Immune Modulators in HBV therapy

Immune modulators in HBV therapy aim to restore or augment the host immune response against the hepatitis B virus (HBV). Chronic HBV infection is often characterized by immune tolerance and exhaustion of HBV-specific immune cells. Immune modulators are being developed to overcome these challenges and enhance the immune system's ability to control or eliminate the virus. Below is a detailed overview of the key immune modulators used in HBV therapy:

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### **1. Innate Immune Agonists**

These agents target the innate immune system, the body's first line of defense, to stimulate antiviral responses.

#### **A. Toll-Like Receptor (TLR) Agonists**

• **Example:** Vesatolimod (GS-9688)
• **Mechanism of Action:** Vesatolimod is a TLR8 agonist that activates innate immune pathways, including cytokine production and the stimulation of antiviral immune responses. TLR agonists can enhance the activity of natural killer (NK) cells and dendritic cells, which play crucial roles in controlling HBV infection.
• **Goal:** Promote antiviral immunity and reduce HBV replication.

#### **B. RIG-I Agonists**

• **Example:** SB-9200
• **Mechanism of Action:** SB-9200 targets RIG-I (Retinoic Acid-Inducible Gene I), a cytoplasmic receptor involved in detecting viral RNA. Activation of RIG-I leads to the production of interferons and other antiviral molecules, enhancing the immune response against HBV.
• **Goal:** Stimulate innate antiviral immunity to suppress HBV replication.

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### **2. Checkpoint Inhibitors**

Checkpoint inhibitors are designed to overcome immune exhaustion by blocking inhibitory signals that suppress T-cell activity.

#### **A. Examples**

• **Nivolumab:** A PD-1 (Programmed Death-1) inhibitor that reactivates exhausted T-cells, enabling them to respond more effectively to HBV infection.
• **ASC22 (Envafolimab):** A PD-L1 (Programmed Death-Ligand 1) inhibitor that works similarly to Nivolumab by reinvigorating HBV-specific T-cell responses.

#### **B. Mechanism of Action**

Checkpoint inhibitors target immune checkpoints such as PD-1/PD-L1 pathways, which are upregulated in chronic HBV infection and contribute to T-cell exhaustion. By blocking these pathways, checkpoint inhibitors restore T-cell function and enhance antiviral immunity.

#### **C. Challenges**

• **Modest HBsAg Decline:** While checkpoint inhibitors can improve immune responses, their effect on reducing HBV surface antigen (HBsAg) levels has been modest.
• **Risk of Autoimmunity:** By reactivating immune cells, checkpoint inhibitors may inadvertently trigger autoimmune responses, posing potential safety concerns.

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### **3. Therapeutic Vaccines**

Therapeutic vaccines aim to break immune tolerance and enhance HBV-specific immune responses, particularly T-cell activity.

#### **A. Examples**

• **GS-4774:** A yeast-based therapeutic vaccine designed to stimulate HBV-specific T-cell responses.
• **INO-1800:** A DNA-based vaccine targeting HBV antigens to elicit strong cellular and humoral immune responses.
• **NASVAC:** A nasal vaccine combining HBV core and surface antigens to boost immunity.
• **BRII-179:** A therapeutic vaccine developed to enhance HBV-specific T-cell responses.

#### **B. Mechanism of Action**

Therapeutic vaccines work by presenting HBV antigens to the immune system in a way that stimulates HBV-specific T-cells. These vaccines aim to overcome immune tolerance and restore the ability of the immune system to recognize and attack HBV-infected cells.

#### **C. Goal**

The primary goal of therapeutic vaccines is to induce a robust and sustained immune response that can help control or eliminate HBV infection.

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### **Overall Goal of Immune Modulators in HBV Therapy**

The ultimate aim of immune modulators is to restore or augment the host immune response to HBV. This includes:

• Breaking immune tolerance.
• Reinvigorating exhausted T-cells.
• Enhancing innate and adaptive immune responses.
• Reducing viral replication and HBsAg levels.

Immune modulators represent a promising avenue for achieving functional cure in HBV patients, particularly when used in combination with other antiviral therapies. However, challenges such as safety concerns, modest efficacy, and variability in patient responses remain areas of active research.