Interleukin-6 (IL-6) was originally described as a B cell growth and differentiation factor. Considerable evidence implicates this cytokine in the progression of cancer, in particular multiple myeloma, a malignancy involving mature or memory B cells. IL-6 has however been implicated in many other cancers including breast cancer. 

The search for new treatments of this disease is important principally due to high levels of breast cancer related mortality. In fact between 1970 and 1994 this disease claimed the lives of well over 1 million women in the US alone making it the 4th most common cause of cancer related death in this region. High levels of mortality are related to both incidence but also to the lack of treatment options. Although first line treatments are effective, particularly for estrogen receptor positive tumors, drug resistance is a common occurrence and once this has occurred the need for more aggressive treatments is required and the prognosis worsens. 

Preventing the development of drug resistance is therefore a key priority. Inhibitors of proteins responsible for drug resistance or molecules with reduced susceptibility to transport by these proteins are therefore currently receiving much attention. US researchers have now linked IL-6 to drug resistance in breast cancer. Breast cancer cells that are sensitive to drug treatment do not express IL-6, whereas high levels of IL-6 are produced by multi drug-resistant breast cancer cells. Expression of the IL-6 gene in drug-sensitive breast cancer cells increases their resistance to drug treatment by activating the CCAAT enhancer-binding protein family of transcription factors and inducing mdr1 gene expression. Thus, the autocrine production of IL-6 by tumor cells is an important factor in determining the susceptibility or resistance of these cells to drug treatment. In addition to preventing multidrug resistance, IL-6 has been reported to reduce estrogen production by breast cancer cells and to increase proliferation, metastasis and ability to destroy bone. IL-6 has also been strongly implicated in cachexia, the rapid loss of body mass in advanced disease.  In view of the variety of beneficial effects related to IL-6 the development of therapies able to block this cytokine should be of immense clinical use. Few IL-6 inhibitors are however in development and this area therefore represents a commercially as well as therapeutically promising area.