Investigating Structural Property Relationships to Enable Repurposing of Pharmaceuticals as Zinc Ionophores

Abstract

The importance of zinc in biology has gained greater recognition in recent years due to its essential contributions to the function of many endogenous enzymes. Disruption of zinc homeostasis may be useful in treating pathological conditions, such as Alzheimer's, and for antiviral purposes. Despite the growth of knowledge and increased interest in zinc, little is known about the structure and function of zinc ionophores. In this study we analyse the Cambridge Structural Database and solution complexation studies found in the literature to identify key functional groups which may confer zinc ionophorism. Pharmaceuticals, nutraceuticals and amino acids with these functionalities were selected to enable us to explore the translatability of ionophoric activity from in vitro assays to cellular systems. We find that although certain species may complex to zinc in the solid and solution states, and may carry ions across simple membrane systems, this does not necessarily translate into ionophoric activity. We propose that the CSD can help refine key functionalities but that ionophoric activity must be confirmed in cellular systems.

Keywords: CSD analysis; drug repurposing; ionophore; ionophorism; zinc.

Figure 1

The distribution of zinc–ligand distances identified from the CSD, grouped by the elements bound to zinc within the ligand, and a summary of results from the Cambridge Structural Database detailing the number of hits of zinc solid-state complexes, stratified by chemical functionality.

Figure 2

Pharmaceuticals, nutraceuticals and amino acids used in this study.

Figure 3

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and selected quinolines. The black arrow indicates the addition of zinc and the red arrow the addition of API.

Figure 4

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and selected flavonoids. The black arrow indicates the addition of zinc and the red arrow the addition of API.

Figure 5

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and selected polyols. The black arrow indicates the addition of zinc and the red arrow the addition of API.

Figure 6

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and selected amino acids. The black arrow indicates the addition of zinc and the red arrow the addition of API.

Figure 7

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and selected imidazole compounds. The black arrow indicates the addition of zinc and the red arrow the addition of API.

Figure 8

Fluorescence intensity of liposomal FluoZin-3 in PBS (0.1 M, pH = 7.4) before and after the addition of zinc and pyrithione.

Figure 9

Cell viability assay performed using Almar blue after A549 cells were incubated with drug for 3 (left) and 7 (right) days.

Figure 10

Fluorescence images of A549 cells incubated stained with FluoZin-3 AM (green) and counterstained using NucBlue Live for nuclei (blue) after 24 h incubation with (a) basal media and (b) with an additional 10 µM zinc chloride added to the media. Magnification ×10.

Figure 11

Fluorescence images of A549 cells incubated stained with FluoZin-3 AM (green) and counterstained using NucBlue Live for nuclei (blue) after 24 h incubation with: (a) 0.125 µM pyrithione and basal zinc, (b) 0.125 µM pyrithione with an additional 10 µM zinc chloride added to the media and (c) 15 µM pyrithione with an additional 10 µM zinc chloride added to the media. Magnification ×10.

Figure 12

Fluorescence images of A549 cells incubated stained with FluoZin-3 AM (green) and counterstained using NucBlue Live for nuclei (blue) after 24 h incubation with: (a) 2.5 µM clioquinol or HCQ and basal zinc, (b) 2.5 µM clioquinol or HCQ with an additional 10 µM zinc chloride added to the media and (c) 100 µM clioquinol or 300 µM HCQ with an additional 10 µM zinc chloride added to the media. Magnification ×10.

Figure 13

Fluorescence images of A549 cells incubated stained with FluoZin-3 AM (green) and counterstained using NucBlue Live for nuclei (blue) after 24 h incubation with: (a) 2.5 µM histidine or cysteine and basal zinc, (b) 2.5 µM histidine or cysteine and with an additional 10 µM zinc added to the media and (c) 300 µM histidine or cysteine with an additional 10 µM zinc added to the media. Magnification ×10.

Figure 14

Fluorescence images of A549 cells incubated stained with FluoZin-3 AM (green) and counterstained using NucBlue Live for nuclei (blue) after 24 h incubation with: (a) 2.5 µM quercetin and basal zinc, (b) 2.5 µM quercetin with an additional 10 µM zinc chloride added to the media and (c) 100 µM quercetin with an additional 10 µM zinc chloride added to the media. Magnification ×10.

Figure 15

Solid state complex of clioquinol (NABMAF) and 8-hydroxyquinoline (AYOCUN) with zinc (purple). Other elements are coloured as follows: carbon (grey), oxygen (red), nitrogen (blue), hydrogen (off-white), chlorine (green) and iodine (purple).

Figure 16

Solid-state complex of morin-5′-sulfonic acid (ZUJTIJ) and quercetin (ASEROI) with zinc (purple). Other elements are coloured as follows: carbon (grey), oxygen (red), nitrogen (blue), hydrogen (off-white), chlorine (green) and sulphur (yellow).

Figure 17

Solid-state complex of 1,3,4,5-tetrahydroxycyclohexanecarboxylic acid (EBAVEI) and tartaric acid (MUYPUU) with zinc (purple). Other elements are coloured as follows: carbon (grey), oxygen (red) and hydrogen (off-white).

Figure 18

Solid-state complex of ascorbic acid and calcium (CAASCO04) and sodium (NAASCB01). Other elements are coloured as follows: carbon (grey), oxygen (red), hydrogen (off-white), calcium (aquamarine) and sodium (purple).

Figure 19

Zinc–histidine crystalline polymorphs obtained from the CSD. Other elements are coloured as follows: carbon (grey), oxygen (red), nitrogen (blue), hydrogen (off-white) and chlorine (green).

Figure 20

Solid-state complex of benzimidazole (AKUGES) and aciclovir (HOPBUJ) with zinc. Other elements are coloured as follows: carbon (grey), oxygen (red), nitrogen (blue), hydrogen (off-white) and chlorine (green).

Figure 21

Solid-state complex of pyrithione (OXPZND) and 2,2′-dithiobis(pyridine N-oxide) (XAMNIL) with zinc. Other elements are coloured as follows: carbon (grey), oxygen (red), nitrogen (blue), hydrogen (off-white), chlorine (green) and sulphur (yellow).