2,3-Diamino propionic acid based chelators for labeling biomolecules with 99mTc(I)

Re(I) and Tc(I) complexes for targeting nitric oxide synthase: influence of the chelator in the affinity for the enzyme

Aiming to design (99m) Tc complexes for probing nitric oxide synthase (NOS) by SPECT, we synthesized conjugates (L4-L6) comprising a NOS-recognizing moiety connected to a diamino-propionic acid (dap) chelating unit. The conjugates led to complexes of the type fac-[M(CO)3 (ĸ(3) -L)] (M = Re/(99m) Tc; Re4/Tc4: L = L4; Re5/Tc5: L = L5; Re6/Tc6: L = L6). Enzymatic studies showed that L4 and L5, but not L6, gave complexes (Re4 and Re5) that are less potent than the conjugates. To rationalize these results, we performed docking and molecular dynamics simulations. The high affinity of L4 and L5 is due to the strong interactions between the dap chelator and polar residues of the binding cavity. These interactions are hampered by metallation resulting in complexes with lower affinity. The higher potency of Re5 compared to Re4 was assigned to the increased bulkiness of Re5 and the presence of additional anchoring groups that better fit the active site and provide more extensive contacts. In turn, Re6 is too bulky and its organometallic tail is oriented toward the peripheral pocket of iNOS, leading to loss of contacts and a lower affinity. These results were compared with our previous results obtained with analogue complexes stabilized by a pyrazolyl-diamine chelating unit.

Orthogonally protected artificial amino acid as tripod ligand for automated peptide synthesis and labeling with [(99m)Tc(OH(2))(3)(CO)(3)](+)

1,2-Diamino-propionic acid (Dap) is a very strong chelator for the [(99m)Tc(CO)(3)](+) core, yielding small and hydrophilic complexes. We prepared the lysine based Dap derivative l-Lys(Dap) in which the ε-NH(2) group was replaced by the tripod through conjugation to its α-carbon. The synthetic strategy produced an orthogonally protected bifunctional chelator (BFC). The -NH(2) group of the α-amino acid portion is Fmoc- and the -NH(2) of Dap are Boc-protected. Fmoc-l-Lys(Dap(Boc)) was either conjugated to the N- and C-terminus of bombesin BBN(7-14) or integrated into the sequence using solid-phase peptide synthesis (SPPS). We also replaced the native lysine in a cyclic RGD peptide with l-Lys(Dap). For all peptides, quantitative labeling with the [(99m)Tc(CO)(3)](+) core at a 10 μM concentration in PBS buffer (pH = 7.4) was achieved. For comparison, the rhenium homologues were prepared from [Re(OH(2))(3)(CO)(3)](+) and Lys(Dap)-BBN(7-14) or cyclo-(RGDyK(Dap)), respectively. Determination of integrin receptor binding showed low to medium nanomolar affinities for various receptor subtypes. The IC(50) of cyclo-(RGDyK(Dap[Re(CO)(3)])) for α(v)β(3) is 7.1 nM as compared to 3.1 nM for nonligated RGD derivative. Biodistribution studies in M21 melanoma bearing nude mice showed reasonable α(v)β(3)-integrin specific tumor uptake. Altogether, orthogonally protected l-Lys(Dap) represents a highly versatile building block for integration in any peptide sequence. Lys(Dap)-precursors allow high-yield (99m)Tc-labeling with [(99m)Tc(OH(2))(3)(CO)(3)](+), forming small and hydrophilic complexes, which in turn leads to peptide radiopharmaceuticals with excellent in vivo characteristics.