Bioactive Bismuth Compounds: Is Their Toxicity a Barrier to Therapeutic Use?

Bismuth compounds are considered relatively non-toxic, with their low solubility in aqueous solutions (e.g., biological fluids) being the major contributing factor to this property. Bismuth derivatives are widely used for the treatment of peptic ulcers, functional dyspepsia, and chronic gastritis. Moreover, the properties of bismuth compounds have also been extensively explored in two main fields of action: antimicrobial and anticancer. Despite the clinical interest of bismuth-based drugs, several side effects have also been reported. In fact, excessive acute ingestion of bismuth, or abuse for an extended period of time, can lead to toxicity. However, evidence has demonstrated that the discontinuation of these compounds usually reverses their toxic effects. Notwithstanding, the continuously growing use of bismuth products suggests that it is indeed part of our environment and our daily lives, which urges a more in-depth review and investigation into its possible undesired activities. Therefore, this review aims to update the pharmaco-toxicological properties of bismuth compounds. A special focus will be given to in vitro, in vivo, and clinical studies exploring their toxicity.

Biological Activities of Bismuth Compounds: An Overview of the New Findings and the Old Challenges Not Yet Overcome

Bismuth-based drugs have been used primarily to treat ulcers caused by Helicobacter pylori and other gastrointestinal ailments. Combined with antibiotics, these drugs also possess synergistic activity, making them ideal for multiple therapy regimens and overcoming bacterial resistance. Compounds based on bismuth have a low cost, are safe for human use, and some of them are also effective against tumoral cells, leishmaniasis, fungi, and viruses. However, these compounds have limited bioavailability in physiological environments. As a result, there is a growing interest in developing new bismuth compounds and approaches to overcome this challenge. Considering the beneficial properties of bismuth and the importance of discovering new drugs, this review focused on the last decade's updates involving bismuth compounds, especially those with potent activity and low toxicity, desirable characteristics for developing new drugs. In addition, bismuth-based compounds with dual activity were also highlighted, as well as their modes of action and structure-activity relationship, among other relevant discoveries. In this way, we hope this review provides a fertile ground for rationalizing new bismuth-based drugs.

Antimicrobial and Antibiofilm Activities of Carvacrol, Amoxicillin and Salicylhydroxamic Acid Alone and in Combination vs. Helicobacter pylori: Towards a New Multi-Targeted Therapy

The World Health Organization has indicated Helicobacter pylori as a high-priority pathogen whose infections urgently require an update of the antibacterial treatments pipeline. Recently, bacterial ureases and carbonic anhydrases (CAs) were found to represent valuable pharmacological targets to inhibit bacterial growth. Hence, we explored the underexploited possibility of developing a multiple-targeted anti-H. pylori therapy by assessing the antimicrobial and antibiofilm activities of a CA inhibitor, carvacrol (CAR), amoxicillin (AMX) and a urease inhibitor (SHA), alone and in combination. Minimal Inhibitory (MIC) and Minimal Bactericidal (MBC) Concentrations of their different combinations were evaluated by checkerboard assay and three different methods were employed to assess their capability to eradicate H. pylori biofilm. Through Transmission Electron Microscopy (TEM) analysis, the mechanism of action of the three compounds alone and together was determined. Interestingly, most combinations were found to strongly inhibit H. pylori growth, resulting in an additive FIC index for both CAR-AMX and CAR-SHA associations, while an indifferent value was recorded for the AMX-SHA association. Greater antimicrobial and antibiofilm efficacy of the combinations CAR-AMX, SHA-AMX and CAR-SHA against H. pylori were found with respect to the same compounds used alone, thereby representing an innovative and promising strategy to counteract H. pylori infections.

