Utilization of Microarrayed Compound Screening (μARCS) to Identify Inhibitors of p56lck Tyrosine Kinase

Protein tyrosine kinases play critical roles in cell signaling and are considered attractive targets for drug discovery. The authors have applied μARCS (microarrayed compound screening) technology to develop a high-throughput screen for finding inhibitors of the p56lck tyrosine kinase. Initial assay development was performed in a homogeneous time-resolved (LANCE™) format in 96-well microplates and then converted into the gel-based μARCS format. The μARCS methodology is a well-less screening format in which 8640 compounds are arrayed on a microplate-sized piece of polystyene and subsequently assayed by placing reagents cast in agarose gels in contact with these compound sheets. A blotting paper soaked with adenosine triphosphate is applied on the gel to initiate the kinase reaction in the gel. Using this screening methodology, 300,000 compounds were screened in less than 40 h. Substantial reagent reduction was achieved by converting this tyrosine kinase assay from a 96-well plate assay to μARCS, resulting in significant cost savings. ( Journal of Biomolecular Screening 2004: 12-21)

βig-h3 Represses T-Cell Activation in Type 1 Diabetes

βig-h3/TGF-βi is a secreted protein capable of binding to both extracellular matrix and cells. Human genetic studies recently revealed that in the tgfbi gene encoding for βig-h3, three single nucleotide polymorphisms were significantly associated with type 1 diabetes (T1D) risk. Pancreatic islets express βig-h3 in physiological conditions, but this expression is reduced in β-cell insult in T1D. Since the integrity of islets is destroyed by autoimmune T lymphocytes, we thought to investigate the impact of βig-h3 on T-cell activation. We show here that βig-h3 inhibits T-cell activation markers as well as cytotoxic molecule production as granzyme B and IFN-γ. Furthermore, βig-h3 inhibits early T-cell receptor signaling by repressing the activation of the early kinase protein Lck. Moreover, βig-h3-treated T cells are unable to induce T1D upon transfer in Rag2 knockout mice. Our study demonstrates for the first time that T-cell activation is modulated by βig-h3, an islet extracellular protein, in order to efficiently avoid autoimmune response.

An immunosuppressive antibody-drug conjugate

We have developed a novel antibody-drug conjugate (ADC) that can selectively deliver the Lck inhibitor dasatinib to human T lymphocytes. This ADC is based on a humanized antibody that selectively binds with high affinity to CXCR4, an antigen that is selectively expressed on hematopoietic cells. The resulting dasatinib-antibody conjugate suppresses T-cell-receptor (TCR)-mediated T-cell activation and cytokine expression with low nM EC50 and has minimal effects on cell viability. This ADC may lead to a new class of selective immunosuppressive drugs with improved safety and extend the ADC strategy to the targeted delivery of kinase inhibitors for indications beyond oncology.

CD8 co-receptor promotes susceptibility of CD8+ T cells to transforming growth factor-β (TGF-β)-mediated suppression

CD8+ T cell function depends on a finely orchestrated balance of activation/suppression signals. While the stimulatory role of the CD8 co-receptor and pleiotropic capabilities of TGF-β have been studied individually, the influence of CD8 co-receptor on TGF-β function in CD8+ T cells is unknown. Here, we show that while CD8 enhances T cell activation, it also enhances susceptibility to TGF-β-mediated immune suppression. Using Jurkat cells expressing a full-length, truncated or no αβCD8 molecule, we demonstrate that cells expressing full-length αβCD8 were highly susceptible, αβCD8-truncated cells were partially susceptible, and CD8-deficient cells were completely resistant to suppression by TGF-β. Additionally, we determined that inhibition of Lck rendered mouse CD8+ T cells highly resistant to TGF-β suppression. Resistance was not associated with TGF-β receptor expression but did correlate with decreased Smad3 and increased Smad7 levels. These findings highlight a previously unrecognized third role for CD8 co-receptor which appears to prepare activated CD8+ T cells for response to TGF-β. Based on the important role which TGF-β-mediated suppression plays in tumor immunology, these findings unveil necessary considerations in formulation of CD8+ T cell-related cancer immunotherapy strategies.

Role of nonreceptor protein tyrosine kinases during phospholipase C-gamma 1-related uterine contractions in the rat

Activated phospholipase C1, produced in response to tyrosine phosphorylation, appears to play an important role during uterine contractions. These studies sought to determine which non-receptor protein tyrosine kinases are involved in the activation of phospholipase C1 in rat uterine tissue. In vitro contraction studies were performed utilizing isoform specific protein tyrosine kinase inhibitors. Western blots were performed utilizing antibodies to phosphotyrosine-phospholipase C1, total phospholipase C1, c-Src kinase and Lck kinase. Spontaneous, stretch-stimulated, and bpV(phen) (tyrosine phosphatase inhibitor) enhanced uterine contractions were significantly suppressed in response to Damnacanthal (Lck kinase inhibitor) and PP1 (c-Src kinase inhibitor). Damnacanthal and PP1 also significantly suppressed bpV(phen)-enhanced tyrosine phosphorylation of phospholipase C1. Western blots confirmed expression of Lck kinase and c-Src kinase in uterine tissue. In conclusion, the Lck and c-Src kinases appear to play an important role in regulating tyrosine phosphorylation of phospholipase C1 and contractile activity in the rat uterus.

Binding is not enough: flexibility is needed for photocrosslinking of Lck kinase by benzophenone photoligands

Benzophenone photophores are employed widely for photoaffinity-labeling studies. Photolabeling with benzophenone, however, is hardly a routine experiment. Even when a photoprobe binds to its target, photocrosslinking does not necessarily occur. This is because photolabeling by benzophenone is affected by many factors other than target-binding, such as conformational flexibility of photoligand. Despite the widespread recognition of such complications, there has been no systematic study to assess the relative importance of individual factors that can affect photolabeling efficiency. In order to gain an insight into this problem, we conducted a structure-activity relationship (SAR) study of benzophenone photoligands for Lck kinase, in which photoligands with varying target-binding affinity and conformational flexibility were compared. The study found that binding-affinity, as indicated by kinase inhibitory potency, did not correlate with photolabeling efficiency. Instead, conformational flexibility was found to be the determining factor for efficient photolabeling by our photoligands. Implication of the current findings, in particular, with regard to selection and optimization of benzophenone photoligands, is discussed.

