Heteronuclear Gd-(99m)Tc Complex of DTPA-Bis(histidylamide) Conjugate as a Bimodal MR/SPECT Imaging Probe

The work describes the synthesis and in vivo application of heterotrimetallic complexes of the type {Gd(H2O)[(M(H2O)(CO)3)2(1)]} {1 = DTPA-bis(histidyl-amide); M = Re (3a); (99m)Tc (3b)} for dual modality MR/SPECT imaging. Here, the DTPA-bis(histidylamide) conjugate functions as a trinucleating chelate incorporating Gd in the DTPA core with Re or (99m)Tc in the pair of histidylamide side arms. The two complexes are chemically equivalent as revealed by HPLC, and their "cocktail mixture" (3a + 3b) has demonstrated itself to be essentially a single bimodal imaging probe. The present system has thus overcome the sensitivity difference problem between MRI and SPECT and paved the way for practical applications.