Radiopharmaceuticals in Prostate Cancer: Systemic “Bone-Seeking” Agents

Radiopharmaceuticals are pharmaceutical agents containing radioisotopes and emitting radiation that may be used for diagnostic or treatment purposes.

A number of small molecules have been used in conjunction with positron emission tomography (PET) scanning for prostate cancer staging. A recent presentation reported on the role of radiopharmaceutical driven imaging, predominately using Ga-PSMA, from the 2018 American Society of Clinical Oncology Annual Meeting.

From a therapeutic perspective, as they are typically given via intravenous infusion, radiopharmaceuticals are systemic radiotherapies, emitting alpha or beta radiation. Radiopharmaceuticals are indicated in patients with castrate-resistant prostate cancer with symptomatic bone metastases. Historically, beta-particle emitting agents including strontium-89 (Metastron®), samarium-153 (Quadramet®), phosphorus-32, and rhenium-186 were used as palliative therapies for patients with symptomatic bone disease.1 In this context, they are quite effective in relieving bony pain,2 however, these agents did not significantly improve survival.3 In contrast, the ALSYMPCA trial, which will be discussed in more detail below, demonstrated an improvement in both overall survival and skeletal-related events for patients receiving the alpha-emitter radium-223.4

In December 2018, the European Association of Nuclear Medicine Focus 1 Meeting reported a consensus regarding the use of molecular imaging and theranostics in prostate cancer.5 A number of relevant conclusions were derived following a systematic review and modified Delphi process. First, traditional diphosphate bone scan and contrast-enhanced computed tomography scan are mentioned but rarely recommended in the majority of patients in clinical guidelines. Second, magnetic resonance imaging and prostate cancer-targeted positron emission tomography are frequently suggested but the specific clinical scenarios in which they are most useful are poorly defined and how they may affect practice are poorly delineated. Third, sodium fluoride-18 positron emission tomography-CT bone scanning is not widely recommended; however, gallium-68 or fluorine-18 PSMA have gained acceptance. Finally, the palliative use of bone-targeting radiopharmaceuticals strontium-89, samarium-153, and rhenium-186 has been supplanted by radium-223, as well as other systemic therapies including docetaxel, abiraterone acetate, enzalutamide, and cabazitaxel.

Radium-223

Radium-223 dichloride (Xofigo®), commonly referred to as radium-223, is a targeted alpha emitter. It functions as a calcium mimetic and selectively binds newly forming bone stroma in regions of high bone turnover in osteoblastic or sclerotic bone metastasis.6 It then emits high-energy alpha particles with a very short range (less than 100 μm).7 This high-energy radiation induces a highly localized cytotoxic effect due to double-stranded DNA breakage.

Initial Phase I and Phase II studies in patients with bone metastasis demonstrated radium-223 to be well tolerated, with minimal myelosuppression.8,9 Phase II trials also demonstrated that radium-223 effectively reduced bone-related pain and improved disease-related biomarkers, including bone alkaline phosphatase and prostate-specific antigen (PSA).9

As a result, the Phase III, Alpharadin in Symptomatic Prostate Cancer Patients (ALSYMPCA) trial was undertaken to assess the efficacy of radium-223 versus placebo in patients with metastatic castration-resistant prostate cancer (mCRPC) and bone metastases across 136 study centers in 19 countries.4 The trial enrolled patients with two or more bone metastases, detected on skeletal scintigraphy, without visceral metastasis who had previously received docetaxel, were docetaxel ineligible or declined docetaxel. Patients were required to have symptomatic disease, based on the requirement for regular analgesics or prior treatment with external beam radiotherapy for cancer-related bone pain in the preceding 12 weeks. Additionally, patients had to have a baseline PSA of at least 5 ng/mL with at least two progressive PSA rises; an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2; life expectancy of at least six months; and adequate hematologic, renal and liver function. Patients who had received chemotherapy within the preceding four weeks were excluded.

A total of 921 patients were enrolled and were randomized in a 2:1 ratio to radium-223 (50 kBq per kilogram of body weight intravenously), once every four weeks, plus best standard of care or placebo plus best standard of care.

The primary study endpoint was overall survival and secondary outcomes included time to first symptomatic skeletal event; biochemical endpoints including time to increase in total alkaline phosphatase level, total alkaline phosphatase response, normalization of alkaline phosphatase, time to increase in PSA; safety endpoints; and quality of life.

An initial, pre-specified, interim analysis was undertaken when 314 deaths had occurred. This demonstrated longer median overall survival among patients who received radium-223 (14.0 months) than those receiving placebo (11.2 months) with a resulting 30% decrease in the risk of death (hazard ratio 0.70, 95% 0.55 to 0.88). A subsequent, updated analysis was performed following 528 deaths. This demonstrated consistent results with longer median overall survival among patients who received radium-223 (14.9 months) than those receiving placebo (11.3 months). Similarly, the updated analysis confirmed a 30% reduction in the risk of death (hazard ratio [HR] 0.70, 95% confidence interval [CI] 0.58 to 0.83) for patients receiving radium-223. This benefit was observed across subgroups including total alkaline phosphatase level at randomization, current bisphosphonate use, previous docetaxel treatment, baseline ECOG score (0/1 vs 2), extent of disease (<6 metastases, 6-20 metastases, >20 metastases, and super scan), and opioid use.