Expression of human tyrosine kinase, Lck, in yeast Saccharomyces cerevisiae: growth suppression and strategy for inhibitor screening

We report the successful expression and detection of a phosphorylated form of human T cell tyrosine kinase, Lck, in Saccharomyes cerevisiae, which leads to growth suppression of the yeast cells. Expression of an inactive Lck mutant resulted in no phosphorylation and no growth suppression, indicating that cell growth inhibition by Lck is due to the activity of the kinase, consistent with the observed tyrosine-phosphorylation of the Lck and yeast host cell proteins. The addition of a known inhibitor of Lck to the cell culture resulted in recovery of cell growth expressing the active Lck, suggesting that the growth inhibition by lck gene expression can be used to screen inhibitors for the gene product. We have extended such approach to Tob, another potential therapeutic target.

N-4-Pyrimidinyl-1H-indazol-4-amine inhibitors of Lck: indazoles as phenol isosteres with improved pharmacokinetics

2,4-Dianilino pyrimidines are well-known inhibitors of tyrosine kinases including lymphocyte specific kinase (Lck). Structure-activity relationships at the 4-position are discussed and rationalised. Examples bearing a 2-methyl-5-hydroxyaniline substituent at the 4-position were especially potent but showed poor oral pharmacokinetics. Replacement of this substituent by 4-amino(5-methyl-1H-indazole) yielded compounds with comparable enzyme potency and improved pharmacokinetic properties.

Synthesis and Src kinase inhibitory activity of a series of 4-[(2,4-dichloro-5-methoxyphenyl)amino]-7-furyl-3-quinolinecarbonitriles

Compound 1 (SKI-606, bosutinib), a 7-alkoxy-4-[(2,4-dichloro-5-methoxyphenyl)amino]-3-quinolinecarbonitrile, is a potent inhibitor of Src kinase activity. We previously reported that analogs of 1 with thiophene groups at C-7 retained the Src activity of the parent compound. The corresponding C-7 furan analogs were prepared and it was found that the 3,5-substituted furan analog had increased activity compared to that of the 2,5-substituted furan isomer. Addition of a methoxy group at C-6 decreased the Src inhibitory activity of the C-7 2,5-substituted furan analog but increased the activity of the C-7 3,5-substituted furan isomer. This compound, 10, was a more potent Src inhibitor than 1 in both enzymatic and cell-based assays. The kinase selectivity profile of 10 was similar to that of 1, with 10 also inhibiting the activity of Abl and Lck. When tested in a solid tumor xenograft model, 10 had comparable oral activity to that of 1.

Association of CD38 with Nonmuscle Myosin Heavy Chain IIA and Lck Is Essential for the Internalization and Activation of CD38

Activation of CD38 in lymphokine-activated killer (LAK) cells involves interleukin-8 (IL8)-mediated protein kinase G (PKG) activation and results in an increase in the sustained intracellular Ca(2+) concentration ([Ca(2+)](i)), cADP-ribose, and LAK cell migration. However, direct phosphorylation or activation of CD38 by PKG has not been observed in vitro. In this study, we examined the molecular mechanism of PKG-mediated activation of CD38. Nonmuscle myosin heavy chain IIA (MHCIIA) was identified as a CD38-associated protein upon IL8 stimulation. The IL8-induced association of MHCIIA with CD38 was dependent on PKG-mediated phosphorylation of MHCIIA. Supporting these observations, IL8- or cell-permeable cGMP analog-induced formation of cADP-ribose, increase in [Ca(2+)](i), and migration of LAK cells were inhibited by treatment with the MHCIIA inhibitor blebbistatin. Binding studies using purified proteins revealed that the association of MHCIIA with CD38 occurred through Lck, a tyrosine kinase. Moreover, these three molecules co-immunoprecipitated upon IL8 stimulation of LAK cells. IL8 treatment of LAK cells resulted in internalization of CD38, which co-localized with MHCIIA and Lck, and blebbistatin blocked internalization of CD38. These findings demonstrate that the association of phospho-MHCIIA with Lck and CD38 is a critical step in the internalization and activation of CD38.

Discovery of 4-amino-5,6-biaryl-furo[2,3-d]pyrimidines as inhibitors of Lck: development of an expedient and divergent synthetic route and preliminary SAR

4-Amino-5,6-biaryl-furo[2,3-d]pyrimidines were identified as potent non-selective inhibitors of Lck. A novel, divergent, and practical synthetic route was developed to access derivatives from bifunctional intermediates. Lead optimization was guided by X-ray crystallographic data, and preliminary SAR led to the identification of compounds with improved cellular potency and selectivity.

Discovery of novel 2,3-diarylfuro[2,3-b]pyridin-4-amines as potent and selective inhibitors of Lck: synthesis, SAR, and pharmacokinetic properties

2,3-Diarylfuro[2,3-b]pyridine-4-amines are a novel class of potent and selective inhibitors of Lck. The discovery, synthesis, and structure activity relationships of this series of inhibitors are reported. The most promising compounds were also profiled to deduce their pharmacokinetic properties.

2-aminothiazole as a novel kinase inhibitor template. Structure-activity relationship studies toward the discovery of N-(2-chloro-6-methylphenyl)-2-[[6-[4-(2-hydroxyethyl)-1- piperazinyl)]-2-methyl-4-pyrimidinyl]amino)]-1,3-thiazole-5-carboxamide (dasatinib, BMS-354825) as a potent pan-Src kinase inhibitor

2-aminothiazole (1) was discovered as a novel Src family kinase inhibitor template through screening of our internal compound collection. Optimization through successive structure-activity relationship iterations identified analogs 2 (Dasatinib, BMS-354825) and 12m as pan-Src inhibitors with nanomolar to subnanomolar potencies in biochemical and cellular assays. Molecular modeling was used to construct a putative binding model for Lck inhibition by this class of compounds. The framework of key hydrogen-bond interactions proposed by this model was in agreement with the subsequent, published crystal structure of 2 bound to structurally similar Abl kinase. The oral efficacy of this class of inhibitors was demonstrated with 12m in inhibiting the proinflammatory cytokine IL-2 ex vivo in mice (ED50 approximately 5 mg/kg) and in reducing TNF levels in an acute murine model of inflammation (90% inhibition in LPS-induced TNFalpha production when dosed orally at 60 mg/kg, 2 h prior to LPS administration). The oral efficacy of 12m was further demonstrated in a chronic model of adjuvant arthritis in rats with established disease when administered orally at 0.3 and 3 mg/kg twice daily. Dasatinib (2) is currently in clinical trials for the treatment of chronic myelogenous leukemia.

