COMPARATIVE MOLECULAR-FIELD ANALYSIS OF IN-VITRO GROWTH-INHIBITION OF L1210 AND HCT-8 CELLS BY SOME PYRAZOLOACRIDINES

In vitro screening of a number of 2-(aminoalkyl)-5-nitropyrazolo[3,4,5-kl]acridines has previously indicated (Sebolt, J. S.; et al. Cancer Res. 1987, 47,4299-4304) that these compounds, in general, exhibit selective cytotoxicity against the human colon adenocarcinoma, HCT-8, cell line, relative to mouse leukemia L1210 cells. Comparative molecular field analysis (CoMFA) was applied to HCT-8 and L1210 growth inhibition assays (IC50s) of a series (44) of the pyrazoloacridine derivatives with the objective of predicting improved solid tumor selectivity. In the absence of crystallographic data, the 9-methoxy derivative (15), which is currently in clinical study, was selected as the template molecular model. Two different structural alignments were tested: an alignment of structures based on root mean square (RMS)-fitting of each structure to 15 was compared with an alternative strategy, steric and electrostatic alignment (SEAL). Somewhat better predictive cross-validation correlations (r2) were obtained with models based on RMS vis-a-vis SEAL alignment for both sets of assays. A large change in lattice spacing, e.g., 2 to 1 angstrom, causes significant variations in the CoMFA results. A shift in the lattice of half of its spacing had a much smaller effect on the CoMFA data for a lattice of 1 A than one of 2 angstrom. The relative contribution of steric and electrostatic fields to both models were about equal, underscoring the importance of both terms. Neither calculated log P nor HOMO and/or LUMO energies contribute to the model. Steric and electrostatic fields of the pyrazoloacridines are the sole relevant descriptors to the structure-activity (cross-validated and conventional) correlations obtained with the cytotoxic data for both the L1210 and HCT-8 cell lines. The cross-validated r2, derived from partial least-squares calculations, indicated considerable predictive capacity for growth inhibition of both the leukemia and solid-tumor data. Evidence for the predictive performance of the CoMFA-derived models is provided in the form of plots of actual vs predicted growth inhibition of L1210 and HCT-8 cells, respectively, by the pyrazoloacridines. The steric and electrostatic features of the QSAR are presented in the form of standard deviation coefficient contour maps of steric and electrostatic fields. The maps indicate that increases or decreases in steric bulk that would enhance growth inhibition of HCT-8 cells would likewise promote growth inhibition of L1210 cells. Contour maps generated to analyze the electrostatic field contributions of the pyrazoloacridines to growth inhibition provide an essentially similar set of results. It is apparent that steric and electrostatic fields alone are inadequate in the CoMFA to characterize the in vitro solid tumor selectivity of the pyrazoloacridines. This points to a need to supplement the cytotoxic data with results of further study that focuses on a quantitative comparison of the potential for differential metabolic activation of the pyrazoloacridines in the two cell lines.