Indolocarbazole

Indolocarbazoles (ICZs) are a class of compounds that are under current study due to their potential as anti-cancer drugs and the prospective number of derivatives and uses found from the basic backbone alone. First isolated in 1977, a wide range of structures and derivatives have been found or developed throughout the world. Due to the extensive number of structures available, this review will focus on the more important groups here while covering their occurrence, biological activity, biosynthesis, and laboratory synthesis.

Indolocarbazoles belong to the alkaloid sub-class of bisindoles. The most frequently isolated indolocarbazoles are Indolo(2,3-a)carbazoles; the most common subgroup of the Indolo(2,3-a)carbazoles are the Indolo(2,3-a)pyrrole(3,4-c)carbazoles. These can be divided into two major classes - halogenated (chlorinated) with a fully oxidized C-7 carbon with only one indole nitrogen containing a β-glycosidic bond and the second class consists of both indole nitrogen glycosylated, non-halogenated, and a fully reduced C-7 carbon.

The first isolated ICZ, dubbed staurosporine (STA) was in 1977 from a culture of Streptomyces staurosporeus found in a soil sample from Iwate Prefecture, Japan. The proper stereochemistry was not proven until 1994. Over the course of the next decade, further study of the compound showed some fungi inhibition, hypotensive activity, and most importantly, a broad protein kinase inhibitor. The next landmark discovery came with the detection of rebeccamycin (REB) in a sample of Lechevalieria aerocolonigenes, again in soil, but this time in a sample from Panama. REB was found to act against leukemia and melanoma in mice, and also against human adenocarcinoma cells.

Since 1977, ICZs have been discovered all over the world in actinomycetes, bacteria commonly found in soil. Numerous forms have tested positive for anti-tumor activity, such as 7-hydroxy-STA and 7-oxo-STA2. Some of the strains from which ICZ compounds have been found are Actinomadura melliaura in Bristol Cove, California, Streptomyces hygroscopicus in Numazu Prefecture, Japan, Micromonospora sp. L-31-CLO-002 from Fuerteventura Island, Canary Islands, Spain, and Actinomadura sp. Strain 007 from Jiaozhou Bay, China. The wide distribution of the various strains that produce these compounds is not surprising due to the number of properties these compounds can take on with limited functionalization on the specie’s part.

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