Heteronuclear Gd- ^99mTc 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(H ₂O)[(M(H ₂O)(CO) ₃) ₂(1)]} {1 = DTPA-bis(histidyl-amide); M = Re (3a); ^99mTc (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 ^99mTc 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.