Adenosine-5'-carboxylic acid peptidyl derivatives as inhibitors of protein kinases

Recent advances in development of hetero-bivalent kinase inhibitors

Identifying a pharmacological agent that targets only one of more than 500 kinases present in humans is an important challenge. One potential solution to this problem is the development of bivalent kinase inhibitors, which consist of two connected fragments, each bind to a dissimilar binding site of the bisubstrate enzyme. The main advantage of bivalent (type V) kinase inhibitors is generating more interactions with target enzymes that can enhance the molecules' selectivity and affinity compared to single-site inhibitors. Earlier type V inhibitors were not suitable for the cellular environment and were mostly used in in vitro studies. However, recently developed bivalent compounds have high kinase affinity, high biological and chemical stability in vivo. This review summarized the hetero-bivalent kinase inhibitors described in the literature from 2014 to the present. We attempted to classify the molecules by serine/threonine and tyrosine kinase inhibitors, and then each target kinase and its hetero-bivalent inhibitor was assessed in depth. In addition, we discussed the analysis of advantages, limitations, and perspectives of bivalent kinase inhibitors compared with the monovalent kinase inhibitors.

Bivalent inhibitors of protein kinases

Protein kinases are key players in a large number of cellular signaling pathways. Dysregulated kinase activity has been implicated in a number of diseases, and members of this enzyme family are of therapeutic interest. However, due to the fact that most inhibitors interact with the highly conserved ATP-binding sites of kinases, it is a significant challenge to develop pharmacological agents that target only one of the greater than 500 kinases present in humans. A potential solution to this problem is the development of bisubstrate and bivalent kinase inhibitors, in which an active site-directed moiety is tethered to another ligand that targets a location outside of the ATP-binding cleft. Because kinase signaling specificity is modulated by regions outside of the ATP-binding site, strategies that exploit these interactions have the potential to provide reagents with high target selectivity. This review highlights examples of kinase interaction sites that can potentially be exploited by bisubstrate and bivalent inhibitors. Furthermore, an overview of efforts to target these interactions with bisubstrate and bivalent inhibitors is provided. Finally, several examples of the successful application of these reagents in a cellular setting are described.

Directed modulation of protein kinase C isozyme selectivity with bisubstrate-based inhibitors

Kinases present an attractive target for drug development, since they are involved in vital cellular processes and are implicated in a variety of diseases, such as cancer and diabetes. However, obtaining selectivity for a specific kinase over others is difficult since many current kinase inhibitors exclusively target the highly conserved kinase ATP binding domain. Previously, a microarray-based strategy to discover so-called bisubstrate-based inhibitors that target the more specific peptide binding groove in addition to the ATP binding site was described. One attractive feature of this strategy is the opportunity to tune the selectivity of these inhibitors by systematically varying components. In an extension to this previous work, this study explores the potential of this guided selectivity modulation, leading to a series of inhibitors with different selectivity profiles against highly homologous protein kinase C (PKC) isozymes. Of the inhibitors studied, most exhibited improved potency and selectivity compared with their constituent parts. Furthermore, the selectivity was found to be tunable either through modification of the pseudosubstrate peptide (peptide binding groove) or the ATP-competitive part (ATP binding site). In a number of cases, the selectivity of the construct could be predicted from the initial peptide substrate profiling experiment. Since this strategy is applicable to all kinase sets, it could be used to rapidly develop uniquely selective inhibitors.

Therapeutic potential of adenosine analogues and conjugates

This review summarizes current knowledge of adenosine analogues and conjugates with promising therapeutic properties. Adenosine is a signaling molecule that triggers numerous physiological responses. It acts through the adenosine receptors (ARs), belonging to the family of G-protein-coupled receptors and widely distributed throughout the body. Moreover, adenosine is involved in key biochemical processes as a part of ATP, the universal energy currency. Thus, compounds that are analogues of adenosine and its conjugates have been extensively studied as potential therapeutics. Many inhibitors of ARs are in clinical trials as promising agents in treatment of inflammation, type 2 diabetes, arrhythmia and as vasodilators used in the myocardial perfusion imaging (MPI) stress test. Furthermore, adenosine analogues revealed high efficacy as enzyme inhibitors, tested for antitrypanosomal action and as bivalent ligands and adenosine-oligoarginine conjugates as inhibitors of protein kinases.

Adenosine analogue-oligo-arginine conjugates (ARCs) serve as high-affinity inhibitors and fluorescence probes of type I cGMP-dependent protein kinase (PKGIalpha)

INTRODUCTION

Type I cGMP-dependent protein kinase (PKGIalpha) belongs to the family of cyclic nucleotide-dependent protein kinases and is one of the main effectors of cGMP. PKGIalpha is involved in regulation of cardiac contractility, vasorelaxation, and blood pressure; hence, the development of potent modulators of PKGIalpha would lead to advances in the treatment of a variety of cardiovascular diseases.

