Amide-modified prenylcysteine based Icmt inhibitors: Structure-activity relationships, kinetic analysis and cellular characterization

Human protein isoprenylcysteine carboxyl methyltransferase (hIcmt) is the enzyme responsible for the α-carboxyl methylation of the C-terminal isoprenylated cysteine of CaaX proteins, including Ras proteins. This specific posttranslational methylation event has been shown to be important for cellular transformation by oncogenic Ras isoforms. This finding led to interest in hIcmt inhibitors as potential anti-cancer agents. Previous analog studies based on N-acetyl-S-farnesylcysteine identified two prenylcysteine-based low micromolar inhibitors (1a and 1b) of hIcmt, each bearing a phenoxyphenyl amide modification. In this study, a focused library of analogs of 1a and 1b was synthesized and screened versus hIcmt, delineating structural features important for inhibition. Kinetic characterization of the most potent analogs 1a and 1b established that both inhibitors exhibited mixed-mode inhibition and that the competitive component predominated. Using the Cheng-Prusoff method, the K(i) values were determined from the IC(50) values. Analog 1a has a K(IC) of 1.4±0.2μM and a K(IU) of 4.8±0.5μM while 1b has a K(IC) of 0.5±0.07μM and a K(IU) of 1.9±0.2μM. Cellular evaluation of 1b revealed that it alters the subcellular localization of GFP-KRas, and also inhibits both Ras activation and Erk phosphorylation in Jurkat cells.     

Synthesis of desthio prenylcysteine analogs: sulfur is important for biological activity

N-Acetyl-S-farnesyl cysteine (AFC) is the minimal synthetic substrate for the enzyme Icmt, which methylates prenylated proteins. The desthio-AFC isostere 2 has been synthesized in racemic form. This analog was not an Icmt substrate, but instead a weak inhibitor with an IC50 of approximately 325 microM.     

Probing the isoprenylcysteine carboxyl methyltransferase (Icmt) binding pocket: sulfonamide modified farnesyl cysteine (SMFC) analogs as Icmt inhibitors

Human isoprenylcysteine carboxyl methyltransferase (hIcmt) is a promising anticancer target as it is important for the post-translational modification of oncogenic Ras proteins. We herein report the synthesis and biochemical activity of 41 farnesyl-cysteine based analogs versus hIcmt. We have demonstrated that the amide linkage of a hIcmt substrate can be replaced by a sulfonamide bond to achieve hIcmt inhibition. The most potent sulfonamide-modified farnesyl cysteine analog was 6ag with an IC₅₀ of 8.8 ± 0.5 μM for hIcmt.     

Transition-state analogs as inhibitors of human and malarial hypoxanthine-guanine phosphoribosyltransferases.

The proposed transition state for hypoxanthine-guanine phosphoribosyltransferases (HGPRTs) has been used to design and synthesize powerful inhibitors that contain features of the transition state. The iminoribitols (1S)-1-(9-deazahypoxanthin-9-yl)-1,4-dideoxy-1,4-imino-D-ribitol 5-phosphate (immucillinHP) and (1S)-1-(9-deazaguanin-9-yl)-1,4-dideoxy-1,4-imino-D-ribitol 5-phosphate (immucillinGP) are the most powerful inhibitors yet reported for both human and malarial HGPRTs. Equilibrium binding constants are >1,000-fold tighter than the binding of the nucleotide substrate. The NMR spectrum of malaria HGXPRT in the Michaelis complex reveals downfield hydrogen-bonded protons. The chemical shifts move farther downfield with bound inhibitor. The inhibitors are lead compounds for species-specific antibiotics against parasitic protozoa. The high-resolution crystal structure of human HGPRT with immucillinGP is reported in the companion paper.     

Amide-substituted farnesylcysteine analogs as inhibitors of human isoprenylcysteine carboxyl methyltransferase

N-Acetyl-S-farnesyl-L-cysteine (AFC) is the minimal substrate for the enzyme isoprenylcysteine carboxyl methyltransferase (Icmt). A series of amide-modified farnesylcysteine analogs were synthesized and screened against human Icmt. From a 23-membered library of compounds, six inhibitors were identified and evaluated further. The adamantyl derivative 7c was the most potent inhibitor with an IC(50) of 12.4 microM.     

Design and synthesis of a new generation of substituted purine hydroxamate analogs as histone deacetylase inhibitors

Histone deacetylase inhibitors have been proved to be great potential for the treatment of cancer. Recently, we designed and modified a series of substituted purine hydroxamate analogs as potent HDAC inhibitors based on our previous studies. The target compounds were investigated for their in vitro HDAC inhibitory activities and anti-proliferative activities. Results indicated that these compounds could effectively inhibit HDAC and possess obvious anti-proliferative activity against tumor cells. Promisingly, target compounds 4m and 4n outperformed SAHA in both enzymatic inhibitory activity and cellular anti-proliferative activity assay.     

