Azamacrocyclic Metal Complexes as CXCR4 Antagonists

Development and therapeutic perspectives of CXCR4 antagonists for disease therapy

Chemokine receptor 4 (CXCR4) is a subtype receptor protein of the GPCR family with a seven-transmembrane structure widely distributed in human tissues. CXCR4 is involved in diseases (e.g., HIV-1 infection), cancer proliferation and metastasis, inflammation signaling pathways, and leukemia, making it a promising drug target. Clinical trials on CXCR4 antagonists mainly focused on peptides and antibodies, with a few small molecule compounds, such as AMD11070 (2) and MSX-122 (3), showing promise in cancer treatment. This perspective discusses the structure-activity relationship (SAR) of CXCR4 and its role in diseases, mainly focusing on the SAR of CXCR4 antagonists. It also explores the standard structural features and target interactions of CXCR4 binding in different disease categories. Furthermore, it investigates various modification strategies to propose potential improvements in the effectiveness of CXCR4 drugs.

Low-molecular-weight anti-HIV-1 agents targeting HIV-1 capsid proteins

The HIV-1 capsid is a shell that encapsulates viral RNA, and forms a conical structure by assembling oligomers of capsid (CA) proteins. Since the CA proteins are highly conserved among many strains of HIV-1, the inhibition of the CA function could be an appropriate goal for suppression of HIV-1 replication, but to date, no drug targeting CA has been developed. Hydrophobic interactions between two CA molecules through Trp184 and Met185 in the protein are known to be indispensable for conformational stabilization of the CA multimer. In our previous study, a small molecule designed by in silico screening as a dipeptide mimic of Trp184 and Met185 in the interaction site was synthesized and found to have significant anti-HIV-1 activity. In the present study, molecules with different scaffolds based on a dipeptide mimic of Trp184 and Met185 have been designed and synthesized. Their significant anti-HIV activity and their advantages compared to the previous compounds were examined. The present results should be useful in the design of novel CA-targeting anti-HIV agents.

Small-Molecule Anti-HIV-1 Agents Based on HIV-1 Capsid Proteins

The capsid of human immunodeficiency virus type 1 (HIV-1) is a shell that encloses viral RNA and is highly conserved among many strains of the virus. It forms a conical structure by assembling oligomers of capsid (CA) proteins. CA dysfunction is expected to be an important target of suppression of HIV-1 replication, and it is important to understand a new mechanism that could lead to the CA dysfunction. A drug targeting CA however, has not been developed to date. Hydrophobic interactions between two CA molecules via Trp184/Met185 in CA were recently reported to be important for stabilization of the multimeric structure of CA. In the present study, a small molecule designed by in silico screening as a dipeptide mimic of Trp184 and Met185 in the interaction site, was synthesized and its significant anti-HIV-1 activity was confirmed. Structure activity relationship (SAR) studies of its derivatives were performed and provided results that are expected to be useful in the future design and development of novel anti-HIV agents targeting CA.

Potent leads based on CA-19L, an anti-HIV active HIV-1 capsid fragment

Several anti-HIV-1 peptides have previously been found among overlapping fragment peptide libraries that contain an octa-arginyl moiety and cover the whole sequence of an HIV-1 capsid (CA) protein. Several derivatives based on a potent CA fragment peptide CA-19L have been synthesized. CA-19L overlaps with the Helix 9 region of the CA protein, which could be important for oligomerization of the CA proteins. Derivatives of CA-19L in which several amino acid residues were added to the N- and C-termini according to the natural CA sequence, were synthesized and their anti-HIV activity was evaluated. Some potent compounds were found, and these potential new anti-HIV agents are expected to be useful as new tools for elucidation of CA functions.

Bivalent HIV-1 fusion inhibitors based on peptidomimetics

Membrane fusion is a valid target for inhibition of HIV-1 replication. A 34-mer fragment peptide (C34), which is contained in the HIV-1 envelope protein gp41, has significant anti-HIV activity. Previously, a dimeric derivative of C34 linked by a disulfide bridge at its C-terminus was found to have more potent anti-HIV activity than the C34 peptide monomer. To date, several peptidomimetic small inhibitors have been reported, but most have lower potency than peptide derivatives related to C34. In the present study we applied this dimerization concept to these peptidomimetic small inhibitors and designed several bivalent peptidomimetic HIV-1 fusion inhibitors. The importance of the length of linkers crosslinking two peptidomimetic compounds was demonstrated and several potent bivalent inhibitors containing tethered peptidomimetics were produced.

