The relationship between high levels of endogenous testosterone or use of testosterone therapy (TTh) with risk of prostate cancer (PCa) remains one of the long-standing controversies in urological research since the past 70 years (1). Some of the contributing factors to this controversy is that PCa is still the second most commonly diagnosed cancer and the sixth most common cause of cancer-related mortality among men worldwide (2,3). Although the five-year survival rate for PCa is extremely high (99%), advanced PCa has a markedly different prognosis with only a 34% five-year survival rate (4), making advanced PCa the clinically relevant outcome.
Another contributing factor has been the increasing interest about the positive and/or negative effects of testosterone in men’s health in the last decade. For instance, low levels of testosterone, or testosterone deficiency (TD), is a common medical condition affecting men, and it is characterized by having total testosterone levels <300 ng/dL (5-8). TD has been linked to a constellation of signs and symptoms that may include sexual dysfunction, decreased bone density and decreased body lean mass (5), as well as other chronic conditions such as cancer and cardiovascular diseases to name a few (9-20). Studies show that approximately 38.7% of men over 45-year-old (18) [24% among men 30–79 years old (8)] demonstrate TD. In the United States, reports show that there are approximately 2.4 million 40–69 years old men with TD (21,22). Yet, a greater concern is the projection that by 2025 approximately 6.5 million American men aged 30 to 80 will suffer from TD (8). In consequence, treatment of TD with TTh has increased dramatically as well; TTh prescriptions increased more than 3-fold, from 0.81% in 2001 to 2.9% in 2011 (23,24). Surprisingly, the controversy about testosterone and PCa is not about TD and PCa, or in conjunction with its treatment (TTh), and PCa, but mainly about the contention that high levels of testosterone and use of TTh play a role in the development or progression of PCa. Therefore, the main objectives were to review the literature on the associations of endogenous [continuous (per 1 unit increment and 5 nmol/L increment) and categorical (high vs. low)] and exogenous testosterone with decreased-, increased-, or null-risk of PCa; and to further evaluate only those studies that reported magnitude of associations from multivariable modeling, that will minimized confounding effects, and their statistical significance.
Criteria to define “decreased-risk”, “increased-risk”, and “null-risk” of PCa groups
In this review, we critically discuss previous and emerging literature on the association between continuous (per 1 unit increment and 5 nmol/L increment) and categorical (high vs. low) total testosterone and use of TTh with PCa from original observational research studies and meta-analyses of randomized-placebo controlled trials [1990–2016]. We mainly focused our search total testosterone due to a limited number of studies that reported free- or bioavailable-testosterone. The review is organized in three parts, where we report the association of (I) endogenous total testosterone [continuous (per 1 unit increment and 5 nmol/L increment)], (II) endogenous total testosterone (categorical high vs. low) and (III) use of TTh (meta-analyses) with decreased-, increased-, or null-risk of incident PCa cases. “Decreased-risk”, “increased-risk”, and “null-risk” of PCa groups were defined based on their magnitude of association, such as odd ratios (ORs), rate ratios or relative risk (RR), or hazard ratios (HRs), plus statistical significant values such as P value <0.05 or 95% confidence intervals (CIs) that does not include value of one. For instance, studies/analyses with “decreased-risk” had a magnitude of association (ORs, RRs, HRs) that is below 1; studies/analyses with “increased-risk” had ORs, RRs or HRs that are higher than 1; or “null-risk” if ORs, RRs and HRs is equal to 1 (or 0.999). These cutoff points were chosen arbitrarily, but magnitudes of association with values so close to the null (i.e., 0.999, 0.998, 1.01, 1.02) have a difficult clinical interpretation. In addition, we considered whether these magnitude of associations reached statistically significance (SS) or not (NS). For consistency, we only presented the magnitude of the association of PCa (aggressive and non-aggressive) for studies/analyses with continuous total testosterone (continuous increment of testosterone and 5 nmol/L increment) and those studies/analyses that categorized total testosterone (high vs. low levels of testosterone). Some studies may be repeated twice because they independently investigated association of testosterone with aggressive and non-aggressive PCa; the rationale for this latter statement stems from studies (25,26) that reported opposite values for low-grade and high-grade (i.e., low-grade OR =1.91 and high-grade OR =0.26) as reporting only one value will be misleading. Therefore, we referred to these as studies/analyses, and they will be counted independently. Studies that reported their findings by comparing low to high categorical groups of total testosterone, we took the reciprocal of the magnitude of the association to present the comparison of high to low categorical groups (this is value only for studies that reported ORs). In addition, we recalculated their 95% CIs to determine their statistical significance. Furthermore, only those studies that adjusted for PCa risk factors in multivariable models were included to minimize confounding effects.
