Forinstance, 18F-VM4-037, a small-molecule radiotracer developedon the sulfonamide pharmacophore, a derivative of the CA ligand ethoxzolamide,failed to localize the ccRCC tumor as it had a high background inthe kidney and liver.29 This was partiallydue to ethoxzolamide-limited selectivity for CAIX over other CA isozymes.The study with a 99mTc-labeled acetazolamide derivative, 99mTc-PHC-102, showed encouraging results in a pilot studyusing a SPECT/CT scanner for identifying substantial tumor and metastasisuptake. Here, CA9 is linked to nonpapillary renal cell carcinoma.