Pilot trial of CRLX101 in patients with advanced, chemotherapy-refractory gastroesophageal cancer

Background

CRLX101 is an investigational nanoparticle-drug conjugate with a camptothecin payload. Preclinical evidence indicated preferential uptake in tumors, and tumor xenograft models demonstrate superiority of CRLX101 over irinotecan. A pilot trial was conducted at recommended phase 2 dosing (RP2D) using the bimonthly schedule to assess preferential uptake of CRLX101 in tumor vs. adjacent normal tissue in endoscopically accessible tumors in chemotherapy-refractory gastroesophageal cancer. Results from the biopsies were previously reported and herein we present the clinical outcomes.

Methods

Patients initiated CRLX101 dosed at RP2D (15 mg/m²) on days 1 and 15 of a 28-day cycle. Detection of preferential CRLX101 tumor uptake was the primary endpoint and objective response rate (ORR) was a secondary endpoint. With a sample size of ten patients, the study had 90% power to detect ≥1 responder if the true response rate is ≥21%.

Results

Between Dec. 2012 and Dec. 2014, ten patients with chemotherapy-refractory (median 2 prior lines of therapy, range 1-4) gastric adenocarcinoma were enrolled. The median time-to-progression was 1.7 months. Best response was seen in one patient with stable disease (SD) for 8 cycles. Only ≥ grade 3 drug-related toxicity occurred in one patient with grade 3 cardiac chest pain who was able to resume therapy after CRLX101 was reduced to 12 mg/m².

Conclusions

Bimonthly CRLX101 demonstrated minimal activity with SD as best response in this heavily pretreated population. Future efforts with CRLX101 in gastric cancer should focus on combination and more dose-intensive strategies given its favorable toxicity profile and evidence of preferential tumor uptake.

Abstract 5135: Sustained and controlled in vivo therapeutic levels of drug payloads in tumors using two separate drug combination platforms: Antibody nanoparticle-drug conjugates and multi-drug nanoparticle-drug conjugates, with the potential for improved drug combinability and anticancer effects

Our nanoparticle-drug conjugates (NDCs), created at Cerulean Pharma Inc. by conjugating drug payloads to our novel β-cyclodextrin-PEG (CDP) copolymer, are designed to significantly mitigate a payload’s limitations by providing sustained drug delivery to the tumor and superior therapeutic index through controlled release kinetics. Cerulean has two NDCs in the clinic, CRLX101 and CRLX301, evidencing the translatability of our technology. CRLX101 has been dosed in over 400 patients and CRLX301 is in ongoing Phase 2a development. A key and differentiating feature of our NDC Platform is our linker technology, which is tailored for an optimal fit with the conjugation functionality of the API drug payloads (e.g., alcohol, carboxylic acid, amine, amide and urea functionality) and customizable to achieve desired drug release profiles. As an illustration of Cerulean’s ability to expand its NDC platform, we will present the biological and pharmacokinetic (PK) data supporting our drug combination platform of antibody nanoparticle-drug conjugates (ANDCs) and multi-drug nanoparticle-drug conjugates (mNDCs). ANDCs combine potentially any NDC with any conjugatable biologic to generate an ANDC with ultra-high drug-antibody ratios (DARs). Using our established NDC linker technology, we generated Herceptin-camptothecin ANDCs with DARs as high as 500, orders of magnitude higher than ADCs, and no more than 5-fold loss in binding (greater than 90% binding up to 300 DAR) compared to native Herceptin in our solution-based HER2 antigen binding assay. In addition, we provide evidence experimentally that rhodamine-labeled ANDCs penetrate and internalize in HER2+ tumor cells. The ANDCs demonstrate tunable in vitro release kinetics and long in vivo half lives in mouse PK studies, including sustained levels of released drug in tumors (greater than 300h) using an ANDC (average DAR 201) delivering an 8 mg/kg camptothecin dose. The observed mouse MTD for the ANDC was at least 3x higher than that of the corresponding NDC. The mNDCs take advantage of our existing NDC linker platform technology to conjugate diverse drug combos including DNA damaging agents (DDA) + DNA damage repair agents (DDR). We generated our POC camptothecin-olaparib mNDCs spanning a range of drug combination ratios (from 1:1 to 1:20) and all demonstrated distinguishable and tunable in vitro release rates. PK study of a 1:1 camptothecin-olaparib mNDC (~8mg/kg each drug) demonstrated very low clearance and sustained levels of released drug in tumor (greater than 72h). Our drug combination platforms, ANDCs and mNDCs, allow delivery of therapeutic agents to their respective biological targets at clinically-relevant doses, greatly increasing the diversity of drug combination possibilities.

Note: This abstract was not presented at the meeting.

Citation Format: Chester A. Metcalf, Derek van der Poll, Liang Zhao, Roy I. Case, Doug Lazarus, Donna Brown, Tiffany Halo, Lata Jayaraman, Christian Peters, Ellen Rohde, Scott Eliasof. Sustained and controlled in vivo therapeutic levels of drug payloads in tumors using two separate drug combination platforms: Antibody nanoparticle-drug conjugates and multi-drug nanoparticle-drug conjugates, with the potential for improved drug combinability and anticancer effects [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 5135. doi:10.1158/1538-7445.AM2017-5135

Targeting Hypoxia-Inducible Factors for Antiangiogenic Cancer Therapy

Hypoxia (low O₂) is a pathobiological hallmark of solid cancers, resulting from the imbalance between cellular O₂ consumption and availability. Hypoxic cancer cells (CCs) stimulate blood vessel sprouting (angiogenesis), aimed at restoring O₂ delivery to the expanding tumor masses through the activation of a transcriptional program mediated by hypoxia-inducible factors (HIFs). Here, we review recent data suggesting that the efficacy of antiangiogenic (AA) therapies is limited in some circumstances by HIF-dependent compensatory responses to increased intratumoral hypoxia. In lieu of this evidence, we discuss the potential of targeting HIFs as a strategy to overcome these instances of AA therapy resistance.

Abstract PR10: Significant improvements in therapeutic index for conjugated payloads using a nanoparticle-drug conjugate (NDC) platform to provide sustained drug release and potentially improved anticancer effects

Cerulean Pharma Inc.'s Dynamic Tumor Targeting™ Platform creates nanoparticle-drug conjugates (NDCs) designed to significantly mitigate a payload's limitations by providing sustained drug delivery to the tumor and superior therapeutic index through controlled release kinetics. By conjugating drug payloads to our novel β-cyclodextrin-PEG copolymer through linker strategies that allow modulation of release and pharmacokinetics (PK), we provide advantages over entrapped nanoparticle strategies, e.g., polymeric nanoparticles and liposomes. Cerulean has two NDCs in the clinic, CRLX101 and CRLX301, evidencing the translatability of our technology. CRLX101 has been dosed in over 350 patients and CRLX301 is in an ongoing Phase 1/2a trial. Our Dynamic Tumor Targeting Platform is applicable to a diverse range of drug payload possibilities, including small molecules with a host of physiochemical properties, i.e., hydrophobic and hydrophilic payloads, and functional groups with chemical handles for conjugation. To illustrate the capabilities of our platform we will present the biological impact of a series of cabazitaxel-containing NDCs with linkers encompassing a diversity of in vitro release rates. In vivo PK studies showed high and sustained levels of released drug in tumor tissues (>168 hrs), and in vivo mouse tumor model studies demonstrated vastly improved efficacy, i.e., tumor regression and significant tumor growth delay, and survival over separately dosed cabazitaxel including efficacy in a taxane-resistant tumor model. Cerulean continues to expand its platform through the development of new and emerging capabilities to treat patients living with cancer. In this regard, we also will address the future evolution of NDCs, including the conjugation of multiple payloads to a single NDC and the development of antibody-conjugated NDCs.

This abstract is also being presented as Poster B43.

Citation Format: Chester Metcalf, III, Derek van der Poll, Liang Zhao, Tiffany Halo, Doug Lazarus, Adam Stockmann, Christian Peters, Donna Brown, Roy Case, Ellen Rohde, Lata Jayaraman, Hongwei Wang, Tiffany Crowell, Adrian Senderowicz, Scott Eliasof. Significant improvements in therapeutic index for conjugated payloads using a nanoparticle-drug conjugate (NDC) platform to provide sustained drug release and potentially improved anticancer effects. [abstract]. In: Proceedings of the AACR Special Conference on Engineering and Physical Sciences in Oncology; 2016 Jun 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2017;77(2 Suppl):Abstract nr PR10.

CRLX101, a Nanoparticle-Drug Conjugate Containing Camptothecin, Improves Rectal Cancer Chemoradiotherapy by Inhibiting DNA Repair and HIF1α

Novel agents are needed to improve chemoradiotherapy for locally advanced rectal cancer. In this study, we assessed the ability of CRLX101, an investigational nanoparticle-drug conjugate containing the payload camptothecin (CPT), to improve therapeutic responses as compared with standard chemotherapy. CRLX101 was evaluated as a radiosensitizer in colorectal cancer cell lines and murine xenograft models. CRLX101 was as potent as CPT in vitro in its ability to radiosensitize cancer cells. Evaluations in vivo demonstrated that the addition of CRLX101 to standard chemoradiotherapy significantly increased therapeutic efficacy by inhibiting DNA repair and HIF1α pathway activation in tumor cells. Notably, CRLX101 was more effective than oxaliplatin at enhancing the efficacy of chemoradiotherapy, with CRLX101 and 5-fluorouracil producing the highest therapeutic efficacy. Gastrointestinal toxicity was also significantly lower for CRLX101 compared with CPT when combined with radiotherapy. Our results offer a preclinical proof of concept for CRLX101 as a modality to improve the outcome of neoadjuvant chemoradiotherapy for rectal cancer treatment, in support of ongoing clinical evaluation of this agent (LCC1315 NCT02010567). Cancer Res; 77(1); 112-22. ©2016 AACR.

