Bivalent IAP antagonists, but not monovalent IAP antagonists, inhibit TNF-mediated NF-κB signaling by degrading TRAF2-associated cIAP1 in cancer cells.

The inhibitor of apoptosis (IAP) proteins have pivotal roles in cell proliferation and differentiation, and antagonizing IAPs in certain cancer cell lines results in induction of cell death. A variety of IAP antagonist compounds targeting the baculovirus IAP protein repeat 3 (BIR3) domain of cIAP1have advanced into clinical trials. Here we sought to compare and contrast the biochemical activities of selected monovalent and bivalent IAP antagonists with the intent of identifying functional differences between these two classes of IAP antagonist drug candidates. The anti-cellular IAP1 (cIAP1) and pro-apoptotic activities of monovalent IAP antagonists were increased by using a single covalent bond to combine the monovalent moieties at the P4 position. In addition, regardless of drug concentration, treatment with monovalent compounds resulted in consistently higher levels of residual cIAP1 compared with that seen following bivalent compound treatment. We found that the remaining residual cIAP1 following monovalent compound treatment was predominantly tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2)-associated cIAP1. As a consequence, bivalent compounds were more effective at inhibiting TNF-induced activation of p65/NF-κB compared with monovalent compounds. Moreover, extension of the linker chain at the P4 position of bivalent compounds resulted in a decreased ability to degrade TRAF2-associated cIAP1 in a manner similar to monovalent compounds. This result implied that specific bivalent IAP antagonists but not monovalent compounds were capable of inducing formation of a cIAP1 E3 ubiquitin ligase complex with the capacity to effectively degrade TRAF2-associated cIAP1. These results further suggested that only certain bivalent IAP antagonists are preferred for the targeting of TNF-dependent signaling for the treatment of cancer or infectious diseases.

Recent advances in the treatment of rhinovirus infections.

Rhinoviruses are the most common causes of viral respiratory infections and complications caused by viral respiratory infections in patients with underlying lung disease. Major recent therapeutic advances include the development of capsid-function inhibitors (pleconaril), inhibitors of 3C protease (AG7088), and recombinant soluble intercellular adhesion molecule (sICAM)-1, all of which exhibit potent antirhinoviral activity in vitro and varying activity in clinical trials. Pleconaril and AG7088 have shown the most promise and are the most advanced in clinical trials.

Human rhinovirus 14 complexed with fragments of active antiviral compounds.

Crystallographic studies of human rhinovirus 14 (HRV14) crystals soaked with fragments of antiviral WIN compounds, at high concentrations (82-200 micrograms/ml), show the compounds bind into the hydrophobic beta-barrel (WIN pocket) of VP1. Two of these short compounds (5-[3,5-dimethyl-4-hydroxyphenyl]-2-methyltetrazole and phenol oxazoline) cause conformational changes in the virus similar to the active, longer WIN compounds. In addition, thermostabilization studies suggest these short WIN compounds provide some stability to the HRV14 capsid. We conclude that the short compounds appear to mimic the cellular cofactors observed in the hydrophobic pocket of VP1 for some picornaviruses. Both cofactors and short WIN compounds bind into the pocket, cause conformational changes in VP1, and provide a small degree of virion stabilization but are unlikely to inhibit attachment. Three specific binding sites for dimethyl sulfoxide (DMSO), used as solvent, were also identified. One of the DMSO molecules binds into the drug binding pocket near the pocket opening, while the other two bind in the canyon near the VP1 protomer-protomer interface.

The structure of human rhinovirus 16.

BACKGROUND: Rhinoviruses and the homologous polioviruses have hydrophobic pockets below their receptor-binding sites, which often contain unidentified electron density ('pocket factors'). Certain antiviral compounds also bind in the pocket, displacing the pocket factor and inhibiting uncoating. However, human rhinovirus (HRV)14, which belongs to the major group of rhinoviruses that use intercellular adhesion molecule-1 (ICAM-1) as a receptor, has an empty pocket. When antiviral compounds bind into the empty pocket of HRV14, the roof of the pocket, which is also the floor of the receptor binding site (the canyon), is deformed, preventing receptor attachment. The role of the pocket in viral infectivity is not known. RESULTS: We have determined the structure of HRV16, another major receptor group rhinovirus serotype, to atomic resolution. Unlike HRV14, the pockets contain electron density resembling a fatty acid, eight or more carbon atoms long. Binding of the antiviral compound WIN 56291 does not cause deformation of the pocket, although it does prevent receptor attachment. CONCLUSIONS: We conjecture that the binding of the receptor to HRV16 can occur only when the pocket is temporarily empty, when it is possible for the canyon floor to be deformed downwards into the pocket. We further propose that the role of the pocket factor is to stabilize virus in transit from one host cell to the next, and that binding of ICAM-1 traps the pocket in the empty state, destabilizing the virus as required for uncoating.

