H3c

Pulegone Menthofuran

Figure 29.17 • Pulegone in pennyroyal.

of the mint family, Labiatae. The dried leaves and flowering tops of the plant contain from 16% to 30% oil, consisting of the monoterpene pulegone (Fig. 29.17).71 The oil also contains tannins, a- and j-pinenes, other terpenes, long-chain alcohols, piperitenones, and paraffin.

The toxicity of pennyroyal is believed to be a result of the pulegone71 in the oil. Cytochrome P450 catalyzes the metabolism of pulegone to yield the toxic metabolite mentho-furan (Fig. 29.17). Possibly, some of the other terpenes undergo oxidation to active metabolites as well. Menthofuran, metabolites of other terpenes, and pulegone itself deplete hepatic glutathione, resulting in liver failure. This mechanistic hypothesis is supported by the fact that administration of acetylcysteine reverses the toxicity.

Pennyroyal has been used as an abortifacient since the time of Pliny the Elder,72 an insect repellent (the terpenes in the oil have citronellal-like properties), an aid to induce menstruation, and a treatment for the symptoms of premenstrual syndrome. It has also been used as a flea repellent on dogs and cats.

When used as an abortifacient, the drug often causes liver failure and hemorrhage, leading to death. Pennyroyal is sometimes used with black cohosh to accelerate the aborti-facient effect. Coma and death have been reported. Pennyroyal is an example of an herb that has no safe uses. It should not be sold.

Herbal Drugs Used in the Treatment of Cancer

Anticancer drugs derived from biological sources are fairly common and are among the most important in the therapeutic armamentarium. Drugs such as doxorubicin, mitomycin C, mithramycin, and bleomycin have been around for a long time and have shed much light on the treatment of cancer. Three plant-derived drugs that have found their way through clinical trials deserve mention here. Two of the most famous are vincristine and vinblastine (Fig. 29.18). These are compounds isolated from the periwinkle plant Catharanthus roseus. The Vinca alkaloids bind tightly to tubulin in cells and interfere with its normal function in spindle formation. The Vinca alkaloids make the tubulin less stable. The net result is metaphase arrest of cell division. Paclitaxel (Taxol, Fig. 29.18) was originally isolated from the needles or bark of the Pacific yew. Because it occurs in vanishingly small concentration in the plant, a semisynthetic method for its production was developed. Taxol binds to tubulin like the Vinca alkaloids, but it makes the tubulin structure hyper-stable so that it cannot function. Again, the net result is metaphase arrest.

Vincristine

Vinblastine

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