Abstract 3721: A camptothecin-containing nanoparticle-drug conjugate combination with DDR agents provides a novel approach to increasing therapeutic index

Topoisomerase I inhibitors are used as standard-of-care chemotherapy in many types of cancer but are associated with significant toxicities. There is potential to improve their efficacy further by combining with inhibitors of the DNA damage response, such as the PARP inhibitor olaparib. However, while preclinical data highlight the improved efficacy of this combination, subsequent clinical trials have struggled due to dose limiting myelotoxicity.

CRLX101 is an investigational nanoparticle-drug conjugate (NDC) containing the payload camptothecin (the most potent topoisomerase I inhibitor known). This agent is preferentially targeted to tumours and demonstrated a favourable toxicity profile in the clinic.

Here, we explored the molecular mechanism and therapeutic potential of combining CRLX101 with either olaparib or the WEE1 inhibitor AZD1775, by testing both efficacy and safety in preclinical models. In vitro studies using NCI-H417a SCLC cells demonstrated that combination with both olaparib and AZD1775 potentiated the efficacy of CRLX101 although by different mechanisms. Cellular analyses revealed that CRLX101 treatment alone predominantly activated ATM-mediated DNA damage response and resulted in late S/G2 cell cycle arrest. Combination with a PARP inhibitor further enhanced the CRLX101-induced DNA damage response and prolonged cell cycle arrest in late S/G2 phase. In contrast, WEE1 inhibition abrogated late S/G2 cell cycle arrest induced by CRLX101, resulting in aberrant mitotic entry and enhanced cell death.

Our in vivo studies using wild type Wistar rat model showed that CRLX101, olaparib and AZD1775, are well tolerated as single agents. However, concurrent combination of CRLX101 with either olaparib or AZD1775 resulted in a dose-dependent decrease in haematological parameters. We investigated sequenced schedules and demonstrated that at a 24h delay between the CRLX101 and olaparib mitigates much of the combined bone marrow toxicity, while improving the efficacy above CRLX101 alone in xenograft tumours from NCI-H417a cells.

Collectively, these preclinical data demonstrate increased anti-tumour efficacy of CRLX101 when combined with DDR inhibitors. The combination schedule for CRLX101 and olaparib identified in our preclinical models as providing an increased therapeutic index has been used to develop protocols to test this combination in a relapsed (2nd line) SCLC human clinical trial (in collaboration with NCI).

Citation Format: Lenka Oplustil O’Connor, Anderson T. Wang, David R. Jones, Rajesh Odedra, Michael Spreadborough, Joanne Wilson, Aaron Smith, Peter Cotton, Jaimini Reens, Jen Barnes, Victoria Sheridan, Scott Eliasof, Andres Tellez, Alan Lau, Claire Sadler, Mark J. O’Connor. A camptothecin-containing nanoparticle-drug conjugate combination with DDR agents provides a novel approach to increasing therapeutic index. [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 3721.

Abstract 1345: Tumor selective localization of CRLX101, an investigational nanoparticle-drug conjugate of camptothecin

CRLX101, an investigational nanoparticle-drug conjugate (NDC) containing the payload camptothecin, is currently being clinically evaluated in multiple treatment-refractory solid tumors. In preclinical models, CRLX101 is believed to release camptothecin in the tumor in a slow and prolonged manner due to its long circulation half-life. CRLX101 has been shown preclinically to be a dual inhibitor of topoisomerase 1 and hypoxia-inducible factor 1α. It has demonstrated striking anti-tumor activity in several different tumor models. Camptothecin itself was identified as an active anti-tumor agent preclinically but was not developed clinically due to its poor tolerability in patients. The development of CRLX101, which has not shown significant toxicity in over 300 patients to date, offers a unique opportunity to improve cancer treatment in a meaningful way.

We hypothesized that CRLX101 utilizes the enhanced permeability and retention (EPR) effect to accumulate selectively in tumors. In this study, we sought to mechanistically dissect the process of CRLX101 entry and accumulation into tumor cells using multiple methods, both in vitro and in xenograft tumors in vivo. Using confocal microscopy, we detected camptothecin fluorescence in CRLX101-treated tumor cells in culture as well as in tumor tissue from mice treated with CRLX101. We can co-localize this camptothecin with intact nanoparticles using an anti-PEG antibody that specifically detects the PEG loops in the NDCs. More recently, we have shown that camptothecin and anti-PEG co-localize specifically in tumors of patients treated with CRLX101 but not in adjoining normal tissue. We can also demonstrate that macropinocytosis and activation of actin polymerization play a role in the process by which tumor cells take up CRLX101. Using an anti-CD31 antibody, we can visualize the distance traversed by CRLX101 from the tumor vasculature over time. We have developed novel analytical methods to precisely quantify both released and CRLX101-conjugated camptothecin over time in CRLX101 treated tumor cells in vitro, as well as in tumor tissue from mice treated with CRLX101 in vivo. Using cell viability assays, we can correlate the kinetics of camptothecin released inside tumor cells to the degree of tumor cell kill. We believe that these data are an important step forward in understanding the precise mechanism(s) underlying selective delivery of CRLX101 into tumor tissue.

Citation Format: Christian G. Peters, Douglas Lazarus, Donna Brown, Ningning Zhang, Adam P. Stockmann, Roy Case, Ellen Rohde, Scott Eliasof, Lata Jayaraman. Tumor selective localization of CRLX101, an investigational nanoparticle-drug conjugate of camptothecin. [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 1345.

Preclinical Efficacy of Bevacizumab with CRLX101, an Investigational Nanoparticle-Drug Conjugate, in Treatment of Metastatic Triple-Negative Breast Cancer

VEGF pathway-targeting antiangiogenic drugs, such as bevacizumab, when combined with chemotherapy have changed clinical practice for the treatment of a broad spectrum of human cancers. However, adaptive resistance often develops, and one major mechanism is elevated tumor hypoxia and upregulated hypoxia-inducible factor-1α (HIF1α) caused by antiangiogenic treatment. Reduced tumor vessel numbers and function following antiangiogenic therapy may also affect intratumoral delivery of concurrently administered chemotherapy. Nonetheless, combining chemotherapy and bevacizumab can lead to improved response rates, progression-free survival, and sometimes, overall survival, the extent of which can partly depend on the chemotherapy backbone. A rational, complementing chemotherapy partner for combination with bevacizumab would not only reduce HIF1α to overcome hypoxia-induced resistance, but also improve tumor perfusion to maintain intratumoral drug delivery. Here, we evaluated bevacizumab and CRLX101, an investigational nanoparticle-drug conjugate containing camptothecin, in preclinical mouse models of orthotopic primary triple-negative breast tumor xenografts, including a patient-derived xenograft. We also evaluated long-term efficacy of CRLX101 and bevacizumab to treat postsurgical, advanced metastatic breast cancer in mice. CRLX101 alone and combined with bevacizumab was highly efficacious, leading to complete tumor regressions, reduced metastasis, and greatly extended survival of mice with metastatic disease. Moreover, CRLX101 led to improved tumor perfusion and reduced hypoxia, as measured by contrast-enhanced ultrasound and photoacoustic imaging. CRLX101 durably suppressed HIF1α, thus potentially counteracting undesirable effects of elevated tumor hypoxia caused by bevacizumab. Our preclinical results show pairing a potent cytotoxic nanoparticle chemotherapeutic that complements and improves concurrent antiangiogenic therapy may be a promising treatment strategy for metastatic breast cancer. Cancer Res; 76(15); 4493-503. ©2016 AACR.

Efficacy of the nanoparticle-drug conjugate CRLX101 in combination with bevacizumab in metastatic renal cell carcinoma: results of an investigator-initiated phase I-IIa clinical trial

BACKGROUND

Anti-angiogenic therapies are effective in metastatic renal cell carcinoma (mRCC), but resistance is inevitable. A dual-inhibition strategy focused on hypoxia-inducible factor (HIF) is hypothesized to be active in this refractory setting. CRLX101 is an investigational camptothecin-containing nanoparticle-drug conjugate (NDC), which durably inhibits HIF1α and HIF2α in preclinical models and in gastric cancer patients. Synergy was observed in the preclinical setting when combining this NDC and anti-angiogenic agents, including bevacizumab.

PATIENTS AND METHODS

Patients with refractory mRCC were treated every 2 weeks with bevacizumab (10 mg/kg) and escalating doses of CRLX101 (12, 15 mg/m(2)) in a 3 + 3 phase I design. An expansion cohort of 10 patients was treated at the recommended phase II dose (RP2D). Patients were treated until progressive disease or prohibitive toxicity. Adverse events (AEs) were assessed using CTCAE v4.0 and clinical outcome using RECIST v1.1.

