A low-molecular-weight inhibitor against the chemokine receptor CXCR4: a strong anti-HIV peptide T140

Structure-activity relationship study on artificial CXCR4 ligands possessing the cyclic pentapeptide scaffold: the exploration of amino acid residues of pentapeptides by substitutions of several aromatic amino acids

Previously, downsizing of a 14-residue peptidic CXCR4 antagonist has led to the development of a highly potent CXCR4 antagonist [cyclo(-d-Tyr(1)-Arg(2)-Arg(3)-Nal(4)-Gly(5)-)]. In the present study, cyclic pentapeptide libraries that were designed by substitutions of several amino acids for d-Tyr(1) and Arg(2) in peptide were prepared and screened to evaluate binding activity for CXCR4. The above structure-activity relationship study led to the finding of several potent CXCR4 ligands.

An antagonist of the chemokine receptor CXCR4 induces mitotic catastrophe in ovarian cancer cells

The chemokine receptor CXCR4 is expressed by malignant cells in ovarian cancer and is implicated in their growth and spread. We report here a unique mechanism of action of a small peptide antagonist of CXCR4 on ovarian cancer cells: induction of cell death by mitotic catastrophe. CTCE-9908 inhibited ovarian cancer cell migration to CXCL12, but on longer incubation, caused cell death in CXCR4-positive cells. CTCE-9908 did not cause apoptosis or cellular senescence, but induced multinucleation, G(2)-M arrest, and abnormal mitosis in ovarian cancer cells. This suggests that cell death was caused by mitotic catastrophe. Using microarray and Western blot analysis, we showed that CTCE-9908 deregulated DNA damage checkpoint proteins and spindle assembly checkpoint proteins at G(2)-M phases of the cell cycle. Combination treatment of CTCE-9908 and the drug paclitaxel led to an additive cytotoxicity that also involved mitotic catastrophe. We conclude that CTCE-9908 has a unique mechanism of action in ovarian cancer cells that seems to be CXCR4 specific.

Staphylococcal superantigen-like 10 inhibits CXCL12-induced human tumor cell migration

PURPOSE

Tumor cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors. CXCR4 is the most widely expressed chemokine receptor in many different types of cancer and has been linked to tumor dissemination and poor prognosis. Several CXCR4 antagonists have been synthesized. A totally novel approach to discover chemokine receptor antagonists is the use of bacteria. Bacteria produce chemokine receptor inhibitors to prevent neutrophil extravasation and migration toward the infection site to escape clearance by innate immune cells. The aim of the current study was to find and identify the mechanism of a bacterial protein that specifically targets CXCR4, a chemokine receptor shared by neutrophils and cancer cells.

EXPERIMENTAL DESIGN

Several staphylococcal proteins were screened for their capacity to prevent binding of a function-blocking antibody against CXCR4.

RESULTS

Staphylococcal superantigen-like 10 was found to bind CXCR4 expressed on human T acute lymphoblastic leukemia, lymphoma, and cervical carcinoma cell lines. It potently inhibited CXCL12-induced calcium mobilization and cell migration.

CONCLUSIONS

Staphylococcal superantigen-like 10 is a potential lead in the development of new anticancer compounds preventing metastasis by targeting CXCR4.

CXCR4 chemokine receptor antagonists: perspectives in SCLC

Small-cell lung cancer (SCLC) is a particularly aggressive form of lung cancer characterized by early and widespread metastases and the ability to rapidly develop resistance against chemotherapeutic agents. Tumor cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokine receptors and adhesion molecules. SCLC cells express high levels of CXCR4 (CD184), a seven-transmembrane G-protein-coupled chemokine receptor. Stromal cells within the bone marrow microenvironment and at extramedullary sites constitutively secrete stromal cell-derived factor-1 (CXCL12), the ligand for CXCR4. Activation of CXCR4 induces SCLC cell migration and adhesion to stromal cells that secrete CXCL12, which in turn provides growth- and drug resistance-signals to the tumor cells. CXCR4 antagonists, such as Plerixafor (AMD3100) and T140 analogues (TN14003/ BKT140), disrupt CXCR4-mediated SCLC cell-adhesion to stromal cells. In stromal cell co-cultures, CXCR4 antagonists also sensitize SCLC cells to cytotoxic drugs, such as etoposide, and thereby antagonize cell adhesion-mediated drug resistance. Therefore, targeting the CXCR4-CXCL12 axis is a novel, attractive therapeutic approach in SCLC. Here, we summarize preclinical data about CXCR4 in SCLC, and the current status of the preclinical and clinical development of CXCR4 antagonists.

HIV entry inhibitors and their potential in HIV therapy

This review discusses recent progress in the development of anti-HIV agents targeting the viral entry process. The three main classes (attachment inhibitors, co-receptor binding inhibitors, and fusion inhibitors) are further broken down by specific mechanism of action and structure. Many of these inhibitors are in advanced clinical trials, including the HIV maturation inhibitor bevirimat, from the authors' laboratories. In addition, the CCR5 inhibitor maraviroc has recently been FDA-approved. Possible roles for these agents in anti-HIV therapy, including treatment of virus resistant to current drugs, are also discussed.

Orally bioavailable isothioureas block function of the chemokine receptor CXCR4 in vitro and in vivo

The interaction of the chemokine receptor CXCR4 with its ligand CXCL12 is involved in many biological processes such as hematopoesis, migration of immune cells, as well as in cancer metastasis. CXCR4 also mediates the infection of T-cells with X4-tropic HIV functioning as a coreceptor for the viral envelope protein gp120. Here, we describe highly potent, selective CXCR4 inhibitors that block CXCR4/CXCL12 interactions in vitro and in vivo as well as the infection of target cells by X4-tropic HIV.

