Melanocortin MC1 receptor in human genetics and model systems

The melanocortin MC₁ receptor is a G-protein coupled receptor expressed in the melanocytes of the skin and hair and is known for its key role in the regulation of human pigmentation. Melanocortin MC₁ receptor activation after ultraviolet radiation exposure results in a switch from the red/yellow pheomelanin to the brown/black eumelanin pigment synthesis within cutaneous melanocytes; this pigment is then transferred to the surrounding keratinocytes of the skin. The increase in melanin maturation and uptake results in tanning of the skin, providing a physical protection of skin cells from ultraviolet radiation induced DNA damage. Melanocortin MC₁ receptor polymorphism is widespread within the Caucasian population and some variant alleles are associated with red hair colour, fair skin, poor tanning and increased risk of skin cancer. Here we will discuss the use of mouse coat colour models, human genetic association studies, and in vitro cell culture studies to determine the complex functions of the melanocortin MC₁ receptor and the molecular mechanisms underlying the association between melanocortin MC₁ receptor variant alleles and the red hair colour phenotype. Recent research indicates that melanocortin MC₁ receptor has many non-pigmentary functions, and that the increased risk of skin cancer conferred by melanocortin MC₁ receptor variant alleles is to some extent independent of pigmentation phenotypes. The use of new transgenic mouse models, the study of novel melanocortin MC₁ receptor response genes and the use of more advanced human skin models such as 3D skin reconstruction may provide key elements in understanding the pharmacogenetics of human melanocortin MC₁ receptor polymorphism.     

DNA methyltransferase inhibitors in cancer: a chemical and therapeutic patent overview and selected clinical studies

Introduction: DNA methylation is an epigenetic modification that modulates gene expression without altering the DNA base sequence. It plays a crucial role in cancer by silencing tumor suppressor genes (TSG). The DNA methyltransferases (DNMT) are the enzymes that catalyze DNA methylation and they are interesting therapeutical targets since DNA methylation is reversible such that an aberrant hypermethylation of DNA can be reverted by inhibition of DNMTs. Today, two drugs are on the market for the treatment of myelodysplastic syndrome, azacitidine and decitabine.

Areas covered: Here, we present a review of the patents describing the chemistry and biological activities of novel DNMT inhibitors and discuss select clinical studies.

Expert opinion: DNMT inhibitors have shown efficacy in clinics. However, highly efficient and specific DNMT inhibitors have not yet been identified. Improving methods will certainly lead to the prediction of novel directly binding inhibitors in the future.     

Targeted delivery of antibody-based therapeutic and imaging agents to CNS tumors: crossing the blood–brain barrier divide

Introduction: Brain tumors are inherently difficult to treat in large part due to the cellular blood–brain barriers (BBBs) that limit the delivery of therapeutics to the tumor tissue from the systemic circulation. Virtually no large molecules, including antibody-based proteins, can penetrate the BBB. With antibodies fast becoming attractive ligands for highly specific molecular targeting to tumor antigens, a variety of methods are being investigated to enhance the access of these agents to intracranial tumors for imaging or therapeutic applications.

Areas covered: This review describes the characteristics of the BBB and the vasculature in brain tumors, described as the blood–brain tumor barrier (BBTB). Antibodies targeted to molecular markers of central nervous system (CNS) tumors will be highlighted, and current strategies for enhancing the delivery of antibodies across these cellular barriers into the brain parenchyma to the tumor will be discussed. Noninvasive imaging approaches to assess BBB/BBTB permeability and/or antibody targeting will be presented as a means of guiding the optimal delivery of targeted agents to brain tumors.

Expert opinion: Preclinical and clinical studies highlight the potential of several approaches in increasing brain tumor delivery across the BBB divide. However, each carries its own risks and challenges. There is tremendous potential in using neuroimaging strategies to assist in understanding and defining the challenges to translating and optimizing molecularly targeted antibody delivery to CNS tumors to improve clinical outcomes.     

Role of microRNAs in gliomagenesis: targeting miRNAs in glioblastoma multiforme therapy

Introduction: Gliomas consist of a very heterogeneous group of malignant tumors, accounting for 50 – 60% of primary brain tumors. Despite all the efforts of cytoreductive surgery in combination with intense chemoradiotherapy, glioblastoma multiforme (GBM, glioma grade IV) still has a dismal prognosis. Current research is focused on molecular targeting to overcome resistance to conventional therapy. MicroRNAs (miRNAs), small non-coding RNAs, represent endogenous agents of RNA interference, dramatically changing expression of target proteins. Their role in brain physiology as well as GBM development has attracted intense research efforts pointing toward therapeutic potential and immediate targeting for sensitization of glioma cells to chemo and/or radiotherapy.

