Poly-ADP-ribose polymerases (PARPs) as a therapeutic target in the treatment of selected cancers

ABSTRACT

Introduction: The implementation of poly-ADP-ribose polymerase (PARP) inhibitors for therapy has created potential treatments for a wide spectrum of malignancies involving DNA damage repair gene abnormalities. PARPs are a group of enzymes that are responsible for detecting and repairing DNA damage and therefore play a key role in maintaining cell function and integrity. PARP inhibitors are drugs that target DNA repair deficiencies. Inhibiting PARP activity in cancer cells causes cell death.

Areas covered: This review summarizes the role of PARP inhibitors in the treatment of cancer. We performed a systematic literature search in February 2019 in the electronic databases PubMed and EMBASE. Our search terms were the following: PARP, PARP inhibitors, PARPi, Poly ADP ribose polymerase, cancer treatment. We discuss PARP inhibitors currently being investigated in cancer clinical trials, their safety profiles, clinical resistance, combined therapeutic approaches and future challenges.

Expert Opinion: The future could bring novel PARP inhibitors with greater DNA trapping potential, better safety profiles and improved combined therapies involving hormonal, chemo-, radio- or immunotherapies. Progress may afford wider indications for PARP inhibitors in the treatment of cancer and the utilization for cancer prevention in high-risk mutation carriers. Research efforts should focus on identifying novel drugs that target DNA repair deficiencies.     

Advances in the approach to novel drug clinical development for breast cancer

Introduction: In the post-genomic era clinical development of new agents to treat breast cancer (BC) can be a real challenge. Different from chemotherapy agents, with a broad but not specific spectrum of activity, novel drugs are being developed as ‘targeted’ agents, potentially benefiting a subgroup of patients. In BC, different clinically identifiable subtypes are now separately addressed in specific clinical trials.

Areas covered: In this review, the authors discuss the clinical development of targeted drugs that have become part of the current treatment of BC. They also highlight the challenges that in other cases determined the failure of promising compounds. Furthermore, the article reports on how combinations of targeted agents have emerged as valid strategies to overcome acquired resistance. It also provides discussion of how ‘old’ therapies can be retargeted to certain patient populations or ‘reinvented’ as safer and more effective with the creation of drug conjugates. They also discuss how novel clinical trial designs are emerging to accelerate the successful matching of targeted drugs to the right patient population.

Expert opinion: It is important not to forget that the development of BC therapeutics is a ‘moving target’, as its biology evolves in time under the pressure of ongoing treatments. There are currently a finite number of resources available for the development of new therapeutics, which means that resources need to be carefully allocated. There is also a need to prioritize clinical trials that can reduce the number of patients who are candidates for expensive treatments.     

Preclinical drug development for childhood cancer

Importance of the field: More effective drugs are needed to treat poor prognosis paediatric malignancies. Development of anticancer agents for childhood cancers faces several unique challenges compared with their adult counterparts.

Areas covered in this review: We demonstrate how recent advances in preclinical drug development may overcome these difficulties and challenges. We explain the role of academia, regulators and industry in this field, address issues with preclinical models and illustrate several examples of biology-driven drug development in childhood cancers.

What the reader will gain: Increased knowledge about preclinical drug development in paediatric oncology including different preclinical models, established preclinical research networks, and relationships among academia, industry and regulators, as illustrated by several examples of targeted agents in childhood solid malignancies.

Take home message: It is anticipated that emerging advanced preclinical models and testing platforms will provide a more efficient, biologically-driven rationale to support the use of targeted therapies in several malignancies such as neuroblastoma, medulloblastoma or high grade glioma which account for the majority of deaths related to childhood cancer.     

Antiviral hyperactivation-limiting therapeutics as a novel class for the treatment of HIV/AIDS: focus on VS411

Introduction: Immune activation plays a central pathogenetic role in both HIV-1 replication and depletion of CD4⁺ T cells leading to disease progression and the onset of the AIDS. While current antiretroviral therapies suppress viral replication to undetectable levels, they do not normalize the excessive level of T-cell activation and proliferation. A new class of anti-HIV-1 drugs known as antiviral hyperactivation-limiting therapeutics (AV-HALTs) combines direct antiviral activity with an antiproliferative action to limit the hyperactivation of the immune system now recognized as the key driver of the progressive loss of CD4⁺ T cells that occurs over the natural course of the HIV-1 infection.

Areas covered: Areas covered include preclinical, Phase I and Phase IIa studies of VS411, the first drug product in a novel class of anti-HIV drugs, AV-HALT agents.

