The effect of backbone cyclization on PK/PD properties of bioactive peptide-peptoid hybrids: The melanocortin agonist paradigm

Rational Design and Multicomponent Synthesis of Lipid-Peptoid Nanocomposites towards a Customized Drug Delivery System Assembly

Nanotechnology has assumed a significant role over the last decade in the development of various technologies applied to health sciences. This becomes even more evident with its application in controlled drug delivery systems. In this context, peptoids are a promising class of compounds for application as nanocarriers in drug delivery systems. These compounds can be obtained efficiently and with highly functionalized structural diversity via the Ugi 4-component reaction (U-4CR). Herein, we report the design of the process control strategy for the future development of lipid-peptoid-based customized drug delivery system assemblies. Over 20 lipid-peptoid nanocomposites were synthesized via the U-4CR in good to excellent yields. These products were successfully submitted to the nanoparticle formation by the emulsification-evaporation process from lipophilic solution and analyzed via Dynamic Light Scattering (DLS). Several molecules generated nanoparticles with a size ≤200 nm, making them good candidates for drug delivery systems, such as in cancer treatment.

Advancement and Strategies for the Development of Peptide-Drug Conjugates: Pharmacokinetic Modulation, Role and Clinical Evidence Against Cancer Management

Currently, many new treatment strategies are being used for the management of cancer. Among them, chemotherapy based on peptides has been of great interest due to the unique features of peptides. This review discusses the role of peptide and peptides analogues in the treatment of cancer, with special emphasis on their pharmacokinetic modulation and research progress. Low molecular weight, targeted drug delivery, enhanced permeability, etc., of the peptide-linked drug conjugates, lead to an increase in the effectiveness of cancer therapy. Various peptides have recently been developed as drugs and vaccines with an altered pharmacokinetic parameter which has subsequently been assessed in different phases of the clinical study. Peptides have made a great impact in the area of cancer therapy and diagnosis. Targeted chemotherapy and drug delivery techniques using peptides are emerging as excellent tools in minimizing problems with conventional chemotherapy. It can be concluded that new advances in using peptides to treat different types of cancer have been shown by different clinical studies indicating that peptides could be used as an ideal therapeutic method in treating cancer due to the novel advantages of peptides. The development of identifying and synthesizing novel peptides could provide a promising choice to patients with cancer.

Improvement on Permeability of Cyclic Peptide/Peptidomimetic: Backbone N-Methylation as A Useful Tool

Peptides have a three-dimensional configuration that can adopt particular conformations for binding to proteins, which are well suited to interact with larger contact surface areas on target proteins. However, low cell permeability is a major challenge in the development of peptide-related drugs. In recent years, backbone N-methylation has been a useful tool for manipulating the permeability of cyclic peptides/peptidomimetics. Backbone N-methylation permits the adjustment of molecule’s conformational space. Several pathways are involved in the drug absorption pathway; the relative importance of each N-methylation to total permeation is likely to differ with intrinsic properties of cyclic peptide/peptidomimetic. Recent studies on the permeability of cyclic peptides/peptidomimetics using the backbone N-methylation strategy and synthetic methodologies will be presented in this review.

Cyclic Peptides as Drugs for Intracellular Targets: The Next Frontier in Peptide Therapeutic Development

Developing macrocyclic peptides that can reach intracellular targets is a significant challenge. This review discusses the most recent strategies used to develop cell permeable cyclic peptides that maintain binding to their biological target inside the cell. Macrocyclic peptides are unique from small molecules because traditional calculated physical properties are unsuccessful for predicting cell membrane permeability. Peptide synthesis and experimental membrane permeability is the only strategy that effectively differentiates between cell permeable and cell impermeable molecules. Discussed are chemical strategies, including backbone N-methylation and stereochemical changes, which have produced molecular scaffolds with improved cell permeability. However, these improvements often come at the expense of biological activity as chemical modifications alter the peptide conformation, frequently impacting the compound's ability to bind to the target. Highlighted is the most promising approach, which involves side-chain alterations that improve cell permeability without impact binding events.

