Biological activity and synthesis of 5,6-dihydroxyindole-2-carboxylic acid - biosynthetic precursor of melanins (microreview).

The microreview considers the biological activity and methods of obtaining natural melanin pigments and their biosynthetic precursor 5,6-dihydroxyindole-2-carboxylic acid. The key methods for the synthesis of 5,6-dihydroxyindole-2-carboxylic acid, published over the past 8 years (2012-2020), are presented. © Springer Science+Business Media, LLC, part of Springer Nature 2021.

Mikhail Aleksandrovich Barabanov was born in 1980 in Sverdlovsk, Russia. In 2007, he defended his PhD thesis in Chemical Sciences, carried out under the guidance of Doctor of Chemical Sciences, Professor V. Ya. Sosnovskikh. Currently, he is a researcher at the Laboratory of Organic Materials at the Postovsky Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences. His research interests: synthesis and biological activity of geroprotectors and endogenous metabolites.

Georgii Sergeevich Martyanov was born in 1998 in Bogdanovich, Russia. He graduated from the Faculty of Chemistry of the Ural Federal University named after the first President of Russia B. N. Yeltsin. At present, he is a research engineer at the Laboratory of Organic Materials at Postovsky Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences. His research interests: synthesis of medications and chemistry of natural substances.

Alexander Viktorovich Pestov was born in 1981 in Sverdlovsk, Russia. In 2007, he defended his PhD thesis in Chemical Sciences and obtained the title of Associate Professor in 2011. At present, he is a senior researcher at the Laboratory of Organic Materials at the Postovsky Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences. His research interests: synthesis and properties of chelating ligands based on functional derivatives of amino acids and amino alcohols.

The properties and uses of melanins

It is known that melanins, natural pigments contained in the human body, are capable of protecting living tissues from UV radiation.[1] On the basis of plant melanins, preparations have been created and patented that have antiviral activity against influenza viruses, herpes simplex virus type 2, HIV-1, and vaccinia virus.[2] In addition, earlier in the USSR, melanin-containing immunomodulators, such as Befungin, were developed to fight cancer, including in patients with stage IV cancer.[3]

Compared with known drugs, natural melanins have a number of advantages as therapeutic agents: a broad spectrum of action, a variety of useful biological activities, low toxicity. However, they also have a number of disadvantages, first of all, the complex process of isolation, as well as the limited solubility in H2O of the pigments themselves and, as a result, low bioavailability. Since melanins are carboxy-containing biopolymers, they must be converted to an anionic form which is better soluble in H2O. Difficulties also arise with the purification of natural melanins. In living organisms, they are present in the form of complexes with proteins from which they must be separated. The thorough cleaning process requires dialysis using large amounts of deionized H2O which increases the cost of the final product.[4]

In turn, the biosynthesis of melanin in mammals begins with phenylalanine which enters the body with food and involves successive stages of oxidation with the participation of enzymes and endogenous H2O2.

At the last stages of melanin biosynthesis, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid

The properties and uses of melanins (continued)

(DHICA) are present, the oxidative combination of which leads to natural eumelanin.[1]

Currently, natural precursors – small molecules from which the body builds its active melanin defense, for example, DHICA – attract the attention of researchers.

Biological activity of DHICA

Due to their low molecular weight and small molecular size, endogenous melanin precursors have advantages over final biopolymers as therapeutic agents. First of all, melanin precursors are distinguished by better penetration into tissues and, as a result, have a more pronounced protective function. Studies on the biological properties of melanin precursors and their analogs began relatively recently, and the analgesic effect of DHICA in mice was soon discovered.[5] Like melanin, DHICA has a cytotoxic effect, in particular on the MT-4 line of tumor cells.[6] Esposito et al. confirmed the previously discovered[7] ability of DHICA to inhibit both RNA-dependent DNA polymerase and HIV-1 integrase.[8] In general, the biological effect of DHICA turned out to be similar to that of melanin and is manifested when it is introduced into the body in smaller quantities than the amount of melanins required for the same effect. However, it is not yet clear whether this is due to the intrinsic activity of DHICA or to the fact that upon entering the cell it is further converted into melanin, which has the corresponding effect. Of the known drugs, the closest structural analog of DHICA is the antiviral drug Arbidol which contains a hydroxyindole carboxylic acid fragment, exhibits a wide spectrum of antiviral activity, and is used in complex therapy for the treatment of SARS coronavirus 2.[9]

DHICA synthesis methods

The classical method for the synthesis of DHICA is the oxidation of 3,4-dihydroxyphenylalanine. Hexacyanoferrates and other compounds are used as an oxidizing agent, while the formed intermediate dopachrome is converted into DHICA by the action of Na2S2O4 [10] or Na2S2O5.[11]

Another method for preparing DHICA is a four-step method involving the Hemetsberger–Knittel indole synthesis.[12] In the first step, the condensation of veratric aldehyde (1) with methyl azidoacetate is carried out. Subsequent thermal generation of nitrene and its cyclization into an indolecarboxylic acid ester followed by hydrolysis gives DHICA methylated at the phenolic hydroxyls. The protective methyl groups are removed with BBr3.[13]

DHICA synthesis methods (continued)

For the synthesis of DHICA, the authors[14] used a route involving the CuI-catalyzed coupling of 2-bromo-4,5-dimethoxybenzaldehyde (2) with ethyl isocyanoacetate which was followed by cyclization leading to an ester of 5,6-dimethoxyindole-2-carboxylic acid. Hydrolysis of the ester group and demethylation using BBr3 afforded the target compound.

To access 5,6-dimethoxyindole-2-carboxylic acid, the key intermediate in the synthesis of DHICA, the classical Fischer indole synthesis is used by rearrangement of [(3,4-dimethoxyphenyl)hydrazono] pyruvic acid ethyl ester into indole-2-carboxylic acid ethyl ester by heating in polyphosphoric acid.[12]

Conclusions

To conclude, the discussed methods for obtaining DHICA are multistep, which makes it urgent to develop new approaches to the synthesis of DHICA in order to increase the availability of chemical precursors for the creation of new medications.