Brief report: RRx-001 is a c-Myc inhibitor that targets cancer stem cells

Glioblastoma Phagocytic Cell Death: Balancing the Opportunities for Therapeutic Manipulation

Macrophages and microglia are professional phagocytes that sense and migrate toward "eat-me" signals. The role of phagocytic cells is to maintain homeostasis by engulfing senescent or apoptotic cells, debris, and abnormally aggregated macromolecules. Usually, dying cells send out "find-me" signals, facilitating the recruitment of phagocytes. Healthy cells can also promote or inhibit the phagocytosis phenomenon of macrophages and microglia by tuning the balance between "eat-me" and "don't-eat-me" signals at different stages in their lifespan, while the "don't-eat-me" signals are often hijacked by tumor cells as a mechanism of immune evasion. Using a combination of bioinformatic analysis and spatial profiling, we delineate the balance of the "don't-eat-me" CD47/SIRPα and "eat-me" CALR/STC1 ligand-receptor interactions to guide therapeutic strategies that are being developed for glioblastoma sequestered in the central nervous system (CNS).

RRx-001: a chimeric triple action NLRP3 inhibitor, Nrf2 inducer, and nitric oxide superagonist

RRx-001 is a shape shifting small molecule with Fast Track designation for the prevention/amelioration of chemoradiation-induced severe oral mucositis (SOM) in newly diagnosed Head and Neck cancer. It has been intentionally developed or "engineered" as a chimeric single molecular entity that targets multiple redox-based mechanisms. Like an antibody drug conjugate (ADC), RRx-001 contains, at one end a "targeting" moiety, which binds to the NLRP3 inflammasome and inhibits it as well as Kelch-like ECH-associated protein 1 (KEAP1), the negative regulator of Nrf2, and, at the other end, a conformationally constrained, dinitro containing 4 membered ring, which fragments under conditions of hypoxia and reduction to release therapeutically active metabolites i.e., the payload. This "payload", which is delivered specifically to hypoperfused and inflamed areas, includes nitric oxide, nitric oxide related species and carbon-centered radicals. As observed with ADCs, RRx-001 contains a backbone amide "linker" attached to a binding site, which correlates with the F_ab region of an antibody, and to the dinitroazetidine payload, which is microenvironmentally activated. However, unlike ADCs, whose large size impacts their pharmacokinetic properties, RRx-001 is a nonpolar small molecule that easily crosses cell membranes and the blood brain barrier (BBB) and distributes systemically. This short review is organized around the de novo design and in vivo pro-oxidant/pro-inflammatory and antioxidant/anti-inflammatory activity of RRx-001, which, in turn, depends on the reduced to oxidized glutathione ratio and the oxygenation status of tissues.

A Review of RRx-001: A Late-Stage Multi-Indication Inhibitor of NLRP3 Activation and Chronic Inflammation

Chronic unresolving inflammation is emerging as a key underlying pathological feature of many if not most diseases ranging from autoimmune conditions to cardiometabolic and neurological disorders. Dysregulated immune and inflammasome activation is thought to be the central driver of unresolving inflammation, which in some ways provides a unified theory of disease pathology and progression. Inflammasomes are a group of large cytosolic protein complexes that, in response to infection- or stress-associated stimuli, oligomerize and assemble to generate a platform for driving inflammation. This occurs through proteolytic activation of caspase-1-mediated inflammatory responses, including cleavage and secretion of the proinflammatory cytokines interleukin (IL)-1β and IL-18, and initiation of pyroptosis, an inflammatory form of cell death. Several inflammasomes have been characterized. The most well-studied is the nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome, so named because the NLRP3 protein in the complex, which is primarily present in immune and inflammatory cells following activation by inflammatory stimuli, belongs to the family of nucleotide-binding and oligomerization domain (Nod) receptor proteins. Several NLRP3 inflammasome inhibitors are in development, all with multi-indication activity. This review discusses the current status, known mechanisms of action, and disease-modifying therapeutic potential of RRx-001, a direct NLRP3 inflammasome inhibitor under investigation in several late-stage anticancer clinical trials, including a phase 3 trial for the treatment of third-line and beyond small cell lung cancer (SCLC), an indication with no treatment, in which RRx-001 is combined with reintroduced chemotherapy from the first line, carboplatin/cisplatin and etoposide (ClinicalTrials.gov Identifier: NCT03699956). Studies from multiple independent groups have now confirmed that RRx-001 is safe and well tolerated in humans. Additionally, emerging evidence in preclinical animal models suggests that RRx-001 could be effective in a wide range of diseases where immune and inflammasome activation drives disease pathology.