Assessment of the secondary endpoints demonstrated a consistent benefit for radium-223. Notably, radium-223 delayed time to first symptomatic skeletal event (median, 15.6 months vs 9.8 months; HR 0.66, 95% CI 0.52 to 0.83). Unlike many systemic therapies, patients who received radium-223 were less likely to experience adverse events than those who received placebo: all adverse events (93% vs 96%), grade 3 or 4 adverse events (56% vs 62%), serious adverse events (47% vs 60%), and treatment-discontinuation as a result of adverse events (16% vs 21%). Finally, patients who received radium-223 were significantly more likely to have an improvement in the quality of life compared to patients receiving a placebo (p=0.02).

The authors subsequently published a pre-planned analysis with stratification according to receipt of prior docetaxel.10 Radium-223 prolonged survival both in patients who had previously received docetaxel (HR 0.70, 95% CI 0.56 to 0.88) and those who had not previously received docetaxel (HR 0.69 (95% CI 0.52 to 0.92).

As both radium-223 and abiraterone acetate11,12 have demonstrated survival benefits in patients with metastatic castrate-resistant prostate cancer, there was interest in combining these two agents. The ERA 223 trial randomized 806 patients with chemotherapy-naïve, metastatic castrate-resistant prostate cancer with bone metastasis to radium-223 or placebo, in addition to abiraterone acetate. Symptomatic skeletal event-free survival was the primary outcome. Somewhat unexpectedly, the trial was unblinded prematurely as more fractures and deaths were identified in the radium-223 arm than among patients receiving placebo. Median skeletal event-free survival was 22.3 months (interquartile range 17.0 to 25.8 months) among patients receiving radium-223 and abiraterone acetate and 26.0 months (interquartile range 21.8 months to 28.3 months) in patients receiving placebo and abiraterone acetate (HR 1.12, 95% CI 0.92 to 1.37). Fractures were more common among patients receiving radium-223 and abiraterone acetate (29%) than those receiving placebo and abiraterone acetate (11%). Thus, the combination of radium-223 and abiraterone acetate is not recommended in combination, however other combinations of agents with radium-223 are currently being tested (ie. enzalutamide).

Role of Radiopharmaceuticals in the AUA Guideline

The American Urological Association Guideline on Castrate-Resistant Prostate Cancer (amended in 2018) defines a number of clinical scenarios in which radiopharmaceuticals may be considered.13

First, among patients with good performance status and have not yet received docetaxel but who are symptomatic (based on a definition requiring regular use of narcotic analgesics for pain that is attributable to documented metastasis), radium-223 may be offered to patients who have symptoms attributable to bony metastatic disease in the absence of visceral disease in addition to standard of care options including abiraterone acetate plus prednisone, enzalutamide, and docetaxel. Patients with symptomatic metastases who decline these standard therapies, alternative treatments including radionuclide therapy (such as strontium-89) may be offered.13

Second, for symptomatic patients with poor performance status who have not previously received docetaxel, there is a relative paucity of direct evidence to inform treatment choices as most patients with poor performance status are excluded from clinical trials. Based on extrapolation from studies in patients with better performance status, the guideline recommends considering aggressive prostate cancer treatment where the functional impairments resulting in poor performance status are directly attributable to prostate cancer. In cases where the poor performance status is related to bony metastatic disease, radium-223 is a recommended option.13

Third, among patients with metastatic castrate-resistant prostate cancer who have previously received docetaxel-based chemotherapy, radium-223 is one of four agents with a proven survival benefit, along with abiraterone acetate plus prednisone, enzalutamide, and cabazitaxel.

Fourth, and finally, in patients with advanced mCRPC who are symptomatic and have poor performance status following previous docetaxel chemotherapy, symptom management is strongly advocated in keeping with the American Society for Clinical Oncology’s guidance regarding the treatment of patients with advanced solid tumors. However, judicious use of radionuclide therapy, along with abiraterone acetate plus prednisone, enzalutamide, ketoconazole plus steroids, are offered within the AUA guidelines13 despite the lack of strong data to support the use of these agents in this patient population.

The Canadian Urologic Association Guidelines similarly recommend radium-223 in patients with metastatic castrate-resistant prostate cancer who have bone pain related to their metastases and no visceral disease.14

New directions

The ALSYMPCA trial was the first to demonstrate that radiopharmaceuticals could improve overall survival, in addition to skeletal-related events,4 in patients with metastatic castrate-resistant prostate cancer. On the basis of this observation, there is an ongoing effort to identify molecular targets for linkage to radiopharmaceuticals. Proposed targets have included prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptors (GRPr),15 These have the advantage of targeting prostate cancer cells, rather than being inherently bone targeting as is the case for current radiopharmaceuticals. Thus far, preliminary data based on prostate-specific membrane antigen targeted beta-emitters such as lutetium-177 suggest a promise to this approach but further work remains prior to the adoption of this approach.1 Further, ongoing research assessing prostate-specific membrane antigen targeted alpha-emitters is ongoing.

Written by: Christopher J.D. Wallis, MD, PhD, Instructor in Urology, Vanderbilt University Medical Center, Nashville, Tennessee; Zachary Klaassen, MD, MSc, Assistant Professor of Urology, Georgia Cancer Center, Augusta University/Medical College of Georgia, Atlanta, Georgia

Published Date: December 2nd, 2019

Written by: Christopher J.D. Wallis, MD, PhD and Zachary Klaassen, MD, MSc
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