Discovery and development of antipicornaviral agents.

The rhinovirus and enterovirus members of the picornavirus family are responsible for the majority of the mild upper respiratory infections most often referred to as the common cold as well as more serious illnesses including myocarditis. Much progress has been made in the identification and development of antiviral agents which specifically target the capsid of the picornaviruses. Preclinical and clinical efficacy data will be discussed on the WIN series of antiviral agents which were the result of a conventional drug discovery approach. The design of new agents is being assisted by the availability of atomic resolution data on the interaction of this class of antiviral agents with the viral capsid.

Application of crystallography to the design of antiviral agents.

A historical review of x-ray crystallography introduces basic concepts and describes the potential of this science to the rational design of antiviral agents. Following a brief introduction to the study of antiviral compounds that inhibit early stages of rhino- and enteroviral infections, the impact of structural studies on rational drug design is discussed in relation to known viral structures.

Treatment of the picornavirus common cold by inhibitors of viral uncoating and attachment.

The human rhinoviruses are the leading cause of the ubiquitous, mild, and self-limiting infections generally referred to as the common cold. Considerable research effort has been expended in the search for well tolerated antiviral agents capable of preventing and treating the common cold. Although no antirhinovirus drug is yet commercially available, considerable progress has been made in the discovery and development of novel, viral specific inhibitors of rhinovirus replication. This report reviews the history and current status of the research that has focused on inhibitors of the early steps in the virus life cycle: attachment to the cellular receptor and uncoating of the viral RNA. Molecules directed at these targets currently possess the greatest potential for generating a safe and efficacious treatment for the rhinovirus common cold.

Binding affinities of structurally related human rhinovirus capsid-binding compounds are related to their activities against human rhinovirus type 14.

The binding affinities (Kds) and the rates of association and dissociation of members of a chemical class of antiviral compounds at their active sites in human rhinovirus type 14 (HRV-14) were determined. On the basis of analysis by LIGAND, a nonlinear curve-fitting program, of saturation binding experiments with HRV-14, the Kds for Win 52084, Win 56590, disoxaril (Win 51711), and Win 54954 were found to be 0.02, 0.02, 0.08, and 0.22 microM, respectively. The independently determined kinetic rates of association and dissociation resulted in calculated Kd values which were in agreement with the Kd values determined in saturation binding experiments. Scatchard plots of each of four compounds for the binding data indicated that approximately 40 to 60 molecules were bound per HRV-14 virion. Hill plots showed no evidence of cooperativity in binding. Furthermore, the antiviral activities (MICs in plaque reduction assays with HRV-14) for this limited series of compounds (n = 4) correlated well (r = 0.997) with the observed Kds. Likewise, the absence of detectable binding of Win 54954 to the drug-resistant mutant HRV-14 (Leu-1188) corresponded to a lack of antiviral activity. The positive relationship between the antiviral activities and the Kds that were determined may have implications for the molecular design of capsid-binding antirhinovirus drugs.

In vitro and in vivo activities of WIN 54954, a new broad-spectrum antipicornavirus drug.

