Scientists are also studying toxins linked to hormone-like substances called growth factors. Many cancer cells have large numbers of receptors for growth factors on their surfaces. This makes growth factors more likely to attach to these cells than those cells with fewer receptors. When these growth factor receptors are stimulated, the cancer cells reproduce and grow faster.
Researchers have learned how to attach growth factors to toxins. When the growth factor-toxin combination reaches the cancer cell's growth factor receptors, it delivers its payload of toxin to kill the cell. The concept behind these growth factor/toxin drugs is much like that of immunotoxins. But because the toxin-carrying growth factor drugs do not contain antibodies, they are not classified as immunotoxins.
The only growth factor/toxin currently approved by the FDA is denileukin diftitox (Ontak). It is the immune system protein known as interleukin-2 (IL-2) attached to a toxin from the germ that causes diphtheria. Denileukin diftitox is used to treat a rare type of skin lymphoma known as mycosis fungoides (or cutaneous T-cell lymphoma). It is also being studied to be used against a number of other cancers.
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