Geldanamycin, a heat-shock protein 90-binding agent, induces thymocyte apoptosis through destabilization of Lck in presence of 12-O-tetradecanoylphorbol 13-acetate

Geldanamycin, a heat-shock protein 90 (Hsp90)-binding agent, modulates various cellular activities. The present study found that, although geldanamycin by itself had no effect on thymocyte viability, it induced apoptosis in thymocytes with a reduction of the mitochondrial transmembrane potential (DeltaPsim) in the presence of 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C (PKC). This apoptosis depended on transcription and translation, and on activation of caspase-8 and -3. Geldanamycin treatment in the presence of TPA also enhanced destabilization of Lck. This destabilization was independent of transcription and translation. It was inhibited, however, by conventional PKC inhibitors, preventing apoptosis. Proteasome inhibitor affected neither the degradation of Lck nor DNA fragmentation, although they inhibited reduction of DeltaPsim. These results suggest that the ubiquitin-proteasome system is not involved in Lck destabilization, and that DeltaPsim reduction is not directly related to the progression of apoptosis. Furthermore, inhibition of Lck in the presence of TPA induced apoptosis in thymocytes. Our findings suggest that Hsp90 modulates thymocyte apoptosis in concert with PKC through the destabilization of Lck and in a caspase-8- and -3-dependent manner.

Discovery of thienopyridines as Src-family selective Lck inhibitors

We describe the identification, SAR, and in vivo pharmacology of a new series of Src-family selective Lck inhibitors. These thienopyridines were designed based on a desire to access the unique residues in the extended hinge region of Lck.

Discovery of aminoquinazolines as potent, orally bioavailable inhibitors of Lck: synthesis, SAR, and in vivo anti-inflammatory activity

The lymphocyte-specific kinase (Lck) is a cytoplasmic tyrosine kinase of the Src family expressed in T cells and natural killer (NK) cells. Genetic evidence in both mice and humans demonstrates that Lck kinase activity is critical for signaling mediated by the T cell receptor (TCR), which leads to normal T cell development and activation. Selective inhibition of Lck is expected to offer a new therapy for the treatment of T-cell-mediated autoimmune and inflammatory disease. Screening of our kinase-preferred collection identified aminoquinazoline 1 as a potent, nonselective inhibitor of Lck and T cell proliferation. In this report, we describe the synthesis and structure-activity relationships of a series of novel aminoquinazolines possessing in vitro mechanism-based potency. Optimized, orally bioavailable compounds 32 and 47 exhibit anti-inflammatory activity (ED(50) of 22 and 11 mg/kg, respectively) in the anti-CD3-induced production of interleukin-2 (IL-2) in mice.

Novel 2-aminopyrimidine carbamates as potent and orally active inhibitors of Lck: synthesis, SAR, and in vivo antiinflammatory activity

The lymphocyte-specific kinase (Lck) is a cytoplasmic tyrosine kinase of the Src family expressed in T cells and NK cells. Genetic evidence in both mice and humans demonstrates that Lck kinase activity is critical for signaling mediated by the T cell receptor (TCR), which leads to normal T cell development and activation. A small molecule inhibitor of Lck is expected to be useful in the treatment of T cell-mediated autoimmune and inflammatory disorders and/or organ transplant rejection. In this paper, we describe the synthesis, structure-activity relationships, and pharmacological characterization of 2-aminopyrimidine carbamates, a new class of compounds with potent and selective inhibition of Lck. The most promising compound of this series, 2,6-dimethylphenyl 2-((3,5-bis(methyloxy)-4-((3-(4-methyl-1-piperazinyl)propyl)oxy)phenyl)amino)-4-pyrimidinyl(2,4-bis(methyloxy)phenyl)carbamate (43) exhibits good activity when evaluated in in vitro assays and in an in vivo model of T cell activation.

The development of novel 1,2-dihydro-pyrimido[4,5-c]pyridazine based inhibitors of lymphocyte specific kinase (Lck)

This communication details the synthesis, biological activity, and proposed binding mode of a novel class of tri-cyclic derivatives of 1,2-dihydro-pyrimido[4,5-c]pyridazines 1 and 2. The most potent analogs disclosed showed low nanomolar activity for the inhibition of Lck kinase.

Structural basis of Src tyrosine kinase inhibition with a new class of potent and selective trisubstituted purine-based compounds

The tyrosine kinase pp60src (Src) is the prototypical member of a family of proteins that participate in a broad array of cellular signal transduction processes, including cell growth, differentiation, survival, adhesion, and migration. Abnormal Src family kinase (SFK) signaling has been linked to several disease states, including osteoporosis and cancer metastases. Src has thus emerged as a molecular target for the discovery of small-molecule inhibitors that regulate Src kinase activity by binding to the ATP pocket within the catalytic domain. Here, we present crystal structures of the kinase domain of Src in complex with two purine-based inhibitors: AP23451, a small-molecule inhibitor designed to inhibit Src-dependent bone resorption, and AP23464, a small-molecule inhibitor designed to inhibit the Src-dependent metastatic spread of cancer. In each case, a trisubstituted purine template core was elaborated using structure-based drug design to yield a potent Src kinase inhibitor. These structures represent early examples of high affinity purine-based Src family kinase-inhibitor complexes, and they provide a detailed view of the specific protein-ligand interactions that lead to potent inhibition of Src. In particular, the 3-hydroxyphenethyl N9 substituent of AP23464 forms unique interactions with the protein that are critical to the picomolar affinity of this compound for Src. The comparison of these new structures with two relevant kinase-inhibitor complexes provides a structural basis for the observed kinase inhibitory selectivity. Further comparisons reveal a concerted induced-fit movement between the N- and C-terminal lobes of the kinase that correlates with the affinity of the ligand. Binding of the most potent inhibitor, AP23464, results in the largest induced-fit movement, which can be directly linked to interactions of the hydrophenethyl N9 substituent with a region at the interface between the two lobes. A less pronounced induced-fit movement is also observed in the Src-AP23451 complex. These new structures illustrate how the combination of structural, computational, and medicinal chemistry can be used to rationalize the process of developing high affinity, selective tyrosine kinase inhibitors as potential therapeutic agents.

