Cytotoxic chemotherapy potentiates the immune response and efficacy of combination CXCR4/PD-1 inhibition in models of pancreatic ductal adenocarcinoma

Purpose

The CXCL12-CXCR4 chemokine axis plays a significant role in modulating T-cell infiltration into the pancreatic tumor microenvironment. Despite promising preclinical findings, clinical trials combining inhibitors of CXCR4 (AMD3100/BL-8040) and anti-programmed death 1/ligand1 (anti-PD1/PD-L1) have failed to improve outcomes.

Experimental Design

We utilized a novel ex vivo autologous patient-derived immune/organoid (PDIO) co-culture system using human peripheral blood mononuclear cells and patient derived tumor organoids, and in vivo the autochthonous LSL-KrasG12D/+; LSL-Trp53R172H/+; Pdx-1-Cre (KPC) pancreatic cancer mouse model to interrogate the effects of either monotherapy or all combinations of gemcitabine, AMD3100, and anit-PD1 on CD8+ T cell activation and survival.

Results

We demonstrate that disruption of the CXCL12-CXCR4 axis using AMD3100 leads to increased migration and activation of CD8+ T-cells. In addition, when combined with the cytotoxic chemotherapy gemcitabine, CXCR4 inhibition further potentiated CD8+ T-cell activation. We next tested the combination of gemcitabine, CXCR4 inhibition, and anti-PD1 in the KPC pancreatic cancer mouse model and demonstrate that this combination markedly impacted the tumor immune microenvironment by increasing infiltration of natural killer cells, the ratio of CD8+ to regulatory T-cells, and tumor cell death while decreasing tumor cell proliferation. Moreover, this combination extended survival in KPC mice.

Conclusions

These findings suggest that combining gemcitabine with CXCR4 inhibiting agents and anti-PD1 therapy controls tumor growth by reducing immunosuppression and potentiating immune cell activation and therefore may represent a novel approach to treating pancreatic cancer.

Using tumor growth rate to inform treatment efficacy in pancreatic adenocarcinoma: From the metastatic to the neoadjuvant setting

726

Background: The development of new treatments in oncology is a long, costly, and, too often, unsuccessful process. Methods for screening agents earlier in development and strategies for conducting smaller randomized controlled trials are needed. Methods: We used a mathematical model of tumor growth kinetics fit to serial radiographic tumor measurements or CA19-9 values to estimate rates of exponential tumor growth [g] and decay [d] during treatment for pancreatic ductal adenocarcinoma (PDAC). Results: We retrospectively collected and analyzed data from 2691 patients with stage III-IV PDAC who were enrolled in five clinical trials or were included in two large real-world data sets from Columbia University Irving Medical Center and Veterans Administration Medical Centers. Using log-rank comparison of Kaplan-Meier plots by quartile of g, we found that in patients with metastatic PDAC g correlates highly with overall (OS) and progression-free survival (PFS) (p<0.001), with slower g associated with improved survival in this population. Pairwise comparisons showed significantly slower median g in the experimental arm versus control arm in the pivotal trials analyzed (p<0.001). At the individual patient level, g was significantly faster for liver metastases as compared to primary pancreatic tumors and g consistently increased towards the end of therapy (often a threefold increase) suggesting development of chemoresistance. In addition to utility in the metastatic setting, a pilot analysis of data from a prospective study of patients treated with gemcitabine + docetaxel + capecitabine (GTX) in the neoadjuvant setting suggests that the emergence of a detectable g during neoadjuvant therapy may portend worse OS following surgery though sample size was small (n=45, median OS with detectable g 13.6 m v 33.1 without detectable g, p =0.35). Conclusions: We applied a tumor growth model to the data of over 2500 patients with PDAC and showed that g is inversely associated with survival in this population. Given the strong association between g and survival, g could be useful in clinical trials as an informative endpoint to expedite the assessment of novel therapies for the treatment of PDAC. Furthermore, g offers valuable patient-level data, including trends in resistance and variations in growth rate by metastatic disease site We plan to evaluate whether the emergence of g during neoadjuvant therapy should be considered a prompt to change treatment regimens.

The tumor microenvironment and immune infiltration landscape of KRAS mutant pancreatic ductal adenocarcinomas (PDAC) compared to colorectal adenocarcinomas (CRC)

4142

Background: The composition of the tumor microenvironment (TME) in PDACs is more heavily driven by mutant (mt) KRAS than any other cancer. How genomic alterations of PDAC including KRAS status affect the immune cell (IC) landscape remains unclear. Thus, we characterized IC types and the prevalence of immuno-oncologic (IO) biomarkers in PDAC by genomic and transcriptomic analysis, and investigated associations of mt KRAS with IC estimates in the TME. Our findings were compared to our previous study in CRC. Methods: A total of 4,142 PDAC and 3,727 CRC with KRAS- mts were analyzed using next-generation DNA sequencing (NextSeq, 592 gene panel or NovaSeq, WES), IHC, and whole transcriptome RNA sequencing (NovaSeq) (Caris Life Sciences, Phoenix, AZ). MSI/MMR was tested by FA, IHC and NGS. TMB-H was classified based on a cut-off of >10 mutations per MB. ICs were estimated by QuantiSeq (Finotello 2019, Genome Medicine) or MCP counter (Betcht 2016, Genome Biology). Significance was determined by X2 and Fisher-Exact and p-adjusted for multiple comparisons (q<0.05). Results: Mutant KRAS was seen in 81% of PDAC and in 48% of CRC. The most common variant was G12D, comprising 43% and 32% of all PDAC and CRC KRAS variants, respectively. The therapeutically actionable KRAS G12C variant comprises 2% and 7% of PDAC and CRC in this cohort, respectively. In PDAC, KRAS mt was associated with lower prevalence of MSI-H/dMMR than KRAS-wildtype (wt); 0.9% vs 1.9%, p=0.027). PDL1 expression was significantly lower in KRAS wt (12%) compared to G12D (19%) and G13X (33%), similar to previous observations in CRC. However, when considering TMB, in PDAC, G12D (1%), G12V (1%) and Q61 (1%) mutations had significantly lower TMB-H than RAS wt tumors (4%); in contradiction to CRC. The immune cell environment of KRAS mt PDAC showed significantly higher infiltration with M1 macrophages and cancer-associated fibroblasts (CAFs), as well as lower M2 macrophages, CD4+ & CD8+ T cells, T-reg, NK, myeloid dendritic and endothelial cells compared to KRAS wt. In CRC, a similar pattern was observed but more pronounced in PDAC. Immune-regulatory markers, were among multiple genes downregulated in KRAS-mt PDAC, including CTLA-4 and LAG3. Overall changes were most pronounced in cases harboring KRAS G12D, G12V, Q61, and rare KRAS variants. Conclusions: The TME of KRAS mt PDAC shows IC patterns similar to KRAS mt CRC. Actionable IO-targets, such as PDL1, are enriched in tumors harboring specific variants of KRAS mt PDAC including the targetable G12C variant. If G12D becomes druggable, it could be targetable in 35% patients with PDAC or 15% in CRC. These results demonstrate that the TME of PDAC and CRC shows immune-cold features. Tailored immunotherapeutic strategies would have to overcome these barriers in KRAS mt PDAC and CRC, possibly in combination with molecularly targeted treatment strategies.

Phase 2 study of azacitidine (AZA) plus pembrolizumab (pembro) as second-line treatment in patients with advanced pancreatic ductal adenocarcinoma

4158

Background: Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related death with a 5-year survival rate of 10%. Novel strategies for advanced PDAC are a critical need. A DNA hypomethylating agent (HMA) increased tumor-infiltrating effector T cells and significantly prolonged survival in combination with immune checkpoint blockade (ICB) in a PDAC mouse model (Gonda et al 2020). We hypothesized that combining HMA with ICB will lead to therapeutic benefit in pts with advanced PDAC. Methods: This is an open-label, single-arm, single-center, Phase 2 trial of AZA plus pembro in pts with unresectable or metastatic PDAC. Pts were treated with AZA 50 mg/m2 subcutaneous daily for 5 days Q4W beginning week 1 day 1 followed by pembro 200 mg IV Q3W starting week 3 day 1. Key eligibility criteria included documented progression on or following first-line systemic treatment, ECOG PS 0-1, and adequate organ function. The primary objective was progression-free survival (PFS). Thirty-one evaluable pts were required to detect an improvement in PFS from 2 mo to 4 mo with a one-sided p-value of 0.05 and 80% power. Secondary endpoints included safety and tolerability, overall response rate (ORR), disease control rate (DCR), and overall survival (OS). Data cutoff was February 10, 2022. Results: Between Oct 2017 and Sept 2021, 36 pts were enrolled (median age 62.5, 75% white, 72% male). At data cutoff, 34 and 31 pts received at least 1 dose of AZA and pembro, respectively. Median PFS for ≥1 dose of AZA was 1.48 mo (95% CI: 1.35, 1.74) and for ≥1 dose of pembro was 1.51 mo (95% CI, 1.38, 3.42). The median OS was 4.67 months. Among the 34 pts, 3 (8.8%) experienced a partial response (PR) by RECIST with a DCR of 32.4%. Of the 3 pts with PR, 2 received 35 doses of pembro and continued beyond 2 years with an ongoing PR. One pt remains on pembro 20 months after study completion. None of the pts with a PR were microsatellite instability-high or tumor mutation burden high, and next-generation sequencing of the 3 cancers identified a BRCA1 variant and POLE variant. Treatment-related AEs (TRAEs) occurred in 20/34 pts (59%), the most common of which were diarrhea and fatigue. Grade ≥3 occurred in 7/34 (21%) including 1 immune-related grade 5 event (encephalitis). Conclusions: AZA plus pembro demonstrated a tolerable safety profile but no PFS benefit compared to historical controls. Our data suggests combined epigenetic therapy and ICB may expand therapeutic options in a subset of pts with PDAC. Further investigation is needed to identify biomarkers to predict response and elucidate effective timing, sequencing, and combination of epigenetic agents. Follow-up for OS is ongoing. Correlative analysis of epigenetic effects and characterization of the tumor immune microenvironment will be reported. Clinical trial information: NCT03264404.

