Abstract 1017: Novel inhibitors of AR-v7 nuclear import: new therapeutic opportunities for CRPC

Reactivation of androgen receptor (AR) signaling by active splice variants (AR-Vs) is one of key drivers of castration resistant prostate cancer (CRPC). AR-v7 is the most prevalent AR-V and its expression has been clinically associated with poor overall survival, resistance to the AR inhibitors enzalutamide and abiraterone, as well as taxane resistance. Given that treatment with AR inhibitors and taxanes are the only effective therapeutic modalities in CRPC, development of specific AR-v7 inhibitors is urgently needed. Mechanistically, AR-v7 re-activates AR signaling by being constitutively active in the nucleus. While taxane chemotherapy inhibits the nuclear import of AR which is significantly associated with clinical outcomes in CRPC, it has no effect on AR-v7 nuclear localization and activity. Mechanistically, AR-v7 lacks the microtubule-binding domain and—unlike AR—does not utilize the canonical importin-α/β pathway, or RanGTP for nuclear import. Using wheat germ agglutinin to block active protein nuclear uptake resulted in AR-v7 cytoplasmic sequestration, indicating a requirement for an alternative transport receptor. Further, mutation of AR-v7 dimerization domain (D-box) led to its cytoplasmic sequestration, indicating that the D-box is also required for nuclear import. As inhibition of AR nuclear import is a clinically validated therapeutic strategy, we developed a novel drug discovery platform to identify compounds that specifically inhibit AR-v7 nuclear import. Using cells stably expressing inducible AR-v7 in conjunction with an enzyme complementation assay we tested 166,000 compounds by high throughput screening (HTS). The robust HTS performance (Z>0.8) together with subsequent counter screens including confirmation and compound titration, cell toxicity, a tertiary imaging based screen, led to identification of lead compounds that inhibit AR-v7 nuclear import. The lead compounds share structural features, across two main chemotypes, which are amenable to structure-activity relationship studies to identify the most desirable compound for in vivo studies. Using newly synthesized compounds from each of the two chemotypes, we showed specific dose-dependent inhibition of AR-v7 nuclear import. Currently, we are testing these compounds, on inhibition of AR-v7 transcriptional activity across several cell models including enzalutamide-resistant cells as well as inhibition of tumor growth in AR-v7 xenograft models. In parallel we are testing the lead compound for potential direct binding to the AR-v7 dimerization domain or to candidate alternative nuclear transport receptors. Further development of our lead small molecules will yield novel chemotypes, with desirable pharmacological properties that target the unique AR-v7 nuclear import pathway and can be clinically combined with existing AR therapies.

Note: This abstract was not presented at the meeting.

Citation Format: Seaho Kim, Mohd Azrin Jamalruddin, Eiman Mukhtar, Michael Miller, Leigh Baxt, Stacia Kargman, Andrew Stamford, Peter Meinke, Paraskevi Giannakakou. Novel inhibitors of AR-v7 nuclear import: new therapeutic opportunities for CRPC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1017.

Abstract 3817: Systems biology identifies that Gleevec reverses taxane resistance in solid tumors by selective inhibition of a novel +Tip microtubule-binding variant

The microtubule (MT) cytoskeleton is a validated therapeutic target in oncology, evidenced by the wide use of taxanes in solid tumors including gastric cancer (GC). Post-hoc analysis of the clinical trial that led to docetaxel approval in GC, revealed that patients with diffuse histological subtype were intrinsically resistant to taxane chemotherapy. Using a panel of GC cell lines intrinsically sensitive or resistant to taxanes, we showed lack of drug-target engagement in the resistant lines, despite unimpaired intracellular drug accumulation and absence of tubulin mutations. We discovered a novel, truncated variant of the MT +TIP binding protein CLIP1, hereafter CLIP1S, which was significantly enriched in the resistant cells. Mass-spec proteomics and 5’RACE showed that CLIP1S lacked the first 150 amino acids, thus, missing the Cap-Gly domain required for MT +TIP localization. Confocal microscopy of endogenous or exogenous tagged proteins revealed that CLIP1S was indeed mislocalized from the +TIP to the MT lattice in contrast to +TIP localization of canonical CLIP1. Stable CLIP1S-Knock Down (KD) entirely reversed taxane-resistance (~300 fold), establishing causation between CLIP1S and taxane resistance. Quantitation of drug-binding kinetics using live-cell imaging of Flutax-2 (fluorescently-labeled taxane) in native cytoskeletons, showed that CLIP1S caused Flutax-2 to have significantly reduced affinity and increased dissociation rates from MTs, as compared with cells expressing only the canonical CLIP1. CLIP1S-KD, fully restored Flutax-2 binding, implicating CLIP1S in impeding taxane-MT interaction. Co-administration of chemical probes specific for the low affinity taxane binding site on MT surface further implicated CLIP1S in partially obstructing the MT pore thereby restricting taxane access in the MT lumen where the high affinity taxane binding site is located. Computational analyses of RNA-seq data from untreated or taxane-treated sensitive and resistant GC cells using a novel bayesian drug-target identification algorithm predicted Imatinib (Gleevec™) as a drug that could overcome CLIP1S mediated taxane resistance. Indeed, imatinib completely reversed taxane resistance, phenocopying the sensitization observed with the CLIP1S-KD. Most importantly, we showed that imatinib reversed taxane resistance by specific inhibition of CLIP1S in a dose-dependent manner as early as 3 h post-treatment. Taken together, these data identify an entirely novel mechanism of taxane resistance that involves obstruction of the MT pore in the presence of a previously unknown +TIP variant. Through systems biology we identified imatinib as the first specific CLIP1S inhibitor, thereby repurposing imatinib as a novel therapeutic to overcome clinical taxane resistance in GC and beyond.

Citation Format: Prashant V. Thakkar, Katsuhiro Kita, Giuseppe Galletti, Neel S. Madhukar, Kyle Cleveland, Isabel Barasoain, Jose Fernando Diaz, Olivier Elemento, Manish A. Shah, Paraskevi Giannakakou. Systems biology identifies that Gleevec reverses taxane resistance in solid tumors by selective inhibition of a novel +Tip microtubule-binding variant [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3817.

