The appearance of the skin is broadly associated with beauty. Therefore, the desire to have a skin looking healthy and free of spots is the goal of many individuals, especially women [1]. Skin pigmentation disorders consist of an overproduction or uneven distribution of melanin, which results in skin spots that mainly affect mainly the face [2]. Such conditions may arise due to numerous factors, including skin aging, sun exposure [3,4], genetic factors [5], ethnicity [6,7], pregnancy [8], disease [9,10], use of certain medicines [11] and others.

The main options for the treatment of skin hyperpigmentation include topical agents, chemical peels, cryotherapy and laser therapy [4]. The majority of the cosmetics utilized in topical treatment of hyperpigmentation contain in their formulation substances such as arbutin, azelaic acid, hydroquinone and kojic acid, with the last two being the main therapeutic options used for depigmenting [12,13]. These substances work by inhibiting the tyrosinase, an enzyme that catalyses the oxidation of tyrosine [12,13]. Others agents, such as dioic acid (derived from oleic acid), ascorbic acid, retinoic acid and soy extracts, act through different mechanisms (Figure 1) [14].

However, the use of hydroquinone and kojic acid are associated with some adverse reactions that may aggravate the appearance of the spots and damage the health of the user. Also, the systemic and collateral effects of these substances have not been fully elucidated [15,16]. For these reasons, the use of cosmetics that contain hydroquinone is prohibited in the European Union and is strictly controlled in the United States by the Food and Drug Administration (FDA) [17]. Instead, dioic acid has been used to treat hyperpigmentation, with good efficacy, but with similar side effects as hydroquinone [18].

The compounds utilized in the treatment of hyperpigmentation usually act as competitive or noncompetitive inhibitors of tyrosinase and thereby prevent the conversion of tyrosine into 3,4-dihydroxyphenylalanine (L-DOPA), and of L-DOPA into dopaquinone, that occur through hydroxylation and oxidation reactions, respectively. Once they block these reaction steps, they also block melanin synthesis [3,13,18,19].

Resveratrol is a polyphenolic phytoalexin that belongs to the stilbenes, which have demonstrated potent antioxidant activity [20,21,22], and numerous pharmacological activities [23,24,25,26], including tyrosinase inhibitory activity. Their hydroxyl derivatives, including oxyresveratrol, also have the same activities and have shown potent inhibitory effects on tyrosinase activity [13,27]. The substances in this drug class act as competitive inhibitors of tyrosinase, in the presence of tyrosine and L-DOPA. After undergoing the enzymatic reaction, its metabolites act as noncompetitive inhibitors of tyrosinase, considering the substrate L-DOPA [13,28]. Furthermore, an investigation using B16-F10 murine melanoma cells showed cellular melanin production was significantly suppressed by resveratrol without any cytotoxicity up to 200 μM [28].

However, some studies have demonstrated that resveratrol does not inhibit the synthesis of melanin to such a degree that enables it to be utilized alone as skin whitening agent in pharmaceutical formulations, and so its use as a coadjuvant in hyperpigmentation treatments is suggested [13,28].

Due to the potential pharmacology presented by resveratrol and its analogues [29], also referred to as stilbene analogs, our research group in a recent effort has been developing the synthesis and biological evaluation of synthetic resveratrol analogs, particularly the aza-stilbenes, bioisosteres [30] of natural stilbenes, with a view to improving the potential of the natural analogues of resveratrol. Such compounds have shown good antitubercular and antioxidant activity [21,31,32], which encouraged us to test the potential of this class of compounds in other areas, such as depigmenting action. Thus, this paper presents a study of the in vitro inhibitory activity of six aza-stilbenes or azaresveratrols analogs on tyrosinase activity.

All the analogs presented inhibitory concentration to 50% of tyrosinase (IC₅₀) values greater than 50% in screening (qualitative assay). The tyrosinase inhibitory activity results demonstrated that the analyzed azaresveratrol analogs A–F showed a greater capacity to inhibit tyrosinase more than kojic acid (p < 0.05) during the first hour of the qualitative assay. In the second hour of quantitative assay, the analogs D, E and F showed inhibitory ability statistically equal to that of kojic acid (p > 0.05) (Table 1).