ROCKET: Phase II Randomized, Active-controlled, Multicenter Trial to Assess the Safety and Efficacy of RRx-001 + Irinotecan vs. Single-agent Regorafenib in Third/Fourth Line Colorectal Cancer

INTRODUCTION

RRx-001 is a novel cysteine-targeted alkylating agent that releases nitric oxide (NO). The primary biological activities of this hybrid molecule include macrophage repolarizing and vascular normalization. The purpose of this clinical trial (ROCKET) (NCT02096354) was to compare the safety and efficacy of the combination therapy RRx-001 + irinotecan vs. regorafenib in third/fourth line colorectal cancer that previously received treatment with irinotecan.

PATIENTS AND METHODS

A total of 34 patients were randomized (24 to RRx-001 + irinotecan (RxI) and 10 to single-agent regorafenib (RegI)) and were the basis for the intention-to-treat analysis (ITT, comprising all 34 patients). RRx-001 treatment was administered as an up-to-2-month "primer" followed by irinotecan for patients randomized to the RRx-001 arm (24). The efficacy and safety data are presented for the 34 patients in the (ITT) efficacy analysis. Therapy consisted of intravenous administration of RRx-001 at 4 mg once weekly for up to 2 months, at which point RRx-001 was discontinued, followed by intravenous infusion of irinotecan at 180 mg/m² on day 1 in a 21-day cycle vs. 160 mg oral regorafenib daily for 3/4 weeks followed at progression, if applicable, by irinotecan 180 mg/m² on day 1 in a 21-day cycle. There were 3 patients (3/24 = 12.5%) with prior single agent irinotecan on the RRx-001 randomized arm and 2 (2/10 = 20%) on the regorafenib randomized arm. Numerous patients had irinotecan combination therapies prior to randomized treatment. There were 15 patients on RRx-001 arm that received irinotecan post-RRx-001 in the randomized trial. There were 5 PRs on RRx-001 plus irinotecan leading to an overall response of 20.8% (5/24). There were 37.5% (9/24) of RRx-001 randomized patients with KRAS mutant type while 60% (6/10) regorafenib randomized patients were of KRAS type mutant. There were only 4 patients with available QOL and Edmonton Symptom Assessment System, an insufficient sample size to allow for any meaningful analysis.

RESULTS

Median patient follow-up was approximately 14.5 months (SD 4.5 months). Median overall survival was 8.6 months for RxI and 4.7 months for RegI. Median progression free survival was 6.1 months for RxI vs. 1.7 months for RegI (a statistically significant result, 2-sided log-rank test, P = .0030). The toxicity profile of RxI was substantially improved compared with RegI.

CONCLUSION

The results of this trial demonstrate improved efficacy of RxI compared with RegI in patients with metastatic colorectal cancer after previous treatment with irinotecan, and late-stage clinical development in this indication is planned on the strength of the observed "signal" accompanied by a sufficient safety profile.

G6PD promotes cell proliferation and dexamethasone resistance in multiple myeloma via increasing anti-oxidant production and activating Wnt/β-catenin pathway

Background

Glucose-6-phosphate dehydrogenase (G6PD) as the rate-limiting enzyme in the pentose phosphate pathway (PPP) is well-established as an aberrantly expressed protein in numerous clinical diseases; however, its role in cancer, specifically in multiple myeloma (MM) remains elusive.

