Synthesis and biological evaluation of tricarbonyl Re(I) and Tc(I) complexes anchored by poly(azolyl)borates: application on the design of radiopharmaceuticals for the targeting of 5-HT1A receptors

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Abstract
The building blocks fac-[99mTc{κ3-HB(timMe)3}(CO)3] and fac-[99mTc{κ3-R(μ-H)B(timMe)2}(CO)3] [R is H (4a), Ph (5a); timMe is 2-mercapto-1-methylimidazolyl] were obtained almost quantitatively by reacting fac-[99mTc(CO)3(H2O)3]+ with the corresponding scorpionate. These compounds cross the intact blood–brain barrier in mice, with significant retention in the case of 4a and 5a. Using 4a as the lead structure, we have synthesized the functionalized complexes fac-[M{κ3-H(μ-H)B(timBu-pip)2}(CO)3] [M is Re (8), 99mTc (8a); timBu-pip is methyl[4-((2-methoxyphenyl)-1-piperazinyl)butyl](2-mercapto-1-methylimidazol-5-yl)methanamide] and fac-[M{κ 3-H(μ-H)B(timMe)(timBu-pip)}(CO)3] [M is Re (9), 99mTc (9a)] and evaluated their potential as radioactive probes for the targeting of brain 5-HT1A serotonergic receptors. The Re complexes exhibit excellent affinity [IC50=0.172 ± 0.003 nM (8); IC50=0.65 ± 0.01 nM (9)] for the 5-HT1A receptor. The radioactive congeners (99mTc) have shown an initial brain uptake of 1.38 ± 0.46%ID g−1 (8a) and 0.43 ± 0.12%ID g−1 (9a), but suffer from a relatively fast washout.

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