The quantitative structure-bioavailability relationship of 232 structurally diverse drugs was studied to evaluate the feasibility of constructing a predictive model for the human oral bioavailability of prospective new medicinal agents. The oral bioavailability determined in human adults was assigned one of four ratings and analyzed in relation to physicochemical and structural factors by the ORMUCS (ordered multicategorical classification method using the simplex technique) method. A systematic examination of various physicochemical parameters relating primarily to absorption, and structural elements which could influence metabolism, was carried out to analyze their effects on the bioavailabilty classification of drugs in the data set. Lipophilicity, expressed as the distribution coefficient at pH 6.5, was found to be a significant factor influencing bioavailability. The observation that acids generally had better bioavailability characteristics than bases, with neutral compounds between, led to the formulation of a new parameter, Delta log D (log D(6.5) - log D(7.4)), which proved to be an important contributor in improving the classification results. The addition of 15 structural descriptors relating primarily to well-known metabolic processes yielded a satisfactory QSAR equation which had a correct classification rate of 71% (97% within one class) and a Spearman rank correlation coefficient (R(s)) of 0.851, despite the diversity of structure and pharmacological activity in the compound set. In leave-one-out tests, an average of 67% of drugs were correctly classified (96% within one class) with an R(s) of 0.812. The relationship formulated identified significant factors influencing bioavailability and assigned them quantitative values expressing their contribution. The predictive power of the model was evaluated using a separate test set of 40 compounds, of which 60% (95% within one class) were correctly classified. Since the necessary physicochemical parameters can be calculated or estimated and the structural descriptors are obtained from an inspection of the structure, the model enables a rough estimate to be made of the prospective human oral bioavailability of unsynthesized compounds. Also, the model has the advantage of transparency in that it indicates which factors may affect bioavailabilty and the extent of that effect. This could be useful in designing compounds which are more bioavailable. Refinement of the model is possible as more bioavailability data becomes available. Potential uses are in drug design, prioritization of compounds for synthesis, and selection for detailed studies of early compound leads in drug discovery programs.

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Corneal permeability data taken from the literature were analyzed for possible quantitative relationships with physicochemical properties. Although a parabolic relationship was obtained with good correlation between lipophilicity, as expressed by the 1-octanol-water partition coefficients, log Poctanol (or the distribution coefficients, log D for ionizable compounds), and the permeability in individual analyses of compound classes such as beta-adrenoceptor blockers and steroids, the correlation was reduced when taken together. However, delta log P (i.e., log Poctanol-log Palkane) correlated inversely with the combined permeability data for beta-blockers and steroids and played a key role as a unifying variable. To a lesser extent, lipophilicity itself also contributes positively to corneal permeation. Even with the addition of miscellaneous compounds such as methanol and ibuprofen, the delta log P and lipophilicity terms were still significant. However, small molecules were likely to be underestimated, which is consistent with penetration via another pathway besides that governed by delta log P and lipophilicity.

:The activity of a number of O-(4,6-dimethoxypyrimidin-2-yl)salicylic acids and their thio analogs inhibiting acetolactate synthase (ALS) preparation was measured. The e †ects of substituents on the salicylic-benzene ring on the inhibitory activity were analyzed quantitatively with physicochemical substituent parameters. For 6-substituted (thio)salicylic acids, the activity was shown to vary parabolically with the "intramolecularÏ steric parameterIn addition, the (E s ). higher steric dimension of substituents in terms of the STERIMOL width or length parameter lowered the activity. The Ðeld-inductive electron-withdrawing property of the 6-substituents in terms of the SwainÈLuptonÈHansch F was favorable for the activity of salicylic acid series. In 5-substituted salicylic acids, the activity was increased by electron-donating substituents with smaller size. The relationships between ALS inhibitory and herbicidal activities were also analyzed with some weed species. Both pre-and post-emergence activities against barnyard grass, Echinochloa crus-galli, were linearly related to the ALS inhibitory activity after allowing for the hydrophobic factor that may contribute to the transport processes. Those against two broad-leaved weed species, Polygonum convolvulus and Abutilon theophrasti were linearly related to the in-vitro activity with no signiÐcant participation of the hydrophobic factor.1998 SCI ( Pestic. Sci., 52, 343È353 (1998)

The N-isobutyl-N-(4-substituted benzyl)-4-halo-2-pentenamides show a potent light-dependent herbicidal activity against the barnyard grasses. The herbicidal mechanism was investigated with assay systems using cotton seedlings and an enzyme preparation from them as well as cultured unicelluar microalgae. Besides the protoporphyrinogen oxidase inhibition, some other mechanism(s) inhibiting step(s) involved in the chlorophyll biosynthetic pathway were suggested as the origin of the phytotoxicity.