Peptoid NPhe4 in AGRP-Based c[Pro1-Arg2-Phe3-Phe4-Xxx5-Ala6-Phe7-DPro8] Scaffolds Maintain Mouse MC4R Antagonist Potency

The melanocortin receptors are involved in numerous physiological functions and are regulated by agonists derived from the proopiomelanocortin gene transcript and two endogenous antagonists, agouti and agouti-related protein (AGRP). The key binding and functional determinant of AGRP, an MC3R and MC4R antagonist, is an Arg-Phe-Phe tripeptide sequence located on an exposed hexapeptide (Arg-Phe-Phe-Asn-Ala-Phe) loop. It has previously been observed that cyclizing this sequence through a DPro-Pro motif (c[Pro¹-Arg²-Phe³-Phe⁴-Asn⁵-Ala⁶-Phe⁷-DPro⁸]) resulted in a macrocyclic scaffold with MC4R antagonist activity, with increased MC4R potency when a diaminopropionic acid (Dap) residue is substituted at position 5. In this report, a series of 11 single-peptoid substitutions were performed in the AGRP-derived macrocycles. While most peptoid substitutions decreased MC4R antagonist potency, it was observed that NPhe⁴ (compounds 4 and 11) or NDab⁵ (diaminobutyric acid, compound 7) maintained MC4R antagonist potency. The NPhe⁴ substitutions also resulted in MC5R antagonist and inverse agonist activity equipotent to the parent scaffolds. These data may be used in the design of future MC4R and MC5R antagonist leads and probes that possess increased metabolic stability due to the presence of peptoid residues.

A Rapid and Efficient Building Block Approach for Click Cyclization of Peptoids

Cyclic peptide-peptoid hybrids possess improved stability and selectivity over linear peptides and are thus better drug candidates. However, their synthesis is far from trivial and is usually difficult to automate. Here we describe a new rapid and efficient approach for the synthesis of click-based cyclic peptide-peptoid hybrids. Our methodology is based on a combination between easily synthesized building blocks, automated microwave assisted solid phase synthesis and bioorthogonal click cyclization. We proved the concept of this method using the INS peptide, which we have previously shown to activate the HIV-1 integrase enzyme. This strategy enabled the rapid synthesis and biophysical evaluation of a library of cyclic peptide-peptoid hybrids derived from HIV-1 integrase in high yield and purity. The new cyclic hybrids showed improved biological activity and were significantly more stable than the original linear INS peptide.

Enhancing Oral Bioavailability of Cyclic RGD Hexa-peptides by the Lipophilic Prodrug Charge Masking Approach: Redirection of Peptide Intestinal Permeability from a Paracellular to Transcellular Pathway

Hydrophilic peptides constitute most of the active peptides. They mostly permeate via tight junctions (paracellular pathway) in the intestine. This permeability mechanism restricts the magnitude of their oral absorption and bioavailability. We hypothesized that concealing the hydrophilic residues of the peptide using the lipophilic prodrug charge masking approach (LPCM) can improve the bioavailability of hydrophilic peptides. To test this hypothesis, a cyclic N-methylated hexapeptide containing Arg-Gly-Asp (RGD) and its prodrug derivatives, masking the Arg and Asp charged side chains, were synthesized. The library was evaluated for intestinal permeability in vitro using the Caco-2 model. Further investigation of metabolic stability ex vivo models in rat plasma, brush border membrane vesicles (BBMVs), and isolated CYP3A4 microsomes and pharmacokinetic studies was performed on a selected peptide and its prodrug (peptide 12). The parent drug analogues were found to have a low permeability rate in vitro, corresponding to atenolol, a marker for paracellular permeability. Moreover, palmitoyl carnitine increased the P_app of peptide 12 by 4-fold, indicating paracellular permeability. The P_app of the prodrug derivatives was much higher than that of their parent peptides. For instance, the P_app of the prodrug 12P was 20-fold higher than the P_app of peptide 12 in the apical to basolateral (AB) direction. Whereas the permeability in the opposite direction (BA of the Caco-2 model) was significantly faster than the P_app AB, indicating the involvement of an efflux system. These results were corroborated when verapamil, a P-gp inhibitor, was added to the Caco-2 model and increased the P_app AB of prodrug 12P by 3-fold. The prodrug 12P was stable in the BBMVs environment, yet degraded quickly (less than 5 min) in the plasma into the parent peptide 12. Pharmacokinetic studies in rats showed an increase in the bioavailability of peptide 12 > 70-fold (from 0.58 ± 0.11% to 43.8 ± 14.9%) after applying the LPCM method to peptide 12 and converting it to the prodrug 12P. To conclude, the LPCM approach converted the absorption mechanism of the polar peptides from a paracellular to transcellular pathway that tremendously affects their oral bioavailability. The LPCM method provides a solution for the poor bioavailability of RGD cyclohexapeptides and paves the way for other active hydrophilic and charged peptides with poor oral bioavailability.