Lead validation and SAR development via chemical similarity searching; application to compounds targeting the pY+3 site of the SH2 domain of p56lck

Compound selection based on chemical similarity has been used to validate active "parent" compounds identified via database searching as viable lead compounds and to obtain initial structure-activity relationships for those leads. Twelve parent compounds that have inhibitory activity against the SH2 domain of the p56 T-cell tyrosine kinase (Lck) are the focus of this study. Lck is involved in the T-cell mediated immune response, and inhibitors of Lck protein-protein interactions could potentially be used to develop novel immunosuppressants. Similarity searches for each parent compound were performed using 2D structural fingerprints on a database containing 1,300,000 commercially available compounds. The inhibitory activity of the selected compounds was assessed using enzyme immunoassay (EIA). In general, the most active parent compounds yield the most high activity similar compounds; however, in two cases low activity parent compounds (i.e. inhibitory activity < 25% at 100 microM) yielded multiple similar compounds with activities > 60%. Such compounds may, therefore, be considered as viable lead compounds for optimization. Structure-activity relationships were explored by examining both ligand structures and their computed bound conformations to the protein. Functional groups common to the active compounds as well as key amino acid residues that form hydrogen bonds with the active compounds were identified. This information will act as the basis for the rational optimization of the lead compounds.

Development of N-4,6-pyrimidine-N-alkyl-N'-phenyl ureas as orally active inhibitors of lymphocyte specific tyrosine kinase

A new class of lymphocyte specific tyrosine kinase (lck) inhibitors based on an N-4,6-pyrimidine-N-alkyl-N'-phenyl urea scaffold is described. Many of these compounds showed low-nanomolar inhibition of lck kinase activity as well as IL-2 synthesis from Jurkat cells. One of these analogs, 7i, was shown to be orally efficacious by in vivo testing in a rat adjuvant-induced arthritis study.

Prostaglandin E2 impairs CD4+ T cell activation by inhibition of lck: implications in Hodgkin's lymphoma

Many tumors, including Hodgkin's lymphoma, are associated with decreased cellular immunity and elevated levels of prostaglandin E(2) (PGE(2)), a known inhibitor of CD4+ T cell activation, suggested to be involved in immune deviation in cancer. To address the molecular mechanisms tumor-derived PGE(2) might have on primary human CD4+ T cells, we used a whole genome-based transcriptional approach and show that PGE(2) severely limited changes of gene expression induced by signaling through the T cell receptor and CD28. This data suggests an interference of PGE(2) at an early step of T cell receptor signaling: indeed, PGE(2) stimulation of T cells leads to inactivation of lck and reduced phosphorylation of ZAP70. Antiapoptotic genes escaped PGE(2)-induced inhibition resulting in partial protection from apoptosis in response to irradiation or Fas-mediated signaling. As a functional consequence, PGE(2)-treated CD4+ T cells are arrested in the cell cycle associated with up-regulation of the cyclin/cyclin-dependent kinase inhibitor p27(kip1). Most importantly, CD4+ T cells in Hodgkin's lymphoma show similar regulation of genes that were altered in vitro by PGE(2) in T cells from healthy individuals. These data strongly suggest that PGE(2) is an important factor leading to CD4+ T cell impairment observed in Hodgkin's lymphoma.

Src family kinase members have a common response to histone deacetylase inhibitors in human colon cancer cells

Histone deacetylase inhibitors (HDIs) induce cell cycle arrest, differentiation and/or apoptosis in numerous cancer cell types and have shown promise in clinical trials. These agents are particularly novel, given their ability to selectively influence gene expression. Previously, we demonstrated that the HDIs butyrate and trichostatin A (TSA) directly repress c-Src proto-oncogene expression in many cancer cell lines. Activation and/or overexpression of c-Src have been frequently observed in numerous malignancies, especially of the colon. Therefore, our observation was particularly interesting since butyrate is a naturally abundant component of the large intestine and has been suggested to be a cancer-preventive agent. However, c-Src is not the only Src family kinase (SFK) member to be implicated in the development of human cancers, including those of the colon. Therefore, the relative expression levels of known SFKs were examined in a panel of human colon cancer cell lines. We found a surprisingly diverse expression pattern but noted that most cell lines expressed relatively high levels of at least 2 SFKs. When the effects of butyrate and TSA were examined in representative cell lines, the expression of all SFKs was repressed in a dose- and time-dependent manner. Further, detailed examination of Lck, Yes and Lyn demonstrated that this repression had a direct effect on transcription and was independent of new protein synthesis. These results mirror our earlier data obtained with c-Src and suggest that SFKs are a major target of HDIs and likely account in part for the anticancer effects of these promising new drugs.

