MAOA-Dependent Activation of Shh-IL6-RANKL Signaling Network Promotes Prostate Cancer Metastasis by Engaging Tumor-Stromal Cell Interactions

Metastasis is a predominant cause of death for prostate cancer (PCa) patients; however, the underlying mechanisms are poorly understood. We report that monoamine oxidase A (MAOA) is a clinically and functionally important mediator of PCa bone and visceral metastases, activating paracrine Shh signaling in tumor-stromal interactions. MAOA provides tumor cell growth advantages in the bone microenvironment by stimulating interleukin-6 (IL6) release from osteoblasts, and triggers skeletal colonization by activating osteoclastogenesis through osteoblast production of RANKL and IL6. MAOA inhibitor treatment effectively reduces metastasis and prolongs mouse survival by disengaging the Shh-IL6-RANKL signaling network in stromal cells in the tumor microenvironment. These findings provide a rationale for targeting MAOA and its associated molecules to treat PCa metastasis.

Luminal and basal subtyping of prostate cancer

3

Background: There is a clear need to develop a clinically relevant molecular subtyping approach for prostate cancer. We hypothesized that prostate cancer can be subtyped based on luminal versus basal lineage. Methods: We applied the PAM50 classifier, which is used clinically to identify luminal and basal cancers in breast cancer, to subtype a total of 3,782 prostate cancer samples using a high-density microarray platform run in a CLIA-certified laboratory. We examined the associations of these subtypes and clinical outcomes. Results: We demonstrate that PAM50 segregates prostate cancer into three reproducible subtypes in both retrospective cohorts and on prospective validation: luminal A (33.3%-34.3%), luminal B (28.5%-32.6%), and basal (34.1%-37.1%). Luminal B prostate cancers exhibited the worst clinical prognoses, followed by basal and luminal A subtypes (10-year biochemical recurrence-free survival: 29/39/41%; distant metastasis-free survival: 53/73/73%; prostate cancer-specific survival: 78/86/89%; overall survival: 69/80/82% respectively) on both univariable and multivariable analyses accounting for standard clinicopathologic prognostic factors. Known luminal lineage markers, such as NKX3.1 and KRT18, and the basal lineage CD49f signature, were enriched in luminal- and basal-like cancers respectively, demonstrating the connection between these subtypes and established prostate cancer biology. While both luminal-like subtypes were associated with increased AR expression and signaling, only luminal B prostate cancers were significantly associated with post-operative response to androgen deprivation therapy (ADT) in a subset analysis matching patients based on clinicopathologic variables (interaction p = 0.006, luminal B 10-year metastasis: 33% (treated) vs. 55% (untreated), non-luminal B: 37% (treated) vs. 21% (untreated)). Conclusions: These findings contribute novel insight into the biology of prostate cancer, and provide translatable clinical tools for personalizing post-operative ADT for patients with prostate cancer. Similar to breast cancer, these findings suggest that luminal/basal subtyping may be useful in treatment selection in prostate cancer.

Phase Ib study of apalutamide (APA) with abiraterone acetate (AA) and prednisone (P) in patients (pts) with metastatic castration-resistant prostate cancer (mCRPC): Update on safety and efficacy

