Chapter 7: Pharmacogenomics & Personalized Drug Response

Pharmacogenomics is a core concept in precision medicine, focusing on how an individual's entire genetic blueprint, or DNA sequence (genome), impacts their physiological response to medications. This approach addresses the widely known interpatient variability in drug response, which is influenced not only by factors like age or disease but profoundly by inherited genetic differences (polymorphisms). While the human genome is 99.9% identical among individuals, the 0.1% variation, often consisting of single-nucleotide polymorphisms (SNPs), determines a patient’s uniqueness, susceptibility to disease, and ability to absorb, distribute, metabolize, and excrete drugs. Understanding these variations is crucial, especially regarding the cytochrome P450 (CYP) superfamily of liver enzymes, which process approximately 75% of all administered drugs. Genetic variations in these enzymes categorize patients into four distinct drug metabolism phenotypes: Ultrarapid Metabolizers (URMs), Extensive Metabolizers (EMs, the normal 1 wild-type), Intermediate Metabolizers (IMs), and Poor Metabolizers (PMs), requiring clinicians—such as nurse practitioners and physician assistants—to tailor treatment to genetic makeup. For instance, the prodrug clopidogrel requires activation by CYP2C19; individuals who are PMs (due to variants like *2 or *3) may experience a lack of therapeutic efficacy when suffering an acute coronary syndrome. Conversely, the potent anticoagulant warfarin is metabolized by CYP2C9. Patients with decreased CYP2C9 activity (*2 or *3 alleles) or certain polymorphisms in the target enzyme VKORC1 (like the A haplotype) face an increased risk of excessive bleeding due to slow clearance of the active drug. Genetic testing in these situations can predict potential adverse drug reactions (ADRs) and guide appropriate dosing. Pharmacogenomics also highlights risks beyond metabolism, such as aminoglycoside antibiotic-induced ototoxicity, which is linked to maternally inherited mitochondrial mutations. Although scientific progress is rapid, the clinical implementation of pharmacogenomics still faces policy obstacles, including ethical, legal, and social issues (ELSIs) related to patient privacy, data protection, and the critical need to educate health care providers.