Role of serum albumin as a carrier of 99mTc-complex to tumor tissue.

To elucidate a factor required for tumor-imaging 99mTc-labeled radiopharmaceuticals, in vivo behaviors of 99mTc-L-cysteine (99mTc-Cys) and 99mTc-2-mercaptoethylamine (99mTc-ME) were compared with that of 99mTc-DL-homocysteine (99mTc-Hcy) which had been found to accumulate in several experimental tumors. When these three complexes were intravenously injected into mice bearing Ehrlich solid tumor, their tumor affinity was found to depend on their binding ability to serum albumin; 99mTc-Hcy, the albumin-binding ability of which was highest of the three, was the most tumor-tropic. When the albumin-bound complexes of these three were injected, their tumor distributions were enlarged. These results suggest the importance of serum albumin in serving as a carrier for the transport of 99mTc-Hcy-related compounds to tumor tissue.