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e14527 Background: A subset of TNBCs have homologous recombination deficiency (HRD) with upregulation of compensatory DNA repair pathways. A combination of TAK-228 and TAK-117 (PIKTOR), investigational oral TORC1/2 and PI3Kα selective inhibitors, respectively, is hypothesized to increase genomic instability (GI) and decrease double strand DNA repair. CTCs from metTNBC pts are being characterized for imaging features that may correlate GI in blood obtained at baseline (BL) prior to beginning PIKTOR, and at disease progression (PD) on PIKTOR. Pts are treated with a cisplatin-based regimen after PD on PIKTOR. Methods: Following IRB-approved informed consent, metTNBC pts received 4 mg PO TAK-228 and 200 mg PO TAK-117 QDx3d QW until PD. Blood samples collected at BL and at PD were sent to Epic Sciences for CTC analyses including enumeration, cell morphology, phenotypic heterogeneity, and GI analysis via a previously developed GI prediction algorithm based on cell phenotypes (Jendrisak et al AACR 2018). Results: The 9 pts treated with PIKTOR to date had mets in lymph nodes (n = 8), lung (n = 5), chest wall (n = 2), and bone (n = 2). Median number of prior regimens for metTNBC was 2; median duration of PIKTOR treatment was 8 wks (range, 5-15). CTCs were detected in 62.5% (5/8) BL and 100% (9/9) PD samples. 2 pts had high phenotypic CTC heterogeneity at both BL and PD (pt#36: 0.69 to 0.64; pt#80: 0.93 to 1.41), 1 pt had increased heterogeneity (pt#75: 0 to 1.68) and 1 pt had decreased heterogeneity (pt#67: 1.26 to 0) post-PIKTOR. 1 pt had predicted phenotypic GI (pGI+) CTCs detected at both BL and PD (pt#80), 1 pt had pGI+ CTCs at BL only (pt#36) and 1 pt at PD only (pt#75). Higher expression of the epithelial marker, cytokeratin (CK), and larger cell size were observed in the PD samples. Conclusions: Following treatment with and PD on PIKTOR, higher CK expression, higher phenotypic heterogeneity, and more pGI+ CTCs were observed in a subset of pts, suggesting that these TN CTCs may have developed epithelial transition and greater GI with PIKTOR treatment. Accrual continues and CTC GI biomarkers will be correlated with pts’ response to subsequent cisplatin-based therapy.

e14033 Background: Immune checkpoint inhibitors (ICIs) are becoming standard treatment options in many indications. However, a substantial portion of patients will not respond. Single biomarkers such as PD-L1 are insufficiently accurate to predict patient benefit. We sought to expand the Epic Sciences non-invasive liquid biopsy platform to identify predictive, peripheral blood biomarkers for ICIs using both molecular analyses of CTCs and immune cell changes. Methods: Blood samples from prostate, kidney, and bladder cancer patients treated with ICIs were collected at baseline and on-therapy and sent to Epic Sciences. Nucleated cells were plated on glass slides and stained with CTC (pan-CK, CD45, PD-L1 and DAPI) and immune panels for activation (CD4, CD8, Ki-67, and DAPI) and exhaustion (CD8, Ki-67, PD-1, Lag-3, Tim-3, and DAPI). Changes in populations of immune cells and circulating tumor cells were assessed using high throughput digital pathology. Results: CTCs were detected in 73% (24/33) patients, of which 12% (4/33) had PD-L1+ CTCs detected. No PD-L1+ CTCs were detected in the nine on-therapy samples tested. Of 14 patients with matched samples, 57% (8/14) patients had an increase in activated CD4+ leukocytes and 36% (5/14) patients had an increase in activated CD8+ leukocytes in on-therapy samples compared to baseline. The exhaustion assay was performed on a subset (6 of 14) of matched samples. In baseline and on-therapy samples, one patient had higher levels of exhausted CD8+ leukocytes compared to healthy donor controls, and two patients had lower levels versus controls. No change in exhausted CD8+ leukocytes was observed from baseline to on-therapy. Conclusions: We developed a liquid biopsy-based platform that can simultaneously measure biomarkers in CTCs and leukocytes from a single peripheral blood sample. Changes in activated and exhausted immune cell populations with ICI treatment were detected and PD-L1 expression on CTCs was evaluated. Efforts to further stratify immune and rare cell populations are ongoing.

