Small Molecule Regulators Targeting NAD+ Biosynthetic Enzymes

Nicotinamide adenine dinucleotide (NAD+) is a key player in many metabolic pathways as an activated carrier of electrons. In addition to being the cofactor for redox reactions, NAD+ also serves as the substrate for various enzymatic transformations such as adenylation and ADP-ribosylation. Maintaining cellular NAD+ homeostasis has been suggested as an effective anti-aging strategy. Given the importance of NAD+ in regulating a broad spectrum of cellular events, small molecules targeting NAD+ metabolism have been pursued as therapeutic interventions for the treatment of mitochondrial disorders and agerelated diseases. In this article, small molecule regulators of NAD+ biosynthetic enzymes will be reviewed. The focus will be given to the discovery and development of these molecules, the mechanism of action as well as their therapeutic potentials.

The footprint of kynurenine pathway in every cancer: a new target for chemotherapy

Treatment of cancers has always been a challenge for physicians. Typically, several groups of anti-cancer medications are needed for effective management of an invasive and metastatic cancer. Recently, therapeutic potentiation of immune system markedly improved treatment of cancers. Kynurenine pathway has an interwoven correlation with immune system. Kynurenine promotes T Reg (regulatory) differentiation, which leads to increased production of anti-inflammatory cytokines and suppression of cytotoxic activity of T cells. Overactivation of kynurenine pathway in cancers provides an immunologically susceptible microenvironment for mutant cells to survive and invade surrounding tissues. Interestingly, kynurenine pathway vigorously interacts with other molecular pathways involved in tumorigenesis. For instance, kynurenine pathway interacts with phospoinosisitide-3 kinase (PI3K), extracellular signal-regulated kinase (ERK), Wnt/β-catenin, P53, bridging integrator 1 (BIN-1), cyclooxygenase 2 (COX-2), cyclin-dependent kinase (CDK) and collagen type XII α1 chain (COL12A1). Overactivation of kynurenine pathway, particularly overactivation of indoleamine 2,3-dioxygenase (IDO) predicts poor prognosis of several cancers such as gastrointestinal cancers, gynecological cancers, hematologic malignancies, breast cancer, lung cancer, glioma, melanoma, prostate cancer and pancreatic cancer. Furthermore, kynurenine increases the invasion, metastasis and chemoresistance of cancer cells. Recently, IDO inhibitors entered clinical trials and successfully passed their safety tests and showed promising therapeutic efficacy for cancers such as melanoma, brain cancer, renal cell carcinoma, prostate cancer and pancreatic cancer. However, a phase III trial of epacadostat, an IDO inhibitor, could not increase the efficacy of treatment with pembrolizumab for melanoma. In this review the expanding knowledge towards kynurenine pathway and its application in each cancer is discussed separately.

Prognostic value of indoleamine 2,3 dioxygenase in patients with higher-risk myelodysplastic syndromes treated with azacytidine

Hypomethylating agents (HMAs) are widely used in patients with higher-risk myelodysplastic syndromes (MDS) not eligible for stem cell transplantation; however, the response rate is <50%. Reliable predictors of response are still missing, and it is a major challenge to develop new treatment strategies. One current approach is the combination of azacytidine (AZA) with checkpoint inhibitors; however, the potential benefit of targeting the immunomodulator indoleamine-2,3-dioxygenase (IDO-1) has not yet been evaluated. We observed moderate to strong IDO-1 expression in 37% of patients with high-risk MDS. IDO-1 positivity was predictive of treatment failure and shorter overall survival. Moreover, IDO-1 positivity correlated inversely with the number of infiltrating CD8⁺ T cells, and IDO-1+ patients failed to show an increase in CD8⁺ T cells under AZA treatment. In vitro experiments confirmed tryptophan catabolism and depletion of CD8⁺ T cells in IDO-1+ MDS, suggesting that IDO-1 expression induces an immunosuppressive microenvironment in MDS, thereby leading to treatment failure under AZA treatment. In conclusion, IDO-1 is expressed in more than one-third of patients with higher-risk MDS, and is predictive of treatment failure and shorter overall survival. Therefore, IDO-1 is emerging as a promising predictor and therapeutic target, especially for combination therapies with HMAs or checkpoint inhibitors.

