11.4 Personalized Medicine

11.4 Personalized Medicine

Quick Answer: Personalized medicine tailors therapy to the individual; precision medicine targets molecular disease drivers; stratified medicine groups patients by biomarker. Pharmacists personalize pharmacokinetically (renal by CrCl/eGFR, hepatic by Child-Pugh, weight-based ABW/IBW/adjusted, age, pregnancy/lactation PLLR) and pharmacogenomically (CYP2C19 + clopidogrel, CYP2D6 + codeine/tamoxifen, HLA-B5701 + abacavir, HLA-B1502 + carbamazepine, TPMT/NUDT15 + thiopurines, DPYD + 5-FU, CYP2C9/VKORC1 + warfarin, SLCO1B1 + simvastatin). TDM personalizes vancomycin (AUC-guided), aminoglycosides (extended interval), and antiepileptics.

Definitions: Personalized, Precision, Stratified

• Personalized medicine — tailoring therapy to individual patient characteristics (clinical, genomic, environmental, lifestyle).
• Precision medicine — targeting the molecular drivers of disease (e.g., HER2+ breast cancer treated with trastuzumab, BCR-ABL CML with imatinib, EGFR-mutant NSCLC with erlotinib/osimertinib).
• Stratified medicine — grouping patients by biomarker to guide therapy selection (e.g., CYP2C19 poor metabolizers and clopidogrel non-response).
• Pharmacogenomics (PGx) — the study of how genomic variation affects drug response; pharmacogenetics traditionally refers to single-gene effects, but the terms are now used interchangeably.

Pharmacokinetic Personalization

Renal Dose Adjustment

Estimate renal function with Cockcroft-Gault (CrCl, mL/min):

CrCl = [(140 − age) × weight (kg)] / (72 × SCr) × (0.85 if female)

Cockcroft-Gault remains the standard for drug dosing despite the rise of eGFR (CKD-EPI), which is used for CKD staging. Many package inserts still reference CrCl. Adjust renally cleared drugs (vancomycin, aminoglycosides, gabapentin, pregabalin, apixaban/rivaroxaban, metformin, lithium, atenolol, sotalol, dabigatran, DOACs in varying degrees). For metformin, the FDA recommends eGFR ≥45 to initiate; continue 30–60 with caution; discontinue <30.

Hepatic Dose Adjustment

The Child-Pugh (Child-Turcotte-Pugh) score classifies cirrhosis severity (A, B, C) using bilirubin, albumin, INR, ascites, and encephalopathy. Many drugs require dose reduction or avoidance in Child-Pugh B and C (e.g., voriconazole contraindicated in Child-Pugh C with reduction in B; tacrolimus and benzodiazepines with caution). For drugs with no clear scoring system, monitor for signs of accumulation and adjust based on clinical response.

Weight-Based Dosing

• Actual body weight (ABW) for most drugs and for underweight patients.
• Ideal body weight (IBW): males = 50 + 2.3 kg per inch over 5 feet; females = 45.5 + 2.3 kg per inch over 5 feet.
• Adjusted body weight (AdjBW) = IBW + 0.4 × (ABW − IBW); used for obese patients when a drug is partly distributed to fat (e.g., vancomycin dosing often uses AdjBW or ABW depending on institution).

Age Considerations

Pediatric: drug clearance is reduced in neonates and young infants because of immature hepatic metabolism and renal function, reaching or exceeding adult clearance per kg by 2–6 months to ~2 years for many drugs. Weight-based dosing must account for maturation (e.g., phenytoin, gabapentin, vancomycin in preterm vs term neonates).

Geriatric: sarcopenia reduces creatinine generation, so SCr may underestimate renal impairment — always calculate CrCl. Increased body fat relative to lean mass raises Vd for lipophilic drugs (diazepam, amiodarone). Reduced hepatic blood flow and Phase I metabolism slow clearance. Falls risk rises with sedatives, anticholinergics, alpha-blockers, and opioids — these appear on the Beers Criteria for potentially inappropriate medications in older adults.