CXCR4 overexpression: An indicator of poor survival and predictor of response to immunotherapy in patients with metastatic colorectal cancer

3546

Background: CXC-chemokine receptor 4 (CXCR4) is a ubiquitous chemokine receptor activated by the CXCL12 ligand and is implicated in tumor invasion, metastasis, and immune cell (IC) trafficking. High CXCR4 expression is associated with poor prognosis in colorectal cancer (CRC). < 10% of metastatic CRC cases harbor microsatellite instability (MSI-H) and demonstrate lower tumor mutation burden (TMB), decreased IC infiltration, and lack of response to current immunotherapy regimens. This study aims to interrogate the role of CXCR4 mRNA expression on the the tumor microenvironment (TME) and its prognostic and predictive value to tailor immunotherapeutic treatment strategies in CRC. Methods: A total of 15,026 CRC samples were analyzed using whole-exome sequencing, whole-transcriptome sequencing, and immunohistochemistry (Caris Life Sciences, Phoenix, AZ). Study cohort was stratified by CXCR4 mRNA expression levels in quartiles (Q1 (low) vs Q4 (high)). IC fraction was calculated by QuantiSeq, and real-world overall survival information was obtained from insurance claims data and calculated from tissue collection time to last day of contact. Statistical significance was determined using chi-square/Fisher-Exact and adjusted for multiple comparisons (q < 0.05). Results: Samples obtained from metastatic sites showed higher CXCR4 mRNA expression than those from primary tumors (22.7 vs 18.6 median transcripts per million (TPM), p < 0.001). CXCR4 mRNA expression was significantly lower in liver metastases than in non-liver metastases (21.2 vs 24.8 TPM, p < 0.001). Median CXCR4 mRNA expression was highest in the consensus molecular subtypes 4 (33.3 TPM) and lowest in 3 (13.0 TPM, p < 0.05). CXCR4 mRNA expression was positively associated with TMB-H, MSI-H/dMMR, and positive PD-L1 IHC status. In the TME, high CXCR4 mRNA expression was observed in tumors with a higher IC infiltration including B cells, M1/M2 macrophages, NK cells, CD8+ T cells and T-regs, regardless of MSI status. High CXCR4 mRNA expression in the primary tumor was associated with poor prognosis (HR 0.77, 95% CI 0.70-0.85; p < 0.001), regardless of MSI-status. In metastatic tumors, low mRNA expression was correlated with improved survival (HR 0.89, 95% CI 0.80-0.99; p = 0.34); however, this did not reach statistical significance in the MSS cohort (HR 0.90, 95% CI 0.80-1.0; p = 0.06). Of note, high CXCR4 mRNA expression was associated with improved survival in all patients with CRC who received pembrolizumab (HR 2.12, 95% CI 1.16-3.91; p = 0.013). Conclusions: This is the largest clinical dataset to date demonstrating high CXCR4 expression as a predictor for poor survival in CRC. Furthermore, high CXCR4 expression was associated with improved outcome after checkpoint inhibition immunotherapy, indicating its strong potential as a predictive biomarker that could inform immunotherapeutic strategies in CRC.

Cost-effectiveness of universal screening for germline BRCA mutations in metastatic pancreatic cancer

536

Background: Germline BRCA1/2 mutations (gBRCAm) increase the risk of pancreatic ductal adenocarcinoma (PDAC). The NCCN 2020 guidelines recommend testing for gBRCAm in metastatic PDAC patients if the patients have a personal history and/or familial history of PDAC (current standard-of-care). However, given the advances made in genetic testing, universal gBRCAm testing for metastatic PDAC patients can be considered. The cost-effectiveness of universal gBRCAm screening has yet to be compared to the current standard-of-care. The purpose of our study was to explore the cost-effectiveness, treatment outcomes, costs, and quality-of-life impact of universal gBRCAm screening. Methods: We developed a decision-analytic mathematical model comparing the cost and health outcomes of universal gBRCAm screening against the current standard-of-care. Inputs for the model were estimated using clinical trial data and published literature. No intervention was used as a comparator. The primary endpoint was incremental cost-effectiveness ratios (ICERs) with a willingness-to-pay (WTP) threshold of $100,000 per quality-adjusted-life-year (QALY). Secondary endpoints included overall survival (OS), progression-free survival (PFS), life-years (LYs) and total cost of care (USD). Results: Universal gBRCAm screening was the cost-effective strategy, totaling incremental QALYs of 1.61 at a cost of $73,682 per QALY when compared to no intervention. A one-way sensitivity analysis found that the standard-of-care becomes the cost-effective strategy when the prevalence of gBRCAm is lowered to 2% of the base case. Conclusions: Our model found that universal gBRCAm screening is cost-effective and even cost-savings for patients with metastatic PDAC. Additional clinical trial data with sufficient follow-up are needed to confirm our findings.[Table: see text]

Improved survival in patients with lung only recurrence after surgical resection of pancreatic ductal adenocarcinoma

608

Background: Recurrence rates after resection of pancreatic ductal adenocarcinoma (PDA) can be up to 80%. Prior data suggests that initial site of recurrence influences prognosis. This study aims to compare survival of patients (pts) with resected PDA by initial site of recurrence. Methods: We retrospectively reviewed the demographics, treatments, recurrence and survival of 717 pts with PDA who underwent resection at Columbia University Irving Medical Center from 2011 to 2020 and were part of a tumor registry. Analyses were performed using Kaplan-Meier and paired T-tests. Results: Of 717 pts with resected PDA, 320 had confirmed recurrence. Median age at diagnosis was 67 years (yrs). Among pts with a single initial recurrence site, 36 recurred in lung, 97 in liver, 95 locally, and 23 in peritoneum. 58 pts had initial recurrence at 2 or more sites. Neoadjuvant treatment had been administered in 42%, 36%, 40%, 35%, and 22% of pts with lung, liver, local, peritoneal, and multiple sites at initial recurrence, respectively (p=0.21). Adjuvant treatment had been administered in 72%, 69%, 76%, 70%, and 72% of pts with lung, liver, local, peritoneal, and multiple sites at initial recurrence, respectively (p=0.88). Pts with initial lung recurrence had a significantly longer median overall survival (mOS), 4.39 yrs, compared to initial recurrence in the liver (1.98 yrs, p=0.02), peritoneum (2.19 yrs, p=0.0002), and at multiple sites (2.66 yrs, p=0.03). A significantly longer time from diagnosis to recurrence was observed in pts who had initial lung recurrence, compared to pts who had initial hepatic, peritoneal or multiple site recurrences. Pts with initial lung recurrence had a significantly longer time from first recurrence to death compared to pts with initial peritoneal recurrence. See Table for summary. Conclusions: Pts with resected PDA with initial pulmonary recurrence experience improved survival compared to those who recur at other distinct or multiple sites. The underlying pathways contributing to this improved survival need to be investigated further.[Table: see text]

Neoadjuvant gemcitabine, docetaxel, and capecitabine results in comparable surgical outcomes to modified FOLFIRINOX in patients with pancreatic ductal adenocarcinoma who also receive radiation: A single institution experience

565

Background: Pancreatic ductal adenocarcinoma (PDAC) has a dismal prognosis with a minority of patients (pts) eligible for curative resection. Currently, systemic treatment options for down-staging pts with borderline resectable or locally advanced PDAC is extrapolated from the metastatic setting and modified FOLFIRINOX (FFX) +/- radiation (RT) is the most widely used regimen. Herein, we report the outcomes of combination gemcitabine, docetaxel, and capecitabine (GTX) +RT as compared to FFX +RT in the neoadjuvant (NA) setting via a single institution retrospective cohort review. Methods: We retrospectively reviewed the outcomes of pts with PDAC who underwent surgical resection at Columbia University Irving Medical Center (CUIMC) between 2011-2020. We evaluated demographics, treatment, clinical, surgical, and pathological outcomes. Statistical analysis includes Kaplan-Meier analysis and paired t-tests. Results: We reviewed 717 pts who underwent surgical resection at CUIMC of which 227 pts were confirmed to have received NA chemotherapy. Of those 227 patients, 133 pts also received RT. In total, 39 pts received GTX+RT and 42 pts received FFX+RT. Median age at diagnosis of pts who received NA GTX+RT or FFX+RT was 65 and 63 years, respectively. All pts were AJCC stage III at diagnosis and ECOG 0 or 1. There was a significantly greater percentage of pts who achieved R0 resection after GTX+RT as compared to FFX+RT, 35 (89.7%) vs 29 (69.0%), respectively (p=0.022). Significantly more pts achieved N0 lymph node status after GTX+RT as compared to FFX+RT, 29 (74.4%) vs 22 (52.4%), respectively (p=0.041). No statistically significant difference was detected in recurrence-free survival (RFS) or median overall survival (mOS) in pts who received GTX+RT and achieved R0 resection as compared to FFX+RT. See Table for summary. Conclusions: GTX appears to be a viable and active NA regimen in Stage III PDAC. In our small cohort study, more patients who received GTX+RT achieved R0 resection and N0 status as compared to FFX+RT. No difference in survival was detected but this may be due to inadequate power or choice of subsequent therapies. Larger prospective studies evaluating GTX+RT as an alternative treatment in the NA setting are warranted.[Table: see text]