Expert opinion: The two drug combination VS411 safely achieved the goals established for the AV-HALT class based on the results of a Phase IIA proof-of-concept study. Additional work is underway to identify and develop new agents that combine the dual attributes of AV-HALTs, direct reduction of both HIV-1 viral load and markers of excessive immune activation, in a single molecule.     

Tubulin inhibitors as novel anticancer agents: an overview on patents (2013-2018)

Introduction: About 20 patents have been published from 2013 to 2018 for developing advanced cancer therapeutics by targeting tubulin polymerization. Currently, there are several tubulin inhibitors that are in the drug development pipeline for various cancers alone or in combination including antibody-conjugated drugs (ACDs).

Areas covered: Important patents focusing on the development of tubulin inhibitors published from 2013 to 2018 are covered. This review mainly focuses on the tubulin inhibitors that are being synthesized and studied in cancer research along with their structures and their phases of development in preclinical and clinical research.

Expert opinion: Regulation of microtubules is important for cell division, cell motility, intracellular transport, and cell shape maintenance. Modulating its activity proved to be very effective in various diseases including different types of cancers. Microtubules are composed of two units, namely, alpha and beta-tubulin, and modifications at these ends affect both its functions and dynamics. A number of compounds that have been designed and synthesized bearing various heterocyclic scaffolds have been proven to modulate its activity and have emerged as potent tubulin inhibitors. This encourages more to study microtubules in order to find a variety of novel, potent compounds as anticancer drugs.     

New phosphatidylinositol 3-kinase inhibitors for cancer

Introduction: Cancer treatment is moving away from conventional cytotoxic drugs and towards agents that target specific proteins important to cancer development or survival. The PI3K signaling axis is an important pathway involved in myriad human malignancies. Inhibition of this axis is a promising therapeutic approach for several cancers.

Areas covered: This article reviews current literature and recent conference proceedings to analyze the rationale for targeting PI3K and its downstream effectors in cancer. Preclinical and clinical results of several PI3K and PI3K–mammalian target of rapamycin (mTOR) inhibitors in early clinical trials, as single agents and in combination with other drugs, are discussed. Thus far, clinical results have been mixed.

Expert opinion: The clinical utility of PI3K and PI3K–mTOR inhibitors will depend on appropriate selection of patients. Mutations in the PI3K pathway may predict sensitivity to PI3K inhibition but they are not reliable biomarkers at this point. Efforts to define predictive biomarkers will probably be the key to finding therapeutic uses for this novel class of anticancer agents.     

Overcoming drug resistance in pancreatic cancer

Introduction: Pancreatic cancer has the worst survival rate of all cancers. The current standard care for metastatic pancreatic cancer is gemcitabine, however, the success of this treatment is poor and overall survival has not improved for decades. Drug resistance (both intrinsic and acquired) is thought to be a major reason for the limited benefit of most pancreatic cancer therapies.

Areas covered: Previous studies have indicated various mechanisms of drug resistance in pancreatic cancer, including changes in individual genes or signaling pathways, the influence of the tumor microenvironment, and the presence of highly resistant stem cells. This review summarizes recent advances in the mechanisms of drug resistance in pancreatic cancer and potential strategies to overcome this.

Expert opinion: Increasing drug delivery efficiency and decreasing drug resistance is the current aim in pancreatic cancer treatment, and will also benefit the treatment of other cancers. Understanding the molecular and cellular basis of drug resistance in pancreatic cancer will lead to the development of novel therapeutic strategies with the potential to sensitize pancreatic cancer to chemotherapy, and to increase the efficacy of current treatments in a wide variety of human cancers.     

Nanocarrier-mediated drugs targeting cancer stem cells: an emerging delivery approach

Introduction: Cancer stem cells (CSCs) play an important role in the development of drug resistance, metastasis and recurrence. Current conventional therapies do not commonly target CSCs. Nanocarrier-based delivery systems targeting cancer cells have entered a new era of treatment, where specific targeting to CSCs may offer superior outcomes to efficient cancer therapies.

Areas covered: This review discusses the involvement of CSCs in tumor progression and relevant mechanisms associated with CSCs resistance to conventional chemo- and radio-therapies. It highlights CSCs-targeted strategies that are either under evaluation or could be explored in the near future, with a focus on various nanocarrier-based delivery systems of drugs and nucleic acids to CSCs. Novel nanocarriers targeting CSCs are presented in a cancer-specific way to provide a current perspective on anti-CSCs therapeutics.

