A number of substituted isoquinolin-1-ones, possible bioisosteres of the 5-aryl substituted 2,3-dihydroimidazo[2,$1-a$]isoquinolines, were synthesized and tested for their antitumor activity against five different human tumor cell lines. O-(3-hydroxyporpyl) substituted compound (15) exhibited the best antitumor activity which is 3-5 times better than 5-[$4^1$-(piperidinomethyl)phenyl]-2,3-dihydroimidazo[2,$1-a$]isoquinoline (1).