Abstract 454: Transferrin receptor identifies a novel circulating tumor cell population in patients with pancreatic cancer with a unique metastasis-associated molecular signature

Pancreatic Ductal Adenocarcinomas (PDAC) is the fourth most common cancer, expected to be the second leading cause of cancer mortality by 2030. Most PDAC patients are diagnosed with metastatic disease, with dismal prognosis of less than 1% 5-year survival. A major challenge in PDAC treatment is the absence of actionable molecular targets and the scarcity of available tumor tissue impeding molecular understanding of PDAC progression. Circulating Tumor Cells (CTCs) provide an easily accessible source for tumor tissue. CellSearchTM is the only FDA-cleared platform for CTC isolation based on positive selection of EpCAM+ cells. However, this method performs poorly in PDAC failing to show any prognostic relevance, likely due to EpCAM down-regulation during epithelial-to-mesenchymal transition (EMT), which precedes metastasis. To overcome this limitation, we tested Transferrin Receptor 1 (TfR) in the identification and isolation of CTCs from PDAC patients. TfR is a cell surface protein that mediates iron uptake, is overexpressed in multiple tumors with sustained expression throughout EMT, making it a suitable candidate for CTC isolation. To test this hypothesis, we first assessed TfR expression in a panel of 9 human PDAC cell lines, and we observed that TfR was expressed in 9/9 (100%) in contrast to 6/9 (66.7%) being EpCAM positive. No TfR expression was detected in healthy donor and PDAC patients’ PBMCs. To assess the performance of TfR in CTC identification and isolation from the peripheral blood of PDAC patients, we collected samples from 37 PDAC patients with stage 4 disease, enriched CTCs by negative CD45 depletion (RossetteSepTM) and subsequently stained live CTCs with TfR, EpCAM, and CD45. CTCs were defined as CD45- cells, positive for TfR+ or EpCAM+. We observed that the number of TfR+ CTCs (median: 148, range 2-4182) was significantly higher than EpCAM+ CTCs (median: 68, range 0-1552) within the same patient and across different patients (P-value 0.007). In addition, serial sampling at baseline (chemotherapy-naïve) and at disease progression from 5 PDAC patients revealed that TfR+CTC enumeration correlated with disease progression radiologically and/or CA19-9 (PDAC tumor marker) levels. In contrast, there was no correlation between the number of EpCAM+CTCs and clinical outcomes.

RNA-Sequencing of isolated pools of TfR+ or EpCAM+ CTCs, followed by gene set enrichment analysis, revealed significant enrichment in oncogenic pathways, such as EMT, G2M, MYC and KRAS in TfR+ CTCs as compared with EpCAM+ CTCs. Taken together, our results reveal that TfR identifies a clinically relevant CTC subpopulation which correlates with disease progression and is molecularly distinct. Ongoing studies are focused on the function of TfR in disease progression and the potential use of TfR+ CTC molecular profiles in personalized PDAC treatment.

Citation Format: Ahmed Halima, Jiaren Zhang, Giuseppe Galletti, Allyson J. Ocean, Paraskevi Giannakakou. Transferrin receptor identifies a novel circulating tumor cell population in patients with pancreatic cancer with a unique metastasis-associated molecular signature [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 454.

Pharmacodynamic study of radium-223 in men with bone metastatic castration resistant prostate cancer

BACKGROUND

Radium-223 is a targeted alpha-particle therapy that improves survival in men with metastatic castration resistant prostate cancer (mCRPC), particularly in men with elevated serum levels of bone alkaline phosphatase (B-ALP). We hypothesized that osteomimicry, a form of epithelial plasticity leading to an osteoblastic phenotype, may contribute to intralesional deposition of radium-223 and subsequent irradiation of the tumor microenvironment.

METHODS

We conducted a pharmacodynamic study (NCT02204943) of radium-223 in men with bone mCRPC. Prior to and three and six months after radium-223 treatment initiation, we collected CTCs and metastatic biopsies for phenotypic characterization and CTC genomic analysis. The primary objective was to describe the impact of radium-223 on the prevalence of CTC B-ALP over time. We measured radium-223 decay products in tumor and surrounding normal bone during treatment. We validated genomic findings in a separate independent study of men with bone metastatic mCRPC (n = 45) and publicly accessible data of metastatic CRPC tissues.

RESULTS

We enrolled 20 men with symptomatic bone predominant mCRPC and treated with radium-223. We observed greater radium-223 radioactivity levels in metastatic bone tumor containing biopsies compared with adjacent normal bone. We found evidence of persistent Cellsearch CTCs and B-ALP (+) CTCs in the majority of men over time during radium-223 therapy despite serum B-ALP normalization. We identified genomic gains in osteoblast mimicry genes including gains of ALPL, osteopontin, SPARC, OB-cadherin and loss of RUNX2, and validated genomic alterations or increased expression at the DNA and RNA level in an independent cohort of 45 men with bone-metastatic CRPC and in 150 metastatic biopsies from men with mCRPC.

CONCLUSIONS

Osteomimicry may contribute in part to the uptake of radium-223 within bone metastases and may thereby enhance the therapeutic benefit of this bone targeting radiotherapy.

Androgen receptor nuclear localization correlates with AR-V7 mRNA expression in circulating tumor cells (CTCs) from metastatic castration resistance prostate cancer patients

Androgen receptor (AR) signaling drives prostate cancer (PC) progression and remains active upon transition to castration resistant prostate cancer (CRPC). Active AR signaling is achieved through the nuclear accumulation of AR following ligand binding and through expression of ligand-independent, constitutively active AR splice variants, such as AR-V7, which is the most commonly expressed variant in metastatic CRPC (mCRPC) patients. Most currently approved PC therapies aim to abrogate AR signaling and activity by inhibiting this ligand-mediated nuclear translocation. In a prospective multi-institutional clinical study, we recently showed that taxane based chemotherapy is also capable of impairing AR nuclear localization (ARNL) in circulating tumor cells (CTCs) from CRPC patients, whereas taxane induced decreases in ARNL were associated with response. Thus, quantitative assessment of ARNL in CTCs can be used to monitor therapeutic response in patients and help guide clinical decisions. Here, we describe the development and implementation of quantitative high throughput (QHT) image analysis algorithms to aid in CTC identification and quantitative assessment of percent ARNL (%ARNL). We applied this algorithm to fifteen CRPC patients at the start of taxane chemotherapy, quantified %ARNL in CTCs, and correlated with expression of AR-V7 mRNA (from CTCs enriched via negative, CD45⁺ depletion of peripheral blood) and with biochemical (prostate specific antigen; PSA) response to taxane chemotherapy. We found that CTCs from AR-V7 positive patients had higher baseline %ARNL compared to CTCs from AR-V7 negative patients, consistent with the constitutive nuclear localization of AR-V7. In addition, lower %ARNL in CTCs at baseline was associated with biochemical response to taxane chemotherapy. High inter- and intra-patient heterogeneity was also observed. As ARNL is required for active AR signaling, the QHT algorithms described herein can provide prognostic and/or predictive value in future clinical studies.

Expression of AR-V7 and ARv567es in Circulating Tumor Cells Correlates with Outcomes to Taxane Therapy in Men with Metastatic Prostate Cancer Treated in TAXYNERGY

PURPOSE

Biomarkers aiding treatment optimization in metastatic castration-resistant prostate cancer (mCRPC) are scarce. The presence or absence of androgen receptor (AR) splice variants, AR-V7 and ARv567es, in mCRPC patient circulating tumor cells (CTC) may be associated with taxane treatment outcomes.Experimental Design: A novel digital droplet PCR (ddPCR) assay assessed AR-splice variant expression in CTCs from patients receiving docetaxel or cabazitaxel in TAXYNERGY (NCT01718353). Patient outcomes were examined according to AR-splice variant expression, including prostate-specific antigen (PSA)50 response and progression-free survival (PFS).