Src Tyrosine Kinases Mediate Crystalline Silica-Induced NF-κB Activation through Tyrosine Phosphorylation of IκB-α and p65 NF-κB in RAW 264.7 Macrophages

Protein tyrosine kinases (PTKs) and mitogen-activated protein kinases (MAPKs) have been demonstrated to play a crucial role in the signaling pathways induced by silica. In the present study, we investigated whether Src family TKs play a role in crystalline silica-induced NF-kappaB activation and whether NF-kappaB activation requires Src TK-dependent MAPK activity in RAW 264.7 cells, a mouse peritoneal macrophage cell line. Selective Src TK inhibitors, damnacanthal or PP1, inhibited silica-induced NF-kappaB activation in a dose-dependent manner. Furthermore, these kinase inhibitors suppressed silica-induced tyrosine phosphorylation of IkappaB-alpha and p65 NF-kappaB. Within a similar time frame, c-Src and Lck were physically associated with IkappaB-alpha and with p65 NF-kappaB. Silica stimulated the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2), but not p38 MAPK and c-Jun NH(2)-terminal kinase 1 and 2 (JNK1/2). Damnacanthal or PP1 substantially blocked the silica-induced activation of ERK1/2. Moreover, PD98059, an inhibitor of ERK1/2, or SB203580, an inhibitor of p38 MAPK, failed to inhibit silica-induced NF-kappaB activation. These results suggest that c-Src and Lck act for silica-induced NF-kappaB activation by mediating the tyrosine phosphorylations of IkappaB-alpha and p65 NF-kappaB. However, the Src TK-dependent activation of ERK1/2 may not be involved in the silica signaling pathway leading to NF-kappaB activation.

Discovery of A-770041, a src-family selective orally active lck inhibitor that prevents organ allograft rejection

We describe the identification, SAR, and pharmacology of the src-family selective lck inhibitor A-770041 that prolongs the survival of major histocompatibility mismatched allografts in models of solid organ transplant rejection for greater than 65 days.

A-770041, a novel and selective small-molecule inhibitor of Lck, prevents heart allograft rejection

Lck, one of eight members of the Src family of tyrosine kinases, is activated after T cell stimulation and is required for T-cell proliferation and interleukin (IL)-2 production. Inhibition of Lck has been a target to prevent lymphocyte activation and acute rejection. Here, we report the pharmacologic characterization of 1-methyl-1H-indole-2-carboxylic acid (4-{1-[4-(4-acetyl-piperazin-l-yl)-cyclohexyl]-4-amino-1H-pyrazolo[3,4-d]pyrimidin-3-yl}-2-methoxy-phenyl)-amide (A-770041), an orally bioavailable pyrazolo[3,4-d]pyrimidine with increased selectivity for Lck compared with previously reported compounds. A-770041 is a 147 nM inhibitor of Lck (1 mM ATP) and is 300-fold selective against Fyn, the other Src family kinase involved in T-cell signaling. Concanavalin A-stimulated IL-2 production in whole blood is inhibited by A-770041 with an EC50 of approximately 80 nM. A-770041 is orally bioavailable (F = 34.1 +/- 7.2% at 10 mg/kg) and has a t(1/2) of 4.1 +/- 0.1 h. Concanavalin A-induced IL-2 production in vivo is inhibited by oral administration of A-770041 (in vivo EC50 = 78 +/- 28 nM). Doses of A-770041 at or above 10 mg/kg/day prevent rejection of hearts transplanted heterotopically in rats from Brown Norway donors to Lewis recipients across a major histocompatibility barrier for least 65 days. Grafts from animals treated with 20 mg/kg/day A-770041 or 10 mg/day Cyclosporin A had minimal microvascular changes or multifocal mononuclear infiltrates. However, mineralization in myocytes from the grafts from A-770041-treated animals was less than animals treated with Cyclosporin A. Lck inhibition is an attractive target to prevent acute rejection.

Synthesis and Src Kinase Inhibitory Activity of 2-Phenyl- and 2-Thienyl-7-phenylaminothieno[3,2-b]pyridine-6-carbonitriles

2-phenyl-7-phenylaminothieno[3,2-b]pyridine-6-carbonitriles were recently reported to be inhibitors of Src kinase activity. In this study we present structure-activity relationships for additional thieno[3,2-b]pyridine-6-carbonitriles, modifying the substituents on the C-2 phenyl and C-7 phenylamino groups. Derivatives with various aminomethyl and aminoethyl substituents on the para position of the C-2 phenyl group retained the activity of the initial analogues. However, direct attachment of an amino group led to decreased activity. A 2,4-dichloro-5-methoxyphenylamino group at C-7 provided superior inhibition of Src enzymatic activity. Replacement of the C-2 phenyl group with a 3,5-substituted thiophene led to improved Src inhibitory activity compared to the parent compound, but other thiophene isomers were less active. One of the analogues reported here exhibited in vivo activity comparable to that of SKI-606, a related 3-quinolinecarbonitrile currently in clinical trials.

Defective Proximal TCR Signaling Inhibits CD8+ Tumor-Infiltrating Lymphocyte Lytic Function

CD8+ tumor-infiltrating lymphocytes (TIL) are severely deficient in cytolysis, a defect that may permit tumor escape from immune-mediated destruction. Because lytic function is dependent upon TCR signaling, we have tested the hypothesis that primary TIL have defective signaling by analysis of the localization and activation status of TIL proteins important in TCR-mediated signaling. Upon conjugate formation with cognate target cells in vitro, TIL do not recruit granzyme B+ granules, the microtubule-organizing center, F-actin, Wiskott-Aldrich syndrome protein, nor proline rich tyrosine kinase-2 to the target cell contact site. In addition, TIL do not flux calcium nor demonstrate proximal tyrosine kinase activity, deficiencies likely to underlie failure to fully activate the lytic machinery. Confocal microscopy and fluorescence resonance energy transfer analyses demonstrate that TIL are triggered by conjugate formation in that the TCR, p56lck, CD3zeta, LFA-1, lipid rafts, ZAP70, and linker for activation of T cells localize at the TIL:tumor cell contact site, and CD43 and CD45 are excluded. However, proximal TCR signaling is blocked upon conjugate formation because the inhibitory motif of p56lck is rapidly phosphorylated (Y505) and COOH-terminal Src kinase is recruited to the contact site, while Src homology 2 domain-containing protein phosphatase 2 is cytoplasmic. Our data support a novel mechanism explaining how tumor-induced inactivation of proximal TCR signaling regulates lytic function of antitumor T cells.

