The P.S.M.A. protein, which can be detected by imaging scans, is almost exclusively on prostate cancer cells, and so the treatment causes less damage to surrounding tissue, said Dr. Oliver Sartor, the trial’s co-principal investigator and medical director of Tulane Cancer Center in New Orleans.
Though the protein is not ubiquitous in prostate tumors, it is found in more than 80 percent of cases. Among patients screened for the trial, 87 percent were P.S.M.A.-positive. Only those men who were positive for the marker were included in the trial.
The study enrolled men with a form of metastatic prostate cancer called castration-resistant prostate cancer. All the patients had disease that progressed despite treatments with chemotherapy and hormonal therapy to suppress and block androgens.
Participants were randomly assigned to receive the experimental treatment, given every six weeks in up to six doses along with standard treatment, or to continue standard care alone, but without chemotherapy or other isotopes.
After a median follow-up period of 20.9 months, patients given the experimental treatment survived for a median of 15.3 months, compared with 11.3 months for those who received only standard care, a reduction of 38 percent.
Their tumors were more likely to shrink, their prostate-specific antigen levels were more likely to fall, and the risk of their cancer progressing was reduced by 60 percent.
Side effects — most commonly fatigue, dry mouth and nausea — were more prevalent among those receiving the compound than among those who did not, but did not appear to significantly affect quality of life, the researchers said.