A Quantitative Method to Distinguish Cytosolic from Endosome-Trapped Cell-Penetrating Peptides

Cell-penetrating peptides are known to penetrate cells through endocytosis and translocation. The two pathways are hardly distinguished in current cell assays. We developed a reliable, simple and robust method to distinguish and quantify independently the two routes. The assay requires (DABCYL) 4-(dimethylaminoazo)benzene-4-carboxylic acid- and (CF) carboxyfluorescein-labeled peptides. When the labeled peptide is intact, the fluorescence signal is weak thanks to the dark quenching property of DABCYL. A 10-fold higher fluorescence signal is measured when the labeled peptide is degraded. By referring to a standard fluorescent curve according to the concentration of the hydrolyzed peptide, we have access to the internalized peptide quantity. Therefore, cell lysis after internalization permits to determine the total quantity of intracellular peptide. The molecular state of the internalized peptide (intact or degraded), depends on its location in cells (cytosol vs endo-lysosomes), and can be blocked by boiling cells. This boiling step results indeed in denaturation and inhibition of the cellular enzymes. The advantage of this method is the possibility to quantify translocation at 37 °C and to compare it to the 4 °C condition, where all endocytosis processes are inhibited. We found that ranking of the translocation efficacy is DABCYL-R6-(ϵCF)K≫DABCYL-R4-(ϵCF)K≥CF-R9.

Influence of Aza-Glycine Substitution on the Internalization of Penetratin

The cell-penetrating peptide (CPP) penetratin has gained much attention over many years due to its potential role as a transporter for a broad range of cargo into cells. The modification of penetratin has been extensively investigated too. Aza-peptides are peptide analogs in which one or more of the amino residues are replaced by a semicarbazide. This substitution results in conformational restrictions and modifications in hydrogen bonding properties, which affect the structure and may lead to enhanced activity and selectivity of the modified peptide. In this work, the Trp residues of penetratin were substituted by aza-glycine or glycine residues to examine the effect of these modifications on the cellular uptake and the internalization mechanism. The substitution of Trp48 or Trp48,56 dramatically reduced the internalization, showing the importance of Trp48 in cellular uptake. Interestingly, while aza-glycine in the position of Trp56 increased the cellular uptake, Gly reduced it. The two Trp-modified derivatives showed altered internalization pathways, too. Based on our knowledge, this is the first study about the effect of aza-amino acid substitution on the cell entry of CPPs. Our results suggest that aza-amino acid insertion is a useful modification to change the internalization of a CPP.

Phosphorylation-Assisted Luciferase Complementation Assay Designed to Monitor Kinase Activity and Kinase-Domain-Mediated Protein-Protein Binding

Protein kinases are key regulators of cell signaling and have been important therapeutic targets for three decades. ATP-competitive drugs directly inhibit the activity of kinases but these enzymes work as part of complex protein networks in which protein-protein interactions (often referred to as kinase docking) may govern a more complex activation pattern. Kinase docking is indispensable for many signaling disease-relevant Ser/Thr kinases and it is mediated by a dedicated surface groove on the kinase domain which is distinct from the substrate-binding pocket. Thus, interfering with kinase docking provides an alternative strategy to control kinases. We describe activity sensors developed for p90 ribosomal S6 kinase (RSK) and mitogen-activated protein kinases (MAPKs: ERK, p38, and JNK) whose substrate phosphorylation is known to depend on kinase-docking-groove-mediated protein-protein binding. The in vitro assays were based on fragment complementation of the NanoBit luciferase, which is facilitated upon substrate motif phosphorylation. The new phosphorylation-assisted luciferase complementation (PhALC) sensors are highly selective and the PhALC assay is a useful tool for the quantitative analysis of kinase activity or kinase docking, and even for high-throughput screening of academic compound collections.

Enhancing Cell Penetration Efficiency of Cyclic Oligoarginines Using Rigid Scaffolds

Delivering therapeutic agents into cells has always been a major challenge. In recent years, cyclization emerged as a tool for designing CPPs to increase their internalization and stability. Cyclic ring(s) can protect the peptide from enzymatic degradation, so cyclic peptides remain intact. Therefore they can be good carrier molecules. In this work, the preparation and investigation of efficient cyclic CPPs are described. Different oligoarginines were designed to conjugate with rigid aromatic scaffolds or form disulfide bonds. The reaction between the scaffolds and the peptides forms stable thioether bonds, constraining the peptide into a cyclic structure. The constructs presented very efficient internalization on cancerous cell lines. Our peptides use more than one endocytic pathway for cellular uptake. In this way, short peptides, which can compete with the penetration of well-known CPPs such as octaarginine (Arg₈), may be synthesized through cyclization.

