A paramagnetic MRI-CEST agent responsive to lactate concentration.

A paramagnetic Yb(III) complex bearing six exchangeable amide protons, [Yb(MBDO3AM)](3+), has been investigated with the aim of developing a MRI-CEST (chemical exchange saturation transfer) contrast agent responsive to the concentration of L-lactate. The complex binds the substrate quantitatively to yield [Yb(MBDO3AM)L-lactate](2+). The exchange between the free and the L-lactate-bound complex is slow on the NMR time scale, and the resonances of their corresponding amide protons are sufficiently separated (more than 10 ppm) to allow their selective irradiation. Therefore, the CEST properties of the two forms can be independently assessed. In turn, the resulting saturation transfer to the bulk water signal is dependent on the L-lactate concentration.