Medicinal chemistry and biomedical applications of bismuth-based compounds and nanoparticles

Bismuth as a relatively non-toxic and inexpensive metal with exceptional properties has numerous biomedical applications. Bismuth-based compounds are used extensively as medicines for the treatment of gastrointestinal disorders including dyspepsia, gastric ulcers and H. pylori infections. Recently, its medicinal application was further extended to potential treatments of viral infection, multidrug resistant microbial infections, cancer and also imaging, drug delivery and biosensing. In this review we have highlighted the unique chemistry and biological chemistry of bismuth-209 as a prelude to sections covering the unique antibacterial activity of bismuth including a description of research undertaken to date to elucidate key molecular mechanisms of action against H. pylori, the development of novel compounds to treat infection from microbes beyond H. pylori and the significant role bismuth compounds can play as resistance breakers. Furthermore we have provided an account of the potential therapeutic application of bismuth-213 in targeted alpha therapy as well as a summary of the biomedical applications of bismuth-based nanoparticles and composites. Ultimately this review aims to provide the state of the art, highlight the untapped biomedical potential of bismuth and encourage original contributions to this exciting and important field.

Synthesis, Structural and Physicochemical Characterization of a Titanium(IV) Compound with the Hydroxamate Ligand N,2-Dihydroxybenzamide

The siderophore organic ligand N,2-dihydroxybenzamide (H2dihybe) incorporates the hydroxamate group, in addition to the phenoxy group in the ortho-position and reveals a very rich coordination chemistry with potential applications in medicine, materials, and physical sciences. The reaction of H2dihybe with TiCl4 in methyl alcohol and KOH yielded the tetranuclear titanium oxo-cluster (TOC) [TiIV4(μ-O)2(HOCH3)4(μ-Hdihybe)4(Hdihybe)4]Cl4∙10H2O∙12CH3OH (1). The titanium compound was characterized by single-crystal X-ray structure analysis, ESI-MS, 13C, and 1H NMR spectroscopy, solid-state and solution UV-Vis, IR vibrational, and luminescence spectroscopies and molecular orbital calculations. The inorganic core Ti4(μ-O)2 of 1 constitutes a rare structural motif for discrete TiIV4 oxo-clusters. High-resolution ESI-MS studies of 1 in methyl alcohol revealed the presence of isotopic distribution patterns which can be attributed to the tetranuclear clusters containing the inorganic core {Ti4(μ-O)2}. Solid-state IR spectroscopy of 1 showed the presence of an intense band at ~800 cm-1 which is absent in the spectrum of the H2dihybe and was attributed to the high-energy ν(Ti2-μ-O) stretching mode. The ν(C=O) in 1 is red-shifted by ~10 cm-1, while the ν(N-O) is blue-shifted by ~20 cm-1 in comparison to H2dihybe. Density Functional Theory (DFT) calculations reveal that in the experimental and theoretically predicted IR absorbance spectra of the ligand and Ti-complex, the main bands observed in the experimental spectra are also present in the calculated spectra supporting the proposed structural model. 1H and 13C NMR solution (CD3OD) studies of 1 reveal that it retains its integrity in CD3OD. The observed NMR changes upon addition of base to a CD3OD solution of 1, are due to an acid-base equilibrium and not a change in the TiIV coordination environment while the decrease in the complex's lability is due to the improved electron-donating properties which arise from the ligand deprotonation. Luminescence spectroscopic studies of 1 in solution reveal a dual narrow luminescence at different excitation wavelengths. The TOC 1 exhibits a band-gap of 1.98 eV which renders it a promising candidate for photocatalytic investigations.

Structural Insights into New Bi(III) Coordination Polymers with Pyridine-2,3-Dicarboxylic Acid: Photoluminescence Properties and Anti-Helicobacter pylori Activity

