It all starts with discovering a new potential drug. This can be done in different ways. Historically, drugs were often discovered by isolating an active ingredient from a traditional medicine (e.g.herbs) or just by chance (the antibiotic penicillin is a good example). Nowadays, often more sophisticated methods, like high-throughput screening of thousands of chemical compounds, are used.
The molecular pathways altered in the disease are investigated and lots of molecules that may influence these pathways are screened. Those showing possible beneficial effects are selected for further research.