The chronically infected cell as a target for the treatment of HIV infection and AIDS.

Infection of the T lymphocyte with HIV results in a cytopathic effect and cell death that has been linked to a selective loss of the helper T-lymphocyte function of the immune system. In addition to acute infection, which leads to cell death, a chronic or persistent infection also occurs. The persistence of these viral reservoirs has been implicated in the progression of HIV infection and AIDS. Rational drug discovery targeted to late-stage events in HIV replication has the potential to yield antiviral agents capable of blocking virus spread by inhibiting the production of infectious virions from these chronic reservoirs. Steve Petteway and colleagues discuss antiviral strategies that target the chronically infected cell, with a focus on HIV protease inhibitors.

Inhibitors of steroid 5 alpha-reductase in benign prostatic hyperplasia, male pattern baldness and acne.

Benign prostatic hyperplasia is an androgen-dependent disease which afflicts a large percentage of males over the age of fifty, and is usually treated by surgery. Dihydrotestosterone, a 5 alpha-reduced metabolite of testosterone, has been implicated as a causative factor in the progression of the disease, largely through the clinical study of males who are genetically deficient in the dihydrotestosterone-producing enzyme, steroid 5 alpha-reductase. As a result, inhibition of this enzyme has become a pharmacological strategy for the treatment of benign prostatic hyperplasia as well as other dihydrotestosterone-related disorders such as acne and male pattern baldness. In this review, Brian Metcalf and colleagues focus on the chemical and kinetic mechanisms of steroid 5 alpha-reductase, and known inhibitors of this enzyme, and discuss the rationale behind the design of a mechanistically distinct class of steroid 5 alpha-reductase inhibitors.

Biochemical and endocrine properties of a mechanism-based inhibitor of aromatase.

The conversion of androgens to estrogens by aromatase represents a major alteration in hormonal expression, and its regulation offers a promising method for therapeutic control of disease processes that are hormonally dependent. The design of suicide inhibitors based on enzyme-activated mechanisms provides an attractive approach for regulation of estrogen biosynthesis. MDL 18,962, (10-2(-propynyl)estr-4-ene-3,17-dione), a C-10 substituted analog of the natural substrate androstenedione, was evaluated as a suicide inhibitor of aromatase. Appropriate kinetic evaluations of MLD 18,962 established it to be a highly potent [inhibition constant (Ki) = 4.5 +/- 1.3 nM] irreversible inhibitor of human placental aromatase. The 2-propynyl group was necessary for time-dependent inactivation, as this activity was lost in related compounds. The inactivation process was specific for aromatase, since other P450 enzyme systems are not inactivated by MDL 18,962. This compound exhibited minimal intrinsic hormonal properties, since weak binding affinities were observed for cytosolic androgen, estrogen, or progestin receptors. The stimulation of ovarian aromatase activity by gonadotropins in immature mice was inhibited in animals implanted with 10-mm MDL 18,962 Silastic implants. This inhibition of estrogen biosynthesis suppressed estrogen-dependent uterine growth in these mice.

alpha-Ethynyl and alpha-vinyl analogues of ornithine as enzyme-activated inhibitors of mammalian ornithine decarboxylase.

alpha-Ethynyl- and alpha-vinylornithine were designed and synthesized as potential enzyme-activated inhibitors of mammalian ornithine decarboxylase. These two new inhibitors produce both immediate and time-dependent inhibition of rat liver ornithine decarboxylase in vitro. The inhibitions exhibition saturation kinetics. The apparent dissociation constants (KI) are 10 and 810 microM, and the times of half-inactivation at infinite concentration of inhibitor (t1/2) are 8.5 and 27 min, respectively, for alpha-ethynyl- and alpha-vinylornithine. In rats, alpha-ethynylornithine causes a rapid dose-dependent decrease of ornithine decarboxylase activity in prostate and, to a lesser extent, in thymus and testis.

cis-4-Carboxy-6-(mercaptomethyl)-3,4,5,6-tetrahydropyrimidin-2(1 H)-one , a potent inhibitor of mammalian dihydroorotase.