Chemogenomics with protein secondary-structure mimetics

During molecular recognition of proteins in biological systems, helices, reverse turns, and beta-sheets are dominant motifs. Often there are therapeutic reasons for blocking such recognition sites, and significant progress has been made by medicinal chemists in the design and synthesis of semirigid molecular scaffolds on which to display amino acid side chains. The basic premise is that preorganization of the competing ligand enhances the binding affinity and potential selectivity of the inhibitor. In this chapter, current progress in these efforts is reviewed.

CXCR4 antagonists: targeting the microenvironment in leukemia and other cancers

Hematopoietic and epithelial cancer cells express CXCR4, a seven-transmembrane G-protein-coupled chemokine receptor. Stromal cells within the bone marrow microenvironment constitutively secrete stromal cell-derived factor-1 (SDF-1/CXCL12), the ligand for CXCR4. Activation of CXCR4 induces leukemia cell trafficking and homing to the marrow microenvironment, where CXCL12 retains leukemia cells in close contact with marrow stromal cells that provide growth and drug resistance signals. CXCR4 antagonists, such as Plerixafor (AMD3100) and T140 analogs, can disrupt adhesive tumor-stroma interactions and mobilize leukemia cells from their protective stromal microenvironment, making them more accessible to conventional drugs. Therefore, targeting the CXCR4-CXCL12 axis is a novel, attractive therapeutic approach that is explored in ongoing clinical trials in leukemia patients. Initially, CXCR4 antagonists were developed for the treatment of HIV, where CXCR4 functions as a co-receptor for virus entry into T cells. Subsequently, CXCR4 antagonists were noticed to induce leukocytosis, and are currently used clinically for mobilization of hematopoietic stem cells. However, because CXCR4 plays a key role in cross-talk between leukemia cells (and a variety of other tumor cells) and their microenvironment, cancer treatment may become the ultimate application of CXCR4 antagonists. Here, we summarize the development of CXCR4 antagonists and their preclinical and clinical activities, focusing on leukemia and other cancers.

Bryostatin-5 blocks stromal cell-derived factor-1 induced chemotaxis via desensitization and down-regulation of cell surface CXCR4 receptors

The chemokine receptor CXCR4 and its ligand, stromal cell-derived factor-1 (SDF-1), play important roles in hematopoiesis regulation, lymphocyte activation, and trafficking, as well as in developmental processes, including organogenesis, vascularization, and embryogenesis. The receptor is also involved in HIV infection and tumor growth and metastasis. Antagonists of CXCR4 have been widely evaluated for drugs against HIV and tumors. In an effort to identify novel CXCR4 antagonists, we screened a small library of compounds derived from marine organisms and found bryostatin-5, which potently inhibits chemotaxis induced by SDF-1 in Jurkat cells. Bryostatin-5 is a member of the macrolactones, and its analogue bryostatin-1 is currently being evaluated in clinical trials for its chemotherapeutic potential. The involvement of bryostatins in the SDF-1/CXCR4 signaling process has never been reported. In this study, we found that bryostatin-5 potently inhibits SDF-1-induced chemotaxis but does not affect serum-induced chemotaxis. Further studies indicate that this inhibitory effect is not due to receptor antagonism but rather to bryostatin-5-induced receptor desensitization and down-regulation of cell surface CXCR4. We also show that these effects are mediated by the activation of conventional protein kinase C.

Chemokine receptor CXCR4 as a therapeutic target for neuroectodermal tumors

Chemokines (chemotactic cytokines) are a family of proteins associated with the trafficking and activation of leukocytes and other cell types in immune surveillance and inflammatory response. Besides their roles in the immune system, they play pleiotropic roles in tumor initiation, promotion, and progression. Chemokines can be classified into four subfamilies of chemokines, CXC, CC, C, or CX3C, based on their number and spacing of conserved cysteine residues near the N-terminus. This CXC subfamily can be further subclassified into two groups, depending on the presence or absence of a tripeptide motif glutamic acid-leucine-arginine (ELR) in the N-terminal domain. ELR(-)CXCL12, which binds to CXCR4 has been frequently implicated in various cancers. Over the past several years, studies have increasingly shown that the CXCR4/CXCL12 axis plays critical roles in tumor progression, such as invasion, angiogenesis, survival, homing to metastatic sites. This review focuses on involvement of CXCR4/CXCL12 interaction in neuroectodermal cancers and their therapeutic potentials. As an attractive therapeutic target of CXCR4/CXCL12 axis for cancer chemotherapy, development history and application of CXCR4 antagonists are described.

Structure-activity relationship study of CXCR4 antagonists bearing the cyclic pentapeptide scaffold: identification of the new pharmacophore

A highly potent CXCR4 antagonist 2 [cyclo (-D-Tyr1-Arg2-Arg3-Nal4-Gly5-)] has previously been identified by screening cyclic pentapeptide libraries that were designed based on pharmacophore residues of a 14-residue peptidic CXCR4 antagonist 1. In the present study, D-Tyr and Arg in peptide 2 were replaced by a bicyclic aromatic amino acid and a cationic amino acid, respectively, and their binding activity for CXCR4 was evaluated for identification of the novel pharmacophore.

