Phase II biotransformation reactions are essential for detoxifying and eliminating xenobiotics, including many pharmaceutical compounds. These reactions typically involve conjugation, the covalent attachment of polar endogenous groups such as glucuronic acid, sulfate, methyl, or acetyl moieties to functional groups introduced during Phase I metabolism. The resulting conjugates are more water-soluble, enabling efficient renal or biliary excretion.
The major classes of Phase II enzymes include thiopurine S-methyltransferase (TPMT), UDP-glucuronosyltransferase (UGT), and N-acetyltransferase 2 (NAT2). Genetic polymorphisms within the genes encoding these enzymes contribute to marked interindividual variability in enzyme activity, drug response, and toxicity.
Individuals with TPMT deficiency due to genetic mutations metabolize thiopurine drugs such as azathioprine and 6-mercaptopurine at a reduced rate. The unmetabolized drugs can accumulate and lead to profound myelosuppression, necessitating genotype-based dose adjustment or alternative therapies.
UGT1A1 polymorphisms, especially the *28 allele, reduce the glucuronidation capacity for drugs such as irinotecan. Impaired metabolism results in elevated levels of the active metabolite SN-38, which is associated with dose-limiting toxicities such as severe diarrhea and neutropenia. Genotype-guided dosing can mitigate these adverse effects.
NAT2 polymorphisms classify individuals as slow, intermediate, or fast acetylators. Slow acetylators experience delayed drug clearance and are at elevated risk for isoniazid-induced hepatotoxicity. In contrast, fast acetylators may not maintain therapeutic drug levels, compromising efficacy. Understanding a patient’s NAT2 genotype supports optimized dosing regimens for isoniazid and similar agents.
Genetic screening for Phase II enzyme polymorphisms enhances precision in drug therapy, minimizing adverse effects while maximizing therapeutic efficacy.
Phase II biotransformation involves transferase enzymes that attach chemical moieties to drugs, enhancing their elimination.
Key enzymes involved in this phase are thiopurine S-methyltransferase, UDP-glucuronosyltransferase, and N-acetyltransferase.
Genetic polymorphisms in these enzymes can significantly influence their activity, resulting in inter-individual variability in drug response and toxicity.
In patients who inherit low or absent TPMT activity due to genetic variations, thiopurines can accumulate and cause life-threatening myelosuppression.
Similarly, polymorphisms causing reduced expression of the UGT1A1 gene are associated with reduced clearance of irinotecan, increasing the risk of severe diarrhea and neutropenia.
NAT2 gene polymorphisms also play a critical role in determining the rate of acetylation. Slow acetylators are at greater risk for isoniazid-induced hepatotoxicity. In contrast, fast acetylators may fail to achieve therapeutic drug concentrations.
Understanding these polymorphisms helps clinicians tailor drug choices and dosages, enhancing safety and improving therapeutic outcomes.
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