Exploratory studies on CA-15L, an anti-HIV active HIV-1 capsid fragment

Utilizing overlapping fragment peptide libraries covering the whole sequence of an HIV-1 capsid (CA) protein with the addition of an octa-arginyl moiety, we had previously found several peptides with anti-HIV-1 activity. Herein, among these potent CA fragment peptides, CA-15L was examined because this peptide sequence overlaps with Helix 7, a helix region of the CA protein, which may be important for oligomerization of the CA proteins. A CA-15L surrogate with hydrophilic residues, and its derivatives, in which amino acid sequences are shifted toward the C-terminus by one or more residues, were synthesized and their anti-HIV activity was evaluated. In addition, its derivatives with substitution for the Ser149 residue were synthesized and their anti-HIV activity was evaluated because Ser149 might be phosphorylated in the step of degradation of CA protein oligomers. Several active compounds were found and might become new anti-HIV agents and new tools for elucidation of CA functions.

Design, synthesis and biological evaluation of low molecular weight CXCR4 ligands

The chemokine receptor CXCR4/stromal cell-derived factor-1 (SDF-1: CXCL12) signaling axis represents a crucial drug target due to its relevance to several diseases such as HIV-1 infection, cancer, leukemia, and rheumatoid arthritis. With the aim of enhancing the binding affinity and anti-HIV activity of a potent CXCR4 ligand as a lead, 23 low molecular weight compounds containing dipicolylamine (Dpa) and cyclam cationic moieties with varying spacers and spatial positioning were designed, synthesized and biologically evaluated. All of the synthesized compounds screened at 1.0 μM in the NanoBRET assay system exhibited >70% inhibition of the binding of a competitive probe TAMRA-Ac-TZ14011 (10 nM) to CXCR4 in the presence of zinc (II) ion. Furthermore, selected compounds 3, 8, 9, 19 and 21 with spatial distances between the next carbon to Dpa and the next carbon to cyclam within the range of 6.5-7.5 Å showed potent binding affinity selective for CXCR4 with IC₅₀ values of 1.6, 7.9, 5.7, 3.5 and 4.5 nM, respectively, with corresponding high anti-HIV activity with EC₅₀s of 28, 13, 21, 28 and 61 nM, respectively, in the presence of zinc (II) ion. Some compounds with remarkably more potent CXCR4-binding affinity than that of an initial lead were obtained. These compounds interact with different but overlapping amino acid residues of CXCR4. The present studies have developed new low molecular weight CXCR4 ligands with high CXCR4-binding and anti-HIV activities, which open avenue into the development of more potent CXCR4 ligands.

The chemical diversity and structure-based evolution of non-peptide CXCR4 antagonists with diverse therapeutic potential

The CXC chemokine receptor 4 (CXCR4) is a highly reserved G-protein coupled 7-transmembrane (TM) chemokine receptor which consists of 352 amino acids. CXCR4 has only one endogenous chemokine ligand of CXCL12, besides several other natural nonchemokine ligands such as extracellular ubiquitin and noncognate ligand of MIF. CXCR4 strongly binds to CXCL12 and the resulting CXCLl2/CXCR4 axis is the molecular basis of their various biological functions, which include: (1) mediating immune and inflammatory response; (2) regulation of hematopoietic stem cell migration and homing; (3) an essential co-receptor for HIV entry into host cells; (4) participation in the process of embryonic development; (5) malignant tumor invasion and metastasis; (6) myocardial infarction, ischemic stroke and acute kidney injury. Correspondingly, CXCR4 antagonists find potential therapeutic applications in HIV infection, as well as hematopoietic stem cell migration, inflammation, immune-related diseases, tumor and ischemic diseases. Recently, great achievements have been made and a number of non-peptide CXCR4 antagonists with diversity scaffolds have been discovered. In this review, the discovery of small molecule CXCR4 antagonists focused on the structures, activities, evolution and development of representative CXCR4 antagonists is comprehensively described. The central role of CXCR4 in diverse cellular signaling pathways and its involvement in several diseases progressions are discussed as well.

