SIB-DOTA: A trifunctional prosthetic group potentially amenable for multi-modal labeling that enhances tumor uptake of internalizing monoclonal antibodies

A major drawback of internalizing monoclonal antibodies (mAbs) radioiodinated with direct electrophilic approaches is that tumor retention of radioactivity is compromised by the rapid washout of iodo-tyrosine, the primary labeled catabolite for mAbs labeled via this strategy. In our continuing efforts to develop more versatile residualizing labels that could overcome this problem, we have designed SIB-DOTA, a prosthetic labeling template that combines the features of the prototypical, dehalogenation-resistant N-succinimidyl 3-iodobenzoate (SIB) with DOTA, a useful macrocyclic chelator for labeling with radiometals. Herein we describe the synthesis of the unlabeled standard of this prosthetic moiety, its protected tin precursor, and radioiodinated SIB-DOTA. An anti-EGFRvIII-reactive mAb, L8A4 was radiolabeled with [131I]SIB-DOTA in 27.1 ± 6.2% (n = 2) conjugation yields and its targeting properties to the same mAb labeled with [125I]SGMIB both in vitro and in vivo using U87MG·ΔEGFR cells and xenografts were compared. In vitro paired-label internalization assays showed that the intracellular radioactivity from [131I]SIB-DOTA-L8A4 was 21.4 ± 0.5% and 26.2 ± 1.1% of initially bound radioactivity at 16 and 24 h, respectively. In comparison, these values for [125I]SGMIB-L8A4 were 16.7 ± 0.5% and 14.9 ± 1.1%. Similarly, the SIB-DOTA prosthetic group provided better tumor targeting in vivo than SGMIB over 8 d period. These results suggest that SIB-DOTA warrants further evaluation as a residualizing agent for labeling internalizing mAbs including those targeted to EGFRvIII.

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