Biosimilar Pharmacokinetics: Study Design, Endpoints & Immunogenicity

Biosimilar pharmacokinetics sit at the centre of a comparative clinical pharmacology. Regulators expect sponsors to show that exposure to the biosimilar is sufficiently similar to the reference product that any meaningful clinical differences are unlikely.

This article outlines the main pharmacokinetic considerations for biosimilar development, including study design, primary PK endpoints, immunogenicity, bioanalytic methods, target-mediated drug disposition, reference product sourcing, and common operational pitfalls that can compromise PK sensitivity.

Developing a biosimilar requires demonstrating that it behaves like the reference biologic. While standard bioequivalence study designs work well for small-molecule generics, biologics have unique characteristics that complicate pharmacokinetic (PK) assessment. Understanding these differences is key to design studies that provide robust, regulator-approved evidence.

Table 1:          Key Differences Between Generics and Biologics

These characteristics mean that biosimilarity cannot be demonstrated on the basis of a single type of evidence. For example, The FDA strongly endorses a totality-of-evidence approach is required: analytical characterisation provides the foundation, while comparative pharmacokinetic (PK) and pharmacodynamic (PD) studies address any residual uncertainty. Together, rigorous analytical data and PK/PD comparisons form the cornerstone of a biosimilarity assessment.

While the route to market for traditional generics is well defined with clear pathways of providing the required physiochemical and pharmacokinetic evidence - the nature and subsequent route to market for biosimilars is considerably more complex. A well-designed PK/PD programme is essential to safely and effectively navigate development. Some of the key differences between traditional bioequivalence studies and those conducted to prove biosimilarity (in terms of pharmacokinetic comparability) are summarised in Table 2, below.

Table 2:          Key differences between proving pharmacokinetic comparability in bioequivalence studies versus biosimilarity studies

CI, Confidence interval. CL, Clearance.

Evaluating biosimilars requires a study design that can sensitively detect pharmacokinetic (PK) differences between the biosimilar and its reference product—often under conditions very different from those used for small-molecule bioequivalence. Because biologics have long half-lives and may provoke immune responses, simple crossover designs are rarely appropriate. Parallel designs are usually preferred, and—when safe—healthy volunteers offer reduced variability and greater PK sensitivity. However, patient populations may be required when disease state, target expression, or concomitant therapies influence PK or immunogenicity. While single-dose studies are generally sufficient, multiple-dose or steady-state studies may be necessary when target-mediated drug disposition (TMDD) or pharmacodynamic factors affect exposure. Ultimately, sample size should be driven by anticipated PK variability, sensitivity requirements, and the potential influence of anti-drug antibodies (ADA), rather than arbitrary defaults, consistent with EMA guidance on biosimilar pharmacokinetic and immunogenicity study design.

PK comparability for biosimilars also requires a broader perspective than traditional bioequivalence assessments. For primary PK endpoints, Cₘₐₓ, AUC₀–t and AUC₀–∞ are typically prespecified, with t½ and CL characterised when warranted. In addition to Cₘₐₓ and AUC, parameters such as half-life (t½) and clearance (CL) must be characterised, with predefined 90% confidence interval limits—commonly 0.80–1.25—agreed with regulators. These equivalence margins should be scientifically justified based on variability and clinical relevance and agreed with regulators in advance. Because immunogenicity can alter both exposure and CL, ADA and neutralising antibodies must be evaluated using validated assays and appropriately timed sampling. Subgroup analyses are essential to determine whether antibody-positive participants display altered PK, and any such findings must be reflected in the overall PK conclusions.

Accurate PK assessment depends on robust bioanalytical methods. For biosimilars, bioanalytical assays must meet ICH M10 validation expectations. Assays should demonstrate appropriate selectivity, sensitivity, and control of matrix effects for the intended concentration range and sample type. Because biologics may bind to endogenous or soluble targets, the assay should clearly specify — and scientifically justify — whether it measures total or free analyte. Potential ADA interference must be understood to ensure that any observed differences in exposure or clearance represent genuine PK behaviour rather than assay artefacts.

