Small Molecules vs. Large Molecules (Biologics) for Antiviral Drug Development

Introduction

Viruses cause some of the most challenging diseases affecting humans, from Ebola, influenza and dengue, to herpes, HIV and emerging pandemic threats. Despite decades of scientific progress, effective antiviral treatments remain severely limited. In fact, the vast majority of viral diseases still have no approved therapeutic drugs. Developing new antivirals is therefore a major global health priority.

Traditionally, most antiviral medicines have been small-molecule drugs - relatively simple chemical compounds that can enter cells and block specific parts of the viral life cycle. These drugs have delivered important breakthroughs and remain the backbone of antiviral therapy today. However, small molecules present some challenges, including unwanted side effects, drug resistance, and limitations in how selectively they can target viruses without affecting healthy cells.

In recent years, large-molecule medicines (known as ‘biologics’) have emerged as another promising approach. These complex protein-based therapies can interact with viruses in different ways and may offer advantages in selectivity, durability, and resistance barriers. Understanding the differences between small molecules and biologics, and how each can contribute to antiviral therapy, is an important step towards designing and bringing to market better treatments for viral diseases.

Early antiviral drug development

The first human antiviral drug, idoxuridine, was approved in 1963 to treat herpes simplex keratitis, a viral eye infection. It was a topical therapy and too toxic for systemic use, but it marked a milestone in antiviral chemotherapy.

Idoxuridine is a small molecule that mimics thymidine, a DNA building block, with an added iodine atom. When incorporated into viral DNA during replication, it disrupts base pairing, creating defective DNA and slowing viral replication. This strategy of using nucleoside analogues to interfere with viral genetic material remains a cornerstone of small-molecule antiviral development.

However, because idoxuridine is not selective for infected cells, it can also affect healthy cells, causing significant toxicity. Its safety warnings include potential genetic defects, reproductive harm, and serious eye irritation. This early experience highlights a key challenge for small molecules: balancing efficacy against viral targets with safety for the host.

Small molecule drugs and adverse effects

Small molecule drugs are low-molecular-weight compounds (typically less than 1,000 Daltons), which allows them to easily enter cells and interact with enzymes, receptors, or DNA. While this accessibility is advantageous for targeting viruses, it also increases the risk of off-target effects - interactions with unintended proteins or tissues.

Adverse effects range from mild (e.g., nausea or drowsiness) to severe (e.g., liver or kidney damage). Key causes of side effects include:

1. Non-selectivity: Binding to proteins similar to the intended target.

2. Metabolism issues: Conversion to toxic byproducts by the liver.

3. Off-target tissue interactions: Affecting organs not meant to be targeted.

4. Dose-related effects: Higher doses increase the likelihood of toxicity.

Pharmacokinetics, or ADME (Absorption, Distribution, Metabolism, Excretion), are studied during drug development as key indicators of how the drug interacts with the body, and the results are used to determine optimal dosing, maximize efficacy, and minimize toxicity. Poor absorption, rapid clearance, accumulation in unintended tissues, or formation of toxic metabolites can cause promising drugs to fail, although small molecules do benefit from decades of research, offering predictable metabolic pathways, chemical stability, and well-established regulatory frameworks.

The problem of drug resistance

Viruses replicate rapidly, and errors during genome replication generate mutations, sometimes resulting in new strains with altered properties. Small molecule drugs often target a single viral enzyme or protein, making them vulnerable to resistance: if a mutation changes the target, the drug may no longer bind to the virus effectively.

Factors that can accelerate resistance include:

1. High replication rate: RNA viruses like HIV, HCV, and influenza replicate billions of times per day, increasing mutation probability.

2. Low-fidelity polymerase: A polymerase is a viral enzyme that copies genetic material. Many RNA viruses lack a “proofreading” capability, making them more prone to reproduce errors.

3. Monotherapy: Using a single drug makes it easier for resistant mutations to become dominant.

4. Incomplete adherence: Skipping doses can create conditions for partially resistant viruses to survive and expand.

Large-molecule antivirals (biologics)

Large-molecule antivirals (often called biologics) are typically proteins or peptides produced in living systems, such as cell cultures or engineered microorganisms. These include monoclonal antibodies, recombinant proteins, and other complex molecules designed to engage very specific viral or host targets. Their size and complexity allow them to interact with larger surface areas, forming multiple contacts with targets. This can result in high specificity, strong binding, and reduced off-target toxicity.

Interferon-alpha is widely recognised as the first biologic antiviral. It was isolated and characterised in 1957, and initially used experimentally for viral infections in the 1960s, before gaining formal approval for clinical use against hepatitis in 1986. interferon-alpha is a naturally occurring signalling protein produced by host cells to induce an antiviral state in neighbouring cells, inhibiting viral replication through multiple interferon-stimulated genes.

The discovery of insulin in 1921 also led to a breakthrough biologic-based treatment. Used to treat diabetes, which was previously fatal, insulin was initially produced from animal pancreatic extracts until the early 80s when biosynthetic human insulin was first manufactured using recombinant DNA technology.

Nowadays, modern biologics can be engineered to target the virus, or host factors critical for viral replication. Monoclonal antibodies, RNA-based therapies and engineered proteins expand antiviral options in ways small molecules cannot, offering mechanisms that are less vulnerable to viral resistance.

Advantages of biologics over small molecules

1. Target specificity and safety: Engineered to recognize precise structures, biologics reduce off-target interactions and classic toxicities.

2. Targeting viral entry and host factors: Biologics can neutralise viruses outside cells, block attachment, or inhibit host factors essential for replication.

3. Reduced risk of resistance: Multiple binding sites or targeting conserved viral regions make it harder for viruses to develop and effective mutations.

4. Longer half-life: Many biologics persist longer in the body, allowing infrequent dosing (weekly or monthly), improving adherence and protection.

Limitations of biologics

Biologics are not without their challenges, however, and these need to be weighed carefully against their advantages:

1. Route of administration: Biologics cannot usually be taken orally; injections or IV infusions are required.

2. Manufacturing complexity and cost: Production requires living systems, stringent conditions, and extensive quality control.

3. Stability and handling: Sensitive to temperature and agitation, requiring cold-chain logistics.

4. Immunogenicity: The immune system may recognize biologics as foreign, potentially neutralising their effects or causing hypersensitivity. Modern protein engineering can minimise this risk, but it cannot eliminate it entirely.

Small molecules and biologics: A complementary toolkit

Small molecules and biologics are often presented as competing approaches to drug development. It is more useful to view them as complementary tools, each approach having a mixture of advantages and challenges. With 95% of human viral diseases still lacking any approved antiviral therapeutic, and the ongoing threat of new pathogens emerging, there is no end of unmet medical need in this space.

At Kimer Med, our research focuses on large-molecule antivirals (proteins) designed to selectively target infected cells, minimising off-target toxicity while maintaining broad antiviral activity. By focusing on a highly conserved viral mechanism (viral dsRNA produced during replication), our aim is to create a family of antiviral therapies to treat many currently unaddressed diseases, and remain resilient to viral evolution.

The future of antiviral therapy may well lie in combination strategies, using fast-acting, orally available small molecule drugs alongside durable, highly specific biologics. Together, these approaches provide the ability to reduce viral load, slow resistance, and improve outcomes for patients worldwide.