173

Background: APA and AA target the androgenreceptor (AR) axis via different mechanisms and may have complementary activity in mCRPC. APA is an advanced AR antagonist that targets the AR ligand-binding domain with high affinity (Clegg. Cancer Res. 2012). APA prevents AR nuclear translocation, DNA binding, and transcription of AR gene targets. This phase 1b study evaluates potential PK interactions between APA and AA + P. Here we report antitumor activity and safety of APA in combination with AA + P from 57 pts versus 29 pts presented at ASCO 2015 (NCT02123758). Methods: Pts with progressive mCRPC and ECOG PS ≤ 2 received AA (1000 mg/d) + P (5 mg BID) on Cycle 1 Day 1 (C1D1) with addition of APA (240 mg/d) on C1D8 in 28-day treatment (tmt) cycles. Efficacy assessment was based on RECIST 1.1 and PCWG2 criteria. Results: 57 pts started tmt on study; median tmt duration was 17 weeks. Median age was 70 years (range, 49-89) and median baseline PSA was 111 µg/L (range, 4-2597). Bone, nodal, and visceral disease were present in 50 (88%), 31 (54%), and 17 (30%) pts, respectively. 29 (51%), 29 (51%), and 23 (40%) pts were previously treated with a taxane, AA, or enzalutamide (ENZ), respectively. 47 pts discontinued tmt: disease progression (n = 39), consent withdrawal (n = 3), physician decision (n = 1), death (n = 1), and other (n = 3). In AA- and ENZ-naïve pts (n = 18), 67% had PSA decline ≥ 50%. In AA- or ENZ-treated pts (n = 39), 15% had PSA decline ≥ 50%. Most commonly reported ( > 10% of pts) drug-related AEs: fatigue (42%), diarrhea (21%), vomiting (21%), nausea (19%), hypokalemia (19%), decreased appetite (16%), hot flush (12%), abdominal pain (12%), and dysgeusia (11%). Grade ≥ 3 drug-related AEs reported in > 1 pt: fatigue (7%), hypokalemia (3.5%), hyponatremia (3.5%), and hypertension (3.5%). 1 pt discontinued study drug for grade 3 fatigue. Conclusions: Interim data indicate that 240 mg/d of APA with 1000 mg/d of AA + P has antitumor activity and an acceptable safety profile in mCRPC. Clinical trial information: NCT02123758. [Table: see text]

Circulating tumor cells in prostate cancer: beyond enumeration

Circulating tumor cells (CTCs) are a population of rare cancer cells that have detached from the primary tumor and/or metastatic lesions and entered the peripheral circulation. Enumeration of CTCs has demonstrated value as a prognostic biomarker, and newer studies have pointed to information beyond enumeration that is of critical importance in prostate cancer. Technologic advances that permit examination of the morphology, function, and molecular content of CTCs have made it possible to measure these factors as part of liquid biopsy. These advances provide a way to study tumor evolution and the development of resistance to therapy. Recent breakthroughs have created new applications for CTCs that will affect the care of patients with prostate cancer.

Applications of circulating tumor cells for prostate cancer

One of the major challenges that clinicians face is in the difficulties of accurately monitoring disease progression. Prostate cancer is among these diseases and greatly affects the health of men globally. Circulating tumor cells (CTCs) are a rare population of cancer cells that have shed from the primary tumor and entered the peripheral circulation. Not until recently, clinical applications of CTCs have been limited to using enumeration as a prognostic tool in Oncology. However, advances in emerging CTC technologies point toward new applications that could revolutionize the field of prostate cancer. It is now possible to study CTCs as components of a liquid biopsy based on morphological phenotypes, biochemical analyses, and genomic profiling. These advances allow us to gain insight into the heterogeneity and dynamics of cancer biology and to further study the mechanisms behind the evolution of therapeutic resistance. These recent developments utilizing CTCs for clinical applications will greatly impact the future of prostate cancer research and pave the way towards personalized care for men.

In Men with Castration-Resistant Prostate Cancer, Visceral Metastases Predict Shorter Overall Survival: What Predicts Visceral Metastases? Results from the SEARCH Database

BACKGROUND

Although visceral metastases (VMs) are widely recognized to portend worse prognoses compared with bone and lymph metastases in men with metastatic castration-resistant prostate cancer (mCRPC), little is known about what predicts VMs and the extent to which men with VMs do worse.

OBJECTIVE

To determine whether men with VMs at initial mCRPC diagnosis have worse overall survival (OS) and identify predictors of VMs.

