Although some people have called it intriguing,  a new “On The Horizon” drug, XL-184, is still very early in its testing.  What makes LX184 intriguing is the just released results from a conference in Berlin.

In very early testing, 19 of 20 men in a phase I trial showed an improvement in their scans used to determine whether the cancer had spread to their bones.  To the surprise of the investigators, in some cases, the bone scans could no longer detect any cancer in men who had already had scans that demonstrated the spread to the bones.  Additionally, the men were able to stop taking the narcotics they were using to control the bone pain.

Before you become to excited, remember, these results are from a very small number of men and there was no control group in the trial design.  Survival was not measured in this trial, so there is no way to know if it is possible for XL184 to prolong life or how long the improvement in bone scans will last.

However, “There’s really no precedent for another drug that does this,” said Dr. Matthew R. Smith, a prostate cancer specialist at the Massachusetts General Hospital.  Additionally, one must remember that bone scans measure bone formation, not the presence of prostate cancer itself. So it cannot be ruled out that XL184 only stopped bone growth without killing the cancer. Pointing to the fact that there were signs that XL184 also controlled tumors outside the bones; Dr. Smith said he doubted that XL184 effect was limited to only stopping bone growth.

Dr. Smith also mentioned that Taxotere has no effect on bone scans and the normally used drugs to treat bones in men with advanced prostate cancer, Zometa and denosumab, only protect bones from incidents like fractures and don’t fight the cancer itself.

It is believed that the underlying mechanism of action of XL184 is to block the formation of blood vessels that feed tumors and also inhibits a protein called MET that has been shown to spur prostate cancer tumor growth.

XL184, is the lead drug candidate of Exelixis, a biotechnology company in South San Francisco, Calif.  Exelixis plans to add many prostate cancer patients to the Midstage clinical trial from which these results were drawn, said Michael M. Morrissey, the company’s chief executive. A late-stage trial could begin as early as next year, he said.

However, there are two active trials, which do include men with prostate cancer, of XL184 currently recruiting subjects.  They are:

NCT00485719: Study of the Safety and Pharmacokinetics of XL765 in Adults With Solid Tumors

NCT00940225 Study of XL184 in Adults With Advanced Malignancies

Joel T Nowak, M.A., M.S.W.