Dual functional immunostimulatory polymeric prodrug carrier with pendent indoximod for enhanced cancer immunochemotherapy

Immunotherapy based on checkpoint blockade has been regarded as one of the most promising approaches towards many types of cancers. However, low response rate hinders its application due to insufficient tumor immunogenicity and immunosuppressive tumor microenvironment. To achieve an overall enhanced therapeutic outcome, we developed a dual-functional immuno-stimulatory polymeric prodrug carrier modified with pendent indoximod, an indoleamine 2,3-dioxygenase (IDO) inhibitor that can be used to reverse immune suppression, for co-delivery of Doxorubicin (Dox), a hydrophobic anticancer agent that can promote immunogenic cell death (ICD) and elicit antitumor immunity. The resulted carrier denoted as POEG-b-PVBIND, consisting of poly (oligo (ethylene glycol) methacrylate) (POEG) hydrophilic blocks and indoximod conjugated hydrophobic blocks, is rationally designed to improve immunotherapy by synergistically modulating the tumor microenvironment (TME). Our data showed that Dox-triggered ICD promoted intra-tumoral infiltration of CD8+ T cells and IFN-γ-production by CD8+ T cells. Meanwhile, cleaved indoximod significantly increased CD8+ T cell infiltration while reducing the immunosuppressive T regulatory cells (Tregs). More importantly, Dox/POEG-b-PVBIND micelles led to significantly improved tumor regression in an orthotopic murine breast cancer model compared to both Dox-loaded POEG-b-PVB micelles (a control inert carrier) and POEG-b-PVBIND micelles alone, confirming combination effect of indoximod and Dox in improving the overall antitumor activity. STATEMENT OF SIGNIFICANCE: Indoleamine 2,3-dioxygenase (IDO) is an enzyme that can induce immune suppressive microenvironment in tumors. As a well-studied IDO inhibitor, indoximod (IND) represents a promising agent for cancer immunotherapy and could be particularly useful in combination with other chemotherapeutic agents. However, three major problems hinder its application: (1) IND is barely soluble in water; (2) IND delivery efficiency is limited (3) simultaneous delivery of two agents into tumor site is still challenging. Currently, most reports largely focus on improving the pharmacokinetic profile of IND alone via different formulations such as IND prodrug and IND nanocrystal. However, there is limited information about IND based co-delivery systems, especially for delivering hydrophobic chemotherapeutic agents. Here, we developed a new dual-functional polymeric prodrug carrier modified with a number of pendent IND units (denoted as POEG-b-PVBIND). POEG-b-PVBIND shows immunostimulatory and antitumor activities by itself. More importantly, POEG-b-PVBIND polymer is able to self-assemble into nano-sized micelles that are highly effective in formulating and codelivering other hydrophobic agents including doxorubicin (Dox), sunitinib (Sun), and daunorubicin (Dau), which can elicit antitumor immunity via promoting immunogenic cell death (ICD). We have shown that our new combination therapy led to a significantly improved antitumor activity in an aggressive murine breast cancer model (4T1.2).

Immune checkpoint blockade and its combination therapy with small-molecule inhibitors for cancer treatment

Initially understood for its physiological maintenance of self-tolerance, the immune checkpoint molecule has recently been recognized as a promising anti-cancer target. There has been considerable interest in the biology and the action mechanism of the immune checkpoint therapy, and their incorporation with other therapeutic regimens. Recently the small-molecule inhibitor (SMI) has been identified as an attractive combination partner for immune checkpoint inhibitors (ICIs) and is becoming a novel direction for the field of combination drug design. In this review, we provide a systematic discussion of the biology and function of major immune checkpoint molecules, and their interactions with corresponding targeting agents. With both preclinical studies and clinical trials, we especially highlight the ICI + SMI combination, with its recent advances as well as its application challenges.

Advanced stage melanoma therapies: Detailing the present and exploring the future

Metastatic melanoma therapies have proliferated over the last ten years. Prior to this, decades passed with only very few drugs available to offer our patients, and even then, those few drugs had minimal survival benefits. Many treatment options emerged over the last ten years with diverse mechanisms of action. Further, combination regimens have demonstrated superiority over monotherapy, especially for targeted agents. Each therapeutic combination possesses different advantages and side effect profiles. In this review, we outline the United States Food and Drug Administration-approved melanoma treatment agents and therapies currently in clinical development, focusing on combination approaches.

LW106, a novel indoleamine 2,3-dioxygenase 1 inhibitor, suppresses tumour progression by limiting stroma-immune crosstalk and cancer stem cell enrichment in tumour micro-environment

BACKGROUND AND PURPOSE

Indoleamine 2,3-dioxygenase 1 (IDO1) is emerging as an important new therapeutic target for treatment of malignant tumours characterized by dysregulated tryptophan metabolism. However, the antitumour efficacy of existing small-molecule inhibitors of IDO1 is still unsatisfactory and the underlying mechanism remains largely undefined. Hence, we discovered a novel potent small-molecule inhibitor of IDO1, LW106, and studied its antitumour effects and the underlying mechanisms in two tumour models.

