Final answer:
In vitro safety tests are first in drug development due to their efficiency, lower costs, and rapid insights into a drug's ADME characteristics while avoiding ethical concerns related to in vivo animal testing. The FDA values in vitro data, considering them predictive of in vivo results, necessitating validation against in vivo data for accuracy.
Step-by-step explanation:
The initial safety tests are done in vitro primarily because these tests offer a reliable screening process while avoiding the ethical and logistical complications associated with in vivo animal studies. In vitro methods, involving the use of human cell lines, tissues, and other biological substrates, can provide early and rapid insights into the absorption, distribution, metabolism, and excretion (ADME) characteristics of compounds with high throughput. The FDA acknowledges the utility of these data as predictive tools for in vivo endpoints, which can obviate the need for certain clinical studies. In addition to ethical considerations, in vitro assays are less costly, require less compound, and can be performed more rapidly compared to their in vivo counterparts. Integrating these assays with subsequent in vivo studies allows for a more comprehensive understanding of drug behavior. Nevertheless, in vitro assays should always be validated against in vivo data to ensure accuracy in predicting clinical outcomes.