Using exposure-response and biomarkers to streamline early drug development

Biomarkers (BMs) are biological measures of PD drug effects or disease markers that may represent clinically significant patient outcomes, either efficacy or toxicity. Their use in drug development, especially as an integral part of PK/PD modeling, has become a popular strategy for optimizing development time and resources. This approach supports quantitative integration of information across different species and throughout the clinical phases I-III. If the BM is based on the mechanism of action (MOA) of the drug, it is expected to follow an exposure-response relationship (E-R). If it is also involved in causal pathways in the pathophysiology of the disease (POD), it may become a surrogate marker (SM). SMs allow prediction of clinical outcomes for different dosing regimens of drug candidates and patient individualization of treatment in clinical practice. Appropriate evaluation of BMs by mechanistic, epidemiological, and clinical pharmacology studies as part of the drug development process allow scientists to establish clinically relevant ER. In early drug development, known ERs for BMs facilitate translation of in vitro findings to in vivo consequences, interspecies PK/PD comparisons, and streamlining of dose-finding phase I and II studies, as well as assessment of new dosing regimen candidates for their likely clinical efficacy and safety, extrapolation of clinical study results to special populations (e.g., pediatrics), and interpretation of exposure differences found in food, drug interaction and special populations studies. Recently, two novel BMs, namely, P50, a measure of ex vivo/in vitro whole blood oxygen affinity and S(pO2), i.e., in vivo pulse oximetry, were used in the development of an allosteric synthetic hemoglobin modifier (SAM), efaproxiral, as PD endpoints; these BMs are based on the MOA of SAMs. Early use of these BMs established excellent in vitro/in vivo PK/PD correlations, appropriate interspecies PK and PD scaling as well as PD-guided phase I and II dose-finding studies. This approach allowed appropriate translation of in vitro and preclinical information along with early identification of sources of PK/PD variability. Frontloading drug development with the identification and use of mechanism-based (MOA/POD) BMs constitutes a rational strategy to quantitatively integrate PK/PD information and optimize dose finding.