Expert opinion: The field of CSCs-targeted therapeutics is still emerging with a few small molecules and macromolecules currently proving efficacy in clinical trials. However considering the complexities of CSCs and existing delivery difficulties in conventional anticancer therapies, CSC-specific delivery systems would face tremendous technical and clinical challenges. Nanocarrier-based approaches have demonstrated significant potential in specific drug delivery and targeting; their success in CSCs-targeted drug delivery would not only significantly enhance anticancer treatment but also address current difficulties associated with cancer resistance, metastasis and recurrence.     

A patent review of RAF kinase inhibitors (2010–2018)

ABSTRACT

Introduction: RAF kinase inhibitors block and regulate RAS/RAF/MEK/ERK signaling, which is a key to tumor treatment. At present, although RAF kinase inhibitors have good efficacy, there are few such drugs with low toxicity, and thus, it is urgent to find novel RAF kinase inhibitors associated with higher activity and fewer adverse reactions. This review highlights the anti-tumor effects of several published RAF kinase inhibitors and might be helpful in providing new ideas for the development of novel drug candidates in the future.

Areas covered: This article covers the pertinent literature published on RAF kinase inhibitors from 2010 to 2018, as well as the potential use of these compounds as therapeutics for cancer.

Expert opinion: To date, many RAF kinase inhibitors with different structures have been studied, many of which have prominent inhibitory activities toward RAF kinase. Further, the specificity of these drugs offers hope for the targeted therapy of tumors. Although RAF kinase inhibition has achieved promising results for the treatment of many cancers, overcoming limitations associated with drug resistance and safety comprises a new direction for the optimization and improvement of RAF kinase inhibitors.     

Antibody drug conjugates under investigation in phase I and phase II clinical trials for gastrointestinal cancer

ABSTRACT

Introduction: Antibody drug conjugates (ADCs) represent a developing class of anticancer therapeutics which are designed to selectively deliver a cytotoxic payload to tumors, while limiting systemic toxicity to healthy tissues. There are several ADCs which are currently in various stages of clinical development for the treatment of gastrointestinal malignancies.

Areas covered: We discuss the biologic rationale and review the clinical experience with ADCs in the treatment of gastrointestinal malignancies, summarizing the pre-clinical and phase I/II clinical trial data that have been completed or are ongoing.

Expert opinion: While there have been significant advances in the development of ADCs since they were first introduced, several challenges remain. These challenges include (i) the selection of an ideal antigen target which is tumor specific and internalized upon binding, (ii) selection of an antibody which has high affinity for its antigen target and low immunogenicity, (iii) selection of a potent payload which is cytotoxic at sub-nanomolar concentrations, and (iv) optimal design of a linker to confer ADC stability with limited off-site toxicity. Efforts are ongoing to address these issues and innovate the ADC technology to improve the safety and efficacy of these agents.     

Recent trends in cancer drug resistance reversal strategies using nanoparticles

Introduction: Resistance to chemotherapy is a major obstacle in the successful amelioration of tumors in many cancer patients. Resistance is either intrinsic or acquired, involving mechanisms such as genetic aberrations, decreased influx and increased efflux of drugs. Strategies for the reversal of resistance involve the alteration of enzymes responsible for drug resistance, the modulation of proteins regulating apoptosis mechanisms and improving the uptake of drugs using nanotechnology. Novel strides in the reversal of drug resistance are emerging, involving the use of nanotechnology, targeting stem cells, etc.

Areas covered: This paper reviews the most recent cancer drug reversal strategies involving nanotechnology for targeting cancer cells and cancer stem cells (CSCs), for enhanced uptake of micro- and macromolecular inhibitors.

Expert opinion: Nanotechnology used in conjunction with existing therapies, such as gene therapy and P-glycoprotein inhibition, has been shown to improve the reversal of drug resistance; the mechanisms involved in this include specific targeting of drugs and nucleotide therapeutics, enhanced cellular uptake of drugs and improved bioavailability of drugs with poor physicochemical characteristics. Important strategies in the reversal of drug resistance include: a multifunctional nanoparticulate system housing a targeting moiety; therapeutics to kill resistant cancer cells and CSCs; cytotoxic drugs and a tumor microenvironment stimuli-responsive element, to release the encapsulated therapeutics.     