Two novel coordination polymers, [Bi2(2,3pydc)2(2,3pydcH)2(H2O)]n (1) and {(Et3NH)2[Bi(2,3pydc)(2,3pydcH)Cl2]}n (2) were prepared using as a prolinker pyridine-2,3-dicarboxylic acid (2,3pydcH2). The obtained complexes were fully characterized by elemental analysis, TG/DTG, FT-IR, solid-state photoluminescence, DFT calculations and single-crystal X-ray diffraction. The obtained complexes crystallized in the triclinic P-1 space group (1) and comprise dimeric units with two crystallographically different Bi(III) centers (polyhedra: distorted pentagonal bipyramid and bicapped trigonal prism) and monoclinic P21/c space group (2) with a distorted monocapped pentagonal bipyramid of Bi(III) center. The various coordination modes of bridging carboxylate ligands are responsible for the formation of 1D chains with 4,5C10 (1) and 2C1 (2) topology. The photoluminescence quantum yield for polymer 2 is 8.36%, which makes it a good candidate for more specific studies towards Bi-based fluorescent materials. Moreover, it was detected that polymer 1 is more than twice as active against H. pylori as polymer 2. It can be concluded that there is an existing relationship between the structure and the antibacterial activity because the presence of chloride and triethylammonium ions in the structure of complex 2 reduces the antibacterial activity.

Water-Soluble Bismuth(III) Polynuclear Tyrosinehydroximate Metallamacrocyclic Complex: Structural Parallels to Lanthanide Metallacrowns

Recently there has been a great deal of interest and associated research into aspects of the coordination chemistry of lanthanides and bismuth-elements that show intriguing common features. This work focuses on the synthesis and characterization of a novel bismuth(III) polynuclear metallamacrocyclic complex derived from aminohydroxamic acid, in order to compare the coordination ability of Bi3+ with the similarly sized La3+ ions. A polynuclear tyrosinehydroximate Bi(OH)[15-MCCu(II)Tyrha-5](NO3)2 (1) was obtained according to the synthetic routes previously described for water-soluble Ln(III)-Cu(II) 15-MC-5 metallacrowns. Correlations between structural parameters of Bi(III) and Ln(III) complexes were analyzed. DFT calculations confirmed the similarity between molecular structures of the model bismuth(III) and lanthanum(III) tyrosinehydroximate 15-metallacrowns-5. Analysis of the electronic structures revealed, however, stronger donor-acceptor interactions between the central ion and the metallamacrocycle in the case of the lanthanum analogue. Thermochromic properties of 1 were studied.

Bismuth(III) Flavonolates: The Impact of Structural Diversity on Antibacterial Activity, Mammalian Cell Viability and Cellular Uptake

A series of homoleptic and heteroleptic bismuth(III) flavonolate complexes derived from six flavonols of varying substitution have been synthesised and structurally characterised. The complexes were evaluated for antibacterial activity towards several problematic Gram-positive (Staphylococcus aureus, methicillin-resistant Staphylococcus aureus (MRSA), and vancomycin-resistant Enterococcus (VRE)) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria. The cell viability of COS-7 (monkey kidney) cells treated with the bismuth flavonolates was also studied to determine the effect of the complexes on mammalian cells. The heteroleptic complexes [BiPh(L)₂ ] (in which L=flavonolate) showed good antibacterial activity towards all of the bacteria but reduced COS-7 cell viability in a concentration-dependent manner. The homoleptic complexes [Bi(L)₃ ] exhibited activity towards the Gram-positive bacteria and showed low toxicity towards the mammalian cell line. Bismuth uptake studies in VRE and COS-7 cells treated with the bismuth flavonolate complexes indicated that Bi accumulation is influenced by both the substitution of the flavonolate ligands and the degree of substitution at the bismuth centre.

Bismuth(III) Thiophosphinates: Understanding How a Small Atomic Change Influences Antibacterial Activity and Mammalian Cell Viability

Diphenylphosphinothioic acid (HSP(=O)Ph2) and diphenylphosphinodithioic acid (HSP(=S)Ph2) have been used to synthesise four BiIII complexes: 1 [Bi(SP(=O)Ph2)3], 2 [BiPh(SP(=O)Ph2)2], 3 [BiPh2(SP(=O)Ph2)], and 4 [Bi(SP(=S)Ph2)3], using BiPh3 and [Bi(OtBu)3] as bismuth sources. The complexes have been characterised by NMR spectroscopy, mass spectrometry, infrared spectroscopy, powder X-ray diffraction, and singe crystal X-ray crystallography (2–4). Biological studies indicated that despite complexes 2 and 3 reducing mammalian cell viability, their antibacterial activity provides a good degree of selectivity towards both Gram positive and Gram negative bacterial strains. The minimum inhibitory concentrations for complexes 2 and 3 are in the range of 0.52–5.5µM towards the bacteria tested. Homoleptic complexes 1 and 4 were generally less active towards both bacterial and mammalian cells.