CPP-E1A fusion peptides inhibit CtBP-mediated transcriptional repression

The carboxyl‐terminal binding proteins (CtBP) are transcriptional corepressors that regulate the expression of multiple epithelial‐specific and pro‐apoptotic genes. Overexpression of CtBP occurs in many human cancers where they promote the epithelial‐to‐mesenchymal transition, stem cell‐like features, and cell survival, while knockdown of CtBP in tumor cells results in p53‐independent apoptosis. CtBPs are recruited to their target genes by binding to a conserved PXDLS peptide motif present in multiple DNA‐binding transcription factors. Disrupting the interaction between CtBP and its transcription factor partners may be a means of altering CtBP‐mediated transcriptional repression and a potential approach for cancer therapies. However, small molecules targeting protein–protein interactions have traditionally been difficult to identify. In this study, we took advantage of the fact that CtBP binds to a conserved peptide motif to explore the feasibility of using peptides containing the PXDLS motif fused to cell‐penetrating peptides (CPP) to inhibit CtBP function. We demonstrate that these peptides disrupt the ability of CtBP to interact with its protein partner, E1A, in an AlphaScreen assay. Moreover, these peptides can enter both lung carcinoma and melanoma cells, disrupt the interaction between CtBP and a transcription factor partner, and inhibit CtBP‐mediated transcriptional repression. Finally, the constitutive expression of one such peptide, Pep1‐E1A‐WT, in a melanoma cell line reverses CtBP‐mediated oncogenic phenotypes including proliferation, migration, and sphere formation and limits tumor growth in vivo. Together, our results suggest that CPP‐fused PXDLS‐containing peptides can potentially be developed into a research tool or therapeutic agent targeting CtBP‐mediated transcriptional events in various biological pathways.

Development of a Novel Backbone Cyclic Peptide Inhibitor of the Innate Immune TLR/IL1R Signaling Protein MyD88

MyD88 is a cytoplasmic adaptor protein that plays a central role in signaling downstream of the TLRs and the IL1R superfamily. We previously demonstrated that MyD88 plays a critical role in EAE, the murine model of multiple sclerosis, and showed that the MyD88 BB-loop decoy peptide RDVLPGT ameliorates EAE. We now designed and screened a library of backbone cyclized peptides based on the linear BB loop peptide, to identify a metabolically stable inhibitor of MyD88 that retains the binding properties of the linear peptide. We identified a novel cyclic peptide protein mimetic that inhibits inflammatory responses to TLR ligands, and NFκB activation in response to IL-1 activation. The inhibitor, c(MyD 4-4), is metabolically stable in comparison to the linear peptide, blocks MyD88 in a specific manner, and inhibits MyD88 function by preventing MyD88 dimerization. Finally, treatment of mice with c(MyD 4-4) reduced the severity of clinical disease in the murine EAE model of multiple sclerosis. Thus, modulation of MyD88-dependent signaling using c(MyD 4-4) is a potential therapeutic strategy to lower innate immune inflammation in autoimmune CNS disease.

Discovery of Potent and Orally Bioavailable Macrocyclic Peptide-Peptoid Hybrid CXCR7 Modulators

The chemokine receptor CXCR7 is an attractive target for a variety of diseases. While several small-molecule modulators of CXCR7 have been reported, peptidic macrocycles may provide advantages in terms of potency, selectivity, and reduced off-target activity. We produced a series of peptidic macrocycles that incorporate an N-linked peptoid functionality where the peptoid group enabled us to explore side-chain diversity well beyond that of natural amino acids. At the same time, theoretical calculations and experimental assays were used to track and reduce the polarity while closely monitoring the physicochemical properties. This strategy led to the discovery of macrocyclic peptide-peptoid hybrids with high CXCR7 binding affinities (K_i < 100 nM) and measurable passive permeability (P_app > 5 × 10^-6 cm/s). Moreover, bioactive peptide 25 (K_i = 9 nM) achieved oral bioavailability of 18% in rats, which was commensurate with the observed plasma clearance values upon intravenous administration.

