Probing the communication between the regulatory and catalytic domains of a protein tyrosine kinase, Csk

Protein tyrosine kinases (PTKs) are important regulators of mammalian cell function and their own activities are tightly regulated. Underlying their tight regulation, all PTKs contain multiple regulatory domains in addition to a catalytic domain. C-terminal Src kinase (Csk) contains a catalytic domain and a regulatory region, consisting of an SH3 and an SH2 domain. In this study, we probed the communication between the regulatory and catalytic domains of Csk. First, kinetic characterization of SH3 and SH2 domain deletion mutants demonstrated that the SH3 and SH2 domains were crucial in maintaining the full activity of Csk, but were not directly involved in Csk recognition of its physiological substrate, Src. Second, highly conserved Trp188, corresponding to a key residue in domain-domain communication in other PTKs, was found to be important for maintaining the active structure of Csk by the presence of the regulatory region, but not required for Csk activation triggered by a phosphopeptide binding to the SH2 domain. Third, structural alignment indicated that the presence of the regulatory domains modulated the conformation of multiple substructures in the catalytic domain, some directly and others remotely. Mutagenic and kinetic studies supported this assignment. This report extended previous studies of Csk domain-domain communication, and provided a foundation for further detailed investigation of this communication.

Bisubstrate analog probes for the insulin receptor protein tyrosine kinase: molecular yardsticks for analyzing catalytic mechanism and inhibitor design

Bisubstrate analogs have the potential to provide enhanced specificity for protein kinase inhibition and tools to understand catalytic mechanism. Previous efforts led to the design of a peptide-ATP conjugate bisubstrate analog utilizing aminophenylalanine in place of tyrosine and a thioacetyl linker to the gamma-phosphate of ATP which was a potent inhibitor of the insulin receptor kinase (IRK). In this study, we have examined the contributions of various electrostatic and structural elements in the bisubstrate analog to IRK binding affinity. Three types of changes (seven specific analogs in all) were introduced: a Tyr isostere of the previous aminophenylalanine moiety, modifications of the spacer between the adenine and the peptide, and deletions and substitutions within the peptide moiety. These studies allowed a direct evaluation of the hydrogen bond strength between the anilino nitrogen of the bisubstrate analog and the enzyme catalytic base Asp and showed that it contributes 2.5 kcal/mol of binding energy, in good agreement with previous predictions. Modifications of the linker length resulted in weakened inhibitory affinity, consistent with the geometric requirements of an enzyme-catalyzed dissociative transition state. Alterations in the peptide motif generally led to diminished inhibitory potency, and only some of these effects could be rationalized based on prior kinetic and structural studies. Taken together, these results suggest that a combination of mechanism-based design and empirical synthetic manipulation will be necessary in producing optimized protein kinase bisubstrate analog inhibitors.

2,6-hexadecadiynoic acid and 2,6-nonadecadiynoic acid: novel synthesized acetylenic fatty acids as potent antifungal agents

The hitherto unknown 2,6-hexadecadiynoic acid, 2,6-nonadecadiynoic acid, and 2,9-hexadecadiynoic acid were synthesized in two steps and in 11-18% overall yields starting from either 1,5-hexadiyne or 1,8-nonadiyne. Among all the compounds 2,6-hexadecadiynoic acid displayed the best overall antifungal activity against both the fluconazole-resistant Candida albicans strains ATCC 14053 and ATCC 60193, with a minimum inhibitory concentration (MIC of 11 microM), and against Cryptococcus neoformans ATCC 66031 (MIC < 5.7 microM). 2,9-Hexadecadiynoic acid did not display any significant cytotoxicity against the fluconazole-resistant C. albicans strains, but it showed fungitoxicity against C. neoformans ATCC 66031 with a MIC value of < 5.8 microM. Other FA, such as 2-hexadecynoic acid, 5-hexadecynoic acid, 9-hexadecynoic acid, and 6-nonadecynoic acid were also synthesized and their antifungal activities compared with those of the novel acetylenic FA. 2-Hexadecynoic acid, a known antifungal FA, exhibited the best antifungal activity (MIC = 9.4 microM) against the fluconazole-resistant C. albicans ATCC 14053 strain, but it showed a MIC value of only 100 microM against C. albicans ATCC 60193. 2,6-Hexadecadiynoic acid and 2-hexadecynoic acid also displayed a MIC of 140-145 microM toward Mycobacterium tuberculosis H37Rv in Middlebrook 7H12 medium. In conclusion, 2,6-hexadecadiynoic acid exhibited the best fungitoxicity profile compared with other analogues. This diynoic FA has the potential to be further evaluated for use in topical antifungal formulations.

