Drugs are generally subdivided into two parts: small molecules and large molecules (or biologics). However, a third class of polypeptide drugs called peptides falls right in between, thus creating an overlap in the benefits of other classes of molecules.
Small molecule drugs are the most common form of therapy, and they are small, simple molecules that are easy to synthesize in the laboratory. These medications are usually taken by mouth, such as pain relievers, and they produce the desired effect quickly. However, due to their chemical composition, the effects last for a few hours at most. Macromolecules, often referred to as biologics, are more complex. They are usually proteins produced by living cells and usually need to be injected into a patient's blood. Common examples are antibodies, vaccines or gene therapy. Unlike small molecules, biologics often share similarities with the body's own molecules and bind very specifically to their targets in the body, making them highly effective against serious diseases with fewer side effects.
A third class of drugs is rapidly gaining popularity: peptide drugs. They are right in between small molecules and biologics. Like small molecules, peptides are usually produced in the laboratory and can exert their intended effect fairly quickly. Meanwhile, like biologics, peptides are usually based on endogenous compounds, composed of amino acids, and are usually well tolerated in patients.
While biologics and peptides are composed of the same building blocks (amino acids), the difference between the two is size. Chains of more than 40 amino acids are considered proteins, such as antibodies, and are classified as biologics, while chains of less than 40 amino acids are called peptides and are classified by the FDA. Despite their differences in size, what peptide drugs and biologics have in common is that they are highly specific for their targets and can produce long-lasting effects.
Some amino acids are the building blocks of naturally occurring peptides and proteins, and in fact, some peptides may occur naturally in the body and are therefore readily available. However, due to their small size, peptides can also be produced synthetically, unlike biologics derived from living cells. Because of their incredible diversity, peptides are used to treat diseases ranging from cancer to diabetes, so it's no surprise that peptide drugs are steadily on the rise.
In general, peptide drug development is considered highly attractive by pharmaceutical companies because of their high likelihood of approval (LoA) or probability of success (POS) in clinical trials compared to small molecules.