First part: association of endogenous total testosterone (continuous increment and 5 nmol/L increment) with decreased-, increased-, or null-risk of PCa
Thirty-one observational studies were identified that investigated the association of endogenous total testosterone with risk of PCa (aggressive and non-aggressive) (25-49). Twenty studies (25,26,34-49), out of the 31, were prospective and previously analyzed in a meta-analysis to determine association of continuous (5 nmol/L increment) endogenous total testosterone with risk of PCa (Figure 1 and Table 1) (50). The remaining 11 studies/analyses (25,27-33), which some may be repeated because they reported findings for both aggressive and non-aggressive PCa in opposite directions (25,26), were a combination of prospective and retrospective studies that focused on the association of continuous total testosterone (per 1 unit increment) with PCa.
None of the twenty prospective studies reported a statistical significant association between continuous [5 nmol/L increment] total testosterone and increased- or decreased-risk of PCa [Figure 1 (numbers 1–20) and Table 1]. In fact, these 20 studies were used for analyses in a meta-analysis (50) that reported a summary RR of 0.99, which also did not reach statistical significance (95% CI =0.96–1.02).
Interestingly, two (27,31) of the 11 remaining studies/analyses, which were a combination of prospective and retrospective studies, showed a significant decreased-risks of PCa for continuous increment of total testosterone [OR =0.706; 95% CI =0.556–0.897 (27) and OR =0.13; 95% CI =0.05–0.35 (31)] [Figure 1 (numbers 21 and 27) and Table 1]. However, there were also two other studies (32,33) that showed significant increased-risks of PCa for continuous increment of total testosterone [HR =2.167; 95% CI =1.291–3.637 (32) and HR =1.31; 95% CI =1.03–1.65 (33)] [Figure 1 (numbers 28 and 31) and Table 1]. The remaining studies reported not significant and “null-risks” values for PCa [Figure 1 (numbers 22, 23, 24, 25, 26,29, 30)] (25,28-30).
Second part: relationship of endogenous total testosterone (categorical high vs. low) with decreased-, increased-, or null-risk of PCa
In this section, we identified 25 studies/analyses that investigated endogenous total testosterone in categorical format high vs. low levels (Figure 2 and Table 2) (25-28,30,38,41,46,51-61). This was a combination of prospective, retrospective, nested-control studies and one meta-analysis of two nested-control studies (52). There were some studies that originally reported low to high levels of testosterone [Figure 2 (numbers 16 to 25) and Table 2 (28,30,57-61)], but we converted these comparisons to high to low and recalculated the 95% CI’s for statistical significance purposes. This conversion was conducted to maintain consistency among all studies about their comparison groups (high vs. low) and because the contention is that high levels of testosterone increase risk of PCa. For the meta-analyses [Figure 2 (number 3) and Table 2] of the two nested-control studies, we only provided the summary OR as the direction of the measure of association of both independent studies is similar to the meta-analyses (52).
