Multitargeted tyrosine kinase inhibition produces discordant changes between 99mTc-MDP bone scans and other disease biomarkers: analysis of a phase II study of sunitinib for metastatic castration-resistant prostate cancer

Prostate cancer microenvironment: multidimensional regulation of immune cells, vascular system, stromal cells, and microbiota

BACKGROUND

Prostate cancer (PCa) is one of the most prevalent malignancies in males worldwide. Increasing research attention has focused on the PCa microenvironment, which plays a crucial role in tumor progression and therapy resistance. This review aims to provide a comprehensive overview of the key components of the PCa microenvironment, including immune cells, vascular systems, stromal cells, and microbiota, and explore their implications for diagnosis and treatment.

METHODS

Keywords such as "prostate cancer", "tumor microenvironment", "immune cells", "vascular system", "stromal cells", and "microbiota" were used for literature retrieval through online databases including PubMed and Web of Science. Studies related to the PCa microenvironment were selected, with a particular focus on those discussing the roles of immune cells, vascular systems, stromal cells, and microbiota in the development, progression, and treatment of PCa. The selection criteria prioritized peer-reviewed articles published in the last five years, aiming to summarize and analyze the latest research advancements and clinical relevance regarding the PCa microenvironment.

RESULTS

The PCa microenvironment is highly complex and dynamic, with immune cells contributing to immunosuppressive conditions, stromal cells promoting tumor growth, and microbiota potentially affecting androgen metabolism. Vascular systems support angiogenesis, which fosters tumor expansion. Understanding these components offers insight into the mechanisms driving PCa progression and opens avenues for novel therapeutic strategies targeting the tumor microenvironment.

CONCLUSIONS

A deeper understanding of the PCa microenvironment is crucial for advancing diagnostic techniques and developing precision therapies. This review highlights the potential of targeting the microenvironment to improve patient outcomes, emphasizing its significance in the broader context of PCa research and treatment innovation.

Superscan Pattern on Bone Scintigraphy: A Comprehensive Review

BACKGROUND/OBJECTIVES

The superscan pattern is a characteristic finding on bone scintigraphy, associated with a variety of metabolic bone diseases, malignancies, and other conditions. This pattern is characterized by a diffuse and intense uptake of radiotracer throughout the entire skeleton. Despite being a relatively rare finding, the superscan pattern can have significant clinical implications.

METHODS

This comprehensive review summarizes the available literature on the superscan pattern, focusing on its pathophysiology, clinical significance, and differential diagnoses. Relevant studies and case reports were analyzed to outline the diagnostic challenges associated with the interpretation of bone scintigraphy featuring the superscan pattern.

RESULTS

The literature highlights the clinical significance of the superscan pattern in various metabolic and oncologic conditions. Misinterpretation of this pattern can lead to diagnostic challenges, especially in distinguishing it from other pathologic conditions. Differential diagnosis remains crucial in the accurate interpretation and subsequent management of patients with this finding.

CONCLUSIONS

This review provides a comprehensive overview of the superscan pattern on bone scintigraphy, aiming to assist clinicians in recognizing and managing this rare yet clinically important finding.

The Multifaceted Role of Aldehyde Dehydrogenases in Prostate Cancer Stem Cells

Cancer stem cells (CSCs) are the only tumor cells possessing self-renewal and differentiation properties, making them an engine of tumor progression and a source of tumor regrowth after treatment. Conventional therapies eliminate most non-CSCs, while CSCs often remain radiation and drug resistant, leading to tumor relapse and metastases. Thus, targeting CSCs might be a powerful tool to overcome tumor resistance and increase the efficiency of current cancer treatment strategies. The identification and isolation of the CSC population based on its high aldehyde dehydrogenase activity (ALDH) is widely accepted for prostate cancer (PCa) and many other solid tumors. In PCa, several ALDH genes contribute to the ALDH activity, which can be measured in the enzymatic assay by converting 4, 4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY) aminoacetaldehyde (BAAA) into the fluorescent product BODIPY-aminoacetate (BAA). Although each ALDH isoform plays an individual role in PCa biology, their mutual functional interplay also contributes to PCa progression. Thus, ALDH proteins are markers and functional regulators of CSC properties, representing an attractive target for cancer treatment. In this review, we discuss the current state of research regarding the role of individual ALDH isoforms in PCa development and progression, their possible therapeutic targeting, and provide an outlook for the future advances in this field.