Chemical Inhibitors when Timing Is Critical: A Pharmacological Concept for the Maturation of T Cell Contacts

Cellular signal transduction proceeds through a complex network of molecular interactions and enzymatic activities. The timing of these molecular events is critical for the propagation of a signal and the generation of a specific cellular response. To define the timing of signalling events, we introduce the combination of high-resolution confocal microscopy with the application of small-molecule inhibitors at various stages of signal transduction in T cells. Inhibitors of Src-family tyrosine kinases and actin dynamics were employed to dissect the role of the lymphocyte-specific tyrosine kinase Lck in the formation and maintenance of T cell receptor/CD3-dependent contacts. Anti-CD3epsilon-coated coverslips served as a highly defined stimulus. The kinetics of the recruitment of the yellow fluorescent protein-tagged signalling protein ZAP-70 were detected by high-resolution confocal microscopy. The analysis revealed that at 5 min after receptor engagement, Lck activity was required for maintenance of contacts. In contrast, after 20 min of receptor engagement, the contacts were Lck-independent. The relevance of the timing of inhibitor application provides a pharmacological concept for the maturation of T cell-substrate contacts.

Relative contributions of desolvation, inter- and intramolecular interactions to binding affinity in protein kinase systems

In several previous studies, we performed sensitivity analysis to gauge the relative importance of different atomic partial charges in determining protein-ligand binding. In this work, we gain further insights by decomposing these results into three contributions: desolvation, intramolecular interactions, and intermolecular interactions, again based on a Poisson continuum electrostatics model. Three protein kinase-inhibitor systems have been analyzed: CDK2-deschloroflavopiridol, PKA-PKI, and LCK-PP2. Although our results point out the importance of specific intermolecular interactions to the binding affinity, they also reveal the remarkable contributions from the solvent-mediated intramolecular interactions in some cases. Thus, it is necessary to look beyond analyzing protein-ligand interactions to understand protein-ligand recognition or to gain insights into designing ligands and proteins. In analyzing the contributions of the three components to the overall binding free energy, the PKA-PKI system with a much larger ligand was found to behave differently from the other two systems with smaller ligands. In the former case, the intermolecular interactions are very favorable, and together with the favorable solvent-mediated intramolecular interactions, they overcome the large desolvation penalties to give a favorable electrostatics contribution to the overall binding affinity. On the other hand, the other two systems with smaller ligands only present modest intermolecular interactions and they are not or are only barely sufficient to overcome the desolvation penalty even with the aid of the favorable intramolecular contributions. As a result, the binding affinity of these two systems do not or only barely benefit from electrostatics contributions.

Discovery of novel 2-(aminoheteroaryl)-thiazole-5-carboxamides as potent and orally active Src-family kinase p56Lck inhibitors

A series of substituted 2-(aminoheteroaryl)-thiazole-5-carboxamide analogs have been synthesized as novel, potent inhibitors of the Src-family kinase p56Lck. Among them, compound 2 displayed superior in vitro potency and excellent in vivo efficacy.

Imatinib inhibits T-cell receptor-mediated T-cell proliferation and activation in a dose-dependent manner

The tyrosine kinase inhibitor imatinib (imatinib, STI571, Glivec, and Gleevec) is increasingly used in patients undergoing allogeneic transplantation for leukemia. However, little is known regarding its potential immunoregulatory effects. Here, we investigate the effect of imatinib on T-cell receptor (TCR)-mediated activation of human T cells. Following stimulation with the anti-CD3 antibody 12F6, proliferation of activated T cells was almost completely inhibited by 10 microM imatinib. Furthermore, antigen-triggered expansion of CD8+ T cells in response to immunodominant cytomegalovirus (CMV) and Epstein-Barr virus (EBV) peptides was significantly reduced. Up-regulation of the activation markers CD25 and CD69 in response to TCR cross-linking was suppressed by imatinib at a mean inhibitory concentration 50% (IC50) of 5.4 microM and 7.3 microM, respectively; interleukin 2 (IL-2) production was also impaired. Analysis of the TCR-induced signaling cascade showed that imatinib substantially reduced tyrosine phosphorylation of ZAP70 and LAT in response to activation through the TCR. Sequence comparisons of all 90 tyrosine kinase genes in the human genome for homology in the adenosine triphosphate (ATP) binding pocket identified LCK, which is required for ZAP70 activation, as a likely target for imatinib. The IC50 for LCK inhibition by imatinib was 0.6 microM to 0.8 microM in an in vitro tyrosine kinase assay. In summary, imatinib can interfere with T-cell activation in vitro, and its impact on the frequency of opportunistic infections and graft-versus-host or graft-versus-leukemia reactions after transplantation should be investigated in clinical trials.

Guanidine alkaloid analogs as inhibitors of HIV-1 Nef interactions with p53, actin, and p56lck

With current anti-HIV treatments targeting only 4 of the 15 HIV proteins, many potential viral vulnerabilities remain unexploited. We report small-molecule inhibitors of the HIV-1 protein Nef. In addition to expanding the anti-HIV arsenal, small-molecule inhibitors against untargeted HIV proteins could be used to dissect key events in the HIV lifecycle. Numerous incompletely characterized interactions between Nef and cellular ligands, for example, present a challenge to understanding molecular events during HIV progression to AIDS. Assays with phage-displayed Nef from HIV(NL4-3) were used to identify a series of guanidine alkaloid-based inhibitors of Nef interactions with p53, actin, and p56(lck). The guanidines, synthetic analogs of batzellidine and crambescidin natural products, inhibit the Nef-ligand interactions with IC(50) values in the low micromolar range. In addition, sensitive in vivo assays for Nef inhibition are reported. Although compounds that are effective in vitro proved to be too cytotoxic for cellular assays, the reported Nef inhibitors provide proof-of-concept for disrupting a new HIV target and offer useful leads for drug development.

Imidazoquinoxaline Src-Family Kinase p56Lck Inhibitors: SAR, QSAR, and the Discovery of (S)-N-(2-Chloro-6-methylphenyl)-2-(3-methyl-1-piperazinyl)imidazo- [1,5-a]pyrido[3,2-e]pyrazin-6-amine (BMS-279700) as a Potent and Orally Active Inhibitor with Excellent in Vivo Antiinflammatory Activity

A series of novel anilino 5-azaimidazoquinoxaline analogues possessing potent in vitro activity against p56Lck and T cell proliferation have been discovered. Subsequent SAR studies led to the identification of compound 4 (BMS-279700) as an orally active lead candidate that blocks the production of proinflammatory cytokines (IL-2 and TNFalpha) in vivo. In addition, an expanded set of imidazoquinoxalines provided several descriptive QSAR models highlighting the influence of significant steric and electronic features. The H-bonding (Met319) contribution to observed binding affinities within a tightly congeneric series was found to be significant.

