Bone tissue is subjected to a continuous process of remodeling based on the coordinated generation and resorption mediated by osteoblasts and osteoclasts, respectively. Osteoclast activity is regulated, among other factors, by the receptor activator of nuclear factor $\kappa$B (RANK) and osteoprotegerin (OPG). The union of RANK with its ligand (RANKL) favors the survival of osteoclasts, whereas the union of RANK with OPG inhibits their activity. Therefore, BMD depends on a cellular and molecular balance.

Osteoporosis is a metabolic bone dysfunction defined by the loss of bone tissue and the deterioration of its microarchitecture. The physiology of bone remodeling is mediated by multiple cytokines and hormones. Estrogens inhibit bone resorption, whereas androgens promote bone formation. An alteration in this balance explains why this group of patients have a loss of BMD and the onset or worsening of osteoporosis [11].

Osteoporosis induced by ADT can be severe: it causes an important deficit in BMD during the first 6–12 months of treatment, 17.3% higher than males untreated for PCa, and it is associated with a higher risk of fracture [12]. The prevalence of osteoporosis in this group of patients varies between 10–40% and increases with age and ADT duration [13, 14]. This reinforces the importance of conducting an adequate evaluation to prevent BMD deficit in all patients that are going to start treatment with ADT [14].

The use of ADT in addition to other systemic treatments in more advanced stages of PCa, such as next generation anti-androgens, glucocorticoids or radium-233 (Ra-233) results in increased bone resorption and, consequently, decreased BMD [15, 16], which translates into a higher risk of suffering SREs [17]. Moreover, bone metastasis related to PCa poses a higher risk of pathologic fractures. It is important to identify those patients at risk given that it has already been reported in multiple clinical trials that the combination of bone-targeted therapies and different treatments for advanced PCa have a positive impact both in pain control and reduction of SREs [18, 19, 20].

Before any treatment for PCa is started, the assessment of BMD is useful for identifying those patients with osteopenia or osteoporosis and for establishing the risk of fracture. There is a direct and progressive association between the loss of BMD and fractures, as the risk doubles for each decrease in standard deviation [21]. The basic tool for evaluating BMD is bone densitometry through a dual energy X-ray absorptiometry of the femoral neck.

The World Health Organization establishes a cut-off point to define osteoporosis (and therefore a high risk of fracture) of 2.5 standard deviations below the average of a female young adult (T-score $\le$2.5), and the same is used for males [22]. Although BMD assessment has high specificity for risk of fracture, its sensitivity is low [23]. For this reason, it is recommended to complement the information provided by bone densitometry with clinical risk factors. There is a well-established association between the risk of bone fracture and other SRES, with the following risk factors: age $>$65, body mass index (BMI) 24, tobacco, alcohol, previous history of falls and fractures, and family history of hip fracture [24].

To help with the evaluation and decision-making in this clinical scenario, various tools have been developed as algorithms that calculate the risk of fracture from a series of risk factors exclusively or with the addition of BMD. For instance, the Fracture Risk Assessment Tool (FRAX) is an algorithm that provides an estimation of the 10-year risk of osteoporotic fracture for each person, with the advantage of being validated for independent cohorts in several countries [25].

Despite the obvious importance of assessing bone health in patients with PCa, there are no clearly established cut-off points to guide the start of bone-targeted therapies or standard intervals to conduct tests during follow-up.

Briot et al. [26] published a series of recommendations to evaluate which patients with PCa under ADT have a risk of fracture and, therefore, would be candidates for treatment with bone-protecting agents (BPA). They propose the evaluation of BMD before the start of ADT and recommend the prescription of antiresorptive therapy if T score –2.5 or between –1.5 and 2.5 with concurrent factors such as BMI 19, age $>$75 years, cardiovascular disease, or frequent falls [26]. Furthermore, European guidelines on the treatment of genitourinary tumors recommend treatment with antiresorptive therapy if the annual bone loss is $>$5% in patients receiving ADT [27].

A reasonable approximation to assess bone health in PCa patients and the start of BPA is the proposal of the European Association of Medical Oncology (ESMO), an adaptation of the recommendations by Hadji et al. [28, 29] or women under aromatase inhibitors (Fig. 1, Ref. [29]).

Fig. 1.

Algorithm for the management of bone health in patients with advanced prostate cancer receiving treatment with androgenic suppression. ADT, androgen deprivation therapy; BMI, body mass index; BMD, bone mineral density. Adapted from Hadji et al., 2017 [29].

Because the FRAX score is not well optimized for PCa patients on ADT, some centres proactively start Vitamin D and bisphosphonates in all patients receiving long term ADT. Studies show that 77% of men on ADT exceed the FRAX threshold for prophylactic therapy hence population based therapy is likely to be of benefit [30].