A series of cis- and trans-4-carboxy-3,4,5,6-tetrahydropyrimidin-2(1H)-ones possessing either a carboxy, hydroxymethyl, or mercaptomethyl substituent at C-6 were prepared and tested for their ability to inhibit mammalian dihydroorotase. Of these compounds, only the cis-6-mercaptomethyl compound, cis-1, was found to be a potent competitive inhibitor of the enzyme (Ki = 140 nM at pH 7.4 and 8.5) when assayed in the direction of dihydro-L-orotate hydrolysis. These results suggest that the inhibition arises from the ligation of the thiolate to the zinc atom which is thought to be located in the enzyme's active site. Although analysis of cis-1 with 2,2'-dithiobis(5-nitrobenzoic acid) revealed significant loss of the free thiol group under enzymatic assay conditions, the addition of the reducing agent, dithiothreitol, to the enzymatic reaction mixtures afforded cis-1 complete protection against this chemical decomposition, as evidenced by lowering of the inhibition constant in the presence of dithiothreitol. Compound cis-1 had no significant antiproliferative activity against B16 melanoma cells in tissue culture, possibly due to the rapid decomposition of the compound or poor permeability into cells.

Steroidal A ring aryl carboxylic acids: a new class of steroid 5 alpha-reductase inhibitors.

A series of 17 beta-carbamoyl-1,3,5(10)-estratriene-3-carboxylic acids has been prepared and evaluated in vitro as inhibitors of human and rat prostatic steroid 5 alpha-reductase (EC 1.3.1.30). Potent inhibition of the human enzyme, in particular, was observed and preliminary studies using rat enzyme suggest that the inhibition results from the formation of an enzyme-NADP(+)-inhibitor complex. The compounds were synthesized from estrone, generally employing a differentiated bis-triflate carbonylation strategy.

Inhibition of steroid 5 alpha-reductase by unsaturated 3-carboxysteroids.

A series of unsaturated steroids bearing a 3-carboxy substituent has been prepared and assayed in vitro as inhibitors of human and rat prostatic steroid 5 alpha-reductase (EC 1.3.1.30). It is proposed that the observed tight binding of the 3-androstene-3-carboxylic acids is due to mimicry of a putative, high-energy, enzyme-bound enolate intermediate formed during the NADPH-dependent conjugate reduction of testosterone by steroid 5 alpha-reductase. These compounds were prepared through palladium(0)-catalyzed carbomethoxylations of enol (trifluoromethyl)sulfonates derived from 3-keto precursors. Modification of A and B ring unsaturation and substitution at C-3, -4, -6, and -11 was explored. Mono- and dialkylcarboxamides were employed as 17 beta side chains to enhance inhibitory activity with the human enzyme.

Synergistic drug interactions of an HIV-1 protease inhibitor with AZT in different in vitro models of HIV-1 infection.

Synthetic peptide mimetic inhibitors of HIV-1 protease effectively block spread of infectious virus in acutely infected T-cells. These compounds also inhibit production of infectious virions from chronically infected T-cell lines. In order to determine the potential for drug interaction effects on antiviral activity, an HIV-1 protease inhibitor (SK&F 108922) and AZT were studied in three different in vitro models of HIV-1 infection of T-cell lines, specifically, (1) acutely infected cells infected at low multiplicity, (2) HIV-1 chronically-infected cells and (3) co-cultivations of chronically infected with non-infected cells. Upon co-treatment, these compounds demonstrated synergy in Molt4 or H9 cells acutely infected with HIV-1 strain IIIB. Either compound alone was a potent inhibitor of HIV-1 in co-cultivations of uninfected and chronically infected cells. In combination treatments of co-cultures, SK&F 108922 demonstrated strong synergy with AZT. Treatment of H9/IIIB chronically infected cells demonstrated no inhibitory effect by AZT treatment (EC50 = > 100 microM) whereas SK&F 108922 was inhibitory (EC50 = 3 microM). Upon co-treatment of H9/IIIB chronically infected cultures with both compounds, the antiviral activity was similar to that of the protease inhibitor alone suggesting no drug interaction. In the co-cultivation experiments, AZT's antiviral effect was most likely due to blocking spread of acute infection to uninfected cells in the culture. No antagonistic effects were observed with AZT and SK&F 108922 co-treatments. These results clearly demonstrate that an HIV-1 protease inhibitor can exert a potent antiviral effect on chronically infected T-cells in contrast to AZT and is capable of potent synergy with AZT in acute and co-culture in vitro infection models.

Studies on the mechanism of steroid 5 alpha-reductase inhibition by 3-carboxy A-ring aryl steroids.