398 Background: Circulating tumor cells (CTCs) are an important emerging biomarker in bladder cancer that allow for a minimally invasive assessment of tumor activity and response to treatment. Characterization of CTC and other single cell populations associated with improved clinical outcomes can help guide treatment recommendations for patients with metastatic bladder cancer. Methods: Patients with metastatic bladder cancer who received treatment with anti-PD-1 or anti-PD-L1 agents were enrolled in this study. Patient response to treatment was assessed by treating physicians according to RECIST v1.1. Blood samples were prospectively collected from patients prior to the initiation of therapy and then while on treatment and shipped to Epic Sciences for processing. All nucleated cells were subjected to immunofluorescent (IF) staining and CTC and leukocyte identification by fluorescent scanners using algorithmic analysis. Kaplan-Meier analysis was utilized to compare time to progression of patients whose CTC and leukocyte values were above and below several pre-determined parameters. Results: A total of 27 patients (median age 74 years, 70% male) were enrolled in this study and were treated with anti-PD-1/PD-L1 agents pembrolizumab (n=15), atezolizumab (n=11), or nivolumab (n=1). For 20 patients who had evaluable responses, objective response rate (ORR) was 2/20 (10%), all partial responses; another 5/20 (35%) had stable disease. Increased CD4% (>8% of total leukocytes) was associated with delayed time to progression (TTP) (p=0.002) whereas increased baseline total CTCs (>2) had a statistically non-significant trend towards shorter TTP (p=0.09). Baseline CD8%, CD4/CD8 ratio and CTC PD-L1 status were not associated with TTP. Conclusions: In a preliminary analysis among metastatic bladder cancer patients treated with immune checkpoint inhibitors, patients with an increased baseline CTC count had a statistically non-significant trend towards shorter TTP whereas increased baseline CD4 cells had an association with delayed TTP. This prospective study is ongoing, and the results will be further validated in larger patient cohorts.

1084Background: The choice between hormonal therapies and chemotherapy is a frequent decision in the care of mBCa pts. We previously developed quantitative measures of phenotypic CTC heterogeneity ...

110 Background: Approximately 15% of men with newly diagnosed prostate cancer (PCa) have high-risk features, many of these patients will recur despite definitive therapy. Better predictive biomarkers could allow for earlier detection of recurrence and change surveillance paradigms. The role of circulating tumor cells (CTCs) as biomarkers in this context is not well defined. Here, we evaluate the ability to detect CTCs from men with high risk, localized PCa after radical prostatectomy (RP) and correlate their presence with prospective clinical data. Methods: Blood samples from 31 patients with high risk, localized PCa were obtained 2-4 months post RP and sent to Epic Sciences on an IRB approved protocol. Nucleated cells were subjected to immunofluorescent (IF) staining for cytokeratin (CK), CD45, and AR N-terminus. CTCs were identified by fluorescent scanners using algorithmic analysis. Cytokeratin expressing (CK+) CTCs were enumerated and subsequently analyzed for AR expression and individually sequenced for copy number variation (CNV) and large scale transitions (LST, a surrogate of genomic instability). Patients were followed prospectively for biochemical recurrence, defined as detectable PSA. Progression free survival (PFS) was calculated using Kaplan-Meier and Cox proportional hazards. Results: CTCs were detected in 87.1% (27/31) samples with an average of 5.6 CTCs/ml (range: 0 – 22.87) detected per patient. AR expression was detected in 12.9% (4/31) of patients. Ninety-nine CTCs from 14 patients were amenable to LST and CNV analyses. 10.1% (10/99) CTCs from 7 patients exhibited higher ( > = 6) LSTs than control WBCs (95% WBCs had LST < 6). Copy number alterations were detected in CTCs in commonly mutated genes in PCa, including AR, MYC, and TP53 amplification and deletions in PTEN and RB1. Patients with higher CTC burdens exhibited a trend toward shorter PFS (hazard ratio: 1.65; 95% confidence interval: 0.7-3.86; p: 0.13). Conclusions: There was a high incidence of CTC detection after RP in this population using a novel platform. We observed a trend toward shorter PFS in those with higher CTC burden. Genomic alterations were detectable in CTCs and consistent with established CNAs in PCa.