Areas covered: This review is focused on the variable role of miRNAs in gliomagenesis and their possible clinical relevance in patient's survival and prognosis. It further addresses the potential application of selected miRNAs as therapeutic targets or agents in GMB, including data from clinical studies in other central nervous system tumors.

Expert opinion: Although miRNA-targeted therapy is still in its initial stage and clinical trials with glioma/brain tumor patients are under recruitment or currently running, several miRNAs have been selected as promising tumor biomarkers, with increased potential to reduce disease progression in combination to conventional first-line therapy for gliomas.     

BRAF inhibitors in BRAF-V600 mutated primary neuroepithelial brain tumors

Introduction: Primary neuroepithelial brain tumors encompass a wide variety of glial and glioneuronal neoplasms. Malignant tumors, tumors located in surgically inaccessible locations (e.g., eloquent brain areas, deep structures, brain stem) and recurrent or progressive tumors pose considerable treatment challenges and are candidates for novel therapeutics based on molecular insights. Small kinase inhibitors of v-RAF murine sarcoma viral oncogene homologue B1 (BRAF) have shown considerable antineoplastic activity in some tumor types harboring activating BRAF-V600 mutations (e.g., melanoma) and promising data are emerging on BRAF inhibitor therapy of mutation-bearing primary brain tumors.

Areas covered: This review summarizes the available data on BRAF-V600 point mutations and the antineoplastic activity and toxicity profiles of BRAF inhibitors in neuroepithelial brain tumors including diffuse gliomas (glioblastomas, astrocytomas, oligodendrogliomas), pilocytic astrocytomas, pleomorphic xanthoastrocytomas and gangliogliomas.

Expert opinion: Activating BRAF-V600 mutations are recurrently found in several glial and glioneuronal brain tumors and the available data indicate that BRAF inhibitors are active and well-tolerated in such tumors. Thus, BRAF inhibitors represent a novel and promising therapeutic opportunity that may alter the disease course of molecularly selected CNS neoplasms in a clinically meaningful way. However, so far the evidence is anecdotal and prospective clinical studies should be conducted.     

The art of attraction: applications of multifunctional magnetic nanomaterials for malignant glioma

Introduction: Malignant gliomas remain one of medicine's most daunting unsolved clinical problems. The development of new technologies is urgently needed to improve the poor prognosis of patients suffering from these brain tumors. Magnetic nanomaterials are appealing due to unique properties that allow for noninvasive brain tumor diagnostics and therapeutics in one multifunctional platform.

Areas covered: We report on the recent advances of magnetic nanomaterials for brain tumor imaging and therapy, with an emphasis on novel approaches and clinical progress. We detail their biomedical applications including brain tumor targeting, MRI contrast enhancement, optical imaging, magnetic hyperthermia, magnetomechanical destruction, drug delivery, gene therapy, as well as tracking of cell-based and viral-based therapies. The clinical cases and obstacles encountered in the use of magnetic nanomaterials for malignant glioma are also examined.

Expert opinion: To accelerate the effective translation of these materials to the clinic as theranostics for brain tumors, limitations such as poor intratumoral distribution, targeting efficiency and nonspecific systemic side effects must be addressed. Future innovations should focus on optimizing and combining the unique therapeutic applications of these magnetic nanomaterials as well as improving the selectivity of the system based on the molecular profiling of tumors.     

Preclinical models for pediatric solid tumor drug discovery: current trends,challenges and the scopes for improvement

Introduction: The enhancement in pediatric cancer survival achieved in the past few decades has been confined to low- and moderate-risk cancers, whereas no notable improvement in survival was observed in high-risk and advanced-stage childhood cancers. High attrition rate of candidate drugs in clinical trials is a major hurdle in the development of effective therapies for pediatric solid tumors. In order to reduce the failure rate of candidate drugs in clinical trials, more effective strategies are needed to enhance the predictability of preclinical testing.

Areas covered: The authors have described the current trends in preclinical drug development for treating pediatric solid tumors. Furthermore, the authors review their limitations and the available remedies, with regards to choice of models, pharmacokinetic considerations and the criteria for assessing the long-term efficacy of a candidate drug.

