Is Less More? Influence of the Coordination Geometry of Copper(II) Picolinate Chelate Complexes on Metabolic Stability

Abstract

A growing number of copper(II) complexes have been identified as suitable candidates for biomedical applications. Here, we show that the biocompatibility and stability of copper(II) complexes can be tuned by directed ligand design and complex geometry. We demonstrate that azamacrocycle-based chelators that envelope copper(II) in a five-coordinate, distorted trigonal-bipyramidal structure are more chemically inert to redox-mediated structural changes than their six-coordinate, Jahn-Teller-distorted counterparts, as evidenced by electrochemical, crystallographic, electron paramagnetic resonance, and density functional theory studies. We further validated our hypothesis of enhanced inertness in vitro and in vivo by employing Cu-64 radiolabeling of bifunctional analogues appended to a prostate-specific membrane antigen targeting dipeptide. The corresponding Cu-64 complexes were tested for stability in vitro and in vivo, with the five-coordinate system demonstrating the greatest metabolic stability among the studied picolinate complex series.

Figure 1.

Coordination complex structures of select, previously reported, six-coordinate chelators for copper(II).

Figure 2.

Copper complexes of nompa, nompa-ac, and mpatcn ligands.

Figure 3.

Structures of bifunctional, targeting-vector-conjugated chelator structures synthesized in this work.

Figure 4.

Views of the X-ray structures of (A) [Cu(nompa)]⁺, (B) [Cu(nompa-ac)], and (C) [Cu₃(mpatcn)₂] with 50% thermal ellipsoid probability. Hydrogen atoms on carbon atoms, along with the ClO₄⁻ counterion for [Cu(nompa)]⁺ and interstitial water molecules for [Cu₃(mpatcn)₂] are omitted for clarity.

Figure 5.

Two lowest-energy-calculated solution structures for (A) [Cu(nompa)]⁺, (B) [Cu(nompa-ac)], and (C) [Cu(mpatcn)]⁻. dSPy: distorted square-pyramidal. SPy: square-pyramidal. dO: distorted octahedral. Energies relative to the lowest-energy structures are given in parentheses (kcal mol⁻¹). See the Supporting Information for the bond metrics and higher-energy structures.

Figure 6.

Experimental (—) and simulated (- - -) X-band EPR spectra (~10 mM, 50:50 H₂O/ethylene glycol, 77 K) of [Cu(nompa)]⁺ (black, bottom), [Cu(nompa-ac)] (blue, middle), and [Cu(mpatcn)]⁻ (red, top) collected at 77 K.

Figure 7.

Cyclic voltammograms of [Cu(nompa)]⁺ (black, bottom), [Cu(nompa-ac)] (blue, middle), and [Cu(mpatcn)]⁻ (red, top) collected at 100 mV s⁻¹. Conditions: 1 mM copper, 100 mM sodium perchlorate, 25 °C, and pH 5.

Figure 8.

Cell binding to PC-3 PiP and PC-3 flu cell lines for each copper-64 complex formed, indicating that target specificity is maintained for each conjugate.

Figure 9.

Comparative biodistribution 2 h postinjection (p.i.) of four copper-64 complexes analyzed in a PSMA^± tumor model using the PC-3 PiP/ flu cell lines (n = 5–6). While all compounds exhibit target-specific tumor uptake, the off-target uptake in PSMA-nonexpressing tumor tissue as well as the liver is increased for picolinate derivatives. Statistical significance determined using the Holm–Sidak method, with α = 0.05.