Methods

In this study, serum metabolites in 70 normal people and 70 newly diagnosed MM patients were analyzed using untargeted metabolomics and the results were verified using ELISA. The survival analysis of multiple clinical datasets was performed to identify a potential target gene in MM. The oncogenic role of G6PD was investigated using lentivirus-based overexpression or knockdown of G6PD using RNAi or an inhibitor in vitro, and in a xenograft mouse model in vivo. The mechanisms of induced Dexamethasone (Dexa)-resistance of G6PD were further explored using the above established MM cell lines in vitro.

Results

Based on the screening of potential genes, PPP was shown to be involved in the occurrence of MM, which was evidenced by the differential expression of serum metabolites of G6P and Dehydroepiandrosterone sulfate (DHEAS, the more stable sulfate ester form of an endogenously uncompetitive G6PD inhibitor known as DHEA). Elevated G6PD promoted MM cell proliferation. Mechanistically, high G6PD expression enhanced enzymatic generation of the antioxidant NADPH via the PPP and decreased the production of reactive oxygen species (ROS), thus inducing the proliferation and Dexa resistance in MM cells. Furthermore, canonical Wnt/β-catenin signaling also participated in regulating G6PD-induced drug resistance and cellular redox levels of ROS. Intriguingly, DHEA treatment could enhance the sensitivity of MM cells to Dexa primarily through augmenting cellular oxidative stress.

Conclusions

Our data demonstrate that G6PD enhances the generation of the enzymatic anti-oxidant NADPH and decreases ROS generation, thereby promoting resistance to Dexa-induced apoptosis via the enzymatic PPP and non-enzymatic Wnt/β-catenin signaling pathway in MM. Targeting G6PD to harness cellular redox may serve as a promising novel strategy for the management of MM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40164-022-00326-6.

BRD4 Targets the KEAP1-Nrf2-G6PD Axis and Suppresses Redox Metabolism in Small Cell Lung Cancer

Accumulating evidence has witnessed the Kelch-like ECH-associated protein 1(KEAP1)- nuclear factor (erythroid-derived 2)-like 2 (Nrf2) axis is the main regulatory factor of cell resistance to endogenous and exogenous oxidative assaults. However, there are few studies addressing the upstream regulatory factors of KEAP1. Herein, bioinformatic analysis suggests bromodomain-containing protein 4 (BRD4) as a potential top transcriptional regulator of KEAP1 in lung cancer. Using molecular and pharmacological approaches, we then discovered that BRD4 can directly bind to the promoter of KEAP1 to activate its transcription and down-regulate the stability of Nrf2 which in turn transcriptionally suppresses glucose-6-phosphate dehydrogenase (G6PD) in small cell lung cancer (SCLC), a highly proliferative and aggressive disease with limited treatment options. In addition, BRD4 could associate with the Nrf2 protein in a non-KEAP1-dependent manner to inhibit Nrf2 activity. Furthermore, simultaneous application of JQ1 and ATRA or RRx-001 yielded synergistic inhibition both in vitro and in vivo. These data suggest metabolic reprogramming by JQ1 treatment improves cell resistance to oxidative stress and might be a resistance mechanism to bromodomain and extra-terminal domain (BET) inhibition therapy. Altogether, our findings provide novel insight into the transcriptional regulatory network of BRD4 and KEAP1 and transcriptional regulation of the pentose phosphate pathway in SCLC.

The Multiple Roles of Glucose-6-Phosphate Dehydrogenase in Tumorigenesis and Cancer Chemoresistance

The pentose phosphate pathway (PPP) is a branch from glycolysis that begins from glucose-6-phosphate (G6P) and ends up with fructose-6-phosphate (F6P) and glyceraldehyde-3-phosphate (GADP). Its primary physiological significance is to provide nicotinamide adenine dinucleotide phosphate (NADPH) and nucleotides for vital activities such as reactive oxygen species (ROS) defense and DNA synthesis. Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping protein with 514 amino acids that is also the rate-limiting enzyme of PPP, catalyzing G6P into 6-phosphogluconolactone (6PGL) and producing the first NADPH of this pathway. Increasing evidence indicates that G6PD is upregulated in diverse cancers, and this dysfunction influences DNA synthesis, DNA repair, cell cycle regulation and redox homeostasis, which provides advantageous conditions for cancer cell growth, epithelial-mesenchymal transition (EMT), invasion, metastasis and chemoresistance. Thus, targeting G6PD by inhibitors has been shown as a promising strategy in treating cancer and reversing chemotherapeutic resistance. In this review, we will summarize the existing knowledge concerning G6PD and discuss its role, regulation and inhibitors in cancer development and chemotherapy resistance.

