Mechanism of tumor transport of 99mTc-DL-homocysteine, a possible tumor-imaging agent.

Abstract

The mechanism of transport of ^99mTc-DL-homocysteine (^99mTc-Hcy), a possible tumor-imaging agent, was studied. In the blood of mice at 10 min after intravenous injection of ^99m Tc-Hcy, most of the radioactivity was distributed in the plasma in both the protein-bound and free forms. The protein-bound form was not appreciably dissociated in physiological saline. Further, when the protein-bound form was injected into tumor-bearing mice, the tumor-distributed radioactivity was about twice that of free ^99mTc-Hcy. The binding protein was considered to be albumin from the result of high performance liquid chromatography analysis. An in vitro experiment showed that the tumor uptake of ¹³¹I-albumin was much less than that of ^99mTc-Hcy-albumin. On the other hand, the radioactivity in the tumor cells of ^99mTc-Hcy-injected mice was found to be mostly that of free ^99mTc-Hcy. These results suggest that a portion of the ^99mTc-Hcy ingected was transported to the tumor tissue as an albumin complex in the blood, then released from the albumin and taken up by the cells.