In this scenario, various trials have evaluated the role of ZA and have found a benefit in the protection from osteoporotic events induced by ADT and the increase in BMD. However, no differences have been reported in terms of prevention of fractures or quality of life. For this reason, ZA is not approved for this specific setting and its use depends on the clinical judgement of the prescribing physician. The doses in these studies were 4 mg every 3 or 12 months [37, 38, 39, 40]. However, a dose of 5 mg annually is recommended as a professional consensus [26]. Oral BPA may be considered as an alternative.

Denosumab is the only drug approved by the Food and Drug Administration and the European Medicines Agency for the prevention of pathologic fractures caused by osteoporosis in patients under ADT. In the pivotal trial that led to this approval, after a follow-up of 36 months, patients treated with denosumab presented less vertebral fractures and an increase in BMD in comparison to the placebo group, with statistically significant differences in both cases [9]. The recommended dose in this setting is 60 mg subcutaneously every six months.

Treatment for patients with non-metastatic castration-resistant prostate cancer (nmCRPC) has drastically changed with the introduction of next generation inhibitors of the androgen receptor. These drugs were approved after showing a significant increase in metastasis-free survival (MFS) compared to placebo in three randomized phase III clinical trials [41, 42, 43]. The results of these three studies suggest that, in patients with nmCRPC these new agents are highly effective at postponing development of metastases. In all three trials the percentage of patients with added treatment with bone-targeted therapies was low: 11% (enzalutamide) vs. 10% (placebo) in PROSPER, 10.2% vs. 9.7% in SPARTAN and 3% vs. 6% in ARAMIS. Therefore, no conclusions can be drawn on the concomitant treatment with both therapies.

The benefit of denosumab has been investigated in a phase III clinical trial that included 1432 patients with CRPC. Of these, 716 were nmCPRC and were treated with a monthly dose of 120 mg of subcutaneous denosumab. A significant increase in MFS was observed when compared to placebo (29.5 vs. 25.2 months; hazard ratio (HR): 0.85; p = 0.028). However, this was at the expense of mandibular osteonecrosis in 5% of participants, which explains why denosumab is not currently approved for the prevention of metastasis in this subset of patients [44].

Regarding bisphosphonates, although preclinical data suggest their antitumor activity, they have not shown a decrease in the appearance of bone metastases in this subgroup of patients [45].

ZA is the only bisphosphonate that has demonstrated a reduction in SREs related to bone metastases in men with metastatic castration-resistant prostate cancer (mCRPC) [28]. Saad et al. published the results of a multicenter randomized trial that compared ZA vs. placebo in this group of patients. The evaluated dose was 4 mg every three weeks and 8 mg every three weeks (which was later reduced to 4 mg) against placebo. Patients in the placebo group had more SREs than those treated with 4 mg (p 0.021) or 8 mg/4 mg of ZA. Pain and need of analgesia were higher in the placebo group, but no differences were found in disease progression, performance status or quality or life. This study defined the tolerance profile of ZA, with 4 mg being the optimal dose compared with 8 mg [46]. After 24 months of treatment, the annual incidence of SREs was significantly higher in the placebo group (p 0.005). Median time to the first SRE was reached in both groups, being six months longer for ZA (p 0.009) [47]. In further analyses, it was observed that ZA reduced the percentage of patients with $\ge$1 SREs and delayed the onset of bone pain in patients who were asymptomatic at the start of the study [48, 49, 50]. ZA also showed a non-statistically significant tendency towards a prolonged overall survival (OS) in patients with mCRPC [51].

Pan et al. [52] evaluated the efficacy and safety of the combination of ZA and docetaxel in CRPC with bone metastases in a prospective study of 105 patients. The combined treatment presented better OS (p = 0.02) and bone MFS (p 0.05) than docetaxel and placebo, with no differences in toxicity [52]. Furthermore, TRAPEZE trial suggested that ZA could have a role as maintenance therapy after docetaxel due to an improved interval free of SRE, mainly after disease progression [53].

Between these two treatments, denosumab has demonstrated to be superior to ZA in patients with bone metastases [54]. Fizazi et al. [10] published the results of a double-blinded, randomized study in patients with mCRPC. 120 mg of subcutaneous denosumab every four weeks were superior to 4 mg of iv ZA. Denosumab significantly reduced the risk of appearance of a first SRE and posterior skeletal events compared to ZA (p 0.008), although no differences in OS were observed. In a subanalysis, denosumab showed a benefit in the delay of pain progression compared to ZA by increasing the time to the onset of moderate-severe pain (p 0.0001) (Table 1) [55].

It is recommended to start ZA or denosumab in patients with CRPC and bone metastases, even if they are asymptomatic, in doses of 4 mg of iv ZA every four weeks and 120 mg of subcutaneous denosumab every month [56].