RESULTS

Twenty-two patients were response-evaluable in an investigator-initiated trial at two academic medical centers. RCC histologies included clear cell (n = 12), papillary (n = 5), chromophobe (n = 2), and unclassified (n = 3). Patients received a median of two prior therapies, with at least one prior vascular endothelial tyrosine kinase inhibitor therapy (VEGF-TKI). No dose-limiting toxicities were observed. Grade ≥3 AEs related to CRLX101 included non-infectious cystitis (5 events), fatigue (3 events), anemia (2 events), diarrhea (2 events), dizziness (2 events), and 7 other individual events. Five of 22 patients (23%) achieved partial responses, including 3 of 12 patients with clear cell histology and 2 of 10 patients (20%) with non-clear cell histology. Twelve of 22 patients (55%) achieved progression-free survival (PFS) of >4 months.

CONCLUSIONS

CRLX101 combined with bevacizumab is safe in mRCC. This combination fulfilled the protocol's predefined threshold for further examination with responses and prolonged PFS in a heavily pretreated population. A randomized phase II clinical trial in mRCC of this combination is ongoing.

Pilot trial of CRLX101 in patients (pts) with advanced, chemotherapy-refractory gastroesophageal cancer (GEC)

44

Background: Camptothecin (CPT) derivatives such as irinotecan have activity in 2nd-line therapy in advanced GEC with reported response rates of 0-15%. CRLX101 is an investigational nanoparticle-drug conjugate (NDC) with a CPT payload. Preclinical evidence indicates preferential uptake in tumors, and animal GEC xenograft models demonstrate superiority of CRLX101 over irinotecan. A pilot trial was conducted at recommended phase 2 dosing (RP2D) to assess preferential uptake of CRLX101 in tumor vs. adjacent normal tissue in endoscopically accessible tumors in patients with chemotherapy-refractory GEC. Data demonstrating preferential tumor uptake of CRLX101 has been presented separately and here we report on the clinical outcomes of patients enrolled. Methods: All pts initiated CRLX101 dosed intravenously at RP2D (15 mg/m2) on days 1 and 15 of a 28-day cycle until disease progression or intolerant toxicity. While detection of preferential CRLX101 tumor uptake was the primary endpoint, with 10 pts enrolled a secondary analysis could be performed with the study having 90% power to detect ≥ 1 responder if the true response rate is ≥ 21%. Responses were assessed using RECIST 1.1. Results: Between Dec. 2012 and Dec. 2014, 10 patients with chemotherapy-refractory (median 2 prior lines of therapy, range 1-4) GEC and adenocarcinoma histology were enrolled and evaluable for response and toxicity. The median time-to-progression was 1.9 mo (range 0.6-8.7 mo). Best response was seen in 1 pt with stable disease (SD) for 8 cycles. Only ≥ grade 3 toxicities related to CRLX101 occurred in a single patient with grade 3 anemia and chest pain who was able to resume therapy without any further toxicity after CRLX101 was reduced to 12 mg/m2. Conclusions: CRLX101 demonstrated minimal activity with SD as best response in this heavily pretreated population. Future efforts with CRLX101 in advanced GEC should focus on combination strategies. Its favorable toxicity profile and evidence of preferential tumor uptake support further clinical research of combining CRLX101 with other targeted therapies such as anti-angiogenesis (ramucirumab) and/or immune checkpoint inhibitors. Clinical trial information: NCT01612546 Clinical trial information: NCT01612546.

Abstract B33: CRLX101, an investigational nanoparticle-drug conjugate, localizes in human tumors and not in adjacent healthy tissue after intravenous dosing

Background: Nanoparticle-based therapeutics are thought to rely on the enhanced permeability and retention effect to preferentially localize in solid tumors and not healthy tissue. These phenomena are rationalized primarily from animal models of the human disease. There is a need to obtain analogous information from humans in order to better understand how nanoparticle-based therapeutics perform in humans. CRLX101 is an investigational nanoparticle drug conjugate (NDC) consisting of a cyclodextrin-containing polymer (CDP) conjugate of the payload camptothecin (CPT). The individual polymer strands self-assemble into nanoparticles (ca. five strands) of approximately 10 to 40 nm diameter and 10 wt% CPT by multiple, interstrand, inclusion complex formation between the cyclodextrin and the CPT molecules. CRLX101 is currently being investigated in phase II trials in patients with renal, ovarian and rectal cancer.

Methods: A phase I clinical trial was performed with CRLX101 at the City of Hope, in patients with advanced or metastatic stomach, gastroesophageal or esophageal cancer. This study was sponsored by City of Hope Medical Center, and funding and CRLX101 was provided by Cerulean Pharma Inc. (ClinicalTrials.gov identifier: NCT01612546). The goal of this study was to test the hypothesis that intact CRLX101 nanoparticles deposit in human tumors and not in normal adjacent tissue after intravenous administration. Tumor and adjacent healthy tissue biopsies were obtained through endoscopic capture from patients who received CRLX101, and analyzed via a number of methodologies.

Results: Both the pre- and post-dosing, healthy tissue samples adjacent to tumors show no evidence of either the NDC or the payload (CPT) contained within the NDC. Similar results are obtained from the pre-dosing tumor samples. However, in 8 of 9 patients that were evaluated, CPT is detected in the tumor tissue by fluorescent microscopy examination of fixed sections. For 3 of these patients, proof of intact CRLX101 NDC is obtained from tissue sections by colocalized, fluorescence from the CPT and a secondary antibody used to stain a PEG specific antibody (binds to the PEG in CRLX101). Following fluorescence imaging, remaining tissues from 3 patients were homogenized and measured for free and CDP conjugated CPT using HPLC. CPT was present in the post-treatment tumor tissue of all 3 patient samples with an average of 96.2±13.1% in the conjugated form, indicating that CRLX101 NDCs are localized in these samples.

Conclusions: Tumor and adjacent healthy tissue biopsies obtained from cancer patients who have received CRLX101show that these NDCs do localize in human tumors and not in adjacent tissues.

Citation Format: Andrew Clark, Devin T. Wiley, Jonathan E. Zuckerman, Paul Webster, Joseph Chao, James Lin, Yun Yen, Mark E. Davis, Scott Eliasof. CRLX101, an investigational nanoparticle-drug conjugate, localizes in human tumors and not in adjacent healthy tissue after intravenous dosing. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B33.

Abstract B37: Selective tumor localization of CRLX101, a novel nanoparticle-drug conjugate

CRLX101, an investigational nanoparticle-drug conjugate (NDC) containing the payload camptothecin conjugated to a biocompatible copolymer of cyclodextrin and polyethylene glycol (PEG), is currently being evaluated clinically in multiple treatment-refractory solid tumors. CRLX101 is a dual inhibitor of topoisomerase 1 (TOPO 1) and hypoxia-inducible factor 1α (HIF-1α) and has been shown pre-clinically to be active in many different tumor types. It has a long circulation half-life and is believed to release camptothecin in a slow and prolonged fashion in the tumor. Camptothecin is an active anti-tumor agent preclinically but was not developed clinically due to its poor tolerability in patients. The development of CRLX101, which has not demonstrated significant toxicity in over 250 patients dosed to date, offers a unique opportunity to improve cancer treatment in a meaningful way.

We have previously published that in gastric cancer patients treated with CRLX101, camptothecin fluorescence can be visualized in tumor tissue but not adjoining normal tissue. Based on these data we hypothesized that CRLX101 utilizes the enhanced permeability and retention (EPR) effect to accumulate selectively in tumors. In this study, we sought to mechanistically dissect the process of CRLX101 entry and accumulation into tumor cells using multiple methods, both in vitro and in xenograft tumors in vivo. Using confocal microscopy, we can detect camptothecin fluorescence in CRLX101- treated tumor cells in culture as well as in tumor tissue from mice treated with CRLX101. We can co-localize this camptothecin with intact nanoparticles using an anti-PEG antibody that specifically detects the PEG loops in the NDCs. We can also demonstrate that macropinocytosis plays a role in the manner by which tumor cells take up CRLX101. Using an anti-CD31 antibody, we can visualize the distance traversed by CRLX101 from the tumor vasculature over time. We have developed novel analytical methods to precisely quantify both released and CRLX101-conjugated camptothecin over time in CRLX101 treated tumor cells in vitro, as well as in tumor tissue from mice treated with CRLX101 in vivo. Using cell viability assays, we can correlate the kinetics of camptothecin released inside tumor cells to the degree of tumor cell kill. We believe that these data are an important step forward in understanding the precise mechanism(s) underlying selective delivery of CRLX101 into tumor tissue.

Citation Format: Christian G. Peters, Douglas Lazarus, Donna Brown, Ningning Zhang, Roy Case, Ellen Rohde, Scott Eliasof, Lata Jayaraman. Selective tumor localization of CRLX101, a novel nanoparticle-drug conjugate. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B37.