DESIGN, SETTING, AND PARTICIPANTS

We analyzed 494 men diagnosed with castration-resistant prostate cancer post-1999 and no known metastases from five Veterans Affairs hospitals of the Shared Equal Access Regional Cancer Hospital (SEARCH) database who later developed metastases. Radiology scans within 30 d of initial metastasis diagnosis were reviewed to collect information on bone, visceral, and lymph node metastases. We analyzed the 236 men who had a computed tomography scan performed.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Predictors of VMs and OS were evaluated using logistic regression and Cox models, respectively.

RESULTS AND LIMITATIONS

Of the 236 mCRPC patients, 38 (16%) had VMs. Regarding VMs, 19 patients (50%), 8 patients (21%), and 16 patients (42%) had metastases in the liver, lungs, and other locations, respectively. VMs were a predictor of OS on crude analysis (hazard ratio [HR]: 1.88; 95% confidence interval [CI], 1.30-2.72; p=0.001) and after risk adjustment (HR: 1.84; 95% CI, 1.24-2.72; p=0.002). Age, year, treatment center, prostate-specific antigen (PSA), and time from CRPC to metastases were significant in predicting OS (all p<0.05). None of the variables tested were associated with having VMs (all p > 0.09). Prospective studies and larger cohorts are needed to validate our findings.

CONCLUSIONS

Demographic, tumor, and PSA kinetic characteristics were not predictive of having VMs, but VMs predicted worse OS.

PATIENT SUMMARY

Because patients with VMs have worse overall survival, further research is needed to develop better biomarkers and thus diagnose those with VMs at earlier stages in their disease course.

Cultured circulating tumor cells and their derived xenografts for personalized oncology

Recent cancer research has demonstrated the existence of circulating tumor cells (CTCs) in cancer patient's blood. Once identified, CTC biomarkers will be invaluable tools for clinical diagnosis, prognosis and treatment. In this review, we propose ex vivo culture as a rational strategy for large scale amplification of the limited numbers of CTCs from a patient sample, to derive enough CTCs for accurate and reproducible characterization of the biophysical, biochemical, gene expressional and behavioral properties of the harvested cells. Because of tumor cell heterogeneity, it is important to amplify all the CTCs in a blood sample for a comprehensive understanding of their role in cancer metastasis. By analyzing critical steps and technical issues in ex vivo CTC culture, we developed a cost-effective and reproducible protocol directly culturing whole peripheral blood mononuclear cells, relying on an assumed survival advantage in CTCs and CTC-like cells over the normal cells to amplify this specified cluster of cancer cells.

Abstract LB-266: Large oncosomes reprogram prostate fibroblasts toward a pro-angiogenic phenotype

Introduction: Cancer cells communicate with different cells in the tumor microenvironment, establishing a supportive stroma that sustains tumor development and facilitates the first steps of metastasis. Extracellular vesicles (EVs) have emerged as key functional mediators of this process. Aim of this study was to determine the mechanism of intercellular communication mediated by the atypically large EVs produced by highly migratory and metastatic tumor cells, referred to as large oncosomes (LO), and prostate fibroblasts (NAF). Methods: Filtration, differential centrifugation followed by iodixanol gradient; flow cytometry and confocal imaging; RNA-seq; kinase assay; TF array; luciferase assay; tube formation; siRNA; RT-qPCR. Results: Active AKT1 is significantly more expressed and functional in LO than in exosomes (Exo). Patients with metastatic disease express abundant active AKT1 in plasma LO. Uptake of LO harboring active AKT1 by NAF results in AKT1 and c-MYC activation. Conditioned media from LO-treated NAF, but not from Exo-treated NAF, promoted endothelial morphogenesis. The Dynamin (DNM) inhibitor Dynasore (Dyn) inhibited LO-uptake, as well as MYC activation and tube formation. Transient silencing of DNM2 significantly reduced LO uptake, suggesting that uptake occurs by phagocytosis. LO treatment increased levels of MYC targets in NAF, suggesting that MYC is involved in LO-induced reprogramming of NAF. Accordingly, MYC expression was higher in activated fibroblasts than NAF, and MYC overexpression in NAF induced hyperplasia in normal prostate epithelium in mice, suggesting MYC activation is an early event in cancer development. Summary/Conclusion: Our results indicate that tumor-derived LO induce a novel, c-MYC mediated, pro-tumorigenic reprogramming of fibroblasts that can be reverted by selectively inhibiting LO uptake.