MYC inhibitors in multiple myeloma

The importance of MYC function in cancer was discovered in the late 1970s when the sequence of the avian retrovirus that causes myelocytic leukemia was identified. Since then, over 40 years of unceasing research have highlighted the significance of this protein in malignant transformation, especially in hematologic diseases. Indeed, some of the earliest connections among the higher expression of proto-oncogenes (such as MYC), genetic rearrangements and their relation to cancer development were made in Burkitt lymphoma, chronic myeloid leukemia and mouse plasmacytomas. Multiple myeloma (MM), in particular, is a plasma cell malignancy strictly associated with MYC deregulation, suggesting that therapeutic strategies against it would be beneficial in treating this disease. However, targeting MYC was - and, somehow, still is - challenging due to its unique properties: lack of defined three-dimensional structure, nuclear localization and absence of a targetable enzymatic pocket. Despite these difficulties, however, many studies have shown the potential therapeutic impact of direct or indirect MYC inhibition. Different molecules have been tested, in fact, in the context of MM. In this review, we summarize the current status of the different compounds, including the results of their clinical testing, and propose to continue with the efforts to identify, repurpose, redesign or improve drug candidates to combine them with standard of care therapies to overcome resistance and enable better management of myeloma treatment.

Discovery of RRx-001, a Myc and CD47 Downregulating Small Molecule with Tumor Targeted Cytotoxicity and Healthy Tissue Cytoprotective Properties in Clinical Development

After extensive screening of aerospace compounds in an effort to source a novel anticancer agent, RRx-001, a first-in-class dinitroazetidine small molecule, was selected for advancement into preclinical and clinical development. RRx-001 is a minimally toxic small molecule with a distinct chemical structure and mechanism of action. The paradox of RRx-001 is that it mediates both antitumor cytotoxicity and normal tissue protection. The question of exactly how RRx-001 does this, and by means of what mechanism(s), depending on the route of delivery, intravenous or intratumoral, are explored. RRx-001 is currently in phase 2 and 3 clinical trials for the treatment of multiple solid tumor malignancies and as a supportive care drug.

Results from a biomarker study to accompany a phase II trial of RRx-001 with reintroduced platinum-based chemotherapy in relapsed small cell carcinoma

Background: In a Phase II study RRx-001 was combined with Etoposide platinum (EP) in previously platinum treated SCLC. We correlated expression of the M2 marker, CD206, on HLA-DR^low/- monocytes, a phenotype that correlates with a poor prognosis, with response to RRx-001. Research design and methods: Patients received 4 mg RRx-001 once weekly until progression followed by the start of EP (etoposide 100 mg/m² IV on days 1-3 of a 21-day cycle and either cisplatin 80 mg/m² IV on day 1 or carboplatin AUC 5-6 IV on day 1). Treatment continued until progression or intolerable toxicity. Peripheral blood was collected in Cell Preparation Tubes with sodium citrate from 14 patients for exploratory studies during screening and after therapy on Days 1, 8, and 15. Peripheral blood mononuclear cells (PBMCs) were isolated from blood by centrifugation and multiparameter flow cytometric analysis was performed. Results: CD206 expression on HLA-DR^low/- monocytes was associated with response to chemotherapy and overall survival. Conclusion: During treatment with RRx-001, reduced expression of the protumorigenic M2 marker CD206 on peripheral monocytes positively correlated with increased response and survival.

In small cell lung cancer patients treated with RRx-001, a downregulator of CD47, decreased expression of PD-L1 on circulating tumor cells significantly correlates with clinical benefit

Background

Small cell lung cancer (SCLC) is the most aggressive lung tumor, characterized by a rapid doubling time and the development of widespread metastases, for which immune checkpoint inhibitors have been approved to overcome T cell anergy. In light of its dismal prognosis, and lack of curative options, new therapies for extensive-disease SCLC are desperately needed.

