Before a potential drug is tested in a clinical trial setting, it is extensively tested in model systems. This can take years and only a few compounds make it to clinical trials. Most of these compounds fail to show effectiveness in model systems or turn out to be unsafe, and their development is discontinued.
It is very important that compounds are tested thoroughly before initiating a clinical trial. This is done firstly to assess whether the proposed mechanism works (proof-of-concept). If it does, additional tests are needed to ensure optimal trials can be designed.