Clint Rosenfeld, PhD, Senior Director, DMPK
Jocelyn Mielke, BA, LAT, Associate Kinetic Scientist
When transitioning from lead optimization to development, there are several factors that should be understood prior to conducting GLP toxicity studies. These include performing a gap analysis which involves identifying the bioanalytical method ranges and selecting appropriate collection intervals to accurately quantify systemic exposure.
Identifying an appropriate bioanalytical method range requires knowledge of the intended doses to be used in combination with a fundamental understanding of the pharmacokinetics. Optimally the bioanalytical range should capture the lowest and highest predicted concentration based on the intended dose levels. The lower limit of quantitation should be sufficient to ensure determination of systemic exposure across the dosing interval for the lowest dose group. The upper limit of quantitation should be sufficient to measure the highest predicted concentration without the need for dilution. Due to limitations with bioanalytical assays, multiple ranges may need to be considered.
In addition to bioanalytical method range determination, identification of the appropriate collection intervals is critical to the assurance of accurate assessment of systemic exposure. Because systemic exposure is used in defining a safe starting dose in man, ensuring that you are accurately assessing exposure is vital to this process. The International Conference on Harmonisation (ICH) provides the following guideline on the assessment of systemic exposure in toxicity studies:
3.3 Justification of time points for sampling
“In each study, the number of time points should be justified on the basis that they are adequate to estimate exposure. The justification should be based on kinetic data gathered from earlier toxicity studies, from pilot or dose range-finding studies, from separate studies in the same animal model or in other models allowing reliable extrapolation.”
To ensure that the time points selected to determine systemic exposure are adequate, several factors should be considered. The time points for collecting samples in toxicokinetic studies should be as frequent as is necessary to quantify systemic exposure, but not so frequent as to interfere with the normal conduct of the study or to cause undue physiological stress to the animals. Time points should be determined on the basis of previous data from PK studies in which the same form of test article was used and a robust sampling scheme has been evaluated (approximately 5 half-lives of data). If data are not available, a separate bridging study should be completed in the relevant species using the same batch of test article that will be utilized in the definitive safety studies. Time point selection should account for exposure assessment in all analytes (parent and metabolites) that will be used to assess safety coverage, as the optimal collection scheme may differ for each analyte. Lastly, MPI Research recommends performing optimization on the aforementioned PK or bridging data (time point selection analysis) in order to demonstrate that the sampling scheme selected for determination of systemic exposure is adequate. Having the appropriate collection schemes will ensure accurate determination of systemic exposure which will help define safety margins that will assist in setting a safe starting dose in the clinic.