This study was done, perhaps, because a Canadian study found in September 2022 that PBT2 does not chelate Cu from amyloid (Alzheimer's Drug PBT2 Interacts with the Amyloid β 1-42 Peptide Differently than Other 8-Hydroxyquinoline Chelating Drugs).

PBT2 is generally regarded as a zinc and iron chelator but as a general metal chelator, it should work also as a copper chelator. This new study below concludes that PBT2 is capable of accessing the Cu amyloid complex, also important in AD.

I am sure that Masters knows both of these studies and evidently is not worried about his PBT2 license.

Int J Mol Sci

. 2023 May 25;24(11):9267.

doi: 10.3390/ijms24119267.

Chelator PBT2 Forms a Ternary Cu²⁺ Complex with β-Amyloid That Has High Stability but Low Specificity

Simon C Drew ^{1 2}

Affiliations expand

• PMID: 37298218

DOI: 10.3390/ijms24119267

Abstract

The metal chelator PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) acts as a terdentate ligand capable of forming binary and ternary Cu²⁺ complexes. It was clinically trialed as an Alzheimer's disease (AD) therapy but failed to progress beyond phase II. The β-amyloid (Aβ) peptide associated with AD was recently concluded to form a unique Cu(Aβ) complex that is inaccessible to PBT2. Herein, it is shown that the species ascribed to this binary Cu(Aβ) complex in fact corresponds to ternary Cu(PBT2)N_Im^Aβ complexes formed by the anchoring of Cu(PBT2) on imine nitrogen (N_Im) donors of His side chains. The primary site of ternary complex formation is His6, with a conditional stepwise formation constant at pH 7.4 (Kc [M^-1]) of logKc = 6.4 ± 0.1, and a second site is supplied by His13 or His14 (logKc = 4.4 ± 0.1). The stability of Cu(PBT2)N_Im^H13/14 is comparable with that of the simplest Cu(PBT2)N_Im complexes involving the N_Im coordination of free imidazole (logKc = 4.22 ± 0.09) and histamine (logKc = 4.00 ± 0.05). The 100-fold larger formation constant for Cu(PBT2)N_Im^H6 indicates that outer-sphere ligand-peptide interactions strongly stabilize its structure. Despite the relatively high stability of Cu(PBT2)N_Im^H6, PBT2 is a promiscuous chelator capable of forming a ternary Cu(PBT2)N_Im complex with any ligand containing an N_Im donor. These ligands include histamine, L-His, and ubiquitous His side chains of peptides and proteins in the extracellular milieu, whose combined effect should outweigh that of a single Cu(PBT2)N_Im^H6 complex regardless of its stability. We therefore conclude that PBT2 is capable of accessing Cu(Aβ) complexes with high stability but low specificity. The results have implications for future AD therapeutic strategies and understanding the role of PBT2 in the bulk transport of transition metal ions. Given the repurposing of PBT2 as a drug for breaking antibiotic resistance, ternary Cu(PBT2)N_Im and analogous Zn(PBT2)N_Im complexes may be relevant to its antimicrobial properties.