Update on the development of HIV entry inhibitors

HIV fusion and entry are two steps in the viral lifecycle that can be targeted by several classes of antiviral drugs. The discovery of chemokines focused the attention on cellular co-receptors used by the virus for entering cells, and on the various steps of such processes that are subject to interactions with small molecules. Intense research has led to a wide range of effective compounds that are able to inhibit these initial steps of viral replication. All steps in the process of HIV entry into the cell may be targeted by specific compounds, grouped into three main classes (attachment inhibitors, co-receptor binding inhibitors and fusion inhibitors), which may be developed as novel antiretrovirals. Thus, several inhibitors of the gp120–CD4 interaction have been discovered (e.g., zintevir and BMS-378806). Small molecule chemokine receptor antagonists acting as HIV entry inhibitors have also been described recently, including those which interact with both the CXCR4 co-receptor (e.g., AMD3100, AMD3465, ALX40-4C, T22, T134 and T140) and CCR5 co-receptor antagonists (TAK-779, TAK-220, E913, AK-602 and NSC 651016 in clinical trials). Recently, a third family of antivirals started to be used clinically (in addition to reverse transcriptase and protease inhibitors), with the advent of enfuvirtide (T20), the first fusion inhibitor to be approved as an anti-HIV agent. Some of these compounds demonstrated in vitro synergism with other classes of antivirals, thus offering the rationale for their combination in therapies for HIV-infected individuals. Many HIV entry and fusion inhibitors are currently being investigated in controlled clinical trials, and a number of them are bioavailable as oral formulations. In 2007, the US FDA approved maraviroc as an anti-HIV agent. Maraviroc is the product of a medicinal chemistry effort initiated following identification of an imidazopyridine CCR5 ligand from a high-throughput screen of the Pfizer compound file. Maraviroc demonstrated potent antiviral activity against all CCR5-tropic HIV-1 viruses tested, including 43 primary isolates from various clades and diverse geographic origin. Maraviroc was active against 200 clinically derived HIV-1 envelope-recombinant pseudoviruses, 100 of which were derived from viruses resistant to existing drug classes. Furthermore, in October 2007, the FDA announced the approval of raltegravir for the treatment of HIV-1 infection as part of combination antiretroviral therapy in treatment-experienced patients with evidence of HIV-1 replication despite optimized background antiretroviral therapy. At present, raltegravir is the only drug in the integrase inhibitor class approved for clinical use. With the approval of raltegravir, oral agents targeting all three constitutive viral enzymes, reverse transcriptase, protease and integrase, are now represented in FDA-approved therapies.

Fluorophore labeling enables imaging and evaluation of specific CXCR4-ligand interaction at the cell membrane for fluorescence-based screening

Development of CXCR4-specific ligands is an important issue in chemotherapy of HIV infection, cancer metastasis, and rheumatoid arthritis, and numerous potential ligands have been developed to date. However, it is difficult to assess their binding mode and specificity because of uncertainties in the structure of the CXCR4-ligand complexes. To address this problem, we have synthesized fluorophore labeled Ac-TZ14011, which is derived from T140, a powerful CXCR4 antagonist. Binding of Ac-TZ14011 to CXCR4 on the cell membrane was observed by fluorescence microscope, and analysis of the binding data produced IC 50 values of several ligands comparable to those obtained in RI-based assays. This fluorescence-based assay is applicable to explore new pharmacophores of CXCR4-specific ligands with high-throughput screening and also to screening of the other GPCR binding ligands.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

Cell-Free Assay of G-Protein-Coupled Receptors Using Fluorescence Polarization

A recently developed nanotechnology, the Integral Molecular lipoparticle, provides an essentially soluble cell-free system in which G-protein-coupled receptors (GPCRs) in their native conformations are concentrated within virus-like particles. As a result, the lipoparticle provides a means to overcome 2 common obstacles to the development of homogeneous, nonradioactive GPCR ligand-binding assays: membrane protein solubilization and low receptor density. The work reported here describes the first application of this nanotechnology to a fluorescence polarization (FP) molecular binding assay format. The GPCR chosen for these studies was the well-studied chemokine receptor CXCR4 for which a peptide ligand (T-22) has been previously characterized. The EC50 determined for the CXCR4-T-22 peptide interaction via FP with CXCR4 lipoparticles (15 nM) is consistent with the IC50 determined for the unlabeled T-22 peptide via competitive binding (59 nM). ( Journal of Biomolecular Screening 2008:424-429)

Identification of novel non-peptide CXCR4 antagonists by ligand-based design approach

The design and synthesis of novel non-peptide CXCR4 antagonists is described. The peptide backbone of highly potent cyclic peptide-based CXCR4 antagonists was entirely replaced by an indole framework, which was expected to reproduce the disposition of the key pharmacophores consistent with those of potential bioactive conformations of the original peptides. A structure-activity relationship study on a series of modified indoles identified novel small-molecule antagonists having three pharmacophore functional groups through the appropriate linkers.

Privileged scaffolds targeting reverse-turn and helix recognition

BACKGROUND

Protein-protein interactions dominate molecular recognition in biologic systems. One major challenge for drug discovery arises from the very large surfaces that are characteristic of many protein-protein interactions.

OBJECTIVES

To identify 'drug-like' small molecule leads capable of modulating protein-protein interactions based on common protein-recognition motifs, such as alpha-helices, beta-strands, reverse-turns and polyproline motifs for example.

OVERVIEW

Many proteins/peptides are unstructured under physiologic conditions and only fold into ordered structures on binding to their cellular targets. Therefore, preorganization of an inhibitor into its protein-bound conformation reduces the entropy of binding and enhances the relative affinity of the inhibitor. Accordingly, this review describes a general strategy to address the challenge based on the 'privileged structure hypothesis' [Che, PhD thesis, Washington University, 2003] that chemical templates capable of mimicking surfaces of protein-recognition motifs are potential privileged scaffolds as small-molecule inhibitors of protein-protein interactions. The authors highlight recent advances in the design of privileged scaffolds targeting reverse-turn and helical recognition.

CONCLUSIONS

Privileged scaffolds targeting common protein-recognition motifs are useful to help elucidate the receptor-bound conformation and to provide non-peptidic, bioavailable substructures suitable for optimization to modulate protein-protein interactions.

Exploratory studies on development of the chemokine receptor CXCR4 antagonists toward downsizing

Seven transmembrane (7TM) G-protein-coupled receptor (GPCR) families are important targets for drug discovery, and specific antagonists for GPCR can accelerate research in the field of medicinal chemistry. The chemokine receptor CXCR4 is a GPCR that possesses a unique ligand CXCL12/stromal cell-derived factor-1 (SDF-1). The interaction between CXCL12 and CXCR4 is essential for the migration of progenitor cells during embryonic development of the cardiovascular, hemopoietic and central nervous systems, and also involved in several intractable disease processes, including HIV infection, cancer cell metastasis, progression of acute and chronic leukemias, rheumatoid arthritis and pulmonary fibrosis. Thus, CXCR4 may be an important therapeutic target in all of these diseases, and various CXCR4 antagonists have been proposed as potential drugs. Fourteen-mer peptides, T140 and its analogs, and downsized cyclic pentapeptides have been developed by us as potent CXCR4 antagonists. This article describes the development of a number of specific CXCR4 antagonists in our laboratory, including downsizing.