Cationic Polypeptoids with Optimized Molecular Characteristics toward Efficient Nonviral Gene Delivery

The rational design of gene vectors relies on the understanding of their structure-property relationship. Polypeptoids, which are structural isomers of natural polypeptides, hold great potential as gene delivery vectors due to their facile preparation, structural tunability, and most importantly, their desirable proteolytic stability. We herein designed a library of polypeptoids with different cationic side-chain terminal groups, degree of polymerizations (DPs), side-chain lengths, and incorporated aliphatic side chains, to unravel the structure-property relationships so that gene delivery efficiency can be maximized and cytotoxicity can be minimized. In HeLa cells, a polypeptoid bearing a primary amine side-chain terminal group exhibited remarkably higher transfection efficiency than that of its analogues containing secondary, tertiary, or quaternary amine groups. Elongation of the polypeptoid backbone length (from 28 to 251 mer) led to enhanced DNA condensation as well as cellular uptake levels, however it also caused higher cytotoxicity. Upon a proper balance between DNA uptake and cytotoxicity, the polypeptoid with a DP of 46 afforded the highest transfection efficiency. Elongating the aliphatic spacer between the backbone and side amine groups enhanced the hydrophobicity of the side chains, which resulted in notably increased membrane activities and transfection efficiency. Further incorporation of hydrophobic decyl side chains led to an improvement in transfection efficiency of ∼6 fold. The top-performing material identified, P11, mediated successful gene transfection under serum-containing conditions, outperforming the commercial transfection reagent poly(ethylenimine) by nearly 4 orders of magnitude. Reflecting its excellent serum-resistant properties, P11 further enabled effective transfection in vivo following intratumoral injection to melanoma-bearing mice. This study will help the rational design of polypeptoid-based gene delivery materials, and the best-performing material identified may provide a potential supplement to existing gene vectors.

Synthesis of acylhydrazino-peptomers, a new class of peptidomimetics, by consecutive Ugi and hydrazino-Ugi reactions

Herein we describe a versatile approach for the synthesis of acylhydrazino-peptomers, a new class of peptidomimetics. The key idea in this approach is based on a simple route using a one-pot hydrazino-Ugi four-component reaction followed by a hydrazinolysis or hydrolysis reaction and subsequent hydrazino-Ugi reaction or classical Ugi reaction for the construction of acyclic acylhydrazino-peptomers. The consecutive multicomponent reactions produced a variety of acylhydrazino-peptomers in moderate to excellent yields (47–90%). These compounds are multifunctional intermediates that can be further functionalized to obtain new peptidomimetics with potential biological activity.

Passive Membrane Permeability in Cyclic Peptomer Scaffolds Is Robust to Extensive Variation in Side Chain Functionality and Backbone Geometry

Synthetic and natural cyclic peptides provide a testing ground for studying membrane permeability in nontraditional drug scaffolds. Cyclic peptomers, which incorporate peptide and N-alkylglycine (peptoid) residues, combine the stereochemical and geometric complexity of peptides with the functional group diversity accessible to peptoids. We synthesized cyclic peptomer libraries by split-pool techniques, separately permuting side chain and backbone geometry, and analyzed their membrane permeabilities using the parallel artificial membrane permeability assay. Nearly half of the side chain permutations had permeability coefficients (P_app) > 1 × 10^-6 cm/s. Some backbone geometries enhanced permeability due to their ability to form more stable intramolecular hydrogen bond networks compared with other scaffolds. These observations suggest that hexameric cyclic peptomers can have good passive permeability even in the context of extensive side chain and backbone variation, and that high permeability can generally be achieved within a relatively wide lipophilicity range.