Solid-phase synthesis of symmetrical 5',5'-dinucleoside mono-, di-, tri-, and tetraphosphodiesters

Four classes of phosphitylating reagents were subjected to reactions with aminomethyl polystyrene resin-bound p-acetoxybenzyl alcohol to yield the corresponding polymer-bound mono-, di-, tri-, and tetraphosphitylating reagents. The solid-phase reagents were reacted with unprotected nucleosides (e.g., thymidine, adenosine, 3'-azido-3'-deoxythymidine, cytidine, or inosine) in the presence of 5-(ethylthio)-1H-tetrazole. Polymer-bound nucleosides underwent oxidation with tert-butyl hydroperoxide, deprotection of cyanoethoxy groups with DBU, and the acidic cleavage, respectively, to afford 5',5'-dinucleoside mono-, di-, tri-, and tetraphosphodiesters in 59-78% yield.

Bilayer disruption and liposome restructuring by a homologous series of small Arg-rich synthetic peptides

The effects of a series of low molecular weight water-soluble cationic linear peptide analogs (LPAs, <1000 MW) with increasing hydrophobic/hydrophilic balance on lipid bilayer phase behavior and permeability were examined using liposomes composed of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and mixed zwitterionic/anionic DPPC/dipalmitoylphosphatidylglycerol (DPPG) lipid bilayers. LPAs were synthesized using a previously reported alkyl linkage strategy as Arg-C(n)-Arg-C(n)-Lys, where C(n) represents the saturated alkyl linkage separating the cationic residues (n=4, 7, or 11) (Ye et al., 2007 [1]). Differential scanning calorimetry results show that the cationic LPAs bound to and disrupted DPPC and, to a greater extent, DPPC/DPPG phase behavior. When added to preformed unilamellar liposomes, the LPAs led to significant structural changes based on cryogenic transmission electron microscopy (cryo-TEM). Coupling cryo-TEM with carboxyfluorescein leakage studies indicate that the LPAs induced permeabilization through bilayer expansion, which caused membrane thinning. The effects were inconsistent with increasing LPA hydrophobicity, which suggests that a cooperative effect between electrostatic binding and hydrophobic insertion determined the location of LPAs within the bilayer and their membrane activity. Our results for LPA-induced membrane disruption correlate with previous breast cancer cell uptake studies that showed minimal LPA-C(4) uptake, but high LPA-C(11) uptake through a non-endocytic mechanism.

Synthesis and biological evaluation of fatty acyl ester derivatives of 2',3'-didehydro-2',3'-dideoxythymidine

A number of 5'-O-fatty acyl derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T) were synthesized and evaluated for anti-HIV activities against cell-free and cell-associated virus, cellular cytotoxicity, and cellular uptake studies. The conjugates were found to be more potent than d4T. Among these conjugates, 5'-O-12-azidododecanoyl derivative of d4T (2), displaying EC(50) = 3.1-22.4 μM, showed 4- to 9-fold higher activities than d4T against cell-free and cell-associated virus. Cellular uptake studies were conducted on CCRF-CEM cell line using 5(6)-carboxyfluorescein derivatives of d4T attached through β-alanine (9) or 12-aminododecanoic acid (10) as linkers. The fluorescein-substituted analog of d4T with long chain length (10) showed 12- to 15-fold higher cellular uptake profile than the corresponding analog with short chain length (9). These studies reveal that conjugation of fatty acids to d4T enhances the cellular uptake and anti-HIV activity of stavudine.

