A method of inhibiting tumor growth is provided, comprising administering an effective dose of a compound of the formula ##STR00001## wherein, independently at each occurrence, w is C or N, v and x are N with H substitution as needed to fulfill open valence sites; y and z are selected from N and C, with H substitution as needed to fulfill open valence sites, with the proviso that each of w, v, x, y and z is not simultaneously C; the ring formed from v, w, x, y and z may be saturated or unsaturated; and R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are selected from hydrogen, alkyl, aryl, alkaryl, aralkyl, heteroalkyl, and heteroaryl; wherein any adjacent two of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 may join together to form a 5, 6 or 7-membered carbocyclic ring or a 5, 6 or 7-membered heterocyclic ring containing at least one atom selected from nitrogen, oxygen and sulfur.