AIM

Representatives of ARC-type compounds previously characterized as potent inhibitors and high-affinity fluorescent probes of PKA catalytic subunit (PKAc) were tested towards PKGIalpha to determine that ARCs could serve as activity regulators and sensors for the latter protein kinase both in vitro and in complex biological systems.

RESULTS

Structure-activity profiling of ARCs with PKGIalpha in vitro demonstrated both similarities as well as differences to corresponding profiling with PKAc, whereas ARC-903 and ARC-668 revealed low nanomolar displacement constants and inhibition IC(50) values with both cyclic nucleotide-dependent kinases. The ability of ARC-based fluorescent probes to penetrate cell plasma membrane was demonstrated in the smooth muscle tissue of rat cerebellum isolated arteries, and the compound with the highest affinity in vitro (ARC-903) showed also potential for in vivo applications, fully abolishing the PKG1alpha-induced vasodilation.

Bisubstrate inhibitors of protein kinases: from principle to practical applications

Bisubstrate inhibitors consist of two conjugated fragments, each targeted to a different binding site of a bisubstrate enzyme. The design of bisubstrate inhibitors presupposes the formation of the ternary complex in the course of the catalyzed reaction. The principle advantage of bisubstrate inhibitors is their ability to generate more interactions with the target enzyme that could result in improved affinity and selectivity of the conjugates, when compared with single-site inhibitors. Among phosphotransferases, the approach was first successfully used for adenylate kinase in 1973. Since then, several types of bisubstrate inhibitors have been developed for protein kinases, including conjugates of peptides with nucleotides, adenosine derivatives and potent ATP-competitive inhibitors. Earlier bisubstrate inhibitors had pharmacokinetic qualities that were unsuitable for cellular experiments and hence were mostly used for in vitro studies. The recently constructed conjugates of adenosine derivatives and D-arginine-rich peptides (ARCs) possess high kinase affinity, high biological and chemical stability and good cell plasma membrane penetrative properties that enable their application in the regulation of cellular protein phosphorylation balances in cell and tissue experiments.

Preparation of novel alkylated arginine derivatives suitable for click-cycloaddition chemistry and their incorporation into pseudosubstrate- and bisubstrate-based kinase inhibitors

Efficient strategies for the introduction of arginine residues featuring acetylene or azide moieties in their side chains are described. The substituents are introduced in a way that maintains the basicity of the guanidine moiety. The methodology can be used e.g. for non-invasive labeling of arginine-containing peptides. Its applicability is demonstrated by the introduction of 'click' handles into a Protein Kinase C (PKC) pseudosubstrate peptide, and the subsequent preparation and evaluation of a novel bisubstrate-based inhibitor based on such a peptide.

Development of selective bisubstrate-based inhibitors against protein kinase C (PKC) isozymes by using dynamic peptide microarrays

Kinase inhibitors are increasingly important in drug development. Because the majority of current inhibitors target the conserved ATP-binding site, selectivity might become an important issue. This could be particularly problematic for the potential drug target protein kinase C (PKC), of which twelve isoforms with high homology exist in humans. A strategy to increase selectivity is to prepare bisubstrate-based inhibitors that target the more selective peptide-binding site in addition to the ATP-binding site. In this paper a generally applicable, rapid methodology is presented to discover such bisubstrate-based leads. Dynamic peptide microarrays were used to find peptide-binding site inhibitors. These were linked with chemoselective click chemistry to an ATP-binding site inhibitor, and this led to novel bisubstrate structures. The peptide microarrays were used to evaluate the resulting inhibitors. Thus, novel bisubstrate-based inhibitors were obtained that were both more potent and selective compared to their constituent parts. The most promising inhibitor has nanomolar affinity and selectivity towards PKCtheta amongst three isozymes.

Staurosporine tethered peptide ligands that target cAMP-dependent protein kinase (PKA): optimization and selectivity profiling

We have recently developed a fragment based selection strategy for targeting kinases, where a small molecule warhead can be non-covalently tethered to a phage-displayed library of peptides. This approach was applied to the conversion of the promiscuous kinase inhibitor, staurosporine, into a potent bivalent ligand for cAMP-dependent protein kinase (PKA). Herein we report a systematic evaluation of this new bivalent ligand (BL); (a) Lineweaver-Burke analysis revealed that the BL, unlike substrate-based bivalent kinase inhibitors, displayed non-competitive inhibition with respect to the peptide substrate, suggesting an allosteric mechanism of action; (b) linker optimization of the BL, afforded one of the most potent, sub-nanomolar, inhibitors of PKA reported to date; (c) the BL was found to be modular, where attachment of active site targeted small molecule warheads in lieu of staurosporine could achieve similar gains in affinity; and (d) profiling studies of both the staurosporine derivative and the BL (amide isostere) against a panel of 90 kinases revealed almost unique enhancement in selectivity against PKA (>5-fold) compared to the starting staurosporine derivative. These combined results provide new insights for BL discovery, which has the potential to provide guidance toward the development of kinase selective reagents while uncovering new allosteric sites on kinases for therapeutic targeting.