Design of tetrapeptide ligands as inhibitors of the Src SH2 domain

Src homology-2 (SH2) domains are noncatalytic motifs containing approximately 100 amino acid residues that are involved in intracellular signal transduction. The phosphotyrosine-containing tetrapeptide pTyr-Glu-Glu-Ile (pYEEI) binds to Src SH2 domain with high affinity (K(d)=100 nM). The development of five classes of tetrapeptides as inhibitors for the Src SH2 domain is described. Peptides were prepared via solid-phase peptide synthesis and tested for affinity to Src SH2 domain using a fluorescence polarization based assay. All of the N-terminal substituted pYEEI derivatives (class II) presented binding affinity (IC(50)=of 2.7-8.6 microM) comparable to pYEEI (IC(50)=6.5 microM) in this assay. C-Terminal substituted pYEEI derivatives (class III) showed a lower binding affinity with IC(50) values of 34-41 microM. Amino-substituted phenylalanine derivatives (class IV) showed weak binding affinities (IC(50)=16-153 microM). Other substitutions on phenyl ring (class I) or the replacement of the phenyl ring with other cyclic groups (class V) dramatically decreased the binding of tetrapeptides to Src SH2 (IC(50)>100 microM). The ability of pYEEI and several of the tetrapeptides to inhibit the growth of cancer cells were assessed in a cell-based proliferation assay in human embryonic kidney (HEK) 293 tumor cells. The binding affinity of several of tested compounds against Src SH2 domain correlates with antiproliferative activity in 293T cells. None of the compounds showed any significant antifungal activity against Candida albicans ATCC 14053 at the maximum tested concentration of 10 microM. Overall, these results provided the structure-activity relationships for some FEEI and YEEI derivatives designed as Src SH2 domain inhibitors.

Peptide amphiphile containing arginine and fatty acyl chains as molecular transporters

Peptide amphiphiles (PAs) are promising tools for the intracellular delivery of numerous drugs. PAs are known to be biodegradable systems. Here, four PA derivatives containing arginine and lysine conjugated with fatty acyl groups with different chain lengths, namely, PA1: R-K(C14)-R, PA2: R-K(C16)-R, PA3: K(C14)-R-K(C14), and PA4: K(C16)-R-K(C16), where C16 = palmitic acid and C14 = myristic acid, were synthesized through Fmoc chemistry. Flow cytometry studies showed that, among all synthesized PAs, only K(C16)-R-K(C16), PA4 was able to enhance the cellular uptake of a fluorescence-labeled anti-HIV drug 2',3'-dideoxy-3'-thiacythidine (F'-3TC, F' = fluorescein) and a biologically important phosphopeptide (F'-PEpYLGLD) in human leukemia cells (CCRF-CEM) after 2 h incubation. For example, the cellular uptake of F'-3TC and F'-PEpYLGLD was enhanced approximately 7.1- and 12.6-fold in the presence of the PA4 compared to those of the drugs alone. Confocal microscopy of F'-3TC and F'-PEpYLGLD loaded PA4 in live cells showed significantly higher intracellular localization than the drug alone in human ovarian cells (SK-OV-3) after 2 h incubation. The high-performance liquid chromatography (HPLC) results showed that loading of Dox by the peptide amphiphile was 56% after 24 h. The loaded Dox was released (34%) within 48 h intracellularly. The circular dichrosim (CD) results exhibited that the secondary structure of the peptide was changed upon interactions with Dox. Mechanistic studies revealed that endocytosis is the major pathway of the internalization. These studies suggest that PAs containing the appropriate sequence of amino acids, chain length, charge, and hydrophobicity can be used as cellular delivery tools for transporting drugs and biomolecules.

Cyclic Peptide Containing Hydrophobic and Positively Charged Residues as a Drug Delivery System for Curcumin

Due to the low water solubility and hydrophobic nature of curcumin, an efficient cellular uptake is critical for its biological activity. We have previously developed a number of homochiral L-cyclic peptides containing arginine and tryptophan as cell-penetrating peptides. Among the synthesized peptides, [WR]5 containing five arginine and five tryptophan residues was found to be the most efficient one. Here, we have compared the application of [WR]5 to improve the intracellular uptake of curcumin by using both peptide-curcumin conjugate and physical mixture (peptide + curcumin) strategies. Flow cytometry results showed that the intracellular uptake of curcumin (50 μM) was enhanced through the physical mixing with [WR]5 by 5.7 folds compared to that of curcumin alone in human leukemia (CCRFCEM) cells after 3 h. When [WR]5 was conjugated with curcumin, the intracellular uptake was enhanced by 4 fold. These data suggest that the physical mixture can work more efficiently in enhancing the cellular delivery of curcumin. Furthermore, the antiproliferative activity of curcumin was enhanced by 20% and ∼13% through the physical mixture and the conjugate, respectively, in CCRF-CEM cells after 72 h.

