The Role of Theranostics in Prostate Cancer

Theranostics in men with metastatic castration-resistant prostate cancer (mCRPC) has been developed to target bone and the tumor itself. Currently, bone-directed targeted alpha therapy with radium-223 (²²³Ra) is the only theranostic agent proven to prolong survival in men with mCRPC who have symptomatic bone metastases and no known visceral metastases. The clinical utility and therapeutic success of ²²³Ra has encouraged the development of other tumor-targeting theranostic agents in mCRPC, primarily targeting prostate-specific membrane antigen (PSMA) with radioligand therapy (RLT). There is increasing evidence of promising response rates and a low toxicity profile with ¹⁷⁷Lu-labeled PSMA RLT in patients with mCRPC. A phase III randomized study of ¹⁷⁷Lu-labeled PSMA RLT has completed accrual and is awaiting results as to whether the drug improves radiographic progression-free survival and overall survival in men with mCRPC receiving standard of care treatments. Additional early clinical trials are investigating the role of tumor-directed targeted alpha therapy with radiotracers such as ²²⁵Ac. In this article, we review the current status of theranostics in prostate cancer, discussing the challenges and opportunities of combination therapies with more conventional agents such as androgen receptor inhibitors, cytotoxic chemotherapy, and immunotherapy.

Prospective Evaluation of Bone Metabolic Markers as Surrogate Markers of Response to Radium-223 Therapy in Metastatic Castration-resistant Prostate Cancer

PURPOSE

Radium-223 is approved for metastatic castration-resistant prostate cancer (mCRPC) based on improved overall survival, and delay in skeletal related events. However, it is not associated with PSA or radiographic response, which poses a challenge in real-time assessment of its efficacy. Surrogate markers of treatment outcomes may facilitate tailoring treatment duration with radium-223, by limiting the duration of therapy with radium-223 in these patients. Here, we sought to investigate the utility of bone metabolic markers (BMMs) as surrogate markers of response to radium-223 in mCRPC.

PATIENTS AND METHODS

A prospective phase II trial of radium-223 plus enzalutamide (RE) versus enzalutamide alone was designed to assess surrogacy of BMMs with respect to response to radium-223. Enzalutamide was used as a comparator in lieu of placebo due to the progressive disease. Co-primary endpoints were relative change in serum BMM N-telopeptide (NTP) levels from baseline to 6 months between the two arms and safety and feasibility of the combination.

RESULTS

Thirty-nine men were randomized to RE (n = 27) or enzalutamide (n = 12). Combination was safe and feasible. Primary endpoint was met. A statistically significant relative change to NTP ratios between arms (0.64, 95% confidence interval, 0.51-0.81; P = 0.00048) favored RE versus enzalutamide. Overall, BMMs decreased with the RE therapy compared with enzalutamide. Improved PSA response rate in RE versus enzalutamide (P = 0.024), correlated with decline in BMMs.

CONCLUSIONS

BMMs declined significantly with combination therapy, and were associated with improved outcomes. Upon external validation, BMMs may emerge as surrogate markers to monitor treatment with radium-223 in real-time.

Radium-223 mechanism of action: implications for use in treatment combinations

The targeted alpha therapy radium-223 (223Ra) can prolong survival in men with castration-resistant prostate cancer (CRPC) who have symptomatic bone metastases and no known visceral metastases. Preclinical studies demonstrate that 223Ra preferentially incorporates into newly formed bone matrix within osteoblastic metastatic lesions. The emitted high-energy alpha particles induce DNA double-strand breaks that might be irreparable and lead to cell death in nearby exposed tumour cells, osteoblasts and osteoclasts. Consequently, tumour growth and abnormal bone formation are inhibited by these direct effects and by the disruption of positive-feedback loops between tumour cells and the bone microenvironment. 223Ra might also modulate immune responses within the bone. The clinical utility of 223Ra has encouraged the development of other anticancer targeted alpha therapies. A thorough understanding of the mechanism of action could inform the design of new combinatorial treatment strategies that might be more efficacious than monotherapy. On the basis of the current mechanistic knowledge and potential clinical benefits, combination therapies of 223Ra with microtubule-stabilizing cytotoxic drugs and agents targeting the androgen receptor axis, immune checkpoint receptors or DNA damage response proteins are being explored in patients with CRPC and metastatic bone disease.