The Balance between Hydrophobicity/Aromaticity and Positively Charged Residues May Influence the Cell Penetration Ability

Cell-penetrating peptides (CPPs) are commonly modified to increase their cellular uptake, alter the mechanism of penetration or enhance their endosomal release. Earlier, we described the internalization enhancement ability of the 4-((4-(dimethylamino)phenyl)azo)benzoyl (Dabcyl) group. We proved that this modification on the N-terminus of tetra- and hexaarginine enhanced their cellular uptake. The introduction of an aromatic ring 4-(aminomethyl) benzoic acid, AMBA) into the peptide backbone has a synergistic effect with Dabcyl, and the tetraarginine derivatives had outstanding cellular uptake. Based on these results, the effect of Dabcyl or Dabcyl-AMBA modification on the internalization of oligoarginines was studied. Oligoarginines were modified with these groups and their internalization was measured using flow cytometry. The concentration dependence of the cellular uptake of selected constructs was compared too. Their internalization mechanism was also examined by using different endocytosis inhibitors. While the effect of the Dabcyl group was optimal for hexaarginine, the Dabcyl-AMBA group increased the cellular uptake in the case of all oligoarginines. All derivatives, with the exception of only tetraarginine, were more effective than the octaarginine control. The internalization mechanism was dependent on the size of the oligoarginine and was independent of the modification. Our findings suggest that these modifications enhanced the internalization of oligoarginines and resulted in novel, very effective CPPs.

Peptide Vaccines in Melanoma: Chemical Approaches towards Improved Immunotherapeutic Efficacy

Cancer of the skin is by far the most common of all cancers. Although the incidence of melanoma is relatively low among skin cancers, it can account for a high number of skin cancer deaths. Since the start of deeper insight into the mechanisms of melanoma tumorigenesis and their strong interaction with the immune system, the development of new therapeutical strategies has been continuously rising. The high number of melanoma cell mutations provides a diverse set of antigens that the immune system can recognize and use to distinguish tumor cells from normal cells. Peptide-based synthetic anti-tumor vaccines are based on tumor antigens that elicit an immune response due to antigen-presenting cells (APCs). Although targeting APCs with peptide antigens is the most important assumption for vaccine development, peptide antigens alone are poorly immunogenic. The immunogenicity of peptide antigens can be improved not only by synthetic modifications but also by the assistance of adjuvants and/or delivery systems. The current review summarizes the different chemical approaches for the development of effective peptide-based vaccines for the immunotherapeutic treatment of advanced melanoma.

Cell-Penetrating Dabcyl-Containing Tetraarginines with Backbone Aromatics as Uptake Enhancers

Cell-penetrating peptides represent an emerging class of carriers capable of effective cellular delivery. This work demonstrates the preparation and investigation of efficient CPPs. We have already shown that the presence of 4-((4-(dimethylamino)phenyl)azo)benzoic acid (Dabcyl) and Trp greatly increase the uptake of oligoarginines. This work is a further step in that direction. We have explored the possibility of employing unnatural, aromatic amino acids, to mimic Trp properties and effects. The added residues allow the introduction of aromaticity, not as a side-chain group, but rather as a part of the sequence. The constructs presented exceptional internalization on various cell lines, with an evident structure-activity relationship. The CPPs were investigated for their entry mechanisms, and our peptides exploit favorable pathways, yet one of the peptides relies highly on direct penetration. Confocal microscopy studies have shown selectivity towards the cell lines, by showing diffuse uptake in FADU cells, while vesicular uptake takes place in SCC-25 cell line. These highly active CPPs have proved their applicability in cargo delivery by successfully delivering antitumor drugs into MCF-7 and MDA-MB-231 cells. The modifications in the sequences allow the preparation of short yet highly effective constructs able to rival the penetration of well-known CPPs such as octaarginine (Arg8).