Neutralizing antibodies decrease the envelope fluidity of HIV-1

For successful penetration of HIV-1, the formation of a fusion pore may be required in order to accumulate critical numbers of fusion-activated gp41 with the help of fluidization of the plasma membrane and viral envelope. An increase in temperature to 40 degrees C after viral adsorption at 25 degrees C enhanced the infectivity by 1.4-fold. The enhanced infectivity was inhibited by an anti-CXCR4 peptide, T140, and anti-V3 monoclonal antibodies (0.5beta and 694/98-D) by post-attachment neutralization, but not by non-neutralizing antibodies (670-30D and 246-D) specific for the C5 of gp120 and cluster I of gp41, respectively. Anti-HLA-II and an anti-HTLV-I gp46 antibody, LAT27, neutralized the molecule-carrying HIV-1(C-2(MT-2)). The anti-V3 antibodies suppressed the fluidity of the HIV-1(C-2) envelope, whereas the non-neutralizing antibodies did not. The anti-HLA-II antibody decreased the envelope fluidity of HIV-1(C-2(MT-2)), but not that of HIV-1(C-2). Therefore, fluidity suppression by these antibodies represents an important neutralization mechanism, in addition to inhibition of viral attachment.

Dual-reporter phenotypic assay for human immunodeficiency viruses

We have established a novel human immunodeficiency virus (HIV) tandem-reporter assay using HIV receptor-transduced NP-2 cells with long terminal repeat-controlled beta-galactosidase, inserted internal ribosome entry site, and secretary alkaline phosphatase genes. This assay allows users to detect replication of clinical isolates, indicating its useful application as an HIV phenotypic assay.

Altered sensitivity of an R5X4 HIV-1 strain 89.6 to coreceptor inhibitors by a single amino acid substitution in the V3 region of gp120

The replication of several R5X4 strains is blocked by single CXCR4 inhibitors such as AMD3100 or T140 although the target cells express both CXCR4 and CCR5 in vitro. To identify which region(s) of the Env are involved in the increased sensitivity to CXCR4 inhibitors, we isolated a T140-escape mutant using R5X4 HIV-1 strain 89.6. An isolated mutant harbored a single amino acid substitution in the V3 region of the Env (arginine 308 to serine R308S). Luciferase-reporter HIV-1 pseudotyped with the mutant Env showed that the substitution conferred total resistance to CXCR4 antagonists but increased sensitivity to a CCR5 antagonist TAK-779 in the infection of the cells expressing both CCR5 and CXCR4. Analyses using the cells expressing a single coreceptor showed that the mutant Env predominantly and efficiently utilized CCR5 rather than CXCR4 while retaining R5X4 phenotype. These results indicated that the sensitivities of the R5X4 strain to coreceptor inhibitors were altered by a single amino acid substitution in the V3 region of gp120.

Tumor angiogenesis is associated with plasma levels of stromal-derived factor-1alpha in patients with multiple myeloma

PURPOSE

Multiple myeloma is an incurable hematologic malignancy characterized by increased bone marrow angiogenesis and extensive lytic bone disease. We have previously shown that elevated levels of stromal-derived factor-1alpha (SDF-1alpha) in peripheral blood plasma are associated with osteolysis in multiple myeloma patients. We have now examined whether SDF-1alpha levels also correlate with angiogenesis.

EXPERIMENTAL DESIGN

We examined the contribution of multiple myeloma plasma cell-derived SDF-1alpha in the stimulation of in vitro angiogenesis using a tube formation assay. We also collected trephine and peripheral blood plasma samples from patients with multiple myeloma to analyze microvessel density and SDF-1alpha levels, respectively.

RESULTS

We show that multiple myeloma plasma cell line-derived conditioned medium containing SDF-1alpha stimulates in vitro angiogenesis. In addition, in a large cohort of patients with multiple myeloma and its precursor condition monoclonal gammopathy of undetermined significance, we confirm previous findings that plasma cell burden correlates with both angiogenesis and plasma levels of SDF-1alpha. We now extend these observations and show the novel finding that peripheral blood plasma levels of SDF-1alpha positively correlate with the degree of bone marrow angiogenesis in multiple myeloma and monoclonal gammopathy of undetermined significance patients.

CONCLUSIONS

High levels of SDF-1alpha produced by multiple myeloma plasma cells promote osteolysis and bone marrow angiogenesis. Therefore, we propose that inhibition of SDF-1alpha may be an effective mechanism by which angiogenesis and osteolysis can be reduced in multiple myeloma patients.

The CXCR4 chemokine receptor in acute and chronic leukaemia: a marrow homing receptor and potential therapeutic target

Chemokine (C-X-C motif) receptor 4 (CXCR4) is essential for homing and maintenance of haematopoietic stem cells in distinct stromal cell niches within the marrow. Chemotactic responsiveness of haematopoietic stem cells is restricted to the ligand for CXCR4, stromal cell-derived factor-1 (SDF-1/CXCL12), which is constitutively secreted by marrow stromal cells. Myeloid and lymphoid leukaemia cells also express CXCR4 that induces leukaemia cell chemotaxis and migration beneath marrow stromal cells. CXCR4 expression levels have a major prognostic impact in acute myeloid leukaemia. There is growing in vitro and in vivo evidence that CXCR4 expression by leukaemia cells allows for homing and their retention within the marrow. As such, leukaemia cells appear to utilise CXCR4 to access niches that are normally restricted to progenitor cells, and thereby reside in a microenvironment that favours their growth and survival. CXCR4- and integrin-mediated contact between leukaemia cells and stromal cells protects leukaemia cells from spontaneous and chemotherapy-induced cell death and therefore may represent a mechanism to explain minimal residual disease and subsequent relapses commonly seen in the treatment of these diseases. This review summarises our current knowledge regarding the importance of CXCR4 in acute and chronic leukaemia, discusses the importance of CXCR4 detection by flow cytometry in the diagnostic workup of leukaemia patients, and introduces the potential role of CXCR4-targeting compounds for the treatment of leukaemia patients.

