Synthesis and biological evaluation of EC20:: A new folate-derived, 99mTc-based radiopharmaceutical

A new peptide derivative of folic acid was designed to efficiently coordinate Tc-99m. This new chelate, referred to as EC20, was found to bind cultured folate receptor (FR)-positive tumor cells in both a time- and concentration-dependent manner with very high affinity (K-D similar to 3 nM). Using an in vitro relative affinity assay, EC20 was also found to effectively compete with H-3-folic acid for cell binding when presented either alone or as a formulated metal chelate. Following intravenous injection into Balb/c mice, Tc-99m-EC20 was rapidly removed from circulation (plasma t(1/2) similar to 4 min) and excreted into the urine in a nonmetabolized form. Data from gamma scintigraphic and quantitative biodistribution studies performed in M109 tumor-bearing Balb/c mice confirmed that 99mTc-EC20 predominantly accumulates in FR-positive tumor and kidney tissues. These results suggest that Tc-99m-EC20 may be clinically useful as a noninvasive radiodiagnostic imaging agent for the detection of FR-positive human cancers.