Redesigning of Cell-Penetrating Peptides to Improve Their Efficacy as a Drug Delivery System

Cell-penetrating peptides (CPP) are promising tools for the transport of a broad range of compounds into cells. Since the discovery of the first members of this peptide family, many other peptides have been identified; nowadays, dozens of these peptides are known. These peptides sometimes have very different chemical–physical properties, but they have similar drawbacks; e.g., non-specific internalization, fast elimination from the body, intracellular/vesicular entrapment. Although our knowledge regarding the mechanism and structure–activity relationship of internalization is growing, the prediction and design of the cell-penetrating properties are challenging. In this review, we focus on the different modifications of well-known CPPs to avoid their drawbacks, as well as how these modifications may increase their internalization and/or change the mechanism of penetration.

Peptide Based Inhibitors of Protein Binding to the Mitogen-Activated Protein Kinase Docking Groove

Mitogen-activated protein kinases (MAPK) are important regulatory units in cells and they take part in the regulation of many cellular functions such as cell division, differentiation or apoptosis. All MAPKs have a shallow docking groove that interacts with linear binding motifs of their substrate proteins and their regulatory proteins such as kinases, phosphatases, scaffolds. Inhibition of these protein–protein interactions may reduce or abolish the activity of the targeted kinase. Based on the wide range of their biological activity, this kind of inhibition can be useful in the treatment of many disorders like tumors, inflammation or undesired cell apoptosis. In this study a linear binding motif from the RHDF1 protein—a 15 amino acids long peptide—was selected for optimization to increase its cellular uptake but retaining its low micromolar binding affinity. First, we synthesized an octaarginine conjugate that showed efficient cellular uptake. Next, we set out to reduce the size of this construct. We were able to decrease the length of the original peptide, and to increase its cellular uptake with specific chemical modifications. These new constructs bound better to ERK2 and p38 kinases than the original peptide and they showed markedly increased cellular uptake. The new octaarginine conjugate and one of the minimized bicyclic derivatives could inhibit the phosphorylation of intracellular ERK or p38. However, the modulation of MAPK phosphorylation levels by these cell-penetrating peptides were complex, despite that in biochemical assays they all inhibited MAPK-substrate binding as well as phosphorylation. The optimized peptides depending on the applied concentration caused an expected decrease, but also some unexpected increase in MAPK phosphorylation patterns in the cell. This possibly reflects the complexity of MAPK docking groove mediated protein–protein interactions including bone fide MAPK clients such activator kinases, deactivating phosphatases or regulatory scaffolds. Thus, our findings with optimized cell-penetrating “inhibitory” peptides highlight the opportunities but also the pitfalls of docking peptide based MAPK activity regulation and call for a better quantitative understanding of MAPK mediated protein–protein interactions in cells.

Influence of the Dabcyl group on the cellular uptake of cationic peptides: short oligoarginines as efficient cell-penetrating peptides

Cell-penetrating peptides (CPPs) are promising delivery vehicles. These short peptides can transport wide range of cargos into cells, although their usage has often limitations. One of them is the endosomatic internalisation and thus the vesicular entrapment. Modifications which increases the direct delivery into the cytosol is highly researched area. Among the oligoarginines the longer ones (n > 6) show efficient internalisation and they are well-known members of CPPs. Herein, we describe the modification of tetra- and hexaarginine with (4-((4-(dimethylamino)phenyl)azo)benzoyl) (Dabcyl) group. This chromophore, which is often used in FRET system increased the internalisation of both peptides, and its effect was more outstanding in case of hexaarginine. The modified hexaarginine may enter into cells more effectively than octaarginine, and showed diffuse distribution besides vesicular transport already at low concentration. The attachment of Dabcyl group not only increases the cellular uptake of the cell-penetrating peptides but it may affect the mechanism of their internalisation. Their conjugates with antitumor drugs were studied on different cells and showed antitumor activity.