Abstract B176: In vitro and in vivo studies demonstrating sustained drug release for multiple anticancer payloads and improved anticancer effects of a cabazitaxel β-cyclodextrin-PEG copolymer-based nanoparticle-drug conjugate (NDC)

We have demonstrated the ability to generate nanoparticle-drug conjugates (NDCs) with tunable and diverse in vitro and in vivo drug release kinetics by the conjugation of multiple anticancer agents (docetaxel, cabazitaxel, and gemcitabine) to a β-cyclodextrin-PEG (CDP) copolymer through a variety of linker strategies. Linker molecules included glycinate, alaninate, hexanoate, and diester linker β-alanine-glycolate, which were conjugated via ester linkages to the anticancer payloads. In vitro release studies demonstrated that the β-alanine-glycolate linker released drug from the CDP copolymer relatively faster than the glycinate and alaninate linkers, while the hexanoate linker showed much slower drug release. Release studies in native and heat-inactivated plasma had no influence on the release kinetics, indicating that drug release was pH- but not enzyme-triggered. The in vitro release profiles for the respective linkers varied slightly (more so in vivo) depending on the API (Active Pharmaceutical Ingredient) payload, demonstrating that release kinetics can be varied through selection of linker molecules and that NDC chemistry is customizable with respect to API. In vivo PK studies with cabazitaxel (CBTX) NDCs demonstrated that the hexanoate-containing NDC (slower releasing linker) led to higher total (conjugated drug + released) drug levels and lower released drug levels compared to the glycinate NDC (faster releasing linker) and the separately dosed cabazitaxel. This likely led to the greater tolerability (i.e., higher MTD) observed for the CBTX-hexanoate NDC. Both the CBTX-hexanoate and -glycinate NDCs led to high and sustained levels of released drug in tumor tissues (>72 hrs). The maximum drug concentrations in the blood (Cmax) of released drug was lower for all NDCs tested compared to similarly dosed parent drug, thereby addressing Cmax-related toxicities. Two CBTX NDCs (hexanoate and glycinate) were chosen for in vivo efficacy studies in mouse tumor models. Both demonstrated vastly improved efficacy (and survival) over the parent drug, cabazitaxel, at similar doses including efficacy against docetaxel-resistant UISO-BCA-1 tumor cells. The CBTX-hexanoate NDC showed a greater therapeutic index (TI) compared to parent drug as well as the CBTX-glycinate NDC.

Citation Format: Chester Metcalf, III, Donna Brown, Jungyeon Hwang, Sujan Kabir, Douglas Lazarus, Pochi Shum, Snehlata Tripathi, Scott Eliasof. In vitro and in vivo studies demonstrating sustained drug release for multiple anticancer payloads and improved anticancer effects of a cabazitaxel β-cyclodextrin-PEG copolymer-based nanoparticle-drug conjugate (NDC). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B176.

Abstract 5515: Neoadjuvant chemoradiotherapy for rectal cancer with CRLX101, an investigational nanoparticle-drug conjugate with a camptothecin payload

Background: There has been great interest in developing novel agents and strategies to improve chemoradiotherapy (CRT) for locally advanced rectal cancer. Irinotecan, a campothecin (CPT) analogue, held high potential, but the combination was clinically infeasible due to severe gastrointestinal toxicities. CRLX101, is an investigational nanoparticle drug conjugate (NDC). Preclinical experiments showed that CRLX101 differentially delivers CPT into cancer cells and appears to durably suppress HIF-1α as well as topoisomerase 1, but with less gastrointestinal toxicities than irinotecan. We therefore hypothesized that the addition of CRLX101 to rectal CRT (5-FU + XRT) may further improve the therapeutic index in this setting.

Methods: Synergy with CRLX101 in combination with either XRT or CRT was studied in vitro (SW480 and HT29 colorectal cancer cell lines) and in vivo (murine flank xenograft models). Skin toxicity and hematologic toxicity were also characterized. In order to test the synergy hypothesis in the clinic, a Phase Ib/II clinical trial (LCCC1315) evaluating the addition of CRLX101 to CRT in the neo-adjuvant treatment of rectal cancer is currently underway. A standard 3 + 3 design is being employed for the phase Ib with a CRLX101 starting dose of 12 mg/m2 in the first cohort escalating to the CRLX101 monotherapy MTD of 15 mg/m2 in the second. The primary phase 2 end-point is the pathological complete response (pCR) rate from treatment.

Results: CRLX101 was found to be as potent as camptothecin in vitro. We have demonstrated that CRLX101 functions by inhibition of both DNA repair and HIF-1α signaling. The addition of CRLX101 to radiotherapy increased and prolonged the number of γH2AX foci, even at 24 hours post radiotherapy. We also confirmed that CRLX101 decreased HIF-1α and its downstream targets VEGF and carbonic anhydrase IX in mice bearing HT29 xenografts. Our findings were further validated in vivo: we demonstrated that both CRLX101+5FU+XRT and CRLX101+XRT delayed tumor growth more than other regimens (p-values < 0.05). More importantly, we found CRT with CRLX101+5FU is significantly more effective than CRT with oxaliplatin+5FU (25 days to double tumor volume vs. 11 days), a regimen that has been extensively studied clinically. Preclinical toxicity studies demonstrated that the addition of CRLX101 did not increase hematologic or skin toxicities. In the ongoing clinical trial, none of the first 6 patients enrolled have experienced dose-limiting toxicities, and 1 out of 3 patients who underwent surgery had a pCR. The other 2 patients had extensive treatment response with minimal residual tumor.

Conclusions: Preclinical data suggests that CRLX101 improves the therapeutic index of CRT for rectal cancer. Preliminary clinical data is encouraging, and supports further clinical assessment of CRLX101+5FU+XRT in patients with locally advanced rectal cancer.

Citation Format: XI TIAN, Minh Nguyen, Henry Foote, Kyle T. Wagner, Hanna K. Sanoff, Autumn J. McRee, Bert H. O'Neil, Benjamin F. Calvo, William A. Blackstock, Joel E. Tepper, Edward Garmey, Scott Eliasof, Andrew Z. Wang. Neoadjuvant chemoradiotherapy for rectal cancer with CRLX101, an investigational nanoparticle-drug conjugate with a camptothecin payload. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5515. doi:10.1158/1538-7445.AM2015-5515

Abstract 1384: CRLX101, an investigational camptothecin-containing nanoparticle-drug conjugate, reverses the HIF-1α-mediated increase in cancer stem cells caused by bevacizumab in a preclinical model of triple-negative breast cancer

Antiangiogenic agents are designed to impede the development of new tumor blood vessels, thereby starving tumors of oxygen and nutrients to ultimately block tumor growth. Despite advances made in developing antiangiogenic therapies, clinical and preclinical data suggest that these drugs have limited efficacy in breast cancer patients. Tumors inevitably develop resistance to antiangiogenic agents and patients frequently fail to demonstrate significantly better overall survival. Acquired resistance to antiangiogenic agents has been attributed in part to the induction of intra-tumoral hypoxia and the concomitant stabilization of hypoxia-inducible factor 1α (HIF-1α), a transcription factor that promotes tumor angiogenesis, invasion, metastasis, and cancer stem cell (CSC) self-renewal. In this preclinical model, we demonstrate that CRLX101, an investigational nanoparticle-drug conjugate (NDC) containing the payload camptothecin (CPT), can reverse the stimulatory effects of hypoxia on the CSC population in an orthotopic triple-negative xenograft model. The payload, camptothecin, a topoisomerase 1 inhibitor, inhibits HIF-1α protein accumulation at concentrations below those that were cytotoxic to tumor cells, and inhibits hypoxia-mediated CSC induction at these low concentrations in vitro. In an orthotopic triple-negative breast cancer model, CRLX101 is synergistic with bevacizumab. Tumor reimplantation experiments confirm that the combination therapy effectively targets both the bulk tumor cell and CSC populations. Results generated from these preclinical studies support the clinical research of combining antiangiogenic agents with CRLX101 to increase patient survival. CRLX101 is currently in phase 2 clinical trials in combination with bevacizumab in both recurrent ovarian cancer and metastatic renal cell carcinoma.

Citation Format: Sarah J. Conley, Trenton L. Baker, Joseph P. Burnet, Rebecca L. Thiesen, Douglas Lazarus, Christian G. Peters, Shawn G. Clouthier, Scott Eliasof, Max S. Wicha. CRLX101, an investigational camptothecin-containing nanoparticle-drug conjugate, reverses the HIF-1α-mediated increase in cancer stem cells caused by bevacizumab in a preclinical model of triple-negative breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1384. doi:10.1158/1538-7445.AM2015-1384

Abstract 4124: Potent anti-tumor and metastatic breast cancer efficacy of bevacizumab with CRLX101, an investigational chemotherapy nanoparticle-drug conjugate that secondarily suppresses HIF-1α

Despite the approval of seven different VEGF-pathway targeting agents, such as bevacizumab, for ten different cancer types, VEGF inhibition has shown only modest survival benefits - especially in breast cancer. One hypothesis for this is the adaptive resistance that emerges, such as increased tumor hypoxia and elevated hypoxia-inducible factor 1α (HIF-1α), which up-regulates genes causing tumor angiogenesis, resistance and metastasis. Here we evaluated bevacizumab paired with a potent cytotoxic investigational drug called CRLX101, a nanoparticle-drug conjugate containing the payload camptothecin that secondarily suppresses HIF-1α. In a preclinical mouse model of primary human breast tumor xenografts grown in the mammary fat pad after cell-line implantation, CRLX101 monotherapy was highly efficacious. More importantly, CRLX101 with bevacizumab resulted in dramatic primary tumor shrinkages and greatly improved mice survival, despite bevacizumab alone having no activity in this model. This potent anti-tumor efficacy was further confirmed in a patient-derived xenograft (PDX) model, where again CRLX101 and bevacizumab led to obvious shrinkage of established primary tumors. HIF-1α suppression was confirmed by immunohistochemistry while changes in tumor hypoxia and perfusion were evaluated using photoacoustic imaging and contrast-enhanced ultrasound, respectively. To better reflect the clinical treatment setting, the combination was next evaluated in our preclinical model of post-surgical, overt metastatic disease. Both CRLX101 monotherapy and CRLX101 in combination with bevacizumab showed shrinkage of existing metastatic masses and prevented the emergence of new metastases. In conclusion, we showed that pairing an anti-angiogenic agent with a potent chemotherapy backbone that is able to suppress HIF-1α up-regulation induced by anti-angiogenic therapy greatly improve efficacy in both primary and metastatic breast cancer models. The data from these preclinical experiments demonstrate that further research of the combination of bevacizumab and other anti-angiogenic drugs with CRLX101 is warranted in solid tumors.

