocompatible, we have explored the linkage of silver to immunoglobulin reduced by ascorbic acid. The linkage of silver was assessed with stable Ag-108 using dial- ysis to quantify the free silver after the reaction of silver and reduced immunoglobulins in various molar ratios (1:1, 1:2, 1:5, 1:10). The silver quantity was estimated gravimetrically after precipitation as chloride. It was observed that using these molar ratios there was negligible silver effiux into the dialysate, suggesting stable linkage. We also assessed the linkage using Ag-110M as radiotracer. The com- parative results with the two techniques are described. Index Entries: Radiosilver-111; radiogold-199; monoclonal anti- bodies; ascorbic acid. *Author to whom all correspondence and reprint requests should be addressed. CeU Biophysics 1 Volumes 24/25, 1994