Until recently, there have been few treatment options for advanced prostate cancer that is resistant to androgen-directed therapies. Newer treatments that target residual androgen production offer some hope of prolonging the interval before chemotherapy, with fewer adverse effects (AEs) and better efficacy. One of those is abiraterone, which blocks extragonadal, testicular, and tumor androgen biosynthesis.
An ongoing multinational phase 3 study is evaluating the clinical benefits of abiraterone plus prednisone vs prednisone alone in patients with progressive metastatic castration-resistant prostate cancer (mCRPC). Follow-up for the study has now exceeded 27 months, giving a good opportunity to evaluate safety and efficacy. Thus, after having reviewed outcomes so far, the independent data-monitoring committee recommended that the study be unblinded and patients be allowed to cross over from prednisone to abiraterone. The researchers reported the results of the third interim analysis, with updated analysis.
Patients were stratified by Eastern Cooperative Oncology Group performance status (ECOG-PS) and randomly assigned to receive abiraterone 1,000 mg plus prednisone 5 mg twice daily or placebo plus prednisone.
Patients who received abiraterone had, compared with those on prednisone, statistically significant improvement in radiographic progression-free survival (PFS), with a median time to disease progression or death of 16.5 months, vs 8.2 months (95% CI, 0.45-0.61).
Overall survival also lengthened, from a median of 35.3 months vs 30.1 months (95% CI, 0.66-0.95).
All secondary endpoints also favored abiraterone over prednisone. For instance, abiraterone treatment delayed the time to the need for opiates for cancer-related pain and the time to initiation of chemotherapy. Abiraterone also delayed the time to deterioration in ECOG-PS and prostate-specific antigen (PSA) progression. Abiraterone more than doubled the PSA response rate: 68% vs 29% with prednisone.
Patients reported more pain relief. Those receiving abiraterone had statistically significant improvement in pain interference (P = .005), although the improvement in mean pain intensity was not significant.
Adverse effects leading to dose modifications or interruption of treatment were reported in 21% of patients on abiraterone, compared with 12% of the prednisone group. Six patients (1%) in each group died of drug-related treatment-emergent AEs. The AEs of “special interest,” the researchers say, included events related to mineralocorticoid excess, such as hypertension, hypokalemia, and fluid retention—all unsurprising, given the known mechanism of action of abiraterone. Grade 3 or 4 AEs with increased alanine aminotransferase and aspartate aminotransferase were more common in the abiraterone group.
The most common subsequent therapy for patients who terminated the study was docetaxel. However, another recent study, from Johns Hopkins researchers in Baltimore, Maryland, indicates the transition warrants caution: The findings suggest a potential cross-resistance between docetaxel and abiraterone.
Their study compared outcomes in 24 men who received abiraterone before docetaxel with 95 who were abiraterone-naïve. Men who were on abiraterone were less likely to achieve a PSA response, and their cancer was more likely to progress.
The researchers concede that their study groups were small; they also say it is possible that differences in disease severity may have influenced the time to progression. However, they say the fact that PSA-PFS was significantly different between the 2 groups (P = .002) supports their initial hypothesis—that is, that abiraterone pretreatment reduces responsiveness to docetaxel.
In spite of its limitations, the researchers say their study represents the only comparative analysis of PSA-PFS and PFS after docetaxel treatment for patients who have or have not received prior abiraterone. Their report, they add, offers the “strongest available evidence to date” of a clinically meaningful cross-resistance between abiraterone and docetaxel. They conclude that their findings provide “valuable information” about which patients are likely to derive the most benefit from docetaxel.
Rathkopf DE, Smith MR, de Bono JS, et al. Eur Urol. 2014;66(5):815-825.
Schewizer MT, Zhou XC, Wang H, et al. Eur Urol. 2014;66(4):646-652.