Citation Format: Elizabeth Pham, Christina R. Lee, Ping Xu, Shan Man, Melissa Yin, F. Stuart Foster, Christian G. Peters, Douglas Lazarus, Scott Eliasof, Robert S. Kerbel. Potent anti-tumor and metastatic breast cancer efficacy of bevacizumab with CRLX101, an investigational chemotherapy nanoparticle-drug conjugate that secondarily suppresses HIF-1α. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4124. doi:10.1158/1538-7445.AM2015-4124

Improving neoadjuvant chemoradiotherapy for rectal cancer with CRLX101, a nanoparticle-drug conjugate, with a camptothecin payload

645

Background: There has been great interest in developing novel agents and strategies to improve chemoradiotherapy (CRT) for locally advanced rectal cancer. Irinotecan, a camptothecin (CPT) analogue, held high potential, but the combination was clinically infeasible due to severe gastrointestinal toxicities. CRLX101, a dynamically tumor targeted nanoparticle-drug conjugate differentially delivers CPT into cancer cells and appears to durably suppress HIF-1α as well as topoisomerase 1, but with less gastrointestinal toxicities than irinotecan. We therefore hypothesized that the addition of CRLX101 to CRT may further improve the therapeutic index in this setting. Methods: CRLX101 was evaluated using murine flank xenograft models (SW480 and HT29) of colorectal cancer alone or in combination with radiotherapy (XRT) and/or 5-FU and compared to CPT plus CRT. Skin toxicity and hematologic toxicity were also studied. In addition, a Phase Ib/II clinical trial (LCCC1315) evaluating the addition of CRLX101 to CRT in the neo-adjuvant treatment of rectal cancer is currently underway. A standard 3 + 3 design was used for the phase Ib with a CRLX101 starting dose of 12 mg/m2 in the first cohort escalating to the CRLX101 monotherapy MTD of 15 mg/m2 in the second. The primary phase 2 end-point is the pathological complete response (pCR) rate from treatment. Results: In the HT29 xenografts model, CRLX101+5FU+XRT delayed tumor growth significantly more than the other treatment regimens evaluated. In the SW480 xenografts model, both CRLX101+5FU+XRT and CRLX101+XRT delayed tumor growth more than other regimens, but there was no statistical advantage to the addition of 5-FU to CRLX101+XRT in this model. The addition of CRLX101 to 5FU+XRT did not increase hematologic or skin toxicities. In the ongoing clinical trial, none of the first 3 patients enrolled (the first dosing cohort) have experienced dose limiting toxicities and 1 out of 3 patients had a pCR. Conclusions: CRLX101 has been shown in preclinical experiments to improve the therapeutic index of CRT for rectal cancer. Preliminary clinical data suggest that CRLX101+5FU+XRT treatment is well tolerated. Clinical trial information: NCT02010567.

Translational impact of nanoparticle-drug conjugate CRLX101 with or without bevacizumab in advanced ovarian cancer

PURPOSE

Increased tumor hypoxia and hence elevated hypoxia-inducible factor-1α (HIF1α) is thought to limit the efficacy of vascular endothelial growth factor (VEGF) pathway-targeting drugs by upregulating adaptive resistance genes. One strategy to counteract this is to combine antiangiogenic drugs with agents able to suppress HIF1α. One such possibility is the investigational drug CRLX101, a nanoparticle-drug conjugate (NDC) containing the payload camptothecin, a known topoisomerase-I poison.

EXPERIMENTAL DESIGN

CRLX101 was evaluated both as a monotherapy and combination with bevacizumab in a preclinical mouse model of advanced metastatic ovarian cancer. These preclinical studies contributed to the rationale for undertaking a phase II clinical study to evaluate CRLX101 monotherapy in patients with advanced platinum-resistant ovarian cancer.

RESULTS

Preclinically, CRLX101 is highly efficacious as a monotherapy when administered at maximum-tolerated doses. Furthermore, chronic low-dose CRLX101 with bevacizumab reduced bevacizumab-induced HIF1α upregulation and resulted in synergistic efficacy, with minimal toxicity in mice. In parallel, initial data reported here from an ongoing phase II clinical study of CRLX101 monotherapy shows measurable tumor reductions in 74% of patients and a 16% RECIST response rate to date.

CONCLUSIONS

Given these preclinical and initial clinical results, further clinical studies are currently evaluating CRLX101 in combination with bevacizumab in ovarian cancer and warrant the evaluation of this therapy combination in other cancer types where HIF1α is implicated in pathogenesis, as it may potentially be able to improve the efficacy of antiangiogenic drugs.

Abstract 4582: CRLX522, a novel dynamic tumor-targeted cabazitaxel nanopharmaceutical

Cabazitaxel is an approved treatment for castration-refractory prostate cancer. As with other taxanes, the therapeutic benefits of cabazitaxel are often compromised by immunosuppression and neurotoxicity. In order to address the obstacles to using cabazitaxel, we developed the dynamic tumor-targeted cabazitaxel nanopharmaceutical CRLX522. This nanopharmaceutical possesses significantly enhanced efficacy and improved pharmacokinetics compared to the parent drug. CRLX522 showed a superior PK profile, with a >400-fold increase in AUC, a significantly longer half-life and a 32-fold higher drug concentration in tumor tissue 72 hours after a single administration versus cabazitaxel alone. The improved PK and biodistribution of CRLX522 translated into superior antitumor activity in murine syngeneic and human xenograft models. In the murine B16.F10 tumor model, median survival at the MTD exceeded 58 days for CRLX522 compared to 26 days for the parent drug cabazitaxel. CRLX522 demonstrated superior efficacy over cabazitaxel with a cure rate of 100% in a xenograft model representing breast cancer tumors, compared to 50% cures for the parent drug cabazitaxel. In summary, CRLX522 represents a new and potent anticancer therapeutic that has the potential to be a best-in-class taxane.

Citation Format: Douglas Lazarus, Sujan Kabir, Scott Eliasof. CRLX522, a novel dynamic tumor-targeted cabazitaxel nanopharmaceutical. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4582. doi:10.1158/1538-7445.AM2014-4582

Interim results of a phase 1b/2a study evaluating the nano pharmaceutical CRLX101 with bevacizumab (bev) in the treatment of patients (pts) with refractory metastatic renal cell carcinoma (mRCC)

412

Background: VHL is inactivated in most clear cell RCC (ccRCC) tumors giving rise to the HIF hypoxia response program and tumor angiogenesis. Antiangiogenic therapies are active in ccRCC, but resistance develops in all pts. Dual inhibition strategies may be needed to overcome resistance. CRLX101, a novel nano pharmaceutical formulation of camptothecin, has been shown in pre-clinical models to target HIF. We combined CRLX101 with bev in the treatment of pts with refractory mRCC in order to determine the safety and the recommended phase 2 dose (RP2D) of the combination, and to determine the therapeutic activity of treatment. Methods: Pts with mRCC refractory to conventional antiangiogenic therapies (all subtypes) were treated every 2 weeks with bev 10 mg/kg and escalating doses of CRLX101 (12 mg/m2, 15 mg/m2) in a standard 3+3 design with an expansion at the RP2D. Pts were treated until progression, death, or prohibitive toxicity. Adverse events were classified using CTCAE v 4.0. Radiographic results were assessed by RECIST v1.1. Results: 10 pts were enrolled on study – 6 clear cell, 2 papillary, 2 chromophobe. Median prior number of therapies was 2. No dose limiting toxicities were observed. CRLX101 at its single-agent RP2D (15 mg/m2) was safely combined with standard bev. 2 episodes of grade 3 non-infectious cystitis and 1 episode of grade 3 hypertension were observed. All other toxicities were grade 1 or 2. The median progression free survival (PFS) was 7.6 months which exceeded our pre-specified threshold of 16 weeks and has triggered 2nd stage accrual. 3 of 9 evaluable pts (33%) achieved a confirmed partial response (PR) including 1 patient with papillary mRCC; 4 pts (44%) displayed stable disease, and 2 had progressive disease as their best overall response. Conclusions: Our results demonstrate that CRLX101 can be safely combined with bev in pts with refractory mRCC. The RP2D is 15 mg/m2 q2w of CRLX101 with standard bev. Preliminary outcomes to date suggest that clinical benefit, manifest as prolonged PFS, is conferred to pts with all histologic subtypes of mRCC. Clinical trial information: NCT01625936.