Development of anti-HIV peptides based on a viral capsid protein

Peptide inhibitors with cell permeability targeting an HIV-1 capsid (CA) protein might make therapeutic by regulating HIV-1 replication. Overlapping fragment peptide libraries covering the whole sequence of an HIV-1 CA protein have been synthesized with the addition of an octa-arginyl moiety to increase their cell permeability. Amongst these peptides, several compounds which inhibit the HIV-1 replication cycle have been found. Conjugation of cell-penetrating functions such as an octa-arginyl group to individual peptides in combination with the addition of chloroquine in cell-based anti-HIV assays was previously proven to be a useful assay method with which to search for active peptides. Anti-HIV assays have been performed in the presence or absence of chloroquine and found that most of compounds have higher anti-HIV activity in the presence, rather than in the absence of chloroquine. Some potent seeds as anti-HIV agents might naturally lie hidden in CA proteins, and could become useful leads to HIV inhibitors.

Bivalent 14-mer peptide ligands of CXCR4 with polyproline linkers with anti-chemotactic activity against Jurkat cells

Interaction of CXCR4 with its endogenous ligand, stromal-cell derived factor-1 (SDF-1)/CXCL12, induces various physiological functions involving chemotaxis. Bivalent ligands with a polyproline helix bearing a cyclic pentapeptide, FC131, were previously shown to have higher binding affinities for CXCR4 than the corresponding monovalent ligands. Bivalent ligands based on a 14-mer peptide T140 derivative with polyproline linkers have been designed and synthesized. The activity of these peptides as well as the effect of bivalency of the ligand on CXCR4 binding has been assessed. The binding affinity of these series of bivalent ligands is increased as the linker length increases up to the 12-/15-mer proline linker. The inhibitory activity against chemotaxis on Jurkat cells also depends on the linker length. The T140-derived bivalent ligands with the 9- and 12-mer proline linkers showed the most effective inhibition against chemotaxis at 1000 nM, which is even higher than that of known CXCR4 antagonists in the monomer structure. The effective metastatic inhibition by bivalent T140 derivatives indicates the therapeutic potential. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Small-Molecule CD4 Mimics Containing Mono-cyclohexyl Moieties as HIV Entry Inhibitors

CD4 mimics are small molecules that inhibit the protein-protein interaction between gp120 and CD4, which is a key interaction for the entry of human immunodeficiency virus (HIV) into host immune cells. In the present study, mono-cyclohexyl-type CD4 mimics were designed to form hydrophobic and electrostatic interactions with Val430 and Asp368 located in the entrance of the Phe43 cavity of gp120, the interaction site of CD4. YIR-329, a novel 1-azaspiro[5.5]undecane derivative with a cyclohexyl ring attached to the piperidine ring, exhibited only slightly weaker anti-HIV activity than a previously described lead HAR-171, and modeling results indicated the formation of advantageous interactions by the para-chlorophenyl moiety of YIR-329. To introduce an electrostatic interaction with Asp368, derivatives with a guanidino group on the piperidine nitrogen atom were synthesized. Mono-cyclohexyl-type CD4 mimics with a guanidino group, such as YIR-819 (N(1) -(4-chlorophenyl)-N(2) -(1-(2-(N-(amidino)glycinamide)ethyl)-2-cyclohexylpiperidin-4-yl)oxalamide) and YIR-821 (1-(2-(5-guanidinovaleramide)ethyl derivative of YIR-819), were identified that exhibit approximately fivefold more potent anti-HIV activity than YIR-329. In combination with a neutralizing antibody, their anti-HIV activities were augmenting. Modeling results suggest that these compounds interact effectively with Val430 and either Asp368 or Asp474 in the gp120 Phe43 cavity. YIR-819 and YIR-821 represent useful lead compounds for the further development of HIV-1 entry inhibitors and could potentially be useful for co-administration with neutralizing antibodies for the treatment of HIV infection and AIDS.

Metal complexes of pyridine-fused macrocyclic polyamines targeting the chemokine receptor CXCR4

The chemokine receptor CXCR4 acts as a key cell surface receptor in HIV infections, multiple forms of cancer, and various other pathologies, such as rheumatoid arthritis and asthma. Macrocyclic polyamines and their metal complexes are known to exert anti-HIV activity, many acting as HIV entry inhibitors by specifically binding to CXCR4. Three series of pyridopentaazacylopentadecanes, in which the pyridine ring is fused to zero, one, or two saturated six-membered rings, were synthesized by manganese(ii)-templated Schiff-base cyclization of triethylenetetramine with various dicarbonyl compounds. By evaluating these macrocyclic polyamines and their complexes with Mn(2+), Cu(2+), Fe(3+), and Zn(2+), we have discovered novel CXCR4-binding compounds. The MnCl2 complex of a new pentaazacyclopentadecane with one fused carbocyclic ring (11) was found to have the greatest potency as an antagonist of the chemokine receptor CXCR4 (IC50: 0.014 μM), as evidenced by inhibiting binding of CXCL12 to PBMCs (peripheral blood mononuclear cells). Consequently, this compound inhibits replication of the CXCR4-using (X4) HIV-1 strain NL4-3 in the TZM-bl cell line with an IC50 value of 0.52 μM and low cytotoxicity (CC50: >100 μM). In addition, 18 other compounds were evaluated for their interaction with CXCR4 via their ability to interfere with ligand chemokine binding and HIV entry and infection. Of these, the metal complexes of the two more hydrophobic series with one or two fused carbocyclic rings exhibited the greatest potency. The Zn(2+) complex 21 was among the most potent, showing that redox activity of the metal center is not associated with CXCR4 antagonist activity.