Cost-effectiveness analysis of platinum-based chemotherapy treatment options for germline BRCA-mutated locally advanced/borderline resectable pancreatic cancer

e16246

Background: Patients with germline BRCA1/2 mutations (gBRCAm) have an increased risk for pancreatic ductal adenocarcinoma (PDAC). The NCCN 2020 guidelines recommend platinum-based chemotherapy (FOLFIRINOX or gemcitabine plus cisplatin) in patients with gBRCAm diagnosed with borderline resectable or locally advanced (BR/LA) PDAC; for patients without progression on chemotherapy, maintenance therapy with a PAPR inhibitor (PARPi) can be considered. FOLFIRINOX and gemcitabine plus cisplatin (GemCis), with or without maintenance PARPi, have not been directly compared in this patient population. The purpose of our study was to compare treatment outcomes, toxicity, costs, and quality-of-life of the two recommended platinum-based regimens, with or without maintenance PARPi (olaparib or veliparib), in patients with gBRCAm and BR/LA PDAC. Methods: We developed a decision-analytic mathematical model comparing the total cost and health outcomes of FOLFIRINOX, FOLFIRINOX + olaparib (FOLFIRNOX-O), GemCis, and GemCis + veliparib (GCV) over twelve years. The inputs for the model were estimated using clinical trial data and published literature. Natural history was used as a comparator. The primary endpoint was incremental cost-effectiveness ratios (ICERs) with a willingness-to-pay (WTP) threshold of $100,000 per quality-adjusted-life-year (QALY). Secondary endpoints included overall survival (OS), progression-free survival (PFS), life-years (LYs) and total cost of care (USD). Results: Both FOLFIRINOX strategies were dominated by the GemCis strategies and thus eliminated from the efficiency frontier. GCV was the strategy that yielded the most life-years (2.05 LYs) but the ICER of $629,697.46 for this regimen far exceeded the WTP threshold. Therefore, GemCis was the cost-effective strategy, totaling incremental QALYs of 1.31 at a cost of $61,228.40 per QALY when compared to natural history. A one-way sensitivity analysis found that the GCV becomes the cost-effective strategy when the cost of maintenance PARPi is lowered to 4% of the base case. Conclusions: Our model found that GemCis is the cost-effective option for patients with gBRCAm and BR/LA PDAC. Neither of the strategies with maintenance PARPi were cost-effective. Additional clinical trial data with adequate follow-up are needed to confirm our findings.[Table: see text]

ARC-8: Phase I/Ib study to evaluate safety and tolerability of AB680 + chemotherapy + zimberelimab (AB122) in patients with treatment-naive metastatic pancreatic adenocarcinoma (mPDAC)

404

Background: AB680, a potent, selective small-molecule inhibitor of soluble and membrane-bound CD73, targets a major pathway of extracellular adenosine production with the aim of eliminating adenosine-mediated immunosuppression within the tumor microenvironment. In mPDAC, programmed cell death protein-1 (PD-1) axis inhibitors have limited clinical activity as monotherapies or combined with standard-of-care (SOC) chemotherapy. KRAS mutations, present in >90% of invasive PDACs, are associated with significantly elevated CD73 expression and poor clinical outcomes. Thus, mPDAC may be particularly sensitive to CD73 inhibition combined with SOC chemotherapy + anti–PD-1 antibody (zimberelimab [Zim]) treatment. Methods: ARC-8 (NCT04104672) is an ongoing phase 1b, dose escalation and expansion study in patients (pts) with treatment-naive mPDAC. In the dose escalation, AB680 (25, 50, 75, or 100 mg) is administered intravenously once every 2 weeks (Q2W) with standard doses of nab-paclitaxel/gemcitabine (NP/Gem) + Zim (240 mg) in a 3+3 design to determine the recommended dose for expansion (RDE). In the dose expansion, AB680 will be administered at the RDE with NP/Gem + Zim. Adverse events (AEs) and dose-limiting toxicities (DLTs) are recorded and graded per NCI CTCAE 5.0. AB680 pharmacokinetics, pharmacodynamics, and biomarker evaluations are also being performed throughout the study. Clinical activity is assessed every 8 weeks per RECIST v1.1. Results: As of 04SEPT2020, enrolled patients include 4 in Cohort 1 (25 mg AB680), 6 in Cohort 2 (50 mg AB680), and 3 in Cohort 3 (75 mg AB680). Initial AE profiles were similar to those observed for NP/Gem. The most common treatment-related AEs were fatigue (n=6, 43%), anemia (n=4, 29%), and neutrophil count decrease (n=4, 29%); anemia (n=2, 14%) was the most common treatment-related grade 3/4 AE. Initial pharmacodynamic data (50 mg dose) indicated excellent peripheral target coverage at AB680 trough concentrations. Best overall responses for 9 evaluable pts were 3 with partial response (1 in Cohort 1; 2 in Cohort 2), including a complete response of a target lesion, and 5 with stable disease (1 in Cohort 1; 4 in Cohort 2). One pt in Cohort 1 discontinued the study due to progressive disease. As of the cutoff date, 1 DLT (Grade 2 autoimmune hepatitis) occurred in Cohort 2; the event completely resolved with steroids and the pt was able to resume study treatment. No DLTs were reported in Cohort 3. Conclusions: Preliminary results from ARC-8 indicate that AB680, the first clinical-stage small-molecule CD73 inhibitor, in combination with SOC chemotherapy + Zim has a manageable safety profile consistent with that expected for each agent alone and demonstrates early signals of clinical activity. Dose escalation to 100 mg AB680 (Cohort 4) is ongoing to inform RDE selection. Clinical trial information: NCT04104672.

MEKiAUTO: A phase I/II open-label study of combination therapy with the MEK inhibitor cobimetinib, Immune-checkpoint blockade with atezolizumab, and the AUTOphagy inhibitor hydroxychloroquine in KRAS-mutated advanced malignancies

TPS450

Background: Pancreatic ductal adenocarcinoma (PDAC) and colorectal carcinoma (CRC) are aggressive diseases which account for the third and second leading causes of cancer-related death, respectively. Limited progress has been made towards effective treatments or cure. K RAS is mutated or amplified in nearly 30% of all cancers, including up to 95% of PDAC and 45% of CRC. Although no successful KRAS directed therapy has been approved to date, data has emerged demonstrating that inhibition of downstream targets of KRAS, namely MEK/ERK, increases autophagic flux in KRAS-mutated tumors. This catabolic process is used by many tumors to maintain viability and recent preclinical studies have shown that combining MEK/ERK and autophagy inhibitors in KRAS-mutated tumors can synergistically suppress cancer cell proliferation. Inhibition of autophagy has demonstrated an increase in antigen presentation which sensitize tumors to immune checkpoint inhibitors. Presented here is a trial-in-progress that will evaluate combination of cobimetinib (MEK inhibitor), atezolizumab (anti-PDL1), and hydroxychloroquine (autophagy inhibitor) in KRAS-mutated advanced malignancies. Methods: This is a phase 1/2 multicenter, open-label study of combination cobimetinib (40-60mg) orally once daily on days 1-21, hydroxychloroquine (600mg) orally twice daily on days 1-28, and atezolizumab 840 mg IV on days 1 and 15 of each 28 day cycle. Patients with histologically confirmed metastatic or unresectable KRAS-mutant adenocarcinoma for which standard curative or meaningful life-prolonging treatment options do not exist or are no longer effective will be enrolled. The primary objective of the phase I portion of this trial, which seeks to enroll 18 patients, is to estimate the maximum tolerated dose (MTD) of these agents using a two-stage time-to-event continual reassessment method (TITE-CRM). The primary objective of the phase II portion of this trial, which seeks to enroll approximately 66-157 patients, is to evaluate the preliminary efficacy of this combination, based on the objective response by 16 weeks. Secondary endpoints include PFS, OS, and safety. Correlative aims include analyses of pre- and on-treatment biopsies with quantitative multiplex immunofluorescence, RNA-sequencing, reverse phase protein array for association with clinical benefit and to determine mechanisms of action/resistance. An interim analysis will be performed at the conclusion of the phase I portion of the study. This study is open with 4 patients enrolled at the time of submission. Clinical trial information: NCT04214418 . This trial is being conducted as part of the imCORE collaboration. Clinical trial information: NCT04214418.

Chemo4MetPanc: A phase II study with combination chemotherapy (gemcitabine and nab-paclitaxel), chemokine (C-X-C) motif receptor 4 inhibitor (motixafortide), and immune checkpoint blockade (cemiplimab) in metastatic treatment naïve pancreas adenocarcinoma (PDAC)

TPS454

Background: Pancreas adenocarcinoma (PDAC) is an aggressive cancer projected to be the second leading cause of cancer-related death in the United States by 2030 for which improved treatment options are desperately needed. Immune checkpoint blockade (ICB) for PDAC has failed as monotherapy in early phase clinical trials likely due to a highly immunosuppressive tumor microenvironment. The CXCR4/CXCL12 axis is a key immune evasion mechanism thought to deter CD8+ T-cells (CTLs) from infiltrating the tumor. We performed a large seven arm survival and biopsy/necropsy study in KPC mice (KrasLSL.G12D/+;p53R172H/+;Pdx1Cretg/+) where we demonstrate that addition of gemcitabine to CXCR4 inhibition in combination with ICB, enhanced tumor stabilization and neoplastic cell death, and improved survival by 50 percent. Multiplex immunofluorescence indicated an increased CTL to regulatory T-cell ratio and clustering of CTLs around neoplastic cells. Presented here is a trial-in progress that will evaluate combination of a CXCR4 inhibitor, ICB, and chemotherapy in treatment naïve patients with PDAC. Methods: This is a multicenter, single arm, open-label phase 2 study of combination motixafortide 1.25mg/kg SC monotherapy for 5 days during priming followed by twice weekly, cemiplimab 350mg IV once every 21 days, gemcitabine 1000mg/m2 IV with nab-paclitaxel 125mg/m2 IV on days 1, 8, and 15 every 28 days. Patients with histologically confirmed metastatic PDAC who have not received prior therapy will be enrolled. The primary endpoint is overall response rate by 16 weeks. A response rate greater than 45% by 16 weeks is considered promising, whereas a response rate of less than 23% is considered not promising. We will use a Simon optimal 2-stage design, where we will enroll 10 patients in the first stage. If 3 or more patients meet the endpoint in the first stage, the study will be expanded to a total of 40 patients. If a total of 14 or more patients achieve CR or PR by 16 weeks, the agent will be considered promising and worthy of further study. Secondary endpoints include safety, mPFS, disease control rate (DCR), and mOS. Correlative aims include analyses of pre- and on-treatment biopsies with quantitative multiplex immunofluorescence, RNA-sequencing, and generation of patient derived organoids for association with clinical benefit and to determine mechanisms of action/resistance. An interim analysis will be performed at the conclusion of the stage I portion of the study. Clinical trial information: NCT04543071.