RESULTS

Of the 54 evaluable patients, 36 (67%) were AR-V7+, 42 (78%) were ARv567es+, 29 (54%) were double positive, and 5 (9%) were double negative. PSA50 response rates at any time were numerically higher for AR-V7- versus AR-V7+ (78% vs. 58%; P = 0.23) and for ARv567es- versus ARv567es+ (92% vs. 57%; P = 0.04) patients. When AR-V mRNA status was correlated with change in nuclear AR from cycle 1 day 1 to day 8 (n = 24), AR-V7+ patients (n = 16) had a 0.4% decrease versus a 12.9% and 26.7% decrease in AR-V7-/ARv567es- (n = 3) and AR-V7-/ARv567es+ (n = 5) patients, respectively, suggesting a dominant role for AR-V7 over ARv567es. Median PFS was 12.02 versus 8.48 months for AR-V7- versus AR-V7+ (HR = 0.38; P = 0.01), and 12.71 versus 7.29 months for ARv567es- versus ARv567es+ (HR = 0.37; P = 0.02). For AR-V7+, AR-V7-/ARv567es+, and AR-V7-/ARv567es- patients, median PFS was 8.48, 11.17, and 16.62 months, respectively (P = 0.0013 for trend).

CONCLUSIONS

Although detection of both CTC-specific AR-V7 and ARv567es by ddPCR influenced taxane outcomes, AR-V7 primarily mediated the prognostic impact. The absence of both variants was associated with the best response and PFS with taxane treatment.See related commentary by Dehm et al., p. 1696.

Prospective Multicenter Validation of Androgen Receptor Splice Variant 7 and Hormone Therapy Resistance in High-Risk Castration-Resistant Prostate Cancer: The PROPHECY Study

PURPOSE

Androgen receptor splice variant 7 (AR-V7) results in a truncated receptor, which leads to ligand-independent constitutive activation that is not inhibited by anti-androgen therapies, including abiraterone or enzalutamide. Given that previous reports suggested that circulating tumor cell (CTC) AR-V7 detection is a poor prognostic indicator for the clinical efficacy of secondary hormone therapies, we conducted a prospective multicenter validation study.

PATIENTS AND METHODS

PROPHECY (ClinicalTrials.gov identifier: NCT02269982) is a multicenter, prospective-blinded study of men with high-risk mCRPC starting abiraterone acetate or enzalutamide treatment. The primary objective was to validate the prognostic significance of baseline CTC AR-V7 on the basis of radiographic or clinical progression free-survival (PFS) by using the Johns Hopkins University modified-AdnaTest CTC AR-V7 mRNA assay and the Epic Sciences CTC nuclear-specific AR-V7 protein assay. Overall survival (OS) and prostate-specific antigen responses were secondary end points.

RESULTS

We enrolled 118 men with mCRPC who were starting abiraterone or enzalutamide treatment. AR-V7 detection by both the Johns Hopkins and Epic AR-V7 assays was independently associated with shorter PFS (hazard ratio, 1.9 [95% CI, 1.1 to 3.3; P = .032] and 2.4 [95% CI, 1.1 to 5.1; P = .020], respectively) and OS (hazard ratio, 4.2 [95% CI, 2.1 to 8.5] and 3.5 [95% CI, 1.6 to 8.1], respectively) after adjusting for CTC number and clinical prognostic factors. Men with AR-V7–positive mCRPC had fewer confirmed prostate-specific antigen responses (0% to 11%) or soft tissue responses (0% to 6%). The observed percentage agreement between the two AR-V7 assays was 82%.

CONCLUSION

Detection of AR-V7 in CTCs by two blood-based assays is independently associated with shorter PFS and OS with abiraterone or enzalutamide, and such men with mCRPC should be offered alternative treatments.

Widespread Mitotic Bookmarking by Histone Marks and Transcription Factors in Pluripotent Stem Cells

During mitosis, transcription is halted and many chromatin features are lost, posing a challenge for the continuity of cell identity, particularly in fast cycling stem cells, which constantly balance self-renewal with differentiation. Here we show that, in pluripotent stem cells, certain histone marks and stem cell regulators remain associated with specific genomic regions of mitotic chromatin, a phenomenon known as mitotic bookmarking. Enhancers of stem cell-related genes are bookmarked by both H3K27ac and the master regulators OCT4, SOX2, and KLF4, while promoters of housekeeping genes retain high levels of mitotic H3K27ac in a cell-type invariant manner. Temporal degradation of OCT4 during mitotic exit compromises its ability both to maintain and induce pluripotency, suggesting that its regulatory function partly depends on its bookmarking activity. Together, our data document a widespread yet specific bookmarking by histone modifications and transcription factors promoting faithful and efficient propagation of stemness after cell division.

Identification of distinct nanoparticles and subsets of extracellular vesicles by asymmetric flow field-flow fractionation

The heterogeneity of exosomal populations has hindered our understanding of their biogenesis, molecular composition, biodistribution and functions. By employing asymmetric flow field-flow fractionation (AF4), we identified two exosome subpopulations (large exosome vesicles, Exo-L, 90-120 nm; small exosome vesicles, Exo-S, 60-80 nm) and discovered an abundant population of non-membranous nanoparticles termed 'exomeres' (~35 nm). Exomere proteomic profiling revealed an enrichment in metabolic enzymes and hypoxia, microtubule and coagulation proteins as well as specific pathways, such as glycolysis and mTOR signalling. Exo-S and Exo-L contained proteins involved in endosomal function and secretion pathways, and mitotic spindle and IL-2/STAT5 signalling pathways, respectively. Exo-S, Exo-L and exomeres each had unique N-glycosylation, protein, lipid, DNA and RNA profiles and biophysical properties. These three nanoparticle subsets demonstrated diverse organ biodistribution patterns, suggesting distinct biological functions. This study demonstrates that AF4 can serve as an improved analytical tool for isolating extracellular vesicles and addressing the complexities of heterogeneous nanoparticle subpopulations.

Using circulating tumor cells to advance precision medicine in prostate cancer

The field of CTC enrichment has seen many emerging technologies in recent years, which have resulted in the identification and monitoring of clinically relevant, CTC-based biomarkers that can be analyzed routinely without invasive procedures. Several molecular platforms have been used to investigate the molecular profile of the disease, from high throughput gene expression analyses down to single cell biological dissection. The established presence of CTC heterogeneity nevertheless constitutes a challenge for cell isolation as the several subpopulations can potentially display different molecular characteristics; in this scenario, careful consideration must be given to the isolation approach, whereas methods that discriminate against certain subpopulations may result in the exclusion of CTCs that carry biological relevance. In the context of prostate cancer (PC), CTC molecular interrogation can enable longitudinal monitoring of key biological features during treatment with substantial clinical impact, as several biomarkers could predict tumor response to AR signaling inhibitors (abiraterone, enzalutamide) or standard chemotherapy (taxanes). Thus, CTCs represent a valuable opportunity to personalize medicine in current clinical practice.

