Joanne J. Bronson

4-Thiazolidinones: Novel Inhibitors of the Bacterial Enzyme MurB

4-Thiazolidinones were synthesized and evaluated for their ability to inhibit the bacterial enzyme MurB. Selected 4-thiazolidinones displayed activity against the enzyme in vitro. This activity, coupled with the design principles of the thiazolidinones, supports the postulate that 4-thiazolidinones may be recognized as diphosphate mimics by a biological selector.

Discovery and Evaluation of BMS-708163, a Potent, Selective and Orally Bioavailable γ-Secretase Inhibitor

During the course of our research efforts to develop a potent and selective γ-secretase inhibitor for the treatment of Alzheimers disease, we investigated a series of carboxamide-substituted sulfonamides. Optimization based on potency, Notch/amyloid-β precursor protein selectivity, and brain efficacy after oral dosing led to the discovery of 4 (BMS-708163). Compound 4 is a potent inhibitor of γ-secretase (Aβ40 IC50 = 0.30 nM), demonstrating a 193-fold selectivity against Notch. Oral administration of 4 significantly reduced Aβ40 levels for sustained periods in brain, plasma, and cerebrospinal fluid in rats and dogs.

Antimicrobial Evaluation of Nocathiacins, a Thiazole Peptide Class of Antibiotics

ABSTRACT Nocathiacins are cyclic thiazolyl peptides with inhibitory activity against gram-positive bacteria. BMS-249524 (nocathiacin I), identified from screening a library of compounds against a multiply antibiotic-resistant Enterococcus faecium strain, was used as a lead chemotype to obtain additional structurally related compounds. The MIC assay results of BMS-249524 and two more water-soluble derivatives, BMS-411886 and BMS-461996, revealed potent in vitro activities against a variety of gram-positive pathogens including methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae, vancomycin intermediate-resistant S. aureus, vancomycin-resistant enterococci, Mycobacterium tuberculosis and Mycobacterium avium. Analysis of killing kinetics revealed that these compounds are bactericidal for S. aureus with at least a 3-log10 reduction of bacterial growth within 6 h of exposure to four times the MICs. Nocathiacin-resistant mutants were characterized by DNA sequence analyses. The mutations mapped to the rplK gene encoding the L11 ribosomal protein in the 50S subunit in a region previously shown to be involved in the binding of related thiazolyl peptide antibiotics. These compounds demonstrated potential for further development as a new class of antibacterial agents with activity against key antibiotic-resistant gram-positive bacterial pathogens.

The Amyloid-β Rise and γ-Secretase Inhibitor Potency Depend on the Level of Substrate Expression

The amyloid-β (Aβ) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-β precursor protein (APP) through consecutive proteolytic cleavages by β-site APP-cleaving enzyme and γ-secretase. Unexpectedly γ-secretase inhibitors can increase the secretion of Aβ peptides under some circumstances. This “Aβ rise” phenomenon, the same inhibitor causing an increase in Aβ at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the Aβ rise depends on the β-secretase-derived C-terminal fragment of APP (βCTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of Aβ, known as “p3,” formed by α-secretase cleavage, did not exhibit a rise. In addition to the Aβ rise, low βCTF or C99 expression decreased γ-secretase inhibitor potency. This “potency shift” may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, γ-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of Aβ under conditions of low substrate expression. The Aβ rise was also observed in rat brain after dosing with the γ-secretase inhibitor BMS-299897. The Aβ rise and potency shift are therefore relevant factors in the development of γ-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the Aβ rise, including the “incomplete processing” and endocytic models, are discussed.

Discovery of the first antibacterial small molecule inhibitors of MurB

A series of imidazolinone analogues was synthesized and shown to possess potent MurB inhibitory as well as good antibacterial activity.

Pharmacodynamics of Selective Inhibition of γ -Secretase by Avagacestat

A hallmark of Alzheimer’s disease (AD) pathology is the accumulation of brain amyloid β-peptide (Aβ), generated by γ-secretase-mediated cleavage of the amyloid precursor protein (APP). Therefore, γ-secretase inhibitors (GSIs) may lower brain Aβ and offer a potential new approach to treat AD. As γ-secretase also cleaves Notch proteins, GSIs can have undesirable effects due to interference with Notch signaling. Avagacestat (BMS-708163) is a GSI developed for selective inhibition of APP over Notch cleavage. Avagacestat inhibition of APP and Notch cleavage was evaluated in cell culture by measuring levels of Aβ and human Notch proteins. In rats, dogs, and humans, selectivity was evaluated by measuring plasma blood concentrations in relation to effects on cerebrospinal fluid (CSF) Aβ levels and Notch-related toxicities. Measurements of Notch-related toxicity included goblet cell metaplasia in the gut, marginal-zone depletion in the spleen, reductions in B cells, and changes in expression of the Notch-regulated hairy and enhancer of split homolog-1 from blood cells. In rats and dogs, acute administration of avagacestat robustly reduced CSF Aβ40 and Aβ42 levels similarly. Chronic administration in rats and dogs, and 28-day, single- and multiple-ascending–dose administration in healthy human subjects caused similar exposure-dependent reductions in CSF Aβ40. Consistent with the 137-fold selectivity measured in cell culture, we identified doses of avagacestat that reduce CSF Aβ levels without causing Notch-related toxicities. Our results demonstrate the selectivity of avagacestat for APP over Notch cleavage, supporting further evaluation of avagacestat for AD therapy.

