Synthesis and characterization of a novel paramagnetic macromolecular complex [Gd(TTDASQ–protamine)]

Abstract

Adenocarcinomas in rats and humans frequently contain perivascular, degranulating mast cells that release heparin. Protamine is a low-molecular weight, cationic polypeptide that binds to heparin and neutralizes its anticoagulant properties. A novel magnetic resonance imaging (MRI) contrast agent containing protamine was synthesized. TTDASQ, the derivative of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), was also synthesized and the kinetic stability of [Gd(TTDASQ)]⁻ chelate containing phosphate buffer and ZnCl₂ to measure the relaxation rate (R₁) at 20 MHz was studied by transmetallation with Zn(II). The water-exchange rate (k_ex²⁹⁸) of [Gd(TTDASQ)]⁻ is 6.4 × 10⁶ s⁻¹ at 25.0 ± 0.1 °C which was obtained from the reduced ¹⁷O relaxation rates (1/T_1r and 1/T_2r) and chemical shift (ω_r) of H₂¹⁷O, and it is compared with that previously reported for the other gadolinium(III) complex, [Gd(DO3ASQ)]. The binding affinity assay showed that the (TTDASQ)₃–pro₁₉ has higher activity toward heparin. On the other hand, the effect of heparin on the relaxivity of the [Gd(TTDASQ)₃–pro₁₉] conjugate shows the binding strength (K_A) is 7669 dm³ mol⁻¹ at pH 7.4 and the relaxivity (r^b₁) of the [Gd(TTDASQ)₃–pro₁₉]–heparin adduct is 30.9 dm³ mmol⁻¹ s⁻¹.