Anti-HIV screening for cell-penetrating peptides using chloroquine and identification of anti-HIV peptides derived from matrix proteins

Previously, compounds which inhibit the HIV-1 replication cycle were found in overlapping peptide libraries covering the whole sequence of an HIV-1 matrix (MA) protein constructed with the addition of an octa-arginyl group. The two top lead compounds are sequential fragments MA-8L and MA-9L. In the present study, the addition of chloroquine in cell-based anti-HIV assays was proven to be an efficient method with which to find anti-HIV compounds among several peptides conjugated by cell-penetrating signals such as an octa-arginyl group: the conjugation of an octa-arginyl group to individual peptides contained in whole proteins in combination with the addition of chloroquine in cells is a useful assay method to search active peptides. To find more potent fragment peptides, individual peptides between MA-8L and MA-9L, having the same peptide chain length but with sequences shifted by one amino acid residue, were synthesized in this paper and their anti-HIV activity was evaluated with an anti-HIV assay using chloroquine. As a result, the peptides in the C-terminal side of the series, which are relatively close to MA-9L, showed more potent inhibitory activity against both X4-HIV-1 and R5-HIV-1 than the peptides in the N-terminal side.

Insights into the mechanism of inhibition of CXCR4: identification of Piperidinylethanamine analogs as anti-HIV-1 inhibitors

The cellular entry of HIV-1 into CD4(+) T cells requires ordered interactions of HIV-1 envelope glycoprotein with C-X-C chemokine receptor type 4 (CXCR4) receptors. However, such interactions, which should be critical for rational structure-based discovery of new CXCR4 inhibitors, remain poorly understood. Here we first determined the effects of amino acid substitutions in CXCR4 on HIV-1NL 4 - 3 glycoprotein-elicited fusion events using site-directed mutagenesis-based fusion assays and identified 11 potentially key amino acid substitutions, including D97A and E288A, which caused >30% reductions in fusion. We subsequently carried out a computational search of a screening library containing ∼604,000 compounds, in order to identify potential CXCR4 inhibitors. The computational search used the shape of IT1t, a known CXCR4 inhibitor, as a reference and employed various algorithms, including shape similarity, isomer generation, and docking against a CXCR4 crystal structure. Sixteen small molecules were identified for biological assays based on their high shape similarity to IT1t, and their putative binding modes formed hydrogen bond interactions with the amino acids identified above. Three compounds with piperidinylethanamine cores showed activity and were resynthesized. One molecule, designated CX6, was shown to significantly inhibit fusion elicited by X4 HIV-1NL 4 - 3 glycoprotein (50% inhibitory concentration [IC50], 1.9 μM), to inhibit Ca(2+) flux elicited by stromal cell-derived factor 1α (SDF-1α) (IC50, 92 nM), and to exert anti-HIV-1 activity (IC50, 1.5 μM). Structural modeling demonstrated that CX6 bound to CXCR4 through hydrogen bond interactions with Asp97 and Glu288. Our study suggests that targeting CXCR4 residues important for fusion elicited by HIV-1 envelope glycoprotein should be a useful and feasible approach to identifying novel CXCR4 inhibitors, and it provides important insights into the mechanism by which small-molecule CXCR4 inhibitors exert their anti-HIV-1 activities.