Support: National Institutes of Health NCI NIH R00 CA131472; NIH UCLA SPORE in Prostate Cancer award P50 CA092131; Avon Foundation Fund 02-2013-043 (to DDV).

Citation Format: Valentina Minciacchi, Cristiana Spinelli, Mariana Reis-Sobreiro, Mandana Zandian, Rosalyn M. Adam, Edwin M. Posadas, Freeman R. Michael, Emanuele Cocucci, Neil Bhowmick, Dolores Di Vizio. Large oncosomes reprogram prostate fibroblasts toward a pro-angiogenic phenotype. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-266.

Abstract 4962: Very-small-nuclear circulating tumor cell (vsnCTC) as a putative biomarker for visceral metastasis in metastatic castration-resistant prostate cancer (mCRPC)

Introduction and Objective: Patients with metastatic castration-resistant prostate cancer (mCRPC) who develop visceral metastases (VM) have poorer clinical outcomes in comparison to those without VM. Currently, VM are discovered late in the clinical course of mCRPC-VM and this aggressive natural history typically culminates in organ failure. There are no existing tests that identify men at risk for VM other than radiography. Our team performed circulating tumor cell (CTC) enumeration using NanoVelcro CTC Assay on prostate cancer patients across the spectrum of metastatic states: no metastasis, non-visceral metastasis, and VM. We identified an association between the presence of very-small-nuclear CTCs (vsnCTCs, DAPI+/Cytokeratin+/CD45- with nuclear size &lt; 8.5μm) and VM. Serial enumeration studies suggested the emergence of vsnCTCs occurred before the radiographic detection of VM, and the change of vsnCTC counts reflected the patients’ disease progression. We then hypothesized that presence of vsnCTC signals the presence of VM and has predictive and prognostic value with respect to VM.

Methods: We identified mCRPC patients who had progressed through next generation hormonal maneuvers such as abiraterone, enzalutamide, or an equivalent drug. Serial blood specimens were used for vsnCTC enumeration using NanoVelcro CTC Assay as previously published. The vsnCTC counts were related to the presence and development of VM (evaluated by radiography) as well as the response to anti-cancer treatment.

Results: Blood specimens were identified from 28 patients who met the eligibility criteria; 15/28 patients presented with VM and 13/28 had bone-only disease at their first CTC enumeration. Five out of 13 non-VM patients developed VM during follow-up, and vsnCTCs were detected 86-196 days prior to radiographic detection of VM (true positive); 4/13 had vsnCTCs detected but no VM was found by the time of analysis (false positive). None of the vsnCTC(-) patients developed VM. vsnCTCs were detected in 20/20 VM patients compared to 4/8 non-VM patients. Reduction of vsnCTC count occurred at initiation of anti-cancer treatment; transition from vsnCTC(-) to vsnCTC(+) was seen prior to progression under the treatment. Of the patients who have VM, 14 passed away at the time of this abstract submission including all the patients converting from non-VM to VM during the time of follow-up. Two out of 8 non-VM patients passed away including one patient who had vsnCTCs detected around 6 months prior to death.

Conclusions: vsnCTCs are associated with the presence of VM. The vsnCTC is a potential biomarker for predicting the development of VM and monitoring the treatment response in mCRPC. Transition from vsnCTC(-) to vsnCTC(+) was associated with the development of VM and progression under the treatment.