Polymer-supported reagents for methylphosphorylation and phosphorylation of carbohydrates

Two polymer-supported reagents for methylphosphorylation and phosphorylation of carbohydrates have been developed. p-Hydroxybenzyl alcohol and beta-mercaptoethanol were immobilized on cross-linked divinylbenzene-polystyrene copolymer and conjugated with methyl N,N-diisopropylchlorophosphoramidite. Carbohydrates were reacted with polymer-bound phosphitylating reagents. Further oxidation, with or without the methoxy group deprotection, and cleavage yielded methylphosphorylated or phosphorylated carbohydrates, respectively.

Membrane-Active Cyclic Amphiphilic Peptides: Broad-Spectrum Antibacterial Activity Alone and in Combination with Antibiotics

We designed a library of 24 cyclic peptides containing arginine (R) and tryptophan (W) residues in a sequential manner [RnWn] (n = 2-7) to study the impact of the hydrophilic/hydrophobic ratio, charge, and ring size on the antibacterial activity against Gram-positive and Gram-negative strains. Among peptides, 5a and 6a demonstrated the highest antimicrobial activity. In combination with 11 commercially available antibiotics, 5a and 6a showed remarkable synergism against a large panel of resistant pathogens. Hemolysis (HC50 = 340 μg/mL) and cell viability against mammalian cells demonstrated the selective lethal action of 5a against bacteria over mammalian cells. Calcein dye leakage and scanning electron microscopy studies revealed the membranolytic effect of 5a. Moreover, the stability in human plasma (t1/2 = 3 h) and the negligible ability of pathogens to develop resistance further reflect the potential of 5a for further development as a peptide-based antibiotic.

Synthesis and Evaluation of Antimicrobial Activity of [R₄W₄K]-Levofloxacin and [R₄W₄K]-Levofloxacin-Q Conjugates

The development of a new class of antibiotics to fight bacterial resistance is a time-consuming effort associated with high-cost and commercial risks. Thus, modification, conjugation or combination of existing antibiotics to enhance their efficacy is a suitable strategy. We have previously reported that the amphiphilic cyclic peptide [R₄W₄] had antibacterial activity with a minimum inhibitory concentration (MIC) of 2.97 µg/mL against Methicillin-resistant Staphylococcus aureus (MRSA). Herein, we hypothesized that conjugation or combination of the amphiphilic cyclic peptide [R₄W₄] with levofloxacin or levofloxacin-Q could improve the antibacterial activity of levofloxacin and levofloxacin-Q. Fmoc/tBu solid-phase chemistry was employed to synthesize conjugates of [R₄W₄K]-levofloxacin-Q and [R₄W₄K]-levofloxacin. The carboxylic acid group of levofloxacin or levofloxacin-Q was conjugated with the amino group of β-alanine attached to lysine in the presence of 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) and N,N-diisopropylethylamine (DIPEA) for 3 h to afford the products. Antibacterial assays were conducted to determine the potency of conjugates [R₄W₄K]-levofloxacin-Q and [R₄W₄K]-levofloxacin against MRSA and Klebsiella pneumoniae. Although levofloxacin-Q was inactive even at a concentration of 128 µg/mL, [R₄W₄K]-levofloxacin-Q conjugate and the corresponding physical mixture showed MIC values of 8 µg/mL and 32 µg/mL against MRSA and Klebsiella pneumonia, respectively, possibly due to the activity of the peptide. On the other hand, [R₄W₄K]-levofloxacin conjugate (MIC = 32 µg/mL and MIC = 128 µg/mL) and the physical mixture (MIC = 8 µg/mL and 32 µg/mL) was less active than levofloxacin (MIC = 2 µg/mL and 4 = µg/mL) against MRSA and Klebsiella pneumoniae, respectively. The data showed that the conjugation of levofloxacin with [R₄W₄K] significantly reduced the antibacterial activity compared to the parent analogs, while [R₄W₄K]-levofloxacin-Q conjugate was more significantly potent than levofloxacin-Q alone.