Conformational preferences of peptide-peptoid hybrid oligomers

Peptomers are oligomeric molecules composed of both α-amino acids and N-substituted glycine monomers, thus creating a hybrid of peptide and peptoid units. Peptomers have been used in several applications such as antimicrobials, protease inhibitors, and antibody mimics. Despite the considerable promise of peptomers as chemically diverse molecular scaffolds, we know little about their conformational tendencies. This lack of knowledge limits the ability to implement computational approaches for peptomer design. Here we computationally evaluate the local structural propensities of the peptide-peptoid linkage. We find some general similarities between the peptide residue conformational preferences and the Ramachandran distribution of residues that precede proline in folded protein structures. However, there are notable differences. For example, several β-turn motifs are disallowed when the i+2 residue is also a peptoid monomer. Significantly, the lowest energy geometry, when dispersion forces are accounted for, corresponds to a "cis-Pro touch-turn" conformation, an unusual turn motif that has been observed at protein catalytic centers and binding sites. The peptomer touch-turn thus represents a useful design element for the construction of folded oligomers capable of molecular recognition and as modules in the assembly of structurally complex peptoid-protein hybrid macromolecules.

Peptide to Peptoid Substitutions Increase Cell Permeability in Cyclic Hexapeptides

The effect of peptide-to-peptoid substitutions on the passive membrane permeability of an N-methylated cyclic hexapeptide is examined. In general, substitutions maintained permeability but increased conformational heterogeneity. Diversification with nonproteinogenic side chains increased permeability up to 3-fold. Additionally, the conformational impact of peptoid substitutions within a β-turn are explored. Based on these results, the strategic incorporation of peptoid residues into cyclic peptides can maintain or improve cell permeability, while increasing access to diverse side-chain functionality.

Peptide splicing in a double-sequence analogue of trypsin inhibitor SFTI-1 substituted in the P₁ positions by peptoid monomers

Recently, we described a process of trypsin-assisted peptide splicing of analogs of trypsin inhibitor SFTI-1, that seems to be very similar to proteasome-catalyzed peptide splicing. Here, we show, for the first time, that a peptide-peptoid hybrid (peptomer) can also be spliced by trypsin. Incubation of a double sequence SFTI-1 analog, containing two peptoid monomers, with equimolar amount of trypsin leads to formation of monocyclic peptomer as the main product. We proved that the peptide bond formed by a peptoid monomer is not only digested by trypsin but also participates in the enzyme-assisted splicing process.

A sequential Ugi multicomponent/Cu-catalyzed azide-alkyne cycloaddition approach for the continuous flow generation of cyclic peptoids

The development of a continuous flow multistep strategy for the synthesis of linear peptoids and their subsequent macrocyclization via Click chemistry is described. The central transformation of this process is an Ugi four-component reaction generating the peptidomimetic core structure. In order to avoid exposure to the often toxic and malodorous isocyanide building blocks, the continuous approach was telescoped by the dehydration of the corresponding formamide. In a concurrent operation, the highly energetic azide moiety required for the subsequent intramolecular copper-catalyzed azide-alkyne cycloaddition (Click reaction) was installed by nucleophilic substitution from a bromide precursor. All steps yielding to the linear core structures can be conveniently coupled without the need for purification steps resulting in a single process generating the desired peptidomimetics in good to excellent yields within a 25 min reaction time. The following macrocyclization was realized in a coil reactor made of copper without any additional additive. A careful process intensification study demonstrated that this transformation occurs quantitatively within 25 min at 140 °C. Depending on the resulting ring strain, either a dimeric or a monomeric form of the cyclic product was obtained.

Melanocortin-4 receptor modulators for the treatment of obesity: a patent analysis (2008–2014)

The central melanocortin system and particularly the melanocortin-4 receptor (MC4R) subtype, plays an important role in the regulation of body weight. The discovery of orally active MC4R agonists suitable for evaluation in human clinical trials as weight loss agents has attracted considerable interest over the past decade, but has proved challenging, in part because of cardiovascular and behavioral side effects. Currently, the only MC4R agonist in clinical trials is a peptide identified as RM-493. To avoid some of the undesirable side effects associated with MC4R activation, new pharmacological approaches for modulating the MC system have been investigated. In this article, we provide a review of the MC4R patent landscape from 2008 to 2014 and analyze the physicochemical properties of compounds described herein.

