Synthesis and evaluation of 105Rhodium(III) complexes derived from diaminodithioether (DADTE) ligands

Abstract

Seven analogoues of tetradentate acyclic diaminodithioether (DADTE)ligands were synthesized and characterized. R₂SC(R₁)₂)CHnN(CH₂)mNCHn((R₁)₂C)SR₂ 222-gdm (L3): m = 1, n = 1, R₁ = -CH₃ and R₂ = -H; 222-gdm-pmBz (L4): m = 1, n = 1, R₁ =-CH₃, R₂ =-CH₂C₆H₅OCH₃; 232-gdm-pmBz (L7): m = 2, n = 1, R₁ = -CH₃, R₂ =-CH₂ C₆H₅OCH₃, 232-gdm-Met (L8): m = 2, n = 1, R₁ =-CH₃, R₂ =-CH₃, 232-Met (L10) m = 2, n = 1, R₁ = -H, R₂ =-CH₃, 323-Met (L12): m = 1, n = 2, R₁ = -H, R₂ =-CH₃ and 333-Met (L14): m = 2, n =2, R₁ = -H, R₂ = -CH₃). Rh(III) complexes of these DADTE ligands were prepared and the effect of the ligand backbone size on the configuration (cis and/or trans) has been studied. 105Rh radiolabeling studies were performed in order to investigate the optimum criteria for an ideal chelate such as high thermodynamic stability and kinetic inertness. Also, it is desirable that a single isomer with low lipophilicity properties is formed after complexation of the chelate. Complexation of 232-gdm-Met with 105Rh yielded a complex that fulfilled the minimum criteria. Coupling studies of 232-gdm-Met to Bombesin(7-14) analogues, Ahx-Met14-BBN(7-14) and Ahx-NLeu14-BBN(7-14), via an alternative approach have been accomplished. Finally, radiolabeling studies with 232-gdm-Met-Ahx-BBN(7-14) analogues were investigated to achieve an efficient 105Rh labeled BBN based target specific radiopharmaceutical. Structural changes on the DADTE ligand system have been effected to provide a Rh(III)-DADTE complex with optimum properties for an ideal chelate for targeted therapy applications. The alternative approach developed to attach the ligands to peptides yields 105Rh labeled Bombesin(7-14) without needing further functionalization on the ligand.