Design, Synthesis, and Evaluation of Homochiral Peptides Containing Arginine and Histidine as Molecular Transporters

Linear (HR)n and cyclic [HR]n peptides (n = 4,5) containing alternate arginine and histidine residues were synthesized. The peptides showed 0⁻15% cytotoxicity at 5⁻100 µM in human ovarian adenocarcinoma (SK-OV-3) cells while they exhibited 0⁻12% toxicity in human leukemia cancer cell line (CCRF-CEM). Among all peptides, cyclic [HR]₄ peptide was able to improve the delivery of a cell impermeable fluorescence-labeled phosphopeptide by two-fold. Fatty acids of different alkyl chain length were attached at the N-terminal of the linear peptide (HR)₄ to improve the molecular transporter property. Addition of fatty acyl chains was expected to help with the permeation of the peptides through the cell membrane. Thus, we synthesized seven fatty acyl derivatives of the linear (HR)₄ peptide. The peptides were synthesized using Fmoc/tBu solid phase peptide chemistry, purified by reverse-phase high-performance liquid chromatography (RP-HPLC) and characterized by matrix-assisted laser desorption/ionization (MALDI) spectrometry. The fatty acyl peptides containing C₈, C12, C14, and C18 alkyl chain did not show cytotoxicity on SK-OV-3 or CCRF-CEM cell lines up to 50 µM concentration; however, at higher concentration (100 µM), they showed mild cytotoxicity. For example, C16-(HR)₄ was also found to reduce the proliferation of SK-OV-3 cells by 11% at 50 µM and C20-(HR)₄ showed mild toxicity at 10 µM, reducing the proliferation of SK-OV-3 cells by 21%. Increase in the length of alkyl chain showed cytotoxicity to the cell lines. C20-(HR)₄ peptide showed better efficiency in translocation of F′-GpYEEI to SK-OV-3 than the phosphopeptide alone. Further investigation of C20-(HR)₄ peptide efficacy showed that the peptide could deliver doxorubicin and epirubicin into SK-OV-3 and also improved the drug antiproliferative ability. These studies provided insights into understanding the structural requirements for optimal cellular delivery of the fatty acyl-(HR)₄ peptide conjugates.

Polymer-bound oxathiaphospholane: a solid-phase reagent for regioselective monothiophosphorylation and monophosphorylation of unprotected nucleosides and carbohydrates

Two polymers bound to N,N-diisopropylamino-1,3,2-oxathiaphospholane were reacted with unprotected carbohydrates and nucleosides in the presence of 1H-tetrazole, followed by oxidation with tert-butyl hydroperoxide or sulfurization with Beaucage's reagent. The 1,3,2-oxathiaphospholane ring-opening with 3-hydroxypropionitrile, followed by treatment with DBU, afforded the corresponding monophosphate and monothiophosphate derivatives, respectively, through the elimination of polymer-bound ethylene episulfide. Reactions using this strategy offer the advantages of high regioselectivity, monosubstitution, and facile isolation and recovery of products.

Efficient Intracellular Delivery of Cell-Impermeable Cargo Molecules by Peptides Containing Tryptophan and Histidine

We have previously evaluated and reported numerous classes of linear and cyclic peptides containing hydrophobic and hydrophilic segments for intracellular delivery of multiple molecular cargos. Herein, a combination of histidine and tryptophan amino acids were designed and evaluated for their efficiency in intracellular delivery of cell-impermeable phosphopeptides and the anti-HIV drug, emtricitabine. Two new decapeptides, with linear and cyclic natures, both containing alternate tryptophan and histidine residues, were synthesized using Fmoc/tBu solid-phase chemistry. The peptides were characterized and purified by using matrix-assisted laser desorption/ionization (MALDI) spectroscopy and high-performance liquid chromatography (HPLC), respectively. These peptides did not show significant toxicity up to 100 µM in ovarian cancer (SK-OV-3) and leukemia cancer (CCRF-CEM) cells. Furthermore, the cellular uptake of a fluorescence (F’)-labeled cell-impermeable phosphopeptide (F’-GpYEEI) was enhanced in the presence of linear (WH)₅ and cyclic [WH]₅ by 2- and 8-fold, respectively, compared to the uptake of the phosphopeptide alone. The cellular uptake was not significantly changed in the presence of endocytosis inhibitors. Furthermore, the intracellular uptake of the fluorescently-labeled anti-HIV drug, emtricitabine (F’-FTC), by linear (WH)₅ and cyclic [WH]₅ in SK-OV-3 cancer cell lines was found to be enhanced by 3.5- and 9-fold, respectively, compared to that of the drug alone. Fluorescent uptake experiments confirmed the localization of F’-GpYEEI-loaded cyclic [WH]₅ intracellularly in the SK-OV-3 cancer cell line after 3 h of incubation. Thus, these data demonstrated that [WH]₅ containing tryptophan and histidine enhanced the cellular uptake of F’-GpYEEI and emtricitabine.