Expert opinion: In many solid tumors, common differences between pediatric and adult cancers have been observed, and therefore, clinical trials for pediatric solid tumors must be conducted on the basis of preclinical observations in pediatric solid tumor models. There is a need to invest in extensive preclinical testing on pediatric solid tumor models. None of the preclinical models can fully recapitulate the human cancers. Therefore, these limitations must be considered while conducting a preclinical trial. The dose and schedule of drugs used for preclinical testing must be clinically relevant. While testing the efficacy of drugs, the markers of apoptosis, drug resistance, hypoxia and tumor-initiating cells can inform us about the long-term therapeutic response of a cancer.     

Targeting miR-205 in breast cancer

As small non-coding regulatory RNAs, microRNAs are capable of silencing gene expression by translational repression or mRNA degradation. Accumulating evidence indicates that deregulation of microRNAs is often associated with human malignancies and suggests a causal role of microRNAs in neoplasia, presumably because microRNAs can function as oncogenes or tumor suppressors. Among them, miR-205 is significantly underexpressed in breast tumors compared with matched normal breast tissue although miR-205 has been shown to be upregulated in some other type of tumors. Furthermore, breast cancer cell lines, including MCF-7 and MDA-MB-231, express a lower level of miR-205 than the non-malignant MCF-10A cells. Ectopic expression of miR-205 significantly inhibits cell proliferation and anchorage-independent growth as well as cell invasion. These findings establish the tumor suppressive role of miR-205, which is probably through direct targeting of oncogenes such as ErbB3 and Zeb1. Therefore, miR-205 may serve as a unique therapeutic target for breast cancer.     

Effects of melanocortins on adrenal gland physiology

The melanocortin-2-receptor (MC₂ receptor), also known as the ACTH receptor, is a critical component of the hypothalamic–pituitary–adrenal axis. The importance of MC₂ receptor in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency, a potentially fatal disease characterised by isolated cortisol deficiency. MC₂receptor mutations cause ~ 25% of cases. The discovery of a MC₂ receptor accessory protein MRAP, mutations of which account for ~ 15%–20% of familial glucocorticoid deficiency, has provided insight into MC₂ receptor trafficking and signalling. MRAP is essential for the functional expression of MC₂ receptor. MRAP2, a novel homolog of MRAP, can also facilitate MC₂ receptor cell surface expression and function. Like MRAP, MRAP2 is a small transmembrane domain glycoprotein capable of homodimerising. In addition, MRAP/MRAP2 can heterodimerise. The presence of MRAP2 adrenal expression suggests a possible role for MRAP2 in adrenal physiology, which has yet to be elucidated. Importantly, new data shows that the MRAPs can interact with all the other melanocortin receptors (MC_1,3,4,5 receptor). In contrast to MC₂ receptor, this interaction results in reduced melanocortin receptor surface expression and signalling. MRAP2 is predominantly expressed in brain. Hypothalamic expression has been demonstrated for both MRAP and MRAP2. The ability of MRAPs to modulate different members of the melanocortin receptor family in a bidirectional manner is intriguing. Furthermore, central nervous system expression of MRAPs points to a role beyond MC₂ receptor mediated adrenal steroidogenesis.     

Radiolabeled (R)-(–)-5-iodo-3′-O-[2-(ε-guanidinohexanoyl)-2-phenylacetyl]-2′-deoxyuridine: A new theranostic for neuroblastoma

Neuroblastoma, the most common extracranial solid tumor in children, accounts for nearly 8% of childhood cancers in the United States. It is a disease with pronounced clinical and biological heterogeneities. The amplification of MYCN, whose key tumorigenic functions include the promotion of proliferation, facilitation of the cell's entry into the S phase, and prevention of cells from leaving the cell cycle, correlates with poor prognosis. Patients with a high proliferation index disease have low survival rates. Neuroblastoma is one of the most radioresponsive of all human tumors. To exploit this radiosensitivity, radioactive guanidine (R)-(–)-5-[¹²⁵I]iodo-3′-O-[2-(ε-guanidinohexanoyl)-2-phenylacetyl]-2′-deoxyuridine ( 9 , GPAID) was designed. This compound enters neuroblastoma cells much like metaiodobenzylguanidine (MIBG). Additionally, it cotargets DNA of proliferating cells, an attribute especially advantageous in the treatment of MYCN-amplified tumors. GPAID was synthesized from the trimethylstannyl precursor with an average yield of >90% at the no-carrier-added specific activities. The norepinephrine transporter-aided delivery of GPAID to neuroblastoma cells was established in the competitive uptake studies with nonradioactive MIBG. The intracellular processing and DNA targeting properties were confirmed in the subcellular distribution experiments. Studies in a mouse model of neuroblastoma demonstrated the therapeutic potential of GPAID. The tin precursor of GPAID can be used to prepare compounds radiolabeled with single-photon emission computed tomography (SPECT)- and positron-emission tomography (PET)-compatible radionuclides. Accordingly, these reagents can function as theranostics useful in the individualized and comprehensive treatment strategies comprising treatment planning and the assessment of tumor responses as well as the targeted molecular radiotherapy employing treatment doses derived from the imaging data.     