Conjugation of adenosine and hexa-(D-arginine) leads to a nanomolar bisubstrate-analog inhibitor of basophilic protein kinases

Conjugates of oligoarginine peptides with adenine, adenosine, adenosine-5'-carboxylic acid, and 5-isoquinolinesulfonic acid were synthesized and characterized as bisubstrate-analog inhibitors of cAMP-dependent protein kinase. Adenosine and adenine derivatives were connected to the N- or C-terminus of peptides containing four to six L- or D-arginine residues via a linker with a length that had been optimized in structure-activity studies. The orientation of the peptide chain strongly affected the activity of compounds incorporating D-arginines. The biligand inhibitor containing Hidaka's H9 isoquinolinesulfonamide connected to the L-peptide had 65 times higher potency than the corresponding adenosine-containing conjugate, while both types of the conjugate comprising D-peptides had similar low nanomolar activity. Two of the most active adenosine- and H9-peptide conjugates were tested in the panel of 52 different kinases. At 1 microM concentration, both compounds showed strong (more than 95%) inhibition of several basophilic AGC kinases, including pharmaceutically important kinases ROCK II and PKB/Akt.

Surface-plasmon-resonance-based biosensor with immobilized bisubstrate analog inhibitor for the determination of affinities of ATP- and protein-competitive ligands of cAMP-dependent protein kinase

Interactions between adenosine-oligoarginine conjugates (ARC), bisubstrate analog inhibitors of protein kinases, and catalytic subunits of cAMP-dependent protein kinase (cAPK Calpha) were characterized with surface-plasmon-resonance-based biosensors. ARC-704 bound to the immobilized kinase with subnanomolar affinity. The immobilization of ARC-704 to the chip surface via streptavidin-biotin complex yielded a high-affinity surface (K(D)=16nM). The bisubstrate character of ARC-704 was demonstrated with various ligands targeted to ATP-binding pocket (ATP and inhibitors H89 and H1152P) and protein-substrate-binding domain of Calpha (RIIalpha and GST-PKIalpha) in competition assays. The experiments performed on surfaces with different immobilization levels of ARC-704 produced similar results. The closeness of the obtained affinities of the tested compounds to the inhibitory potencies and affinities of the compounds measured with other methods demonstrates the applicability of the chip with the immobilized biligand inhibitor for the characterization of both ATP- and substrate protein-competitive ligands of basophilic protein kinases.

Kinetic analysis of inhibition of cAMP-dependent protein kinase catalytic subunit by the peptide-nucleoside conjugate AdcAhxArg6

Kinetic analysis of the inhibition of the phosphorylation of Kemptide, (LRRASLG), catalyzed by the catalytic subunit of cAMP-dependent protein kinase, by a peptide-nucleoside conjugate inhibitor AdcAhxArg6 was carried out over a wide range of ATP and peptide concentrations. A simple procedure was proposed for characterization of the interaction of this inhibitor with the free enzyme, and with the enzyme-ATP and enzyme-peptide complexes. The second-order rate constants, calculated from the steady-state reaction kinetics, were used for this analysis to avoid the complications related to the complex catalytic mechanism of the protein kinase catalyzed reaction.