Strategic approaches to optimizing peptide ADME properties

Development of peptide drugs is challenging but also quite rewarding. Five blockbuster peptide drugs are currently on the market, and six new peptides received first marketing approval as new molecular entities in 2012. Although peptides only represent 2% of the drug market, the market is growing twice as quickly and might soon occupy a larger niche. Natural peptides typically have poor absorption, distribution, metabolism, and excretion (ADME) properties with rapid clearance, short half-life, low permeability, and sometimes low solubility. Strategies have been developed to improve peptide drugability through enhancing permeability, reducing proteolysis and renal clearance, and prolonging half-life. In vivo, in vitro, and in silico tools are available to evaluate ADME properties of peptides, and structural modification strategies are in place to improve peptide developability.

Cyclic carbo-isosteric depsipeptides and peptides as a novel class of peptidomimetics

A novel and highly efficient cyclization method has been developed to access a new class of cyclic carbo-isosteric depsipeptides and carbo-isosteric peptides. Our strategy requires easily accessible C-terminal methyl ketone ester or amide functionalized linear precursors as starting materials. The well-known reductive amination has then been used to afford cyclic tetra- to octa-pseudopeptides via a selective intramolecular formation of a glycine peptidomimetic unit under moderate dilution.

Inhibition of inositol monophosphatase (IMPase) at the calbindin-D28k binding site: molecular and behavioral aspects

Bipolar-disorder (manic-depressive illness) is a severe chronic illness affecting ∼1% of the adult population. It is treated with mood-stabilizers, the prototypic one being lithium-salts (lithium), but it has life threatening side-effects and a significant number of patients fail to respond. The lithium-inhibitable enzyme inositol-monophosphatase (IMPase) is one of the viable targets for lithium's mechanism of action. Calbindin-D28k (calbindin) up-regulates IMPase activity. The IMPase-calbindincomplex was modeled using the program MolFit. The in-silico model indicated that the 55-66 amino-acid segment of IMPase anchors calbindin via Lys59 and Lys61 with a glutamate in between (Lys-Glu-Lys motif) and that the motif interacts with residues Asp24 and Asp26 of calbindin. We found that differently from wildtype calbindin, IMPase was not activated by mutated calbindin in which Asp24 and Asp26 were replaced by alanine. Calbindin's effect was significantly reduced by a linear peptide with the sequence of amino acids 58-63 of IMPase (peptide 1) and by six amino-acid linear peptides including at least part of the Lys-Glu-Lys motif. The three amino-acid peptide Lys-Glu-Lys or five amino-acid linear peptides containing this motif were ineffective. Mice administered peptide 1 intracerebroventricularly exhibited a significant anti-depressant-like reduced immobility in the forced-swim test. Based on the sequence of peptide 1, and to potentially increase the peptide's stability, cyclic and linear pre-cyclic analog peptides were synthesized. One cyclic peptide and one linear pre-cyclic analog peptide inhibited calbindin-activated brain IMPase activity in-vitro. Our findings may lead to the development of molecules capable of inhibiting IMPase activity at an alternative site than that of lithium.

Impact of physiochemical properties on pharmacokinetics of protein therapeutics

Physicochemical properties, such as molecular weight, size, partition coefficient, acid dissociation constant and solubility have a great impact on pharmacokinetics of traditional small molecule drugs and substantially used in development of small drugs. However, predicting pharmacokinetic fate (absorption, distribution, metabolism and elimination) of protein therapeutics from their physicochemical parameters is extremely difficult due to the macromolecular nature of therapeutic proteins and peptides. Their structural complexity and immunogenicity are other contributing factors that determine their biological fate. Therefore, to develop generalized strategies concerning development of therapeutic proteins and peptides are highly challenging. However, reviewing the literature, authors found that physiochemical properties, such as molecular weight, charge and structural modification are having great impact on pharmacokinetics of protein therapeutics and an attempt is made to provide the major findings in this manuscript. This manuscript will serve to provide some bases for developing protein therapeutics with desired pharmacokinetic profile.