Bismuth Phosphinates in Bi-Nanocellulose Composites and their Efficacy towards Multi-Drug Resistant Bacteria

A series of poorly soluble phenyl bis-phosphinato bismuth(III) complexes [BiPh(OP(=O)R¹ R² )₂ ] (R¹ =R² =Ph; R¹ =R² =p-OMePh; R¹ =R² =m-NO₂ Ph; R¹ =Ph, R² =H; R¹ =R² =Me) have been synthesised and characterised, and shown to have effective antibacterial activity against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE). The bismuth complexes were incorporated into microfibrillated (nano-) cellulose generating a bismuth-cellulose composite as paper sheets. Antibacterial evaluation indicates that the Bi-cellulose materials have analogous or greater activity against Gram positive bacteria when compared with commercial silver based additives: silver sulfadiazine loaded at 0.43 wt % into nanocellulose produces a 10 mm zone of inhibition on the surface of agar plates containing S. aureus whereas [BiPh(OP(=O)Ph₂ )₂ ] loaded at 0.34 wt % produces an 18 mm zone of inhibition. These phenyl bis-phosphinato bismuth(III) complexes show potential to be applied in materials in healthcare facilities, to inhibit the growth of bacteria capable of causing serious disease.

Do bismuth complexes hold promise as antileishmanial drugs?

Even after 70 years, pentavalent antimonials sodium stibogluconate and meglumine antimoniate remain the most important and cost-effective antileishmanial drugs. However, the drugs cannot be delivered orally and treatment involves intravascular or intramuscular injections for 28 days under strict medical monitoring due to the toxicity of Sb(III). The main alternatives, amphotericin B, pentamidine and miltefosine, are expensive and not without their own problems. Bismuth sits below antimony in the periodic table and is considered to be relatively nontoxic to humans while being capable of providing powerful antimicrobial activity. This review describes recent efforts into developing antileishmanial Bi(III) and Bi(V) drugs, which may resemble Sb analogs in effect and mode-of-action while providing lower mammalian cell toxicity and opportunities of oral delivery. Within the last 10 years, various studies concerning bismuth-based compounds as potential antileishmanial agents have been published. This review seeks to summarize the relevant studies and draw a conclusion as to whether bismuth complexes have the potential to be effective drugs.

Exposure and nephrotoxicity concern of bismuth with the occurrence of autophagy

Metal nanoparticles or metal-based compounds have drawn attention in various fields ranging from industry to medicine because of their unique physicochemical properties. Bismuth (Bi) compounds and nanomaterials have been commonly used in alloys, electronic industry, batteries, and as flame retardants as well as for anti- Helicobacter pylori therapy, while the nanomaterial form has great potential for computed tomography imaging and thermotherapy, both of which will be introduced in this review. Although Bi was used for several decades, there is a lack of detailed information concerning their toxicity and mechanisms on human health. We described the toxicity of Bi on the kidney that seemed to be relatively known by researchers, while the mechanisms remain unclear. Recently, our group has found that Bi compounds, including bismuth nitrate (BN) and Bi nanomaterials, can induce autophagy in kidney cells. We also extended our findings by selecting five Bi compounds, and the results showed that BN, bismuth oxychloride, bismuth citrate, colloidal bismuth subcitrate, and Bi nanomaterials all induced slight cytotoxicity accompanied with autophagy. Although the role of autophagy in Bi-induced cytotoxicity and kidney injury is under investigation by us, autophagy may help with the exploration of the mechanisms of nephrotoxicity by Bi.

Novel class of Bi(iii) hydroxamato complexes: synthesis, urease inhibitory activity and activity against H. pylori

A novel class of bismuth hydroxamato complexes of type Bi(L_-1H)₂·NO₃ (where L is hydroxamic acid) were specifically designed to and demonstrated to inhibit urease, H. pylori's Achilles heel.