Discovery of the antiviral activities of pharmacologic cyclin-dependent kinase inhibitors: from basic to applied science

The opportunities and challenges of epigenetic approaches to manage herpes simplex infections

INTRODUCTION

Despite the existence of antivirals that potently and efficiently inhibit the replication of herpes simplex virus 1 and 2 (HSV-1, -2), their ability to establish and maintain, and reactivate from, latency has precluded the development of curative therapies. Several groups are exploring the opportunities of targeting epigenetic regulation to permanently silence latent HSV genomes or induce their simultaneous reactivation in the presence of antivirals to flush the latent reservoirs, as has been explored for HIV.

AREAS COVERED

This review covers the basic principles of epigenetic regulation with an emphasis on those mechanisms relevant to the regulation of herpes simplex viruses, as well as the current knowledge on the regulation of lytic infections and the establishment and maintenance of, and reactivation from, latency, with an emphasis on epigenetic regulation. The differences with the epigenetic regulation of viral and cellular gene expression are highlighted as are the effects of known epigenetic regulators on herpes simplex viruses. The major limitations of current models to the development of novel antiviral strategies targeting latency are highlighted.

EXPERT OPINION

We provide an update on the epigenetic regulation during lytic and latent HSV-1 infection, highlighting the commonalities and differences with cellular gene expression and the potential of epigenetic drugs as antivirals, including the opportunities, challenges, and potential future directions.

Combined Treatment with Host-Directed and Anticytomegaloviral Kinase Inhibitors: Mechanisms, Synergisms and Drug Resistance Barriers

Despite the availability of currently approved antiviral drugs, infections with human cytomegalovirus (HCMV) still cause clinically challenging, sometimes life-threatening situations. There is an urgent need for enhanced anti-HCMV drugs that offer improved efficacy, reduced dosages and options for long-term treatment without risk of the development of viral drug resistance. Recently, we reported the pronounced anti-HCMV efficacy of pharmacological inhibitors of cyclin-dependent kinases (CDKs), in particular, the potential of utilizing drug synergies upon combination treatment with inhibitors of host CDKs and the viral CDK-like kinase pUL97 (vCDK/pUL97). Here, we expand this finding by further assessing the in vitro synergistic antiviral interaction between vCDK and CDK inhibitors towards HCMV as well as non-human cytomegaloviruses. An extension of this synergy approach was achieved in vivo by using the recombinant MCMV-UL97/mouse model, confirming the high potential of combination treatment with the clinically approved vCDK inhibitor maribavir (MBV) and the developmental CDK7 inhibitor LDC4297. Moreover, mechanistic aspects of this synergistic drug combination were illustrated on the levels of intracellular viral protein transport and viral genome replication. The analysis of viral drug resistance did not reveal resistance formation in the case of MBV + LDC4297 combination treatment. Spanning various investigational levels, these new results strongly support our concept, employing the great potential of anti-HCMV synergistic drug treatment.

In Vitro, Molecular Docking and In Silico ADME/Tox Studies of Emodin and Chrysophanol against Human Colorectal and Cervical Carcinoma

Anthraquinones (AQs) are present in foods, dietary supplements, pharmaceuticals, and traditional treatments and have a wide spectrum of pharmacological activities. In the search for anti-cancer drugs, AQ derivatives are an important class. In this study, anthraquinone aglycons chrysophanol (Chr), emodin (EM) and FDA-approved anticancer drug fluorouracil were analyzed by molecular docking studies against receptor molecules caspase-3, apoptosis regulator Bcl-2, TRAF2 and NCK-interacting protein kinase (TNIK) and cyclin-dependent protein kinase 2 (CDK2) as novel candidates for future anticancer therapeutic development. The ADMET SAR database was used to predict the toxicity profile and pharmacokinetics of the Chr and EM. Furthermore, in silico results were validated by the in vitro anticancer activity against HCT-116 and HeLa cell lines to determine the anticancer effect. According to the docking studies simulated by the docking program AutoDock Vina 4.0, Chr and EM had good binding energies against the target proteins. It has been observed that Chr and EM show stronger molecular interaction than that of the FDA-approved anticancer drug fluorouracil. In the in vitro results, Chr and EM demonstrated promising anticancer activity in HCT-116 and HeLa cells. These findings lay the groundwork for the potential use of Chr and EM in the treatment of human colorectal and cervical carcinomas.

