Prostate cancer (PCa) is an androgen-dependent disease. Androgen action is mediated by the androgen receptor (AR), a hormone-activated transcription factor that plays a central role in the biology and progression of PCa [1]. AR amplification and mutations are the most important molecular alterations leading to the onset of castration-resistant PCa (CRPC). Testosterone is the natural growth factor for PCa. Administration of testosterone increases prostate growth in animal models and castration of patients results in regression of advanced PCa. However, from the literature data a direct relationship between circulating testosterone levels and PCa aggressiveness and prognosis is not evident in all circumstances and patient settings.

It is actually clear that the long-held belief that high serum androgen concentrations increase the PCa risk can no longer be supported [2]. In untreated PCa patients multiple studies have reported that lower serum testosterone concentrations are associated with worrisome features such as high grade and higher stage at presentation, positive surgical margins after prostatectomy and worse overall survival [3]. These studies suggest that PCas are stimulated to dedifferentiate in a testosterone-deficient environment, leading to more aggressive tumors. However, three studies demonstrated that serum testosterone levels measured in PCa patients after 6 months of luteinizing hormone releasing hormone analogue (LHRH-A) therapy were inversely correlated with the patient outcome [4–6]. Serum testosterone in these studies was measured with automatic techniques that are imprecise in assessing testosterone levels at low concentration ranges [4]. These concerns notwithstanding, all these studies concurrently demonstrated that considering only patients with serum testosterone within the castrate range, the attainment of serum testosterone levels as low as possible (i.e., below 20 ng/dl) was associated with a trend of better outcome.

A recently published paper by Ryan et al. [7] provided further insights into the intriguing relationship between serum testosterone and PCa growth. These authors showed that serum testosterone levels measured with mass spectrometry (an ultrasensitive technique that is the reference method for measuring serum testosterone at the castrate range) were prognostic in patients with CRPC recruited in the COU-AA-301 trial and these patients were subsequently randomized to receive abiraterone plus prednisone or prednisone alone. Longer survival was observed for patients with serum testosterone above median compared with below median. These results are consistent with previous data showing that higher androstenedione levels in patients with CRPC predicted better survival compared with patients with lower levels [8]. This scenario is therefore very similar to that observed in hormone-naive PCa; it seems that lower testosterone levels are poorly prognostic both in hormone-naive and CRPC. The only difference is that in the CRPC patients tumor growth occurs in a very low testosterone milieu and an ultrasensitive method of testosterone measurement is required to detect this phenomenon.

These data suggest that the biological adaptation of PCa to castrate testosterone levels is very complex and testosterone may maintain a differentiating role also when the tumor progresses on androgen deprivation therapy. The mechanisms by which more elevated serum testosterone seems to select less-aggressive tumor clones in the castration-resistant state, however, are not clear. Upregulation of c-MET [9] and stimulation of the neuroendocrine differentiation [10] induced by greater testosterone inhibition may both be contributory.

Putting together the pieces of the puzzle, it seems that serum testosterone during conventional androgen deprivation therapy has an opposite prognostic significance according to whether the measurement is performed when the patients are still responding to treatment or when they become castrate resistant. If this hypothesis is true, then CRPC patients with baseline testosterone levels above the median may be patients in whom previous endocrine therapy was less efficacious than in CRPC patients with testosterone levels below the median. We believe that this issue deserves to be explored and the patients enrolled in the abiraterone Phase III study are the best candidates for such analyses.

Noteworthy, in the Ryan et al. study [7] serum testosterone in CRPC was not only prognostic but also predictive of abiraterone efficacy. A more pronounced treatment effect of abiraterone over prednisone, in fact, was observed in patients with lower androgens as opposed to their counterpart. As the authors pointed out, AR amplification may explain why, in the subset with low androgen levels, the survival of patients treated with abiraterone was better than those receiving prednisone. However, it is not clear why higher testosterone levels, although associated with a greater overall survival, seemed to be predictive of lower abiraterone efficacy over prednisone.

The relationship of decline in androgen levels with PSA decline, which is an ongoing analysis, will provide further important elements to better understand this discrepancy. In addition, in our small personal experience [berruti a et al., unpublished data] we have shown that testosterone measured with an ultrasensitive technique after abiraterone therapy is undectectable in many cases, but not in all cases, so it will be interesting to know whether detectable testosterone levels during abiraterone has a prognostic significance or not and whether there is a correlation with pretreatment testosterone levels.

Notewothy, glucocorticoid receptors have been recently demonstrated to confer resistance to antiandrogen therapies [11]. Since the patients enrolled in the COU-AA-301 trial had previously received prednisone in association with docetaxel and continued receiving prednisone either in association with abiraterone or in the control arm, this may represent a confounder in the interpretation of the prognostic and predictive role of serum testosterone in this series.

In conclusion, the role of serum testosterone in the modulation of PCa growth is complex and the molecular alterations favoring a more indolent disease course in the presence of higher serum testosterone levels in both in hormone-naive and castrate-resistant patients needs to be elucidated and is an exciting field for future research.