CXCR4-derived synthetic peptides inducing anti-HIV-1 antibodies

Despite almost 30 years since the identification of the human immunodeficiency virus type I (HIV-1), development of effective AIDS vaccines has been hindered by the high mutability of HIV-1. The HIV-1 co-receptors CCR5 and CXCR4 are genetically stable, but viral proteins may mutate rapidly during the course of infection. CXCR4 is a seven transmembrane G protein-coupled receptor, possessing an N-terminal region (NT) and three extracellular loops (ECL1-3). Previous studies have shown that the CXCR4-ED-derived peptides inhibit the entry of HIV-1 by interacting with gp120, an HIV-1 envelope glycoprotein. In the present study, antigenicity of CXCR4-derived peptides has been investigated and the anti-HIV-1 effects of induced antisera have been assessed. It was found that CXCR4-ED-derived antigen molecules immunize mice, showing that the linear peptides have higher antigenicity than the cyclic peptides. The L1- and L2-induced antisera inhibited the HIV-1 entry significantly, while anti-N1 antibodies have no inhibitory activity. This study produced promising examples for the design of AIDS vaccines which target the human protein and can overcome mutability of HIV-1.

Anti-HIV-1 peptide derivatives based on the HIV-1 Co-receptor CXCR4

The human immunodeficiency virus type 1 (HIV-1) uses CD4 and the co-receptor CCR5 or CXCR4 in the process of cell entry. The negatively charged extracellular domains of CXCR4 (CXCR4-ED) interact with positive charges on the V3 loop of gp120, facilitating binding via electrostatic interactions. The presence of highly conserved positively charged residues in the V3 loop suggests that CXCR4-ED-derived inhibitors might be broadly effective inhibitors. Synthetic peptide derivatives were evaluated for anti-HIV-1 activity. The 39-mer extracellular N-terminal region (NT) was divided into three fragments with 10-mer overlapping sites (N1-N3), and these linear peptides were synthesized. Peptide N1 contains Met 1-Asp 20 and shows significant anti-HIV-1 activity. Extracellular loops 1 and 2 (ECL1 and 2) were mimicked by cyclic peptides C1 and C2, which were synthesized by chemoselective cyclization. Cyclic peptides C1 and C2 show higher anti-HIV-1 activity than their linear peptide counterparts, L1 and L2. The cytotoxicities of C1 and C2 are lower than those of L1 and L2. These results indicate that Met 1-Asp 20 segments of the NT and cyclic peptides of ECL1 and ECL2 are potent anti-HIV-1 drug candidates.

Benzenesulfonamides: a unique class of chemokine receptor type 4 inhibitors

The interaction of CXCR4 with CXCL12 (SDF-1) plays a critical role in cancer metastasis by facilitating the homing of tumor cells to metastatic sites. Based on our previously published work on CXCR4 antagonists, we have synthesized a series of aryl sulfonamides that inhibit the CXCR4/CXCL12 interaction. Analogue bioactivities were assessed with binding affinity and Matrigel invasion assays. Computer modeling was employed to evaluate a selection of the new analogues docked into the CXCR4 X-ray structure and to rationalize discrepancies between the affinity and Matrigel in vitro assays. A lead compound displays nanomolar potency in the binding affinity assay (IC(50)=8.0 nM) and the Matrigel invasion assay (100 % blockade of invasion at 10 nM). These data demonstrate that benzenesulfonamides are a unique class of CXCR4 inhibitors with high potency.

Imaging agents for the chemokine receptor 4 (CXCR4)

The interaction between the chemokine receptor 4 (CXCR4) and stromal cell-derived factor-1 (SDF-1, also known as CXCL12) is a natural regulatory process in the human body. However, CXCR4 over-expression is also found in diseases such as cancer, where it plays a role in, among others, the metastatic spread. For this reason it is an interesting biomarker for the field of diagnostic oncology, and therefore, it is gaining increasing interest for applications in molecular imaging. Especially "small-molecule" imaging agents based on T140, FC131 and AMD3100 have been extensively studied. SDF-1, antibodies, pepducins and bioluminescence have also been used to visualize CXCR4. In this critical review reported CXCR4 targeting imaging agents are described based on their affinity, specificity and biodistribution. The level wherein CXCR4 is up-regulated in cancer patients and its relation to the different cell lines and animal models used to evaluate the efficacy of the imaging agents is also discussed (221 references).

Pharmacophore-based small molecule CXCR4 ligands

Low molecular weight CXCR4 ligands were developed based on the peptide T140, which has previously been identified as a potent CXCR4 antagonist. Some compounds with naphthyl, fluorobenzyl and pyridyl moieties as pharmacophore groups in the molecule showed significant CXCR4-binding activity and anti-HIV activity. Structure-activity relationships were studied and characteristics of each of these three moieties necessary for CXCR4 binding were defined. In this way, CXCR4 ligands with two types of recognition modes for CXCR4 have been found.