inhibits cerebral synthesis of BH2 (reduced biopterin) from GTP; 2'-deoxy derivative of formamidopyrimidine ribotide (FPyd-P3); structure of FPyD-P3 given in Neurochem Res 1978 Feb;3(1):69-88; chemical name given in first source incorrect (amino omitted in ribosylamino & 6-hydroxy instead of 4-OH)