The American Society of Clinical Oncology meeting in Chicago began with more promising, if somewhat controversial, prostate cancer data for Johnson & Johnson's Zytiga.

Results presented at ASCO from a Phase III study which was unblinded in March due to its effectiveness in patients with castration-resistant prostate cancer, were interim analyses showing that Zytiga plus prednisone showed a statistically significant improvement in radiographic progression-free survival compared to patients treated with placebo plus prednisone. The pill was approved in the USA and Europe last year for the treatment of metastatic CRPC who have received prior chemotherapy.

J&J noted that the median rPFS in the control arm was 8.3 months but had not yet been reached in the Zytiga arm because progression events were occurring more slowly in the latter. However, lead investigator Charles Ryan estimated that patients on Zytiga would have at least 16 months rPFS.

The results also showed "a trend for increased median overall survival", which J&J estimated to be about a 33% improvement compared with placebo (or nine months), but did not reach statistical significance; patients in the placebo group lived for a median of 27.2 months.

Some observers have expressed concern about the trial being unblinded on the just the possibility, however high, that Zytiga would have shown a statistically significant benefit if trial had been completed. Nevertheless, most analysts seem to believe regulators will be satisfied by the data from the halted study to back expanding the Zytiga approval.

Dr Ryan, of the UCSF Helen Diller Family Comprehensive Cancer Center, said "this is an important study with all clinically relevant endpoints favouring treatment with abiraterone acetate plus prednisone". He added that it is "also the first to suggest that inhibiting androgen production significantly delays initiation of cytotoxic chemotherapy".

Attendees at ASCO were wondering what the effect will be on Dendreon Corp and its expensive infusion Provenge (sipuleucel-T), approved by the US Food and Drug Administration in April 2010 for advanced prostate cancer. That approval was based on data showing overall survival rate of 4.1 months but the vaccine has struggled to make much of a mark saleswise.

By Kevin Grogan in Chicago