On the Biosynthesis of the Phainanoids, A New Class of Immunosuppressive Steroids

            The phainanoids are a recently reported class of oxidatively modified triterpenoids isolated from Phyllanthus hainanensis, a lovely shrub that is indigenous to the Hainan island of China. The architectural framework of the phainanoids includes two rare and intricate spirocyclic motifs. One of those, an eastern spiro-fused benzofurancyclobutanone, is unprecedented amongst naturally occurring steroids. Plants in the genus Phyllanthus have long been used used in traditional Chinese and Ayurvedic medicine for the treatment of infections, diabetes and hepatitis B. The phainanoids were shown to exhibit potent immunosuppressive bioactivity in in vitro experiments that tested the compounds’ abilities to inhibit the proliferation of T and B lymphocytes. New immunosuppressants that are devoid of liver and renal toxicity offer tremendous therapeutic potential as experimental candidates for the treatment of organ transplant and other immunological-associated ailments such as rheumatoid arthritis. Phainanoid F displayed impressive single-digit nanomolar potency against both T and B cells, which far exceeds the immunosuppressive activity of the well-known transplant drug, cyclosporin A.

            While the western A-ring-annulated cyclobutane system is quite unique, particularly amongst steroidal natural products, the right-hand side of the phainanoids shares significant structural homology with a known class of triterpenoids isolated from the Dichapetalaceae family of plants. The dichapetalins are dammarane-type triterpenoids that contain a 2-phenylpyrano system fused to the steroid A-ring. The C17 side chain of dichapetalin M (shown below), comprised of a gamma-butyrolactone-spiroketal, is very similar, in terms of atom connectivity, to the eastern substructure of the six newly identified phainanoid natural products. This leads one to postulate about a potential biogenic relationship between the two triterpenoid structural classes. Indeed, in 2008, five highly complex dichapetalin-type triterpenoids, coined the ‘acutissimatriterpenes,’ were isolated from a plant of the genus Phyllanthus (The phainanoids were also isolated from a Phyllanthus species). The dichapetalins were shown to exhibit cytotoxic activity against several cancer cell lines, but no immunosuppressive activity was reported.

            The eastern gamma-butyrolactone-spiroketal moiety of the phainanoids and dichapetalins is likely derived from enzymatic oxidation of several positions along the prototypical dammarane/cholestane C17 side chain. Oxidation of the C20 substituent to an ester sets up a hypothetical spiroketalization/lactonization event (shown below) that ultimately furnishes this ornate spirocyclic structural motif.

            The biosynthetic origin of the novel western spiro-fused benzofurancyclobutanone is less obvious. However, given the high degree of structural similarity between dichapetalin M and the phainanoid class of triterpenoids, along with their common occurrence in Phyllanthus species of flowering plants, it is tempting to propose dichapetalin M as a biogenetic precursor to the phainanoids. Hypothetically speaking, oxidation of the ortho-position of the eastern phenyl ring of dichapetalin M could produce the benzofuran moiety of the phainanoids. Subsequently, an allylic radical adjacent to the benzofuran heteroatom could induce fragmentation of the carbon-oxygen bond of the pyran system, resulting in a ring contraction to fashion the unique phainanoid cyclobutane. It is noteworthy that all of the carbon atoms of the phainanoid benzofurancyclobutanone are contained within the 2-phenylpyrano substructure of the dichapetalins. Installation of a single oxygen atom, along with the aforementioned radical-induced ring contraction and C-O bond fragmentation (shown below) seems like a plausible biosynthetic pathway leading to the new triterpenoid class of immunosuppressive compounds. Hopefully, future biosynthetic studies will be conducted to reveal the precise nature of the biogenetic origin of the phainanoids.