Structure-Based Design of Lipophilic Quinazoline Inhibitors of Thymidylate Synthase

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Abstract
To develop novel lipophilic thymidylate synthase (TS) inhibitors, the X-ray structure of Escherichia coli TS in ternary complex with FdUMP and the inhibitor 10-propargyl-5,8-dideazafolic acid (CB3717) was used as a basis for structure-based design. A total of 31 novel lipophilic TS inhibitors, lacking a glutamate residue, were synthesized; 26 of them had in common a N-((3,4-dihydro-2-methyl-6-quinazolinyl)methyl)-N-prop-2-ynylaniline structure in which the aniline was appropriately substituted with simple lipophilic substituents either in position 3 or 4, or in both. Compounds were tested for their inhibition of E. coli TS and human TS and also for their inhibition of the growth in tissue culture of a murine leukemia, a human leukemia, and a thymidine kinase-deficient human adenocarcinoma. The crystal structures of five inhibitors complexed with E. coli TS were determined. Five main conclusions are drawn from this study. (i) A 3-substituent such as CF3, iodo, or ethynyl enhances binding by up to 1 order of magnitude and in the case of CF3 was proven to fill a nearby pocket in the enzyme. (ii) A simple strongly electron-withdrawing substituent such as NO2 or CF3SO2 in the 4-position enhances binding by 2 orders of magnitude; it is hypothesized that the transannular dipole so induced interacts favorably with the protein. (iii) Attempts to combine the enhancements of i and ii in the same molecule were generally unsuccessful. (iv) A 4-C6H5SO2 substituent provided both electron withdrawal and a van der Waal's interaction of the phenyl group with a hydrophobic surface at the mouth of the active site. The inhibition (Kis = 12 nM) of human TS by this compound, 7n, showed that C6H5SO2 provided virtually as much binding affinity as the CO-glutamate which it had replaced. (v) The series of compounds were poorly water soluble, and also the potent TS inhibition shown by several of them did not translate into good cytotoxicity. Compounds with large cyclic groups linked to position 4 by an SO or SO2 group did, however, have IC50's in the range 1−5 μM. Of these, 4-(N-((3,4-dihydro-2-methyl-6-quinazolinyl)methyl)-N-prop-2-ynylamino)phenyl phenyl sulfone, 7n, had IC50's of about 1 μM and was chosen for further elaboration.