CA9 and cancer: In search of ligands with high selectivityand affinity toward cancer-associated CAIX, our laboratory previouslydesigned, synthesized, and tested over 1000 sulfonamide compounds.One of them, VD11-4-2, demonstrated a very strong binding affinityto CAIX (dissociation constant Kd = 30pM) and remarkable selectivity over other CA isozymes.9 To make our inhibitor suitable for cancer imaging, we synthesizeda compound AZ21-6 that bears an NIR-783 fluorophore attachedto the inhibitor via a 3-mer polyethylene glycol linker chain (markedin pink) via an amine group (marked in blue) (Scheme 1).