A structure-activity approach to drug action in pharmacology: (A) Structure-activity relationships involved in DT-diaphorase (NQO1) mediated reduction kinetics and mode of action of anticancer bioreductive benzoquinone alkylating agents. (B) Structural c

Objectives. Study 1 and 2: To investigate the effects of functional group substitutions on the benzoquinone ring of the bioreductive benzoquinone alkylating agent (BM), on DT-diaphorase (DTD) mediated reduction kinetics, cytotoxicity and DNA cross-link or strand break formation. Study 3: To evaluate the inhibitory potency of NMDA receptor antagonists on energy-dependent amantadine uptake by renal organic cation transporters.; Methods. Study 1: Using purified human DTD, the rates of DTD mediated reduction of BM analogs with electron-donating (MBM, MeBM, m-MeBM), electron-withdrawing (CBM, FBM) and sterically bulky group (PBM, m-PBM, m-TBM) substitutions were determined in vitro. Cytotoxic activity was measured via the MTT assay with/without the DTD inhibitor, dicoumarol. Study 2: DNA cross-link and strand break formation were assessed by agarose gel assays after reduction of the BM analogs by DTD. Study 3: Inhibition of [3H]amantadine uptake by structurally similar NMDA receptor antagonists were determined in rat proximal and distal renal tubules.; Results. Study 1: Steric, rather than electronic effects, were more important in modifying the rate of reduction by DTD. Electron-donating groups increased redox cycling of the reduced products vs. BM. Reduction by DTD was an activating pathway for MBM. Study 2: DNA damage produced by the BM analogs displayed a rank order of MeBM≈MBM > m-MeBM≈PBM > BM > CBM > FBM > m-PBM≈m-TBM for DNA cross-link formation, and MeBM > MBM > m-MeBM > PBM > BM≈CBM > FBM > m-PBM≈m-TBM for strand break formation. Study 3: Steric hindrance around the ionized amino group of the cyclohexane ring appeared to prevent bicarbonate-mediated organic cation transport, and MRZ 2/579 displayed a novel distal tubule selectivity of inhibition. Conclusions. Study 1 and 2: Steric effects were more important than electronic effects in decreasing the rate of reduction by DTD. Sterically bulky group substitutions at the C6 position of the quinone were more important in decreasing the rate of reduction by DTD than substitutions at the C5 position. DTD mediated DNA cross-link and strand break formation positively correlated with the cytotoxic activity of the BM analogs. Study 3: A steric mechanism of bicarbonate-mediated transport inhibition is proposed, while the selective distal tubule inhibition of MRZ 2/579 may be utilized to determine the relative importance of distal vs. proximal renal tubule organic cation transporters.