Methods

RRx-001 is a small molecule Myc inhibitor and down-regulates CD47 expression on tumor cells. We evaluated the programmed death-ligand 1 (PD-L1) status of circulating tumor cells (CTCs) pre and post RRx-001 treatment in a phase 2 clinical trial, called QUADRUPLE THREAT, where patients with previously treated SCLC received RRx-001 in combination with a platinum doublet. The trial was registered with ClinicalTrials.gov, number NCT02489903. Fourteen patients with SCLC were analyzed to investigate the association between clinical outcome and PD-L1 expression on CTCs pre and post RRx-001. The correlation between the binary clinical outcome (clinical benefit vs. progressive disease) and the change of PD-L1 expression on CTCs after RRx-001 was analyzed using a logistic regression adjusting for baseline PD-L1 expression.

Results

The logistic model McFadden goodness of fit score was 0.477. The logistic model analyzing the association between decreased PD-L1 expression on CTCs after RRx-001 and response to reintroduced platinum doublet had an approximate 92.8% accuracy in its prediction of clinical benefit. The estimated receiver operating characteristic (ROC) displayed a ROC area under the curve (AUC) of 0.93 (95% confidence interval, 0.78–0.99).

Conclusions

These results suggest that PD-L1 expression on CTCs decreased after RRx-001 was significantly correlated with response to reintroduced platinum-based doublet therapy. Monitoring PD-L1 expression on CTCs during RRx-001 treatment may serve as a biomarker to predict response to RRx-001-based cancer therapy.

The development of small-molecule inhibitors targeting CD47

Immunotherapy has become an indispensable part of cancer treatment. A pivotal phagocytosis checkpoint, named cluster of differentiation 47 (CD47), which functions as 'don't eat me' signal to protect cells from phagocytosis upon interaction with signal regulatory protein alpha (SIRPα) on macrophages, has recently attracted much attention. Numerous antibodies targeting the CD47/SIRPα axis have shown encouraging efficacy in clinical trials. Meanwhile, studies on small-molecule inhibitors that interfere with CD47/SIRPα interaction or regulate CD47 expression are also in full swing. In this review, we summarize the small-molecule inhibitors interrupting the binding of CD47/SIRPα and regulating CD47 at the transcriptional, translational, and post-translational modification (PTM) levels. We provide perspectives and strategies for targeting the CD47/SIRPα phagocytosis checkpoint.

Just eat it: A review of CD47 and SIRP-α antagonism

The mammalian immune system consists of two distinct arms, nonspecific innate and more specific adaptive, with the innate immune response as the first line of defense and protection, which primes and amplifies subsequent adaptive responses. On the basis of this binary immune interplay, stimulation of T cells through checkpoint inhibitors (CIs), which bypasses innate involvement, seems likely to engender suboptimal or incomplete anticancer immunity, given that the successful induction of effect or responses depends on two-way innate/adaptive coordination. Indeed, the majority of patients-70%-80%, do not respond to CIs, which is potentially problematic if access to more optimal standard therapies is withheld or delayed in favor of ineffective or only marginally effective anti-PD-1/PD-L1 treatment. Therefore, stimulation of the innate immune response in combination with CIs (or other inducers of T cell cytotoxicity) has the potential to make the immune system "whole" and thereby to enhance and broaden the anti-tumor activity of PD-1/PD-L1 inhibitors for example, in relatively nonimmunogenic or "cold" tumor types. A critical innate macrophage immune checkpoint and druggable target is the antiphagocytic and "marker of self" CD47-SIRPα pathway, which is co-opted by cancer cells to mediate escape from immune-mediated clearance and checkpoint inhibition. This review summarizes the status of key CD47 antagonists in clinical trials, including the biologics, Hu5F9-G4 (5F9), TTI-621, and ALX148, as well as the small molecule, RRx-001, now in a Phase 3 clinical trial, which has not been previously included in CD47-SIRPα reviews focused on biologics. Hu5F9-G4 (5F9), TTI-621, ALX148, and RRx-001 are chosen as compounds with potentially promising data that have advanced the farthest in clinical development.