Therapeutic potential of the chemokine receptor CXCR4 antagonists as multifunctional agents

The chemokine receptor CXCR4 possesses multiple critical functions in normal and pathologic physiology. CXCR4 is a G-protein-coupled receptor that transduces signals of its endogenous ligand, the chemokine CXCL12 (stromal cell-derived factor-1, SDF-1). The interaction between CXCL12 and CXCR4 plays an important role in the migration of progenitors during embryologic development of the cardiovascular, hemopoietic, central nervous systems, and so on. This interaction is also known to be involved in several intractable disease processes, including HIV infection, cancer cell metastasis, leukemia cell progression, rheumatoid arthritis (RA), and pulmonary fibrosis. It is conjectured that this interaction may be a critical therapeutic target in all of these diseases, and several CXCR4 antagonists have been proposed as potential drugs. Fourteen-mer peptides, T140 and its analogues, were previously developed in our laboratory as specific CXCR4 antagonists that were identified as HIV-entry inhibitors, anti-cancer-metastatic agents, anti-chronic lymphocytic/acute lymphoblastic leukemia agents, and anti-RA agents. Cyclic pentapeptides, such as FC131 [cyclo(D-Tyr-Arg-Arg-L-3-(2-naphthyl)alanine-Gly)], were also previously found as CXCR4 antagonist leads based on pharmacophores of T140. This review article describes the elucidation of multiple functions of CXCR4 antagonists and the development of a number of low-molecular weight CXCR4 antagonists involving FC131 analogues and other compounds with different scaffolds including linear-type structures.

Design and synthesis of all diastereomers of cyclic pseudo-dipeptides as mimics of cyclic CXCR4 pentapeptide antagonists

The four diastereomers of 2,5-bis[(3-guanidino)propyl]-1-[3-(4-hydroxyphenyl)propionyl]-7-(2-naphthylacetyl)-1,4,7-triazacycloundec-9-en-3-one (-) and of 2,5-bis[(3-guanidino)propyl]-1-(4-hydroxyphenylacetyl)-7-(2-naphthylacetyl)-1,4,7-triazacycloundec-9-en-3-one (-) were synthesized by a divergent methodology from l- and D-glutamic acids. The 11-membered ring core was made by ring closing metathesis of linear bis(allylamines), and the guanidyl functions were introduced by a simultaneous double Mitsunobu reaction using bis(Boc)guanidine. These compounds were designed to mimic cyclic pentapeptide FC131 (c[Gly-D-Tyr-Arg-Arg-Nal]).

Stromal-derived factor-1 (CXCL12) regulates laminar position of Cajal-Retzius cells in normal and dysplastic brains

Normal brain development requires a series of highly complex and interrelated steps. This process presents many opportunities for errors to occur, which could result in developmental defects in the brain, clinically referred to as malformations of cortical development. The marginal zone and Cajal-Retzius cells are key players in cortical development and are established early, yet there is little understanding of the factors resulting in the disruption of the marginal zone in many types of cortical malformation syndromes. We showed previously that treatment with methylazoxymethanol in rats causes marginal zone dysplasia with displacement of Cajal-Retzius cells to deeper cortical layers. Here we establish that loss of activity of the chemokine stromal-derived factor-1 (SDF1) (CXCL12), which is expressed by the leptomeninges, is necessary and sufficient to cause marginal zone disorganization in this widely used teratogenic animal model. We also found that mice with mutations in the main receptor for SDF1 (CXCR4) have Cajal-Retzius cells displaced to deeper cortical layers. Furthermore, by inhibiting SDF1 signaling in utero by intraventricular injection of a receptor antagonist, we establish that SDF1 signaling is required for the maintenance of Cajal-Retzius cell position in the marginal zone during normal cortical development. Our data imply that cortical layering is not a static process, but rather requires input from locally produced molecular cues for maintenance, and that complex syndromes of cortical malformation as a result of environmental insults may still be amenable to explanation by interruption of specific molecular signaling pathways.

HIV co-receptor inhibitors as novel class of anti-HIV drugs

Entry inhibitors constitute a new class of drugs to treat infection by human immunodeficiency virus type 1 (HIV-1). The first member of this class, enfuvirtide, previously known as T-20 and targeting gp41, has now been licensed for therapeutic use. Several other entry inhibitors are in various stages of pre-clinical or clinical development. In this review we focus on the chemokine receptor inhibitors targeting CCR5 and CXCR4 that are the main HIV co-receptors for viral entry.

Peptide antimicrobial agents

Antimicrobial host defense peptides are produced by all complex organisms as well as some microbes and have diverse and complex antimicrobial activities. Collectively these peptides demonstrate a broad range of antiviral and antibacterial activities and modes of action, and it is important to distinguish between direct microbicidal and indirect activities against such pathogens. The structural requirements of peptides for antiviral and antibacterial activities are evaluated in light of the diverse set of primary and secondary structures described for host defense peptides. Peptides with antifungal and antiparasitic activities are discussed in less detail, although the broad-spectrum activities of such peptides indicate that they are important host defense molecules. Knowledge regarding the relationship between peptide structure and function as well as their mechanism of action is being applied in the design of antimicrobial peptide variants as potential novel therapeutic agents.

