In order to develop new oral
bronchodilators, a series of novel imidazol[4,5-c][1,8]naphthyridin-4(5H)-ones 5 were designed and synthesized. Some of these new heterocycles exhibited more potent
bronchodilator activity in vitro and in vivo than
theophylline. With respect to modification at the 5-position, both phenyl and n-butyl substitution produced potent activity. Though bulk tolerance at N-3 is observed with short and small lipophilic groups, any substitution at the other positions and transformations of the parent skeleton eliminated activity. Thus 5-phenyl-1H-imidazo[4,5-c][1,8]naphthyridin-4(5H)-one (23) (
KF17625), which satisfied these conditions, was selected for further studies (
antigen inhalation-induced
bronchospasm model; minimum effective dose (MED) = 1 mg/kg, po;
antigen-induced contraction of trachea (the Schultz-Dale reaction), IC50 = 2.2 microM). Compound 23 inhibited
carbachol-,
histamine-, or
leukotriene D4-induced contraction and relaxed spontaneous tone in guinea pig isolated tracheal preparations with, 4- to 16-fold greater potency than
aminophylline. Thus it appeared to relax directly the airway smooth muscle. 23 did not have any influence on
adenosine binding
at 10 microM, but inhibited canine tracheal
phosphodiesterase (PDE) IV (IC50 = 12 microM) and
concanavalin-A-induced histamine release from rat mast cells (44% inhibition
at 10 microM). Although the detailed mechanisms of these compounds remain to be elucidated, this series of novel tricyclic heterocycles represents a new class of
bronchodilator.