Synthesis, in vitro anti-HIV activity, and biological stability of 5'-O-myristoyl analogue derivatives of 3'-fluoro-2',3'-dideoxythymidine (FLT) as potential bifunctional prodrugs of FLT

A group of 5'-O-myristoyl analogue derivatives of FLT (2) were evaluated as potential anti-HIV agents that were designed to serve as prodrugs to FLT. 3'-Fluoro-2',3'-dideoxy-5'-O-(12-methoxydodecanoyl)thymidine (4) (EC50 = 3.8 nM) and 3'-fluoro-2',3'-dideoxy-5'-O-(12-azidododecanoyl)thymidine (8) (EC50 = 2.8 nM) were the most effective anti-HIV-1 agents. There was a linear correlation between Log P and HPLC Log retention time for the 5'-O-FLT esters. The in vitro enzymatic hydrolysis half-life (t1/2), among the group of esters (3-8) in porcine liver esterase, rat plasma and rat brain homogenate was longer for 3'-fluoro-2',3'-dideoxy-5'-O-(myristoyl)thymidine (7), with t1/2 values of 20.3, 4.6 and 17.5 min, respectively.

Synthesis and in vitro characterization of novel dextran-methylprednisolone conjugates with peptide linkers: effects of linker length on hydrolytic and enzymatic release of methylprednisolone and its peptidyl intermediates

To control the rate of release of methylprednisolone (MP) in lysosomes, new dextran-MP conjugates with peptide linkers were synthesized and characterized. Methylprednisolone succinate (MPS) was attached to dextran 25 kDa using linkers with 1-5 Gly residues. The release characteristics of the conjugates in pH 4.0 and 7.4 buffers, blood, liver lysosomes, and various lysosomal proteinases were determined using a size-exclusion and/or a newly developed reversed-phase HPLC method capable of simultaneous quantitation of MP, MPS, and all five possible MPS-peptidyl intermediates. We synthesized conjugates with >or=90% purity and 6.9-9.5% (w/w) degree of MP substitution. The conjugates were stable at pH 4.0, but released MP and intact MPS-peptidyl intermediates in the pH 7.4 buffer and rat blood, with faster degradation rates for longer linkers. Rat lysosomal fractions degraded the conjugates to MP and all the possible intermediates also at a rate directly proportional to the length of the peptide. Whereas the degradation of the conjugates by cysteine peptidases (papain or cathepsin B) was relatively substantial, no degradation was observed in the presence of aspartic (cathepsin D) or serine (trypsin) proteinases, which do not cleave peptide bonds with Gly. These newly developed dextran conjugates of MP show promise for controlled delivery of MP in lysosomes.

Amphiphilic bicyclic peptides as cellular delivery agents

Two bicyclic peptides composed of tryptophan and arginine residues were synthesized from monocyclic peptide building blocks and evaluated as cellular delivery agents. [W5G]-(triazole)-[KR5] and [W5E]-(β-Ala)-[KR5] containing triazole and β-alanine linkers improved the cellular delivery of fluorescein (F')-labeled phosphopeptide F'-GpYEEI (F'-PP) by 7.6- and 19.3-fold, respectively, in human ovarian adenocarcinoma cells. However, parent monocyclic peptide [R5 ] and monocyclic peptide [WR]4 only enhanced the cellular uptake of the phosphopeptide by only 1.3- and 3.7-fold, respectively. Confocal microscopy showed that the corresponding fluorescein-labeled bicyclic peptide F'-[KW4E]-(β-Ala)-[KR5] was localized in the cytosol and nucleus. Studying the cellular uptake of F'-[KW4E]-(β-Ala)-[KR5] in the presence of endocytosis inhibitors indicated that the clathrin- and caveolin-dependent endocytosis are the main pathways for cellular uptake. The bicyclic peptide was able to improve antiproliferative activity of doxorubicin by 20 %. These data suggest that this bicyclic peptide can be utilized as a new class of cell-penetrating peptides and cellular delivery tools.

ATP-phosphopeptide conjugates as inhibitors of Src tyrosine kinases

A number of Src SH2 domain inhibitors enhance the kinase catalytic activity by switching the closed inactive to the open active conformation. ATP-phosphopeptide conjugates were designed and synthesized as Src tyrosine kinase inhibitors based on a tetrapeptide sequence pTyr-Glu-Glu-Ile (pYEEI) and ATP to block the SH2 domain signaling and substrate phosphorylation by ATP, respectively. In general, ATP-phosphopeptide conjugates with optimal linkers such as compounds 5 and 7 (K(i) = 1.7-2.6 microM) showed higher binding affinities to the ATP-binding site relative to the other ATP-phosphopeptide conjugates having short or long linkers, 1-4 and 6, (K(i) = 10.1-16.1 microM) and ATP (K(m) = 74 microM). These ATP-phosphopeptide conjugates may serve as novel templates for designing protein tyrosine kinase inhibitors to block SH2 mediated protein-protein interactions and to counter the activation of enzyme that resulted from the SH2 inhibition.