Novel nitric oxide synthase inhibitors: a patent review

Introduction: Knowledge of NO and its function in cell signaling has rapidly developed since its biological effects were first described in 1977. It is formed from l-arginine by NOS isoforms (nNOS, iNOS and eNOS). These enzymes are products of separate genes, encoded on three different chromosomes and responsible for regulating a variety of functions within cells and tissues. NOS isoforms are currently under investigation as targets for novel therapeutics in especially neurodegenerative disorders, inflammation and pain. Many important questions regarding these messengers and signaling molecules remain to be answered.

Areas covered: This review gives an overview of patents covering drug-like inhibitors for the NOS isoforms filed and published within the last 6 years, up to September 2010, as well as insight into recent highlights in this area.

Expert opinion: The NOS isoforms are attractive targets in drug design for various pathological conditions and have received considerable interest over recent years. With the advances in molecular biology, modeling software, synthesis, bioassays, and our understanding of the NOS enzymes and the function of NO, novel bioavailable and highly selective drug therapies utilizing this mode of action may soon see the light.     

The DNA helicase–primase complex as a target for herpes viral infection

Introduction: The Herpesviridae are responsible for debilitating acute and chronic infections, and some members of this family are associated with human cancers. Conventional anti-herpesviral therapy targets the viral DNA polymerase and has been extremely successful; however, the emergence of drug-resistant virus strains, especially in neonates and immunocompromised patients, underscores the need for continued development of anti-herpes drugs. In this article, we explore an alternative target for antiviral therapy, the HSV helicase/primase complex.

Areas covered: This review addresses the current state of knowledge of HSV DNA replication and the important roles played by the herpesvirus helicase– primase complex. In the last 10 years several helicase/primase inhibitors (HPIs) have been described, and in this article, we discuss and contrast these new agents with established inhibitors.

Expert opinion: The outstanding safety profile of existing nucleoside analogues for α-herpesvirus infection make the development of new therapeutic agents a challenge. Currently used nucleoside analogues exhibit few side effects and have low occurrence of clinically relevant resistance. For HCMV, however, existing drugs have significant toxicity issues and the frequency of drug resistance is high, and no antiviral therapies are available for EBV and KSHV. The development of new anti-herpesvirus drugs is thus well worth pursuing especially for immunocompromised patients and those who develop drug-resistant infections. Although the HPIs are promising, limitations to their development into a successful drug strategy remain.     

Targeting the liver kinase B1/AMP-activated protein kinase pathway as a therapeutic strategy for hematological malignancies

Introduction: Despite considerable advances, several hematological malignancies remain incurable with standard treatments. Therefore, there is a need for novel targeted and less toxic therapies, particularly for patients who develop resistance to traditional chemotherapeutic drugs. The liver kinase B1 (LKB1)/AMP-activated protein kinase (AMPK) signaling pathway has recently emerged as a tumor suppressor axis. A critical point is that the LKB1/AMPK network remains functional in a wide range of cancers and could be stimulated by drugs, such as N,N-dimethylimidodicarbonimidic diamide (metformin) or 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR).

Areas covered: The literature data show that drugs activating LKB1/AMPK signaling induced cell cycle arrest, caspase-dependent apoptosis or autophagy in hematopoietic tumors. Moreover, metformin effectively inhibited mammalian target of rapamycin complex 1 (mTORC1)-controlled oncogenetic protein translation, which does not occur with allosteric mTORC1 inhibitors, such as rapamycin and its derivatives. Metformin was also capable of targeting leukemic stem cells, the most relevant target for leukemia eradication.

Expert opinion: Data emerging from preclinical settings suggest that the LKB1/AMPK pathway is critically involved in regulating proliferation and survival of malignant hematopoietic cells. Thus, it is proposed that drugs activating the LKB1/AMPK axis may offer a novel and less toxic treatment option for some types of hematological malignancies.     

Development of drugs for Epstein–Barr virus using high-throughput in silico virtual screening

Importance of the field: Epstein–Barr virus (EBV) is a ubiquitous human herpesvirus that is causally associated with endemic forms of Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease in immunosuppressed individuals. On a global scale, EBV infects > 90% of the adult population and is responsible for ～ 1% of all human cancers. To date, there is no efficacious drug or therapy for the treatment of EBV infection and EBV-related diseases.