Identification of Non-Phosphate-Containing Small Molecular Weight Inhibitors of the Tyrosine Kinase p56 Lck SH2 Domain via in Silico Screening against the pY + 3 Binding Site

The protein p56 lymphoid T cell tyrosine kinase (Lck) is predominantly expressed in T lymphocytes where it plays a critical role in T-cell-mediated immune response. Lck participates in phosphotyrosine-dependent protein-protein interactions through its modular binding unit, the Src homology-2 (SH2) domain. Accordingly, virtual screening methods combined with experimental assays were used to identify small molecular weight nonpeptidic compounds that block Lck SH2 domain-dependent interactions. Virtual screening included scoring normalization procedures and postdocking structural clustering that is shown to facilitate the selection of active compounds. By targeting the well-defined hydrophobic binding pocket known to impart specificity on Lck-protein interactions (i.e., pY + 3 site), inhibitors of the Lck SH2 domain were discovered that omit the phosphotyrosine (pY) or related moieties. The 34 out of 196 computationally selected compounds were shown to inhibit Lck SH2 domain association with phosphorylated immunoreceptor tyrosine based activation motifs peptide. Twenty-four of the active compounds were further tested for their ability to modulate biological function. Thirteen of these compounds showed inhibitory activity in mixed lymphocyte culture assay. Fluorescence titration experiments on four of these active compounds further verified their binding to the SH2 domain. Because of their simple chemical structures, these small organic compounds have the potential to act as lead compounds for the development of novel immunosuppressant drugs.

4-thiophenoxy-N-(3,4,5-trialkoxyphenyl)pyrimidine-2-amines as potent and selective inhibitors of the T-cell tyrosine kinase p56lck

We have developed a series of 4-thiophenoxy-N-(3,4,5-trialkoxyphenyl) pyrimidine-2-amines as potent and selective inhibitors of p56lck tyrosine kinase activity. In particular, the most potent inhibitor shows cellular activity in T-cell receptor (TCR) stimulated models of cytokine release, which suggests an immunomodulatory role for this class of inhibitor.

A-420983: a potent, orally active inhibitor of lck with efficacy in a model of transplant rejection

We have identified the pyrazolo[3,4-d]pyrimidine A-420983 (compound 7) as a potent inhibitor of lck. A-420983 exhibits oral efficacy in animal models of delayed-type hypersensitivity and organ transplant rejection.

Superantigen stimulation reveals the contribution of Lck to negative regulation of T cell activation

The conventional paradigm of T cell activation through the TCR states that Lck plays a critical activating role in this signaling process. However, the T cell response to bacterial superantigens does not require Lck. In this study we report that not only is Lck dispensable for T cell activation by superantigens, but it actively inhibits this signaling pathway. Disruption of Lck function, either by repression of Lck gene expression or by selective pharmacologic inhibitors of Lck, led to increased IL-2 production in response to superantigen stimulation. This negative regulatory effect of Lck on superantigen-induced T cell responses required the kinase activity of Lck and correlated with early TCR signaling, but was independent of immunological synapse formation and TCR internalization. Our data demonstrate that the multistage role of Lck in T cell signaling includes the activation of a negative regulatory pathway of T cell activation.

General Model for Estimation of the Inhibition of Protein Kinases Using Monte Carlo Simulations

Monte Carlo statistical mechanics simulations were used in combination with the extended linear response (ELR) approach to develop a model to predict the activities of kinase inhibitors. One hundred forty eight inhibitors of three protein kinases, cyclin-dependent kinase 2 (CDK2), lymphocyte-specific kinase (Lck), and p38 mitogen-activated protein kinase were considered. The inhibitor sets for the individual kinases were analyzed first, and ELR models using only three descriptors were obtained with correlation coefficients, r(2), of 0.7-0.8. Models for each pair of kinases were then developed and used to predict the activities of the inhibitors for the remaining kinase with resultant q(2) values of 0.71 (CDK2), 0.70 (Lck), and 0.54 (p38). Finally, the three datasets were combined to yield a general ELR model for kinase inhibition; with just three physically reasonable descriptors, EXX, DeltaHB(total), and DeltaSASA, the r(2) and leave-one-out q(2) are 0.69 and 0.67. The optimization of the model was confirmed using a genetic algorithm. The descriptors reflect the structural requirements for strong inhibition: good steric and electrostatic complementarities between inhibitor and protein, limited loss of hydrogen bonds for the inhibitor upon binding, and increased burial of surface area of the inhibitor.

QSAR study of flavonoid derivatives as p56lck tyrosinkinase inhibitors

QSAR studies on 104 flavonoid derivatives as p56lck protein tyrosine kinase inhibitors were performed using hydration energy and logP as predictor parameters. The results obtained demonstrate in detail, which specify that hydration energy and hydrophobic parameters of the compounds play a significant role in developing QSAR models. The significance of presence and absence of substituents on particular position is successfully explored with the help of indicator parameters. The results are critically discussed on the basis of multiple linear regression parameters.

A New Strategy for the Construction of the Imidazo[1,5-a]quinoxalin-4-one Ring System and Its Application to the Efficient Synthesis of BMS-238497, a Novel and Potent Lck Inhibitor

A new efficient strategy was developed for the construction of the imidazo[1,5-a]quinoxalin-4-one ring system. The new method involves condensation of o-nitroaniline with glyoxylate in methanol followed by treatment of the resulting alpha-(o-nitroanilino)-alpha-methoxy acetate with tosylmethyl isocyanide (TosMIC) reagent to give 1-(o-nitrophenyl)imidazole-5-carboxylate. Reductive cyclization of the nitro imidazole carboxylate afforded imidazo[1,5-a]quinoxalin-4-one in three steps and 60% overall yield. The new method was successfully applied to the synthesis of BMS-238497, a novel and potent Lck inhibitor.