Cysteine and arginine-rich peptides as molecular carriers

A number of linear and cyclic peptides containing alternative arginine and cysteine residues, namely linear (CR)3, linear (CR)4, linear (CR)5, cyclic [CR]4, and cyclic [CR]5, were synthesized. The peptides were evaluated for their ability to deliver two molecular cargos, fluorescence-labeled cell-impermeable negatively charged phosphopeptide (F'-GpYEEI) and fluorescence-labeled lamivudine (F'-3TC), intracellularly in human leukemia cancer (CCRF-CEM) cells. We investigated the role of cyclization and the number of amino acids in improving the transporting ability of the peptides. The flow cytometry studies suggested that the synthesized peptides were able to work efficiently as transporters for both cargos. Among all compounds, cyclic [CR]4 was found to be the most efficient peptide in transporting the cargo into cells. For instance, the cellular uptake of F'-3TC (5μM) and F'-GpYEEI (5μM) was enhanced by 16- and 20-fold, respectively, in the presence of cyclic [CR]4 compared to that of the parent compound alone. The mechanism of F'-GpYEEI uptake by cells was found to be energy-independent. The results showed that the number of amino acids and their cyclic nature can impact the efficiency of the peptide in transporting the molecular cargos.

O-Aryl α,β-d-ribofuranosides: synthesis & highly efficient biocatalytic separation of anomers and evaluation of their Src kinase inhibitory activity

A series of peracetylated O-aryl α,β-d-ribofuranosides have been synthesized and an efficient biocatalytic methodology has been developed for the separation of their anomers which was otherwise almost impossible by column chromatographic or other techniques. The incubation of 2,3,5-tri-O-acetyl-1-O-aryl-α,β-d-ribofuranoside with Lipozyme® TL IM immobilized on silica led to the selective deacetylation of only one acetoxy group, viz the C-5'-O-acetoxy group of the α-anomer over the other acetoxy groups derived from the two secondary hydroxyl groups present in the molecule and also over three acetoxy groups (derived from one primary and two secondary hydroxyls of the β-anomer). This methodology led to the easy synthesis of both, α- and β-anomers of O-aryl d-ribofuranosides. All the arylribofuranosides were screened for inhibition of Src kinase. 1-O-(3-Methoxyphenyl)-β-d-ribofuranoside exhibited the highest activity for inhibition of Src kinase (IC(50)=95.0μM).

Synthesis and structure-activity relationships of linear and conformationally constrained peptide analogues of CIYKYY as Src tyrosine kinase inhibitors

A series of peptide analogues of Ac-CIYKYY (1) were synthesized by functional group modifications in peptide side chains or by introducing conformational constraints, to improve the inhibitory potency against active Src kinase. Ac-CIYKF(4-NO2)Y (2, IC50 = 0.53 microM) and conformationally constrained peptide 31 (IC50 = 0.28 microM) exhibited 750- and 1400-fold higher inhibitory activities, respectively, versus that of 1 (IC50 = 400 microM). Compound 2 exhibited a partial competitive inhibition pattern against ATP.

Synthesis of polymer-bound 4-acetoxy-3-phenylbenzaldehyde derivatives: applications in solid-phase organic synthesis

Aminomethyl polystyrene resin was reacted with 4-(5'-formyl-2'-hydroxyphenyl)benzoic acid and 4-(5'-formyl-2'-hydroxyphenyl)phenyl propionic acid, respectively, in the presence of 1-hydroxybenzotriazole and 1,3-diisopropylcarbodiimide to yield polymer-bound benzaldehydes. The phenolic group in resins was acetylated with acetic anhydride to afford two polymer-bound 4-acetoxybenzaldehydes. The reductive amination of polymer-bound linkers by amines and sodium triacetoxyborohydride, followed by sulfonylation with arylsulfonyl chloride derivatives in the presence of pyridine and the cleavage with TFA/DCM/H2O, produced pure sulfonamides.