Personalized viral-based prime/boost immunotherapy targeting patient-specific or shared neoantigens: Immunogenicity, safety, and efficacy results from two ongoing phase I studies

3137

Background: Neoantigens are key targets of a tumor-specific immune response and CD8 T cells targeting neoantigens drive clinical benefit in patients (pts) treated with checkpoint inhibitors. Methods: Two Phase I studies are being conducted to assess the safety, immunogenicity, and early clinical activity of a viral-based neoantigen-targeting prime/boost immunotherapy aimed at maximizing the CD8 T cell response. Both studies use a chimpanzee adenovirus prime followed by increasing doses of repeat boosts with a self-amplifying mRNA in combination with IV nivolumab +/- SC ipilimumab. In the first study, GO-004, patient-specific neoantigens are predicted using Gritstone's EDGE model and incorporated into both prime/boost vectors. In GO-005, shared neoantigens derived from common driver mutations (including several from KRAS) are encoded in off-the-shelf prime/boost vectors. Results: To date, 12 pts have been treated: 6 pts with GEA, NSCLC, or MSS-CRC (GO-004) and 6 pts with NSCLC, MSS-CRC, or PDA (GO-005) with all pts receiving IV nivolumab and 5 pts also receiving SC ipilimumab. Nine pts continue to receive study treatment. No DLTs have been observed. Treatment-related AEs are reversible and include Grade 1/2 fever (7/12), injection site reactions (4/12), fatigue (3/12), diarrhea (2/12), hypotension (2/12), pruritus (2/12), skin reactions (2/12), anorexia (1/12), dyspnea (1/12), hyponatremia (1/12), infusion-related reactions (1/12), myalgia (1/12), and asymptomatic Grade 3 CK elevation (1/12). At the time of analysis, 8 of 12 pts with ≥ 1 radiographic assessment have a best response of stable disease (SD) (3) and progressive disease (PD) (4), and one pt with no evaluable disease at baseline continues on study > 8 months. In GO-005, 1 pt with SD has a 20% reduction in tumor dimensions that correlates with a decrease in ctDNA. In 4 pts in GO-004 analyzed to date, all pts showed substantial neoantigen-specific CD8 T cell responses to multiple neoantigens after priming which increase further in 2 of 3 pts analyzed after subsequent boosts. In GO-005, 1 of 3 pts showed a robust KRAS G12C-specific CD8 T cell response. Induced T cells express IFNg and granzyme B, consistent with an effector response. Conclusions: Taken together, these early data support the tolerability of a viral-based prime/boost immunotherapy, demonstrate marked immunogenicity, and are consistent with potential clinical activity. Additional pts and data at higher dose levels will be presented. Clinical trial information: NCT03639714, NCT03953235 .

Cost-effectiveness of neoadjuvant FOLFIRINOX versus gemcitabine plus nab-paclitaxel in locally advanced/borderline resectable pancreatic cancer patients

e16793

Background: The 2019 NCCN guidelines recommend neoadjuvant FOLFIRINOX or neoadjuvant gemcitabine plus nab-paclitaxel (G-nP) for locally advanced and borderline resectable pancreatic ductal adenocarcinoma (BR/LA PDAC). Neoadjuvant FOLFIRINOX and G-nP have yet to be directly compared in a prospective, randomized trial with BR/LA PDAC patients. The purpose of our study was to incorporate treatment outcomes, toxicity profiles, costs, and quality-of-life measures to further inform clinical decision-making. Methods: We developed a decision-analytic mathematical model to compare the total cost and health outcomes of neoadjuvant FOLFIRINOX against G-nP over twelve years. Adjuvant gemcitabine (GEM) was used as a comparator. The inputs for the model were estimated using clinical trial data and published literature. We used single-institution retrospective studies to estimate our survival data in the absence of a prospective trial. The primary endpoint was incremental cost-effectiveness ratios (ICERs) with a willingness-to-pay (WTP) threshold of $100,000 per quality-adjusted-life-year (QALY). Secondary endpoints included overall (OS) and progression-free survival (PFS), total cost of care (USD), QALYs, patient resection rate, and monthly treatment-related adverse events (TRAE) costs (USD). Results: FOLFIRINOX was the cost-effective strategy, totaling incremental QALYs of 0.21 at a cost of $52,845.96 per QALY when compared to G-nP. G-nP was also on the efficiency frontier with an ICER of $46,430.73 compared to GEM. More patients received resection with FOLFIRINOX (82.15% vs. 72.40%), but had higher TRAE costs than G-nP ($12,051.26 vs. $4,666.97). A one-way sensitivity analysis found that the FOLFIRINOX ICER exceeds the WTP threshold when TRAE costs are higher and resection rates are lower. Conclusions: Our modeling analysis finds FOLFIRNOX is the cost-effective treatment compared to G-nP for BR/LA PDAC despite having a higher cost of total care due to TRAE costs. Trial data with sufficient follow-up are needed to confirm our findings. [Table: see text]

Use of a real-world data approach to rapidly generate outcomes data following a case study of a novel treatment combination in pancreatic adenocarcinoma

e16735

Background: We use a real-world data approach to report on safety and benefits on metastatic pancreatic cancer pts who were treated with a MEK inhibitor plus hydroxychloroquine (HCQ) after exhausting all other treatment options. MEK inhibition acts on the KRAS pathway, which in turn increases autophagy as a resistance mechanism, furthermore, HCQ inhibits autophagy causing a cytotoxic effect. This combination was shown to diminish tumor volume in xenograft mouse models and a partial response in one heavily pre-treated patients was reported. Methods: XCELSIOR is an IRB approved, patient-centric, real-world data and outcomes registry for developing operational and analytic methods in precision oncology. Searching the XCELSIOR database, we identified 14 pts for whom this regimen had been considered. As part of their participation in XCELSIOR, these patients shared access to their full medical records, which were collected, processed, and abstracted into a 21 CFR 11 compliant database for analysis. We additionally collected de-identified data on 12 pts treated with this combination from five academic centers. Three more patients are expected to start treatment soon. Results: Between March 2018 and January 2020, 15 patients treated with the trametinib/HCQ combination and 3 patients treated with cobimetinib/HCQ were identified in XCELSIOR and five academic institutions. The median age at diagnosis was 64 (range 43-74) and 56% were male. For patients treated with trametinib/HCQ, the median time on treatment was 67 days (range 5-172 days), 11 patients were treated for more than 30 days (median time 97 days). The median PFS for this group was 2.9 months and the median OS was 7.4 months. The clinical benefit rate was 60% for the 10 evaluable patients treated with trametinib/HCQ, 1 patient had a partial response (previously published), 5 had stable disease (for at least 8 weeks) and 4 had progressive disease (physician reported). 2/3 patients treated with cobimetinib/HCQ were on treatment for more than 30 days and all three had progressive disease within 7 weeks. The most common side effects were Grade 1 fatigue and Grade 1/2 rash for both combinations. An additional 3 patients will start treatment soon and will be included in the analysis. Conclusions: Combinatorial MEK and autophagy inhibition was well tolerated in heavily treated metastatic pancreatic cancer patients. Trametinib/HCQ demonstrates some clinical benefit for this group. We demonstrate the feasibility of utilizing real-world data in precision oncology. Clinical trial information: NCT03793088 .

A phase I study to evaluate the safety and tolerability of AB680 combination therapy in participants with gastrointestinal malignancies

TPS788

Background: Metastatic pancreatic ductal adenocarcinoma (mPDAC) expresses very high levels of CD73 among tumor types, and CD73 expression level is a known poor prognostic factor in PDAC. Adenosine, a product of AMP breakdown by CD73, is highly immunosuppressive against effector T & NK cells in the tumor microenvironment. AB680 is the first clinical-stage small-molecule CD73 inhibitor, which is highly potent, pharmacodynamically active, and safe in healthy volunteer dose escalation studies. Targeting the adenosine pathway in combination with standard of care regimens may have a more profound effect on activating and inducing sustained anti-tumor immunity. Methods: This is a Phase 1/1b, open-label, dose-escalation, and dose-expansion study to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and clinical activity of AB680 in combination with AB122 (anti-PD-1 antibody) and standard chemotherapy (nab-paclitaxel [NP] and gemcitabine [Gem]) in participants with first line (1L) mPDAC. In the dose-escalation Ph1 portion, increasing dose levels of AB680 are administered every 2 weeks (Q2W) in combination with AB122 (240 mg Q2W) and NP/Gem (Gem 1000 mg/m2 + NP 125 mg/m2 IV on Days 1, 8, and 15 of each 28-day cycle). Up to 30 participants may be evaluated in Ph1 dose-escalation. In the dose-expansion Ph1b portion, AB680 will be administered at the recommended dose for expansion in combination with AB122 and NP/Gem in up to 40 participants. Adverse events will be graded according to NCI CTCAE 5.0 and antitumor activity assessed using RECIST v1.1. Conclusions: This Ph1/1b study will be the first to target the adenosine axis using a highly potent small-molecule inhibitor of CD73, AB680, in 1L mPDAC in combination with standard of care chemotherapy (NP/Gem) and a PD-1 antibody (AB122). Future results will be shared in upcoming scientific conferences. Clinical trial information: NCT04104672.