Pregnancy Pharmacokinetics

Pregnancy physiologically changes pharmacokinetics: increased GFR (50–60% rise) raises clearance of renally cleared drugs (e.g., lamotrigine, digoxin, lithium, beta-lactams — often need dose increase or more frequent dosing); increased Vd from plasma volume expansion; decreased albumin increases free fraction of highly bound drugs; accelerated CYP450 (CYP2D6, CYP3A4 induction) and slower CYP1A2 change metabolite ratios; placental transfer and teratogenic risk must be weighed. The FDA Pregnancy and Lactation Labeling Rule (PLLR) replaced letter categories (A, B, C, D, X) in 2015 with three narrative subsections: Pregnancy (risk summary, clinical considerations, data), Lactation (risk summary, clinical considerations, data), and Females and Males of Reproductive Potential (pregnancy testing, contraception, infertility).

Lactation

The older Hale's L1–L5 safety categories are still referenced in some references:

• L1 (safest) — compatible (e.g., acetaminophen, ibuprofen, penicillins).
• L2 (safer) — moderately safe; used in limited numbers of breastfeeding mothers without adverse effects.
• L3 (moderately safe) — no controlled studies; potential risk; weigh benefit-risk.
• L4 (possibly hazardous) — risk confirmed; only if no safer alternative.
• L5 (contraindicated) — significant risk; contraindicated (e.g., radioactive isotopes, cocaine, certain chemotherapy).

Assess infant risk by drug properties: high oral bioavailability, low protein binding, low molecular weight, high lipid solubility, long half-life, and relative infant dose >10% all increase exposure. The InfantRisk Center and LactMed (NLM/NIH) are authoritative resources.

Pharmacogenetic-Guided Therapy

Pharmacogenomic biomarkers fall into two main categories: drug-metabolizing enzyme variants (CYP2D6, CYP2C19, CYP2C9, TPMT, NUDT15, DPYD) and drug-target or hypersensitivity markers (HLA-B5701, HLA-B1502, VKORC1, SLCO1B1).

CYP2C19 and Clopidogrel

Clopidogrel is a prodrug activated by CYP2C19. **CYP2C19 poor metabolizers (PM, *2/*2 or 2/3) have reduced active metabolite formation and increased stent thrombosis and CV events. CPIC and FDA recommend alternative antiplatelet (e.g., prasugrel, ticagrelor) for PMs undergoing PCI. The *2 allele is common in East Asian populations (~15% PM frequency).

CYP2D6 and Codeine

Codeine is activated by CYP2D6 to morphine. CYP2D6 ultrarapid metabolizers (UM) can produce life-threatening or fatal morphine overdose (especially breastfeeding infants of UM mothers). CYP2D6 poor metabolizers (PM) have inadequate analgesia. FDA boxed warning and CPIC recommend avoiding codeine in UM and PM; consider alternatives (morphine, hydromorphone, non-opioid) for PM; tramadol and oxycodone are partially CYP2D6-dependent and share some risk.

CYP2D6 and Tamoxifen

Tamoxifen is activated by CYP2D6 to endoxifen. CYP2D6 PM premenopausal women have reduced endoxifen and increased recurrence; CPIC recommends considering an aromatase inhibitor (e.g., exemestane, anastrozole, letrozole) in PM premenopausal women, with ovarian suppression if needed. Postmenopausal women generally receive an AI regardless.

HLA-B*5701 and Abacavir

Abacavir hypersensitivity (fever, rash, GI, respiratory) is strongly associated with HLA-B*5701. Screen before initiating abacavir; if positive, avoid abacavir and document the allergy. Re-challenge after a true HSR can be fatal.

HLA-B*1502 and Carbamazepine

HLA-B*1502 is strongly associated with carbamazepine-induced SJS/TEN, particularly in patients of Han Chinese, Thai, Malaysian, Indonesian, Filipino, and South Asian ancestry. CPIC and FDA recommend screening patients of Asian ancestry before carbamazepine; if positive, avoid carbamazepine and structurally related agents (oxcarbazepine, eslicarbazepine) when feasible. The marker is less predictive in European, African, and Japanese populations.

TPMT and NUDT15 with Thiopurines

Azathioprine, 6-mercaptopurine (6-MP), and thioguanine are inactivated by thiopurine S-methyltransferase (TPMT) and NUDT15. Patients with low or absent TPMT/NUDT15 activity are at high risk of severe, potentially fatal myelosuppression. Test TPMT and NUDT15 before starting; in deficient patients, reduce starting dose to 10% of standard (or use alternative therapy). NUDT15 variants are more common in Asian populations.

