The androgen receptor antagonist apalutamide is highly effective for treating nonmetastatic castration-resistant prostate cancer that carries high risk for metastasis based on a rapidly rising prostate-specific antigen (PSA) level, according to results of the randomized phase 3 SPARTAN trial.
“Metastatic castration-resistant prostate cancer is a uniformly fatal disease, with a median survival of around 2.5 years. The aim of this study was to see if the development of metastases in the transition from nonmetastatic to metastatic castration-resistant prostate cancer could be slowed,” lead study author Eric J. Small, MD, professor of medicine at the University of California, San Francisco, commented in a press briefing held in advance of the 2018 Genitourinary Cancers Symposium.
He and his coinvestigators randomized 1,207 men with high-risk nonmetastatic castration-resistant prostate cancer 2:1 to apalutamide (an investigational next-generation competitive inhibitor of the androgen receptor, also known as ARN-509) or placebo, with continuation of androgen-deprivation therapy in both groups. After development of metastases, patients were offered open-label abiraterone (Zytiga), the standard of care for this population, with prednisone.
Main results showed that, compared with placebo, apalutamide prolonged the time to metastasis (ascertained by central review of standard imaging) or death by more than 2 years, Dr. Small reported. The difference translated to a nearly three-fourths reduction in the risk of these events. In addition, the drug was well tolerated and had a rate of serious adverse events similar to that seen with placebo.
“This is a positive trial,” he summarized. “Overall, these data suggest that apalutamide should now be considered as a new standard of care for men with high-risk nonmetastatic castration-resistant prostate cancer.”
The metastasis-free survival findings led to unblinding of the trial in July 2017, and all patients were offered open-label apalutamide. Full results of SPARTAN will be reported later this week at the symposium, which is sponsored by the American Society of Clinical Oncology, ASTRO, and the Society of Urologic Oncology.
Parsing the findings
“Until the results of studies presented at this meeting, there was really no obvious standard of care for these patients,” commented ASCO Expert and presscast moderator Sumanta K. Pal, MD. The gain in metastasis-free survival in SPARTAN was “very clinically meaningful.”
Results of the similar PROSPER trial, which compared enzalutamide (Xtandi) with placebo, will also be presented at the symposium, he noted. “We know from a press release issued in September of 2017 that this study also showed a significant delay in the time to onset of metastatic disease. It will be interesting to juxtapose these data once available. Enzalutamide is a drug familiar to the prostate cancer community given existing approvals in the setting of more advanced disease. The familiarity that oncologists already have with enzalutamide may help with clinical adoption.”
Ultimately, imaging advances that allow for earlier detection of metastases may shrink the population of men with nonmetastatic castration-resistant prostate cancer, according to Dr. Pal, who is also a medical oncologist and codirector of the Kidney Cancer Program at City of Hope, Duarte, Calif.
“The SPARTAN trial used a more conventional imaging approach, with CT scans and technetium bone scans,” he pointed out. “While it’s true that this is the current standard, imaging techniques such as fluciclovine PET, PSMA [prostate-specific membrane antigen] PET, may potentially improve our ability to detect disease spread earlier and thereby change our management strategy.”
Study details
The patients with nonmetastatic castration-resistant prostate cancer enrolled in SPARTAN had a calculated PSA-doubling time of 10 months or less, a feature identifying those most at risk for developing metastases.
At a median follow-up of 20.3 months, median metastasis-free survival was 40.5 months with apalutamide versus 16.2 months with placebo (hazard ratio, 0.28; P less than .001), Dr. Small reported. An interim analysis of overall survival showed a trend favoring apalutamide (P = .07).
Fully 61% of patients in the apalutamide group were still on treatment at data cutoff, compared with 30% of placebo-treated patients.
Both groups had low rates of discontinuation attributable to adverse events (10.7% and 6.3%). Apalutamide-treated patients did not have any decrease from baseline in patient-reported health-related quality of life scores, which were similar to those of placebo-treated counterparts.
Dr. Small disclosed that he has a consulting or advisory role with Fortis, Gilead Sciences, and Valeant Pharmaceuticals International; has stock and other ownership interests with Fortis and Harpoon Therapeutics; and receives honoraria from Janssen-Cilag; in addition, his institution receives research funding from Janssen. The study was funded by Aragon Pharmaceuticals, a wholly-owned subsidiary of Johnson & Johnson.