Citation Format: Jie-Fu Chen, Hao Ho, Elisabeth Hodara, Alexander Ureno, Ann Go, Elizabeth Kaufman, Margarit Sievert, Daniel J. Luthringer, Jiaoti Huang, Leland Chung, Zunfu Ke, Ker-Chau Li, Hsian-Rong Tseng, Edwin M. Posadas. Very-small-nuclear circulating tumor cell (vsnCTC) as a putative biomarker for visceral metastasis in metastatic castration-resistant prostate cancer (mCRPC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4962.

Clinical Applications of NanoVelcro Rare-Cell Assays for Detection and Characterization of Circulating Tumor Cells

Liquid biopsy of tumor through isolation of circulating tumor cells (CTCs) allows non-invasive, repetitive, and systemic sampling of disease. Although detecting and enumerating CTCs is of prognostic significance in metastatic cancer, it is conceivable that performing molecular and functional characterization on CTCs will reveal unprecedented insight into the pathogenic mechanisms driving lethal disease. Nanomaterial-embedded cancer diagnostic platforms, i.e., NanoVelcro CTC Assays represent a unique rare-cell sorting method that enables detection isolation, and characterization of CTCs in peripheral blood, providing an opportunity to noninvasively monitor disease progression in individual cancer patients. Over the past decade, a series of NanoVelcro CTC Assays has been demonstrated for exploring the full potential of CTCs as a clinical biomarker, including CTC enumeration, phenotyping, genotyping and expression profiling. In this review article, the authors will briefly introduce the development of three generations of NanoVelcro CTC Assays, and highlight the clinical applications of each generation for various types of solid cancers, including prostate cancer, pancreatic cancer, lung cancer, and melanoma.

Saracatinib as a metastasis inhibitor in metastatic castration-resistant prostate cancer: A University of Chicago Phase 2 Consortium and DOD/PCF Prostate Cancer Clinical Trials Consortium Study

BACKGROUND

Fyn is a kinase that is upregulated in a subset of metastatic castration-resistant prostate cancer. Saracatinib potently inhibits Fyn activation. We have noted a relationship between Fyn expression and directional motility, a cellular process related to metastasis. As such we hypothesized that treatment with saracatinib would increase the time required to develop new metastatic lesions.

METHODS

Patients with metastatic castration-resistant prostate cancer that had progressed after docetaxel were eligible for enrollment. This study was executed as a randomized discontinuation trial. During a lead-in phase of two 28-Day cycles, all patients received saracatinib. Afterward, patients with radiographically stable disease were randomized to either saracatinib or placebo. Patients continued treatment until evidence of new metastasis.

RESULTS

Thirty-one patients were treated. Only 26% of patients had stable disease after 8 weeks and thus proceeded to randomization. This required early termination of the study for futility. The 70% of patients who progressed after the lead-in phase exhibited expansion of existing lesions or decompensation due to clinical progression without new metastatic lesions. Fatigue was reported in more than 25% of patients (all grades) with only two patients experiencing grade 3 toxicity. Other grade 3 adverse events included dehydration, thrombocytopenia, and weakness.

CONCLUSIONS

This study was unable to determine if saracatinib had potential as metastasis inhibitor. Metastasis inhibition by saracatinib may still be viable in an earlier time in the disease history.

Phase 1b Study of Abiraterone Acetate Plus Prednisone and Docetaxel in Patients with Metastatic Castration-resistant Prostate Cancer