Abstract 4646: A novel truncated variant of the hematopoietic Hβ-1 tubulin isotype with implications for stem cell biology

The αβ-tubulin dimer is the building block of the microtubule cytoskeleton, which is important for many cellular functions. Several α and β tubulin isotypes have been identified so far; Hβ-1 tubulin (human β tubulin Class VI) is a hematopoietic-specific tubulin isotype whose expression is restricted to megakaryocytes and platelets. TCGA analysis showed that β-1 is highly and specifically expressed in acute myeloid leukemia (AML). β-1 may therefore be an attractive target for the development of novel therapeutics for AML. We developed a highly-specific polyclonal antibody against the last 8 C-terminal amino acids and analyzed a panel of AML cell lines and primary samples by western blot. We identified the presence of a novel truncated 35kD band, distinct from the canonical 50kD tubulin. This 35kD band, which we have named as Acute Leukemia Isoform β1 (ALIBI), was present in 10 of the 19 AML cell lines tested as well as in a large number of AML clinical samples, as evidenced by immunoblotting, and corroborated by RNA-sequencing. We determined that ALIBI was a splice variant of Hb1 tubulin, lacking exons 1 and 2 with partial retention of exon 3 and intron 3. Mass array methylation analysis revealed differential promoter and intragenic methylation pattern in canonical b1 tubulin versus ALIBI, suggesting that there is an epigenetic switch that determines the expression of each tubulin isoform. Immunofluorescence analysis revealed that ALIBI, unlike all tubulin isotypes, does not incorporate into microtubules but instead accumulates in the nucleus and in distinct puncta in the cytoplasm. Exogenous expression of ALIBI, also resulted in a similar cellular distribution. Using a murine leukemia mouse model we found significant enrichment of ALIBI in the leukemic stem cell fraction, while in normal mouse development ALIBI was present in fetal liver and hematopoietic progenitor cells. Based on this observation, as well as the relationship between leukemic stem cells and embryonic stem cells, we probed for the presence of ALIBI within murine embryonic stem cells (mESCs) and found ALIBI to be present in cultured mESCs, and down-regulated upon differentiation when full length b1 tubulin was expressed. Taken together, these data suggest that ALIBI has an important, heretofore unrecognized, role in stem cell biology. However, much remains unknown regarding the function of ALIBI within the nucleus, the factors that regulate its expression over the canonical Hβ1 isoform, and most importantly its precise role in progenitor or stem cells. Understanding the function and regulation of ALIBI will allow us to understand a novel role of tubulin in stem cell biology and open multiple avenues for application of this knowledge.

Citation Format: Yang Bai, Paul Basciano, Xi Li, Jason Matakas, Silvana Di Giandomenico, Siddhartha Sen, Todd Evans, Joseph Scandura, Monica Guzman, Paraskevi Giannakakou. A novel truncated variant of the hematopoietic Hβ-1 tubulin isotype with implications for stem cell biology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4646. doi:10.1158/1538-7445.AM2017-4646

Abstract 1590: High intranuclear mobility of AR-v7 reveals distinct mode of transcriptional activity in prostate cancer with important therapeutic implications

It is well established that androgen receptor (AR) signaling, is a key driver of prostate cancer (PC) growth and metastatic progression. Therefore, androgen deprivation therapy (ADT) is the first line of treatment for PC. However, most patients develop castration resistant prostate cancer (CRPC). Interestingly, AR signaling remains active in CRPC, due to the expression of transcriptionally active AR splice variants (AR-Vs), which lack the ligand binding domain (LBD) and constitutively translocate to the nucleus even in castrate conditions. AR-v7 is the most prevalent AR-V expressed in about 60% of CRPC tumors. AR-v7 expression was clinically correlated with poor prognosis of CRPC patients and with resistance to next-generation AR signaling inhibitors, which are part of standard clinical care. Therefore, inhibition of AR-v7 function is urgently needed for the treatment of CRPC. Currently, there is no therapeutic modality that can inhibit AR-v7 expression or activity. Mechanistically, AR-v7 transcriptional targets largely overlap with those of AR-fl, with the exception of a few AR-v7 unique targets. However, the exact mechanism by which transcription is activated by AR-v7 is not known. In this study we sought to investigate the mechanisms underlying the transcriptional activity of the ligand-independent AR-v7 in comparison to liganded AR-fl. We used live cell imaging to monitor the dynamics and intranuclear mobility of fluorescently-tagged AR-v7 or AR-fl. Fluorescent recovery after photobleaching (FRAP) revealed that AR-v7 intranuclear mobility was significantly faster than that of liganded AR-fl, with t1/2 3s versus several minutes, respectively. To precisely map the spatial distribution and chromatin-binding kinetics of AR-fl and AR-v7, we generated expression plasmids with AR tagged with green-to-red mEos4 photo-convertible proteins. Photoconversion of a discrete, subnuclear pool of AR-fl in the presence of its ligand (R1881) followed by 1 hr of time-lapse imaging in 5 min intervals, revealed absence of mobility indicating tight chromatin binding and active transcription. In contrast, similar photoconversion experiment for AR-v7, revealed immediate redistribution throughout the nucleus in less than 9 s, suggesting a “hit-and-run” mode of interaction with DNA with uncertain transcriptional output. QRT-PCR of endogenous target genes and ARE-mcherry reporter assays showed similar transcriptional activity of the two proteins. We are currently investigating the relationship between rates of intranuclear mobility and transcriptional activity and the mechanisms underlying the distinct mobility patterns. These data suggest that AR-v7 has a distinct mode of interaction with DNA and gene promoters, which may identify novel targetable pathways for its inhibition in CRPC.

Citation Format: Seaho Kim, Mohd Azrin Jamalruddin, Paraskevi Giannakakou. High intranuclear mobility of AR-v7 reveals distinct mode of transcriptional activity in prostate cancer with important therapeutic implications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1590. doi:10.1158/1538-7445.AM2017-1590

Abstract 1713: Transferrin receptor 1 (TfR) as marker for circulating tumor cells (CTCs) identification in NSCLC

CTCs are used as a surrogate source of tumor material in solid tumors. Clinical applications of CTCs as liquid biopsy comprise the monitoring of molecular alterations during tumor progression and dynamic evaluation of molecular markers of treatment response. The FDA-cleared method to isolate CTCs in cancer patients (Cell Search) is based on positive selection of EpCAM expressing cells. However, this approach performs poorly in non-small cell lung cancer (NSCLC) as it identifies CTCs in only 7% of the subjects, failing to show any prognostic relevance.

Down-regulation/loss of epithelial markers to isolate (EpCAM) and identify (cytokeratin, CK) CTCs could in part explain the low CTC yield obtained in NSCLC with approaches based on epithelial markers expression. To overcome this challenge we used size-based CTC enrichment (ISET filters) from NSCLC patients’ peripheral blood. As a positive identifier of CTCs we used transferrin receptor 1 (TfR) which is a cell membrane-associated protein, that mediates intracellular iron uptake, and which is expressed at low levels in many normal tissues but over-expressed in cancer cells.

