Kit formulation development for 99mTc labeling of a novel single chain antibody fragment

Objectives We have reported a novel rapidly internalizing single chain antibody fragment (scFv) targeting all subtypes of mesothelioma that is promising for targeted imaging and therapy. The objective of the study was to develop a kit formulation for 99mTc labeling.

Methods The scFv was engineered to contain a cystein tag to accommodate the specific conjugation of the chelator,HYNIC, and subsequent 99mTc labeling. The labeling conditions including various co-ligands were optimized to allow instant one-pot quantitative labeling. The labeled product was evaluated in vitro for cell binding capability and stability in phosphate buffered saline (room temperature) and serum at 37 degree C for up to 24 hours. Animal studies in tumor bearing mice were also performed to confirm tumor targeting in vivo.

Results The optimized one-pot formulation provided not only high labeling yield (>95%) but remarkable specific activity (1.8 x 10⁷ Ci/mol). The 99mTc labeled scFv was stable in both phosphate buffered saline and serum for up to 24 hours with 98% and 91% found intact respectively. The in vitro cell study confirmed the labeled scFv maintained the binding capability with a Kd at 27nM. The animal studies in tumor bearing mice showed high tumor uptake at 16.9%ID/g even at 24h post-injection along with rapid blood clearance (0.18%ID/g) and kidney excretion (44%ID/g) providing very high contrast.

Conclusions A kit formulation for 99mTc labeling based on specific HYNIC conjugation and labeling was successfully developed for a novel scFv which is potential to translate into clinical use.

Research Support This work is partially supported by NIH/NCI R01CA 135358-01 and Grant #IRG-97-150-10 from the American Cancer Society