Biodistribution of a 67Ga-labeled anti-TNF VHH single-domain antibody containing a bacterial albumin-binding domain (Zag)

Introduction: Small domain antibodies (sdAbs) present high potential for both molecular in vivo imaging and therapy. Owing to the low molecular weight they are rapidly cleared from blood circulation, and new strategies to extend their half-lifes are needed for therapeutic applications. We have selected a bacterial albumin-binding domain (ABD) from protein Zag to be fused to an anti-tumor necrosis factor (TNF) single variable-domain heavy-chain region antibody (VHH) to delay blood clearance, and evaluated the biodistribution profile of the fusion protein. Methods: The anti-TNF VHH and the fusion protein VHH-Zag were conjugated to S-2-(4-isothiocyanatobenzyl)1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA). The anti-TNF and albumin-binding properties of the conjugates NOTA-VHH and NOTA-VHH-Zag were assessed by enzyme-linked immunosorbent assay (ELISA). The radioconjugates Ga-67-NOTA-VHH and Ga-67-NOTA-VHH-Zag were obtained by reaction of (GaCl3)-Ga-67 with the corresponding conjugates at room temperature. Biodistribution studies were performed in healthy female CD-1 mice. Results: The immunoreactivity of the VHH-based proteins is preserved upon conjugation to NOTA as well as after radiometallation. The radiochemical purity of the radioconjugates was higher than 95% as determined by ITLC-SG after purification by gel filtration. The biodistribution studies showed that the Zag domain affected the pharmacokinetic properties of VHH, with impressive differences in blood clearance (0.028 +/- 0.004 vs 1.7 +/- 0.8 % I.A./g) and total excretion (97.8 +/- 0.6 vs 25.5 +/- 2.1 % I.A.) for Ga-67-NOTA-VHH and Ga-67-NOTAVHH-Zag, respectively, at 24 h p.i. Conclusion: The Zag domain prolonged the circulation time of VHH by reducing the blood clearance of the labeled fusion protein Ga-67-NOTA-VHH-Zag. In this way, the anti-TNF VHH in fusion with the Zag ABD presents a higher therapeutic potential than the unmodified VHH. (C) 2014 Elsevier Inc. All rights reserved.