Aromatic and heteroaromatic amines (ArNH(2)) represent a class of potential mutagens that after being metabolically activated covalently modify DNA. Activation of ArNH(2) in many cases starts with N-hydroxylation by P450 enzymes, primarily CYP1A2. Poor understanding of structure-mutagenicity relationships of ArNH(2) limits their use in drug discovery programs. Key factors that facilitate activation of ArNH(2) are revealed by exploring their reaction intermediates in CYP1A2 using DFT calculations. On the basis of these calculations and extensive analysis of structure-mutagenicity data, we suggest that mutagenic metabolites are generated by ferric peroxo intermediate, (CYP1A2)Fe(III)-OO(-), in a three-step heterolytic mechanism. First, the distal oxygen of the oxidant abstracts proton from H-bonded ArNH(2). The subsequent proximal protonation of the resulting (CYP1A2)Fe(III)-OOH weakens both the O-O and the O-H bonds of the oxidant. Heterolytic cleavage of the O-O bond leads to N-hydroxylation of ArNH(-) via S(N)2 mechanism, whereas cleavage of the O-H bond results in release of hydroperoxy radical. Thus, our proposed reaction offers a mechanistic explanation for previous observations that metabolism of aromatic amines could cause oxidative stress. The primary drivers for mutagenic potency of ArNH(2) are (i) binding affinity of ArNH(2) in the productive binding mode within the CYP1A2 substrate cavity, (ii) resonance stabilization of the anionic forms of ArNH(2), and (iii) exothermicity of proton-assisted heterolytic cleavage of N-O bonds of hydroxylamines and their bioconjugates. This leads to a strategy for designing mutagenicity free ArNH(2): Structural alterations in ArNH(2), which disrupt geometric compatibility with CYP1A2, hinder proton abstraction, or strongly destabilize the nitrenium ion, in this order of priority, prevent genotoxicity.

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The pulmonary absorption of nine low-molecular-weight (225-430 Da) drugs (atenolol, budesonide, enalaprilat, enalapril, formoterol, losartan, metoprolol, propranolol and terbutaline) and one high-molecular-weight membrane permeability marker compound (FITC-dextran 10000 Da) was investigated using the isolated, perfused and ventilated rat lung (IPL). The relationships between pulmonary transport characteristics, epithelial permeability of Caco-2 cell monolayers and drug physicochemical properties were evaluated using multivariate data analysis. Finally, an in vitro-in vivo correlation was made using in vivo rat lung absorption data. The absorption half-life of the investigated drugs ranged from 2 to 59 min, and the extent of absorption from 21 to 94% in 2 h in the isolated perfused rat lung model. The apparent first-order absorption rate constant in IPL (ka(lung)) was found to correlate to the apparent permeability (P(app)) of Caco-2 cell monolayers (r = 0.87), cLog D(7.4) (r = 0.70), cLog P, and to the molecular polar surface area (%PSA) (r = -0.79) of the drugs. A Partial Least Squares (PLS)-model for prediction of the absorption rate (log ka(lung)) from the descriptors log P(app), %PSA and cLogD(7.4) was found (Q2 = 0.74, R2 = 0.78). Furthermore, a strong in vitro-in vivo correlation (r = 0.98) was found for the in vitro (IPL) drug absorption half-life and the pulmonary absorption half-life obtained in rats in vivo, based on a sub-set of five compounds.

The inhibition of the hERG channel by noncardiovascular drugs is a side effect that severely impedes the development of new medications. To increase hERG selectivity of preclinical compounds, we recommend the study of nondesolvation related interactions with the intended target and hERG using a baseline lipophilicity relationship approach. While this approach is conventionally used in studies of potency, we demonstrate here that it can help in selectivity issues. Studies of hERG selectivity in four in-house classes of chemokine receptor (CCR) antagonists suggest that the selectivity is improved most effectively by structural alterations that increase the lipophilicity-adjusted primary potency, pIC 50 (CCR) - Log D. Fragment-based QSAR analysis is performed using the lipophilicity-adjusted hERG potency, pIC 50 (hERG) - Log D, to identify moieties that form nonhydrophobic interactions with the hERG channel. These moieties, which erode hERG selectivity, can then be avoided. A novel two-dimensional fragment-based QSAR analysis helps visualizing the lipophilicity-adjusted hERG and CCR potencies within chemical series.