Coadministration of docetaxel and abiraterone acetate plus prednisone (AA + P) may benefit patients with metastatic castration-resistant prostate cancer (mCRPC) because of complementary mechanisms of action. COU-AA-206 was a phase 1b study to determine the safe dose combination of docetaxel and AA + P in three cohorts of chemotherapy-naïve mCRPC patients. Twenty-two patients received escalating doses of docetaxel plus AA + P. The primary endpoint was the proportion of patients with a dose-limiting toxicity (DLT) between weeks 2 and 7. The recommended phase 2 dose (RP2D) was the highest safe combination of docetaxel plus AA + P. Prostate-specific antigen (PSA) changes and intensive pharmacokinetic parameters for each drug were evaluated. Docetaxel 75mg/m² + AA 1000mg + P 10mg was deemed the RP2D, with DLT in one of six patients. PSA declines from baseline of ≥50% and ≥90% were observed for 85.7% and 66.7% of patients, respectively. During median follow-up of 14.5 mo, eight patients had PSA progression and six had radiographic progression or died. Systemic exposure was comparable for docetaxel and abiraterone when given alone or in combination. Studies are ongoing to confirm the efficacy of potent androgen receptor-targeted therapy plus taxane in early mCRPC.

PATIENT SUMMARY

The combination of hormonal therapy and chemotherapy may improve outcomes in men with metastatic prostate cancer. This study demonstrates the ability to combine the hormonal therapy agent abiraterone acetate, plus prednisone, and the chemotherapy drug docetaxel with an acceptable side effect profile. A high rate of prostate-specific antigen decline was seen, but the study was small and additional research is needed before this becomes a standard approach.

Very small nuclear circulating tumor cell (vsnCTC) as a putative biomarker for visceral metastasis in metastatic castration-resistant prostate cancer (mCRPC)

64

Background: Patients with metastatic castration-resistant prostate cancer (mCRPC) who develop visceral metastasis (VM) have a poorer clinical outcome in comparison to those without VM. Their clinical course is aggressive and culminates in organ failure as this process is often discovered late in the disease course. There are no existing tests that identify men at risk for VM. Our team has identified an association between the presence of very small nuclear circulating tumor cells (vsnCTCs) and VM. We hypothesized that vsnCTC that can predict the development of VM and monitor the response to anticancer treatment. Methods: In our database we identified mCRPC patients who had progressed through next generation hormonal maneuvers such as abiraterone, enzalutamide, or an equivalent drug. Serial blood specimens were used for vsnCTC enumeration using published methods. The vsnCTC counts were related to the presence and development of VM as well as the response to anticancer treatment. Results: Blood specimens were identified from 28 patients who met the eligibility criteria; 16/28 patients presented with VM and 12/28 had bone-only disease at their first CTC enumeration. Five out of 12 non-VM patients developed VM during follow-up, and vsnCTCs were detected 86-196 days prior to radiographic detection of VM (true positive); 3/12 had vsnCTCs detected but no VM was found by the time of analysis (false positive). None of the vsnCTC(-) patients developed VM. vsnCTCs were detected in 21/21 VM patients compared to 3/12 non-VM patients. Reduction of vsnCTC count occurred at initiation of anticancer treatment; transition from vsnCTC(-) to vsnCTC(+) was seen prior to progression under the treatment. Conclusions: vsnCTC is associated with the presence of VM and is a potential biomarker for predicting the development of VM and monitoring the treatment response in mCRPC. Transition from vsnCTC(-) to vsnCTC(+) was associated with the development of VM and progression under the treatment.

A phase 2 study of BIND-014 (PSMA-targeted docetaxel nanoparticle) administered to patients with chemotherapy-naïve metastatic castration-resistant prostate cancer (mCRPC)