Topoisomerases inhibition and DNA binding mode of daunomycin-oligoarginine conjugate

Cancer is a major health issue adsorbing the attention of a biomedical research. To fight this disease, new drugs are developed, specifically tailored to target biological pathways or peculiar components of the tumour cells. Particularly interesting is the use of intercalating agents as drugs capable to bind DNA and inhibit enzymes involved in DNA metabolism. Anthracyclines are the most commonly used anticancer drugs. In particular, daunomycin is used to cancer treatment by exploiting its ability to intercalate DNA and inhibit the activity of DNA topoisomerases implicated in the replication processes. Unfortunately, clinical application of anthracyclines is limited by their side effects. The conjugation with specific carriers could affect the selectivity and reduce side effect by improving stability and/or cellular uptake properties. We here report the biochemical characterisation of a daunomycin oligopeptide conjugate containing six residues of arginine, by the analysis of its fluorescence properties, DNA interaction and topoisomerases inhibitory effects.

The effect of conjugation on antitumor activity of vindoline derivatives with octaarginine, a cell-penetrating peptide

Some Vinca alkaloids (eg, vinblastine, vincristine) have been widely used as antitumor drugs for a long time. Unfortunately, vindoline, a main alkaloid component of Catharanthus roseus (L.) G. Don, itself, has no antitumor activity. In our novel research program, we have prepared and identified new vindoline derivatives with moderate cytostatic activity. Here, we describe the effect of conjugation of vindoline derivative with oligoarginine (tetra-, hexa-, or octapeptides) cell-penetrating peptides on the cytostatic activity in vitro and in vivo. Br-Vindoline-(l)-Trp-OH attached to the N-terminus of octaarginine was the most effective compound in vitro on HL-60 cell line. Analysis of the in vitro activity of two isomer conjugates (Br-vindoline-(l)-Trp-Arg₈ and Br-vindoline-(d)-Trp-Arg₈ suggests the covalent attachment of the vindoline derivatives to octaarginine increased the antitumor activity significantly against P388 and C26 tumour cells in vitro. The cytostatic effect was dependent on the presence and configuration of Trp in the conjugate as well as on the cell line studied. The configuration of Trp notably influenced the activity on C26 and P388 cells: conjugate with (l)-Trp was more active than conjugate with the (d)-isomer. In contrast, conjugates had very similar effect on both the HL-60 and MDA-MB-231 cells. In preliminary experiments, conjugate Br-vindoline-(l)-Trp-Arg₈ exhibited some inhibitory effect on the tumor growth in P388 mouse leukemia tumor-bearing mice. Our results indicate that the conjugation of modified vindoline could result in an effective compound even with in vivo antitumor activity.

Cell-penetrating conjugates of pentaglutamylated methotrexate as potential anticancer drugs against resistant tumor cells

The emerging resistance of tumor cells against methotrexate (MTX) is one of the major limitations of the MTX treatment of tumorous diseases. The disturbance in the polyglutamation which is a main step in the mechanism of methotrexate action is often the reason of the resistance. Delivery of polyglutamylated MTX into cells may evade the mechanisms that are responsible for drug resistance. In this study conjugates of methotrexate and its pentaglutamylated derivatives with cell-penetrating peptides - penetratin and octaarginine - were investigated. The cellular-uptake and in vitro cytostatic activity of conjugates were examined on breast cancer cell cultures (MDA-MB-231 as resistant and MCF-7 as sensitive cell culture). These cell cultures showed very different behaviour towards the conjugates. Although the presence of pentaglutamyl moiety significantly decreased the internalisation of conjugates, some of them were significantly active in vitro. All of the conjugates were able to penetrate in some extent into both cell types, but only the conjugates of penetratin showed in vitro cytostatic activity. The most effective conjugates were the MTX-Glu5-Penetratin(desMet) and MTX-Glu5-GFLG-Penetratin(desMet). The latter was effective on both cell cultures while the former was active only on the resistant tumor cells. Our results suggest that the translocation of polyglutamylated MTX may be a new way to treat sensitive and more importantly resistant tumors. While both penetratin and octaarginine peptides were successfully used to deliver several kinds of cargos earlier in our case the activity of penetratin conjugates was more pronounced.