Dose selection is another essential PK consideration. Where TMDD exists, choosing a dose on the ascending portion of the exposure–response curve helps reveal meaningful differences in disposition between products. This choice should consider the density of expression of the pharmacological target in the selected population. Sampling must be planned to capture non-linear kinetics and fully describe TMDD behaviour, with sufficient timepoints to detect exposure differences between treatment arms, allowing the subsequent interpretation of PK parameters to be both accurate and comparable across treatment arms.

Finally, the practical realities of biosimilar development introduce additional PK considerations. Differences in protein concentration between test and reference formulations may require appropriate adjustment when interpreting PK parameters. Where multiple regional reference products exist, sourcing strategies must be planned in advance. If more than one reference product is used, analytical comparisons or PK bridging studies may be necessary to justify data pooling or cross-regional interpretation.

Operational pitfalls can undermine even well-designed biosimilar PK studies. Batch-to-batch variability in protein content, aggregates or impurities can distort exposure estimates if not tightly controlled and documented across test and reference batches. Device-related factors, including prefilled syringe or autoinjector fill volumes, dead-space and injection speed, may introduce systematic differences between arms if devices are not aligned or handled consistently.

Inconsistent administration route or injection site, wide or poorly enforced sampling windows, and misaligned ADA sampling relative to PK timepoints can all reduce the ability to detect genuine exposure differences. Data handling issues, such as ad hoc outlier exclusion or inconsistent rules for values below the limit of quantification, further complicate interpretation. Proactive planning, detailed site training and clear data handling conventions should therefore be treated as integral components of the PK risk assessment rather than afterthoughts.

Taken together, these pharmacokinetic considerations—spanning study design, immunogenicity assessment, bioanalytics, TMDD, and reference product strategy—form the foundation of a scientifically sound biosimilar development programme. In the UK, MHRA guidance further specifies that confirmatory PK studies should be powered to demonstrate equivalence, with prespecified margins and appropriate subgroup analyses for ADA-positive versus ADA-negative participants. Careful, regulator-aligned planning across all these elements subsequently ensures that PK studies provide the sensitivity and reliability needed to support biosimilarity.

For sponsors and development teams, the goal is a PK package that is sensitive, well-justified, and operationally robust, so that residual uncertainty is minimised before confirmatory efficacy studies. By treating biosimilar pharmacokinetics as a structured, evidence-driven exercise rather than a generic bioequivalence repeat, teams can generate data that support confident regulatory and clinical decision-making.

Quanticate’s statistical consultancy and programming team design, analyse, and interpret biosimilar pharmacokinetic and immunogenicity studies with regulator-ready documentation. If you need support with biosimilar PK study design, analysis, or reporting, please submit an RFI and a member of our business development team will be in touch shortly.

FAQs

How do biosimilar drugs work?
Biosimilars are designed to match the reference biologic in structure, function, and clinical effect. They bind the same target, trigger the same biological responses, and aim to reproduce the safety and efficacy profile of the original drug. Regulatory approval depends on demonstrating no clinically meaningful differences from the reference product.

What is the problem with biosimilars?
Unlike small-molecule generics, biosimilars are large and complex. Minor differences in structure, glycosylation, or impurities can affect safety, efficacy, or immunogenicity. Establishing comparability to the reference product is therefore more challenging and requires careful pharmacokinetic, pharmacodynamic, and immunogenicity evaluation.

What are the 4 stages of pharmacokinetics?
Pharmacokinetics describes the journey of a drug through the body, typically divided into four stages: absorption (how the drug enters systemic circulation), distribution (how it spreads through tissues and fluids), metabolism (how it is chemically modified, usually in the liver), and excretion (how it is eliminated, mainly via urine or faeces).