Nanotechnology in Medicine. Группа авторов

Chitosan‐alginate nanofibers with 1–3% wt gentamicin significantly enhanced skin regeneration in mice model by stimulating the formation of a thicker dermis, increasing collagen deposition, and increasing the formation of new blood vessels and hair follicles Bakhsheshi‐Rad et al. (2019)

2.4 Safety of Microbial Biopolymers Used in Nanoscale‐Systems for Therapeutic Applications

      Several synthetic polymers are produced from aromatic monomers, mainly bearing a benzene ring. This characteristic often makes them more toxic and less biocompatible, which restrict their applications as biomaterials. The metabolism of benzene (and also some of its derivatives) in mammals and in microorganisms is a subject of study since decades, but its biotransformation and the mechanisms that lead to toxicity are still not fully understood. In mammals and in some microorganisms, the metabolism of benzene and its derivatives occurs through the family of cytochrome P450 (CYP) enzymes, responsible for the insertion of oxygen atoms in these lipophilic compounds aiming at increasing their solubility in aqueous medium (Santos et al. 2017).

In humans, benzene is metabolized mainly in the liver (primary metabolism) and subsequently in the bone marrow (secondary metabolism) (Dougherty et al. 2008; Barata‐Silva et al. 2014). In the first stage of benzene metabolization, the action of a monooxygenase of the cytochrome P450 complex (CYP) is observed, leading to the formation of benzene oxides (epoxy benzene or oxepin), considered reactive. Benzene oxides can covalently bind to proteins and DNA, forming precancerous adducts, or can be metabolized to muconaldehydes and benzoquinones, which are also reactive compounds. The formation of free radicals is also possible, leading to oxidative stress (Figure 2.4) (Chaney and Carlson 1995; Ross 1996; Snyder and Hedli 1996; Monks et al. 2010; Moro et al. 2013; Barata‐Silva et al. 2014).

      In later stages of benzene metabolism, the formation of phenolic compounds such as phenol, catechol, and hydroquinone is observed. The toxicity of these compounds, in particular, of hydroquinones, can be highlighted, since they are precursors of myelotoxic compounds and inhibitors of the ribonucleotide reductase, essential for the DNA biosynthesis. Benzoquinones and benzene triol, formed in the bone marrow, can conjugate with sulfates and glucuronic acids, and are capable of forming adducts with DNA generating mutations favoring carcinogenesis processes (Figure 2.4) (Loureiro et al. 2002; Poirier 2004; Hartwig 2010; Monks et al. 2010; McHale et al. 2012; Snyder 2012; Barata‐Silva et al. 2014).

Figure 2.4 Main pathways of the metabolism of benzene and its derivatives in humans. *MPO, Myeloperoxidase; **NQO1, Quinone oxidoreductase 1 NADPH dependent.

      Sources: Based on Chaney and Carlson (1995), Ross (1996), Monks et al. (2010) and Moro et al. (2013).

The biotransformation reactions of benzene (via muconic acid and phenol) are explained by the need for obtaining water‐soluble compounds for urinary excretion. Human exposure to benzene, in concentrations between 0.1 and 10 ppm, results in urinary metabolite profiles with 70–85% phenol, 5–10% hydroquinone, 5–10% trans, trans‐muconic acid and catechol, and less than 1% S‐phenylmercapturic acid (Qu et al. 2002; Kim et al. 2006; Barata‐Silva et al. 2014). This is a clear indication of the toxicity of oxygenated compounds derived from benzene.

      As for the microbial metabolism of benzene and its derivatives, the anaerobic pathway is a slow process and its biochemical mechanism has not been fully described (Coates et al. 1996). In general, the metabolic pathways of hydrocarbon degradation involve an aerobic metabolism carried out by bacteria, lignin‐degrading fungi (lignolytics), and non‐lignolytic fungi (Jacques et al. 2007).

In some fungi, the metabolism of benzene and its derivatives occurs in a similar way as humans, using the same metabolic reactions (via muconic acid and phenol) (Figure 2.5). However, the phenolic compounds originated here are conjugated with O‐glycosides and O‐glucoronides (Cerniglia 1984). The reactions occurring in any of these phases can be considered as detoxification measures if the products generated are less toxic than the original hydrocarbons (Cerniglia et al. 1985).

Lignolytic fungi have alternative pathways for the degradation of aromatic compounds. This degradation occurs through the action of extracellular enzymes of low specificity, such as lignin peroxidase, manganese peroxidase, and laccase (Leonowicz et al. 1999). In this process, benzene and its derivatives are degraded, with the formation of quinones (Cerniglia 1997). Some lignolytic fungi have the ability to metabolize quinone PAHs by cleaving aromatic rings with subsequent breaking and formation of carbon dioxide (through the Krebs cycle reactions) (Hammel 1995) (Figure 2.5).

Figure 2.5 Aerobic biodegradation pathways of aromatic compounds conducted by bacteria and fungi.

      Source: Based on Cerniglia (1984, 1997). *, Products resulting from reactions of the Krebs cycle.

      Understanding the microbial degradation of benzenes and other aromatic compounds present in synthetic polymers is very important considering that these materials will be disposed, and biodegradation can generate compounds that are even more toxic or that may impact other living organisms of the environment.

      The metabolism of biopolymers such as exopolysaccharides (EPSs) and polyesters does not generate toxic and reactive compounds, as is the case of aromatic/synthetic polymers. The metabolization of an EPS or microbial polyesters leads to the formation of organic acids, most often found in the Krebs cycle (Eggers and Steinbuchel 2013). Both humans and capable microorganisms can degrade biopolymers by well‐known metabolic pathways, such as glycolysis, respiratory chain, Lynen cycle (β‐oxidation of fatty acids), among others. The metabolism of biopolymers is generally complete, leading to carbon dioxide and water, but if it is not, it generates compounds of low environmental toxicity.

2.5 Conclusions

The exploitation of microbial polymers for the design of novel therapeutic systems has evolved radically during the last decades, as they correspond to tunable, nontoxic, biodegradable, and biocompatible materials. Processing such materials in the form of nanofibers and nanoparticles is a very promising strategy that may provide more effective and convenient routes of administration, leading to sophisticated macromolecular materials for pharmaceutical and biomedical purposes. The applications of nanomaterials based on microbial polymers in biomedicine have become a large subject area, with original materials continuously being described in literature. As a consequence, commercial biomedical nanosystems based on exopolysaccharides (mainly pullulan and bacterial cellulose) and polyhydroxyalkanoates (mainly poly[hydroxybutyrate]), are expected to appear more frequently, as they may offer excellent prospects.

References

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      2 Alhariri, M., Azghani, A., and Omri, A. (2013). Liposomal antibiotics for the treatment of infectious diseases. Expert Opinion on Drug Delivery