The objective of this study was to develop an acylation agent for the radioiodination of monoclonal antibodies that would maximize retention of the label in tumor cells following receptor- or antigen-mediated internalization. The strategy taken was to add a polar substituent to the labeled aromatic ring to impede transport of labeled catabolites across lysosomal and cell membranes after antibody degradation. Preparation of unlabeled N-succinimidyl 4-guanidinomethyl-3-iodobenzoate (SGMIB) was achieved in six steps from 3-iodo-4-methylbenzoic acid. Preparation of 4-guanidinomethyl-3-[131I]iodobenzoic acid from the silicon precursor, 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylsilylbenzoic acid proceeded in less than 5% radiochemical yield. A more successful approach was to prepare [131I]SGMIB directly from the tin precursor, N-succinimidyl 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylstannylbenzoate, which was achieved in 60-65% radiochemical yield. A rapidly internalizing anti-epidermal growth factor receptor variant III antibody L8A4 was labeled using [131I]SGMIB in 65% conjugation efficiency and with preservation of immunoreactivity. Paired-label in vitro internalization assays demonstrated that the amount of radioactivity retained in cells after internalization for L8A4 labeled with [131I]SGMIB was 3-4-fold higher than that for L8A4 labeled with 125I using either Iodogen or [125I]SIPC. Catabolite assays documented that the increased retention of radioiodine in tumor cells for antibody labeled using [131I]SGMIB was due to positively charged, low molecular weight species. These results suggest that [131I]SGMIB warrants further evaluation as a reagent for labeling internalizing antibodies.

The aims of this investigation were to develop a no-carrier-added (nca) synthesis of (4-[18F]-fluoro-3-iodobenzyl)guanidine ([18F]FIBG) and to evaluate its potential as an MIBG analogue useful for positron emission tomography. [18F]FIBG was prepared in four steps starting from 4-cyano-2-iodo-N,N,N-trimethylanilinium trifluoromethanesulfonate in 5% decay-corrected radiochemical yield in a total synthesis time of 130 min. The specific activity was more than 1500 Ci per mmol. In vitro binding studies showed that the percent binding of [18F]FIBG to SK-N-SH human neuroblastoma cells remained constant over a 3-log activity range and was similar to that of nca [131I]MIBG. Specific and high uptake of FIBG was also seen in mouse heart and adrenals. The in vitro and in vivo properties of [18F]FIBG suggest that this compound may be a useful positron-emitting analogue of MIBG.

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Two radiolabeled analogues of 6-benzyloxy-9H-purin-2-ylamine (O(6)-benzylguanine; BG) potentially useful in the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase (AGT) were synthesized. Fluorine-18 labeling of the known 6-(4-fluoro-benzyloxy)-9H-purin-2-ylamine (FBG; 6) was accomplished by the condensation of 4-[(18)F]fluorobenzyl alcohol with 2-aminopurin-6-yltrimethylammonium chloride (4) or 2-amino-6-chloropurine in average decay-corrected radiochemical yields of 40 and 25%, respectively. Unlabeled 6-(3-iodo-benzyloxy)-9H-purin-2-ylamine (IBG; 7) was prepared from 4 and 3-iodobenzyl alcohol. Radioiodination of 9, prepared from 7 in two steps, and subsequent deprotection gave [(131)I]7 in about 70% overall radiochemical yield. The IC(50) values for the inactivation of AGT from CHO cells transfected with pCMV-AGT were 15 nM for IBG and 50 nM for FBG. The binding of [(18)F]6 and [(131)I]7 to purified AGT was specific and saturable with both exhibiting similar IC(50) values (5-6 microM).