New strategies for the medical treatment of prostate cancer.

Androgen is a major growth factor in the normal prostate and determines the overall number of prostate cells. Metastatic prostate cancer, while initially responsive to androgen ablation, eventually becomes hormone-refractory and resistant to many treatments. Unfortunately, there are very few agents in the preclinical stage with a seemingly promising future for hormone-refractory prostate cancer (HRPC) that are actually taken through the complete drug development process, including US Food and Drug Administration approval. Many novel strategies under investigation for treating HRPC target metastatic prostate cancer cells that are neither androgen-dependent nor in the proliferative state. Examples of therapies that target this so-called "Achilles' heel" of HRPC include immune therapy, gene therapy, angiogenesis inhibition, and activation of programmed cell death. Unique properties of HRPC allow for the development of novel treatments that target prostate-specific antigen (PSA), human glandular kallikrein-2, or prostate-specific membrane antigen. An inactive prodrug with a thapsigargin analogue, a sesquiterpene lactone from the plant Thapsia garganica, is currently under investigation specifically for the targeted therapy of HRPC. Preclinical data suggest the PSA-targeting abilities of this novel therapy are associated with a nearly complete cessation of tumour growth with minimal toxicity.