Host-Directed Antiviral Therapy

Antiviral drugs have traditionally been developed by directly targeting essential viral components. However, this strategy often fails due to the rapid generation of drug-resistant viruses. Recent genome-wide approaches, such as those employing small interfering RNA (siRNA) or clustered regularly interspaced short palindromic repeats (CRISPR) or those using small molecule chemical inhibitors targeting the cellular "kinome," have been used successfully to identify cellular factors that can support virus replication. Since some of these cellular factors are critical for virus replication, but are dispensable for the host, they can serve as novel targets for antiviral drug development. In addition, potentiation of immune responses, regulation of cytokine storms, and modulation of epigenetic changes upon virus infections are also feasible approaches to control infections. Because it is less likely that viruses will mutate to replace missing cellular functions, the chance of generating drug-resistant mutants with host-targeted inhibitor approaches is minimized. However, drug resistance against some host-directed agents can, in fact, occur under certain circumstances, such as long-term selection pressure of a host-directed antiviral agent that can allow the virus the opportunity to adapt to use an alternate host factor or to alter its affinity toward the target that confers resistance. This review describes novel approaches for antiviral drug development with a focus on host-directed therapies and the potential mechanisms that may account for the acquisition of antiviral drug resistance against host-directed agents.

Recruitment of cyclin-dependent kinase 9 to nuclear compartments during cytomegalovirus late replication: importance of an interaction between viral pUL69 and cyclin T1

Cyclin-dependent protein kinases (CDKs) are important regulators of cellular processes and are functionally integrated into the replication of human cytomegalovirus (HCMV). Recently, a regulatory impact of CDK activity on the viral mRNA export factor pUL69 was shown. Here, specific aspects of the mode of interaction between CDK9/cyclin T1 and pUL69 are described. Intracellular localization was studied in the presence of a novel selective CDK9 inhibitor, R22, which exerts anti-cytomegaloviral activity in vitro. A pronounced R22-induced formation of nuclear speckled aggregation of pUL69 was demonstrated. Multi-labelling confocal laser-scanning microscopy revealed that CDK9 and cyclin T1 co-localized perfectly with pUL69 in individual speckles. The effects were similar to those described recently for the broad CDK inhibitor roscovitine. Co-immunoprecipitation and yeast two-hybrid analyses showed that cyclin T1 interacted with both CDK9 and pUL69. The interaction region of pUL69 for cyclin T1 could be attributed to aa 269-487. Moreover, another component of CDK inhibitor-induced speckled aggregates was identified with RNA polymerase II, supporting earlier reports that strongly suggested an association of pUL69 with transcription complexes. Interestingly, when using a UL69-deleted recombinant HCMV, no speckled aggregates were formed by CDK inhibitor treatment. This indicated that pUL69 is the defining component of aggregates and generally may represent a crucial viral interactor of cyclin T1. In conclusion, these data emphasize that HCMV inter-regulation with CDK9/cyclin T1 is at least partly based on a pUL69-cylin T1 interaction, thus contributing to the importance of CDK9 for HCMV replication.

Five years of progress on cyclin-dependent kinases and other cellular proteins as potential targets for antiviral drugs

In 1997-1998, the pharmacological cyclin-dependent kinase (CDK) inhibitors (PCIs) were independently discovered to inhibit replication of human cytomegalovirus, herpes simplex virus type 1 and HIV-1. The results from small clinical trials against cancer were then suggesting that PCIs could be safe enough to be used clinically. It was thus hypothesized that PCIs could have the potential to be developed as novel antivirals targeting cellular proteins. Consequently, Antiviral Chemistry & Chemotherapy published in 2001 the first review on the potential of CDKs, and cellular proteins in general, as potential targets for antivirals. The viral functions inhibited by PCIs, or their cellular targets, were then just starting to be characterized. The antiviral spectrum of PCIs and their effects on viral disease were still mostly untested. Even their actual specificity was not yet completely characterized. In addition, cellular proteins were not accepted as valid targets for antivirals. Significant progress has been made in the last 5 years in understanding the antiviral activities of PCIs and the potential roles of cellular proteins in general as targets for antivirals. The first clinical trials of the antiviral activities of PCIs and other inhibitors of cellular protein kinases have now been scheduled. Herein, we review the progress made since the publication of the first review on PCIs as potential antiviral drugs and on CDKs, and cellular proteins in general, as potential targets for antiviral drugs. We also highlight the major issues that still need to be addressed before PCIs or other drugs targeting cellular proteins can be developed as clinical antivirals.