Fighting Cancer with Immunotoxins
Cancer occurs when abnormal cells in one part of the body go rogue, replicating uncontrollably and potentially spreading to other parts of the body. Can bacterial toxins help control these rogue cells? In the form of immunotoxins, they can.
Immunotoxins consist of an antibody fused to a toxin protein from plants or bacteria, namely Corynebacterium diphtheriae (diphtheria toxin) and Pseudomonas aeruginosa (exotoxin A). Both diphtheria toxin and exotoxin A have similar, well-characterized mechanisms-they modify mammalian elongation factor 2 (eEF-2) to inhibit protein translation, leading to cell death. The antibody portion of immunotoxins targets a receptor highly expressed by cancer cells (the specific receptor varies depending on the immunotoxin and cancer type). When the antibody binds the receptor, the toxin can enter and kill the cancer cell.
Immunotoxins have had some clinical successes. The first immunotoxin, denileukin diftitox (Ontak®), was approved in 1999 by the U.S. Food and Drug Administration (FDA) for treatment of cutaneous T-cell lymphoma (CTCL), a rare form of non-Hodgkin lymphoma. Denileukin diftitox consisted of a fragment of the cytokine IL-2-which targets the IL-2 receptor highly expressed on malignant T cells-bound to portions of the diphtheria toxin. It was eventually withdrawn from the market due to production issues and to enable manufacturing improvements. Recently, Citius Pharmaceuticals created a new version of the therapy (I/ONTAK®) that has better purity and bioactivity compared to the original, and shows promise for treating persistent or recurrent CTCL.
Another example, moxetumomab pasudotox (Lumoxiti®), relies on the cytotoxic activity of exotoxin A. It consists of an antibody targeting CD22, a protein highly expressed on malignant B cells, joined with an exotoxin A-derived peptide. A phase 3 clinical trial demonstrated that, out of 80 adult patients with hairy cell leukemia (HCL, a B cell cancer), 75% responded to treatment with moxetumomab pasudotox, with 41% exhibiting a complete response (i.e., no evidence of HCL in the bone marrow or blood by the end of treatment). The product has since been FDA-approved for treatment of HCL.
Immunotoxins do have some drawbacks. For example, they are seen as "foreign" by the body's immune system, which may lead to the development of neutralizing antibodies that hinder their efficacy. Researchers are exploring how to reduce the immunogenicity of immunotoxins, such as by modifying the antibody and toxin domains and/or administering immunomodulatory drugs in conjunction with the toxin-based therapies.
Botulinum Toxin: A Cosmetic and Medical Wonder
Botulinum toxin (BT) is a potent neurotoxin produced by the foodborne bacterial pathogen, Clostridium botulinum. Though there are 7 serotypes of BT (A, B, C1, D, E, F and G), they all work by blocking the release of acetylcholine, a neurotransmitter, from presynaptic motor neurons, thus preventing the neurons from firing. The result: muscle paralysis.
While ingestion of BT can lead to botulism, a life-threatening disease, its paralytic properties have also been exploited for clinical use. In this sense, BT is popularly known for its cosmetic applications; it is often referred to as Botox®, which is one of several products derived from BT serotypes A or B. Facial BT injections are used to smooth "frown lines," forehead wrinkles and crow's feet, among other aesthetic facial tweaks. It has also been explored for off-label dermatological indications, including hair loss and psoriasis. These products contain extremely small quantities of purified BT-less than 1 gram of purified toxin is enough to produce the world's supply of Botox® products for a year.
In addition to its cosmetic uses, BT therapy has been approved by the FDA for an array of medical indications. The toxin's ability to relax muscles makes it advantageous for conditions characterized by muscle hyperactivity, including various forms of dystonia (involuntary muscle contractions) and urinary incontinence. When injected into exocrine gland tissue, BT can help control hyperhidrosis (excessive sweating) and sialorrhea (excessive saliva production). Routine injections (i.e., every 3 months) of BT into various regions along the head and neck can also dampen migraine pain, likely by blocking the release of compounds (e.g., neuropeptides and inflammatory peptides) involved in chronic migraine development. Recent investigations suggest facial injections of BT may also be effective for managing depression.
Toxoid Vaccines
While toxin-mediated antibody production is undesirable in some cases (e.g., in the context of immunotoxin therapy), scientists have capitalized on this feature to generate vaccines against the bacterial pathogens Clostridium tetani (causes tetanus) and C. diphtheriae (causes diphtheria). As mentioned, diphtheria toxin inhibits protein synthesis. C. tetani produces tetanospasmin, a toxin that prevents release of inhibitory neurotransmitters from neurons in the central nervous system, leading to neuronal hyperactivity and muscle contraction.
Vaccines for diphtheria and tetanus contain inactivated versions of the C. diphtheriae and C. tetani toxins (i.e., toxoids). The toxoids, along with antigens from Bordatella pertussis (the bacterium that causes pertussis, or whooping cough), make up the DTaP vaccine that is administered to children starting at 2 months of age. Another version of the vaccine, TdaP, is administered to adolescents and adults 10 years of age and older, with booster doses recommended every 10 years. When immune cells come across the toxoids, they generate antibodies that can recognize and neutralize the real toxins during infection.
To that end, diphtheria and tetanus toxoid vaccines have been instrumental in controlling these once prevalent diseases. For example, there were 100,000-200,000 cases of diphtheria in the U.S. in the 1920s before widespread vaccination. In 2021, there was only 1 reported case of diphtheria in the U.S. Tetanus cases have similarly declined.
Bacterial Toxin-Antitoxin Systems: New Antimicrobial Targets?
Bacteria don't just produce toxins that harm host cells, but also their own. These toxins are part of toxin-antitoxin (TA) systems present in many bacteria, with type II TA systems being the most well-characterized (pathogens like Mycobacterium tuberculosis, Staphylococcus aureus and Neisseria gonorrhoeae all have them). In these systems, intracellular toxins are bound to protein antitoxins to form a complex. Under certain conditions, the antitoxin is degraded by proteases, releasing the toxin to halt critical cell processes (e.g., inhibit DNA gyrase activity, cleave RNA, among others). Why? There are multiple hypotheses. For example, TA systems may be part of a stress response to halt bacterial growth under stressful conditions (e.g., nutrient starvation), then kick-start it again when conditions improve.
Whatever their purpose, scientists are exploring how type II TA systems could serve as targets for antimicrobials-in other words, is it possible to use toxins against the bacteria that produce them? Several strategies have been investigated, including generating peptides that disrupt or prevent formation of TA protein complexes, or that prevent translation of the antitoxin, thus preventing the antitoxin from neutralizing the toxin. For example, scientists designed peptides that inhibit antitoxin translation for the mazEF and hipBA TA systems (which cleave mRNA and prevent tRNA synthesis, respectively), in Escherichia coli, leading to growth arrest.
Given TA systems are ubiquitous in the bacterial world, researchers are also studying how to specifically target the TA systems of pathogens while leaving harmless/beneficial microbes within the body unharmed, allowing for narrow-spectrum treatments. However, most studies sit squarely in the realm of in vitro and preclinical experimentation. More work is needed to understand the applicability and feasibility of TA-targeting antimicrobials.
The Bottom Line
Bacterial toxins are, well, toxic. However, researchers can modify and strategically use them for beneficial purposes-the toxins can be both our friends and foes. With this in mind, scientists have, and will continue, to explore how to harness bacterial toxins to advance health and prevent disease.
Microbial toxins can help more than humans! Bacteria living inside soil fungus produce toxins that can protect their host from tiny predators.