Development of a peptidase-resistant substrate for single-cell measurement of protein kinase B activation

An iterative design strategy using three criteria was utilized to develop a peptidase-resistant substrate peptide for protein kinase B. Libraries of peptides possessing non-native amino acids were screened for time to 50% phosphorylation, degradation half-life within a lysate, and appearance of a dominant fragment. The lead peptide possessed a half-life of 92 ± 7 and 16 ± 2 min in HeLa and LNCaP cytosolic lysates, respectively, representing a 4.6- and 2.7-fold lifetime improvement over that of the starting peptide. The redesigned peptide possessed a 4.5-fold improvement in phosphorylation efficiency compared to the starting peptide. The same peptide fragments were formed when the lead peptide was incubated in a lysate or loaded into single cells although the fragments formed in significantly different ratios suggesting that distinct peptidases metabolized the peptide in the two preparations. The rate of peptide degradation and phosphorylation was on average 0.1 ± 0.2 zmol pg(-1) s(-1) and 0.04 ± 0.08 zmol pg(-1) s(-1), respectively, for single LNCaP cells loaded with 4 ± 8 μM of peptide. Peptidase-resistant kinase substrates should find widespread utility in both lysate-based and single-cell assays of kinase activity.

Synthesis and properties of cyclic gomesin and analogues

Gomesin (Gm) was the first antimicrobial peptide (AMP) isolated from the hemocytes of a spider, the Brazilian mygalomorph Acanthoscurria gomesiana. We have been studying the properties of this interesting AMP, which also displays anticancer, antimalarial, anticryptococcal and anti-Leishmania activities. In the present study, the total syntheses of backbone-cyclized analogues of Gm (two disulfide bonds), [Cys(Acm)(2,15)]-Gm (one disulfide bond) and [Thr(2,6,11,15),(D)-Pro(9)]-Gm (no disulfide bonds) were accomplished, and the impact of cyclization on their properties was examined. The consequence of simultaneous deletion of pGlu(1) and Arg(16) -Glu-Arg(18) -NH(2) on Gm antimicrobial activity and structure was also analyzed. The results obtained showed that the synthetic route that includes peptide backbone cyclization on resin was advantageous and that a combination of 20% DMSO/NMP, EDC/HOBt, 60 °C and conventional heating appears to be particularly suitable for backbone cyclization of bioactive peptides. The biological properties of the Gm analogues clearly revealed that the N-terminal amino acid pGlu(1) and the amidated C-terminal tripeptide Arg(16) -Glu-Arg(18) -NH(2) play a major role in the interaction of Gm with the target membranes. Moreover, backbone cyclization practically did not affect the stability of the peptides in human serum; it also did not affect or enhanced hemolytic activity, but induced selectivity and, in some cases, discrete enhancements of antimicrobial activity and salt tolerance. Because of its high therapeutic index, easy synthesis and lower cost, the [Thr(2,6,11,15),(D)-Pro(9)]-Gm analogue remains the best active Gm-derived AMP developed so far; nevertheless, its elevated instability in human serum may limit its therapeutic potential.

Metabolism of peptide reporters in cell lysates and single cells

The stability of an Abl kinase substrate peptide in a cytosolic lysate and in single cells was characterized. In the cytosolic lysate, the starting peptide was metabolized at an average initial rate of 1.7 ± 0.3 zmol pg(-1) s(-1) with a t(1/2) of 1.3 min. Five different fragments formed over time; however, a dominant cleavage site was identified. Multiple rational design cycles were utilized to develop a lead peptide with a phenylalanine and alanine replaced by an (N-methyl)phenylalanine and isoleucine, respectively, to attain cytosolic peptidase resistance while maintaining Abl substrate efficacy. This lead peptide possessed a 15-fold greater lifetime in the cytosolic lysate while attaining a 7-fold improvement in k(cat) as an Abl kinase substrate compared to the starting peptide. However, when loaded into single cells, the starting peptide and lead peptide possessed nearly identical degradation rates and an altered pattern of fragmentation relative to that in cell lysates. Preferential accumulation of a fragment with cleavage at an Ala-Ala bond in single cells suggested that dissimilar peptidases act on the peptides in the lysate versus single cells. A design strategy for peptide stabilization, analogous to that demonstrated for the lysate, should be effective for stabilization in single cells.