We first analyzed TfR protein expression by immunofluorescence in a panel of NSCLC cell lines and in healthy donor leukocytes. While all NSCLC cells lines analyzed were positive for TfR expression, none of the leukocyte expressed the receptor. Moreover, TfR expression was detected also in EpCAM negative NSCLC cell lines. To determine the clinical applicability of this novel CTC identifier, we determined TfR expression in CTCs isolated from peripheral blood of 35 metastatic NSCLC patients using the ISET filter technology. The isolated CTCs were stained for TfR, CK, CD45 and DAPI. For each patient, one additional ISET filter was stained with Giemsa for morphologic analysis by a pathologist. By using the classic panel of CTC identifiers markers (CK+/CD45-/DAPI+), CTCs were identified in 4/34 (11%) patients, while by using TfR as positive identifier (TfR+/CD45-/DAPI+) CTCs were identified in 31/35 (88%) subjects. The morphologic review of Giemsa stained filters confirmed the presence of tumor cells in 28/34 (82%) samples [0-217 CTCs/sample]. Interestingly, patients with > 6 TfR+ CTCs had a worse overall survival (OS) than patients with < 6 TfR+ CTCs [p=0.048 Log Rank (Mantel-Cox)]. OS did not significantly differ using the same cutoff with CTCs defined based on CK or Giemsa staining.

Overall, our data indicate that TfR is a promising biomarker for the detection of CTCs in NSCLC CTCs, superior to CK or EpCAM. Our data also suggest that TfR may potentially identify CTCs subpopulations with a significant prognostic role in NSCLC. We are currently isolating TfR+ CTCs from early stage and metastatic NSCLC patients for further molecular characterization and determination of clinical significance.

Citation Format: Giuseppe Galletti, Galatea Kallergi, Ashish Saxena, Despoina Aggouraki, Christos Stournaras, Vassilis Georgoulias, Timothy E. McGraw, Nasser Altorki, Paraskevi Giannakakou. Transferrin receptor 1 (TfR) as marker for circulating tumor cells (CTCs) identification in NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1713. doi:10.1158/1538-7445.AM2017-1713

Abstract 4169: A novel, short isoform of the +Tip microtubule binding protein CLIP170 confers taxane resistance in gastric cancer

The microtubule (MT) cytoskeleton is one of the most validated therapeutic targets in clinical oncology, as evidenced by the wide clinical activity of taxanes and other MT inhibitors. However, patients with intrinsic drug resistance do not benefit from taxane chemotherapy, such as patients with diffuse gastric cancer (GC). Currently, the molecular mechanisms underlying clinical taxane resistance remain poorly elucidated. Using a panel of GC cell lines with intrinsic sensitivity or resistance to taxanes, we showed lack of drug-target engagement in the resistant lines, despite unimpaired intracellular accumulation of the drug and in the absence of tubulin mutations or altered tubulin isotype expression. Herein, we show for the first time that the resistant cells expressed a novel, short isoform of the MT plus-end binding protein, CLIP170 (CLIP1). CLIP1 binds to the plus-ends of microtubules (+TIPs) and regulates dynein-mediated MT-based trafficking and the tubulin tyrosination cycle. We showed by both mass-spec proteomics and 5’-RACE that the short CLIP1 isoform (CLIP1S) lacks the first 150 amino acids, thus, missing the first Cap-Gly (Cytoskeleton-Associated Protein-Glycine) domain, which is required for proper +TIP localization. Indeed, confocal microscopy experiments showed that CLIP1S was mislocalized to the microtubule lattice in contrast to the canonical comet-like pattern of CLIP1 seen in taxane sensitive cell lines. CLIP1S expression was specifically correlated with taxane resistance (docetaxel, cabazitaxel) as no correlation was observed with other DNA-damaging agents. Since, CLIP1S expression has never been reported before, to establish causation, we stably knocked down CLIP1 and CLIP1S in taxane-sensitive and resistant cells, respectively. CLIP1SKD entirely reversed taxane-resistance (~300 fold) while CLIP1KD had no effect in the taxane-sensitive cell lines. These data suggest a gain-of-function of CLIP1S that leads to taxane resistance. To study the binding kinetics of taxanes to MTs in the presence or absence of CLIP1S we performed live cell imaging of native cytoskeletons using fluorescently-labeled paclitaxel (Flutax). Our data revealed significantly faster dissociation rates of Flutax from MTs in the resistant cells, indicating transient interaction with MTs. Taxane binding to MTs is a two-step process. First taxanes bind to the MT-pore low affinity surface site, which then facilitates access to the high affinity luminal site. Using a small molecule that binds only to MT-pore site, we showed that we showed that taxol binding to the pore is significantly slower in resistant cells expressing CLIP1S, which together with mislocalization of this variant along the MT lattice suggests that it obstructs access to the MT-pore thus restricting entry of taxane into the lumen of microtubules. This finding will have profound implications for taxane resistance as well as microtubule biology broadly.

Citation Format: Prashant V. Thakkar, Katsuhiro Kita, Giuseppe Galletti, Kyle Cleveland, Isabel Barasoain, Jose Fernando Diaz, Manish A. Shah, Paraskevi Giannakakou. A novel, short isoform of the +Tip microtubule binding protein CLIP170 confers taxane resistance in gastric cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4169. doi:10.1158/1538-7445.AM2017-4169

Abstract 5634: Characterization of molecular mechanisms of cytoplasmic trafficking and nuclear translocation of AR splice variants ARv7

Androgen receptor (AR) signaling is critical to not only hormone-sensitive but also advanced castration-resistant prostate cancer. AR inhibitors (abiraterone and enzalutamide), the next generation of androgen deprivation therapy (ADT) have been used for metastatic castration-resistant PC (mCRPC) treatment, however, the majority of patients progress due to the development of drug resistance. AR variants has emerged as one of the mechanisms of resistance to these drugs. ARv7 and Arv567 splice variants that found lacking the ligand -binding domain are constitutively active in the nucleus and thus restore AR function despite AR inhibitors treatment. We have reported that microtubules and dynein motor protein is required as transportation system for AR for its nuclear translocation and activity and that taxanes inhibit AR signaling downstream of microtubule inhibitors. In addition, we identified that the AR hinge region mediates binding to microtubules is present in ARv567 but missing from ARv7. ARv7 is expressed in about 60% of CRPC patients and has been shown to confer clinical resistance to next generation AR signaling inhibitors. Currently, there is no therapeutic modality targeting specifically ARv7 expression or function. Using fluorescent recovery after photobleaching experiments (FRAP) we have shown that ARv7 translocation to the nucleus occurs much faster than AR-FL (t1/2 11s and 23 s respectively) and that its nuclear import is independent of the importin a/b pathway utilized by AR-FL and proteins with canonical NLS. Furthermore, transient expression of the Q69L RanGTP mutant protein, which acts as dominant negative by occluding the nucleoporins, had minimal effect on ARv7 nuclear import, while it completely blocked AR-FL and AR-v567. Collectively, these data suggest that the nuclear import pathway for ARv7 is distinct from that utilized by AR-FL. We believe that ARv7 undoubtedly uses facilitated transport because of its size (75 kDa), therefore we are tackling three possibilities of nuclear translocation of ARv7. First, ARv7 could interact directly with FG repeat nucleoporins (without receptor) as reported for beta catenin. We are investigating this by the use of importin β-(71-876) mutant cell line for in vitro studies which blocks all members of the importin family. In addition, we are investigating the role of karyopherin transport receptors by individual knockdown experiments coupled with live cell imaging and kinetic quantitation of ARv7 nucleocytoplasmic trafficking. Further, ARv7could translocate to nucleus by utilizes a piggyback on another protein with a classical nuclear localization signals. Overexpression of glutathione S-transferase- importin β binding domain fragment which will block all importin β pathways will exclude this mechanism. Elucidation of this mechanism will be vital to facilitate design an alternative therapeutic modality to halt AR signaling for CRPC treatment.

