Context: New drug approvals in the United States must be supported by substantial evidence from “adequate and well-controlled” trials. The Food and Drug Administration (FDA) has flexibility in how it applies this standard.

Methods: The authors conducted a systematic literature review of studies evaluating the design and outcomes of the key trials supporting new drug approvals in the United States. They extracted data on the trial characteristics, endpoint types, and expedited regulatory pathways.

Findings: Among 48 publications eligible for inclusion, 30 covered trial characteristics, 23 covered surrogate measures, and 30 covered regulatory pathways. Trends point toward less frequent randomization, double-blinding, and active controls, with variation by drug type and indication. Surrogate measures are becoming more common but are not consistently well correlated with clinical outcomes. Drugs approved through expedited regulatory pathways often have less rigorous trial design characteristics.

Conclusions: The characteristics of trials used to approve new drugs have evolved over the past two decades along with greater use of expedited regulatory pathways and changes in the nature of drugs being evaluated. While flexibility in regulatory standards is important, policy changes can emphasize high-quality data collection before or after FDA approval.

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