Ribavirin

Overview

Ribavirin was first synthesized in 1970 by ICN Pharmaceuticals (now “Valent International Pharmaceuticals”). In 1986, its first major use was the treatment of RSV (respiratory syncitial virus) infections in pediatric patients, but since its FDA approval in 1998, it has primarily been used as a component in treating Hepatitis C. The treatment was modified and approved in 2002 by the FDA by combining it with interferon alfa2b. ^[1]

Structure & Function

While the main function of Ribavirin is to treat Hepatitis C and RSV, Ribavirin alone is not enough to treat these diseases and is commonly combined with interferon alfa2b. Ribavirin contains antiviral activity which inhibits DNA/RNA synthesis.The structure of Ribavirin resembles the structure of the nucleoside guanosine. Like guanosine, ribavirin is also water soluble and is able to mimic other purines as well. However, a key difference between the structure of ribavirin and the purine nucleosides is that it’s heterocyclic base contains only one ring, as opposed to purines which have two. Despite this, it is able to go through similar mechanisms as that of nucleosides such as phosphorylating into a triphosphate. It’s structural similarity to the common nucleoside guanosine may suggest how the drug can inhibit DNA/RNA synthesis through purine mimicry.

Disease

Hepatitis C

Hepatitis C is an infectious disease that affects the liver due to the Hepatitis C virus (HCV). This disease can be acute or chronic and can even lead to death. By binding to the gC1qR receptor, HCV proteins are able to effectively inhibit the differentiation of helper T cells. In addition, HCV core proteins work by preventing the synthesis of the antiviral interferon IFN-γ. Thus, weakening the body’s immunity and making it susceptible to infection. Ribavirin is used in combination with peginterferon to treat Hepatitis C.^[2] By adding pegylated interferon-alpha to ribavirin, the drug had a longer half life, which required only single weekly dosing for Hepatitis C treatment. ^[3] Polyethylene glycol (PEG) is covalently attached with two types:1. P-INFa-2b and 2. P-INFa-2a. While P-INFa-2b is linear and subject to hydrolysis upon injection, P-INFa-2a is branched and circulated the molecule as a whole. The limited distribution of P-INFa-2a results in a longer half life. ^[4]

Pneumonia

Respiratory syncytial virus is responsible for viral pneumonia. This infection causes the air sacs in one or both of the lungs to become inflamed and potentially filled with fluid or pus. In infants, children, and adults over the age of 65, pneumonia can be deadly. It has been shown that Ribavirin can treat viral pneumonia by preventing transcription of respiratory syncytial virus. ^[5]

Mechanism

Ribavirin, when administered in combination with pegylated interferon alpha, induces an antiviral state in host cells, resulting in reduced virus replication rates and activation of the host immune system. It is thought that Ribavirin acts as an antiviral agent by disrupting viral RNA synthesis, which would impact both transcription and genome replication in the Hepatitis C virus. However, the exact mechanism by which the drug interferes with this process is unknown. Several theories have been proposed to explain the effect of Ribavirin in inhibiting the replication of the Hepatitis C virus ^[3].

Immunomodulation by Ribavirin may be responsible for the drug’s antiviral properties. It has been suggested that the natural CD4+ helper T cell response may be altered in the presence of Ribavirin. It is thought that ribavirin may enhance the T helper 1 response, resulting in greater clearance of virus ^[6]. However, there is conflicting evidence suggesting that the T helper 2 response may be implicated in this process instead ^[7]. Another possible mechanism involves the enhancement of interferon-stimulated gene (ISG) expression by Ribavirin. When a cell becomes infected with a virus, it may release interferons. Interferons are signaling molecules that function in a paracrine fashion to induce an antiviral state in neighboring cells, protecting them from infection. Ribavirin is thought to enhance the interferon signaling pathway ^[8], resulting in a wider antiviral response. This theory has been supported in studies using cell culture models (Thomas et al). It has also been suggested that the relationship between Ribavirin and inosine 5’-monophosphate dehydrogenase (IMPDH) may impact this process. IMPDH play a significant role in the guanine nucleotide synthesis pathway. It results in the conversion of inosine 5’-monophosphate to xanthine 5’-monophosphate, which is an intermediate for the nucleotide guanosine (Shu et al). Therefore, modulation of IMPDH activity affects a cell’s reservoir of guanosine. Ribavirin has been shown to function as a competitive inhibitor for IMPDH (Streeter et al). Because guanosine triphosphate (GTP) plays a critical role in the viral genome replication process, inhibition of IMPDH would result in the prevention of viral replication.

Ribavirin has also been shown to act as an inhibitor for eIF4E, a protein of the translation initiation complex (Kentsis et al). Because of its structural similarity to guanosine, it mimics the 7-methyl guanosine mRNA cap, preventing translation. This would result in reduced capacity for viral replication within an infected cell. The Hepatitis C viral genome is replicated by RNA-dependent RNA polymerase (RdRp). A modified form of Ribavirin, Ribavirin 5’-triphosphate (RTP) is also believed to directly inhibit RdRp activity (Graci et al), resulting in lower rates of genome replication. If Ribavirin is converted to the monophosphate form, RMP, it is believed to be incorporated into the viral genome, functioning as a mutagen.