233

Background: BIND-014 is a novel PSMA-targeted Accurin (polymeric nanoparticle) that contains docetaxel (D). BIND-014 is anticipated to improve the therapeutic index of D by increasing its intratumoral concentration and duration of exposure. In a phase 1 study, BIND-014 was generally well-tolerated and displayed anti-tumor effects, including in two patients with chemotherapy-naïve mCRPC. Methods: A phase 2 study was conducted of BIND-014 administered by a 60-min IV infusion at 60 mg/m² on day 1 of a 21-day cycle in combination with 5 mg of prednisone twice per day to patients with chemotherapy-naïve mCRPC. Prior treatment included abiraterone acetate (AA, 48%), enzalutamide (E, 12%) or both (14%). Of these 31 patients 19% progressed within ≤ 6 months on prior AA and/or E. The primary endpoint was radiographic progression-free survival (rPFS) using PCWG2 for bone and RECIST v1.1. Other endpoints included prostate specific antigen (PSA) and circulating tumor cell count (CTC) conversions, objective response rate (ORR), and overall survival (OS). Results: Forty-two patients were enrolled and received a median of 6 doses (range 1 – 21). Median rPFS was 7.1 months (95% CI: 2.6 – 9.9) with 4 patients (10%) censored. PSA response ( ≥ 50% reduction from baseline) was observed in 12 of 40 patients (30%). CTC conversions (from ≥ 5 CTCs/7.5 mL of blood at baseline to < 5 CTCs) occurred in 13 of 26 patients (50%). ORR was 32% with 3 confirmed responses (1 complete response and 2 partial responses) and 3 unconfirmed responses out of 19 patients with measurable disease. Estimated median OS was 13.4 months (95% CI: 9.9 – 18.6) with 14 patients (33%) censored. The most common hematological treatment-related adverse events (TRAEs) observed were lymphopenia (26%), and anemia (19%).Fatigue (69%), nausea (55%), diarrhea (45%), dyspnea (36%), and neuropathy (33%) were the most common non-hematological TRAEs. The most common TRAEs were generally grade 1 and 2. Conclusions: BIND-014 administered at 60 mg/m² on day 1 of a 21-day cycle is clinically active and well-tolerated when administered to patients with chemotherapy-naïve mCRPC including those with prior AA and/or E. Clinical trial information: NCT01812746.

A phase II study of cabozantinib in metastatic castration-resistant prostate cancer (mCRPC) with visceral metastases (VM) with very small nuclear circulating tumor cell (vsnCTC) association studies

208

Background: Cabozantinib is a multi-targeted tyrosine kinase inhibitor (TKI) that has recognized clinical activity in mCRPC. While phase 3 studies did not show a significant improvement in overall survival (OS), patients with VM had improved OS compared to those with only osseous metastases in subgroup analysis. Our group has also identified a subgroup of CTCs strongly associated with the presence of VM (Chen, Cancer 2015). We hypothesized that cabozantinib would yield clinical benefit and changes in vsnCTC counts would reflect disease behavior. Methods: This study was designed as an open label study of cabozantinib 60 mg daily, for mCRPC patients with visceral disease. The primary endpoint is clinical benefit rate (radiographic stable disease + partial response (PR) by RECIST 1.1) at 12 weeks (CBR12). Serum PSA concentrations were not used as a primary endpoint. CTCs were enumerated by NanoVelcro CTC Assay and characterized using published methods. Results: Thirteen patients have completed at least 8 weeks of treatment. In this group, 38% had pulmonary metastases; 77% had hepatic metastases; CBR12 was 92% at the time of this report. A quarter of the patients experienced a PR. Mean time to radiographic progression for responders was 38 weeks (19-57). Discordant responses were noted between VM and bone scans. Grade 3 or higher toxicities were compatible with prior studies and observed in 50% of patients. In this advanced mCRPC and relatively heavily pre-treated population, treatment discontinuation due to toxicity was 33%. Eleven of 13 patients were assessable for CTC and imaging. Of the 11 patients, 7 had vsnCTCs detected at baseline. All 7 patients reported subjective improvement that was associated with a decrease in vsnCTCs by 6-8 weeks. In patients who had a PR, no vsnCTCs were detected at 6 weeks while on active treatment. An increase in vsnCTCs was seen prior to visceral progression. Conclusions: Cabozantinib is a TKI with activity in mCRPC with visceral metastases. Toxicity is consistent with previous reports and manageable. The NanoVelcro vsnCTC assay may be a predictive marker of response. Clinical trial information: NCT01834651.