Study details
The current phase 3 randomized TRITON3 clinical trial was conducted to confirm the earlier findings and to expand upon the data in mCRPC. The participants in this trial were patients with mCRPC who had specific gene alterations, including BRCA and ATM alterations, who had experienced disease progression after androgen receptor–directed therapy but who had not yet received chemotherapy.
A total of 270 men were assigned to receive rucaparib (600 mg twice daily); 135 patients received their physician’s choice of medication. Within the two study arms, 302 patients had a BRCA alteration, and 103 patients had an ATM alteration. The ITT population consisted of all the patients who had been randomly assigned to either of the two groups. A prespecified subgroup included patients with a BRCA alteration.
The primary outcome was the median duration of imaging-based PSF, as determined through independent review. Key secondary outcomes were overall survival and objective response rate.
The most common adverse events in the rucaparib group were fatigue, nausea, and anemia or decreased hemoglobin. In the control group, the most common adverse events were fatigue, diarrhea, and neuropathy. The most common events of grade 3 or higher were anemia or decreased hemoglobin, neutropenia or a decreased neutrophil count, and fatigue in the rucaparib group, and fatigue and neutropenia or a decreased neutrophil count among control patients.
No changes in standard of care
In a discussion of the study, Elena Castro, MD, PhD, of the Instituto de Investigación Biomédica de Málaga, Campanillas, Spain, emphasized that there is a clear benefit from the use of PARP inhibitors (such as rucaparib) for patients with BRCA alterations.
However, she highlighted the absence of convincing overall survival data and the absence of a clear benefit on PFS in the subgroup of patients with ATM alterations.
“These data raise several questions,” she noted, “such as, do patients with ATM alterations benefit at all? And should PARP inhibitors [such as rucaparib] precede or follow docetaxel therapy?”
Because of the high crossover rate, it may be possible to evaluate the directionality of docetaxel followed by PARP inhibitors and the other way around, she suggested.
Dr. Castro said that patients with BRCA alterations benefit from PARP inhibitors and are likely to derive more benefit from them than from taxanes.
“But those with ATM alterations are unlikely to benefit from rucaparib more than from taxanes,” she said.
In a comment, Hank Ng, MD, medical oncologist, NYU Langone Perlmutter Cancer Center, New York, said he is not convinced that the findings from TRITON 3 represent a new standard of care in BRCA 1/2 mutations or ATM.
“Currently, we know that, for patients with prostate cancer with BRCA1/2 or ATM, the standard of care is an androgen receptor pathway inhibitor (ARPI), such as abiraterone or enzalutamide, then docetaxel, and then a PARP inhibitor like rucaparib,” he said.
(Currently, rucaparib is indicated for use in patients with mCRPC with BRCA alterations after they have already received an ARPI and taxane-based chemotherapy.)
Dr. Ng also questioned the control arm of the TRITON 3 trial. All the participants in the trial had already experienced disease progression after treatment with a second-generation ARPI. But the physician’s choice of therapy allowed them to move on to another ARPI or to docetaxel.
Dr. NG commented that, “in almost all cases, after progression of one ARPI, switching to another ARPI does not provide much benefit – from what is visible from this abstract – and only 56% patients received docetaxel, and thus 44% received a not-beneficial treatment,” he said.
“I am not sure what the docetaxel subgroup showed, but potentially, if those numbers are convincing, we could move this [rucaparib] ahead of docetaxel,” he speculated.
However, he also pointed out that an overall survival benefit has not yet been shown; so far, the benefit that has been shown is with respect to imaging-based PFS.
Dr. Ng does agree that rucaparib is indicated in the second line after progression with one ARPI for patients who are not candidates for chemotherapy. “But this has not yet shown me that we should absolutely be offering rucaparib before docetaxel,” he said.
TRITON3 was supported by Clovis Oncology, manufacturer of rucaparib. Dr. Bryce has relationships with Bayer, Foundation Medicine, Janssen, Merck, Myovant Sciences, and Novartis and holds a patent for therapeutic targeting of cancer patients with NRG1 rearrangements. Dr. Castro has relationships with Astellas Pharma, AstraZeneca, Bayer, Clovis Oncology, Janssen-Cilag, Merck, MSD Oncology, Novartis, Pfizer, and Roche.
A version of this article first appeared on Medscape.com.