Molecular imaging of bone metastases using bone targeted tracers

Molecular imaging using bone targeted tracers has been used in clinical practice for almost fifty years and still plays an essential role in the diagnosis and follow-up of bone metastases. It includes both [99mTc]bisphosphonates for bone scan and [18F]NaF for positron emission tomography/computed tomography (PET/CT) which are very sensitive to detect osteoblastic activity, but it is important to consider several aspects to increase the specificity of reported findings (such as specific tracer characteristics and mechanism of action, patient's clinical history, common metastatic patterns, changes after treatment, limitations of the technique, variations and pitfalls). This will enable useful information for clinical management being provided in the report. Furthermore, iatrogenic skeletal adverse events are common and they should also be identified, as they have impact on patient's quality of life. This review makes a brief summary of the mechanism of action of bone targeted tracers, followed by a discussion of classic patterns of bone metastasis, treatment response assessment and iatrogenic skeletal complications. The value of hybrid imaging techniques with bone targeted tracers, including single photon emission computed tomography and PET/CT is also explored. The final part summarizes new bone targeted tracers with superior imaging characteristics that are being developed, and which may further enhance the applications of radionuclide bone imaging.

Phase III Study of Cabozantinib in Previously Treated Metastatic Castration-Resistant Prostate Cancer: COMET-1

Purpose

Cabozantinib is an inhibitor of kinases, including MET and vascular endothelial growth factor receptors, and has shown activity in men with previously treated metastatic castration-resistant prostate cancer (mCRPC). This blinded phase III trial compared cabozantinib with prednisone in patients with mCRPC.

Patients and Methods

Men with progressive mCRPC after docetaxel and abiraterone and/or enzalutamide were randomly assigned at a two-to-one ratio to cabozantinib 60 mg once per day or prednisone 5 mg twice per day. The primary end point was overall survival (OS). Bone scan response (BSR) at week 12 as assessed by independent review committee was the secondary end point; radiographic progression-free survival (rPFS) and effects on circulating tumor cells (CTCs), bone biomarkers, serum prostate-specific antigen (PSA), and symptomatic skeletal events (SSEs) were exploratory assessments.

Results

A total of 1,028 patients were randomly assigned to cabozantinib (n = 682) or prednisone (n = 346). Median OS was 11.0 months with cabozantinib and 9.8 months with prednisone (hazard ratio, 0.90; 95% CI, 0.76 to 1.06; stratified log-rank P = .213). BSR at week 12 favored cabozantinib (42% v 3%; stratified Cochran-Mantel-Haenszel P < .001). rPFS was improved in the cabozantinib group (median, 5.6 v 2.8 months; hazard ratio, 0.48; 95% CI, 0.40 to 0.57; stratified log-rank P < .001). Cabozantinib was associated with improvements in CTC conversion, bone biomarkers, and post–random assignment incidence of SSEs but not PSA outcomes. Grade 3 to 4 adverse events and discontinuations because of adverse events were higher with cabozantinib than with prednisone (71% v 56% and 33% v 12%, respectively).

Conclusion

Cabozantinib did not significantly improve OS compared with prednisone in heavily treated patients with mCRPC and progressive disease after docetaxel and abiraterone and/or enzalutamide. Cabozantinib had some activity in improving BSR, rPFS, SSEs, CTC conversions, and bone biomarkers but not PSA outcomes.

Cabozantinib resolves bone scans in tumor-naïve mice harboring skeletal injuries

The receptor tyrosine kinase inhibitor cabozantinib (XL184, BMS-907351 Cometriq) has displayed impressive clinical activity against several indications, culminating in its recent approval for medullary thyroid cancer. Among malignancies with tropism for the bone (prostate, breast), one striking feature of early clinical reports about this drug has been the rapid and complete resolution of bone scans, a phenomenon almost never observed even among therapies already shown to confer survival benefit. In castration-resistant prostate cancer, not all conventional response indicators change as dramatically posttreatment, raising the possibility that cabozantinib may impair the ability of bone-seeking radionuclides to integrate within the remodeling bone. To test this hypothesis, we surgically induced bone remodeling via physical insult in non-tumor-bearing mice and performed 18F-sodium fluoride (18F-NaF) positron emission tomographic (PET) and technetium 99m-methylene diphosphonate (99mTc-MDP) single-photon emission computed tomographic (SPECT) scans pre- and posttreatment with cabozantinib and related inhibitors. A consistent reduction in the accumulation of either radiotracer at the site of bone remodeling was observed in animals treated with cabozantinib. Given that cabozantinib is known to inhibit several receptor tyrosine kinases, we drugged animals with various permutations of more selective inhibitors to attempt to refine the molecular basis of bone scan resolution. Neither the vascular endothelial growth factor receptor (VEGFR) inhibitor axitinib, the MET inhibitor crizotinib, nor the combination was capable of inhibiting 18F-NaF accumulation at known bioactive doses. In summary, although the mechanism by which cabozantinib suppresses radionuclide incorporation into foci undergoing bone remodeling remains unknown, that this phenomenon occurs in tumor-naïve models indicates that caution should be exercised in interpreting the clinical significance of this event.

