5.6 : Phase I Reactions: Hydrolytic Reactions

Hydrolysis, a cornerstone of phase I biotransformation reactions, uses water to cleave chemical bonds. This process is pivotal in drug metabolism, generating more polar metabolites that can be easily excreted.

An important hydrolytic reaction is ester hydrolysis. Ester bonds, often found in prodrugs, are broken down, increasing the solubility of drugs like aspirin and lidocaine for more straightforward elimination. Amide hydrolysis is another critical reaction, targeting amide bonds prevalent in numerous drugs. Secondary amides like acetaminophen and tertiary amides like lidocaine undergo hydrolysis, resulting in corresponding carboxylic acids and amines. Hydrazides like isoniazid also experience hydrolysis, yielding carboxylic acid and hydrazine.

Nonaromatic heterocycles, including imidazoles and pyrazoles, can also be hydrolyzed. For instance, metronidazole transforms into the respective carboxylic acid and alcohol through hydrolysis. Hydrolytic dehalogenation, removing a halogen atom from a molecule, represents another hydrolytic mechanism. Chloramphenicol, for example, can be hydrolyzed into its corresponding carboxylic acid and alcohol. Miscellaneous hydrolytic reactions extend to the hydrolysis of thioesters like captopril, which transforms into the respective carboxylic acid and thiol.

Hydrolysis significantly contributes to phase I biotransformation reactions in drug metabolism. The process enhances the polarity and solubility of metabolites, facilitating their excretion and playing an indispensable role in drug elimination.