Bioisosteres in Medicinal Chemistry

Bioisosteres

Bioisosteres are molecular entities that result from substituting an atom or a group of atoms with an alternative, yet similar, atom or group of atoms. This concept is widely employed in drug design and medicinal chemistry, as bioisosteric replacement aims to generate new molecules that maintain similar biological properties to the parent compound. The concept of bioisosteres can be traced back to the work of Irving Langmuir in 1919, who defined a bioisostere as a compound or group of atoms with an equal number of atoms and electrons.

The primary goal of bioisosteric replacement is to maintain or enhance the biological activity of the parent compound while optimising its physicochemical, pharmacokinetic, and toxicological properties. For example, researchers can modify a molecule’s size, shape, lipophilicity, and electronic properties by replacing specific atoms or functional groups, influencing its interaction with biological targets, absorption, distribution, metabolism, and excretion.

There are several reasons why bioisosteric replacements are employed in drug design. First, they can improve the potency and selectivity of a lead compound by fine-tuning its interactions with the target protein. This can be achieved by modifying the molecule’s hydrogen bonding pattern, charge distribution, or steric properties to optimise its binding to the target.

Second, bioisosteric replacements can be employed to modulate the physicochemical properties of a chemical compound, such as solubility, lipophilicity, and membrane permeability. These modifications can influence the compound’s pharmacokinetic properties, improving absorption, distribution, and bioavailability.

Third, bioisosteres can reduce the potential for toxic side effects or drug-drug interactions. By modifying a molecule’s metabolism or eliminating reactive functional groups, researchers can minimise the formation of toxic metabolites or decrease the likelihood of undesirable interactions with other medications.

Moreover, bioisosteric replacements can be applied to address intellectual property concerns. By altering the chemical structure of a patented compound, researchers can design novel molecules that retain the desired biological activity while circumventing existing patents.

There are two main types of bioisosteres: classical and non-classical. Classical bioisosteres are groups of atoms with similar electronic and steric properties, such as substituting a hydrogen atom with a fluorine atom. Non-classical bioisosteres, on the other hand, involve more complex structural modifications, such as ring replacements or conformational changes.