Evaluation of Tc-labeled modified serum albumin for tumor detection

Serum albumin (SA) modified and labeled with ¹³¹l-tyramine N-1′-desoxysorbitol (¹³¹I-TDS) has been shown to localize in tumors Sinn et al., (1990) Nucl. Med. Biol. Part B 17, 819–827]. We prepared similar TDS complexes labeled with ^99mTc and evaluated their potential for tumor imaging. Derivatization of SA with TDS was optimized using cyanuric chloride or 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDAC) as coupling agents. A high TDS loading yield of 38 mol/mol SA was obtained with the latter reagent. Modified SA (8 and 38 mol TDS/mol SA) were labeled with ^99mTc via the stannous reduction method and injected i.v. into EMT-6 tumor bearing mice. ¹²⁵I-TDS-SA (8 mol ¹²⁵I-TDS/mol SA) revealed a high tumor uptake of 10% ID/g at 3 h post-injection. The ^99mTc-labeled SA and TDS-SA complexes lacked tumor specificity, instead TDS loading of SA resulted in increased liver/spleen uptake, suggesting colloid formation. This study confirms the potential of modified SA for tumor imaging but highlights the importance of choice of radioisotope, as well as site of attachment of the radiolabel to the modified SA for optimal tumor localization.