New pemetrexed-peptide conjugates: synthesis, characterization and in vitro cytostatic effect on non-small cell lung carcinoma (NCI-H358) and human leukemia (HL-60) cells

Pemetrexed (Pem) is a novel antimetabolite type of anticancer drug that demonstrated promising clinical activity in a wide variety of solid tumors, including non-small cell lung carcinoma and malignant pleural mesothelioma. It inhibits enzymes involved in the folate pathway, for which the presence of its free carboxylic groups is necessary. The heteroaromatic ring system of Pem has a modifiable amino group, which opens a possibility to apply a new strategy to conjugate Pem to carrier molecules. Considering this as well as the necessity of untouched carboxylic groups of Pem in the new conjugates, we developed a new synthesis strategy. Here, we describe the synthesis and the characterization of new Pem-peptide conjugates in which cell-penetrating octaarginine or/and lung-targeting H-Ile-Glu-Leu-Leu-Gln-Ala-Arg-NH(2) peptide is attached to the drug by thioether bond. The conjugates characterized by RP-HPLC and MS exhibited cytostatic effect in vitro on non-small cell lung carcinoma as well as on human leukemia cell lines. The IC(50) values of the conjugates were similar, but the conjugates with H-Ile-Glu-Leu-Leu-Gln-Ala-Arg-NH(2) sequence were slightly more effective. Our data show that the in vitro cytostatic effect of the free Pem was essentially maintained after conjugation with cell-penetrating or cell-targeting peptides. Thus, the conjugation strategy reported could lead to the development of a new generation of active Pem conjugates.

A new daunomycin-peptide conjugate: synthesis, characterization and the effect on the protein expression profile of HL-60 cells in vitro

Daunomycin (Dau) is a DNA-binding antineoplastic agent in the treatment of various types of cancer, such as osteosarcomas and acute myeloid leukemia. One approach to improve its selectivity and to decrease the side effects is the conjugation of Dau with oligopeptide carriers, which might alter the drug uptake and intracellular fate. Here, we report on the synthesis, characterization, and in vitro biological properties of a novel conjugate in which Dau is attached, via an oxime bond, to one of the cancer specific small peptides (LTVSPWY) selected from a random phage peptide library. The in vitro cytostatic effect and cellular uptake of Dau═Aoa-LTVSPWY-NH(2) conjugate were studied on various human cancer cell lines expressing different levels of ErbB2 receptor which could be targeted by the peptide. We found that the new daunomycin-peptide conjugate is highly cytostatic and could be taken up efficiently by the human cancer cells studied. However, the conjugate was less effective than the free drug itself. RP-HPLC data indicate that the conjugate is stable at least for 24 h in the pH 2.5-7.0 range of buffers, as well as in cell culture medium. The conjugate in the presence of rat liver lysosomal homogenate, as indicated by LC-MS analysis, could be degraded. The smallest, Dau-containing metabolite (Dau═Aoa-Leu-OH) identified and prepared expresses DNA-binding ability. In order to get insight on the potential mechanism of action, we compared the protein expression profile of HL-60 human leukemia cells after treatment with the free and peptide conjugated daunomycin. Proteomic analysis suggests that the expression of several proteins has been altered. This includes three proteins, whose expression was lower (tubulin β chain) or markedly higher (proliferating cell nuclear antigen and protein kinase C inhibitor protein 1) after administration of cells with Dau-conjugate vs free drug.

Synthesis and in vitro antitumor effect of vinblastine derivative-oligoarginine conjugates

Vinblastine is a widely used anticancer drug with undesired side effects. Its conjugation with carrier molecules could be an efficient strategy to reduce these side effects. Besides this, the conjugate could exhibit increased efficiency against resistant cells, e.g., due to the altered internalization pathway. Oligoarginines, as cell-penetrating peptides, can transport covalently attached compounds into different kinds of cells and enhance the efficiency of those compounds. We report here the coupling of vinblastine through its carboxyl group at position 16 with the N-terminal amino function of L-Trp methyl ester. After hydrolysis of the ester group, 17-desacetylvinblastineTrp was conjugated to the N-terminal amino group of oligoarginine via the C-terminal carboxyl group of the Trp moiety in solution. The antitumor effect of conjugates was studied on sensitive and resistant human leukemia (HL-60) cells in vitro. Our data suggest that all conjugates investigated possess an antiproliferative effect against the studied cells. However, the effect was dependent on the number of Arg residues in the conjugates: Arg₈ > Arg₆ ≫ Arg₄. The conjugate with Arg₈ exhibited similar efficicacy as compared with free 17-desacetylvinblastineTrp. The in vitro studies also showed that the tubulin binding ability of vinblastine was essentially preserved even in the octaarginine conjugate. We also observed that two isomers were formed during conjugation. These isomers showed different levels of activity against tubulin polymerization in vitro and in vivo. The 17-desacetylvinblastineTrp-Arg₈-1 isomer conjugate possessed high selectivity against the mitotic spindles. HRMS and NMR data suggest that 17-desacetylvinblastineTrp-Arg₈-1 and 17-desacetylvinblastineTrp-Arg₈-2 are epimers at the tryptophan α carbon atom.