Phase Ib/II open-label, randomized evaluation of 2L atezolizumab (atezo) + BL-8040 versus control in MORPHEUS-pancreatic ductal adenocarcinoma (M-PDAC) and MORPHEUS-gastric cancer (M-GC)

712

Background: The MORPHEUS platform comprises multiple Ph Ib/II trials to identify early efficacy signals and safety of treatment (tx) combinations across cancers. Due to the immune-mediated effects of BL-8040, a high-affinity antagonist for CXCR4, it was tested with atezo (anti-PD-L1) in pts with advanced/metastatic (m) PDAC and GC. Methods: In 2 separate randomized trials, pts with mPDAC or advanced/mGC who progressed on 1L chemo received either atezo + BL-8040 (BL-8040 1.25 mg/kg SC D1-5, then BL-8040 1.25 mg/kg SC TIW + atezo 1200 mg IV Q3W) or control tx (M-PDAC: mFOLFOX-6 or gemcitabine + nab-paclitaxel; M-GC: paclitaxel + ramucirumab). Primary endpoints were investigator-assessed ORR per RECIST 1.1 and safety. Results: Efficacy from evaluable pts followed for ≥18 wks in M-PDAC and ≥8 wks in M-GC is summarized in the table; 24-wk M-GC data will be presented. There were 15 safety-evaluable pts in each M-PDAC arm, as well as 13 in the atezo + BL-8040 and 12 in the control arm of M-GC. Gr 3-5 AEs were seen in 47% of pts on atezo + BL-8040 and 67% on control in M-PDAC, and 77% on atezo + BL-8040 and 67% on control in M-GC. Tx-related SAEs in M-PDAC occurred in 7% of pts on atezo + BL-8040 and 20% on control, and in M-GC, in 8% of pts on control. No Gr 5 AEs occurred in atezo + BL-8040 arms. Tx-related AEs led to 7% and 8% of pts discontinuing tx in the M-PDAC and M-GC control arms, respectively, and 15% discontinuing BL-8040 in M-GC due to Gr 3 injection-related reactions. Biomarker and PK data will be presented. Conclusions: Atezo + BL-8040 had limited efficacy for PDAC or GC. Tx-related AEs with atezo + BL-8040 were consistent with each agent’s known safety profile. Clinical trial information: NCT03281369; NCT03193190 . [Table: see text]

Ligand-inducible, prostate stem cell antigen (PSCA)-directed GoCAR-T cells in advanced solid tumors: Preliminary results with cyclophosphamide (Cy) ± fludarabine (Flu) lymphodepletion (LD)

2536

Background: Cell-surface protein PSCA is upregulated in many solid tumors and correlates with disease stage. BPX-601, an autologous T-cell product expressing a PSCA-CD3ζ CAR and a rimiducid (Rim)-inducible MyD88/CD40 co-activation switch to augment T-cell proliferation and persistence, is designed to have enhanced efficacy in solid tumors vs traditional CARs. This ongoing first-in-human study assesses safety, biologic, and clinical activity of BPX-601+Rim in PSCA+ cancers. Updated results, including those from patients (pts) who underwent LD with Flu/Cy, are presented. Methods: BP-012 is a 2-part, open-label trial. Part 1 is a 3+3 dose escalation of BPX-601 (1.25–5.0x106 cells/kg; Day [D] 0) given prior to a single, fixed Rim dose (0.4 mg/kg; D7) in pts with previously treated PSCA+ metastatic pancreatic, gastric, or prostate cancers with measurable disease. Results: As of Jan-22-2019, 15 pts have received BPX-601±Rim. Two pts at the highest cell dose received Flu/Cy for LD on D−5 to D−3 before BPX-601; LD after Flu/Cy was 96.6% and 84.3%. Thirteen pts received Cy alone on D−3; in these pts, LD ranged from 0–68.6%. Rapid cell expansion by D4 was observed in all pts with peak vector copy number 8.3-fold higher with Flu/Cy (n = 2) vs Cy LD (n = 13). Serum IP-10, IL-6 and TNFα increased > 2-fold from baseline in ≥1 pt in all Rim cohorts, with 3- to 20-fold Rim-dependent cell expansion in 6 pts. No CRS or DLTs were reported. After Rim, one Flu/Cy pt experienced a serious Grade 2 AE (encephalopathy) related to BPX-601+Rim that resolved with IV steroids; despite time-matched nonserious Grade 1 pyrexia, the pt had no other CRS symptoms. After BPX-601+Rim and ≥1 scan, best responses were 8 SD and 3 PD (1 non-evaluable). With a median follow-up of 9.8 wks, time to next treatment (tx) after BPX-601 ranged from 2.7–22.1 wks (n = 8) and ongoing tx-free intervals range from 9.1–30.1 wks (n = 4). Conclusions: BPX-601+Rim was well-tolerated with manageable safety and early evidence of enhanced CAR T-cell expansion and prolonged persistence after Flu/Cy vs Cy. Additional pts will undergo Flu/Cy LD prior to BPX-601 with single- and repeat-dose Rim. Clinical trial information: NCT02744287.

Phase 1 combination therapy with pexidartinib (PEX) and sirolimus (S) to target tumor-associated macrophages in pigmented villonodular synovitis, malignant peripheral nerve sheath tumors, and other soft tissue sarcomas

11055

Background: No effective therapy exists for unresectable malignant peripheral nerve sheath tumors (MPNSTs). We previously reported that the combination of PEX and the mTOR inhibitor S synergistically inhibited MPNST growth (CCR 20: 3146, 2014) by depleting M2 TAMs and by inhibiting receptor tyrosine kinases (RTKs), including c-KIT, PDGFR, CSF1R. We characterized the safety, tolerability, recommended phase 2 dose (RP2D) of PEX plus S in all sarcoma sub-types. Methods: Patients (pts) received PEX plus S orally in 28 days cycle as per Table. The RP2D was determined using the time-to-event continual reassessment method (TITE-CRM) in advanced sarcoma who have progressed on standard therapy. DLT was defined as any need for a dose reduction. Results: 24 pts were accrued (Acr) of which 18 were evaluable (MPNST – 6, pigmented villonodular synovitis (PVNS) – 3, leiomyosarcoma – 5, and other – 9). The mean age was 46y, 56% were male, and 67% had greater than 2 prior therapies. Most common ( > 20%) grade 2 or higher TEAEs were anemia (33%), WBC count decrease (28%), fatigue, neutropenia, and lymphopenia (22% each). There were 5 dose limiting toxicities (DLT): 2 for elevated LFTs both of which resolved with dose reduction, 2 for supra-therapeutic S trough levels, and 1 for grade 5 dehydration at dose level (DL) 3. Four subjects experienced a partial response (PR; -44% to -77% by RECIST, 18 – 61 wks on therapy). Seven subjects experienced stable disease (SD; +19.7% to -20.7% by RECIST; 9.4 – 30 wks on therapy). Five subjects progressed on therapy and two subjects experienced early DLTs and did not undergo tumor assessment. The RP2D is DL 3 (S 2mg/PEX 1000mg) with an estimated probability of DLT of 26.7% as determined by TITE-CRM. This recommendation is based on a target DLT rate of 25%. TAMs and immune subtypes from available tissue specimens and historical controls will be presented. Conclusions: 1000mg of PEX in combination with 2mg of S daily has an acceptable safety profile. Objective responses and durable SD was observed in PNVS and MPNST patients justifying proceeding with a multi-center single arm phase 2 study in advanced MPNST. Clinical trial information: NCT02584647. [Table: see text]

Second-line treatment of metastatic gastric cancer in the era of predictive biomarkers: A cost-effectiveness analysis

e15517

Background: Gastric cancer is the third leading cause of cancer-related mortality, with only a 30% five-year survival rate. Patients who progress after one round of systemic therapy face an especially poor prognosis. The National Comprehensive Cancer Network guidelines include both pembrolizumab (PEM) and ramucirumab plus paclitaxel (RAM/PAC) as second-line (2L) therapy for gastric cancer based on data from the Phase II KEYNOTE-059 and Phase III RAINBOW trials, respectively. Recently, the Phase III KEYNOTE-061 trial reported on the effectiveness of PEM for patients with programmed death-ligand 1 (PD-L1) expression and high microsatellite instability (MSI-H). Given the high prices of targeted therapies, it is important to determine if cost-effectiveness is possible using personalized treatment strategies. The aim of this study was to assess the cost-effectiveness of these regimens in both the general patient population and specific biomarker populations. Methods: A decision-analytic (Markov) model was constructed using data from the KEYNOTE-059, KEYNOTE-061, RAINBOW, and REGARD trials. The analysis compared PEM and RAM/PAC for all patients, as well as PEM for patients based on MSI status or PD-L1 expression (combined positive score of 1% or 10%) in the 2L setting. Comparators were paclitaxel monotherapy (PAC) for all patients and best supportive care (BSC) for all patients. Costs (USD) and utility values were estimated from Medicare and the literature. The primary outcome was the incremental cost-effectiveness ratio (ICER) with a willingness-to-pay (WTP) threshold of $100,000. Results: The only cost-effective strategy was PAC monotherapy for all patients, with an ICER of $53,705/QALY. PEM for MSI-H patients and RAM/PAC for microsatellite stable patients was the most effective strategy (greatest QALYs), but was not cost-effective with an ICER of $1,074,620/QALY. Conclusions: Despite their effectiveness, PEM and RAM/PAC are not cost-effective as 2L treatments for metastatic gastric cancer. Although personalizing treatment based on biomarkers improved cost-effectiveness, the ICERs surpassed the WTP threshold at current drug prices.

