Finding a chink in the armor: Update, limitations, and challenges toward successful antivirals against flaviviruses

Abstract

Flaviviruses have caused large epidemics and ongoing outbreaks for centuries. They are now distributed in every continent infecting up to millions of people annually and may emerge to cause future epidemics. Some of the viruses from this group cause severe illnesses ranging from hemorrhagic to neurological manifestations. Despite decades of research, there are currently no approved antiviral drugs against flaviviruses, urging for new strategies and antiviral targets. In recent years, integrated omics data-based drug repurposing paired with novel drug validation methodologies and appropriate animal models has substantially aided in the discovery of new antiviral medicines. Here, we aim to review the latest progress in the development of both new and repurposed (i) direct-acting antivirals; (ii) host-targeting antivirals; and (iii) multitarget antivirals against flaviviruses, which have been evaluated both in vitro and in vivo, with an emphasis on their targets and mechanisms. The search yielded 37 compounds that have been evaluated for their efficacy against flaviviruses in animal models; 20 of them are repurposed drugs, and the majority of them exhibit broad-spectrum antiviral activity. The review also highlighted the major limitations and challenges faced in the current in vitro and in vivo evaluations that hamper the development of successful antiviral drugs for flaviviruses. We provided an analysis of what can be learned from some of the approved antiviral drugs as well as drugs that failed clinical trials. Potent in vitro and in vivo antiviral efficacy alone does not warrant successful antiviral drugs; current gaps in studies need to be addressed to improve efficacy and safety in clinical trials.

Fig 1. Drugs targeting the different steps in the flavivirus life cycle.

Direct-acting antivirals*, host-targeting antivirals**, and multitarget antivirals*** that inhibit the different steps of the virus lifecycle discussed in this review are indicated in the red boxes. Flaviviruses consist of 3 structural proteins (C, prM or M, and E) and 7 NS proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5), each playing distinctive roles in the different stages of the flavivirus life cycle and modulation of innate immune responses. During the virus replication cycle, the viral proteins interact with numerous host factors to modulate cellular pathways and induce infection. The flavivirus life cycle is outlined in Fig 1. (1) The E glycoprotein of the mature virion binds to the host cell membrane receptors and enters host cells via clathrin-mediated endocytosis [3]. (2) The acidic environment of the endosome triggers major structural changes in the E glycoprotein, inducing fusion of the host endosomal membrane with the viral E protein [38]. This is followed by the uncoating of the NC and the releasing of the viral genomic RNA into the cytoplasm. (3) The positive-sense viral genome (+ssRNA) is translated by ribosomes to form the viral polyprotein, which is then cleaved into structural and NS proteins by viral serine protease (NS2B-NS3) and host–cells peptidases/signalases in the ER [39]. (4) The NS proteins associate with intracellular membranes located on the surface of the ER to form viral replication complexes. The NS5 protein-derived RdRp synthesizes a complementary minus-strand RNA from genomic RNA for the synthesis of new positive-strand viral RNA [40]. (5) The newly synthesized viral RNA will then associate with the C protein in the ER to form a NC, leading to the formation of immature virions [41]. (6) Maturation and exocytosis. The assembled immature, noninfectious virion will then bud off the ER and travels through the secretory pathway to the TGN, where furin-mediated proteolysis occurs. Then, the mature virions are released from the cell surface via exocytosis [39]. C, capsid; E, envelope; ER, endoplasmic reticulum; M, membrane; NC, nucleocapsid; NS, nonstructural; prM, premembrane; RdRp, RNA-dependent RNA polymerase; TGN, trans-Golgi network.