Discovery and initial SAR of 2-amino-5-carboxamidothiazoles as inhibitors of the Src-family kinase p56Lck

A novel series of 2-amino-5-carboxamidothiazoles were identified as inhibitors of Lck. Structure-activity studies demonstrate the structural requirements for potent Lck activity. Cyclopropylamide 11d is a potent Lck inhibitor having sub-micromolar activity in a PBL proliferation assay.

Inhibition of the activity of SRC and Abl tyrosine protein kinases by the binding of the Wiskott-Aldrich syndrome protein

The Wiscott-Aldrich syndrome protein, WASP, is an effector through which cdc42, a Rho family GTPase, regulates the actin cytoskeleton in hematopoietic cells. We have found that WASP binds readily to a number of tyrosine protein kinases including the Src kinases and the Abl kinase when the proteins are coexpressed during transient transfection. Binding inhibited the activity of each of these kinases strikingly, both in vitro and in vivo. Surprisingly, the binding was not due to an interaction between the proline-rich domain of WASP and the SH3 domain of these kinases. Rather, residues 83-93 in WASP were found to bind to the catalytic domains of the kinases. Binding did not decrease the affinity of Src kinases for either ATP or a peptide substrate noticeably. Rather, the V(max) of substrate phosphorylation was reduced by the binding of the peptide. This inhibition represents a novel form of regulation of protein kinase activity and suggests that that the isolation of small molecules that exploit this inhibitory mechanism may be possible.

Discovery of 2-amino-heteroaryl-benzothiazole-6-anilides as potent p56(lck) inhibitors

A series of structurally novel benzothiazole based small molecule inhibitors of p56(lck) was prepared to elucidate their structure-activity relationships (SAR), selectivity and cell activity in the T-cell proliferation assay. BMS-350751 (2) and BMS-358233 (3) are identified as potent Lck inhibitors with excellent cellular activities against T-cell proliferation.

Cutting edge: a chemical genetic system for the analysis of kinases regulating T cell development

To understand the regulatory activities of kinases in vivo requires their study across a biologically relevant window of activity. To this end, ATP analog-sensitive kinase alleles (ASKAs) specifically sensitive to a competitive inhibitor have been developed. This article tests whether ASKA technology can be applied to complex immunological systems, such as lymphoid development. The results show that when applied to reaggregate thymic organ culture, novel p56(Lck) ASKAs readily expose a dose-dependent correlation of thymocyte development with a range of p56(Lck) activity. By regulating kinase activity, rather than amounts of RNA or protein, ASKA technology offers a general means for assessing the quantitative contributions to immunology of numerous kinases emerging from genomics analyses. It can obviate the generation of multiple lines of mice expressing different levels of kinase transgenes and should permit specific biological effects to be associated with defined biochemical activities.

Molecular design, synthesis, and structure-Activity relationships leading to the potent and selective p56(lck) inhibitor BMS-243117

A series of structurally novel benzothiazole based small molecule inhibitors of p56(lck) were prepared to elucidate their structure-activity relationships (SARs), selectivity and cell activity in the T-cell proliferation assay. BMS-243117 (compound 2) is identified as a potent, and selective Lck inhibitor with good cellular activity (IC(50)=1.1 microM) against T-cell proliferation.

Dopamine inhibits cytokine release and expression of tyrosine kinases, Lck and Fyn in activated T cells

The effect of dopamine (DA) on the release of cytokines from activated human T cells has been evaluated to analyze the mechanism by which physiological concentration of dopamine inhibits T cell proliferation. Dopamine inhibited anti-CD3 mAb-induced release of both Th1 and Th2 cytokines, IL2, IFN-gamma and IL4 from T cells by specific class of dopamine receptors. This action of dopamine was mediated by a new mechanism. Dopamine suppressed non-receptor tyrosine kinases, Lck and Fyn expression which are the initial and pivotal signaling steps in T cell receptor (TCR) mediated different down stream signaling cascades, leading to cytokine release and subsequent clonal expansion of these immune effector cells.

Inhibitory effects of rosmarinic acid on Lck SH2 domain binding to a synthetic phosphopeptide

In the course of screening inhibitors from the methanol (MeOH) extracts of 168 medicinal plants against lymphocyte cell-specific kinase (Lck) Src -homology 2 (SH2) binding to a synthetic phosphotyrosine-containing peptide (phosphopeptide), we isolated rosmarinic acid from the MeOH extract of Prunella vulgaris, which showed specific inhibitory activity. The IC 50 value for Lck SH2 binding to phosphopeptide (SGSGEEPQpYEEIPI) of hamster polyomavirus middle-sized tumor (hmT pY324) was 7 microM. However, even at concentrations of 0.1 to 1000 microM, no significant inhibitions were observed against other SH2 domains binding such as the growth factor receptor binding protein 2 (Grb2) SH2 domain to phosphopeptide of Shc and phospholipase Cgamma1 (PLCgamma1) SH2 domain to translational elongation factor 1alpha (EF1alpha) C-terminal. Rosmarinic acid inhibited interleukin-2 (IL-2) gene expression by 50 % at a concentration of 8 microM in Jurkat cells stimulated with anti-CD3 and anti-CD4 antibodies. FK506 and cyclosporin A (CsA) employed as positive controls showed less than 30 % inhibition at the same concentration. In addition, rosmarinic acid inhibited the intracellular [Ca 2+] i increase in Jurkat cells after T cell activation in a dose-dependent manner at concentrations of 1.4 to 140 microM of rosmarinic acid, which is one of the earliest responses of antigen-specific T cell receptor (TCR) and of the upstream pathway of IL-2 expression. Taken together, these results suggest that rosmarinic acid has the potential to specifically inhibit Lck SH2 domain binding to its cognate ligand, including ZAP-70, Cbl, HS-1, and PLCgamma1, and Lck-dependent Ca 2+ signaling pathway of its downstream effector and finally to modulate IL-2 gene expression after T cell activation.