Flavones and related compounds as inhibitors of protein tyrosine kinases
Aberrant tyrosine protein kinase activity has been implicated in the formation and maintenance of malignancy and so presents a potential target for cancer chemotherapy. Quercetin, a naturally occuring flavonoid, inhibits the tyrosine protein kinase encoded by the Rous sarcoma virus but also exhibits many other effects. Analogues of this compound were synthesised by the acylation of suitable 2-hydroxyacetophenones with appropriately substituted aromatic (or alicyclic) acid chlorides, followed by base catalysed rearrangement to the 1-(2-hydroxyphenyl)-3-phenylpropan-1,3-diones. Acid catalysed ring closure furnished flavones. The majority of the 1-(2-hydroxyphenyl)-3-phenylpropan-1,3-diones were shown by NMR to exist in the enol form. This was supported by the crystal structure of 1-(2-hydroxy-4-methoxyphenyl)-3-phenylpropan-1,3-dione. In contrast, 1.(4,6-dimethoxy-2-hydroxyphenyl)-3-phenylpropan-1,3-dione did not exhibit keto-enol tautomerism in the NMR spectrum and was shown in its crystal structure to assume a twisted conformation. Assessment of the biological activity of the analogues of quercetin was carried out using whole cells and the kinase domain of the tyrosine protein kinase encoded by the Abelson murine leukaemia virus, ptab150 kinase. Single cell suspension cultures and clonogenic potential of murine fibroblasts transformed by the Abelson Murine leukaemia virus (ANN-1 cells) did not indicate the existence of any structure activity relationship required for cytotoxicity or cytostasis. No selective toxicity was apparent when the `normal' parent cell line, (3T3), was used to assess the cytotoxic potential of quercetin. The ICS50 for these compounds were generally in the region of 1-100M. The potential for these compounds to inhibit ptab150 kinase was determined. A definite substitution requirement emerged from these experiments indicating a necessity for substituents in the A ring or in the 3-position of the flavone nucleus. Kinetic data showed these inhibitors to be competitive for ATP.