COMPARATIVE MOLECULAR-FIELD ANALYSIS OF THE ANTITUMOR-ACTIVITY OF 9H-THIOXANTHEN-9-ONE DERIVATIVES AGAINST PANCREATIC DUCTAL CARCINOMA-03

The present study establishes correlations of in vivo growth inhibition of a solid tumor, pancreatic ductal adenocarcinoma (Panc 03), of mice with the steric and electrostatic fields and the hydrophobic parameter log P of a series (32) of 1-[[2-(dialkylamino)alkyl]amino]-9H-thioxanthen-9-ones by the 3D-QSAR method comparative molecular field analysis (CoMFA). The template molecular model was hycanthone methanesulfonate (19), the structure of which had been established previously by X-ray crystallography. The hycanthone base is protonated at the terminal nitrogen N(2), and an intramolecular hydrogen bond is present between the proximal nitrogen N(1) and carbonyl oxygen O(1) atoms. Crystallographic data also indicate a planar arrangement of bonds around N(1). However, the molecular geometry of 19, optimized by semiempirical molecular orbital methods (PM3,MNDO,AM1), showed the expected trigonal-pyramidal configuration for N(1). A comparison of MO and ab initio methods applied to a model compound, 1-amino-9H-thioxanthen-9-one, led to the selection of PM3 as the method for full geometry optimization of first the cationic and then the neutral forms of 1-32, whereas AM1 provided atomic charges for these same structures save those incorporating a sulfonamide moiety (5, 7, 20, 25, 26, 29, 31, and 32). Acceptable values for the latter were obtained from ab initio calculations. Structures were aligned by minimizing root-mean-square (rms) differences in the fitting of structures to 19 using the FIT option of SYBYL. An alternative strategy of alignment, steric and electrostatic alignment (SEAL), was invoked to provide a comparison of statistical data generated with the rms alignment. The rms-fit alignment of structures produced slightly better cross-validated and conventional r2 Values than those generated with the SEAL method. In addition, the rms-fit data indicate that a shift in the lattice of one-half of its spacing has a much smaller effect on the CoMFA data for a lattice of 1 angstrom than one of 2 angstrom. Inclusion of log P in a CoMFA of the neutral structures effected a small (ca. 8-10%) but significant improvement in cross-validated r2 values. The relative contributions of the hydrophobic effects and the steric and electrostatic fields to the conventional r2 values were 16%, 42%, and 42%, respectively. By contrast, incorporation of frontier molecular orbital (HOMO and LUMO) energies or their gaps in the PLS analyses failed to enhance correlation coefficients derived for either the charged or uncharged compounds. Graphical results of the non-cross-validated CoMFA studies of the cationic structures are shown in the form of three-dimensional coefficient contour maps that delineate the steric and electrostatic features of the model. Figure 4 presents a view of the major steric fractions of the CoMFA contour map for an active (5) and inactive (6) analogue. Maps of the electrostatic field (Figure 5) indicate areas where positive or negative interactions favor tumor-growth inhibition. The present findings indicate, in accord with the rationale for CoMFA, that the interactions, which seem most appropriate for describing the anticancer activities of the 9H-thioxanthen-9-one derivatives, are noncovalent.