A strategy to minimize reactive metabolite formation: discovery of (S)-4-(1-cyclopropyl-2-methoxyethyl)-6-[6-(difluoromethoxy)-2,5-dimethylpyridin-3-ylamino]-5-oxo-4,5-dihydropyrazine-2-carbonitrile as a potent, orally bioavailable corticotropin-releasing factor-1 receptor antagonist.

Detailed metabolic characterization of 8, an earlier lead pyrazinone-based corticotropin-releasing factor-1 (CRF(1)) receptor antagonist, revealed that this compound formed significant levels of reactive metabolites, as measured by in vivo and in vitro biotransformation studies. This was of particular concern due to the body of evidence suggesting that reactive metabolites may be involved in idiosyncratic drug reactions. Further optimization of the structure-activity relationships and in vivo properties of pyrazinone-based CRF(1) receptor antagonists and studies to assess the formation of reactive metabolites led to the discovery of 19e, a high affinity CRF(1) receptor antagonist (IC(50) = 0.86 nM) wherein GSH adducts were estimated to be only 0.1% of the total amount of drug-related material excreted through bile and urine, indicating low levels of reactive metabolite formation in vivo. A novel 6-(difluoromethoxy)-2,5-dimethylpyridin-3-amine group in 19e contributed to the potency and improved in vivo properties of this compound and related analogues. 19e had excellent pharmacokinetic properties in rats and dogs and showed efficacy in the defensive withdrawal model of anxiety in rats. The lowest efficacious dose was 1.8 mg/kg. The results of a two-week rat safety study with 19e indicated that this compound was well-tolerated.

Synthesis and SAR of indole-and 7-azaindole-1,3-dicarboxamide hydroxyethylamine inhibitors of BACE-1.

Heterocyclic replacement of the isophthalamide phenyl ring in hydroxyethylamine (HEA) BACE-1 inhibitors was explored. A variety of indole-1,3-dicarboxamide HEAs exhibited potent BACE-1 enzyme inhibition, but displayed poor cellular activity. Improvements in cellular activity and aspartic protease selectivity were observed for 7-azaindole-1,3-dicarboxamide HEAs. A methylprolinol-bearing derivative (10n) demonstrated robust reductions in rat plasma Aβ levels, but did not lower rat brain Aβ due to poor central exposure. The same analog exhibited a high efflux ratio in a bidirectional Caco-2 assay and was likely a substrate of the efflux transporter P-glycoprotein. X-ray crystal structures are reported for two indole HEAs in complex with BACE-1.

Biochemical Pharmacology of Acyclic Nucleotide Analogues

Our studies have shown that the acyclic nucleotide analogues PMEA and HPMPC are able to penetrate into cells and are then activated to mono- and diphosphate derivatives. The latter correspond to triphosphate analogues and presumably serve an important role in the biological activity exerted by these antiviral agents. In support of this idea, the inhibitory effect of PMEApp on HIV reverse transcriptase has been demonstrated with both RNA and DNA template-primer systems. Further studies will be undertaken to determine the effect of HPMPCpp on viral DNA polymerases. Whereas the metabolism of PMEA in CEM cells gives rise to only PMEAp and PMEApp, additional metabolites were obtained in MRC-5 cells; the identity of these metabolites remains to be determined. In the case of HPMPC, a third metabolite was obtained in addition to HPMPCp and HPMPCpp, which has been tentatively assigned as a phosphate-choline adduct by analogy with activation of cytosine-based nucleoside derivatives. The metabolism of HPMPC was unchanged between uninfected and infected cells, indicating that viral enzymes are not necessary for the activation of HPMPC. The long intracellular half-lives of the HPMPC metabolites may have implications for the antiviral efficacy of this compound. The persistence of activated metabolites suggests that infrequent dosing may be possible due to a prolonged antiviral effect. Our results on the effectiveness of infrequent dosing schedules with HPMPC in the treatment of HSV 2 infections in mice support this hypothesis. It is also possible that HPMPCp-choline may serve as a reservoir for HPMPC and therefore for the presumed active metabolite HPMPCpp.

Synthesis and activity of a C-8 keto pleuromutilin derivative

A C-8 keto pleuromutilin derivative has been synthesized from the biotransformation product 8-hydroxy mutilin. A key step in the process was the selective oxidation at C-8 of 8-hydroxy mutilin using tetrapropylammonium perruthenate. The presence of the C-8 keto group precipitated interesting intramolecular chemistry to afford a compound (10) with a novel pleuromutilin-derived ring system.