Podophyllotoxin

Medical uses

Podophyllotoxin possesses a large number of medical applications and can be used as a cathartic, purgative, antiviral agent, vesicant, and antihelminthic. Additionally, podophyllotoxin and its derivatives are leads for anti-tumor agents such as etoposide and teniposide.

Adverse effects

The most common side effects near the application site are skin reactions, including burning, redness, pain, itching, swelling. Application can be immediately followed by burning or itching. Small sores, itching and peeling skin can also follow.

Neither podophyllin resin nor podophyllotoxin lotions or gels are used during pregnancy because these medications can be harmful to the fetus.

Overdose and toxicity

While podophyllin, the herbal extract from which podophyllotoxin is derived, can cause enteritis and potentially fatal CNS depression, podophyllotoxin has been shown to be safe, with minimal toxicity even in large doses.

Mechanism of action

Podophyllotoxin destabilizes microtubules. In contrast, some of its derivatives display binding activity to the enzyme topoisomerase II during the late S and early G2 stage. For instance, etoposide binds and stabilizes the temporary DNA break caused by the enzyme, disrupts the reparation of the break through which the double-stranded DNA passes, and consequently stops DNA unwinding and replication. Mutants resistant to either podophyllotoxin, or to its topoisomerase II inhibitory derivatives such as etoposide (VP-16), have been described in Chinese hamster cells. The mutually exclusive cross-resistance patterns of these mutants provide a highly specific mean to distinguish the two kinds of podophyllotoxin derivatives. Mutant Chinese hamster cells resistant to podophyllotoxin are affected in a protein P1 that was later identified as the mammalian HSP60 or chaperonin protein.

Structural characteristic

The structure of podophyllotoxin was first elucidated in the 1930s. Podophyllotoxin bears four consecutive chiral centers, labelled C-1 through C-4 in the following image. The molecule also contains four almost planar fused rings. The podophyllotoxin molecule includes a number of oxygen containing functional groups: an alcohol, a lactone, three methoxy groups, and an acetal.

Derivatives of podophyllotoxin are synthesized as properties of the rings and carbon 1 through 4 are diversified. For example, ring A is not essential to antimitotic activity. Aromatization of ring C leads to loss of activity, possibly from ring E no longer being placed on the axial position. In addition, the stereochemistry at C-2 and C-3 configures a trans-lactone, which has more activity than the cis counterpart. Chirality at C-1 is also important as it implies an axial position for ring E.

Biosynthesis

The biosynthetic route of podophyllotoxin was not completely eludicidated for many years; however, in September 2015, the identity of the six missing enzymes in podophyllotoxin biosynthesis were reported for the first time. Several prior studies have suggested a common pathway starting from coniferyl alcohol being converted to (+)-pinoresinol in the presence of a one-electron oxidant through dimerization of stereospecific radical intermediate. Pinoresinol is subsequently reduced in the presence of co-factor NADPH to first lariciresinol, and ultimately secoisolariciresinol. Lactonization on secoisolariciresinol gives rise to matairesinol. Secoisolariciresinol is assumed to be converted to yatein through appropriate quinomethane intermediates, leading to podophyllotoxin.

A sequence of enzymes involved has been reported to be dirigent protein (DIR), to convert coniferyl alcohol to (+)-pinocresol, which is converted by pinocresol-lariciresinol reductase (PLR) to (-)-secoisolariciresinol, which is converted by sericoisolariciresinol dehydrogenase (SDH) to (-)-matairesinol, which is converted by CYP719A23 to (-)-pluviatolide, which is likely converted by Phex13114 (OMT1) to (-)-yatein, which is converted by Phex30848 (2-ODD) to (-)-deoxypodophyllotoxin. Though not proceeding through the last step of producing podophyllotoxin itself, a combination of six genes from the mayapple enabled production of the etoposide aglycone in tobacco plants.

Natural abundance

It is present at concentrations of 0.3 to 1.0% by mass in the rhizome of American Mayapple (Podophyllum peltatum). Another common source of podophyllotoxin is the rhizomes of Sinopodophyllum hexandrum Royle (Berberidaceae).

It is biosynthesized from two molecules of coniferyl alcohol by phenolic oxidative coupling and a series of oxidations, reductions and methylations.