Discovery, optimization and biodistribution of an Affibody molecule for imaging of CD69

Due to the wide scale of inflammatory processes in different types of disease, more sensitive and specific biomarkers are required to improve prevention and treatment. Cluster of differentiation 69 (CD69) is one of the earliest cell surface proteins expressed by activated leukocytes. Here we characterize and optimize potential new imaging probes, Affibody molecules targeting CD69 for imaging of activated immune cells. Analysis of candidates isolated in a previously performed selection from a Z variant E. coli library to the recombinant extracellular domain of human CD69, identified one cross-reactive Z variant with affinity to murine and human CD69. Affinity maturation was performed by randomization of the primary Z variant, followed by selections from the library. The resulting Z variants were evaluated for affinity towards human and murine CD69 and thermal stability. The in vivo biodistribution was assessed by SPECT/CT in rats following conjugation of the Z variants by a DOTA chelator and radiolabeling with Indium-111. A primary Z variant with a K_d of approximately 50 nM affinity to human and murine CD69 was identified. Affinity maturation generated 5 additional Z variants with improved or similar affinity. All clones exhibited suitable stability. Radiolabeling and in vivo biodistribution in rat demonstrated rapid renal clearance for all variants, while the background uptake and washout varied. The variant Z_CD69:4 had the highest affinity for human and murine CD69 (34 nM) as well as the lowest in vivo background binding. In summary, we describe the discovery, optimization and evaluation of novel Affibody molecules with affinity for CD69. Affibody molecule Z_CD69:4 is suitable for further development for imaging of activated immune cells.