Sharks and Cancer
Carl A. Luer, Ph.D.
Mote Marine Laboratory
The mystique concerning sharks and their relatively low susceptibility to disease has existed for more than a century. Of particular fascination is the incidence of tumors found in sharks and their relatives in the wild. When compared with the reasonably high incidence of tumors among the bony fishes, the frequency of tumors found among the cartilaginous fishes is impressively low and has been documented in two classic review articles covering fish as well as other lower animals (Schlumberger and Lucke, 1948; Wellings, 1969). The literature sited by these reviews documented 15 elasmobranch neoplasms (new and abnormal formation of tissue), including eight from sharks and seven from skates/rays. Based on the published descriptions of the 15 lesions, seven were clearly benign, while five showed infiltration of surrounding tissue and were possibly malignant. The remaining three proved not to be tumors, two being proliferative responses to parasites, while one was a fibrous response to a wound.
In 1965, the National Cancer Institute established the Registry of Tumors in Lower Animals (RTLA) in an attempt to compile data reflecting the trends and types of tumors occurring in invertebrate and poikilothermic vertebrate animals. From 1965 to 1991, the RTLA compiled a list of 20 "neoplasms" from cartilaginous fishes (14 from shark species, four from skates or rays, and two from chimaeras). Of the 20 lesions, however, eight are now recognized as goiters (not thyroid adenomas, as listed) and at least one is a fibrous response to foreign matter (not a fibroma, as listed). The remaining eleven represent lesions of both benign and malignant fates. In 1999, RTLA Director John Harshbarger and Gary Ostrander of Johns Hopkins University presented a poster at the Eastern Fish Health Workshop, updating the number of elasmobranch neoplasms in the RTLA to 35, with malignant tumors accounting for only about one-fourth of that number.
Unfortunately, the relatively low incidence of tumors is often overstated by the media as well as by enterprising food supplement marketers, who claim that "…sharks don't get cancer" because their cartilaginous skeleton contains factors that inhibit cancer by preventing blood vessels from spreading to developing tumor cells and that shark cartilage powder when taken orally or rectally will likewise protect the consumer from cancer. In reality, there is no evidence that cartilage plays an active role in conferring any disease resistance to sharks or any other living animal. Any naturally occurring ingredient in cartilage that might inhibit vascular penetration is there to keep the cartilage free of blood vessels, just as in any vertebrate animal cartilage, and is in no way passively released in the animal to have a vascular inhibitory effect anywhere else in that animal.
Studies to investigate the potential of shark cartilage as a cancer therapy include both animal studies and clinical trials. Oral doses of shark cartilage had no retarding effect on the growth of implanted primary tumors in mice, nor did they inhibit metastatic spread to the lungs (Horsman et al., 1998). Similarly, studies with humans have concluded that shark cartilage is inactive in patients with advanced stage carcinoma, specifically in breast, colorectal, lung, and prostate cancer (Miller et al., 1998; Loprinzi et al., 2005). The clinical trials also documented that treatment with shark cartilage is not without adverse side effects, including gastrointestinal toxicity.
This does not mean, however, that molecular components with the ability to inhibit the vascularization of cancer cells couldn't form the basis for developing a useful therapeutic drug. Efforts to identify molecules with this type of "anti-angiogenic" activity (not necessarily from cartilage) have been extremely active for some time. But this type of therapy can only be successful when engineered into a drug, or some other form of treatment, where an effective concentration of the active molecule(s) can be delivered directly to the target tissue. It is naive to think that oral consumption of ground cartilage will achieve this result.
List of References
Do Sharks Hold Secret to Human Cancer Fight? (National Geographic News Online)
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