Abstract PR09: Synergistic activity of CRLX101, a nanopharmaceutical in Phase II clinical trials, with antiangiogenic therapies mediated through HIF-1alpha inhibition: A translational research program

Background: Antiangiogenic drugs reduce blood flow to tumors and thereby inhibit tumor growth by starving tumors of oxygen and nutrients. However, antiangiogenic drugs have achieved limited success as monotherapies, in part because of their induction of hypoxia and the concomitant up-regulation of hypoxia-inducible factor 1α (HIF-1α), now well implicated in the promotion of tumor angiogenesis, invasion, metastasis, and cancer stem cell formation. We describe here a translational research program to investigate whether the efficacy of antiangiogenic drugs can be improved through combination with CRLX101, a camptothecin (CPT) containing nanopharmaceutical that inhibits both topoisomerase-1 and HIF-1α.

Material and methods: We will present preclinical and clinical projects conducted across several major research institutions intended to demonstrate the anti-HIF-1α activity of CRLX101, the capacity of this drug to block the epithelial-mesenchymal transition (EMT) and the formation of cancer stem cells, and the synergistic activity of CRLX101 given in combination with antiangiogenic drugs. We will further describe two ongoing clinical trials evaluating these hypotheses, one at the University of Pennsylvania in advanced renal cell carcinoma (RCC) and one at the Massachusetts General Hospital in relapsed ovarian cancer following progression through prior platinum-containing chemotherapy.

Results: A single dose of CRLX101 durably inhibits HIF-1α protein levels across multiple tumor types. Evaluation of CRLX101 in combination with bevacizumab, aflibercept or pazopanib in the A2780 ovarian xenograft tumor model demonstrates synergistic inhibition of tumor growth inhibition as well as increases in the rate of long-term survivorship. While all three antiangiogenic drugs alone increased HIF-1α protein levels, levels were inhibited in response to combination with CRLX101. In clinical evaluations, a CRLX101-bevacizumab combination appears safe and well tolerated with no dose limiting toxicities observed to date. Notable tumor decreases and long periods of progression free survival have been noted among patients treated with CRLX101-based mono and combination therapy.

Conclusions: Results generated through this translational research program suggest that CRLX101 can overcome HIF-1α-mediated acquired resistance to antiangiogenic drugs, supporting the use of CRLX101 in combination with antiangiogenic drugs as an exciting new paradigm for the treatment of cancer.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):PR09.

Citation Format: Scott Eliasof, Sarah Conley, Stephen M. Keefe, Robert Kerbel, Carolyn N. Krasner, Douglas Lazarus, Christian Peters, Elizabeth Pham, Max S. Wicha, Edward G. Garmey. Synergistic activity of CRLX101, a nanopharmaceutical in Phase II clinical trials, with antiangiogenic therapies mediated through HIF-1alpha inhibition: A translational research program. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr PR09.

Abstract 376: CRLX101, a nanopharmaceutical in phase 2 clinical trials, is synergistic with antiangiogenic treatment through HIF-1α inhibition in an ovarian cancer xenograft model

Antiangiogenic drugs reduce blood flow to tumors and thereby inhibit tumor growth by starving tumors of oxygen and nutrients. However, antiangiogenic agents have achieved limited success as monotherapies. This may be in part because the reduced blood flow that follows this therapy induces tumor hypoxia and up-regulates hypoxia-inducible factor 1α (HIF-1α), well implicated in the promotion of tumor angiogenesis, invasion, and metastasis. We investigated whether the efficacy of antiangiogenic drugs could be improved by combining them with an inhibitor of HIF-1α. Previous investigators showed that HIF-1α can be inhibited by camptothecin (CPT). Therefore, we sought to demonstrate that CRLX101, a self-assembling nanopharmaceutical that delivers the topoisomerase-1 inhibitor CPT, is an inhibitor of HIF-1α and that CRLX101 efficacy is synergistic with antiangiogenic treatment. CRLX101 is designed to deliver therapeutically relevant concentrations of CPT to tumors over an extended period of time. We have previously presented data demonstrating dramatic CRLX101-mediated inhibition of tumor growth in 20 different xenograft models and CRLX101 is currently being evaluated in phase 2 clinical trials as a monotherapy for NSCLC, ovarian and gastric cancer and in combination with bevacizumab in renal cell carcinoma. First, we demonstrated that CRLX101 is an inhibitor of HIF-1α. A single administration of CRLX101 inhibited HIF-1α protein in 7/8 different xenograft models tested, representing ovarian, lung, renal, colorectal and prostate tumor types. In the HCT-116 colon xenograft tumor, for example, HIF-1α was inhibited up to 90% for at least 7 days. Treatment by CRLX101 also decreased the mRNA of all HIF-1α-regulated genes analyzed, including VEGF, CA9, GLUT-1, GLUT-3 and CD31. Inhibition of these genes occurred only in tumors in which HIF-1α levels were inhibited. Second, combining CRLX101 with bevacizumab (BEV) demonstrated a synergistic effect on tumor growth inhibition and improved survival in the A2780 ovarian xenograft tumor model. Specifically, CRLX101 alone, at a sub-maximal dose of 5 mg/kg, produced 2/10 complete responses (CRs), BEV alone at 5 mg/kg produced 2/10 CRs and the combination of the two resulted in 6/9 CRs. BEV alone increased HIF-1α protein, consistent with BEV-induced hypoxia, whereas CRLX101 alone inhibited HIF-1α protein. When the two drugs were combined, inhibition of HIF-1α was significantly greater than CRLX101 monotherapy. The synergistic activity of CRLX101 in combination with bevacizumab suggests that CRLX101 may be able to overcome HIF-mediated acquired resistance to antiangiogenic drugs through inhibition of HIF-1α, in addition to its topoisomerase 1 inhibition. These findings support the use of CRLX101 in combination with antiangiogenic drugs as an exciting new paradigm for the treatment of cancer.

Citation Format: Douglas Lazarus, Christian Peters, Ketan Deotale, Scott Eliasof. CRLX101, a nanopharmaceutical in phase 2 clinical trials, is synergistic with antiangiogenic treatment through HIF-1α inhibition in an ovarian cancer xenograft model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 376. doi:10.1158/1538-7445.AM2013-376

Abstract 2419: A phase Ib-IIa study evaluating the nanopharmaceutical CRLX101 in combination with bevacizumab in the treatment of patients (pts.) with advanced renal cell carcinoma

Background:

Topoisomerase-1 (topo-1)-inhibiting agents demonstrate activity across multiple cancer types but use of these agents remains limited by insufficient tumor exposure and toxicity. CRLX101, a novel cyclodextrin-containing polymer conjugate of camptothecin (CPT) that self-assembles into nanoparticles, delivers sustained levels of active CPT into cancer cells while substantially reducing systemic exposure. In vitro and in vivo data suggest superior activity of CRLX101 compared to approved agents in multiple animal tumor models. A monotherapy maximum tolerated dose (MTD)/recommended phase 2 dose (RP2D) of 15 mg/m2 IV every 2 weeks (wks.) was reported previously and over 100 cancer pts. have now been treated with CRLX101 across three ongoing phase 2 clinical trials. To be presented separately at this conference, CRLX101 further appears to inhibit HIF1α, a hypoxia-induced transcription factor implicated in tumor angiogenesis, invasion, and metastasis. We hypothesize that observed synergistic activity between CRLX101 and the VEGF-inhibiting monoclonal antibody bevacizumab occurs as a result of overcoming HIF-mediated resistance to VEGF-inhibition. Clear cell renal cell carcinoma (ccRCC), accounting for approximately 85% of RCC, provides an ideal clinical setting in which to evaluate the potential of this drug-drug combination. The most commonly observed genetic abnormality in ccRCC results in higher intracellular levels of HIF1α. Bevacizumab was chosen as a combination partner here because of its proven activity in the treatment of RCC as well as a history of successful combination with chemotherapy partners including topo-1 inhibitors.

Methods:

This clinical trial examines the combination of bevacizumab and CRLX101 in the treatment of pts. with advanced metastatic RCC. The primary objective is to determine the RP2D of CRLX101 administered in ombination with bevacizumab. In a preliminary stage 1b, a standard 3+3 dose-escalation design is being employed and no fewer than 6 pts. will be evaluated at the MTD. A secondary objective is to evaluate progression-free survival (PFS). Based on a preliminary test for futility, the study will be terminated if ≤ 3 of the initial 12 pts. survive without progression to 16 wks. Otherwise, an additional 10 pts. will be enrolled for a total of 22 pts. and the combination will be viewed as active and worthy of further examination if the PFS rate at 16 wks. is ≥ 50%. Additional exploratory endpoints include a comparison of bone marrow histology and molecular features among pts. treated with 12 mg/m2 vs. 15 mg/m2 of CRLX101. Finally, tumor expression of CAIX, a well-studied surrogate for HIF1α activity, will be measured using 124I-cG250, a CAIX antibody, with real-time PET imaging employed to determine the degree of CRLX101-mediated HIF1α modulation.

