The proteases expressed by the HIV-1 and HIV-2 viruses process the polyproteins encoded by the viral genomes into the mature proteins required for virion replication and assembly. Eight analogs of haloperidol have been synthesized that cause time-dependent inactivation of the HIV-1 protease and, in six cases, HIV-2 protease. The IC50 values for the analogues are comparable to that of haloperidol itself. Enzyme inactivation is due to the presence of an epoxide in two of the analogues and carbonyl-conjugated double or triple bonds in the others. Irreversible inactivation is confirmed by the failure to recover activity when one of the inhibitors is removed from the medium. At pH 8.0, the agents inactivate the HIV-1 protease 4-80 times more rapidly thanthe HIV-2 protease. Faster inactivationof the HIV-1 protease is consistent with alkylation of cysteine residues because the HIV-1 protease has four such residues whereas the HIV-2 protease has none. Inactivation of the HIV-2 protease requires modification of non-cysteineresidues. The similarities in the rates of inactivation of the HIV-2 protease by sixagents that have intrinsically different reactivities toward nucleophiles suggest that the rate-limiting step in the inactivation process is not the alkylation reaction itself. At least fiveof theagents inhibit polyprotein processing in an ex vivo cell assay system, but they are also toxic to the cells.

The HIV-1 aspartyl protease (HIV-1 PR), for which a high-resolution three-dimensional structure is available, is a dimer of two identical subunits of 99 amino acids each. 1-4 The HIV-2 protease (HIV-2 PR) is 39-44% identical in sequence and, as shown by X-ray crystallography, is an analogous dimer. 6 The proteases are required to process the polypeptides encoded by the viral gag and pol genes and are therefore essential for viral maturation. 6 Mutation of the catalytic aspartate at position 25 of HIV-1 PR yields an inactive enzyme and results in noninfectious virions. 1’7 The protease is therefore under intense inves-tigation as a target for the development of therapeutic agents for AIDS. Peptides that inhibit the protease in vitro with nanomolar K\values havebeen synthesized and shown to inhibit gag polyprotein processing8-10 and to reduce viral iiifectivity in cultured T4 cells. 11 However, the pharmacokinetic difficulties inherent in the use of peptide-based agents as pharmaceutical agents12 make the development of non-peptide inhibitors of the HIV pro-teases highly desirable. A structure-based, computer-assisted search procedure has identified haloperidol (1), a clinically employed antipsychotic agent that readily crosses the blood-brain barrier, 13 as an inhibitor of HIV-1 PR with apparent Ki= 100 jtM (IC50= 125 mM). 14 The crystal structure of a thioketal derivative of haloperidol bound in the active site of HIV-1 PR has been determined to highresolution. 16 The K\value of haloperidol, however, is approximately 1000 times higher than the serum concentrations achieved in patients treated with the drug. 13’16 Haloperidol is toxic at elevated concentrations and therefore is not itself a viable drug for the treatment of AIDS. Nevertheless, itsspecificity for the HIV pro-teases, 14 its attractive pharmacokinetic properties, and