Folding transitions in calpain activator peptides studied by solution NMR spectroscopy

Calpastatin, the endogenous inhibitor of calpain, a cysteine protease in eukaryotic cells, is an intrinsically unstructured protein, which upon binding to the enzyme goes through a conformational change. Peptides calpA (SGKSGMDAALDDLIDTLGG) and calpC (SKPIGPDDAIDALSSDFTS), corresponding to the two conserved subdomains of calpastatin, are known to activate calpain and increase the Ca(2+) sensitivity of the enzyme. Using solution NMR spectroscopy, here we show that calpA and calpC are disordered in water but assume an alpha-helical conformation in 50% CD(3)OH. The position and length of the helices are in agreement with those described in the literature for the bound state of the corresponding segments of calpastatin suggesting that the latter might be structurally primed for the interaction with its target. According to our data, the presence of Ca(2+) induces a backbone rearrangement in the peptides, an effect that may contribute to setting the fine conformational balance required for the interaction of the peptides with calpain.

New ferrocene containing peptide conjugates: synthesis and effect on human leukemia (HL-60) cells

Data reported in this article describe the synthesis of Arg-rich oligopeptide conjugates of ferrocenecarboxylic acid on solid support with two different strategies and for the first time, the successful preparation of peptide conjugates of ferrocenylacrylic acid in solution. The antitumor effect of conjugates was analyzed by MTT assay in vitro. We demonstrated that ferrocenylacrylic acid possessing an enone (--CH==CH--CO--) moiety exhibited remarkable antiproliferative effect against human leukemia cells (HL-60) in vitro, but its effect was not improved by conjugation with hexa- or octaarginines. However, we observed highly increased water-solubility. In contrast, the results provide evidence that conjugation of ferrocenecarboxylic acid to Arg(n) (n = 6, 8) improved not only its water-solubility, but also antitumor effect on human leukemia cells in vitro.

Medium-sized peptides as built in carriers for biologically active compounds

A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section.

On the sequential determinants of calpain cleavage

The structural clues of substrate recognition by calpain are incompletely understood. In this study, 106 cleavage sites in substrate proteins compiled from the literature have been analyzed to dissect the signal for calpain cleavage and also to enable the design of an ideal calpain substrate and interfere with calpain action via site-directed mutagenesis. In general, our data underline the importance of the primary structure of the substrate around the scissile bond in the recognition process. Significant amino acid preferences were found to extend over 11 residues around the scissile bond, from P(4) to P(7)'. In compliance with earlier data, preferred residues in the P(2) position are Leu, Thr, and Val, and in P(1) Lys, Tyr, and Arg. In position P(1) ', small hydrophilic residues, Ser and to a lesser extent Thr and Ala, occur most often. Pro dominates the region flanking the P(2)-P(1)' segment, i.e. positions P(3) and P(2)'-P(4)'; most notable is its occurrence 5.59 times above chance in P(3)'. Intriguingly, the segment C-terminal to the cleavage site resembles the consensus inhibitory region of calpastatin, the specific inhibitor of the enzyme. Further, the position of the scissile bond correlates with certain sequential attributes, such as secondary structure and PEST score, which, along with the amino acid preferences, suggests that calpain cleaves within rather disordered segments of proteins. The amino acid preferences were confirmed by site-directed mutagenesis of the autolysis sites of Drosophila calpain B; when amino acids at key positions were changed to less preferred ones, autolytic cleavage shifted to other, adjacent sites. Based on these preferences, a new fluorogenic calpain substrate, DABCYLTPLKSPPPSPR-EDANS, was designed and synthesized. In the case of micro- and m-calpain, this substrate is kinetically superior to commercially available ones, and it can be used for the in vivo assessment of the activity of these ubiquitous mammalian calpains.