Regarding other treatments available in mCRPC, Ra-223 is an alpha emitter radionucleotide tailored for the treatment of patients with osteoblastic metastases, with benefits in OS and SREs, and manageable side effects. Various trials have studied its combination with BPA and/or next generation anti-androgens [57].

The ERA 223 trial combined abiraterone plus prednisone with Ra-223 or placebo in patients with mCRPC who had not received previous chemotherapy [17]. The study showed, contrary to expectations, that the addition of Ra-223 did not improve SRE-free survival and was, in fact, associated with a higher risk of fracture, mainly in bone sites with no metastases. However, post-hoc analysis showed that the increase in bone fractures was particularly observed in patients not receiving BPA. These findings reinforce the importance of adequate monitoring bone health and the use of BPA [58]. These results are not replicated with the combination of enzalutamide and Ra-223 as the results of ERA 223 trial has led to the mandatory use of BPS in the phase III EORTC-1333/PEACE III trial, which treats patients with Ra-223 plus enzalutamide vs. enzalutamide alone. An interim safety analysis was performed and confirmed that, in absence of BPA, fracture risk was increased and, more importantly, the risk remains almost nonexistent with a preventive continuous administration of BPA. Again, this highlights the value of BPA in this patient population [57].

There are data that support the concomitant administration of Ra-223 with osteoclast-targeted agents. The phase III study ALSYMPCA recruited 921 patients with mCRPC with symptomatic bone metastasis and no visceral lesions and randomized them to receive 50 KBq/Kg of Ra-223 every four weeks or placebo [59]. OS, the primary endpoint of this study, was reported as 14.9 months in the experimental arm vs. 11.3 in the placebo arm (HR: 0.70; p 0.001). 41% of patients received bisphosphonates and a delay in the onset of SREs was observed in this subgroup (19.6 vs. 10.2 months; HR: 0.49; p = 0.00048). At present, international guidelines recommend that, in men with mCRPC previously treated with docetaxel (or unsuitable for docetaxel) and one anti-androgen and who present bone pain, Ra-223 should be considered as a treatment option to prolong life, reduce symptomatic SREs and improve quality of life. Concomitant use with abiraterone and prednisone is advised against due to an increased risk of fracture [60].

As for the new anti-androgens, a post hoc analysis of the COU-AA-302 trial reported that, in patients with no previous chemotherapy, bone-directed therapies administered concomitantly with abiraterone and prednisone were associated with improved clinical results compared to previous studies in terms of OS, time to worsening in performance status and pain control [18]. Likewise, the PREVAIL study reported a 29% reduction in the risk of death and prolonged time to the first SRE in patients with no previous chemotherapy who received enzalutamide [61].

ZA has been evaluated in patients with metastatic hormone-sensitive prostate cancer (mHSPC). In the STAMPEDE trial, the addition of ZA to standard of care (SOC) treatment was not associated with an improvement in OS, disease free survival or delay in SRE [19]. Similar results were obtained in the CALGB 90202 study, which evaluated ADT plus ZA versus ADT plus placebo, and found no differences in the time to the first SRE, OS or grade $\ge$3 toxicity [62]. The ZAPCA trial also reported comparable results [63] (Table 1). There are still no published data on the role of denosumab in this setting.

For the reasons above, the use of ZA or denosumab is not approved for clinical practice in mHSPC for purposes of disease control.

In contrast, several clinical trials have shown benefits in OS and radiographic progression-free survival (rPFS) associated with next generation hormonal therapies [64, 65, 66, 67]. The ARCHES trial [68] and the ENZAMET trial laid the groundwork for the use of enzalutamide in patients with mHSPC. This last study reported an increase in 3-year OS (80% vs. 72%; HR: 0.39; p 0.001) and rPFS (p = 0.005) in favor of enzalutamide. In fact, in the ENZAMET study, the use of bone-directed therapies was used as a stratification factor (9.8% of patients). In the subgroup analysis, no clear clinical benefit was observed for OS (HR: 1.77; 95% confidence interval (CI) 0.83–3.77; p = 0.006) [66]. Similarly, the TITAN trial reported a benefit in the apalutamide group compared to placebo, both in OS (82.4% vs. 73.5%; HR: 0.67; 95% CI 0.51–0.89; p = 0.005) and rPFS (68.2% vs. 47.5%; HR: 0.48; 95% CI 0.39–0.60; p 0.001), although no significant differences in time to first SRE were observed [67]. At present, there are no randomized prospective studies on the sequence of hormonal therapies than can allow us to define the optimal timing for these types of treatments or their combination with bone-directed therapies. Additional studies are needed to solve an issue that is appearing more and more frequently in the management of these patients.