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It has been shown previously by our group and others that a series of four disulfide benzamides with cellular anti-human immunodeficiency virus (HIV) activity can eject zinc from HIV type 1 nucleocapsid protein (NCp7) in vitro while analogs without antiviral activity do not. We also found that the zinc ejection activity correlates with the loss of the ability of NCp7 to bind to HIV psi RNA in vitro. These observations indicate that the antiviral disulfide benzamides may act at a novel retroviral target of action, i.e., the nucleocapsid protein. The present studies examine the relationship among disulfide benzamide structure, in vitro NCp7 zinc ejection activity, and antiviral activity for a larger series of compounds. All of the antiviral disulfide benzamides were found to eject NCp7 zinc, while some disulfide benzamides with zinc ejection activity are not antiviral. Utilizing the thiol reagent 5,5'-dithiobis(2-nitrobenzoic acid), it was determined that the o-amido-phenyl disulfides being studied cyclize in aqueous solution to form benzisothiazolones. A series of benzisothiazolones, which are stable in solution in the absence of dithiothreitol, were found to eject NCp7 zinc at a rate similar to that of their disulfide benzamide analogs and to possess similar antiviral activity. It was also found that the relative rates of HIV inactivation by various disulfide benzamides and benzisothiazolones correlate with their relative kinetic rates of NCp7 zinc ejection, which is consistent with the nucleocapsid protein being the target of action of these compounds.

Several disulfide benzamides have been shown to possess wide-spectrum antiretroviral activity in cell culture at low micromolar to submicromolar concentrations, inhibiting human immunodeficiency virus (HIV) type 1 (HIV-1) clinical and drug-resistant strains along with HIV-2 and simian immunodeficiency virus [Rice, W. G., Supko, J. G., Malspeis, L., Buckheit, R. W., Jr., Clanton, D., Bu, M., Graham, L., Schaeffer, C. A., Turpin, J. A., Domagala, J., Gogliotti, R., Bader, J. P., Halliday, S. M., Coren, L., Sowder, R. C., II, Arthur, L. O. & Henderson, L. E. (1995) Science 270, 1194-1197]. Rice and coworkers have proposed that the compounds act by "attacking" the two zinc fingers of HIV nucleocapsid protein. Shown here is evidence that low micromolar concentrations of the anti-HIV disulfide benzamides eject zinc from HIV nucleocapsid protein (NCp7) in vitro, as monitored by the zinc-specific fluorescent probe N-(6-methoxy-8-quinoyl)-p-toluenesulfonamide (TSQ). Structurally similar disulfide benzamides that do not inhibit HIV-1 in culture do not eject zinc, nor do analogs of the antiviral compounds with the disulfide replaced with a methylene sulfide. The kinetics of NCp7 zinc ejection by disulfide benzamides were found to be nonsaturable and biexponential, with the rate of ejection from the C-terminal zinc finger 7-fold faster than that from the N-terminal. The antiviral compounds were found to inhibit the zinc-dependent binding of NCp7 to HIV psi RNA, as studied by gel-shift assays, and the data correlated well with the zinc ejection data. Anti-HIV disulfide benzamides specifically eject NCp7 zinc and abolish the protein's ability to bind psi RNA in vitro, providing evidence for a possible antiretroviral mechanism of action of these compounds. Congeners of this class are under advanced preclinical evaluation as a potential chemotherapy for acquired immunodeficiency syndrome.

Using molecular modeling and the information derived from the X-ray crystal structure of HIV-1 protease (HIV PR) complexed with the pyran-2-one 1, a series of (4-hydroxy-6-phenyl-2-oxo-2H-pyran-3-yl)thiomethanes was designed and analyzed as novel, nonpeptidic inhibitors of HIV PR. Structure-activity studies led to the discovery of inhibitor 19 having (RS)-1-(cyclopentylthio)-3-methylbutyl functionalization at the C-3 position, which exhibited a Kc of 33 nM. A X-ray crystallographic structure of 19 bound to HIV PR showed that structural water-301 (inhibitor-flap-bridging water) was displaced by the inhibitor. Interestingly, the enol moiety of the pyran-2-one formed a hydrogen bond directly with Asp125 and with Asp25 via a bridging water molecule, thus illustrating a unique mode of active site binding by an HIV PR inhibitor. The pendant cyclopentyl and isobutyl groups of 19 occupied the S1' and S2' binding sites, respectively, whereas the 6-phenyl group occupied a region in between the S1 and S3 pockets of HIV PR. Selected compounds were tested for antiviral activity on H9 cells infected with HIV-1IIIb. A correlation between enzymatic activity and antiviral activity was not found in this series. The best antiviral compound in this series, 18, contained (RS)-3-[cyclopentyl(cyclopentylthio)methyl] functionalization at the C-3 position of the pyran-2-one ring and exhibited a CIC50 of 14 microM and TC50 of 70 microM. These studies demonstrate that potent enzyme inhibition can be achieved by inhibitors that span only three subsites.

A novel series of nonpeptidic compounds that contain a biphenyl carboxylic acid group have been shown to inhibit HIV-1 protease. The active compounds, most of which are highly soluble, have IC50 values in the range of 3.4-74 microM. The structure-inhibitory activity relationship demonstrates the necessity of the biphenyl carboxylic acid group for inhibition, which is enhanced by the presence of a sulfone group and by halogenation of an adjacent phenyl group. A double reciprocal plot of inhibition data on two of the compounds clearly shows that the inhibition occurs in a competitive manner, with Ki values of 1.1 and 3.4 microM. Inhibition by several of the compounds was found to be reversible and fast-binding, while one of the biphenyl carboxylic acids inhibits in a reversible slow-binding manner. Time-dependent inhibition studies were conducted on this compound, and it was determined to have the kinetic values of kon = 0.18 microM-1min-1, koff = 9.7 x 10(-2)min-1, and Ki = 0.14 microM. Thus, the slow-binding inhibitor is the most potent in the series. Molecular modeling has provided information on a possible binding mode for two different biphenyl carboxylic acid inhibitors of HIV-1 protease.

The nonpeptide compounds C1 (4-hydroxy-3-(3-phenoxypropyl)-1-benzopyran-2-one) and P1 (4-hydroxy-6-phenyl-3-(phenylthio)pyran-2-one) are structurally novel low micromolar inhibitors of the protease of human immunodeficiency virus type 1 (HIV-1). Kinetic analysis revealed that both compounds are competitive inhibitors, with Ki values of 1.0 microM (C1) and 1.1 microM (P1), that act in a reversible fast-binding manner. Structural analogs of both compounds indicate that the pyran-2-one group, the 4-hydroxyl group and substitution at the 3 position are all necessary for inhibitory activity. These two pyranones provide excellent initial compounds in the development of therapeutically effective HIV-1 protease inhibitors, since they are small achiral nonpeptide molecules more easily synthesized and with potentially better pharmacological characteristics than peptide inhibitors of the protease.