What Is Antiviral Therapy?

Introduction

Viruses are composed of nucleic acids (such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)) surrounded by a protein coat. Viruses are invisible to the naked eye, and can only be seen using an electron microscope. Though viruses share characteristic structural features, they cause a diverse range of diseases. These diseases can be short-term or long-term, exhibit varying levels of severity, and can affect virtually any part of our bodies. Some examples of well-known viruses include human immunodeficiency virus (HIV),  hepatitis A, B and C, rhinovirus, poliovirus, and SARS-CoV-2 (or coronavirus).¹ Viruses are very common, and they have infected billions of people throughout history. Even today, they continue to affect millions of people and are responsible for many deaths.^{2 3}

Alongside vaccinations, antiviral therapy has served as a tool to minimise the impact of viral infections by reducing disease severity and mortality.² Having said that, the development of antiviral therapies to curb the rising rates of viruses has not yet been achieved. This task is becoming increasingly difficult due to viruses developing resistance to antiviral therapy.³ Nonetheless, antiviral therapy has proved to be  lifesaving for many patients, such as in the case of HIV.⁴

Types of antiviral therapy

There are various forms of antiviral therapy which can be classified depending on their mechanism of action. These different therapies are suitable for treating a range of viruses. Some of the most common classes are listed below:

• Nucleotide/nucleoside analogues: nucleotide/nucleoside analogues are molecular structures that resemble the building blocks of DNA or RNA. These drugs can alter the biochemical processes occurring in virus-infected cells and stop viruses from replicating.^5,6,7
• Protease inhibitors: protease inhibitors reduce the activity of proteases, which are enzymes that support viral replication.^8,9
• Fusion inhibitors: Fusion inhibitors prevent viruses from fusing with our cells. By preventing said fusion, viruses are unable to enter cells and therefore are unable to infect them.¹⁰
• Polymerase inhibitors: Polymerase inhibitors reduce the activity of polymerases,  which are enzymes essential for viral replication.¹¹
• Immunomodulatory agents: When viruses enter our bodies, they activate our immune system - causing various biological processes to occur as an act of defence against the virus.¹² Immunomodulatory agents alter how the immune system responds to viruses, enhancing the biological processes that fight viruses attacking the body.¹³

Uses of antiviral therapy

Antiviral therapy can be used to treat many types of viruses, which vary widely in their severity.  There are numerous viruses that can be treated with antiviral therapy. Some of the more prominent of these are mentioned below:⁴  

• Influenza: influenza refers to a group of flu-causing viruses. This group includes influenza A, B, C and D viruses. Typically, these viruses are transmitted through droplets, such as those produced from talking, coughing, and sneezing. Influenza typically causes fever, headache, aches, faintness, tiredness, and coughs.¹⁴
• Human immunodeficiency virus: HIV is typically transmitted through sexual activity, although it can also be spread via infected needles or blood transfusions. HIV kills our white blood cells which help the body fight against foreign substances and disease, making the body more vulnerable to sickness. If not immediately treated with antiviral therapy, HIV can progress to acquired immunodeficiency syndrome (AIDS). This can be fatal, as AIDS results in the severe weakening of the immune system.¹⁵
• Hepatitis viruses: these viruses (hepatitis A, B and C) are responsible for hepatitis. They have varying routes of transmission and can cause a range of signs and symptoms. Though they have differences, each of these viruses leads to inflammation of the liver. Some of these viruses can be prevented by vaccines, while others require antiviral therapy.¹⁶
• Herpes simplex virus: herpes simplex virus (HSV) refers to two herpes viruses, HSV1 and HSV2. HSV1 typically causes lesions on and around the mouth, whilst HSV2 tends to cause genital lesions and infections in newborns. HSV is one of the most common viruses that infect humans.¹⁷
• Cytomegalovirus: cytomegalovirus (CMV) is a herpesvirus that can be acquired through exposure to bodily fluids. CMV often causes no signs and symptoms in healthy patients, though those with a weakened immune system are vulnerable to more serious illness.¹⁸

Resistance to antiviral therapy

Resistance to antiviral therapy has become a major problem, as it leads to certain types of antiviral therapy becoming ineffective. Resistance to antiviral therapy occurs through certain viruses mutating so that they can resist an antiviral therapy that was once able to successfully treat them. 