WIN 54954 (5-[5-[2,6-dichloro-4-(4,5-dihydro-2-oxazolyl)phenoxy]pentyl]-3- methylisoxazole) is a new member of the class of broad-spectrum antipicornavirus compounds known to bind in a hydrophobic pocket within virion capsid protein VP1. In plaque reduction assays, WIN 54954 reduced plaque formation of 50 of 52 rhinovirus serotypes (MICs ranged from 0.007 to 2.2 micrograms/ml). A concentration of 0.28 microgram/ml was effective in inhibiting 80% of the 52 serotypes tested (EC80). WIN 54954 was also effective in inhibiting 15 commonly isolated enteroviruses, with an EC80 of 0.06 microgram/ml. Furthermore, WIN 54954 was effective in reducing the yield of two selected enteroviruses in cell culture by 90% at concentrations approximately equal to their MICs. The therapeutic efficacy of intragastrically administered WIN 54954 was assessed in suckling mice infected with coxsackievirus A-9 or echovirus type 9 (Barty) 2.5 days prior to initiation of therapy. Single daily doses of 2 and 100 mg/kg protected 50% of the mice from developing paralysis (PD50) following infection with coxsackievirus A-9 and echovirus-9, respectively. At the PD50 doses for these two viruses, levels of WIN 54954 in serum were maintained above the in vitro MICs for a significant portion of the dosing interval. The dose-dependent reduction in viral titers observed in coxsackievirus A-9-infected mice correlated well with the therapeutic dose response. The potency and spectrum of WIN 54954 make it a potentially useful compound for the treatment of human enterovirus and rhinovirus infections.

Crystal structure of human rhinovirus serotype 1A (HRV1A).

The structure of human rhinovirus 1A (HRV1A) has been determined to 3.2 A resolution using phase refinement and extension by symmetry averaging starting with phases at 5 A resolution calculated from the known human rhinovirus 14 (HRV14) structure. The polypeptide backbone structures of HRV1A and HRV14 are similar, but the exposed surfaces are rather different. Differential charge distribution of amino acid residues in the "canyon", the putative receptor binding site, provides a possible explanation for the difference in minor versus major receptor group specificities, represented by HRV1A and HRV14, respectively. The hydrophobic pocket in VP1, into which antiviral compounds bind, is in an "open" conformation similar to that observed in drug-bound HRV14. Drug binding in HRV1A does not induce extensive conformational changes, in contrast to the case of HRV14.

Genetic and molecular analyses of spontaneous mutants of human rhinovirus 14 that are resistant to an antiviral compound.

Spontaneous mutants of human rhinovirus 14 resistant to WIN 52084, an antiviral compound that inhibits attachment to cells, were isolated by selecting plaques that developed when wild-type virus was plated in the presence of high (2 micrograms/ml) or low (0.1 to 0.4 micrograms/ml) concentrations of the compound. Two classes of drug resistance were observed: a high-resistance (HR) class with a frequency of about 4 x 10(-5), and a low-resistance (LR) class with a 10- to 30-fold-higher frequency. The RNA genomes of 56 HR mutants and 13 LR mutants were sequenced in regions encoding the drug-binding site. The HR mutations mapped to only 2 of the 16 amino acid residues that form the walls of the drug-binding pocket. The side chains of these two residues point directly into the pocket and were invariably replaced by bulkier groups. These findings, and patterns of resistance to related WIN compounds, support the concept that HR mutations may hinder the entry or seating of drug within the binding pocket. In contrast, all of the LR mutations mapped to portions of the polypeptide chain near the canyon floor that move when the drug is inserted. Because several LR mutations partially reverse the attachment-inhibiting effect of WIN compounds, these mutants provide useful tools for studying the regions of the capsid structure involved in attachment. This paper shows that the method of escape mutant analysis, previously used to identify antibody binding sites on human rhinovirus 14, is also applicable to analysis of antiviral drug activity.

Conformational change in the floor of the human rhinovirus canyon blocks adsorption to HeLa cell receptors.