Molecular modelling and cytotoxicity of substituted anthraquinones as inhibitors of human telomerase

Molecular modelling has been carried out for a number of amine-functionalised anthraquinone derivatives to determine their extent of binding to G-tetraplex DNA and their ability to inhibit the enzymes telomerase and Taq polymerase. The results are compared to data obtained from a modified TRAP assay and show good correlation between the two methods. The findings suggest that anthraquinone derivatives of this type inhibit telomerase by stabilisation of four-stranded tetraplex structures associated with guanine-rich telomeric DNA regions.     

Synthesis and biological evaluation of NAD analogs as human pyridine nucleotide adenylyltransferase inhibitors

NAD analogs modified at the ribose adenylyl moiety, named N-2'-MeAD and Na-2'-MeAD, were synthesized as ligands of pyridine nucleotide (NMN/NaMN) adenylyltransferase (NMNAT). Both dinucleotides resulted selective inhibitors against human NMNAT-3 isoenzyme.     

Biaryl substituted hydantoin compounds as TACE inhibitors

We disclose further optimization of hydantoin TNF-α convertase enzyme (TACE) inhibitors. SAR with respect to the non-prime region of TACE active site was explored. A series of biaryl substituted hydantoin compounds was shown to have sub-nanomolar K_i, good rat PK, and good selectivity versus MMP-1, -2, -3, -7, -9, and -13.     

Thiophene substituted acylguanidines as BACE1 inhibitors

A series of thiophene-substituted acylguanidines were designed from a pyrrole substituted acylguanidine HTS lead. This template allowed a greater flexibility, through differential Suzuki couplings, to explore the binding site of BACE1 and to enhance the inhibitory potencies. This exploration provided a 25-fold enhancement in potency to yield compound 10a, which was 150 nM in a BACE1 FRET assay.     

Synthesis and evaluation of aplysinopsin analogs as inhibitors of human monoamine oxidase A and B

Aplysinopsins are tryptophan-derived natural products that have been isolated from a variety of marine organisms. Previous studies have shown aplysinopsin analogs to possess a variety of biological activities, including modulation of neurotransmissions. A series of fifty aplysinopsin analogs was synthesized and assayed for monoamine oxidase A and B inhibitory activity. Three compounds displayed significant MAO inhibitory activity and selectivity. The compound (E)-5-[(6-bromo-1H-indol-3-yl)methylene]-2-imino-1,3-dimethylimidazolidin-4-one (3x) possessed an IC(50) of 5.6 nM at MAO-A and had a selectivity index of 80.24. An SAR study revealed that multiple N-methylations, one of which should be at position N-2', and bromination at C-5 or C-6 are important factors for MAO-A potency and selectivity.     

New aromatic substituted pyrazoles as selective inhibitors of human adipocyte fatty acid-binding protein

a-FABP is indespensible in inflammation and may serve as a new potential drug target for inflammation related diseases. We have successfully designed and synthesized a series of aromatic substituted pyrazoles as new human a-FABP inhibitors. The compounds strongly bound to the hydrophobic binding pocket of a-FABP, while showed significantly lower binding affinities to the closely related homologue protein h-FABP. The most potent and selective compound 5g bound to a-FABP with an apparent K_i value below 1.0 nM, while did not inhibit h-FABP at 50 μM and thus represents one of the most potent and selective a-FABP inhibitors to date. The strong binding capacity of these inhibitors was further validated by their effective blockade of inflammatory responses as determined by the production of pro-inflammatory cytokines upon LPS stimulation. Compound 5g may serve as a lead compound for developing new effective therapeutic agent for prevention and treatment of atherosclerosis, type 2 diabetes and other inflammatory and metabolic related diseases.     

Synthesis of N-alkyl substituted indolocarbazoles as potent inhibitors of human cytomegalovirus replication.

The synthesis and antiviral evaluation of unsymmetrical indolocarbazole derivatives of Arcyriaflavin A, substituted with a range of alkyl groups at the indole nitrogen, is described. Structure-activity relationships in this series against human cytomegalovirus (HCMV) replication in cell culture are reported. Compound 4b was identified as potent inhibitor of HCMV (IC(50)=19 nM), which retained activity against a range of HCMV strains including ganciclovir resistant isolates.     

Synthesis and kinetic analysis of some phosphonate analogs of cyclophostin as inhibitors of human acetylcholinesterase

Two new monocyclic analogs of the natural AChE inhibitor cyclophostin and two exocyclic enol phosphates were synthesized. The potencies and mechanisms of inhibition of the bicyclic and monocyclic enol phosphonates and the exocyclic enol phosphates toward human AChE are examined. One diastereoisomer of the bicyclic phosphonate exhibits an IC₅₀ of 3 μM. Potency is only preserved when the cyclic enol phosphonate is intact and conjugated to an ester. Kinetic analysis indicates both a binding and a slow inactivation step for all active compounds. Mass spectrometric analysis indicates that the active site Ser is indeed phosphorylated by the bicyclic phosphonate.     