Insight into the binding mode for cyclopentapeptide antagonists of the CXCR4 receptor

The finding that the chemokine receptor CXCR4 is involved in T-cell HIV entry has encouraged the development of antiretroviral drugs targeting this receptor. Additional evidence that CXCR4 plays a crucial role in both angiogenesis and metastasis provides further motivation for the development of a CXCR4 inhibitor for therapeutic applications in oncology. To facilitate the design of such ligands, we have investigated the possible binding modes for cyclopentapeptide CXCR4 antagonists by docking 11 high/medium affinity cyclopentapeptides to a developed three-dimensional model of the CXCR4 G-protein-coupled receptor's transmembrane region. These ligands, expected to bind in the same mode to the receptor, were docked in the previously deduced receptor-bound conformation [Våbenøet al., in press; doi 10.1002/bip.20508]. Ligand-receptor complexes were generated using an automated docking procedure that allowed ligand flexibility. By comparing the resulting ligand poses, only two binding modes common for all 11 compounds were identified. Inspection of these two ligand-receptor complexes identified several CXCR4 contact residues shown by mutation to be interaction sites for ligands and important for HIV gp120 binding. Thus, the results provide further insight into the mechanism by which these cyclopentapeptides block HIV entry as well as a basis for rational design of CXCR4 mutants to map potential contacts with small peptide ligands.

The therapeutic potential of CXCR4 antagonists in the treatment of HIV infection, cancer metastasis and rheumatoid arthritis

CXCR4 is the receptor of the chemokine CXCL12, which is involved in progression and metastasis of several types of cancer cells, HIV infection and rheumatoid arthritis. The authors developed selective CXCR4 antagonists, T22 and T140, initially as anti-HIV agents, which inhibit T cell line-tropic (X4-) HIV-1 infection through their specific binding to CXCR4. Recently, T140 analogues have also been shown to inhibit CXCL12-induced migration of breast cancer cells, leukaemia T cells, pancreatic cancer cells, small cell lung cancer cells, chronic lymphocytic leukaemia B cells, pre-B acute lymphoblastic leukaemia cells and so on in vitro. Biostable T140 analogues significantly suppressed pulmonary metastasis of breast cancer cells and melanoma cells in mice. Furthermore, these compounds significantly suppressed the delayed-type hypersensitivity response induced by sheep red blood cells and collagen-induced arthritis, which represent in vivo mouse models of arthritis. Thus, T140 analogues proved to be attractive lead compounds for chemotherapy of these problematic diseases. This article reviews recent research on T140 analogues, referring to several other CXCR4 antagonists.

CXCR4 and CCR5 mediate homing of primitive bone marrow-derived hematopoietic cells to the postnatal thymus

Factors governing the entry of cells into the postnatal thymus are poorly understood. We aimed to define molecular mechanisms mediating the homing of bone marrow cells to the thymus using a sublethally irradiated in vivo murine model. Entry of unfractionated and lineage-depleted bone marrow cells to the thymus, but not bone marrow, was a Galphai-mediated phenomenon. Lineage-depleted cells that had homed to the thymus expressed abundant CXCR4 and CCR5 mRNA, alone of 17 chemokine receptors evaluated by QPCR. Thymic-homed cells were distinct from cells that had homed to bone marrow in expression of CXCR4 and CCR5 by mRNA quantification and cell-surface expression of protein. Abrogation of CXCR4 and CCR5 function by genetic, antibody, or pharmacologic means impaired homing of lineage-depleted cells to the thymus, although not in a synergistic manner, implying interdependency of these receptors in the homing process. Competitive repopulation experiments demonstrated that inhibiting CXCR4-mediated homing adversely affected the double-negative cell pool at 2 weeks, suggesting that cells with prothymocytic activity may in part home via CXCR4. Overall, our data demonstrate differential homing mechanisms governing entry of unfractionated and lineage-depleted cells to irradiated bone marrow or thymus, with thymic homing of immature cells being pertussis-sensitive and mediated by the chemokine receptors CXCR4 and CCR5.

Identification of a New Class of Low Molecular Weight Antagonists against the Chemokine Receptor CXCR4 Having the Dipicolylamine−Zinc(II) Complex Structure

Several low molecular weight nonpeptide compounds having the dipicolylamine-zinc(II) complex structure were identified as potent and selective antagonists of the chemokine receptor CXCR4. These compounds showed strong inhibitory activity against CXCL12 binding to CXCR4, and the top compound exhibited significant anti-HIV activity. Zinc(II)-dipicolylamine unit-containing compounds proved to be useful and attractive lead compounds for chemotherapy of these diseases as nonpeptide CXCR4 antagonists possessing the novel scaffold structure.

Development of a 111In-labeled peptide derivative targeting a chemokine receptor, CXCR4, for imaging tumors

The chemokine receptor CXCR4 is highly expressed in tumor cells and plays an important role in tumor metastasis. The aim of this study was to develop a radiopharmaceutical for the imaging of CXCR4-expressing tumors in vivo. Based on structure-activity relationships, we designed a 14-residue peptidic CXCR4 inhibitor, Ac-TZ14011, as a precursor for radiolabeled peptides. For 111In-labeling, diethylenetriaminepentaacetic acid (DTPA) was attached to the side chain of d-Lys(8) which is distant from the residues indispensable for the antagonistic activity. In-DTPA-Ac-TZ14011 inhibited the binding of a natural ligand, stromal cell-derived factor-1alpha, to CXCR4 in a concentration-dependent manner with an IC50 of 7.9 nM (Ac-TZ14011: 1.2 nM). In biodistribution experiments, more 111In-DTPA-Ac-TZ14011 accumulated in the CXCR4-expressing tumor than in blood or muscle. Furthermore, the tumor-to-blood and tumor-to-muscle ratios were significantly reduced by coinjection of Ac-TZ14011, indicating a CXCR4-mediated accumulation in tumor. These findings suggested that 111In-DTPA-Ac-TZ14011 would be a potential agent for the imaging of CXCR4 expression in metastatic tumors in vivo.