Asthma phenotypes in children and stratified pharmacological treatment regimens

Introduction: Asthma is the most common inflammatory disease in childhood. The interaction of genetic, environmental and host factors may contribute to the development of childhood asthma and defines its progress, including persistence and severity. Until now, various classifications of childhood asthma phenotypes have been suggested based on patient’s age during onset of symptoms, type of inflammatory cells, response to treatment and disease severity. Many efforts have been carried out to identify childhood asthma phenotypes and to clarify which are the risk factors that define asthma prediction and the response to therapy. The identification of asthma phenotypes has not only prognostic but also therapeutic role. However, the classification of asthma phenotypes is complex due to the heterogeneity of the disease.

Areas covered: The current childhood asthma phenotypes and the new therapeutic strategies for each phenotype are reviewed.

Expert commentary: There are multiple phenotypes in childhood asthma and it is crucial to define them before the initiation of personalized treatment. Both the therapeutic strategy and monitoring should follow the recent guidelines.     

Melanocortin MC4 receptor expression sites and local function

The melanocortin MC₄ receptor plays an important role in energy metabolism, but also affects blood pressure, heart rate and erectile function. Localization of the receptors that fulfill these distinct roles is only partially known. Mapping of the melanocortin MC₄ receptor has been stymied by the absence of a functional antibody. Several groups have examined mRNA expression of the melanocortin MC₄ receptor in the rodent brain and transgenic approaches have also been utilized to visualize melanocortin MC₄ receptor expression sites within the brain. Ligand expression and binding studies have provided additional information on the areas of the brain where this elusive receptor is functionally expressed. Finally, microinjection of melanocortin MC₄ receptor ligands in specific nuclei has further served to elucidate the function of melanocortin MC₄ receptors in these nuclei. These combined approaches have helped link the anatomy and function of this receptor, such as the role of paraventricular hypothalamic nucleus melanocortin MC₄ receptor in the regulation of food intake. Intriguingly, however, numerous expression-sites have been identified that have not been linked to a specific receptor function such as those along the optic tract and olfactory tubercle. Further research is needed to clarify the function of the melanocortin MC₄ receptor at these sites.     

Whole Brain Radiation-Induced Cognitive Impairment: Pathophysiological Mechanisms and Therapeutic Targets

Radiation therapy, the most commonly used for the treatment of brain tumors, has been shown to be of major significance in tu-mor control and survival rate of brain tumor patients. About 200,000 patients with brain tumor are treated with either partial large field or whole brain radiation every year in the United States. The use of radiation therapy for treatment of brain tumors, however, may lead to devastating functional deficits in brain several months to years after treatment. In particular, whole brain radiation therapy results in a significant reduction in learning and memory in brain tumor patients as long-term consequences of treatment. Although a number of in vitro and in vivo studies have demonstrated the pathogenesis of radiation-mediated brain injury, the cel-lular and molecular mechanisms by which radiation induces damage to normal tissue in brain remain largely unknown. Therefore, this review focuses on the pathophysiological mechanisms of whole brain radiation-induced cognitive impairment and the iden-tification of novel therapeutic targets. Specifically, we review the current knowledge about the effects of whole brain radiation on pro-oxidative and pro-inflammatory pathways, matrix metalloproteinases (MMPs)/tissue inhibitors of metalloproteinases (TIMPs) system and extracellular matrix (ECM), and physiological angiogenesis in brain. These studies may provide a foundation for defin-ing a new cellular and molecular basis related to the etiology of cognitive impairment that occurs among patients in response to whole brain radiation therapy. It may also lead to new opportunities for therapeutic interventions for brain tumor patients who are undergoing whole brain radiation therapy.     

