Comment from Huilin Shao

Thank you for the opportunity to comment on the use of plausible mechanism frameworks for evaluating drug effectiveness and safety in individualized therapies. In the past decade, the paradigm of treatment has shifted to targeted individualized therapies. Measuring defined interactions between drug molecules and their intended targets (i.e., drug–target engagement) can thus provide a promising opportunity for activity-based evaluation of drug effects. As the guidance rightly suggests, assessment of biomarker(s) that directly measure target engagement or primary pharmacodynamics is likely essential. While drug–target engagement is commonly measured during the phase of drug discovery and development (in cell lines and animal models), it cannot be readily characterized in patients due to limitations of current measurement technologies. These approaches, including thermal shift and proteomics assays, are universally complex and require extensive sample processing on large sample amounts (e.g., repeat tissue biopsies). As a result, there is an intense interest to develop safe and informative blood tests to characterize drug–target engagement for personalized treatment evaluation. Extracellular vesicles (EVs) have recently emerged as an attractive blood biomarker. EVs are nanoscale membrane vesicles (mostly <200 nm in diameter) actively secreted by a variety of mammalian cells. These vesicles abound in blood and carry reflective molecular constituents of their parent cells. Our recent studies found that EVs contain drug-bound proteins (Pan et al. Nature Nanotechnology 2021, Chemical Society Reviews 2023); capturing this repertoire of blood-borne information could thus present a transformative approach to molecularly characterize drug–target interactions across different cell systems. In conjunction with conventional blood pharmacokinetic/pharmacodynamic analyses which measure total drug concentration and ensemble biochemical responses in blood, the EV-based blood assay interrogates multiparametric drug–target interactions (multi-EVs and multi-targets). The approach not only establishes reflective metrics to accurately capture drug effects in different systems (e.g., on-target treatment effects and off-target side effects) but can also be performed directly and rapidly on blood samples. As part of the plausible mechanism framework, such minimally-invasive blood assays to objectively stratify patients and monitor treatment efficacy are critical for the clinical evaluation of individualized therapies.