Chronic inflammatory processes can be considered an important health problem contributing to the development of multiple diseases such as diabetes, hypertension, arthritis, autoimmune diseases, cancer, Alzheimer's, among others [[1], [2], [3], [4]]. The evolution of these diseases is in part related to the overexpression of the enzyme's cyclooxygenase-2 (COX-2) and 5-Lipoxygenase (5-LOX), which are responsible for metabolizing the substrate arachidonic acid (AA) to prostaglandins (PG) and leukotrienes (LT) mediators of inflammation, respectively [5,6]. In this sense, the pro-inflammatory enzyme 5-LOX has a non-hemic iron atom in the active site, coordinated with amino acids such as His367, His372, His550, Asn554 and C-terminal Ile673, which varies between two oxidation states, as ferrous (Fe(II)-inactive) to the ferric active state (Fe(III)) [7,8].

Currently, different inhibitors have been designed to reduce the inflammatory effects, in addition to preventing the diseases mentioned before [5,[9], [10], [11], [12]]. For 5-LOX enzyme, its inhibitors are associated with different deactivation mechanisms [13], such as redox inhibitors (reducing non-hemic iron to its inactive state Fe(II)), iron chelator (chelating the metal center), non-redox (blocking the active site and competing with the AA substrate) and allosteric inhibitors (binding at the allosteric site of 5-LOX, nearby amino acids Arg101 and Asp166). Some examples of molecules with these mechanisms are nordihydroguaiaretic acid (NDGA) [14,15], zileuton [16], ZD-2138 [17] and AKBA [14,18], respectively. To date, zileuton is the only of these molecules that has been approved by the FDA (“food and drug administration”) for the treatment of a different disease (bronchial asthma) [16], however, it presents problems of hepatotoxicity and poor pharmacokinetics [19].

In this sense, polyphenolic compounds have been tested in different preclinical cellular and animal models, showing anti-inflammatory [20] and antioxidant [21] responses. Some examples of the above are NDGA [15], curcumin [22] and caffeic acid [10,23], which present 5-LOX activity associated with a chelating/redox deactivation mechanism. Additionally, Schiff base derivatives have exhibited different biological properties, such as anticancer [24], antioxidant [25], and anti-inflammatory activity [26]. Recently, Lamie et al. [26] reported the synthesis of a series of N-substituted indole Schiff bases for dual inhibition of the proinflammatory enzymes cyclooxygenase-2 and 5-lipoxygenase, which present a micromolar capacity inhibitory and reduce cytotoxicity in normal human cells.

In recent decades, various research groups have focused their efforts on functionalizing organometallic complexes with bioactive fragments (as part of bioorganometallic chemistry) given their remarkable and diverse therapeutic properties, including anticancer [[27], [28], [29]], antiparasitic [30], antioxidant [31], etc. In this context, the organometallic ferrocenyl fragment possesses outstanding electronic properties (reversible redox couple Fe(III)/Fe(II) ferricinium/ferrocene), synthetic adaptability, low toxicity, stability in air and in aqueous media, as well as great compatibility with biological media, being an interesting alternative to be used in the modification of bioactive molecules [[32], [33], [34]]. Concerning organometallic inhibitors of 5-LOX, our research group has reported the first examples of ferrocenyl derivatives functionalized with catechol fragments (present in the NDGA inhibitor) linked by a thiazole heterocyclic spacer [35], which presented a powerful inhibition against 5-LOX with an IC50 of 0.87 μM, comparable with the commercial drug zileuton (IC50 = 0.92 μM). Through in-silico, in-vitro and electrochemical studies, it was possible to associate this inhibitory potency with a cooperative redox mechanism (reduction of catalytic iron to its inactive Fe(II) state). Additionally, recent studies of ferrocene and ferrocenyl precursors (aldehyde, carboxylic acid, and amine) on the 5-LOX inhibition have demonstrated that only the carboxylic acid complex has a mixed inhibition with an IC50 of 2.5 μM, concluding that both the presence of the organometallic and inhibitory pharmacophore fragments of 5-LOX play a cooperative role in enzyme inhibition [36].

In this contribution, we synthesized ferrocenyl Schiff base complexes with a phenolic moiety (catechol and vanillin) to study their potential application as inhibitors of 5-human lipoxygenase (5 h-LOX). DPPH and FRAP essays, as well as, kinetic studies were used to elucidate the inhibition mechanism, while electrochemical measurements confirmed that the inhibitory activity is due to the complex-enzyme interaction instead of hydrolysis products.