A phase II study of GW786034 (pazopanib) with or without bicalutamide in patients with castration-resistant prostate cancer

INTRODUCTION

Pazopanib is an oral vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitor. In this randomized, open label phase II study, pazopanib alone or in combination with bicalutamide was evaluated in patients with chemotherapy-naive castration-resistant prostate cancer (CRPC).

PATIENTS AND METHODS

Patients received either pazopanib 800 mg daily (arm A) or pazopanib 800 mg plus bicalutamide 50 mg daily (arm B). A 2-stage study design was used, and the primary endpoint was prostate-specific antigen (PSA) response rate (defined as a confirmed ≥ 50% decline from baseline).

RESULTS

A total of 23 patients (arm A, 10; arm B, 13) were accrued. The main grade 3+ toxicities were hypertension, fatigue, decreased lymphocytes, and increased alanine transaminase. Owing to significant toxicity, the protocol was amended after the first 11 patients and the pazopanib starting dose was reduced to 600 mg daily. In arm A, of 9 evaluable patients, there was 1 patient (11%) with a PSA response, 3 (33%) with stable PSA, and 5 (56%) with PSA progression; in arm B, of 12 evaluable patients, there were 2 patients (17%) with PSA responses, 6 (50%) with stable PSA, and 4 (33%) with PSA progression. Median progression-free survival was similar in both arms at 7.3 months (95% CI, 2.5 months to not reached). Long-term stable disease was seen in 4 patients who remained on treatment for 18 months (arm A), 26 months (arm A), 35 months (arm B), and 52 months (arm B).

CONCLUSION

In this unselected patient population, pazopanib either alone or in combination with bicalutamide failed to show sufficient activity to warrant further evaluation. However, 4 patients had long-term benefit, suggesting that targeting the VEGFR pathway may still be relevant in selected patients and emphasizing the need for improved predictive markers for patients with CRPC.

Targeting metastatic castration-resistant prostate cancer: mechanisms of progression and novel early therapeutic approaches

INTRODUCTION

Advances in clinical research have led to official approval of several new treatments for metastatic prostate cancer in the last three years: sipuleucel-T, cabazitaxel, abiraterone acetate, radium-223 and enzalutamide. Although these agents have all been shown to improve overall survival in randomized Phase III trials, metastatic castration-resistant prostate cancer (mCRPC) remains incurable.

AREAS COVERED

First, the review summarizes the current literature on the biology of mCRPC. The emerging data are increasing our understanding of the mechanisms that underlie the pathogenesis of castrate resistance and where future treatment might be headed. In the second part of the review, the authors assess the future directions in disease therapy. Indeed, novel selected therapeutic approaches, including novel agents and combinatorial therapies, are showing promising early results.

EXPERT OPINION

Targeting different molecular pathways in combination with immunotherapy can be a promising direction in metastatic castration prostate cancer treatment. However, several challenges still exist including elucidating the optimal use and sequencing of these new agents. There are also challenges in both the design and the interpretation of the results from clinical trials.

Randomized, Placebo-Controlled, Phase III Trial of Sunitinib Plus Prednisone Versus Prednisone Alone in Progressive, Metastatic, Castration-Resistant Prostate Cancer

Purpose

We evaluated angiogenesis-targeted sunitinib therapy in a randomized, double-blind trial of metastatic castration-resistant prostate cancer (mCRPC).

Patients and Methods

Men with progressive mCRPC after docetaxel-based chemotherapy were randomly assigned 2:1 to receive sunitinib 37.5 mg/d continuously or placebo. Patients also received oral prednisone 5 mg twice daily. The primary end point was overall survival (OS); secondary end points included progression-free survival (PFS). Two interim analyses were planned.

Results

Overall, 873 patients were randomly assigned to receive sunitinib (n = 584) or placebo (n = 289). The independent data monitoring committee stopped the study for futility after the second interim analysis. After a median overall follow-up of 8.7 months, median OS was 13.1 months and 11.8 months for sunitinib and placebo, respectively (hazard ratio [HR], 0.914; 95% CI, 0.762 to 1.097; stratified log-rank test, P = .168). PFS was significantly improved in the sunitinib arm (median 5.6 v 4.1 months; HR, 0.725; 95% CI, 0.591 to 0.890; stratified log-rank test, P < .001). Toxicity and rates of discontinuations because of adverse events (AEs; 27% v 7%) were greater with sunitinib than placebo. The most common treatment-related grade 3/4 AEs were fatigue (9% v 1%), asthenia (8% v 2%), and hand–foot syndrome (7% v 0%). Frequent treatment-emergent grade 3/4 hematologic abnormalities were lymphopenia (20% v 11%), anemia (9% v 8%), and neutropenia (6% v < 1%).