Phase II multi-institutional study of nivolumab (Nivo), cabiralizumab (Cabira), and stereotactic body radiotherapy (SBRT) for locally advanced unresectable pancreatic cancer (LAUPC)

TPS4163

Background: Treatment of LAUPC most commonly involves chemotherapy +/- RT. Patients(pts) who can be downstaged and undergo R0 resection have significant improvement in overall survival, but conventional chemoRT converts < 10% of patients with LAUPC. If the effects of RT can be augmented then higher R0 resection rates may be achieved and improve survival. In pre-clinical models, RT leads to increased expression of M-CSF from pancreatic tumor cells and marked immune suppression within the tumor microenvironment via expansion of tumor associated macrophages (TAMs). Concurrent blockade of M-CSF with RT reduces TAM infiltration, prevents the generation of tumor promoting T cell populations, and increases the therapeutic effect of RT. RT also induces up-regulation of PD-L1 in TAMs, another mode of immune suppression that can account for RT resistance in LAUPC. (Seifert et al. 2016). These data suggest the efficacy of RT in LAUPC is limited by its promotion of innate and adaptive immune suppression. CSF1R blockade with Cabira combined with PD-1 blockade with Nivo may enhance the efficacy of SBRT by reprogramming the TAM compartment in tumors, thereby preventing an immune suppressive phenotype and augmenting T-cell mediated anti-tumor response. Methods: Single arm phase II study designed to evaluate safety, tolerability, and surgical resection rate in LAUPC pts treated with concurrent Nivo, Cabira, and SBRT. Exploratory endpoints include immune changes within blood and tissue following treatment and correlation with clinical endpoints. Key eligibility: completion of 2- 6 months standard induction chemotherapy, normal organ and marrow function, pre- and on-treatment biopsy, and PS ≤ 1. Following initial biopsy and placement of fiducials, Cabira 4mg/kg and Nivo 240mg are given D1 of every 14 day cycle. SBRT 6.6 Gy x 5 consecutive fractions starts D8. After 2 cycles, repeat biopsy and imaging is performed. Treatment with Cabira and Nivo continues every 2 weeks and imaging is done every 8 weeks, at which time pt is assessed for surgical resection. If pt is downstaged, treatment is discontinued and pt proceeds to surgery. Preliminary 6 pt safety cohort is monitored for unacceptable toxicities. If < 3 unacceptable toxicities in the first 6 subjects enrolled, then plan for expansion phase with 14 more pts. As of abstract submission, 3 pts have been enrolled. Clinical trial information: NCT03599362.

Ligand-inducible, prostate stem cell antigen (PSCA)-directed GoCAR-T cells in advanced solid tumors: Preliminary results from a dose escalation

283

Background: PSCA, a cell surface protein, is upregulated in many solid tumors and correlates with disease stage. BPX601 is an autologous, T-cell product engineered to contain a PSCA-CD3ζ CAR plus the small molecule rimiducid (Rim)-inducible MyD88/CD40 costimulatory domain. BPX601 is optimized for antigen-directed and independent T cell activation, proliferation and persistence, potentially enhancing efficacy in solid tumors versus traditional CARs. This first-in-human study assesses the safety, biological and clinical activity of BPX601 plus Rim in select PSCA-positive cancers. Methods: NCT02744287 is a two-part, open-label trial. Part 1 is an ongoing 3+3 cell dose escalation to identify the recommended BPX601 cell dose (Day 0) given in combination with a fixed, single Rim dose (0.4 mg/kg; Day 7). Eligibility criteria include previously treated metastatic pancreatic cancer (mPDAC) with measurable disease & positive PSCA expression. Results: Patients received only cyclophosphamide (CTX) for lymphodepletion (LD) within three days before BPX601 infusion. Nine adults have been treated across three cell dose levels (cells/kg): 1.25x106 (cells only), 1.25x106+Rim, 2.5x106+Rim. All had mPDAC with ≥ two prior therapies. Common AEs were fatigue and nausea. No DLTs, related SAEs, neurotoxicity or CRS events were reported. Rapid cell engraftment by Day 4 was observed in all patients. No evidence of LD with CTX was seen. Of six patients that received Rim: two had cell expansion 10- to 20-fold within seven days; two had cell persistence > three weeks; all had elevated serum cytokines (IP-10, TNFα) correlated with cell expansion. Best response after ≥ one scan was 4 SD ≥ eight weeks with two minor responses (not confirmed; one patient had matched CA19-9 decrease) and 2 PD. Disease control without new therapy was 16 and > 11 weeks (ongoing) in one and two patients, respectively. Conclusions: BPX601 with single-dose Rim was well-tolerated and resulted in enhanced T cell expansion and prolonged persistence in some patients despite lack of LD. Evidence of clinical benefit in this heavily pretreated mPDAC population was seen. Part 2 is planned to open soon and will include CTX/fludarabine LD to maximize engraftment as well as gastric and prostate cancers. Clinical trial information: NCT02744287.

Current and Emerging Therapies in Metastatic Pancreatic Cancer

Targeted therapies and immunotherapy have changed the face of multiple solid malignancies, including metastatic melanoma and lung cancer, but no such therapies exist for pancreatic ductal adenocarcinoma (PDAC) despite the knowledge of key mutations and an increasing understanding of the tumor microenvironment. Until now, most clinical studies have not been biomarker driven in this highly immunosuppressive and heterogeneous cancer. Ongoing basic and translational studies are better classifying the disease in hopes of identifying critical pathways that distinguish the unique PDAC subtypes, which will lead to personalized therapies. In this review, we discuss the current treatment options for metastatic pancreatic cancer and highlight current ongoing clinical trials, which aim to target the stroma and the immune microenvironment either alone or in combination with standard chemotherapy. Identifying biomarkers and key resistance pathways and targeting these pathways in a personalized manner in combination with chemotherapy are likely to yield a more immediate and durable clinical benefit. Clin Cancer Res; 23(7); 1670-8. ©2017 AACRSee all articles in this CCR Focus section, "Pancreatic Cancer: Challenge and Inspiration."

MORPHEUS: A phase Ib/II multi-trial platform evaluating the efficacy and safety of cancer immunotherapy (CIT)-based combinations in patients (pts) with gastric or pancreatic cancer

TPS530

Background: CIT has demonstrated significant survival benefits across multiple cancers. Despite remarkable success, only subsets of pts derive clinical benefit from CIT monotherapy. Thus, CIT combinations may be needed to address the mechanisms that allow cancers to escape antitumor immunity. However, a large number of potential combinations would have to be tested to identify an effective CIT combination regimen. The MORPHEUS platform consists of multiple, global, open-label, randomized Phase Ib/II trials designed to assess the impact of CIT combination therapies in pts with different tumor types. The randomized trial designs allow comparison of a single control arm vs multiple CIT combination arms. The trials will aid development of CIT combinations by identifying early signals and have the flexibility to open arms with new CIT combinations and close arms that show minimal clinical activity or unacceptable toxicity. Various CIT combinations that simultaneously enhance immune cell priming and activation, tumor infiltration and/or recognition of tumor cells for elimination will be evaluated. Here, we will describe Phase Ib/II trials in pts with gastric or gastroesophageal junction cancer (GC) and metastatic pancreatic ductal adenocarcinoma (mPDAC), both of which represent populations with high unmet medical need. Methods: MORPHEUS-GC (NCT03281369) will enroll 2 cohorts including pts with advanced unresectable or metastatic GC who have not received prior chemotherapy (chemo) or have progressed on platinum- or fluoropyrimidine-based chemo. MORPHEUS-PDAC (NCT03193190) will enroll pts with mPDAC who have progressed on prior chemo. Further eligibility criteria details will be provided. Pts will be randomized to one of the CIT combination arms or a control arm (up to 8 arms across 2 cohorts in GC; 4 arms in PDAC). Pts experiencing loss of clinical benefit or unacceptable toxicity may be eligible to continue on a different CIT combination arm. Primary endpoints are investigator-assessed ORR per RECIST v1.1 and safety. Secondary endpoints include PFS, OS, DCR and DOR. Multiple exploratory biomarkers will also be examined. Clinical trial information: NCT03281369, NCT03193190.

Phase II open-label, single-center study evaluating safety and efficacy of pembrolizumab following induction with the hypomethylating agent azacitidine in patients with advanced pancreatic cancer after failure of first-line therapy

TPS534

Background: First-line cytotoxic chemotherapy improves survival for patients with metastatic pancreatic adenocarcinoma (PDA) but the majority will progress after a median of 5-7 months. CD8 T-cells are thought to be the major anti-tumor effector cells and their density in resected pancreatic tumors correlates with survival. Our published pre-clinical data in the KPC-Brca1 mouse model shows that treatment with a hypomethylating agent inhibits tumor growth (Shakya, Gonda et al., Cancer Res, 2013) and, in recent data (B.T. and T.G. in preparation), combining a hypomethylating agent with an immune checkpoint inhibitor leads to slowing of tumor growth and increased survival in KPC mice. We propose a novel approach to improve response rates with immune checkpoint blockade in pancreatic cancer by utilizing a hypomethylating agent to prime the tumor and its microenvironment, thereby increasing the number of intratumoral effector T cells, decreasing the immunosuppressive cell population, and influencing stromal-tumor cell interactions. Methods: Thirty-one evaluable subjects with advanced pancreatic cancer will be enrolled in this phase II study to evaluate the efficacy and safety of pembrolizumab following induction with azacitidine in the second-line setting. All subjects will have received a single line of chemotherapy for locally advanced or metastatic PDA prior to study enrollment. Subjects must have available baseline biopsy or archival tissue for analysis. Low-dose azacitidine (50 mg/m2 subcutaneous) will be given daily for 5 days every 4 weeks, and pembrolizumab 200 mg IV will be administered starting on day 15 and then continued every 3 weeks. Subjects will undergo an on-treatment biopsy during week 8. The primary efficacy endpoint will be PFS according to RECIST v1.1 criteria. There will be an accelerated phase Ib dose determination component with an initial 6 subjects. Secondary endpoints will include safety/tolerability, ORR, DOR, and OS. Multiple correlative analyses utilizing tissue and serum samples will be performed. Clinical trial information: NCT03264404.

