Bound to shock: protection from lethal endotoxemic shock by a novel, nontoxic, alkylpolyamine lipopolysaccharide sequestrant

Lipopolysaccharide-binding alkylpolyamine DS-96 inhibits Chlamydia trachomatis infection by blocking attachment and entry

Vaginally delivered microbicides are being developed to offer women self-initiated protection against transmission of sexually transmitted infections such as Chlamydia trachomatis. A small molecule, DS-96, rationally designed for high affinity to Escherichia coli lipid A, was previously demonstrated to bind and neutralize lipopolysaccharide (LPS) from a wide variety of Gram-negative bacteria (D. Sil et al., Antimicrob. Agents Chemother. 51: 2811-2819, 2007, doi:10.1128/AAC.00200-07). Aside from the lack of the repeating O antigen, chlamydial lipooligosaccharide (LOS) shares general molecular architecture features with E. coli LPS. Importantly, the portion of lipid A where the interaction with DS-96 is expected to take place is well conserved between the two organisms, leading to the hypothesis that DS-96 inhibits Chlamydia infection by binding to LOS and compromising the function. In this study, antichlamydial activity of DS-96 was examined in cell culture. DS-96 inhibited the intercellular growth of Chlamydia in a dose-dependent manner and offered a high level of inhibition at a relatively low concentration (8 μM). The data also revealed that infectious elementary bodies (EBs) were predominantly blocked at the attachment step, as indicated by the reduced number of EBs associated with the host cell surface following pretreatment. Of those EBs that were capable of attachment, the vast majority was unable to gain entry into the host cell. Inhibition of EB attachment and entry by DS-96 suggests that Chlamydia LOS is critical to these processes during the developmental cycle. Importantly, given the low association of host toxicity previously reported by Sil et al., DS-96 is expected to perform well in animal studies as an active antichlamydial compound in a vaginal microbicide.

The Methanol Extract of Azadirachta indica A. Juss Leaf Protects Mice Against Lethal Endotoxemia and Sepsis

In the present study, the inhibitory effect of neem leaf extract (NLE) on lipopolysaccaride (LPS)-induced nitric oxide (NO) and tumor necrosis factor-α (TNF-α) production was examined both in vitro and in vivo. In vitro study revealed that NLE treatment (100 μg/ml) inhibits LPS (100 ng/ml)-induced NO production by 96% and TNF-α production by 32%. The reduction in NO production is probably conferred by the complete suppression of inducible nitric oxide synthase (iNOS) expression. Interestingly, in vivo NLE significantly improved the survival rate of mice in an experimental sepsis model. Administration of NLE (100 mg/kg) 24 h before LPS treatment (20 mg/kg) improved the survival rate of mice by 60%. The inhibition of plasma NO and TNF-α production by NLE is likely to account for the improved survival of mice. Our results suggest that NLE may present a promising avenue in the development of therapeutic agents for the treatment of inflammatory diseases.

Biophysical mechanisms of the neutralization of endotoxins by lipopolyamines

Endotoxins (lipopolysaccharides, LPS) are one of the strongest immunostimulators in nature, responsible for beneficial effects at low, and pathophysiological effects at high concentrations, the latter frequently leading to sepsis and septic shock associated with high mortality in critical care settings. There are no drugs specifically targeting the pathophysiology of sepsis, and new therapeutic agents are therefore urgently needed. The lipopolyamines are a novel class of small molecules designed to sequester and neutralize LPS. To understand the mechanisms underlying the binding and neutralization of LPS toxicity, we have performed detailed biophysical analyses of the interactions of LPS with candidate lipopolyamines which differ in their potencies of LPS neutralization. We examined gel-to-liquid crystalline phase behavior of LPS and of its supramolecular aggregate structures in the absence and presence of lipopolyamines, the ability of such compounds to incorporate into different membrane systems, and the thermodynamics of the LPS:lipopolyamine binding. We have found that the mechanisms which govern the inactivation process of LPS obey similar rules as found for other active endotoxin neutralizers such as certain antimicrobial peptides.

