The Development of a pH-sensitive contrast agent for NMR1H imaging

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Abstract
Fe(III)meso‐tetra(4‐sulfonatophenyl)porphine (Fe‐TPPS,) has been investigated as a potential pH‐sensitive nuclear magnetic resonance (NMR) proton image contrast agent. Relaxation rates (1/T1, and 1/T2) of water protons were measured as a function of pH and concentration of Fe‐TPPS, in phosphate‐buffered isotonic saline. Transverse relaxation rates (1/T2) did not change appreciably with pH above 6.0. Longitudinal relaxation rates (1/T1) increased significantly between pH 7.75 and 5.75. This effect was more pronounced with the increasing concentration of Fe‐TPPS, from 0.1 to 1. 5 mM and is attributed to a pH‐dependent equilibrium between the high‐spin (S = 5/2) Fe–TPPS4 monomer at low pH and the antiferromagnetically coupled μ‐oxodimer O–(FeTPPS4)2 at high pH. The efficacy of this pH‐dependent contrast agent is demonstrated in vitro. © 1987 Academic Press, Inc.