Grape seed proanthocyanidins reactivate silenced tumor suppressor genes in human skin cancer cells by targeting epigenetic regulators

Grape seed proanthocyanidins (GSPs) have been shown to have anti-skin carcinogenic effects in in vitro and in vivo models. However, the precise epigenetic molecular mechanisms remain unexplored. This study was designed to investigate whether GSPs reactivate silenced tumor suppressor genes following epigenetic modifications in skin cancer cells. For this purpose, A431 and SCC13 human squamous cell carcinoma cell lines were used as in vitro models. The effects of GSPs on DNA methylation, histone modifications and tumor suppressor gene expressions were studied in these cell lines using enzyme activity assays, western blotting, dot-blot analysis and real-time polymerase chain reaction (RT-PCR). We found that treatment of A431 and SCC13 cells with GSPs decreased the levels of: (i) global DNA methylation, (ii) 5-methylcytosine, (iii) DNA methyltransferase (DNMT) activity and (iv) messenger RNA (mRNA) and protein levels of DNMT1, DNMT3a and DNMT3b in these cells. Similar effects were noted when these cancer cells were treated identically with 5-aza-2′-deoxycytidine, an inhibitor of DNA methylation. GSPs decreased histone deacetylase activity, increased levels of acetylated lysines 9 and 14 on histone H3 (H3-Lys 9 and 14) and acetylated lysines 5, 12 and 16 on histone H4, and reduced the levels of methylated H3-Lys 9. Further, GSP treatment resulted in re-expression of the mRNA and proteins of silenced tumor suppressor genes, RASSF1A, p16^INK4a and Cip1/p21. Together, this study provides a new insight into the epigenetic mechanisms of GSPs and may have significant implications for epigenetic therapy in the treatment/prevention of skin cancers in humans.     

DNA methyltransferase inhibitors: an updated patent review (2012-2015)

Introduction: DNA methyltransferases (DNMTs), important enzymes involved in epigenetic regulation of gene expression, represent promising targets in cancer therapy. DNMT inhibitors (DNMTi), which can modulate the aberrant DNA methylation pattern in a reversible way via inhibiting DNMT activity, have attracted significant attention in recent years.

Areas covered: This review outlines the newly patented inhibitors targeting DNMTs, mainly incorporating small molecular inhibitors and oligonucleotide derivatives. The chemical structures, biological activity, and the encouraging clinical research in progress are delineated in detail.

Expert opinion: Two drugs, azacitidine and decitabine, have evidently shown efficacy in hematologic malignancies, yet do not work well on solid tumors, have low specificity, substantial toxicity, and poor bioavailability. With the rapid advancement in systems biology, drug combinations, such as DNMTi, in conjugation with histone deacetylase inhibitors (HDACi) or immunotherapy, probably serve as an efficient way of implementing epigenetic therapy. Meanwhile, the resolved autoinhibitory structures of DNMTs afford a novel strategy for targeting the protein-protein interface involved in the autoinhi-bitory interactions. The molecular mechanism underlying the conformational transitions would also shed new light on the design of allosteric inhibitors. Both strategies would produce inhibitors with more selectivity compared to nucleotide derivatives.     

Liposome-Mediated Therapy of Intracranial Brain Tumors in a Rat Model

Purpose. Malignant brain tumors represent a serious therapeutic challenge, and survival often is low. We investigated the delivery of doxorubicin (DXR) to rat brain tumors in situ vialiposomes, to test the hypothesis that intact liposomes undergo deposition in intracranial tumor through a compromised blood-tumor vasculature. Both therapeutic effect and intra-tumor drug carrier distribution were evaluated to identify variables in carrier-mediated delivery having impact on therapy. Methods. The rat 9L gliosarcoma tumor was implanted orthotopically in Fischer 344 rats in the caudate-putamen region. The tumor-bearing rats were treated with DXR, either free or encapsulated in long-circulating, sterically-stabilized liposomes. Anti-tumor efficacy was assessed by survival time. In parallel, liposomes labeled with a fluorescent phospholipid analog were injected into tumor-bearing rats. At predetermined intervals, the brains were perfused with fixative, sectioned, and imaged with laser scanning confocal microscope (LSCM) to investigate the integrity of the tumor vascular bed and the intratumor deposition of liposomes. Results. Free DXR given in 3 weekly iv injections was ineffective in increasing the life span of tumor-bearing rats at cumulative doses 17 mg/kg, and at the highest dose (17 mg/kg) decreased survival slightly, compared to saline-treated controls. In contrast, DXR encapsulated in long-circulating liposomes mediated significant increases in life span at 17 mg/kg. Rats showed a 29% percent increase in median survival, respectively, compared to saline-control animals. The delay of treatment after tumor implantation was a major determinant of therapeutic effect. Fluorescent liposomes were deposited preferentially in tumor rather than normal brain, and were distributed non-uniformly, in close proximity to tumor blood vessels. Conclusions. Liposomes can be used to enhance delivery of drugs to brain tumors and increase therapeutic effect. The therapeutic effect may arise from release of drug from liposomes extravasated in discrete regions of the tumor vasculature and the extravascular space.     