A phase I study of DKN-01 (D), an anti-DKK1 monoclonal antibody, in combination with gemcitabine (G) and cisplatin (C) in patients (pts) for first-line therapy with advanced biliary tract cancer (BTC)

4075

Background: DKK1 is a secreted modulator of Wnt signaling often expressed in tumors, including BTC. DKK1 expression in BTC is associated with advanced stage and shorter survival. Depletion of DKK1 has efficacy in BTC xenograft models, inhibits cell invasion, and decreases MMP9 and VEGF-C expression, known promoters of metastasis and angiogenesis. D is a humanized monoclonal antibody against DKK1. This study evaluated the safety and efficacy of D in combination with GC in pts with advanced BTC. Methods: In Part A, pts received D at either 150 or 300 mg (and 300 mg D in Part B expansion) with 1000 mg/m2 G and 25 mg/m2 C on days 1 and 8 of each 21-day cycle. Response assessed every 2 cycles using RECISTv1.1. Results: 27 pts were enrolled; 4 dosed at 150 mg and 23 dosed at 300 mg. Median age: 65; Female: 74%; White: 85%. Gallbladder cancer 37%, intrahepatic cholangiocarcinoma 59%. 3 pts had prior G: 2 pts with adjuvant G; 1 pt with 2 prior regimens. Median number of cycles with D: 8 (range 1, 17). Median duration on study 6.8 mos; 8 pts still on therapy. No dose limiting toxicities or D-related serious adverse events have been observed. 24 pts (89%) had grade 3/4 treatment emergent adverse events (TEAEs); events in ≥ 3 pts include: neutropenia (n = 19), leukopenia (n = 9), thrombocytopenia (n = 9), hyperbilirubinemia (n = 6), anemia (n = 5), AST/ALT elevation (n = 4), and ALP elevation, bacteremia, hypertension, and hyponatremia (n = 3 each). The MTD of D + GC was 300 mg. At the MTD; 7 pts had a confirmed partial response (PR), 14 pts had stable disease > 6 weeks, and 1 pt had progressive disease. Both overall and MTD median PFS were 9.4 mos (95% CI 4.6, NE); median overall survival and duration of response were not reached. Conclusions: The addition of D (300 mg) to GC demonstrated a preliminary PFS of 9.4 mos and disease control rate of 96% with a 32% PR rate in pts with BTC. D + GC is well tolerated with no new emerging safety trends. Clinical trial information: NCT02375880.

A phase I study of LY3022855, a colony-stimulating factor-1 receptor (CSF-1R) inhibitor, in patients (pts) with advanced solid tumors

2523

Background: Binding of CSF-1 to the CSF-1 receptor (CSF-1R) results in proliferation, differentiation, and migration of monocytes/macrophages. Intratumoral infiltration with macrophages correlates with increased invasiveness, growth, and immunosuppression. LY3022855 (LY) is a human IgG1 antibody (mAb) targeting CSF-1R. Methods: Eligible pts (ECOG 0-2) with advanced solid tumors were enrolled. Mandatory pre and post-treatment biopsies were obtained. LY was given on a 6-week cycle. Two escalation regimens (Part A: weight-based dosing; Part B: flat dosing) were investigated in a 3+3 design. Primary objective was to establish the safety and characterize the pharmacokinetics (PK) of LY. Secondary objectives were to establish recommended phase 2 dose (RP2D) and to characterize pharmacodynamics (PD). Results: As of Sept 6, 2016, 35 cancer pts (colorectal 14; lung 4; pancreas 3; others 14) were treated (29 in Part A; 6 in Part B) with median treatment duration 4 weeks (range 1-21). Common treatment-emergent adverse events (TEAEs) were fatigue (54%), hypoalbuminemia (40%), nausea (37%), AST increase (37%), anemia (34%), anorexia (34%), creatine kinase elevation (29%), and constipation (23%). Most common grade (G) 3/4 TEAEs were anemia (11%), fatigue (11%), ascites (9%), and lymphocyte count decrease (9%). 3/28 evaluable pts had DLTs: G3 left ventricular systolic dysfunction (1), G4 rhabdomyolysis and G4 acute renal failure (1), and G3 pancreatitis (1). Eight treatment unrelated deaths were reported. One pt (adenoid cystic carcinoma) had stable disease (~3 mo as of last visit), 19 pts had progressive disease, and 15 pts were non-evaluable for response assessment. PK profile of LY was consistent with IgG1 mAbs. An interim analysis following completion of Part A demonstrated a lack of relationship between weight and clearance, prompting evaluation of non-weight based dosing. PD analyses revealed dose-dependent increases in serum CSF-1 levels as well as suppression of circulating non-classical monocytes (CD14dim CD16bright), indicating biologic activity at studied doses. Conclusions: RP2D for LY monotherapy has been determined. Detailed PK and PD data will be presented. Clinical trial information: NCT01346358.

Application of molecular biology to individualize therapy for patients with liposarcoma

Liposarcomas are one the most common of over 50 histologic subtypes of soft tissue sarcomas that are mostly resistant to chemotherapy. Histologically, liposarcomas themselves are heterogeneous and fall into four distinct subtypes: well-differentiated/atypical lipomatous tumor, dedifferentiated liposarcoma, myxoid (round cell) liposarcoma, and pleomorphic liposarcoma. Surgical resection with negative margins remains the mainstay for definitive treatment for operable disease. For unresectable disease, retrospective studies have identified myxoid (round cell) and pleomorphic sarcomas to be relatively responsive to chemotherapy. Recent studies have identified distinct genetic aberrations that not only aid in the diagnosis of particular liposarcoma subtypes, but represent actionable targets as they are considered central to disease pathogenesis. Cyclin-dependent kinase 4 (CDK4) and murine double minute 2 (MDM2) are overexpressed in well-differentiated and dedifferentiated liposarcomas and offer tantalizing opportunities that are being pursued in clinical trials. Myxoid (round cell) liposarcomas appear to be sensitive to trabectedin, which is currently under U.S. Food and Drug Administration (FDA) review. Liposarcomas do not represent a uniform disease and understanding the underlying molecular mechanism will help not only in accurate diagnosis but in selecting the appropriate treatment.

Time to ATRA in suspected newly diagnosed acute promyelocytic leukemia and association with early death rate at a non-cancer center institution: Are we meeting the target?

6615

Background: ATRA administration in suspected APL patients (sAPL) is thought to impact early death rate (EDR) (Tallman and Manji, Blood Cells Mol Dis. 2011). Delay in ATRA therapy at specialized centers was associated with EDR (Altman et al Blood 2011). EDR within this study was significantly lower compared to the SEER database (12% vs 17.3%) and hence may not reflect overall delay in ATRA therapy. We determined time to ATRA therapy in sAPL, and fraction of sAPL that did not have disease. Methods: Retrospective analysis of patients that received ATRA for newly diagnosed s APL between 01/01/98-12/31/11 at Albany Medical Center. Time to hematologist evaluation, ATRA ordered and administered, and mortality data was collected from cancer registry, medical record, and chemo-pharmacy database. Results: A total of 39 patients with newly diagnosed sAPL were administered ATRA (46% male, mean age 50y). APL diagnosis: 29/39 (75%) true APL (APL); 9/10 ATRA-treated non-APL (A-nAPL); and one patient for whom cytogenetic data unavailable. EDR amongst APL was 5/29 (17%) compared to 2/9 (22%) within A-nAPL. Time variables were compared between APL patients that died early (<30d) to those that survived 30 days, and included: time to hematologist response (0.9d vs. 0.4d); time to ATRA ordered (2.9d vs. 2.0d); time to ATRA administered (3.4d v. 2.2d); and time elapsed between hematologist response to ATRA administered (2.5d v. 1.9d), respectively. Cryoprecipitate was administered to 1/5 (20%) patients who expired within 30d compared to 10/23 (43%) who survived. Overall mortality for APL was 9/29 (31%) compared to 4/9 (44%) for A-nAPL group. Compared to recent reports, time to ATRA administration in our institution was later (2.0d vs. 2.4d). Conclusions: Our data indicate that APL patients who survived 30d received ATRA early. EDR at our institution is comparable to that reported by SEER database and may be attributed to delay in ATRA administration. Higher fraction of patients that survived 30d received cryoprecipitate. Hence aggressive blood product support may contribute to improved survival. Timing at which these products were administered is currently being evaluated.