Differential expression of CXCR4 is associated with the metastatic potential of human non-small cell lung cancer cells

PURPOSE

To evaluate the relation between CXCR4 expression and the presence of metastatic disease in human non-small cell lung cancer (NSCLC) patients and investigate whether modulation of CXCR4 expression could serve as a potential pathway in preventing metastasis of NSCLC.

EXPERIMENTAL DESIGN

CXCR4 expression in 36 patients with NSCLC and 10 normal lung tissues was detected by real-time PCR and immunohistochemistry. CXCR4 expression in two human NSCLC clones (95C and 95D) with different metastatic potential was determined by real-time PCR and flow cytometry. 95C and 95D cells were transfected with the plasmid DNA containing CXCR4 coding gene or CXCR4 antisense nucleotide fragment, respectively, and the effects on in vitro cell migration, invasion, and adhesion and in vivo metastasis were measured.

RESULTS

Up-regulated expression of CXCR4 was detected in 34 tumors, which were further divided into 17 high expression cancers and 17 low expression cancers by their staining intensities. High CXCR4 tumors (13 of 17) were more prone to clinical metastasis in comparison with low expression tumors. CXCR4 was differentially expressed in 95C and 95D cells with low or high metastatic potential, and the surface expression of CXCR4 were 50% up-regulated or down-regulated following the stable transfection. The metastatic potential of NSCLC in vitro, such as migration, invasion, and adhesion, were significantly enhanced or impaired. In addition, neutralizing the interactions of stromal cell-derived factor-1/CXCR4 in vitro with CXCR4-specific antibodies inhibited the CXCR4-dependent migration, invasion, and adhesion. Furthermore, s.c. inoculation of lung cancer cells with low expression of CXCR4 in nude mice showed 0- to 2-fold decrease in lung metastatic foci than that with high expression of CXCR4.

CONCLUSIONS

Differential expression of CXCR4 is associated with the metastatic potential of human NSCLC, raising the possibility that blockade of CXCR4/stromal cell-derived factor-1 interaction may lead the way to design novel therapeutic tools for the treatment of metastatic NSCLC patients.

A minimalistic 3D pharmacophore model for cyclopentapeptide CXCR4 antagonists

Because of its involvement in HIV entry, the chemokine receptor CXCR4 is an attractive target for antiretroviral drugs. Despite the large number of CXCR4 inhibitors studied, the 3D pharmacophore for binding to CXCR4 remains elusive, mainly as a result of conformational flexibility inherent in the identified ligands. In the present study, an exhaustive systematic exploration of the conformational space for a series of analogs of FC131, a cyclopentapeptide CXCR4 antagonist, has been performed. By comparing the resulting low-energy conformations using different sets of atoms, specific conformational features common only to the high/medium affinity compounds were identified. These features included the spatial arrangement of three pharmacophoric side chains as well as the orientation of a specific backbone amide bond. Together these features represent a minimalistic 3D pharmacophore model for binding of the cyclopentapeptide antagonists to CXCR4. The model enables rationalization of the experimental affinity data for this class of compounds as well as for the peptidomimetic KRH-1636.

Development of a linear type of low molecular weight CXCR4 antagonists based on T140 analogs

A linear type of several low molecular weight CXCR4 antagonists were developed based on T140 analogs, which were previously found to be strong CXCR4 antagonists that block X4-HIV-1 entry and have inhibitory activities against cancer metastasis/progression and rheumatoid arthritis.

Mutations in both env and gag genes are required for HIV-1 resistance to the polysulfonic dendrimer SPL2923, as corroborated by chimeric virus technology

A drug-resistant NL4.3/SPL2923 strain has previously been generated by in vitro selection of HIV-1(NL4.3) in the presence of the polysulfonic dendrimer SPL2923 and mutations were reported in its gp120 gene (Witvrouw et al., 2000). Here, we further analysed the (cross) resistance profile of NL4.3/SPL2923. NL4.3/SPL2923 was found to contain additional mutations in gp41 and showed reduced susceptibility to SPL2923, dextran sulfate (DS) and enfuvirtide. To delineate to what extent the mutations in each env gene were accountable for the phenotypic (cross) resistance of NL4.3/SPL2923, the gp120-, gp41- and gp160-sequences derived from this strain were placed into a wild-type background using env chimeric virus technology (CVT). The cross resistance of NL4.3/SPL2923 towards DS was fully reproduced following gp160-recombination, while it was only partially reproduced following gp120- or gp41-recombination. The mutations in gp41 of NL4.3/SPL2923 were sufficient to reproduce the cross resistance to enfuvirtide. Unexpectedly, the reduced sensitivity towards SPL2923 was not fully reproduced after gp160-recombination. The search for mutations in NL4.3/SPL2923 in viral genes other than env revealed several mutations in the gene encoding the HIV p17 matrix protein (MA) and one mutation in the gene encoding the p24 capsid protein (CA). In order to analyse the impact of the gag mutations alone and in combination with the mutations in env on the phenotypic resistance towards SPL2923, we developed a novel p17- and p17/gp160-CVT. Phenotypic analysis of the NL4.3/SPL2923 p17- and p17/gp160-recombined strains indicated that the mutations in both env and gag have to be present to fully reproduce the resistance of NL4.3/SPL2923 towards SPL2923.

Structure-activity relationship studies on CXCR4 antagonists having cyclic pentapeptide scaffolds

Structure-activity relationship studies on CXCR4 antagonists, which were previously found by using cyclic pentapeptide libraries, were performed to optimize side-chain functional groups, involving conformationally constrained analogues. In addition, a new lead of cyclic pentapeptides with the introduction of a novel pharmacophore was developed.