Bone Health Issues in Patients with Prostate Cancer: An Evidence-Based Review

Bone health in prostate cancer patients represents a prerequisite for acceptable quality of life and optimal outcome of this disease. The major threat for bone health in prostate cancer displays cancer treatment induced bone loss as well as the development of bone metastases. In recent years, several new pharmaceuticals targeting bone metabolism such as denosumab or androgen pathway targeting drugs (abiraterone acetate and enzalutamide) have been approved for the treatment of progressive disease aiming to interrupt the vicious circle of bone metastasis and aberrant bone resorption. This development raised the awareness of the pivotal role of bone health in prostate cancer and introduced (symptomatic) skeletal related events as an important end point in recent clinical trials. Bone targeted drugs have become standard of care in patients with metastatic castration resistant prostate cancer, their role in metastatic hormone sensitive prostate cancer has been discussed controversely. In oligometastatic prostate cancer patients several promising approaches in metastasis directed therapy, including conventional surgery, stereotactic ablative radiation and image-guided single-fraction robotic stereotactic radiosurgery (CyberKnife®) were launched but are not in routine clinical use until now caused by sparse clinical evidence.

Bone-Targeting Radiopharmaceuticals as Monotherapy or Combined With Chemotherapy in Patients With Castration-Resistant Prostate Cancer Metastatic to Bone

In patients with metastatic castration-resistant prostate cancer, bone is the most common site for metastases. Because of their osteoblastic character, these lesions are very suitable for treatment with bone-seeking radiopharmaceuticals (RPs). Nowadays, radium-223-chloride is the only RP with a proven benefit in overall survival, whereas the β-emitting RPs are used for pain palliation. In the past, many trials that investigated RPs alone, or in combination with chemotherapy have been performed. Because of different designs, characteristics of included patients, and chemotherapeutical and RP regimens, interpretation of the promising data and positioning of RPs in the treatment of metastatic prostate cancer has become difficult. In this review, we provide an overview of the existing data per RP with a focus on the different RPs in combination with chemotherapy. Furthermore, we aim to clarify the benefits on pain response and quality of life. Finally, we focus on the optimal timing and use of biomarkers in the treatment of patients with castration-resistant prostate cancer with RPs.

α-Emitters for Radiotherapy: From Basic Radiochemistry to Clinical Studies-Part 2

The use of radioactive sources to deliver cytotoxic ionizing radiation to disease sites dates back to the early 20th century, with the discovery of radium and its physiologic effects. α-emitters are of particular interest in the field of clinical oncology for radiotherapy applications. The first part of this review explored the basic radiochemistry, high cell-killing potency, and availability of α-emitting radionuclides, together with hurdles such as radiolabeling methods and daughter redistribution. The second part of this review will give an overview of the most promising and current uses of α-emitters in preclinical and clinical studies.

Current treatment strategies for advanced prostate cancer

During the past decade, treatment strategies for patients with advanced prostate cancer involving stage IV (T4N0M0, N1M0 or M1) hormone-sensitive prostate cancer and recurrent prostate cancer after treatment with curative intent, as well as castration-resistant prostate cancer, have extensively evolved with the introduction and approval of several new agents including sipuleucel-T, radium-223, abiraterone, enzalutamide and cabazitaxel, all of which have shown significant improvement on overall survival. The appropriate use of these agents and the proper sequencing of these agents are still not optimized. The results of several recently reported randomized controlled trials and retrospective studies could assist in developing a treatment strategy for advanced prostate cancer. In addition, prospective studies and molecular characterization of tumors to address these issues are ongoing.

Radionuclides in oncology clinical practice – review of the literature

Radionuclide therapy is a promising type of targeted therapy for cancer and its use is becoming more common in several types of malignant tumors.