The human epidermal growth factor receptor (HER) family of receptor tyrosine 
kinases is part of a network of pathways that are involved in the development 
and progression of prostate cancer. HER-kinase receptors include epidermal 
growth factor receptor (EGFR), HER2, HER3, and HER4, which must combine as 
dimers to affect signaling. Different combinations of receptors produce 
different qualities and levels of pathway activation. Among HER-family 
receptors, HER2 activation is particularly important in breast cancer, as HER2 
gene amplification is associated with a distinct clinical course and response to 
treatment with a HER2-directed therapy (trastuzumab). Although HER2 can be 
over-expressed in prostate cancer, there is no clinical data to support the use 
of trastuzumab for prostate cancer patients. Preclinical and clinical data show 
that the activation of the HER-kinase axis is important for the progression of 
prostate cancer to androgen-independent disease. Data points towards the 
importance of inhibiting multiple members of the HER-kinase family to achieve 
more complete blockade of this axis for cancers other than HER2-overexpressing 
breast cancer. Multiple pharmaceutical agents that block the HER-kinase axis are 
currently being tested for patients with prostate cancer. These include 
antibodies, tyrosine kinase inhibitors, and novel strategies which seek to 
decrease HER2 expression.