THE ASSESSMENT OF SYSTEMIC EXPOSURE IN TOXICITY STUDIES
S3A
1 INTRODUCTION
This Note for Guidance focuses on toxicokinetics in the development of pharmaceutical products for human use. It defines toxicokinetics as the generation of pharmacokinetic data relevant to non-clinical toxicity studies, aiding in interpreting toxicology findings and their safety implications. The guidance emphasizes integrating pharmacokinetics into toxicity testing, allowing for optimal study design and minimizing animal use by reducing duplication. Toxicokinetics assists in understanding the kinetics of therapeutic agents under toxicity study conditions, contributing to risk and safety assessments without rigid procedures. The necessity for toxicokinetic data is determined on a case-by-case basis, following a flexible approach to ensure adequate information for safety evaluations.
2. Objectives of toxicokinetics and criteria that can be determined
Toxicokinetics primarily aims to describe systemic exposure in animals concerning dose level and toxicity study duration. Secondary objectives include relating exposure to toxicological outcomes, aiding species and treatment regimen selection in non-clinical studies and informing the design of future toxicity studies based on initial findings.
Achieving these objectives involves deriving pharmacokinetic parameters from measurements taken at specific time points during studies. Typically, these measurements include plasma, whole blood, or serum concentrations of the parent compound and/or its metabolites, selected on a case-by-case basis. The most common parameters for assessing exposure in toxicokinetics are AUC, Cmax, and C(time). For certain compounds, calculating exposure based on unbound plasma protein concentration may be more appropriate.
These data may be obtained from all animals on a toxicity study, in representative subgroups, in satellite groups (see 3.5 and Note 1) or in separate studies.
Toxicity studies that can benefit from toxicokinetic information encompass single and repeated-dose toxicity studies, as well as reproductive, genotoxicity, and carcinogenicity studies. This information is also valuable for evaluating the implications of changes to the clinical route of administration.
3. General principles to be considered
In the design of individual studies, general principles must be observed, particularly for toxicity studies governed by Good Laboratory Practice (GLP). Toxicokinetics associated with these studies should also adhere to GLP. Additionally, toxicokinetic studies should be designed retrospectively to produce specific data under conditions that closely resemble those of the toxicity studies when required for safety evaluation.
3.2 Quantification of exposure
The quantification of systemic exposure is crucial for assessing the burden on test species and understanding toxicity variations across different species, doses, and sexes. It can be represented by plasma concentrations or the area under the curve (AUC) of the parent compound and/or its metabolites. Studies may also focus on tissue concentrations. When designing toxicity studies, relevant human exposure at therapeutic doses must be considered, along with potential species differences in the pharmacodynamics of the compound.
Pharmacodynamic effects or toxicity may also serve as supporting evidence of exposure, and in some cases can supplant pharmacokinetic measures.
Toxicokinetic monitoring in toxicity studies determines the exposure levels achieved and identifies non-linear, dose-related changes. This information facilitates better interspecies comparisons than basic dose/body weight or surface area assessments.
3.3 Justification of time points for sampling
The collection of body fluids in concomitant toxicokinetic studies should occur frequently enough to estimate exposure without causing undue stress to the animals. The justification for the number of time points should rely on kinetic data from previous toxicity studies or relevant pilot studies in comparable animal models.
3.4 Contribution to the setting of dose levels in order to produce adequate exposure
The dose levels in toxicity studies are primarily determined by toxicology findings and pharmacodynamic responses of test species, influenced by certain toxicokinetic principles.
3.4.1 Low dose levels
In toxicity studies, animal exposure should ideally match or slightly exceed the maximum expected dose in patients, although this is not always feasible. Low doses often depend on toxicology considerations, but systemic exposure must still be assessed.
3.4.2 Intermediate dose levels
Exposure at intermediate dose levels should generally reflect a suitable multiple or fraction of exposures at lower or higher dose levels, depending on the study’s objectives.
3.4.3 High dose levels
The determination of high dose levels in toxicity studies typically relies on toxicological factors, but the achieved exposure at these doses must also be evaluated. If toxicokinetic data suggest that a compound’s absorption constrains exposure, the lowest dose that yields maximum exposure should be regarded as the top dose level, barring other limitations. Careful interpretation of findings is crucial, especially with non-linear kinetics, though such kinetics shouldn’t automatically limit doses or undermine study results; they can provide valuable insights into the dose-exposure relationship.
3.5 Extent of exposure assessment in toxicity studies
In toxicity studies, proper estimation of systemic exposure requires an adequate number of animals and dose groups to inform risk assessment. Toxicokinetics may be assessed in all or a representative portion of main study animals or special satellite groups. While large animals typically provide samples for toxicokinetic data, smaller species may need dedicated satellite groups. The minimum number of animals needed should suffice for reliable data. Estimation of exposure is usually conducted in both male and female animals unless justified otherwise. Toxicokinetic data from different duration studies is not mandatory if the dosing regimen remains unchanged.
3.6 Complicating factors in exposure interpretation
Estimating exposure can aid in interpreting toxicity studies and comparing with human exposure, but there are important caveats. Species differences in protein binding, tissue uptake, receptor properties, and metabolic profiles must be considered. For highly protein-bound compounds, exposure may be better expressed as free (unbound) concentrations. Additionally, the pharmacological and toxicological effects of metabolites and the antigenicity of biotechnology products may complicate interpretations. It is also critical to recognize that high levels of the compound or its metabolites can accumulate in specific organs or tissues, even at low plasma concentrations.
3.7 Route of administration
The toxicokinetic strategy for alternative routes of administration, such as inhalation or parenteral delivery, should depend on the pharmacokinetic properties of the substance. When transitioning a drug from one route (e.g., oral) to another (e.g., intravenous), it is essential to determine if this change affects the safety margin. This involves comparing systemic exposure measures like AUC and Cmax between current and proposed routes. If the new route significantly increases these measures or alters metabolic pathways, previous safety assessments may need reevaluation. However, if exposure remains similar, further non-clinical toxicity studies may center on local toxicity.
3.8 Determination of metabolites
A primary objective of toxicokinetics is to explain systemic exposure to compounds in toxicology species, with specific circumstances making metabolite concentration measurement in plasma or other body fluids crucial.
When the administered compound acts as a ‘pro-drug’ and the delivered metabolite is acknowledged to be the primary active entity.
When the compound is metabolised to one or more pharmacologically or toxicologically active metabolites which could make a significant contribution to tissue/organ responses.
When a compound is extensively metabolized, measuring plasma or tissue concentrations of a major metabolite is the most practical way to estimate exposure in toxicity studies.
3.9 Statistical evaluation of data
The data must enable an adequate assessment of exposure. Due to significant variability in kinetic parameters and limited animal numbers, precise statistical analysis is typically unnecessary. Mean or median values and variability estimates are important, but individual animal data may sometimes hold greater significance than group statistics.
If data transformation (e.g. logarithmic) is performed, a rationale should be provided.
3.10 Analytical methods
Integration of pharmacokinetics into toxicity testing requires the early development of analytical methods, with ongoing reviews of analytes and matrices based on metabolism and species differences. Analytical methods must be specific, accurate, and precise, with a suitable limit of quantification for the expected concentration range. The chosen analyte and matrix, often plasma, serum, or whole blood, must be clearly defined, and potential interferences from endogenous components investigated. For drugs that are mixtures of enantiomers, justification for the analyte choice is necessary. Ideally, non-clinical and clinical studies should assess the same analyte and matrix, and any differing assay methods must be appropriately validated.
Reference:
https://database.ich.org/sites/default/files/S3A_Guideline.pdf
Dr Subramanian S Iyer
