Bifunctional Chelates Optimized for Molecular MRI

Important requirements for exogenous dyes or contrast agents in magnetic resonance imaging (MRI) include an effective concentration of paramagnetic or superparamagnetic ions at the target to be imaged. We report the concise synthesis and characterization of several new enantiopure bifunctional derivatives of (α¹<i>R</i>,α⁴<i>R</i>,α⁷<i>R</i>,α¹⁰<i>R</i>)-α¹,α⁴,α⁷,α¹⁰-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTMA) (and their 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) analogues as controls) that can be covalently attached to a contrast agent delivery system using either click or peptide coupling chemistry. Gd complexes of these derivatives can be attached to delivery systems while maintaining optimal water residence time for increased molecular imaging sensitivity. Long chain biotin (LC-biotin) derivatives of the Eu­(III) and Gd­(III) chelates associated with avidin are used to demonstrate higher efficiencies. Variable-temperature relaxometry, ¹⁷O NMR, and nuclear magnetic resonance dispersion (NMRD) spectroscopy used on the complexes and biotin–avidin adducts measure the influence of water residence time and rotational correlation time on constrained and unconstrained systems. The Gd­(III)-DOTMA derivative has a shorter water residence time than the Gd­(III)-DOTA derivative. Compared to the constrained Gd­(III)-DOTA derivatives, the rotationally constrained Gd­(III)-DOTMA derivative has ∼40% higher relaxivity at 37 °C, which could increase its sensitivity as an MRI agent as well as reduce the dose of the targeting agent