Abstract: Various substituted nitrogen heterocyclic derivatives and their pharmaceutically acceptable salt derivatives are provided for use as medicaments, and particularly, as antimitotic, anti-viral, anti-cancer, anti-degenerative, immunosuppressive, and anti-microbial drugs or vaccines. These heterocyclic derivatives can be used as an active agent in a pharmaceutical, as well as a diagnostic utility. To this end, several families of heterocyclic derivatives are provided including pyrrolopyrimidines, pyrazolopyrimidines, purines, and imidazopyridines. In particular, certain tri-substituted and tetra-substituted purines and pyrazolopyrimidines and their deaza analogues are provided for inhibiting cyclin-dependent kinase (“cdk”) proteins, viruses, and immunostimulation.