The mechanism of interaction between two 3-carboxy A-ring aryl steroids, 17 beta-(N,N-diisopropylcarboxamide)-estra-1,3,5(10)-triene-3-carboxy lic acid (1) and 17 beta-(N-t-butylcarboxamide)-estra-1,3,5(10)-triene-3-carboxylic acid (2), with rat hepatic and human prostatic steroid 5 alpha-reductases has been investigated. Dead-end inhibition plots with 1 and 2 versus both substrates (testosterone and NADPH) were linear-uncompetitive using either enzyme, while double-inhibition analyses indicated cooperative binding to enzyme between NADP+ and 1 or 2. These results were interpreted within the ordered kinetic mechanism of steroid 5 alpha-reductase to result from the preferential association of 3-carboxy A-ring aryl steroids to the enzyme-NADP+ complex. The direct displacement by 2 of a radioligand known to associate to this same enzyme form provides further support for these conclusions.

Epristeride is a selective and specific uncompetitive inhibitor of human steroid 5 alpha-reductase isoform 2.

Specificity of an enzyme inhibitor can have profound implications upon the compound's therapeutic potential, utility and safety profile. As potent inhibitors of human steroid 5 alpha-reductase (SR) the 3-androstene-3-carboxylic acids, or steroidal acrylates, may be useful in treatment of diseases such as benign prostatic hyperplasia for which 5 alpha-dihydrotestosterone (DHT) appears to be a causative agent. To determine its specificity profile, the interactions of a representative compound from this class, N-(t-butyl)androst-3,5-diene-17 beta-carboxamide-3-carboxylic acid (epristeride, SK&F 105657), have been studied with 7 other steroid processing enzymes and 5 steroid hormone receptors. The affinity of epristeride for each of these 12 potential targets was found to be at least 1000-fold weaker than that for SR, the intended target. In addition, using samples of the individually expressed two known forms of human SRs, epristeride has been shown to be a selective inhibitor of the recombinant human SR type 2, the predominant activity found in the prostate of man. Nonetheless, the mechanisms of SR inhibition for both isoenzymes involve formation of a ternary complex with epristeride, NADP+, and enzyme. Epristeride, consequently, has been shown to be an uncompetitive inhibitor versus steroid substrate of both human SR isoenzymes. These results suggest that this 3-androstene-3-carboxylic acid is a specific and selective inhibitor of the human type 2 SR, and that epristeride is an attractive compound for further investigation as a safe and effective therapeutic agent in the potential treatment of disease states associated with DHT-induced tissue growth.

Inhibition of pyridine-nucleotide-dependent enzymes by pyrazoles. Synthesis and enzymology of a novel A-ring pyrazole steroid.

A novel A-ring pyrazole steroid, 2,3-bisaza-A-nor-1,5(10)-estradien-17 beta-ol (3), was synthesized as a potential inhibitor of steroidal NAD(P)H-dependent oxidoreductases. Compound 3 proved to be a potent inhibitor of 3(17)beta-hydroxysteroid dehydrogenase (from P. testosteroni) exhibiting a Ki of 90 +/- 20 nM. The activities of 3 alpha,20 beta-hydroxysteroid dehydrogenase (from S. hydrogenans), steroid-5 alpha-reductase (from rat prostate), and 3 alpha-hydroxysteroid dehydrogenase (from rat liver) were unaffected by pyrazole 3. Dead end inhibition studies indicate an ordered binding of cofactor prior to substrate or pyrazole inhibitor.

Synthesis of a steroidal A ring aromatic sulfonic acid as an inhibitor of steroid 5 alpha-reductase.

The synthesis of 17 beta-(N,N-diisopropylcarbamoyl)estra-1,3,5(10)-triene-3-sulfonic acid (3) has been accomplished. Sulfonate 3 was designed as a novel inhibitor of human steroid 5 alpha-reductase based on considerations of enzyme mechanisms, and exhibits an inhibition constant in the low nanomolar range.

Purification and properties of the catalytic domain of human 3-hydroxy-3-methylglutaryl-CoA reductase expressed in Escherichia coli.