Exquisite selectivity for human toll-like receptor 8 in substituted furo[2,3-c]quinolines

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds may be promising candidate adjuvants. We synthesized and evaluated hitherto unexplored furo[2,3-c]quinolines and regioisomeric furo[3,2-c]quinolines derived via a tandem, one-pot Sonogashira coupling and intramolecular 5-endo-dig cyclization strategy in a panel of primary screens. We observed a pure TLR8-agonistic activity profile in select furo[2,3-c]quinolines, with maximal potency conferred by a C2-butyl group (EC50 = 1.6 μM); shorter, longer, or substituted homologues as well as compounds bearing C1 substitutions were inactive, which was rationalized by docking studies using the recently described crystal structure of human TLR8. The best-in-class compound displayed prominent proinflammatory cytokine induction (including interleukin-12 and interleukin-18), but was bereft of interferon-α inducing properties, confirming its high selectivity for human TLR8.

Bacterial cell wall compounds as promising targets of antimicrobial agents I. Antimicrobial peptides and lipopolyamines

The first barrier that an antimicrobial agent must overcome when interacting with its target is the microbial cell wall. In the case of Gram-negative bacteria, additional to the cytoplasmic membrane and the peptidoglycan layer, an outer membrane (OM) is the outermost barrier. The OM has an asymmetric distribution of the lipids with phospholipids and lipopolysaccharide (LPS) located in the inner and outer leaflets, respectively. In contrast, Gram-positive bacteria lack OM and possess a much thicker peptidoglycan layer compared to their Gram-negative counterparts. An additional class of amphiphiles exists in Gram-positives, the lipoteichoic acids (LTA), which may represent important structural components. These long molecules cross-bridge the entire cell envelope with their lipid component inserting into the outer leaflet of the cytoplasmic membrane and the teichoic acid portion penetrating into the peptidoglycan layer. Furthermore, both classes of bacteria have other important amphiphiles, such as lipoproteins, whose importance has become evident only recently. It is not known yet whether any of these amphiphilic components are able to stimulate the immune system under physiological conditions as constituents of intact bacteria. However, all of them have a very high pro-inflammatory activity when released from the cell. Such a release may take place through the interaction with the immune system, or with antibiotics (particularly with those targeting cell wall components), or simply by the bacterial division. Therefore, a given antimicrobial agent must ideally have a double character, namely, it must overcome the bacterial cell wall barrier, without inducing the liberation of the pro-inflammatory amphiphiles. Here, new data are presented which describe the development and use of membrane-active antimicrobial agents, in particular antimicrobial peptides (AMPs) and lipopolyamines. In this way, essential progress was achieved, in particular with respect to the inhibition of deleterious consequences of bacterial infections such as severe sepsis and septic shock.

Potent adjuvanticity of a pure TLR7-agonistic imidazoquinoline dendrimer

Engagement of toll-like receptors (TLRs) serve to link innate immune responses with adaptive immunity and can be exploited as powerful vaccine adjuvants for eliciting both primary and anamnestic immune responses. TLR7 agonists are highly immunostimulatory without inducing dominant proinflammatory cytokine responses. We synthesized a dendrimeric molecule bearing six units of a potent TLR7/TLR8 dual-agonistic imidazoquinoline to explore if multimerization of TLR7/8 would result in altered activity profiles. A complete loss of TLR8-stimulatory activity with selective retention of the TLR7-agonistic activity was observed in the dendrimer. This was reflected by a complete absence of TLR8-driven proinflammatory cytokine and interferon (IFN)-γ induction in human PBMCs, with preservation of TLR7-driven IFN-α induction. The dendrimer was found to be superior to the imidazoquinoline monomer in inducing high titers of high-affinity antibodies to bovine α-lactalbumin. Additionally, epitope mapping experiments showed that the dendrimer induced immunoreactivity to more contiguous peptide epitopes along the amino acid sequence of the model antigen.

