Introduction
Viral infections have plagued humankind and contributed immensely to elevated mortality and morbidity. Due to the debilitating nature of viral diseases, scientists have developed ways to ameliorate and alleviate their impact by developing vaccines and antivirals. However, it appears that control and prevention of viral infections is still a daunting task owing to compounding factors, such as zoonosis and mutation (Russell et al. 2006). In this view, there is a need to generate diverse antiviral drugs. Therefore, this literature review delineates different types of antiviral drugs and their targets.
Types of Antiviral Drugs and Their Targets
The anti-HIV drugs in use are a combination of three agents, which are non-nucleoside reverse transcriptase inhibitors (NNRTIs), nucleoside reverse transcriptase inhibitors (NRTIs), and protease inhibitors (PIs). These drugs target viral replication by inhibiting proteases and reverse transcriptases (Bean 2005). NRTIs (zidovudine, abacavir) are structural analogs of natural nucleosides that viral DNA polymerase incorporates into HIV RNA, hence, inhibiting replication. NNRTIs bind directly to reverse transcriptase, at a site downstream of the catalytic site, thus, inhibiting its activity. PIs, interfere with protease activity that is important in breaking down complex proteins and generating virions.
Chronic Hepatitis B has seven antiviral drugs recognized by the Food and Drug Administration. These antiviral drugs are three nucleoside analogs (lamivudine, entecavir, and telbivudine), IFN-α, pegylated IFN-α, and two nucleoside analogs (Clercq 2010). The nucleoside and nucleotide analogs target HBV DNA polymerase while other strategies, such as RNA interference, that target viral RNA exist.
The herpes simplex virus (HSV) manifests in neuronal cells and exhibits latency, with or without symptoms. Billau, Thouvenot, and Morfin (2009) aver that limited drugs for HSV, which majorly target the viral DNA polymerase, remain the standard drugs three decades after their discovery. Cases of resistance amongst immunocompromised patients underscore the need to come up with new chemotherapies that are highly potent against both the wild type and acyclovir-resistant HSV strains.
The frequent outbreaks of H5N1 in Asia and the possibility of zoonosis, have left virologists worried. Acute shortage of antiviral drugs, the occurrence of epidemics, and increasing drug resistance limit the control and prevention of viruses (Hayden & DeJong 2011; Jones et al. 2006). According to Jones et al. (2006), influenza A antiviral drugs belong to two classes, adamantine derivatives (amantadine and rimantadine) and neuraminidase inhibitors (zanamivir and oseltamivir). Neuraminidase derivatives impede the activities of neuraminidase, while adamantine derivatives block the viral M2 ion channel. However, resistance has rendered the usage of these antivirals obsolete, hence, prompting virologists to develop other drugs such as entry blockers.
Conclusion
Antiviral drugs have proven useful in curbing myriad viral diseases. Their mode of action inhibits viral DNA polymerase, proteases important in processing pro-virions into mature virions, viral receptors used in attachment to host cell, and enzymes such as neuraminidases. However, increased cases of antiviral resistance undermine the usefulness of antivirals due to viral intrinsic mechanisms that circumvent the effects of antivirals. Therefore, there is a need for increased pharmacovigilance and the development of efficacious agents.
References
Bean, P 2005, ‘New drug targets for HIV’, Clinical Infectious Diseases, vol. 41, no 1, pp. 96-100.
Billau, G, Thouvenot, G & Morfin, F, 2009, ‘Drug targets in herpes simplex and Epstein Barr virus infections’, Infectious Disorder Drug Targets, vol. 9, no 2, pp.117- 125.
Clercq, E, Geoffrey, F, Suzanne, K & Johan, N 2010, ‘Antiviral treatment of chronic hepatitis b virus infections’, Viruses, vol. 2, no 6, pp. 1279-1305.
Hayden, F & DeJong, M 2011, ’Emerging influenza antiviral resistance threats’, Journal of Infectious Diseases, vol. 203, no 1, pp 6-10.
Jones, J, Elizabeth, T, Bultmann, H, Curtis, B & Schultz-cherry, S 2006, ‘Inhibition of influenza virus infection by a novel antiviral peptide that targets viral attachment to cells’, Journal of Virology, vol. 80, no 24, pp. 11960-11967.
Russel, R, Haire, L, Stevens, D, Collins, P, Lin, Y, Blackburn, M, Hay, A Gamblin, S & Skehel, J 2006, ’The structure of H5N1 avian influenza neuraminidase suggests new opportunities for drug design’, Nature, vol. 443, no. pp. 45-49.