Areas covered in this review: In this review, we discuss the existing anti-EBV inhibitors and those under development. We discuss the value of different molecular targets, including EBV lytic DNA replication enzymes as well as proteins that are expressed exclusively during latent infection, such as EBV nuclear antigen 1 (EBNA-1) and latent membrane protein 1. As the atomic structure of the EBNA-1 DNA binding domain has been described, it is an attractive target for in silico methods of drug design and small molecule screening. We discuss the use of computational methods that can greatly facilitate the development of novel inhibitors and how in silico screening methods can be applied to target proteins with known structures, such as EBNA-1, to treat EBV infection and disease.

What the reader will gain: The reader is familiarized with the problems in targeting of EBV for inhibition by small molecules and how computational methods can greatly facilitate this process.

Take home message: Despite the impressive efficacy of nucleoside analogs for the treatment of herpesvirus lytic infection, there remain few effective treatments for latent infections. As EBV latent infection persists within and contributes to the formation of EBV-associated cancers, targeting EBV latent proteins is an unmet medical need. High-throughput in silico screening can accelerate the process of drug discovery for novel and selective agents that inhibit EBV latent infection and associated disease.     

Investigational drugs for the treatment of Zika virus infection: a preclinical and clinical update

ABSTRACT

Introduction: The Zika virus (ZIKV) infection results in severe neurological complications and has emerged as a threat to public health worldwide. No drugs or vaccines are available for use in the clinic and the need for novel and effective therapeutic agents is urgent.

Areas covered: This review describes the latest progress of antiviral development for the treatment of ZIKV infection; it primarily focuses on the literature describing 20 potential anti-ZIKV drugs/agents currently being tested in vivo or in clinical trials. The paper also discusses the need for novel ZIKV inhibitors and the critical issues for successful antiviral drug development.

Expert opinion: So far, 20 compounds have been tested in vivo and three in the clinical trials; progressing these compounds to the clinic is a challenge. Novel ZIKV inhibitors that target virus or host factors are urgently needed. Knowledge-driven drug repurposing, structure-based discovery, RNA interference, long noncoding RNAs, miRNAs, and peptide inhibitors may pave the way for the discovery of such novel agents.     

Targeted approaches to childhood cancer: progress in drug discovery and development

Introduction: Cancer is a leading cause of death in childhood. Encouraging progress has been made in the treatment of childhood malignancies, but there is an unmet need for new drugs to improve survival and reduce treatment-associated toxicities. Drug development in paediatric oncology has specific requirements with regard to the patient population and the regulatory background and presents several unique challenges that need addressing.

Areas covered: This review discusses the current framework of paediatric oncology drug development and some of the specific challenges in pre-clinical and clinical research. The authors discuss the recent developments in the targeting of various signalling pathways. These pathways represent a selection of targets that have been identified by pre-clinical and clinical investigators to be highly relevant in paediatric malignancies.

Expert opinion: The development of targeted agents in paediatric oncology must be driven by knowledge of tumour biology. Predictive and pharmacodynamic biomarkers should be incorporated within paediatric early clinical trials wherever possible. Faster dose-escalation, limited numbers of cohorts and novel adaptive designs can help to make paediatric early clinical trials more efficient. Close collaboration between academic/clinical researchers, the pharmaceutical industry, regulatory bodies and parent groups are crucial in overcoming the challenges associated with paediatric oncology drug development.     

In pursuit of a moving target: nanotherapeutics for the treatment of non-Hodgkin B-cell lymphoma

Introduction: Non-Hodgkin lymphoma (NHL) is diagnosed in 70,000 Americans annually. Chemotherapy was the standard course of treatment until the addition of the monoclonal antibody (mAb) drug, rituximab, to therapy regimens in 1997. Although disease prognosis has improved dramatically since that time, nearly 20,000 patients succumb annually to the disease, with an average life expectancy beyond diagnosis of only 12 years. The advent of nanomedicine may fulfill the remaining need for novel therapy capable of eradicating solid tumor and disseminated B-cell lymphomas.

Areas covered: This review details the current landscape of B-cell NHL and nanoparticles now being developed for its treatment. Specifically, we discuss lipid, polymer and metal nanoparticles that deliver an array of drugs, including toxins, chemotherapeutic agents and nucleic acids.

Expert opinion: Because B-cell malignancies have responded quite well to new components in multi-drug regimens, nanomedicines that are mechanistically distinct from existing therapies hold significant promise. In our opinion, advancement of these technologies into the clinic will likely require significantly more effective targeting systems coupled with a better understanding of lymphoma biology. Furthermore, it is important for researchers to recognize the genetic and phenotypic heterogeneity of NHL and to develop therapeutic strategies for distinct subsets of NHL before attempting to generalize approaches.