Bridging the gap between in vitro and in vivo imaging: Isostructural Re and ^99mTc complexes for correlating fluorescence and radioimaging studies

A bifunctional ligand that is capable of forming Re and ^99mTc complexes as complementary fluorescent and radioactive probes was developed. The tridentate bis(quinoline) amine ligand, which is referred to as the SAACQ system, was prepared in a single step from Fmoc protected lysine in high yield. Reaction of the SAACQ ligand with [Re(CO)₃Br₃]^2- resulted in the formation of the SAACQ-(Re(CO)₃)⁺complex which exhibits favorable fluorescence properties including a long lifetime and a large Stoke's shift. Because the SAACQ ligand is derived from an amino acid, it can readily be linked to or incorporated within peptides as a means of targeting the probe to specific receptors. To demonstrate this feature, the SAACQ ligand and the SAACQ-Re complex were incorporated into fMLFG, a peptide that binds to the formyl peptide receptor (FPR). Uptake of the fMLF[(SAACQ-Re(CO)₃)⁺]G conjugate into human leukocytes in vitro was visualized by fluorescence microscopy, and the observed distribution of the peptide was similar to that of a well-established fluorescent FPR probe. The corresponding Tc complex, fMLF[(SAACQ-^99mTc(CO)₃)⁺]G, was prepared in excellent yield from [^99mTc(CO)₃(OH₂)₃]⁺, which affords the opportunity to correlate the results of the microscopy experiments with in vivo radioimaging studies because the probes are isostructural.