Three fragments of the cDNA encoding human 3-hydroxy-3-methylglutaryl-CoA reductase, all incorporating the majority of the catalytic domain of the protein, were subcloned into Escherichia coli expression vectors containing the pL promoter. The two larger expressed fragments (58 and 52 kDa) were soluble and had enzymatic activity, while the smallest (48 kDa) was insoluble. The two active fragments were purified by a combination of conventional techniques and affinity chromatography. A number of properties of the two enzymes were compared including specific activity, kinetic parameters, relative solubility, and cold lability. The 52-kDa enzyme was observed to change from a dimeric to monomeric form and to lose activity at 4 degrees C. In contrast, the 58-kDa enzyme was found to be much less cold labile, and was dimeric at both 20 and 4 degrees C. In order to resolve the number of subunits required to form an active site, the number of inhibitor binding sites for a known inhibitor was determined to be one per subunit in the 58-kDa enzyme.

Time-dependent inhibition of aromatase in trophoblastic tumor cells in tissue culture.

Human choriocarcinoma trophoblast cells (JAr line) were utilized as whole cell preparations in tissue culture to evaluate the effects of mechanism-based inactivation or "suicide inhibition" of estrogen biosynthesis. A C-19 acetylenic analog (MDL 18,962) of the substrate, androstenedione, was evaluated in competitive and time-dependent assays. Product formation was determined by accumulation of 3H2O resulting from the stereo-specific elimination of 1 beta-tritium from the androgen substrate. Trophoblast cells exhibited initial linear kinetics for at least 3 h following addition of [1-3H]androstenedione. The Km for androstenedione was 35.1 nM with Vmax of 3.7 pmol/h/10(6) trophoblast cells. Kinetic analysis of time-dependent inhibition of aromatase in trophoblast cells revealed an apparent Ki of 0.6 nM for MDL 18,962 and at t 1/2 of inactivation of 26 min at infinite inhibitor concentration. These studies suggest that a suicide aromatase inhibitor can cause irreversible inhibition of estrogen biosynthesis in intact trophoblast cells. In the presence of 1 nM or 10 nM MDL 18,962, trophoblast cells exhibited initial linear kinetics for estrogen biosynthesis during the first hour following co-incubation with inhibitor and 33 nM substrate. During the subsequent 30 min the rate of estrogen biosynthesis precipitously declined from 104 +/- 24 fmol/min/10(6) cells in control cells to 24 +/- 13 and 8 +/- 4 fmol/min/10(6) trophoblasts treated with 1 or 10 nM MDL 18,962, respectively. This significant decrease in aromatase activity (P less than or equal to 0.01) implied irreversible inactivation, which was supported by prolonged inhibition of aromatase activity in trophoblast cells incubated for 6-48 h following removal of medium containing 3 nM or 30 nM MDL 18,962.

Mechanism of the stereospectific irreversible inhibition of bacterial glutamic acid decarboxylase by (R)-(--)-4-aminohex-5-ynoic acid, an analogue of 4-aminobutyric acid.

4-Aminohex-5-ynoic acid inhibits bacterial glutamic acid decarboxylase in a time-dependent irreversible manner. The inhibition is stereospecific and requires the abstraction of the propargylic hydrogen from 4(R)-(--)-4-aminohex-5-ynoic acid. This leads to the generation of a reactive alkylating agent in the active site which can react with a nucleophilic residue. At complete inhibition, there is incorporation of one molecule of inhibitor per pyridoxal binding site. If the decarboxylation of glutamate occurs with retention of configuration, the irreversible inhibition of this enzyme by the 4-(R) isomer can be rationalized on the basis of reversibility of the protonation step in the normal catalytic mechanism.

4-amino-hex-5-enoic acid, a selective catalytic inhibitor of 4-aminobutyric-acid aminotransferase in mammalian brain.

Incubation of rat brain 4-aminobutyrate aminotransferase with 4-amino-hex-5-enoic acid, a substrate analog of 4-aminobutyric acid, results in a time-dependent irreversible loss of enzymatic activity. In the presence of 0.1 mM inhibitor the half-life of the inactivation process is approximately 6 min. Low concentrations of L-glutamic acid or 4-aminobutyric acid protect against this inactivation, while 2-oxoglutarate prevents this protection, suggesting that only the pyridoxal form of the enzyme is susceptible to inhibition by 4-amino-hex-5-enoic acid. The irreversible inhibition of mammalian 4-aminobutyrate aminotransferase by 4-amino-hex-5-enoic acid is selective. There is no inhibition of this enzyme from Pseudomonas fluorescens with the inhibitor at mM concentrations. Even at 10 mM there is no irreversible inhibition of mammalian glutamate decarboxylase or of aspartate aminotransferase, while alanine aminotransferase is inhibited over 500 times more slowly than rat brain 4-aminobutyrate transaminase.