Many well-established drugs have fused heterocyclic structures containing a 4-aminopyridine structural fragment. A few examples are summarized in Fig.1 and include the 9-aminoacridines quinacrine (antimalarial) and amsacrine (anticancer). The 9-amino-1,2,3,4-tetrahydroacridine derivative tacrine was the first acetylcholinester- ase inhibitor registered as
an anti-Alzheimer drug, and its derivative bistacrine is currently in clinical development. Other
related com-pounds of interest are the antimalarial 4-aminoquinoline derivatives chloroquine and amodiaquine. Related 4-aminoquinazoline moie- ties are present in the anticancer drug gefitinib and in a huge number of compounds showing a variety of pharmacological activities. Finally, the essential role of aminoheterocycles as building blocks for many important diversity-generating reactions has also to be taken into account.