Simplified staurosporine analogs as potent JAK3 inhibitors

Simplification of bottom ring and regioselective functionalization of the indolocarbazole unit of staurosporine (2) are described. The modification led to a new series of simplified staurosporine analogs, which exhibited significant inhibitory activity against Janus kinase 3 (JAK3). The structure-activity relationships (SAR) are discussed and a proposed binding model is also highlighted.     

Development of radamide analogs as Grp94 inhibitors

Hsp90 isoform-selective inhibition is highly desired as it can potentially avoid the toxic side-effects of pan-inhibition. The current study developed selective inhibitors of one such isoform, Grp94, predicated on the chimeric and pan-Hsp90 inhibitor, radamide (RDA). Replacement of the quinone moiety of RDA with a phenyl ring (2) was found to be better suited for Grp94 inhibition as it can fully interact with a unique hydrophobic pocket present in Grp94. An extensive SAR for this scaffold showed that substitutions at the 2- and 4-positions (8 and 27, respectively) manifested excellent Grp94 affinity and selectivity. Introduction of heteroatoms into the ring also proved beneficial, with a 2-pyridine derivative (38) exhibiting the highest Grp94 affinity (K_d = 820 nM). Subsequent cell-based assays showed that these Grp94 inhibitors inhibit migration of the metastatic breast cancer cell line, MDA-MB-231, as well as exhibit an anti-proliferative affect against the multiple myeloma cell line, RPMI 8226.     

Triethylated chromones with substituted naphthalenes as tubulin inhibitors

Previously synthesized 2-(benzo[b]thiophene-3′-yl)-6,8,8-triethyldesmosdumotin B (1, TEDB-TB) and 2-(naphth-1′-yl)-6,8,8-triethyldesmosdumotin B (2) showed potent activity against multiple human tumor cell lines, including a multidrug-resistant (MDR) subline, by targeting spindle formation and/or the microtubule network. Consequently, ester analogues of hydroxylated naphthyl substituted TEBDs (3–5) were prepared and evaluated for their effects on tumor cell proliferation and on tubulin assembly. Among all new compounds, compound 6, a 4′-acetoxynaphthalen-1′-yl derivative, displayed the most potent antiproliferative activity (IC₅₀ 0.2–5.7 μM). Selected analogues were confirmed to be tubulin assembly inhibitors in cell-free and cell-based assays using MDR tumor cells. The new analogues partially inhibited colchicine binding to tubulin, suggesting their binding mode would be different from that of colchicine. This observation was supported by computational docking model analyses. Thus, the newly synthesized triethylated chromones with esterified naphthalene groups have good potential for development as a new class of mitotic inhibitors that target tubulin.     

Non-charged thiamine analogs as inhibitors of enzyme transketolase

Inhibition of the thiamine-utilizing enzyme transketolase (TK) has been linked with diminished tumor cell proliferation. Most thiamine antagonists have a permanent positive charge on the B-ring, and it has been suggested that this charge is required for diphosphorylation by thiamine pyrophosphokinase (TPPK) and binding to TK. We sought to make neutral thiazolium replacements that would be substrates for TPPK, while not necessarily needing thiamine transporters (ThTr1 and ThTr2) for cell penetration. The synthesis, SAR, and structure-based rationale for highly potent non-thiazolium TK antagonists are presented.     

Novel Roflumilast analogs as soft PDE4 inhibitors

PDE4 inhibitors are of high interest for treatment of a wide range of inflammatory or autoimmune diseases. Their potential however has not yet been realized due to target-associated side effects, resulting in a low therapeutic window. We herein report the design, synthesis and evaluation of novel PDE4 inhibitors containing a γ-lactone structure. Such molecules are designed to undergo metabolic inactivation when entering circulation, thereby limiting systemic exposure and reducing the risk for side effects. The resulting inhibitors were highly active on both PDE4B1 and PDE4D2 and underwent rapid degradation in human plasma by paraoxonase 1. In contrast, their metabolites displayed markedly reduced permeability and/or on-target activity.     

Heteroaryl substituted bis-trifluoromethyl carbinols as malonyl-CoA decarboxylase inhibitors

A series of heteroaryl-substituted bis-trifluoromethyl carbinols were prepared and evaluated as malonyl-CoA decarboxylase (MCD) inhibitors. Some thiazole-based derivatives showed potent in vitro MCD inhibitory activities and significantly increased glucose oxidation rates in isolated working rat hearts.