What is an Immunotoxin?
An immunotoxin is a type of targeted therapy that uses a targeted binding domain, like an antibody, linked to a highly potent cytotoxic agent or toxin. The primary goal of an immunotoxin is to deliver the toxin directly to specific, typically diseased, cells while minimizing harm to healthy tissue. Upon binding to the target cell's surface, the immunotoxin is internalized, and the toxin component is released into the cytosol, where it works to induce cell death, often through the inhibition of protein synthesis.
Immunotoxins are classified in several ways, most commonly by their manufacturing method and the origin of their toxic component. This classification helps in understanding their characteristics, advantages, and drawbacks.
Classification by Manufacturing Method
The development of immunotoxins has progressed significantly over the decades, leading to a classification based on their production method.
Chemically Conjugated Immunotoxins
Chemically conjugated immunotoxins, often referred to as first-generation or second-generation, are created by chemically linking a toxin to an antibody.
- First-generation: These involved conjugating a whole antibody to a whole or a catalytic subunit of a toxin. They were often heterogeneous, resulting in variability in the number of toxin molecules per antibody and inconsistent efficacy and toxicity. The use of whole antibodies also meant potential host immunogenicity against the foreign antibody component.
- Second-generation: These improved upon the first by using antibody fragments, which generally have better tumor penetration and pharmacokinetics. However, they were still produced via chemical linking, retaining many of the heterogeneity and immunogenicity issues.
Recombinant Immunotoxins
Recombinant immunotoxins (RITs) are a significant advancement, constructed using genetic engineering techniques. A DNA sequence encoding an antibody fragment (often a single-chain variable fragment, or scFv) is fused with a DNA sequence for a truncated, modified toxin. The resulting fusion protein is then produced in a host, such as E. coli.
Key features of RITs include:
- Homogeneity: They are highly uniform in their structure and composition, leading to more predictable pharmacological properties.
- Specificity: Using engineered antibody fragments (e.g., scFv) enhances specific binding to the target antigen.
- Tunability: Genetic engineering allows for modifications to reduce immunogenicity, improve stability, and alter the cytotoxic potency.
- Examples: Denileukin diftitox (historical, DT-based) and Moxetumomab pasudotox (approved, PE-based) are prominent examples of RITs.
Classification by Toxin Origin
Another major classification is based on the source of the cytotoxic payload.
Bacterial Toxins
These are derived from potent bacterial proteins and are engineered for safe and targeted delivery. The most commonly used include:
- Pseudomonas exotoxin A (PE): A highly potent toxin from Pseudomonas aeruginosa. Domains responsible for native receptor binding are removed, and a domain is added to facilitate targeting. Truncated versions like PE38 and PE24 are frequently used. PE inactivates eukaryotic elongation factor 2 (eEF2) via ADP-ribosylation, halting protein synthesis and causing cell death.
- Diphtheria Toxin (DT): Produced by Corynebacterium diphtheriae, DT also functions by ADP-ribosylating eEF2. The targeting domain is removed, and the catalytic domain is used in immunotoxin constructs. Denileukin diftitox is a DT-based example.
Plant Toxins
These come from various plant species and typically function as ribosome-inactivating proteins (RIPs), halting protein synthesis by damaging ribosomal RNA.
- Ricin A chain (RTA): Derived from the castor bean (Ricinus communis), the A chain is the cytotoxic component. The native binding chain is removed to reduce systemic toxicity.
- Gelonin: A RIP from the plant Gelonium multiflorum. It is a single-chain protein, simplifying its use in immunotoxin construction.
- Saporin: A RIP derived from the soapwort plant (Saponaria officinalis).
Human Toxins
One of the most recent advancements involves using human-derived proteins to reduce the immunogenicity associated with bacterial and plant toxins.
- Ribonucleases (RNases): Human pancreatic RNase is one example that can be engineered into immunotoxins. Its smaller size and lower immunogenicity offer advantages.
- Granzyme B (GrB): A serine protease involved in apoptosis within the human immune system. GrB-based immunotoxins can initiate programmed cell death in target cells.
Challenges and Advances in Immunotoxin Therapy
Developing effective immunotoxins has faced several challenges, most notably immunogenicity and off-target toxicities.
- Immunogenicity: The foreign nature of bacterial and plant toxins can provoke a strong immune response in patients, leading to the development of neutralizing antibodies. This can limit the effectiveness of repeat treatments. Strategies to overcome this include de-immunizing the toxin (mutating immunogenic epitopes) and combining therapy with immunosuppressive agents.
- Vascular Leak Syndrome (VLS): A significant dose-limiting toxicity for some immunotoxins, VLS results from damage to endothelial cells. Modern engineering of the toxin component, including deletion of specific domains, has helped reduce this effect.
Immunotoxins vs. Antibody-Drug Conjugates (ADCs)
While both are antibody-based targeted therapies, immunotoxins and ADCs differ fundamentally in their toxic payload and manufacturing.
| Feature | Immunotoxins | Antibody-Drug Conjugates (ADCs) |
|---|---|---|
| Payload | Large protein toxins (e.g., PE, Ricin A, RNases). | Small-molecule cytotoxic drugs (e.g., auristatins, maytansinoids). |
| Structure | Recombinant protein fusion or chemically conjugated protein. | Antibody chemically linked to a small-molecule drug via a chemical linker. |
| Mechanism | Inhibits protein synthesis (eEF2 inactivation or ribosomal damage). | Varies, but often targets cell cycle or DNA, e.g., microtubule disruption. |
| Immunogenicity | Can be highly immunogenic, especially bacterial toxins, though reduced in modern designs. | Generally lower than protein-based immunotoxins, though immunogenicity can still occur against the antibody or drug. |
| Production | Primarily recombinant expression in bacteria, followed by purification. | Chemical synthesis and conjugation after antibody production. |
| Toxicity Profile | Potential for specific toxicities like vascular leak syndrome. | Potential for off-target toxicity if the linker is unstable, releasing the payload prematurely. |
Conclusion
Immunotoxins have evolved significantly from their early, chemically-conjugated forms to highly specific and potent recombinant versions. The different types, classified by their construction and toxin origin, each offer unique advantages and challenges. While bacterial and plant toxins offer extreme potency, they come with the hurdle of immunogenicity. The emergence of human-derived toxins represents a promising path toward less immunogenic, repeatable treatments. The success of agents like moxetumomab pasudotox showcases the potential for immunotoxins in treating specific malignancies. As research continues to refine engineering strategies, address challenges like immunogenicity and vascular leak syndrome, and explore new toxin and targeting moieties, immunotoxins are poised to become an increasingly important class of targeted cancer therapy. For more in-depth scientific reviews on immunotoxin therapy, reputable sources like the National Institutes of Health provide comprehensive overviews.