Prospective identification of cardiac progenitors by a novel single cell-based cardiomyocyte induction

Dissection of cardiomyocyte differentiation process at the cellular level is indispensable in the research for cardiac development and regeneration. Previously, we have established an embryonic stem cell differentiation system that reproduces early vascular development from progenitor cells that express Flk1, a vascular endothelial growth factor receptor, by the combinatory application of 2-dimensional culture and flowcytometry. Here we show that cardiomyocytes can be successfully induced from a single Flk1+ cell on 2-dimensional culture, enabling the direct observation of differentiating cardiomyocytes and the prospective identification of cardiac progenitor potentials. Flk1+ cells could give rise to cardiomyocytes, as well as endothelial cells, from a single cell by the co-culture on OP9 stroma cells in a fusion-independent manner. Among the cell populations in intermediate stages from Flk1+ cells to cardiomyocytes, Flk1+/CXCR4+/vascular endothelial cadherin- cells were cardiac-specific progenitors at the single cell level. Noggin, a bone morphogenetic protein inhibitor, abolished cardiomyocyte differentiation by inhibiting the cardiac progenitor induction. However, wnt inhibitors Dkk-1 or Frizzled-8/Fc chimeric protein augmented, but wnt3a inhibited, cardiomyocyte differentiation. In vitro reproduction of cardiomyocyte differentiation process should be a potent tool for the cellular and molecular elucidation of cardiac development, which would provide various targets for cardiac regeneration.

HIV chemokine receptor inhibitors as novel anti-HIV drugs

The chemokine receptors CXCR4 and CCR5 are the main coreceptors used by the T-cell-tropic (CXCR4-using, X4) and macrophage-tropic (CCR5-using, R5) HIV-1 strains, respectively, for entering their CD4+ target cells. In this review, we focus on the function of these chemokine receptors in HIV infection and their role as novel targets for viral inhibition. Besides some modified chemokines with antiviral activity, several low-molecular weight CCR5 and CXCR4 antagonistic compounds have been described with potent antiviral activity. The best CXCR4 antagonists described are the bicyclam derivatives, which consistently block X4 but also R5/X4 viral replication in PBMCs. We believe that chemokine receptor antagonists will become important new antiviral drugs to combat AIDS. Both CXCR4 and CCR5 chemokine receptor inhibitors will be needed in combination and even in combinations of antiviral drugs that also target other aspects of the HIV replication cycle to obtain optimum antiviral therapeutic effects.

Identification of novel low molecular weight CXCR4 antagonists by structural tuning of cyclic tetrapeptide scaffolds

A highly potent CXCR4 antagonist, compound 2, was previously found by using two orthogonal cyclic pentapeptide libraries involving conformation-based and sequence-based libraries based on the pharmacophore of a 14-mer peptidic antagonist, 1. Herein, cyclic tetrapeptides derived from replacements of the dipeptide unit (Nal-Gly) with a gamma-amino acid and pseudopeptides cyclized by disulfide and olefin bridges were synthesized to find novel scaffold structures different from that of cyclic pentapeptides. These compounds contain a reduced number of peptide bonds compared to compound 2. Furthermore, several analogues with chemical modification of the side chain of Arg(4) in 2 were also prepared. From these, several new leads possessing high to moderate CXCR4-antagonistic activity were characterized.

Influence of membrane fluidity on human immunodeficiency virus type 1 entry

For penetration of human immunodeficiency virus type 1 (HIV-1), formation of fusion-pores might be required for accumulating critical numbers of fusion-activated gp41, followed by multiple-site binding of gp120 with receptors, with the help of fluidization of the plasma membrane and viral envelope. Correlation between HIV-1 infectivity and fluidity was observed by treatment of fluidity-modulators, indicating that infectivity was dependent on fluidity. A 5% decrease in fluidity suppressed the HIV-1 infectivity by 56%. Contrarily, a 5% increase in fluidity augmented the infectivity by 2.4-fold. An increased temperature of 40 degrees C or treatment of 0.2% xylocaine after viral adsorption at room temperature enhanced the infectivity by 2.6- and 1.5-fold, respectively. These were inhibited by anti-CXCR4 peptide, implying that multiple-site binding was accelerated at 40 degrees C or by xylocaine. Thus, fluidity of both the plasma membrane and viral envelope was required to form the fusion-pore and to complete the entry of HIV-1.

Elevated serum levels of stromal-derived factor-1alpha are associated with increased osteoclast activity and osteolytic bone disease in multiple myeloma patients

Multiple myeloma (MM) is an incurable plasma cell (PC) malignancy able to mediate massive destruction of the axial and craniofacial skeleton. The aim of this study was to investigate the role of the potent chemokine, stromal-derived factor-1alpha (SDF-1alpha) in the recruitment of osteoclast precursors to the bone marrow. Our studies show that MM PC produce significant levels of SDF-1alpha protein and exhibit elevated plasma levels of SDF-1alpha when compared with normal, age-matched subjects. The level of SDF-1alpha positively correlated with the presence of multiple radiological bone lesions in individuals with MM, suggesting a potential role for SDF-1alpha in osteoclast precursor recruitment and activation. To examine this further, peripheral blood-derived CD14+ osteoclast precursors were cultured in an in vitro osteoclast-potentiating culture system in the presence of recombinant human SDF-1alpha. Although failing to stimulate an increase in TRAP+, multinucleated osteoclast formation, our studies show that SDF-1alpha mediated a dramatic increase in both the number and the size of the resorption lacunae formed. The increased osteoclast motility and activation in response to SDF-1alpha was associated with an increase in the expression of a number of osteoclast activation-related genes, including RANKL, RANK, TRAP, MMP-9, CA-II, and Cathepsin K. Importantly, the small-molecule CXCR4-specific inhibitor, 4F-Benzoyl-TE14011 (T140), effectively blocked osteoclast formation stimulated by the myeloma cell line, RPMI-8226. Based on these findings, we believe that the synthesis of high levels of SDF-1alpha by MM PC may serve to recruit osteoclast precursors to local sites within the bone marrow and enhance their motility and bone-resorbing activity. Therefore, we propose that inhibition of the CXCR4-SDF-1alpha axis may provide an effective means of treatment for MM-induced osteolysis.