"Cancer prevention, inhibition, and regression are the most note worthy attributes of the MTs [Monoterpenes]. D-limonene (DL) and perillyl alcohol (POH) have been shown to be chemopreventive against mammary, liver, lung, UV-induced skin cancer and chemotherapeutic against both experimental mammary and pancreatic tumours. Perillyl alcohol stands out as effective against human pancreatic cancer, colon, liver to reduce vein graft intimal hyperplasia, as chemopreventive against colon carcinogenesis, prostate and lung cancer."


D-limonene is one of the most common terpenes in nature. It is a major constituent in several citrus oils (orange, lemon, mandarin, lime, and grapefruit). D-limonene is listed in the Code of Federal Regulations as generally recognized as safe (GRAS) for a flavoring agent and can be found in common food items such as fruit juices, soft drinks, baked goods, ice cream, and pudding. D-limonene is considered to have fairly low toxicity. It has been tested for carcinogenicity in mice and rats. Although initial results showed d-limonene increased the incidence of renal tubular tumors in male rats, female rats and mice in both genders showed no evidence of any tumor. Subsequent studies have determined how these tumors occur and established that d-limonene does not pose a mutagenic, carcinogenic, or nephrotoxic risk to humans. In humans, d-limonene has demonstrated low toxicity after single and repeated dosing for up to one year. Being an excellent solvent of cholesterol, d-limonene has been used clinically to dissolve cholesterol-containing gallstones. Because of its gastric acid neutralizing effect and its support of normal peristalsis, it has also been used for relief of heartburn. D-limonene has well-established chemopreventive activity against many types of cancers. Evidence from a phase I clinical trial shows a partial response in a patient with breast cancer and stable disease for more than six months in three patients with colorectal cancer.

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London's Charing Cross Hospital has recently begun a Phase I trial of limonene in pancreatic and colorectal cancer. Limonene is found in the essential oils of orange peel and other citrus fruits, in mint, and in dill, caraway, and celery seed oils, as well as lemongrass and other oils. The anti-tumor effect, with low toxicity, was first reported in rodent research in 1984. In the animal studies, limonene both inhibited tumor formation and caused regression of existing tumors. Two mechanisms of action are being studied, both fairly complex. One particularly intriguing possibility is that limonene may re-activate a natural anti-carcinogenic process which is genetically determined. If true, this suggests that limonene, and possibly other terpenes, might be able to counteract a genetic predisposition for cancer as shown in mammary tumor research in rodents. [McNamee, D., Limonene trial in cancer, The Lancet, Vol. 342, Sept. 25, 1993, p. 801]

Article copyright American Botanical Council.
By Rob McCaleb