The African Swine Fever Virus (ASFV) Topoisomerase II as a Target for Viral Prevention and Control

African swine fever (ASF) is, once more, spreading throughout the world. After its recent reintroduction in Georgia, it quickly reached many neighboring countries in Eastern Europe. It was also detected in Asia, infecting China, the world's biggest pig producer, and spreading to many of the surrounding countries. Without any vaccine or effective treatment currently available, new strategies for the control of the disease are mandatory. Its etiological agent, the African swine fever virus (ASFV), has been shown to code for a type II DNA topoisomerase. These are enzymes capable of modulating the topology of DNA molecules, known to be essential in unicellular and multicellular organisms, and constitute targets in antibacterial and anti-cancer treatments. In this review, we summarize most of what is known about this viral enzyme, pP1192R, and discuss about its possible role(s) during infection. Given the essential role of type II topoisomerases in cells, the data so far suggest that pP1192R is likely to be equally essential for the virus and thus a promising target for the elaboration of a replication-defective virus, which could provide the basis for an effective vaccine. Furthermore, the use of inhibitors could be considered to control the spread of the infection during outbreaks and therefore limit the spreading of the disease.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

Synthesis and evaluation of acridine- and acridone-based anti-herpes agents with topoisomerase activity

The discovery of new non-nucleoside antiviral compounds is of significant and growing interest for treating herpes virus infections due to the emergence of nucleoside-resistant strains. Using a whole cell virus-induced cytopathogenic assay, we tested a series of substituted triaryl heterocyclic compounds including acridones, xanthones, and acridines. The compounds which showed activity against Herpes Simplex-1 and/or Herpes Simplex-2 were further assayed for inhibition of topoisomerase activity to gain insight into the mechanism of action. The results indicate that the acridine analogs bearing substituted carboxamides and bulky 9-amino functionalities are able to inhibit herpes infections as well as inhibit topoisomerase II relaxation of supercoiled DNA. Given the mechanism of action of amsacrine (a closely related, well-studied 9-amino substituted acridine), the compounds were further tested in a DNA topoisomerase II cleavage assay to determine if the compounds function as poisons. The results show that the acridines synthesized in this study function through a different mechanism to that of amsacrine, most likely by blocking topoisomerase binding to DNA (akin to that of aclarubicin). This not only suggests a unique mechanism of action in treating herpes virus infections, but also may be of great interest in the development of anticancer agents that target topoisomerase II activity.

Inhibition of DNA topoisomerase II catalytic activity by the antiviral agents 7-chloro-1,3-dihydroxyacridone and 1,3,7-trihydroxyacridone

Previously we reported that the antiproliferative and antiviral actions of 7-chloro-1,3 dihydroxyacridone (compound 1) and its derivatives may be mediated through the inhibition of mammalian DNA topoisomerase II. In the present work, we have extended our investigation into the mechanism of topoisomerase II inhibition by these agents. Both compound 1 and its 7-OH derivative, compound 2, inhibited topoisomerase II catalytic activity in vitro, yet neither agent affected the activity of topoisomerase I. DNA unwinding assays indicated that compound 1 and compound 2 bound to DNA, although no correlation was found between DNA unwinding and topoisomerase II catalytic inhibition. Neither agent enhanced topoisomerase II-mediated DNA cleavage in vitro; however, both compound 1 and compound 2 antagonized breaks induced by etoposide and amsacrine. Experiments indicate that interference with etoposide-stimulated breaks results from inhibition of topoisomerase II * DNA binding by compound 1. These findings suggest that compound 1 and its derivatives may represent a novel structural class of topoisomerase II catalytic inhibitors.

Inhibitory effects of podophyllotoxin derivatives on herpes simplex virus replication

Podophyllotoxin and its derivatives were examined for inhibitory effects on the replication of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), including acyclovir-resistant virus and clinical isolates. Deoxypodophyllotoxin (RD4-6266) proved to be a highly potent and selective inhibitor of all HSV strains in MRC-5 cells. EC50 values of RD4-6283 (in which the methylenedioxy ring A is modified) for HSV-1 and -2 were inferior to those of deoxypodophyllotoxin. However, podorhizol (RD4-6277) and 5'-methoxy-podorhizol (RD4-6276), in which ring C is absent, did not inhibit HSV replication. Moreover, RD4-6266 also inhibited the production of infectious virus particles of HSV-1 KOS strain and HSV-2 G strain. In contrast, none of the podophyllotoxin derivatives were found to have an antiviral effect against influenza A virus, respiratory syncytial virus or human cytomegalovirus in doses not toxic to the cells.