Analog D presented the greatest tyrosinase inhibition potency (IC₅₀ = 28.66 μg/mL), followed by analogs A, E, B, F and C, respectively. Furthermore, it showed a more stable IC₅₀ in the quantitative assay compared with kojic acid. The performance of analog D may be associated to its known antioxidant capacity, as demonstrated in a previous study conducted by our group [21]. The presence of a hydroxyl group at the para-position of the aromatic ring thus appears to be critical to good antioxidant activity, as well as tyrosinase inhibitors. This is an expected result since resveratrol has hydroxyl groups in its structure that are directly linked to its antioxidant activity, as described in the literature [33]. This suggests that in the presence of analog D, the formation of reactive oxygen species, responsible for oxidation of L-DOPA to dopaquinone was more difficult [3,18,19].

The lower tyrosinase inhibitory activity demonstrated by analog E compared to analog D, may be due to the insertion of the hydroxyl on the phenolic ring in the ortho-position. Molecules containing a para-hydroxyl (position 4) were more effective than molecules substituted in the ortho- or meta- (positions 1 and 2, respectively) [33,34,35]. Satooka and Kubo [28] described that the presence of a hydroxyl at position C4’ is essential for the inhibition of tyrosinase.

Regarding the analog F, its antioxidant potential assigned to a para-hydroxyl may have been diminished due to the methoxyl group inserted in the meta-position adjacent to the hydroxyl, and it increases the period of induction of oxidation compared to a non-methoxylated control [35,36].

However, Scotti et al. [37] have indicated that rings with ortho- and/or para-hydroxyl group substituents that also have other nitrogen or oxygen-containing substituents retain their antioxidant capacity due to resonance effects between electron pairs and a phenoxy radical, as seen in analog A.

The analog C presents a nitro group at position C4'. This compound may chelate metals and block the action of tyrosinase due to the unshared pair of electrons in its molecular structure that is able to complex with copper [38]. This happens because tyrosinase is a copper-protein enzymatic complex that requires copper ions to promote the redox reactions, essential in production of melanin [39]. The insertion of a disubstituted amine (analog B) on the parent molecule was not an useful modification for the tyrosinase inhibitory activity [20].

Phenolic compounds, such as the analogs tested, form relatively stable intermediates because of the resonance of the aromatic ring present in their structure [20]. This could explain the greater stability of the molecules proposed in this study compared to kojic acid.

Bernard and Berthon [13] determinated that the IC₅₀ for resveratrol was 57.05 µg/mL. Therefore, the analogs A, D and E showed IC₅₀ values lower than resveratrol, which demonstrated the great inhibitory potency of the analogs, superior to the natural compound. These three compounds, like resveratrol that has oxygen-containing groups in its structure, suggest that changes to the basic structure of resveratrol, where the CH grouping was replaced by a nitrogen atom, resulted in an increase in tyrosinase inhibitory effects (Figure 2).

Although only six analogs have been analyzed in this study, it is possible to observe that molecules with polar groups, such as hydroxyl, confer a higher tyrosinase inhibitory activity than analogs with fewer polar groups such as methoxyl and amine, highlighting the importance of a polar substituent on the molecule.

According to the data, para-hydroxylation was the molecular modification that gave the best evaluated tyrosinase inhibitory activity. However, this was lower than that of the reference standard tested. Resveratrol analogs may be important compounds to provide skin whitening and some analogs tested (A, D and E) showed lower IC₅₀ values than resveratrol, the natural compound. Some of these aza-stilbenes were evaluated in recent studies for their antioxidant [21,32] and antituberculous effects [30] and together with the present results, indicate that these molecules may have pharmacological utility in a near future. Studies of the ratio between the in vitro behavior of the new analogs and their in vivo activity may contribute to the development of more effective skin whitening treatments.