Lanthanide Monoporphyrinates and Complexes With a Schiff Base Derivative: Thermodynamic Stability and Bioimaging Capabilities

Clinical applications ranging from magnetic resonance imaging to cancer therapy are more relying on the use of lanthanide coordination compounds. However, the coordination properties based on the electronic configuration of lanthanide ions can be easily sequestered by serum proteins and consequently induce unwanted side effects and/or toxicity if leaked from the bioprobe/prodrug. The knowledge of the thermodynamic and kinetic stability of the complexes used in these applications is therefore essential. To close this gap, we present here the determination of the stability constants via spectrometric method of three lanthanide monoporphyrinates capped with Kläui's ligand, LnL1 (Ln = Gd, Er, Yb), whose stability has not been studied yet, under physiological conditions (37 ^oC, pH 7.5); the monoporphyrinates are found to be more stable than corresponding EDTA chelates, but significantly less stable than corresponding macrocyclic DOTA chelates. In addition, we turn our attention to less stable bimetallic chelates with a Zn-containing Schiff base derivative, LnL3 (Ln= Nd, Gd, Yb, LogK_C = 4.8–5.1), and explore the potentiality of LnL1 and LnL3 complexes as NIR imaging probes.