Researchers create test to predict response to Xtandi, Zytiga

Researchers have created a $65 test designed to show which prostate cancer patients will respond to Pfizer’s Xtandi and Johnson & Johnson’s Zytiga. Retrospective analysis of plasma DNA using the droplet digital PCR assay suggest the drugs are less effective in patients with multiple copies of the androgen receptor gene.

Xtandi and Zytiga have established themselves in prostate cancer treatment pathways by inhibiting the androgen receptor. However, the drugs work better in some patients than others. And, as there is no approved test to show who is likely to respond, some patients are currently taking medicines that are unlikely to tackle their cancers, resulting in poorer outcomes for them and wasting money for the healthcare system.

Researchers working in the United Kingdom think they have found a way to stratify patients by the likelihood they will respond to Xtandi and Zytiga.

“We have developed a robust test that can be used in the clinic to pick out which men with advanced prostate cancer are likely to respond to [Zytiga] and [Xtandi], and which men might need alternative treatments. Our method costs less than £50 ($65), is quick to provide results, and can be implemented in hospital laboratories across the [National Health Service],” Gerhardt Attard, M.D., consultant medical oncologist at The Royal Marsden, said in a statement.

The test uses droplet digital PCR to look at the androgen receptor genetics of prostate cancer patients. When applied retrospectively to samples from 171 prostate cancer patients, the assay found that men with multiple copies of the androgen receptor gene were four times more likely to die during the study.

An analysis of the genetics and outcomes of another 94 patients associated the presence of one or two copies of the gene to longer responses to the drugs. Patients with more copies had eight-fold shorter response durations than people with just one or two.

The results from the retrospective analyses have encouraged the researchers to push ahead with further evaluations of the test. Attard and his collaborators plan to put the PCR assay through a prospective clinical trial to show it can predict which patients will respond. If successful, the prospective study could lead to the test being used to stratify prostate cancer patients in the U.K. and elsewhere.