Incidence of colorectal premalignant and malignant neoplasm on colonoscopy in an in-patient setting

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Background: Colorectal cancer (CRC) continues to be a leading cause of death and the third most common cancer in both genders. Numerous studies have shown that routine screening for colon cancer reduces CRC incidence and mortality by detecting premalignant lesions early. Hence screening colonoscopy in the outpatient setting has been the standard of care. In contrast, the incidence of pre-malignant (PMN) and malignant neoplasms (MN) identified during a colonoscopy due to an acute presentation in an in-patient setting is currently unknown. We evaluate the incidence of PMN and MN lesions identified in patients that underwent in-patient colonoscopy for anemia, gastrointestinal bleed (GIB), and or abdominal pain. Methods: A retrospective review of all gastrointestinal consultations (GIC) from January 1st, 2009 until December 31st, 2009 at a single institution. GIC as determined by the requesting provider for anemia, abdominal pain, and/or gastrointestinal bleed (GIB) were included. Results: There were 291 GICs for anemia, abdominal pain, GIB, or a combination thereof resulting in 218 colonoscopies. A total of 15 (6.9%) PMN and MN lesions were identified. Pathological evaluation reveled 13 (86.7%) of these lesions to be consistent with that of adenocarcinoma. The remaining 2 showed squamous cell cancer morphology. PMN/MNs were identified in patients with GIB and anemia (46.7%), anemia alone (20%), GIB and abdominal pain (20%), and GIB alone (13.3%). There was no significant difference in lesions found between genders (46.7% male). Conclusions: Routine outpatient screening colonoscopy is the standard of care with the goal of resecting early stage cancerous lesions resulting in cure. However, there is paucity of data on the incidence of these lesions being identified in an in-patient setting where colonoscopy is performed for GI bleed, anemia, abdominal pain, or a combination thereof. We demonstrate that colonoscopy resulted in identification of PMN/MNs in a significant proportion of patients with afore mentioned specific complaints. Interestingly, most lesions were identified in patients with GIB and anemia, suggesting that a thorough evaluation be undertaken as it may herald an underlying neoplasm.

Card10 is a novel caspase recruitment domain/membrane-associated guanylate kinase family member that interacts with BCL10 and activates NF-kappa B

BCL10 belongs to the caspase recruitment domain (CARD) family of proteins that regulate apoptosis and NF-kappaB signaling pathways. Analysis of BCL10-deficient mice has revealed that BCL10 mediates NF-kappaB activation by antigen receptors in B and T cells. We recently identified a subclass of CARD proteins (CARD9, CARD11, and CARD14) that may function to connect BCL10 to multiple upstream signaling pathways. We report here that CARD10 is a novel BCL10 interactor that belongs to the membrane-associated guanylate kinase family, a class of proteins that function to organize signaling complexes at plasma membranes. When expressed in cells, CARD10 binds to BCL10 and signals the activation of NF-kappaB through its N-terminal effector CARD domain. We propose that CARD10 functions as a molecular scaffold for the assembly of a BCL10 signaling complex that activates NF-kappaB.

Human CARD12 Is a Novel CED4/Apaf-1 Family Member That Induces Apoptosis

The CED4/Apaf-1 family of proteins functions as critical regulators of apoptosis and NF-kappaB signaling pathways. A novel human member of this family, called CARD12, was identified that induces apoptosis when expressed in cells. CARD12 is most similar in structure to the CED4/Apaf-1 family member CARD4, and is comprised of an N-terminal caspase recruitment domain (CARD), a central nucleotide-binding site (NBS), and a C-terminal domain of leucine-rich repeats (LRR). The CARD domain of CARD12 interacts selectively with the CARD domain of ASC, a recently identified proapoptotic protein. In addition, CARD12 coprecipitates caspase-1, a caspase that participates in both apoptotic signaling and cytokine processing. CARD12 may assemble with proapoptotic CARD proteins to coordinate the activation of downstream apoptotic and inflammatory signaling pathways.

Apoptosis in motion. An apical, P35-insensitive caspase mediates programmed cell death in insect cells

Activation of caspases by proteolytic processing is a critical step during apoptosis in metazoans. Here we use high resolution time lapse microscopy to show a tight link between caspase activation and the morphological events delineating apoptosis in cultured SF21 cells from the moth Spodoptera frugiperda, a model insect system. The principal effector caspase, Sf-caspase-1, is proteolytically activated during SF21 apoptosis. To define the potential role of initiator caspases in vivo, we tested the effect of cell-permeable peptide inhibitors on pro-Sf-caspase-1 processing. Anti-caspase peptide analogues prevented apoptosis induced by diverse signals, including UV radiation and baculovirus infection. IETD-fmk potently inhibited the initial processing of pro-Sf-caspase-1 at the junction (TETD-G) of the large and small subunit, a cleavage that is blocked by inhibitor of apoptosis Op-IAP but not pancaspase inhibitor P35. Because Sf-caspase-1 was inhibited poorly by IETD-CHO, our data indicated that the protease responsible for the first step in pro-Sf-caspase-1 activation is a distinct apical caspase. Thus, Sf-caspase-1 activation is mediated by a novel, P35-resistant caspase. These findings support the hypothesis that apoptosis in insects, like that in mammals, involves a cascade of caspase activations.

The BIR motifs mediate dominant interference and oligomerization of inhibitor of apoptosis Op-IAP

The defining structural motif of the inhibitor of apoptosis (iap) protein family is the BIR (baculovirus iap repeat), a highly conserved zinc coordination domain of approximately 70 residues. Although the BIR is required for inhibitor-of-apoptosis (IAP) function, including caspase inhibition, its molecular role in antiapoptotic activity in vivo is unknown. To define the function of the BIRs, we investigated the activity of these structural motifs within Op-IAP, an efficient, virus-derived IAP. We report here that Op-IAP(1-216), a loss-of-function truncation which contains two BIRs but lacks the C-terminal RING motif, potently interfered with Op-IAP's capacity to block apoptosis induced by diverse stimuli. In contrast, Op-IAP(1-216) had no effect on apoptotic suppression by caspase inhibitor P35. Consistent with a mechanism of dominant inhibition that involves direct interaction between Op-IAP(1-216) and full-length Op-IAP, both proteins formed an immunoprecipitable complex in vivo. Op-IAP also self-associated. In contrast, the RING motif-containing truncation Op-IAP(183-268) failed to interact with or interfere with Op-IAP function. Substitution of conserved residues within BIR 2 caused loss of dominant inhibition by Op-IAP(1-216) and coincided with loss of interaction with Op-IAP. Thus, residues encompassing the BIRs mediate dominant inhibition and oligomerization of Op-IAP. Consistent with dominant interference by interaction with an endogenous cellular IAP, Op-IAP(1-216) also lowered the survival threshold of cultured insect cells. Taken together, these data suggest a new model wherein the antiapoptotic function of IAP requires homo-oligomerization, which in turn mediates specific interactions with cellular apoptotic effectors.

Baculovirus inhibitor of apoptosis functions at or upstream of the apoptotic suppressor P35 to prevent programmed cell death

Members of the inhibitor of apoptosis (iap) gene family prevent programmed cell death induced by multiple signals in diverse organisms, suggesting that they act at a conserved step in the apoptotic pathway. To investigate the molecular mechanism of iap function, we expressed epitope-tagged Op-iap, the prototype viral iap from Orgyia pseudotsugata nuclear polyhedrosis virus, by using novel baculovirus recombinants and stably transfected insect cell lines. Epitope-tagged Op-iap blocked both virus- and UV radiation-induced apoptosis. With or without apoptotic stimuli, Op-IAP protein (31 kDa) cofractionated with cellular membranes and the cytosol, suggesting a cytoplasmic site of action. To identify the step(s) at which Op-iap blocks apoptosis, we monitored the effect of Op-iap expression on in vivo activation of the insect CED-3/ICE death proteases (caspases). Op-iap prevented in vivo caspase-mediated cleavage of the baculovirus substrate inhibitor P35 and blocked caspase activity upon viral infection or UV irradiation. However, unlike the stoichiometric inhibitor P35, Op-IAP failed to affect activated caspase as determined by in vitro protease assays. These findings provide the first biochemical evidence that Op-iap blocks activation of the host caspase or inhibits its activity by a mechanism distinct from P35. Moreover, as suggested by the capacity of Op-iap to block apoptosis induced by diverse signals, including virus infection and UV radiation, iap functions at a central point at or upstream from steps involving the death proteases.

Proteolytic release of membrane-bound angiotensin-converting enzyme: role of the juxtamembrane stalk sequence

Many structurally and functionally diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors, and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE; human testis isozyme) or one of four juxtamembrane (stalk) mutants containing either deletions of 17, 24, and 47 residues (ACE-JM delta 17, -JM delta 24, and -JM delta 47, respectively) or a substitution of 26 stalk residues with a 20-residue sequence from the stalk of the low-density lipoprotein receptor (ACE-JMLDL). The C termini of released, soluble WT-ACE and ACE-JM delta 17 and -JMLDL were determined by MALDI-TOF mass spectrometry analyses of C-terminal peptides generated by CNBr cleavage. Observed masses of 4264 (WT-ACE) and 4269 (ACE-JM delta 17) are in good agreement with an expected mass of 4262 for the C-terminal CNBr peptide ending at Arg-627, indicating cleavage at the Arg-627/Ser-628 bond in both WT-ACE and ACE-JM delta 17, at distances of 24 and 10 residues from the membrane, respectively. Data for ACE-JM delta 24 are also consistent with cleavage at or near Arg-627. For ACE-JMLDL, in which the native cleavage site is absent, observed masses of 4372 and 4542 are in close agreement with expected masses of 4371 and 4542 for peptides ending at Ala-628 and Gly-630, respectively, indicating cleavages at 17 or 15 residues from the membrane. These data indicate that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, cleavage at a distance of 10 residues from the membrane is more favorable, as ACE-JM delta 17 is cleaved 12-fold faster than WT-ACE. In contrast, ACE-JM delta 24 is released 17-fold slower, suggesting that a minimum distance from the membrane must be preserved. This is supported by results with the ACE-JM delta 47 mutant, which is membrane-bound but not cleaved, likely because the entire stalk has been deleted. Finally, soluble full-length (anchor-plus) WT-ACE is not cleaved when incubated with various CHO cell fractions or intact CHO cells. On the basis of these and other data, we propose that the CHO cell MPSP that solubilizes ACE (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain; and (4) may have a weak preference for cleavage at Arg/Lys-X bonds.