Protein kinase structure and function analysis with chemical tools

Protein kinases are the largest enzyme superfamily involved in cell signal transduction and represent therapeutic targets for a range of diseases. There have been intensive efforts from many labs to understand their catalytic mechanisms, discover inhibitors and discern their cellular functions. In this review, we will describe two approaches developed to analyze protein kinases: bisubstrate analog inhibition and phosphonate analog utilization. Both of these methods have been used in combination with the protein semisynthesis method expressed protein ligation to advance our understanding of kinase-substrate interactions and functional elucidation of phosphorylation. Previous work on the nature of the protein kinase mechanism suggests it follows a dissociative transition state. A bisubstrate analog was designed against the insulin receptor kinase to mimic the geometry of a dissociative transition state reaction coordinate distance. This bisubstrate compound proved to be a potent inhibitor against the insulin receptor kinase and occupied both peptide and nucleotide binding sites. Bisubstrate compounds with altered hydrogen bonding potential as well as varying spacers between the adenine and the peptide demonstrate the importance of the original design features. We have also shown that related bisubstrate analogs can be used to potently block serine/threonine kinases including protein kinase A. Since many protein kinases recognize folded protein substrates for efficient phosphorylation, it was advantageous to incorporate the peptide-ATP conjugates into protein structures. Using expressed protein ligation, a Src-ATP conjugate was produced and shown to be a high affinity ligand for the Csk tyrosine kinase. Nonhydrolyzable mimics of phosphoSer/phosphoTyr can be useful in examining the functionality of phosphorylation events. Using expressed protein ligation, we have employed phosphonomethylene phenylalanine and phosphonomethylene alanine to probe the phosphorylation of Tyr and Ser, respectively. These tools have permitted an analysis of the SH2-phosphatases (SHP1 and SHP2), revealing a novel intramolecular stimulation of catalytic activity mediated by the corresponding phosphorylation events. They have also been used to characterize the cellular regulation of the melatonin rhythm enzyme by phosphorylation.

Increasing the Kinase Specificity of K252a by Protein Surface Recognition

[reaction: see text] Here we describe a miniature protein (1) that presents the cAMP-dependent protein kinase (PKA) recognition epitope found within the heat-stable Protein Kinase Inhibitor protein (PKI) and a miniature protein conjugate (1-K252a) in which 1 is joined covalently to the high-affinity but nonselective kinase inhibitor K252a. Miniature protein 1 recognizes PKA with an affinity that rivals that of PKI and, in the context of 1-K252a, leads to a dramatic increase in kinase specificity.

Liquid-phase synthesis of a pegylated adenosine-oligoarginine conjugate, cell-permeable inhibitor of cAMP-dependent protein kinase

An adenosine-oligoarginine conjugate (ARC) was assembled in a stepwise manner on a poly(ethylene glycol) carrier. The pegylated conjugate inhibited cAMP-dependent protein kinase with IC(50)=460 nM and the cellular uptake of its BODIPY FL derivative was demonstrated and compared to that of free ARC with fluorescence microscopy.

Identification of the ability of highly charged nanomolar inhibitors of protein kinases to cross plasma membranes and carry a protein into cells

A fluorescently labeled adenosine-oligoarginine conjugate (ARC), nanomolar bisubstrate analogue-type inhibitor of basophilic protein kinases PKA and PKC, readily enters cells of different origin and localizes into cytoplasm and nucleus. Moreover, the biotinylated derivative of ARC is able to deliver avidin, a non-covalently attached protein cargo, into cells.

Designing bisubstrate analog inhibitors for protein kinases

Protein kinases play critical roles in signal transduction pathways by transmitting extracellular signals across the cell membrane to distant locations in the cytoplasm and the nucleus. The development of protein kinase inhibitors has been hindered by the broad overlapping substrate specificities exhibited by these enzymes. The design of bisubstrate analog inhibitors could provide for the enhancement of specificity and potency in protein kinase inhibition. Bisubstrate analog inhibitors form a special group of protein kinase inhibitors that mimic two natural substrates/ligands and that simultaneously associate with two regions of given kinases. Most bisubstrate analogs have been designed to mimic the phosphate donor (ATP) and the acceptor components (Ser-, Thr-, or Tyr-containing peptides). Recent studies have emphasized the importance of maintaining a specific distance between these two components to achieve potent inhibition. In this review, we present a discussion of the methods for designing protein kinase inhibitors by mechanism-based approaches. Emphasis is given to bivalent approaches, with an interpretation of what has been learned from more and less successful examples. Future challenges in this area are also highlighted.

Bi-substrate analogue ligands for affinity chromatography of protein kinases

Novel affinity ligands, consisting of ATP-resembling part coupled with specificity determining peptide fragment, were proposed for purification of protein kinases. Following this approach affinity sorbents based on two closely similar ligands AdoC-Aoc-Arg4-Lys and AdoC-Aoc-Arg4-NH(CH2)6NH2, where AdoC stands for adenosine-5'-carboxylic acid and Aoc for amino-octanoic acid, were synthesized and tested for purification of recombinant protein kinase A catalytic subunit directly from crude cell extract. Elution of the enzyme with MgATP as well as L-arginine yielded homogeneous protein kinase A preparation in a single purification step. Also protein kinase A from pig heart homogenate was selectively isolated using MgATP as eluting agent. Protein kinase with acidic specificity determinant (CK2) as well as other proteins possessing nucleotide binding site (L-type pyruvate kinase) or sites for wide variety of different ligands (bovine serum albumin) did not bind to the column, pointing to high selectivity of the bi-functional binding mode of the affinity ligand.