Conclusion

The addition of sunitinib to prednisone did not improve OS compared with placebo in docetaxel-refractory mCRPC. The role of antiangiogenic therapy in mCRPC remains investigational.

Recent progress in pharmaceutical therapies for castration-resistant prostate cancer

Since 2010, six drugs have been approved for the treatment of castration-resistant prostate cancer, i.e., CYP17 inhibitor Abiraterone, androgen receptor antagonist Enzalutamide, cytotoxic agent Cabazitaxel, vaccine Sipuleucel-T, antibody Denosumab against receptor activator of nuclear factor kappa B ligand and radiopharmaceutical Alpharadin. All these drugs demonstrate improvement on overall survival, expect for Denosumab, which increases the bone mineral density of patients under androgen deprivation therapy and prolongs bone-metastasis-free survival. Besides further CYP17 inhibitors (Orteronel, Galeterone, VT-464 and CFG920), androgen receptor antagonists (ARN-509, ODM-201, AZD-3514 and EZN-4176) and vaccine Prostvac, more drug candidates with various mechanisms or new indications of launched drugs are currently under evaluation in different stages of clinical trials, including various kinase inhibitors and platinum complexes. Some novel strategies have also been proposed aimed at further potentiation of antitumor effects or reduction of side effects and complications related to treatments. Under these flourishing circumstances, more investigations should be performed on the optimal combination or the sequence of treatments needed to delay or reverse possible resistance and thus maximize the clinical benefits for the patients.

A dose-ranging study of cabozantinib in men with castration-resistant prostate cancer and bone metastases

BACKGROUND

Cabozantinib is an oral MET/VEGFR2 inhibitor. A recent phase II study of cabozantinib (100 mg daily) showed improved bone scans in subjects with metastatic castration-resistant prostate cancer (mCRPC), but adverse events (AE) caused frequent dose reductions. This study was designed to determine the efficacy and tolerability of cabozantinib at lower starting doses.

EXPERIMENTAL DESIGN

An adaptive design was used to determine the lowest active daily dose among 60, 40, and 20 mg. The primary endpoint was week 6 bone scan response, defined as ≥30% decrease in bone scan lesion area. The secondary endpoint was change in circulating tumor cells (CTC).

RESULTS

Among 11 evaluable subjects enrolled at 40 mg, there were 9 partial responses (PR), 1 complete response, and 1 stable disease (SD). Of 10 subjects subsequently enrolled at 20 mg, there were 1 PR, 5 SDs, and 4 with progressive disease. Among 13 subjects enrolled on the 40 mg expansion cohort, there were 6 PRs and 7 SDs. No subjects required dose reduction or treatment interruption at 6 or 12 weeks; 3 subjects at dose level 0 discontinued due to AEs by 12 weeks. At 40 mg, median treatment duration was 27 weeks. 58% of subjects with ≥5 CTCs/7.5 mL at baseline converted to <5.

CONCLUSIONS

Cabozantinib 40 mg daily was associated with a high rate of bone scan response. Cabozantinib 40 mg daily was associated with better tolerability than previously reported for cabozantinib 100 mg daily. These observations informed the design of phase III studies of cabozantinib in mCRPC.

Targeting MET and vascular endothelial growth factor receptor signaling in castration-resistant prostate cancer

Effective management of bone metastases in men with castration-resistant prostate cancer (CRPC) remains an important unmet medical need. MET and vascular endothelial growth factor receptor (VEGFR) are rational targets for intervention in CRPC. Clinical trials involving agents that inhibit one but not both pathways have reported modest activity and no improvement in overall survival. Cabozantinib is an oral multitargeted tyrosine kinase inhibitor that inhibits both MET and VEGFR-2. A phase II randomized discontinuation study involving subjects with CRPC demonstrated that cabozantinib therapy is associated with improvement in bone scans, bone turnover markers, and pain response, but with significant adverse events leading to dose reduction and treatment discontinuation. Lower doses of cabozantinib retain high levels of activity with less toxicity. Ongoing phase III clinical trials will define the role of cabozantinib in CRPC. We summarize the rationale for targeting MET and VEGFR pathways in CRPC and the clinical data available to date.