DPYD and Fluoropyrimidines

DPD (dihydropyrimidine dehydrogenase, encoded by DPYD) inactivates 5-FU and capecitabine. DPYD deficiency causes severe, sometimes fatal, mucositis, neutropenia, diarrhea, and hand-foot syndrome. CPIC recommends dose reduction for partial deficiency and avoidance for complete deficiency. Test before starting 5-FU or capecitabine, particularly in patients with personal or family history of severe toxicity.

CYP2C9 and VKORC1 with Warfarin

CYP2C9 metabolizes S-warfarin (the more potent enantiomer); VKORC1 is the pharmacologic target. The FDA-recommended warfarin dosing algorithm combines **CYP2C9 2 and 3 and VKORC1 −1639G>A genotype with clinical factors (age, weight, amiodarone use) to estimate starting and maintenance doses. VKORC1 AA homozygotes typically need the lowest doses; CYP2C9 PM patients also need lower doses and more gradual titration.

SLCO1B1 and Simvastatin

SLCO1B1 encodes the hepatic uptake transporter OATP1B1. The SLCO1B1 c.521TC and CC genotypes reduce simvastatin hepatic uptake, raising plasma simvastatin acid levels and myopathy risk. CPIC recommends dose limitation or alternative statin (e.g., pravastatin, rosuvastatin) for CC carriers. The FDA label limits simvastatin 80 mg.

Therapeutic Drug Monitoring (TDM) as Personalization

Vancomycin AUC-Guided Dosing

Vancomycin exposure is best characterized by the AUC/MIC ratio. The 2020 revised vancomycin consensus guideline moved from trough-only targets to AUC-guided dosing with a target AUC/MIC 400–600 (assuming MIC ≤1). Trough-only targets (15–20 mg/L) were inferior and led to AKI; AUC-guided dosing reduces nephrotoxicity while maintaining efficacy. Bayesian dose-forecasting software integrates two measured levels with population PK to estimate AUC.

Aminoglycoside Extended-Interval Dosing

Aminoglycosides (gentamicin, tobramycin, amikacin) exhibit concentration-dependent killing and a post-antibiotic effect that favors extended-interval dosing (e.g., 5–7 mg/kg gentamicin/tobramycin q24h). Monitor with a random level 6–14 hours post-dose and apply the Hartford nomogram to determine the next interval (q24, q36, or q48h). Traditional peak/trough monitoring is used for endocarditis synergistic dosing (low-dose gentamicin) and for unstable renal function.

Antiepileptic Drug Levels

Anti-epileptic drug (AED) TDM distinguishes non-adherence (subtherapeutic level despite prescription), toxicity (supratherapeutic level with side effects), and breakthrough seizures despite therapeutic level (consider dose increase, alternative agent, or non-pharmacologic factors). Key AEDs monitored: phenytoin (note nonlinear kinetics and albumin binding — correct for hypoalbuminemia), valproate, carbamazepine (autoinduction), lamotrigine (altered by hormonal contraceptives and pregnancy), phenobarbital, levetiracetam (renal clearance).

Other TDM Targets

• Digoxin (target 0.5–0.9 ng/mL in heart failure, higher historically in AF).
• Lithium (acute mania ~0.8–1.2 mEq/L; maintenance 0.6–1.0).
• Immunosuppressants (tacrolimus, cyclosporine, sirolimus — trough ranges by transplant type).
• Antiarrhythmics (amiodarone, procainamide, quinidine).

Pharmacometabolomics and Pharmacoproteomics

Pharmacometabolomics profiles an individual's metabolite signatures (lipidomics, amino acids, organic acids) to predict response or toxicity. Pharmacoproteomics uses protein expression patterns to guide therapy selection, particularly in oncology. These are emerging fields and not yet routine clinical tools, but FPGEE may ask their definitions.

Ethical and Health Equity Considerations

PGx testing raises equity concerns. Access barriers — cost, insurance coverage, geographic location, language — limit uptake. Genomic databases historically over-represent European-ancestry populations, so variant interpretations are less reliable for African, Asian, Indigenous, and admixed populations, risking misclassification (e.g., a variant common and benign in one population may be misread as pathogenic in another). Direct-to-consumer testing may yield variants without counseling. Pharmacists advocate for equitable access, culturally competent pre-test counseling, diverse database representation, and avoidance of deterministic interpretations. The Genetic Information Nondiscrimination Act (GINA) prohibits genetic discrimination in employment and health insurance (life, disability, and long-term care insurance are not covered).