Citation Format: Eiman Mukhtar, Seaho Kim, Paraskevi Giannakakou. Characterization of molecular mechanisms of cytoplasmic trafficking and nuclear translocation of AR splice variants ARv7 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5634. doi:10.1158/1538-7445.AM2017-5634

Abstract 2736: A digital droplet PCR assay for the quantitation of androgen receptor and splice variant expression in CTCs from metastatic castration resistant prostate cancer patients

Prostate cancer (PC) is the second leading cause of cancer death in men in the US. The aberrant functioning of androgen receptor signaling is the central driving force behind prostatic tumorigenesis and its transition into metastatic castration resistant disease. Hence, androgen deprivation therapy (ADT) is the first line of treatment for PC patients. However, many patients progress becoming resistant to ADT therapy, due to the expression of AR splice variants (AR-Vs), which lack the ligand binding domain and are constitutively active in the nucleus. Expression of the AR splice variant, AR-v7, in circulating tumor cells (CTCs) isolated from the blood of PC patients was correlated with resistance to enzalutamide and abiraterone, which are the next generation AR signaling inhibitors in CRPC. Further, there is evidence that AR-Vs may convey cross-resistance, not only to enzalutamide and abiraterone, but also to taxanes, highlighting that their assessment in the clinic may have clinical utility.

We developed a novel, specific and highly sensitive assay to measure mRNA expression of the AR full length (AR-FL) and the splice variants ARv7 and ARv567es, by using Droplet Digital PCR in CTCs isolated from CRPC patients. The analytical specificity of the assay was determined by transfecting cells with plasmids encoding AR-FL, AR-v7 and AR-v567 and showed that each probe detected signal only in cells expressing the respective transcript. No signal was detected against genomic DNA, indicating lack of non-specific binding. Also, the assay detected endogenous expression of AR-FL and AR-v7 in VCAP or 22RV1 cells, while no variant expression was detected in healthy donor blood. The analytical sensitivity of the assay was determined in a series of serial dilution experiments that showed sensitivity down to single cell.

We then used this assay to determine the clinical prevalence and expression pattern of each of these variants in CTCs from about 200 mCRPC patient samples and blood from 40 healthy donors. CTCs were enriched by EpCAM- or PSMA-based positive selection or CD45 negative depletion in an antigen-agnostic manner. AR-FL was detected in ~80% of mCRPC samples irrespective of CTC-enrichment technology. AR-v7 was expressed in 65% of the samples in which in CTCs were enriched either by PSMA-positive selection or by negative depletion. In contrast, EpCAM-based CTC enrichment showed lower AR-v7 expression both in terms of expression levels and prevalence. In addition, CTC enrichment following negative depletion showed that 30% of the samples had higher AR-v7 expression levels as compared to AR-FL. This expression pattern was not observed in the samples using EpCAM-based selection. Collectively, these data suggest distinct CTC subpopulations are present in CRPC patient samples, with differential expression of AR-Vs that could have important predictive and prognostic implications.

Citation Format: Ada Gjyrezi, Giuseppe Galletti, Areti Strati, Seaho Kim, Evi Lianidou, David M. Nanus, Jun Luo, Emmanuel Antonarakis, Scott T. Tagawa, Andrew Armstrong, Paraskevi Giannakakou. A digital droplet PCR assay for the quantitation of androgen receptor and splice variant expression in CTCs from metastatic castration resistant prostate cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2736. doi:10.1158/1538-7445.AM2017-2736

Randomized, Noncomparative, Phase II Trial of Early Switch From Docetaxel to Cabazitaxel or Vice Versa, With Integrated Biomarker Analysis, in Men With Chemotherapy-Naïve, Metastatic, Castration-Resistant Prostate Cancer

Purpose The TAXYNERGY trial ( ClinicalTrials.gov identifier: NCT01718353) evaluated clinical benefit from early taxane switch and circulating tumor cell (CTC) biomarkers to interrogate mechanisms of sensitivity or resistance to taxanes in men with chemotherapy-naïve, metastatic, castration-resistant prostate cancer. Patients and Methods Patients were randomly assigned 2:1 to docetaxel or cabazitaxel. Men who did not achieve ≥ 30% prostate-specific antigen (PSA) decline by cycle 4 (C4) switched taxane. The primary clinical endpoint was confirmed ≥ 50% PSA decline versus historical control (TAX327). The primary biomarker endpoint was analysis of post-treatment CTCs to confirm the hypothesis that clinical response was associated with taxane drug-target engagement, evidenced by decreased percent androgen receptor nuclear localization (%ARNL) and increased microtubule bundling. Results Sixty-three patients were randomly assigned to docetaxel (n = 41) or cabazitaxel (n = 22); 44.4% received prior potent androgen receptor-targeted therapy. Overall, 35 patients (55.6%) had confirmed ≥ 50% PSA responses, exceeding the historical control rate of 45.4% (TAX327). Of 61 treated patients, 33 (54.1%) had ≥ 30% PSA declines by C4 and did not switch taxane, 15 patients (24.6%) who did not achieve ≥ 30% PSA declines by C4 switched taxane, and 13 patients (21.3%) discontinued therapy before or at C4. Of patients switching taxane, 46.7% subsequently achieved ≥ 50% PSA decrease. In 26 CTC-evaluable patients, taxane-induced decrease in %ARNL (cycle 1 day 1 v cycle 1 day 8) was associated with a higher rate of ≥ 50% PSA decrease at C4 ( P = .009). Median composite progression-free survival was 9.1 months (95% CI, 4.9 to 11.7 months); median overall survival was not reached at 14 months. Common grade 3 or 4 adverse events included fatigue (13.1%) and febrile neutropenia (11.5%). Conclusion The early taxane switch strategy was associated with improved PSA response rates versus TAX327. Taxane-induced shifts in %ARNL may serve as an early biomarker of clinical benefit in patients treated with taxanes.

