The Food and Drug Administration (FDA) recently introduced the Exploratory Investigational New Drug Guidance to expedite
the clinical evaluation of new therapeutic and imaging agents. Early clinical studies performed under the auspices of this
guidance, so-called “Phase 0” trials, have been initiated at the National Cancer Institute to integrate qualified pharmacodynamic
biomarker assays into first-in-human cancer clinical trials of molecularly targeted agents. The goal of this integration is to
perform molecular proof-of-concept investigations at the earliest stage of cancer drug development. Phase 0 trials do not offer
any possibility of patient benefit; instead, intensive, real-time pharmacodynamic and pharmacokinetic analyses of patient tumor
samples and/or surrogate tissues are performed to inform subsequent trials. Phase 0 studies do not replace formal Phase I drug
safety testing and require a substantial investment of resources in assay development early on; however, they offer the promise
of more rational selection of agents for further, large-scale development as well as the molecular identification of potential
therapeutic failures early in the development process.

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