6.4 Bioavailability, Bioequivalence, Biosimilars, and Pharmacogenomics

Bioavailability: Definitions and Metrics

Bioavailability is the fraction of administered dose reaching systemic circulation unchanged, plus the rate at which it appears. Three parameters characterize a plasma concentration-time curve:

• Cmax — peak plasma concentration; relates to peak pharmacologic effect and acute toxicity.
• Tmax — time to Cmax; reflects absorption rate.
• AUC (area under the curve) — total exposure; proportional to absorbed dose when clearance is constant.

Absolute bioavailability compares an extravascular route (typically oral) to IV: F = (AUC_po · Dose_iv) / (AUC_iv · Dose_po). IV F is by definition 1.0. Relative bioavailability compares two non-IV formulations (e.g., a generic tablet to a brand tablet).

AUC is computed numerically with the linear trapezoidal rule between sampled time points, then extrapolated to infinity by adding C_last / k, where k is the terminal elimination rate constant. Non-compartmental analysis is the regulatory standard because it makes no structural model assumptions.

Bioequivalence and FDA Standards

Two products are bioequivalent if their rate and extent of absorption are so similar that they are expected to produce the same clinical effect and safety profile. FDA requires a two-way crossover study in healthy volunteers under fasting or fed conditions, with single and multiple doses as appropriate.

The decision rule: the 90% confidence interval of the geometric mean test/reference ratio for both AUC and Cmax must lie within 0.80 to 1.25 (in natural log scale this is a symmetric interval of ±ln(1.25)). The 80-125% bounds are not the same as a 20% allowable difference — they are a symmetric region on the log scale that the FDA adopted to control Type I and Type II error for both directions of inequality. Tmax differences are assessed only descriptively, except for products where onset matters (e.g., analgesics).

A drug product meeting bioequivalence standards can be substituted at the pharmacy under FDA's AB-rating (Orange Book). Therapeutic equivalence also requires the same active ingredient, dosage form, strength, route, and labeling.

Biosimilars vs Generics

A generic small-molecule drug is chemically identical to its reference listed drug and can be substituted without prescriber notification after FDA AB-rating.

A biosimilar is a biologic product that is highly similar to the reference biologic, with no clinically meaningful differences in safety, purity, or potency. Because biologics are produced in living cells, post-translational modifications (glycosylation, deamidation, aggregation) cannot be perfectly replicated.

Biosimilars are approved through the 351(k) pathway of the Biologics Price Competition and Innovation Act (BPCIA). They require analytical, animal, and clinical studies (often a pharmacokinetic/pharmacodynamic equivalence study in a sensitive indication). Extrapolation of indications is permitted if the mechanism of action and safety are expected to be the same across the reference product's approved uses.

Interchangeability is a higher FDA standard: an interchangeable biosimilar may be substituted for the reference product by a pharmacist without prescriber intervention, subject to state law. Immunogenicity remains a residual risk; anti-drug antibodies can reduce efficacy or cause hypersensitivity, so post-marketing surveillance is mandatory.

Pharmacogenomics: Population-Based Genetic Variation

Pharmacogenomics (PGx) studies how genome variation affects drug response. The FDA labels many drugs with PGx information; the Clinical Pharmacogenetics Implementation Consortium (CPIC) publishes peer-reviewed dosing guidelines.

Warfarin Dosing Algorithm

Warfarin maintenance dose depends on CYP2C9 (metabolism of S-warfarin) and VKORC1 (target enzyme sensitivity):

• **CYP2C9 1/1 — normal metabolism; *2/*3 variants reduce enzyme activity by ~30% and ~80%, lowering dose requirements.
• VKORC1 -1639G>A — A allele increases sensitivity; AA homozygotes need ~25-30% lower doses than GG homozygotes.

FDA labeling recommends lower starting doses (e.g., 5 mg or less) for patients with one or more CYP2C9 variant alleles or the VKORC1 AA genotype. CPIC combines both into a dosing table spanning 0.5-7 mg/day. Genotyping is most useful at therapy initiation; INR remains the dynamic measure once dosing is underway.

Tumor Genotyping for Targeted Therapy

Somatic tumor mutations drive therapy selection in oncology:

• HER2 amplification — trastuzumab, pertuzumab, ado-trastuzumab emtansine in breast and gastric cancer; test by IHC/FISH before therapy.
• EGFR mutations (exon 19 deletion, L858R) — erlotinib, gefitinib, osimertinib in non-small cell lung cancer; T790M predicts first-generation TKI resistance and osimertinib sensitivity.
• KRAS G12C — sotorasib, adagrasib in NSCLC; historically "undruggable."
• BRAF V600E — vemurafenib, dabrafenib (with trametinib) in melanoma, NSCLC, anaplastic thyroid; combined with MEK inhibition to delay resistance.
• BCR-ABL — imatinib, dasatinib, nilotinib in CML.
• PD-L1 expression (TPS) — pembrolizumab first-line NSCLC if TPS ≥ 50% without actionable driver mutation.

Preemptive Genotyping in Practice

Preemptive genotyping sequences a panel of relevant genes before prescribing, embedding results in the EHR so that alerts fire when a relevant drug is ordered. Preemptive panels cover CYP2D6, CYP2C19, CYP2C9, CYP3A5, TPMT, NUDT15, SLCO1B1 (statin myopathy risk), HLA-B5701, HLA-B1502, DPYD, and G6PD. CPIC Level A and B guidelines prioritize drug-gene pairs with strong evidence and actionable alternatives. FDA drug labeling uses terms "Test before dosing," "Recommended use," or "Informational" to communicate PGx strength.