AR nuclear localization and microtubule bundling as markers of docetaxel and cabazitaxel sensitivity in metastatic castration-resistant prostate cancer (mCRPC): Prospective biomarker analysis from TAXYNERGY

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Background: A better understanding of taxane sensitivity/resistance in mCRPC is needed to optimize treatment. Preclinically, taxane efficacy has been linked to the ability of microtubules (MT) to inhibit AR nuclear trafficking. In this prospective biomarker study, we used circulating tumor cells (CTCs) from patients (pts) in TAXYNERGY to perform real-time analysis of AR nuclear localization (ARNL) and MT stabilization (bundling; MTB) in order to predict taxane sensitivity. Methods: TAXYNERGY (NCT01718353) is a phase 2 trial randomizing chemo-naïve mCRPC pts 2:1 to docetaxel or cabazitaxel, with a switch to the alternative taxane in the absence of a ≥ 30% PSA drop by C4. Here we present the co-primary biomarker endpoints. CTCs at baseline (C1D1) were compared to CTCs after 1 week of taxane treatment (C1D8), and were analyzed by multiplex confocal microscopy for %ARNL (integrated AR intensity in the cell and nuclear areas) and MTB (assessed for increase compared to C1D1 on a scale from 0–3 from no to most MTB increase). Associations between %ARNL and MTB with clinical outcomes were sought. Results: Of 63 randomized pts, 26 had evaluable CTCs both at C1D1 and C1D8. At C1D8, mean %ARNL was significantly lower in pts achieving a ≥ 50% PSA drop by C4 vs those without (44% vs 64%; p = 0.004). A taxane-induced decrease in mean %ARNL (C1D8 vs C1D1) was associated with a higher rate of ≥ 50% PSA response (73% vs 13%; p = 0.009); mean %ARNL decreased by 18% in responders and increased by 2% in non-responders (p = 0.02). Finally, a taxane-induced increase in mean MTB trended higher in pts achieving a ≥ 30% PSA drop by C4 vs those without (0.69 vs 0.09; p = 0.09); increase in mean MTB score was indicative of response and observed in pts who did not require a taxane switch after C4 (0.75 vs 0.09; p = 0.06). Conclusions: We provide the first prospective data suggesting that taxane-induced shifts in ARNL and MTB (measured in CTCs) may serve as an early biomarker of taxane sensitivity. Consistent with preclinical data, AR nuclear exclusion caused by microtubule bundling may be a clinically-actionable marker of taxane efficacy. Funding: Sanofi Genzyme. Clinical trial information: NCT01718353.

Abstract 2932: Impaired taxane binding to MT pore sites mediates intrinsic drug resistance in diffuse gastric cancer

Successful clinical application of taxanes (microtubule-stabilizers) is limited due to intrinsic or acquired drug resistance. Thus, it is critical to unveil the molecular mechanisms of taxane resistance to significantly improve clinical outcomes. Retrospective analysis of the TAX-325 gastric cancer (GC) trial revealed that the addition of docetaxel (DTX) to cisplatin/fluorouracil increased progression-free plus overall survival, in intestinal (INT) but not in diffuse (DIF) GC subtypes. Our preclinical data confirmed that DIF GC cell lines are intrinsically resistant to taxanes. The incidence of DTX resistance in DIF GC cell lines was 2.5 times higher than INT GC cell lines. Drug efflux, tubulin posttranslational modification and differential β-tubulin isotype expression were ruled out as potential mechanisms of intrinsic taxane resistance. Thus, a new molecular mechanism must underlie the intrinsic taxane resistance in DIF GC. To quantify the kinetics of taxol binding to cellular MTs, we treated both DIF and INT groups of GC lines with the fluorescein-conjugated paclitaxel analog, Flutax-2. The Flutax-2 staining intensity of cellular MTs was assessed by live-cell confocal microscopy at different time points. Following a 3h incubation, there was less than 20% decrease in Flutax-2 intensity in the sensitive cell lines, compared to 59∼89% decrease in the resistant cell lines. These data suggested different binding kinetics between sensitive and resistant cells. We added 1μM Flutax-2 to sensitive/resistant cells’ native cytoskeletons for 0∼60 seconds to determine the association rate (kon) of Flutax-2 binding to MTs. The kon of Flutax-2 in the sensitive cell lines was significantly higher in sensitive (5.8×104M−1s−1) versus resistant cells (0.3∼2.6×104M−1s−1). Next, we measured the dissociation rates (koff) by competing the pre-bound Flutax-2 with DTX (0∼600 seconds). Although 20∼40% faster koff was observed in resistant cells, it appeared that the association rate of Flutax-2 was the dominant factor of differential taxane binding to MTs in GC cells. The binding mode of taxanes to MTs involves 1) binding to MT pores and 2) internalization to the high-affinity binding site at the MT lumen. To differentiate between the two, we used hexaflutax, which binds exclusively to the MT pores. We observed hexaflutax decorating radial MT arrays in sensitive but not in the resistant cells. As tubulin mutations around the high-affinity taxane binding site or tubulin posttranslational modifications/differential expression of β-tubulin isotype is not the case in DIF GC cell lines, our data suggest that modifications of the MT pore conformation or occlusion of the pore site is responsible for the intrinsic taxane resistance in DIF GC. Determining the origin of the defect at the pores will help design better MT-stabilizing drugs to overcome chemo-resistance, the major obstacle hindering overall survival of patients.

Citation Format: Katsuhiro Kita, Giuseppe Galletti, Kyle Cleveland, Chao Zhang, Isabel Barasoain, J. Fernando Díaz, Doron Betel, Manish A. Shah, Paraskevi Giannakakou. Impaired taxane binding to MT pore sites mediates intrinsic drug resistance in diffuse gastric cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2932.

Abstract 2125: ERG and AR-v7 involvement in taxane resistance of metastatic castration-resistant prostate cancers

While taxane-class drugs remains the only chemotherapy agents shown to improve survival of patients with metastatic castration-resistant prostate cancer (mCRPC), most patients with mCRPC ultimately become refractory due to the development of drug resistance. The molecular mechanisms involved in this resistance remain an unmet need. We previously showed that ERG, an oncogenic transcription factor, directly interacts with soluble tubulin dimers impairing the ability of taxanes to bind to microtubules in vitro and in circulating tumor cells (CTCs) from CRPC patients. We also demonstrated that expression of AR-v7 confers taxane resistance in vitro and in animal models of CRPC, owing to the lack of the microtubule-binding hinge domain which renders AR-v7 insensitive to microtubule stabilization by the taxanes. The purpose of the present study is to evaluate the impact of ERG and AR-v7 co-expression in prostate cells and to characterize their cooperative role in mediating taxane resistance. We generated multiple isogenic cell lines over-expressing ERG alone or in combination with AR-v7, and engineered VCaP cells (harboring endogenous ERG rearrangement and expressing AR-v7) to express a TetOn-shRNA targeting ERG mRNA. We also developed and optimized a Digital Droplet PCR (DD-PCR) assay to quantify the expression of ERG derived from the TMPRSS2-ERG gene fusion and of AR-v7. We tested the sensitivity and specificity of the assays using VCaP cells spiked into healthy donor blood samples. We detected ERG and AR-v7 co-expression in single VCaP cells. We also found that ERG and AR-v7 form a protein complex and we are testing their potential co-binding at common target genes using ChIP-seq. To determine the clinical relevance of our findings we queried available RNAseq data from benign prostate, locally advanced hormone naïve prostate cancer and a subset of the Stand-up-to-Cancer cohort of CRPC patients. We found high levels of AR-v7 expression in CRPC samples and in none of the benign or PCa samples and a co-overexpression of AR-v7 and ERG with 75% of ERG positive CRPC also expressing AR-v7. We are currently using our assays to characterize ERG and AR-v7 co-expression in CTCs of mCRPC patients samples collected at baseline, on treatment and relapse- as part of a fully-enrolled clinical trial (TAXYNERGY). We expect an enrichment overtime of CTCs that are positive for both ERG and AR-v7. In conclusion, determining ERG and AR-v7 status in mCRPC patients and their involvement in taxane resistance mechanism will aid refer ERG and AR-v7 positive mCRPC patients to an effective therapy.