Citation Format: Stephen M. Keefe, Roger B. Cohen, Scott Eliasof, Edward G. Garmey, Meliessa Hennessy, Kristine M. Mykulowicz, Daniel A. Pryma, Naomi Haas. A phase Ib-IIa study evaluating the nanopharmaceutical CRLX101 in combination with bevacizumab in the treatment of patients (pts.) with advanced renal cell carcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2419. doi:10.1158/1538-7445.AM2013-2419

Abstract C38: CRLX101 shows significant tumor penetration and superior activity compared to first- and second-line agents in NSCLC xenograft tumor models

Introduction: CRLX101 is a tumor-targeted nanopharmaceutical designed to localize and accumulate in tumors. A randomized Phase 2 study is ongoing evaluating safety and efficacy of CRLX101 as a second- and third-line single agent therapy for the treatment of advanced non-small cell lung cancer (NSCLC). CRLX101 has previously demonstrated broad spectrum anti-tumor activity in xenograft tumor models of lung, ovarian, colorectal, pancreatic, breast and lymphoma. Here we present efficacy data of CRLX101 in NSCLC tumor models representative of clinical subgroups with limited therapeutic options, including squamous cell, and KRAS and EGFR mutants. Additionally, we present microscopy data illustrating that CRLX101 penetrates tumor tissues and is taken up by cancer cells.

Experimental procedures: CRLX101 is composed of camptothecin, a highly potent topoisomerase 1 inhibitor, conjugated to a cyclodextrin-poly(ethylene glycol) co-polymer and self-assembles into nanoparticles of 20–50 nm in diameter. For in vivo efficacy, tumor-bearing mice were administered either CRLX101 or a comparator at their respective maximum-tolerated dose (MTD) and schedule, and tumor growth inhibition and survival were measured. For tumor localization studies, xenograft tumors were excised and CRLX101 was visualized with standard staining methods. Summary of data: In H1299 NSCLC model, CRLX101 showed superior activity over approved 1st and 2nd line agents resulting in 100% (10/10) cures. Gemcitabine and topotecan achieved 50% (5/10) or 10% (1/10) cures, respectively; and no cure (0/10) was observed with docetaxel, erlotinib or pemetrexed. In H520 squamous-type NSCLC model, CRLX101 showed a greater median survival and tumor growth delay than carboplatin, docetaxel or gemcitabine. Finally, CRLX101 showed significant tumor growth delay activity and survival improvements in G12C and G12S KRAS mutation models (H2122 and A549 respectively) and in a T790M EGFR mutation model (H1975). BrdU staining of tumor sections from H1299 NSCLC xenografts treated with CRLX101 demonstrated that the nanoparticle penetrated deep and uniformly in the tumor tissues and inhibited cell proliferation.

Conclusions: CRLX101 is a novel nanopharmaceutical that has shown encouraging activity in advanced NSCLC patients in a Phase 1/2b clinical trial. Our preclinical data suggests that CRLX101 may be efficacious in patient population harboring mutations and drug resistance that render them refractory to currently approved agents. Our imaging data further support that CRLX101 can potentially achieve superior anti-tumor activity by localizing and penetrating deep into tumor tissues and releasing camptothecin inside tumor cells. In sum, our pre-clinical and clinical data together support that CRLX101 is potentially a versatile therapeutic option for the management of advanced NSCLC patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C38.

CRLX101 (formerly IT-101)-A Novel Nanopharmaceutical of Camptothecin in Clinical Development

CRLX101 (formerly IT-101) is a first-in-class nanopharmaceutical, currently in Phase 2a development, which has been developed by covalently conjugating camptothecin (CPT) to a linear, cyclodextrin-polyethylene glycol (CD-PEG) co-polymer that self-assembles into nanoparticles. As a nanometer-scale drug carrier system, the cyclodextrin polymeric nanoparticle technology, referred to as “CDP”, has unique design features and capabilities. Specifically, CRLX101 preclinical and clinical data confirm that CDP can address not only solubility, formulation, toxicity, and pharmacokinetic challenges associated with administration of CPT, but more importantly, can impart unique biological properties that enhance CPT pharmacodynamics and efficacy.

Small molecules, big players: the National Cancer Institute's Initiative for Chemical Genetics

In 2002, the National Cancer Institute created the Initiative for Chemical Genetics (ICG), to enable public research using small molecules to accelerate the discovery of cancer-relevant small-molecule probes. The ICG is a public-access research facility consisting of a tightly integrated team of synthetic and analytical chemists, assay developers, high-throughput screening and automation engineers, computational scientists, and software developers. The ICG seeks to facilitate the cross-fertilization of synthetic chemistry and cancer biology by creating a research environment in which new scientific collaborations are possible. To date, the ICG has interacted with 76 biology laboratories from 39 institutions and more than a dozen organic synthetic chemistry laboratories around the country and in Canada. All chemistry and screening data are deposited into the ChemBank web site (http://chembank.broad.harvard.edu/) and are available to the entire research community within a year of generation. ChemBank is both a data repository and a data analysis environment, facilitating the exploration of chemical and biological information across many different assays and small molecules. This report outlines how the ICG functions, how researchers can take advantage of its screening, chemistry and informatic capabilities, and provides a brief summary of some of the many important research findings.

WAY-855 (3-amino-tricyclo[2.2.1.02.6]heptane-1,3-dicarboxylic acid): a novel, EAAT2-preferring, nonsubstrate inhibitor of high-affinity glutamate uptake

The pharmacological profile of a novel glutamate transport inhibitor, WAY-855 (3-amino-tricyclo[2.2.1.0(2.6)]heptane-1,3-dicarboxylic acid), on the activity of the human forebrain glutamate transporters EAAT1, EAAT2 and EAAT3 expressed in stable mammalian cell lines and in Xenopus laevis oocytes is presented. WAY-855 inhibited glutamate uptake mediated by all three subtypes in a concentration-dependent manner, with preferential inhibition of the CNS-predominant EAAT2 subtype in both cells and oocytes. IC50 values for EAAT2 and EAAT3 inhibition in cells were 2.2 and 24.5 microM, respectively, while EAAT1 activity was inhibited by 50% at 100 microM (IC50 values determined in oocytes were 1.3 microM (EAAT2), 52.5 microM (EAAT3) and 125.9 microM (EAAT1)). Application of WAY-855 to EAAT-expressing oocytes failed to induce a transporter current, and the compound failed to exchange with accumulated [3H]d-aspartate in synaptosomes consistent with a nonsubstrate inhibitor. WAY-855 inhibited d-aspartate uptake into cortical synaptosomes by a competitive mechanism, and with similar potency to that observed for the cloned EAAT2. WAY-855 failed to agonise or antagonise ionotropic glutamate receptors in cultured hippocampal neurones, or the human metabotropic glutamate receptor subtype 4 expressed in a stable cell line. WAY-855 represents a novel structure in glutamate transporter pharmacology, and exploration of this structure might provide insights into the discrimination between EAAT2 and other EAAT subtypes.

Sulfhydryl modification of V449C in the glutamate transporter EAAT1 abolishes substrate transport but not the substrate-gated anion conductance

Excitatory amino acid transporters (EAATs) buffer and remove synaptically released L-glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. Here, we demonstrate that modification of a cysteine substituted within a C-terminal domain of EAAT1 abolishes transport in both the forward and reverse directions without affecting activation of the anion conductance. EC(50)s for L-glutamate and sodium are significantly lower after modification, consistent with kinetic models of the transport cycle that link anion channel gating to an early step in substrate translocation. Also, decreasing the pH from 7.5 to 6.5 decreases the EC(50) for L-glutamate to activate the anion conductance, without affecting the EC(50) for the entire transport cycle. These findings demonstrate for the first time a structural separation of transport and the uncoupled anion flux. Moreover, they shed light on some controversial aspects of the EAAT transport cycle, including the kinetics of proton binding and anion conductance activation.

Pharmacological characterization of threo-3-methylglutamic acid with excitatory amino acid transporters in native and recombinant systems

The glutamate analog (+/-) threo-3-methylglutamate (T3MG) has recently been reported to inhibit the EAAT2 but not EAAT1 subtype of high-affinity, Na(+)-dependent excitatory amino acid transporter (EAAT). We have examined the effects of T3MG on glutamate-elicited currents mediated by EAATs 1-4 expressed in Xenopus oocytes and on the transport of radiolabeled substrate in mammalian cell lines expressing EAATs 1-3. T3MG was found to be an inhibitor of EAAT2 and EAAT4 but a weak inhibitor of EAAT1 and EAAT3. T3MG competitively inhibited uptake of D-[(3)H]-aspartate into both cortical and cerebellar synaptosomes with a similar potency, consistent with its inhibitory activity on the cloned EAAT2 and EAAT4 subtypes. In addition, T3MG produced substrate-like currents in oocytes expressing EAAT4 but not EAAT2. However, T3MG was unable to elicit heteroexchange of preloaded D-[(3)H]-aspartate in cerebellar synaptosomes, inconsistent with the behavior of a substrate inhibitor. Finally, T3MG acts as a poor ionotropic glutamate receptor agonist in cultured hippocampal neurons: concentrations greater than 100 microM T3MG were required to elicit significant NMDA receptor-mediated currents. Thus, T3MG represents a pharmacological tool for the study of not only the predominant EAAT2 subtype but also the EAAT4 subtype highly expressed in cerebellum.

