7-Benzylamino-6-chloro-2-piperazino-4-pyrrolidinopteridine (7a) is a potent inhibitor of the cAMP-specific phosphodiesterase isoenzyme family PDE4 and induces growth inhibition in a panel of tumor cell lines. In this study, we describe a synthesis that yields 7a and novel derivatives free of positional isomers. The synthesis of alkylamino substituted pteridines is based on the successive nucleophilic aromatic substitution of the chlorine atoms of 2,4,6, 7-tetrachloropteridine. For the reaction with secondary amines, the positional order of reactivity was found to be C4 > C7 > C2 > C6. Final structural proof is given by X-ray crystallography. To unravel structural elements of 7a crucial for the interaction with the target enzyme, the compound was modified systematically. The impact of the modifications on activity was tested by evaluating the ability of the compounds to inhibit cAMP hydrolysis by cAMP-specific phosphodiesterase (PDE4) purified from the solid human large cell lung tumor xenograft LXFL529. Growth inhibitory properties were determined by in vitro treatment of the respective cell line LXFL529L using the sulforhodamine B assay (SRB). The results show that for high activity, the heterocyclic substituent in position 2 of the pteridine ring system requires the presence of a basic nitrogen in 4'-position, as represented by piperazine.