Citation Format: Adeline Berger, Seaho Kim, Ada Gjyrezi, Giuseppe Galletti, Andrea Sboner, Mark A. Rubin, Scott Tagawa, Paraskevi Giannakakou, David S. rickman. ERG and AR-v7 involvement in taxane resistance of metastatic castration-resistant prostate cancers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2125.

Abstract 4938: Molecular mechanisms of intrinsic resistance to taxanes in gastric cancer

Gastric cancer (GC) is histologically divided into intestinal (INT) and diffuse (DIF) clinical subtypes. Even though these two GC groups are structurally and biologically different, this classification is not used to inform choice of treatment.

Taxanes (paclitaxel, docetaxel (DTX) and cabazitaxel) are widely used for cancer treatment and for GC specifically the TAX-325 study revealed therapeutic benefit when DTX was added to the standard chemotherapy regimen. Despite these improvements, however, patients often exhibit intrinsic or acquired resistance to DTX adversely affecting patient survival. Yet, the molecular basis of clinical drug resistance remains poorly elucidated posing a major barrier for the effective treatment of GC patients.

We performed a post-hoc analysis of the TAX-325 study to examine the potential influence of GC subtypes in clinical response to DTX. We classified randomized patients as diffuse or non-diffuse histology and correlated histology with clinical outcomes using a Cox proportional hazards model. Non-diffuse GC showed a significant improvement in overall survival with the addition of DTX (12.1 v 8.8 mo, p = 0.002), whereas diffuse histology was not associated with an improvement in survival (8.3 v 8.5 mo, p = 0.66). To investigate the molecular mechanism of GC DIF subtype resistance to taxanes, we used a panel of 12 GC cell lines representative of both subtypes (4 intestinal subtype, 8 diffuse subtype). DTX cytotoxicity assays revealed that similarly to what we observed clinically, 63% (5/8) of the DIF GC cell lines were resistant (IC50 > 600 nM) to DTX compared to 25% (1/4) of the INT GC cell lines. Further functional studies revealed that there was minimal DTX drug-target engagement in the DIF GC cells, assessed by confocal microscopy of the microtubule (MT) network and tubulin polymerization assays. These results suggested that DTX interaction with its target, MT, was impaired in the DIF GC cell lines. To rule out multi-drug resistance (MDR) as potential cause of intrinsic DTX resistance in these cells we performed flow cytometric evaluation of P-glycoprotein and found that all of the DIF GC cell lines were negative. Additionally, drug accumulation studies with C-14 radiolabeled DTX revealed that the drug accumulated intracellularly in all of GC cell lines in our panel.

Next generation sequencing of our panel of untreated or DTX-treated GC cell lines revealed 84 significantly differentially expressed genes in drug-sensitive cell lines but no changes in resistant cells. Our analysis showed a significant enrichment and a transcriptional co-regulation after treatment of genes encoding for kinesins, motor proteins associated with MT, in DTX-sensitive cells but not in DTX-resistant cells.

These studies will provide novel insights into mechanism of drug resistance and sensitivity and will ultimately allow us to design more effective targeted therapies to overcome chemo-resistance and eventually prolong patient survival.

Citation Format: Giuseppe Galletti, Chao Zhang, Kyle Cleveland, Doron Betel, Manish Shah, Paraskevi Giannakakou. Molecular mechanisms of intrinsic resistance to taxanes in gastric cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4938.

Abstract 3486: Dissecting the mechanism of AR-v7 nuclear translocation in prostate cancer

Continuous androgen receptor (AR) signaling is the key driver of castration-resistant prostate cancer (CRPC), despite prior androgen deprivation therapy (ADT). Potent, next-generation AR signaling inhibitors, such as abiraterone and enzalutamide have been recently added to the standard of care in the treatment of prostate cancer. However, resistance to these drugs inevitably emerges and is mediated by adaptive mechanisms that restore AR function, in the form of constitutively active, ligand-independent AR-variant (AR-V) overexpression. The microtubule-targeting drugs Docetaxel (DTX) and Cabazitaxel (CTX) are the only chemotherapy that significantly improves survival of CRPC patients. We have shown that full-length AR (AR-fl) binds microtubules (MTs) via its hinge domain, and utilizes them as tracks to facilitate its nuclear translocation. Taxanes keep AR-fl inactive in the cytoplasm as a result of MT stabilization. Of the two most prevalent AR-Vs, ARv567 contains the hinge region and is sensitive to taxane treatment while AR-v7, expressed in 60% of CRPC patients, lacks the hinge region and does not depend on MTs for its nuclear translocation. Therefore, ARv7 nuclear translocation is MT-independent and neither taxane treatment nor the disruption of dynein motor complex inhibited its nuclear localization. Thus, AR-v7 expression confers taxane resistance in vivo, in addition to next-generation AR inhibitors. Hence, inhibition of AR-v7 nuclear translocation and activity is critical to overcome drug resistance to current therapeutic modalities. However, the mechanism of ARv7 nuclear translocation is not clearly understood. Previously, we showed that ARv7 exhibited faster kinetics of nuclear translocation compared to AR-fl. Additionally, AR-v7 nuclear import is independent of the importin-α/β pathway, which is utilized by the MT-dependent, AR-fl and ARv567. Upon disruption of the Ran-GTPase nuclear import pathway, by overexpression of the catalytic mutant Q69L form of Ran, we observed that the nuclear import of both AR-fl and AR-V567 was significantly impaired, while ARv7 nuclear import was only partially affected. In this study, we hypothesized that molecular dynamics of AR nuclear import and export determines overall spatiotemporal localization of AR and its activity. To test this hypothesis, we generated photo-convertible AR-fl and AR-Vs (AR-v567 and AR-v7) fused to mEos4b construct, which enables us to track protein mobilization in the region of interest after photo-conversion in live cells. We are currently studying the nuclear import and export kinetics of both AR-fl and AR variants (AR-v567 and AR-v7) alone and in combination using the photo-conversion methodology. Elucidation of the distinct pathway(s) that mediate ARv7 nuclear import and/or export will allow the rationale design of effective ARv7 inhibitors or the development of co-targeting strategies in combination with taxanes and AR signaling inhibitors.

Citation Format: Seaho Kim, Mohd Azrin Jamalruddin, Luigi Portella, Paraskevi Giannakakou. Dissecting the mechanism of AR-v7 nuclear translocation in prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3486.