EXPERIMENTAL APPROACH

C57BL6 mice, athymic nude mice or Ido1^-/- mice were inoculated with IDO1-expressing and -nonexpressing tumour cells and treated with vehicle, epacadostat or increasing doses of LW106. Xenografted tumours, plasma, spleens and other vital organs were harvested and subjected to kynurenine/tryptophan measurement and flow cytometric, histological and immunohistochemical analyses.

KEY RESULTS

LW106 dose-dependently inhibited the outgrowth of xenografted tumours that were inoculated in C57BL6 mice but not nude mice or Ido1^-/- mice, showing a stronger antitumour efficacy than epacadostat, an existing IDO1 inhibitor. LW106 substantially elevated intratumoural infiltration of proliferative T_eff cells, while reducing recruitment of proliferative T_reg cells and non-haematopoietic stromal cells such as endothelial cells and cancer-associated fibroblasts. LW106 treatment resulted in a reduced subpopulation of cancer stem cells (CSCs) in xenografted tumours in which fewer proliferative/invasive tumour cells and more apoptotic tumour cells were observed.

CONCLUSIONS AND IMPLICATIONS

LW106 inhibits tumour outgrowth by limiting stroma-immune crosstalk and CSC enrichment in the tumour micro-environment. LW106 has potential as a immunotherapeutic agent for use in combination with immune checkpoint inhibitors and (or) chemotherapeutic drugs for cancer treatment.

How Cancers Escape Immune Destruction and Mechanisms of Action for the New Significantly Active Immune Therapies: Helping Nonimmunologists Decipher Recent Advances

With the Food and Drug Administration and other worldwide regulatory authorities' approval of ipilimumab (Yervoy), sipuleucel-T (Provenge), nivolumab (Opdivo), and pembrolizumab (Keytruda), oncologic therapy has now moved into noncancer cell targets within the immune system. For many nonimmunologists, understanding how these vastly different therapies work to improve survival, like no other therapies have in the past, is a challenge. The present report reviews the normal function of the immune system, how cancers escape the normal immune system, and how these new therapies improve immune system reactions against cancers.

IMPLICATIONS FOR PRACTICE

Oncologists have tremendous experience with therapies that target the cancer cells. New biologic agents have been rapidly introduced recently that target not cancer cells, but the patient's immune cells. The mechanisms of action of these immune-based biologic agents are within the host immune system. To understand these new biologic therapies, basic knowledge of normal and abnormal immune function is essential. The present report explains the up-to-date basic immune normal and abnormal function and prepares the oncologist to understand how the new drugs work, why they work, and why there are associated adverse events.

Targeting the indoleamine 2,3-dioxygenase pathway in cancer

Tumor cells escape the immune surveillance system of the host through a process called immune tolerance. Immunotherapy targets molecules that serve as checks and balances in the regulation of immune response. Indoleamine-2,3-dioxygenase (IDO) is an intracellular enzyme, which through the process of tryptophan depletion exerts an immunosuppressive effect, facilitating immune escape of tumors. This review summarizes our current knowledge on IDO expression in malignancies, the IDO inhibitors that are currently available and those under clinical development.

Microenvironmental clues for glioma immunotherapy

Gliomas have been viewed for decades as inaccessible for a meaningful antitumor immune response as they grow in a sanctuary site protected from infiltrating immune cells. Moreover, the glioma microenvironment constitutes a hostile environment for an efficient antitumor immune response as glioma-derived factors such as transforming growth factor β and catabolites of the essential amino acid tryptophan paralyze T-cell function. There is growing evidence from preclinical and clinical studies that a meaningful antitumor immunity exists in glioma patients and that it can be activated by vaccination strategies. As a consequence, the concept of glioma immunotherapy appears to be experiencing a renaissance with the first phase 3 randomized immunotherapy trials entering the clinical arena. On the basis of encouraging results from other tumor entities using immunostimulatory approaches by blocking endogenous T-cell suppressive pathways mediated by cytotoxic T-lymphocyte antigen 4 or programmed cell death protein 1/programmed cell death protein 1 ligand 1 with humanized antibodies, there is now a realistic and promising option to combine active immunotherapy with agents blocking the immunosuppressive microenvironment in patients with gliomas to allow a peripheral antitumor immune response induced by vaccination to become effective. Here we review the current clinical and preclinical evidence of antimicroenvironment immunotherapeutic strategies in gliomas.