Association analysis of DNA methyltransferases in IgA nephropathy

DNA methylation, especially DNA methyltransferases (DNMTs), is involved in the pathogenesis of many autoimmune diseases through regulating immune function. This study aimed to explore the potential role of DNMTs in IgA nephropathy (IgAN). We evaluated mRNA expressions of DNMT1, DNMT3A, DNMT3B along with β1,3-galactosyltransferase (C1GALT1) in peripheral blood mononuclear cells (PBMCs), and measured galactose-deficient IgA1 (Gd-IgA1) levels in plasma. The expression intensity of DNMT1 and DNMT3B in the renal specimen of IgAN patients were also detected. Results showed DNMT3B, not DNMT1 or DNMT3A, was notably increased in IgAN patients compared to controls and associated with pathologic types. However, DNMT1 and C1GALT1 were found positively correlated with estimated glomerular filtration rate (eGFR) and negatively correlated with 24 h Urine protein in IgAN patients. No association was found between DNMT1 and Gd-IgA1. The expressions of DNMT3B and DNMT1 were barely observed in IgAN renal biopsy specimens. In conclusion, for the first time, we identified the relations of DNMTs and C1GALT1 to the clinical state and pathology of IgAN patients, which provide new clues for IgAN.     

Recent developments on immunotherapy for brain cancer

Introduction: Brain tumors are a unique class of cancers since they are anatomically shielded from normal immunosurveillance by the blood–brain barrier, lack a normal lymphatic drainage system and reside in a potently immunosuppressive environment. Of the primary brain cancers, glioblastoma multiforme (GBM) is the most common and aggressive in adults. Although treatment options include surgery, radiation and chemotherapy, the average lifespan of GBM patients remains at only 14.6 months post-diagnosis.

Areas covered: A review of key cellular and molecular immune system mediators in the context of brain tumors including TGF-β, cytotoxic T cells, Tregs, CTLA-4, PD-1 and IDO is discussed. In addition, prognostic factors, currently utilized immunotherapeutic strategies, ongoing clinical trials and a discussion of new or potential immunotherapies for brain tumor patients are considered.

Expert opinion: Current drugs that improve the quality of life and overall survival in patients with brain tumors, especially for GBM, are poorly effective. This disease requires a reanalysis of currently accepted treatment strategies, as well as newly designed approaches. Here, we review the fundamental aspects of immunosuppression in brain tumors, new and promising immunotherapeutic drugs as well as combinatorial strategies that focus on the simultaneous inhibition of immunosuppressive hubs, both in immune and brain tumor cells, which is critical to consider for achieving future success for the treatment of this devastating disease.     

Targeting the TCR signaling checkpoint: a therapeutic strategy to reactivate memory T cells in the tumor microenvironment

Background: One of the major conundrums in cancer immunotherapy is why human tumors are not rejected, and progress despite the presence of inflammatory leukocytes in the tumor microenvironment. While studies addressing the mechanisms responsible for the failure of immunocompetent cells to control tumor progression have shed considerable light upon this issue, not enough is known about the mechanisms contributing to the regulation of tumor-associated T cells in the microenvironment of human tumors. Objective: A persistent and robust response is likely to be required for the complete eradication of tumors. Such a durable immune response will require the development and persistence of functional tumor-reactive memory T cells. Methods: Various studies have investigated the mechanisms of suppression in the tumor microenvironment. However, very few studies have investigated the hyporesponsiveness of tumor-associated T cells at the molecular level. This review focuses on the hyporesponsiveness of tumor-associated T cells and how this relates to the T cell receptor signaling cascade. Results/conclusions: We postulate that this hyporesponsiveness results from a normal regulatory mechanism or TCR signaling checkpoint that is initiated by the persistence of antigen. If the TCR checkpoint is defined, it will be possible to design therapeutic strategies that reverse the TCR arrest. Essentially, this will reactivate the T cells in situ, leading to the killing and lysis of tumors locally, the release of tumor antigens and the generation of a systemic antitumor immunity.