YY1 regulates cancer cell immune resistance by modulating PD-L1 expression

Recent advances in the treatment of various cancers have resulted in the adaptation of several novel immunotherapeutic strategies. Notably, the recent intervention through immune checkpoint inhibitors has resulted in significant clinical responses and prolongation of survival in patients with several therapy-resistant cancers (melanoma, lung, bladder, etc.). This intervention was mediated by various antibodies directed against inhibitory receptors expressed on cytotoxic T-cells or against corresponding ligands expressed on tumor cells and other cells in the tumor microenvironment (TME). However, the clinical responses were only observed in a subset of the treated patients; it was not clear why the remaining patients did not respond to checkpoint inhibitor therapies. One hypothesis stated that the levels of PD-L1 expression correlated with poor clinical responses to cell-mediated anti-tumor immunotherapy. Hence, exploring the underlying mechanisms that regulate PD-L1 expression on tumor cells is one approach to target such mechanisms to reduce PD-L1 expression and, therefore, sensitize the resistant tumor cells to respond to PD-1/PD-L1 antibody treatments. Various investigations revealed that the overexpression of the transcription factor Yin Yang 1 (YY1) in most cancers is involved in the regulation of tumor cells' resistance to cell-mediated immunotherapies. We, therefore, hypothesized that the role of YY1 in cancer immune resistance may be correlated with PD-L1 overexpression on cancer cells. This hypothesis was investigated and analysis of the reported literature revealed that several signaling crosstalk pathways exist between the regulations of both YY1 and PD-L1 expressions. Such pathways include p53, miR34a, STAT3, NF-kB, PI3K/AKT/mTOR, c-Myc, and COX-2. Noteworthy, many clinical and pre-clinical drugs have been utilized to target these above pathways in various cancers independent of their roles in the regulation of PD-L1 expression. Therefore, the direct inhibition of YY1 and/or the use of the above targeted drugs in combination with checkpoint inhibitors should result in enhancing the cell-mediated anti-tumor cell response and also reverse the resistance observed with the use of checkpoint inhibitors alone.

RRx-001 Acts as a Dual Small Molecule Checkpoint Inhibitor by Downregulating CD47 on Cancer Cells and SIRP-α on Monocytes/Macrophages

RRx-001 is a pleiotropic anticancer agent in phase III clinical trials, which polarizes tumor-associated macrophages from a low phagocytic M2 phenotype to a high phagocytic M1 phenotype. One of the ways in which tumors promote M2 polarization and evade macrophage-mediated destruction is through upregulation of CD47 expression. As a myeloid-specific immune checkpoint, CD47 interacts with signal-regulatory protein alpha (SIRPα) on macrophages and monocytes to prevent phagocytosis. Herein, the effect of RRx-001 on CD47 and SIRPα expression was evaluated as well as its activity in vivo in macrophage-depleted tumors. In vitro, RRx-001 was found to decrease the expression levels of CD47 and SIRPα on tumor cells and monocytes/macrophages, respectively, reducing the phagocytosis inhibitory function of the CD47/SIRPα interaction. In vivo, macrophage depletion by clodronate in an A549 xenograft-bearing mouse model attenuated the ability of RRx-001 to suppress tumor growth, which suggests that the presence of infiltrated macrophages in the tumor microenvironment is a sine qua non condition for the antitumor activity of RRx-001. Furthermore, these in vitro effects translate into significant antitumor activity in mouse models of lung cancer. Importantly, unlike anti-CD47 antibodies, RRx-001, which has been evaluated in close to 300 patients in 9 clinical trials, is not associated with any hematologic toxicities. On the basis of demonstrated antitumor activity and minimal toxicity in phase II clinical trials, RRx-001 has received clearance from the FDA and the EMA for phase III, multicenter studies in subjects with relapsed/refractory solid tumors.