Localization and function of five glutamate transporters cloned from the salamander retina

Glutamate is the major excitatory neurotransmitter in the vertebrate retina. Native glutamate transporters have been well characterized in several retinal neurons, particularly from the salamander retina. We have cloned five distinct glutamate transporters from the salamander retina and examined their localization and functional properties: sEAAT1, sEEAAT2A, sEAAT2B, sEAAT5A and sEAAT5B. sEAAT1 is a homologue of the glutamate transporter EAAT1 (GLAST), sEAAT2A and sEAAT2B are homologues of EAAT2 (GLT-1) and sEAAT5A and sEAAT5B are homologues of the recently cloned human retinal glutamate transporter EAAT5. Localization was determined by immunocytochemical techniques using antibodies directed at portions of the highly divergent carboxy terminal. Glutamate transporters were found in glial, photoreceptor, bipolar, amacrine and ganglion cells. The pharmacology and ionic dependence were determined by two-electrode voltage clamp recordings from Xenopus laevis oocytes which had previously been injected with one of the glutamate transporter mRNAs. Each of the transporters behaved in a manner consistent with a glutamate transporter and there were some distinguishing characteristics which make it possible to link the function in native cells with the behavior of the cloned transporters in this study.

Excitatory amino acid transporters of the salamander retina: identification, localization, and function

The rapid re-uptake of extracellular glutamate mediated by a family of high-affinity glutamate transporter proteins is essential to continued glutamatergic signaling and neuronal viability, but the contributions of individual transporter subtypes toward cellular physiology are poorly understood. Because the physiology of glutamate transport in the salamander retina has been well described, we have examined the expression and function of glutamate transporter subtypes in this preparation. cDNAs encoding five distinct salamander excitatory amino acid transporter (sEAAT) subtypes were isolated, and their molecular properties and distributions of expression were compared. We report evidence that at least four distinct sEAAT subtypes are expressed in glial (Müller) cells. In addition, four of the five transporter subtypes are localized in neurons throughout the retina. The brightest immunostaining was seen in the synaptic regions of the inner and outer plexiform layers and in the outer nuclear layer. Using electrophysiological measurements in the Xenopus oocyte expression system, we also examined the pharmacology and ionic dependence of the four expressing transporter subtypes that make it possible to distinguish, on the basis of functional behavior, among the various subtypes. Although no simple correlation between transporter subtype and retinal cell physiology can be made, the diverse population of sEAAT transporter subtypes with unique localization and functional properties indicates that glutamate transporters play a wide variety of roles in retinal function and are likely to underlie both the uptake of glutamate by Müller cells and the glutamate-elicited chloride conductance involved in signal transduction by photoreceptors and bipolar cells.

Excitatory amino acid transporter 5, a retinal glutamate transporter coupled to a chloride conductance

Although a glutamate-gated chloride conductance with the properties of a sodium-dependent glutamate transporter has been described in vertebrate retinal photoreceptors and bipolar cells, the molecular species underlying this conductance has not yet been identified. We now report the cloning and functional characterization of a human excitatory amino acid transporter, EAAT5, expressed primarily in retina. Although EAAT5 shares the structural homologies of the EAAT gene family, one novel feature of the EAAT5 sequence is a carboxy-terminal motif identified previously in N-methyl-D-aspartate receptors and potassium channels and shown to confer interactions with a family of synaptic proteins that promote ion channel clustering. Functional properties of EAAT5 were examined in the Xenopus oocyte expression system by measuring radiolabeled glutamate flux and two-electrode voltage clamp recording. EAAT5-mediated L-glutamate uptake is sodium- and voltage-dependent and chloride-independent. Transporter currents elicited by glutamate are also sodium- and voltage-dependent, but ion substitution experiments suggest that this current is largely carried by chloride ions. These properties of EAAT5 are similar to the glutamate-elicited chloride conductances previously described in retinal neurons, suggesting that the EAAT5-associated chloride conductance may participate in visual processing.

Rapid AMPA receptor desensitization in catfish cone horizontal cells

AMPA and NMDA type glutamate receptors were studied in isolated catfish cone horizontal cells using the whole-cell and outside-out patch-recording techniques. In whole-cell recordings, cyclothiazide (CTZ) enhanced the peak current in response to glutamate (in the presence of NMDA receptor antagonists). In patch recordings, currents evoked by rapid and maintained applications of glutamate desensitized with a time constant of one millisecond. CTZ blocked this rapid desensitization. Recovery from desensitization of the AMPA receptors was rapid, having a time constant of 8.65 ms. In contrast, the whole-cell and patch responses to applications of NMDA were much smaller than the AMPA receptor responses and did not desensitize. The relative contribution of these two receptor subtypes depends critically on the condition of the synapse; if glutamate levels are tonically present, the NMDA receptors contribute significantly to the postsynaptic response. If glutamate levels fall rapidly following the release of a single quantum of glutamate, then AMPA receptor currents will dominate the postsynaptic response.

Retinal glial cell glutamate transporter is coupled to an anionic conductance

Application of L-glutamate to retinal glial (Müller) cells results in an inwardly rectifying current due to the net influx of one positive charge per molecule of glutamate transported into the cell. However, at positive potentials an outward current can be elicited by glutamate. This outward current is eliminated by removal of external chloride ions. Substitution of external chloride with the anions thiocyanate, perchlorate, nitrate, and iodide, which are known to be more permeant at other chloride channels, results in a considerably larger glutamate-elicited outward current at positive potentials. The large outward current in external nitrate has the same ionic dependence, apparent affinity for L-glutamate, and pharmacology as the glutamate transporter previously reported to exist in these cells. Varying the concentration of external nitrate shifts the reversal potential in a manner consistent with a conductance permeable to nitrate. Together, these results suggest that the glutamate transporter in retinal glial cells is associated with an anionic conductance. This anionic conductance may be important for preventing a reduction in the rate of transport due the depolarization that would otherwise occur as a result of electrogenic glutamate uptake.

Characterization of the glutamate transporter in retinal cones of the tiger salamander

L-Glutamate elicits an inwardly rectifying current at hyperpolarized potentials in isolated retinal cones of the tiger salamander, as measured under whole-cell patch clamp. Evidence presented in this article supports the notion that cones possess a high-affinity glutamate transporter. This glutamate-elicited current shows no desensitization over a period of several minutes, and has an affinity (Km) of 10 microM. The inward current is mimicked by the amino acids L-aspartate, D-aspartate, L-cysteate, and to a lesser extent D-glutamate. It is neither blocked by the glutamate receptor antagonists kynurenic acid (1 mM), 6-cyano-7-nitroquinoxaline-2,3-dione (100 microM), or 2-amino-5-phosphonovalerate (100 microM), nor elicited by the glutamate receptor agonists (100 microM each) kainate, quisqualate, NMDA, or 2-amino-4-phosphonobutyrate. The glutamate-elicited current was reduced by the glutamate transport blockers dihydrokainate (DHKA), DL-threo-beta-hydroxyaspartate (beta HA), and L-trans-pyrrolidine-2,4-dicarboxylic acid. When glutamate was present on both sides of the membrane, the blockers reduced both uptake and release; the blocker-sensitive current as a function of membrane potential represents the transport current-voltage relation (I-V), and the reversal potential of the I-V represents the transporter equilibrium potential. This potential was a function of the equilibrium potential for glutamate. DHKA and beta HA depolarized horizontal cells in a retinal slice, and abolished their light responses, suggesting that in the absence of glutamate transport, glutamate concentrations in the cleft rise to a level that saturates the postsynaptic receptors. The high capacity of the cone glutamate transporter is well suited for the rapid removal of glutamate from the synaptic cleft required for the signaling of a light onset to postsynaptic cells.

Neural interactions mediating the detection of motion in the retina of the tiger salamander

The neural circuitry underlying movement detection was inferred from studies of amacrine cells under whole-cell patch clamp in retinal slices. Cells were identified by Lucifer yellow staining. Synaptic inputs were driven by "puffing" transmitter substances at the dendrites of presynaptic cells. Spatial sensitivity profiles for amacrine cells were measured by puffing transmitter substances along the lateral spread of their processes. Synaptic pathways were separated and identified with appropriate pre- and postsynaptic pharmacological blocking agents. Two distinct amacrine cell types were found: one with narrow spread of processes that received sustained excitatory synaptic current, the other with very wide spread of processes that received transient excitatory synaptic currents. The transient currents found only in the wide-field amacrine cell were formed presynaptically at GABAB receptors. They could be blocked with baclofen, a GABAB agonist, and their time course was extended by AVA, a GABAB antagonist. Baclofen and AVA had no direct affect upon the wide-field amacrine cell, but picrotoxin blocked a separate, direct GABA input to this cell. The narrow-field amacrine cell was shown to be GABAergic by counterstaining with anti-GABA antiserum after it was filled with Lucifer yellow. Its narrow, spatial profile and sustained synaptic input are properties that closely match those of the GABAergic antagonistic signal that forms transient activity (described above), suggesting that the narrow-field amacrine cell itself is the source of the GABAergic interaction mediating transient activity in the inner plexiform layer (IPL). Other work has shown a GABAB sensitivity at some bipolar terminals, suggesting a population of bipolars as the probable site of interaction mediating transient action. The results suggest that two local populations of amacrine cell types (sustained and transient) interact with the two populations of bipolar cell types (transient forming and nontransient forming). These interactions underlie the formation of the change-detecting subunits. We suggest that local populations of these subunits converge to form the receptive fields of movement-detecting ganglion cells.