A series of eight antiviral compounds complexed with human rhinovirus 14 (HRV-14) were previously shown to displace segments of polypeptide chains in the floor of the "canyon" by as much as 0.45 nm in C-alpha positions from the native conformation (J. Badger, I. Minor, M. J. Kremer, M. A. Oliveira, T. J. Smith, J. P. Griffith, D. M. A. Guerin, S. Krishnaswamy, M. Luo, M. G. Rossman, M. A. McKinlay, G. D. Diana, F. J. Dutko, M. Fancher, R. R. Rueckert, and B. A. Heinz, Proc. Natl. Acad. Sci. USA 85:3304-3308, 1988). Because the canyon is thought to serve as the viral receptor-binding site (M. G. Rossmann, E. Arnold, J. W. Erickson, E. A. Frankenberger, J. P. Griffith, H. J. Hecht, J. E. Johnson, G. Kamer, M. Luo, A. G. Mosser, R. R. Rueckert, B. Sherry, and G. Vriend, Nature [London] 317:145-153, 1985; M. G. Rossmann and R. R. Rueckert, Microbiol. Sci. 4:206-214, 1987), these compounds were assessed for their ability to block adsorption of HRV-14 to HeLa cell membrane receptors. In parallel experiments, the compounds were assessed directly for antiviral activity in an in vitro plaque reduction assay in intact HeLa cells. All eight compounds blocked the adsorption of 50% of HRV-14 at approximately the same concentration required to reduce the number of visible plaques by 50% (MIC). A structurally related compound which was inactive in the plaque reduction assay had no effect on HRV-14 binding. A drug-resistant mutant of HRV-14 (Leu-1188), which was less sensitive to the eight compounds in plaque reduction assays was similarly less sensitive in the adsorption assay. We propose that the conformational changes in the floor of the HRV-14 canyon induced by these compounds substantially decrease adsorption of the virion to its receptor. These results provide further evidence for the role of the HRV canyon in receptor binding.

Three-dimensional structures of drug-resistant mutants of human rhinovirus 14.

Mutants of human rhinovirus 14 were isolated and characterized by searching for resistance to compounds that inhibit viral uncoating. The portions of the RNA that code for amino acids that surround the antiviral compound binding site were sequenced. X-ray analysis of two of these mutants, 1188 Val----Leu and 1199 Cys----Tyr, shows that these were single-site substitutions which would sterically hinder drug binding. Differences in the resistance of mutant viruses to various antiviral compounds may be rationalized in terms of the three-dimensional structures of these mutants. Predictions of the structures of mutant rhinovirus 14 with the substitutions 1188 Val----Leu, 1199 Cys----Tyr and 1199 Cys----Trp in VP1 were made using a molecular dynamics technique. The predicted structure of the 1199 Cys----Tyr mutant was consistent with the electron density map, while the 1188 Val----Leu prediction was not. Large (up to 1.4 A) conformational differences between native rhinovirus 14 and the 1199 Cys----Tyr mutant occurred in main-chain atoms near the mutation site. These changes, as well as the orientation of the 1199 tyrosine side-chain, were correctly predicted by the molecular dynamics calculation. The structure of the predicted 1199 Cys----Trp mutation is consistent with the drug-resistant properties of this virus.

Clearance of a persistent human enterovirus infection of the mouse central nervous system by the antiviral agent disoxaril.

Enteroviruses can cause persistent central nervous system (CNS) infections in agammaglobulinemic individuals. Because these infections are rarely cured by passive administration of antibody, a chemotherapeutic approach would be advantageous. In this study, the efficacy of the antienterovirus (and antipicornavirus) drug disoxaril was demonstrated in a murine model of persistent enterovirus infection. Disoxaril is a hydrophobic antiviral compound that blocks picornavirus uncoating. The W-2 strain of human poliovirus type 2 (PV2) persists in the CNS of immunosuppressed mice and causes late paralysis. Mice were inoculated intracerebrally with PV2, immunosuppressed with cyclophosphamide, and treated intragastrically with disoxaril at 50, 100, or 200 mg/kg per day in two divided doses beginning on postinfection day 20. At 200 mg/kg per day, disoxaril significantly decreased the incidence of clinical disease, i.e., paralysis and death. Assays for virus revealed more rapid clearance of virus from the CNS in the drug-treated group. No drug-associated toxicity was observed. Residual isolates of virus were not drug-resistant, suggesting that the appearance of drug resistance during prolonged treatment may not be a clinical problem.

Synthesis and structure-activity studies of some disubstituted phenylisoxazoles against human picornavirus.

A number of 2,6-disubstituted analogues of disoxaril, a broad spectrum antipicornavirus agent, have been prepared and evaluated against several rhinovirus serotypes. A QSAR study revealed that the mean MIC (MIC) against five rhinovirus serotypes correlated well with log P. The 2,6-dichloro analogue, 15, was highly effective in vitro against rhinoviruses with an MIC80 of 0.3 microM, as well as against several enteroviruses, and was also effective in preventing paralysis in mice infected with coxsackievirus A-9.