Mechanisms of resistance to antiviral therapy vary according to the type of virus. Viruses that contain RNA mutate much more than viruses with DNA. Furthermore, some types of antiviral therapy are more susceptible to resistance than others. Some patients who undergo several types of antiviral therapy, such as in the case of HIV patients, are less likely to experience viral resistance as it is unlikely that viruses will mutate enough to produce resistance to the different types of antiviral therapy.⁴

Side effects of antiviral therapy

Antiviral therapy can cause a range of side effects, depending on the type of drug being used and the types of antiviral therapy a patient is receiving. Ultimately, the side effects of antiviral therapy vary from person to person, and they can affect various bodily systems. Typically, side effects will depend on the dose being taken and can be reversed by lowering the dosage or stopping treatment altogether.¹⁹

It is important to note that certain side effects are less likely to occur if patients take specific measures. For example, staying hydrated can help prevent damage to the kidney and can reduce other symptoms associated with kidney damage. Additionally, maintaining balanced electrolyte levels can help patients avoid disorders of the brain and heart.²⁰

Gastrointestinal side effects

These side effects affect the stomach and the intestines. Common gastrointestinal side effects of antiviral therapy include:²⁰

• Diarrhoea
• Nausea
• Vomiting 

Haematological side effects

These side effects affect the blood. Blood disorders are among the more common side effects of antiviral therapy, and they tend to be related to bone marrow suppression. The bone marrow is responsible for the production of the cells that make up blood. These side effects include:^{19 20}

• Anaemia (low red blood cell count)
• Leukopenia (low white blood cell count)

Optic side effects

Some types of antiviral therapy, such as the drug fomivirsen, can increase the pressure within the eye. This can result in several side effects that impact vision:²⁰

• Blurred vision
• Subconjunctival haemorrhage
• Retinal detachment
• Macula oedema

Neuropsychiatric side effects

Neuropsychiatric side effects refer to those affecting the brain and mental functions. These include:¹⁹

• Cognitive impairment
• Irritability
• Depression
• Mania
• Headaches
• Sleep disturbances
• Dizziness
• Hallucinations
• Seizures

Summary

Antiviral therapy is a key treatment for a range of viruses and has played a key role in preventing viral infections from becoming fatal. There are various types of antiviral therapy available, which can be classified depending on their mechanism of action. Some viruses have adapted to antiviral treatments by mutating so that they can survive and resist antiviral therapy. Taking multiple types of antiviral therapy, such as in the case of HIV patients, can reduce the likelihood of viral resistance emerging. 

Antiviral therapy can cause numerous side effects that impact various systems in our bodies, though these are typically dose-dependent and reversible. Gastrointestinal and haematological side effects are the most common.  Despite these side effects and increasing resistance to these treatments, antiviral therapy is a very useful treatment for viral infections and one that has saved many lives.

FAQ’s

What types of viruses can be treated with antiviral therapy?

A range of viruses can be treated with antiviral therapy.  Examples include influenza viruses, HIV, hepatitis viruses, herpes simplex virus, and cytomegalovirus.⁴

Are antiviral therapies effective in treating all viruses?

Not all viruses can be treated with antiviral therapy. Furthermore, resistance to antiviral therapy has caused some types of antiviral therapy to be less effective in treating certain viruses.⁴

How is antiviral therapy different from antibiotics?

The purpose of antiviral therapy is to treat viral diseases, whereas antibiotics are used to treat bacterial infections.^4,21

Can antiviral therapy prevent viral infections?

Antiviral therapy can be used as a tool to prevent viral infections in patients who possess risk factors for these infections.⁴