A radioconjugate composed of PU-AD, a synthetic purine-scaffold inhibitor of the molecular chaperone heat shock protein 90 (Hsp90), conjugated to the radioisotope iodine I 124, with potential imaging activity upon positron emission tomography (PET). Upon administration of iodine I 124 PU-AD, the PU-AD moiety selectively binds to cancer cells expressing stress-induced Hsp90 (stress Hsp90). Upon PET, Hsp90-overexpressing cancer cells can be visualized and patients who may benefit from PU-AD therapy can be identified. This radioconjugate may also be used to determine the pharmacokinetics of the therapeutic agent PU-AD. PU-AD is able to cross the blood-brain-barrier (BBB) and specifically targets stress Hsp90, as seen in certain conditions, such as cancer and neurodegenerative diseases, while normal housekeeping Hsp90 complexes are not targeted by PU-AD at dose levels administered for imaging. Additionally, housekeeping Hsp90 complexes are only targeted at doses that are much larger than that are needed to exert an anticancer effect. Check for active clinical trials using this agent.