Antibodies have been used as medicines for a wide range of diseases. However, there is a group of target molecules that researchers are struggeling to find antibodies against. These target molecules are a group of membrane proteins – more specifically, ion channels and G-protein coupled receptors (GPCRs). They are found in the membranes of our cells. Because the proteins are located in the membrane, only a small portion of the proteins protrude to outside the cell. It is this part of the membrane proteins that you want to find antibodies against.
While being hard to target, the membrane proteins are interesting because a wide range of diseases can be treated by interacting with these channels and receptors. An example is chronic pain, which could be alleviated by blocking the activity of pain receptors. This is why researchers and pharmaceutical companies are very interested in finding antibodies that can recognize these ion channels and GPCRs.
Evolution has spent millions of years developing molecules that can bind to these ion channels and GPCRs. Venom from snakes, scorpions, spiders and many other animals contain toxins that can bind and block these membrane proteins. These toxins have evolved over millions of years, as it has been beneficial for animals to affect these channels, based on their important functions in the body, such as in nerve signaling. An example of this is paralysis of enemies or prey, as is the case with the black mamba’s venom. These ion channel-affecting toxins are small proteins that are unique in their structure because they contain many cysteines. Cysteines are amino acids that form disulfide bridges in proteins. The disulfide bridges turn these toxins into very small and compact proteins – also known as “knottins”. The names stems from the protein structure resembling a knot.
Researchers from a biotech company in the UK (Maxion Therapeutics) have been working for the past 10 years to harness the natural ability of knottins to target channels and receptors that antibodies have historically been challenged to target. They have done this by fusing knottins with antibodies. More specifically, they have cloned DNA sequences encoding different knottins into the part of the antibody responsible for antigen recognition. In this way, they have created antibodies with specificity to ion channels or GPCRs based on knottins.
But you might be thinking “why go to all that trouble, why not just use knottins directly as a treatment”. However, it’s not as simple as just isolating the genes that code for the different knottins and producing them for medical use. The pharmaceutical industry is regulated by several different regulatory instances that make sure that ensures, that new drugs entering the marked have been thoroughly tested. Proteins such as knottins have many characteristics that are unfavorable for the use as medicine; they are not of human origin, meaning the human immune system are more likely to fight them, as if they were an unwelcome infection. In addition, the toxins are very small and are therefore quickly filtered and excreted from the body through the kidneys. We say that the toxins have a short half-life (the time it takes for the body to halve the available amount of the drug). Toxins are also difficult to produce and the method for their production is not standardized, as it is for antibodies.
Therefore, the combination of antibodies and knottins, in what are called “knotbodies”, is a combination that maintains specificity towards ion channels/GPCRs while retaining good drug-like properties (Figure 25). These properties come from the antibody and include: standardized production, high safety and long half-life.