Eight out of 25 studies/analyses reported an increased-risk of PCa for men with high levels of testosterone compared to low [Figure 2 (numbers 1–4, 10, 12, 14, 19)] (25,26,38,46,51-53,59), but only four were statistically significant (numbers 1–4) (46,51-53). The remaining 17 studies/analyses showed a decreased-risk of PCa after comparing high vs. low levels of testosterone [Figure 2 (numbers 5–9,11, 13, 15, 16, 17, 18, 19 and 20–25) and Table 2] (25-28,30,38,41,54-61), but only 11 were statistically significant [Figure 2 (numbers 6–9,13, 16, 17, 18, 20, 21, 25) and Table 2] (26,27,54-61).
Third part: association of use of TTh with decreased-, increased-, or null-risk of PCa in meta-analyses of randomized placebo-controlled trials
As of to date, two meta-analyses of randomized placebo-controlled trials that investigated use of TTh with risk of PCa have been conducted (Figure 3 and Table 3) (50,62). The first meta-analyses (62) consisted of 8 trials, and they were separated in two groups: first group (n=5 trials) consisted of men who used TTh for less than 12 months and the second group (n=3 trials) consisted of men who used TTh between 12 and 36 months. Findings from the first group reported a non-significant reduced-risk of PCa (summary OR<12 months =0.74; 95% CI =0.25–4.65). However, results from the second group were considered null-risk of PCa (summary OR12-36 months =0.99; 95% CI =0.24–4.02), possibly due to the few number of trials (3 trials) and small sample size of PCa cases (n=6). The most recent meta-analyses (50), which included 11 trials, reported a reduced-risk of PCa for men who used TTh for less than 12 months, but it was not statistically significant (summary OR<12 months =0.87; 95% CI =0.30–2.5).
The contention that high levels of testosterone or that the use of TTh increases the risk of PCa doesn’t seem to be supported with the present body of literature, which were adjusted for potential PCa risk factors. A significant number of observational studies (prospective and retrospective) that investigated the association of endogenous (continuous per 1 unit increment or 5 ng/mL increment) total testosterone with PCa were not statistically significant. Most of these studies reported magnitudes of association that were null. On the other hand, those observational studies that categorized endogenous total testosterone (high vs. low) and investigated its relationship with PCa seemed to show a different picture; there were more studies/analyses reporting a reduced-risk of PCa (17 out of 25) than increased-risk of PCa (8 out of 25) among men with high levels of total testosterone compared to those with low levels. Most of these reduced-risks findings were statistically significant (11 out of 17) than the increased-risk of PCa ones (4 out of 8). Meta-analyses of randomized placebo-controlled trials that investigated use of TTh and its association with PCa were more consistent on reporting magnitude of associations of reduced-risks of PCa, but they did not reach statistical significance.
Although the contention of high levels of testosterone is challenged by the body of literature presented, we can’t conclude or promote that there is “zero” risk by using TTh or increasing levels of testosterone among PCa patients. It is important to note that any kind of epidemiologic study design experience different limitations such as sample size/power (mainly in randomized trials), no randomization (i.e., observational studies), no or limited follow-up data, different methodology on data collection and different definitions of exposures and outcomes to conduct pooled-analysis. In addition, comparison among studies is challenging as some studies define high vs. low levels of testosterone with different cutoff points or develop different criteria to define aggressive and non-aggressive PCa. Therefore, there is a need further studies with the adequate power, follow-up data, epidemiological and clinicopathological data that can provide a better explanation about the safety and protective effects of high levels of endogenous testosterone or use of TTh in the natural history of PCa and in men’s health.
Furthermore, it seems that the long-standing controversy between endogenous/exogenous testosterone and PCa will remain for some time. First, the evolution of PCa research shows that advanced PCa is the most clinical relevant outcome (4). However, there are few studies, observational and randomized controlled trials, focusing on advanced PCa and endogenous/exogenous testosterone. In addition, there are significant racial disparities of PCa showing African-American men with the highest incident and mortality rates (63,64); yet, there are limited number of studies investigating the relationship between endogenous/exogenous testosterone and PCa among African-American men (65-67). These can be relevant research topics that should be addressed in future studies.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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