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e17512 Background: Very-small-nuclear circulating tumor cells (vsnCTCs) as a subset of CTCs with nuclear size < 8.5 μm associated with visceral metastases (VM) in advanced metastatic, castration-resistant prostate cancer (mCRPC). As VM predicts foreshortened survival, we hypothesized that vsnCTCs would be directly associated with poor overall survival (OS). Methods: Clinically annotated, blood samples from patients with mCRPC with survival follow-up within the Translational Oncology Program Blood & Biospecimen Bank were selected for analysis. CTCs were isolated and analyzed using the NanoVelcro assay. The nuclear size from all CTCs from each patient sample was compared to OS and progression-free survival (PFS) from the time of the blood draw. Results: A total of 76 patients had samples suitable for analysis. Sixty-six (87%) had CTCs; 49 (64%) were vsnCTC+ (≥1 vsnCTC). vsnCTCs were more common in mCRPC patients with previous androgen receptor signaling inhibitor (ARSI) therapy and/or 2 or more lines of treatment. OS was significantly shorter for vsnCTC+ than vsnCTC- patients (median 34 vs. 149 weeks; log-rank HR = 2.6; 95% CI = 1.5 to 4.5; P = 0.0006). Fifty patient samples were available for PFS analysis (i.e. drawn within 4 weeks of starting therapy). vsnCTC+ patients experienced more rapid progression than vsnCTC- patients (median 12 vs. 26 weeks, log-rank HR = 2.2, 95% CI = 1.3 to 4.0; P = 0.004). Multivariable Cox regression revealed that vsnCTC status was independently associated with OS and PFS. P-spline plot analysis showed that the HR of OS increased as the minimum CTC nuclear size decreased. The minimum CTC nuclear size was also independently associated with OS and PFS in a multivariable Cox regression analysis. Patients with prior use of androgen receptor signaling inhibitor (ARSI) therapy had significantly smaller minimum CTC nuclear size compared to those without prior use of ARSI. Average and median nuclear size did not strongly associate with OS or PFS. Conclusions: vsnCTC+ patients are at risk for more rapid clinical progression and have greater risk of death than vsnCTC-. We posit that the vsnCTC may be a biomarker of aggressive mCRPC with foreshortened survival. Interestingly, the CTCs with the smallest nuclei appear to best reflect the observed clinical behavior. This has potential importance in optimizing therapeutic choices and may point toward a unique biology relating nuclear size to aggressive molecular features. Our group continues to explore the biology underlying the vsnCTC.

168 Background: Circulating tumor cells (CTCs) have arisen as contemporary noninvasive prognostic biomarkers for prostate cancer (PC). Previously, a subgroup of PC CTCs, with particularly small nuclei (

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.

11027 Background: In prostate cancer (PCa), morphologic classification remains a standard clinical practice in pathology. It has been shown that nuclear size and shape in tumor sections correlate w...