Imbuing aqueous solubility to amphotericin B and nystatin with a vitamin

Aqueous solubilities of many drugs in current clinical use are very low, necessitating formulations that often present problems for parenteral administration, including toxicities due to the excipients used. Recognizing that pharmacologically active compounds frequently possess amines, we asked whether pyridoxal phosphate (PLP), an inoccuous, water-soluble vitamin, could be utilized to form prodrug-like complexes via the formation of imine or iminium adducts and whether the vitamin would impart solubilizing properties to such complexes. Direct spectroscopic and crystallographic data obtained using model primary and secondary amines showed that PLP forms stable imine adducts with primary amines under entirely aqueous conditions and at physiologic pH, while no reaction was observed for secondary amines; the basis of the exceptional stability appears to be a consequence of favorable H-bond interactions of the imine nitrogen with the 5-OH group of PLP. Amphotericin B and nystatin in their native forms display marked aqueous insolubility and possess lone primary amines. We were able to utilize PLP in achieving excellent solubilization of both of these antifungal agents, surpassing aqueous solubilities of 100 mg/mL. In in vitro bioassays, both polyenes in their PLP-adducted form display attenuated antifungal potencies which are attributable to "prodrug-like" complexes. These results point to the utility of excipient-free, entirely aqueous formulations of amphotericin B for parenteral use, and may also be extended to other primary amine-bearing compounds exhibiting poor aqueous solubility.

Development of small-molecule endotoxin sequestering agents

Sepsis, otherwise referred to as "blood poisoning" is a serious clinical problem, the incidence of which continues to rise in the US and worldwide despite advances in antimicrobial chemotherapy. The primary trigger in Gram-negative sepsis is endotoxin, a lipopolysaccharide (LPS) constituent of the outer membrane of all Gram-negative bacteria. The structurally highly conserved glycolipid called lipid A is the active moiety of LPS. Lipid A is composed of a hydrophilic, bis-phosphorylated di-glucosamine backbone, and a hydrophobic polyacyl domain. The bis-anionic, amphiphilic nature of lipid A enables it to interact with a variety of cationic hydrophobic ligands, including polymyxin B, a toxic peptide antibiotic which binds to lipid A and neutralizes endotoxicity. Having determined the structural basis of the interaction of polymyxin B with lipid A, our long-term goal has been to rationally design non-peptidic, nontoxic, small-molecule LPS-sequestrants. Our efforts began with defining the central pharmacophore that determined LPS-recognition and -neutralization properties in small molecules, which led to the discovery of a novel lipopolyamine lead, DS-96. DS-96 is an effective LPS-neutralizer, rivaling polymyxin B in a panel of vitro assays, as well as in protecting animals against endotoxicosis. Structure-activity relationships in our effort to rationally design endotoxin sequestering agents, preclinical assessment of hits and leads, and approaches to overcoming issues with toxicity are described in this chapter.

Synthesis of a highly water-soluble derivative of amphotericin B with attenuated proinflammatory activity

Amphotericin B (AmB), a well-known polyene antifungal agent, displays a marked tendency to self-associate and, as a consequence, exhibits very poor solubility in water. The therapeutic index of AmB is low and is associated with significant dose-related nephrotoxicity, as well as acute, infusion-related febrile reactions. Reports in the literature indicate that the toxicity of AmB may be related to the physical state of the drug. Reaction of AmB in dimethylformamide with bis(dimethylaminopropyl)carbodiimide yielded an unexpected N-alkylguanidine/N-acylurea bis-adduct of AmB which was highly water-soluble. The absorption spectrum of the AmB derivative in water indicated excellent monomerization, and the antifungal activities of reference AmB and its water-soluble derivative against Candida albicans were found to be virtually identical. Furthermore, the water-soluble adduct is significantly less active in engaging TLR4, which would suggest that the adduct may be less proinflammatory.

Controlling plasma protein binding: structural correlates of interactions of hydrophobic polyamine endotoxin sequestrants with human serum albumin

Hydrophobically substituted polyamine compounds, particularly N-acyl or N-alkyl derivatives of homospermine, are potent endotoxin (lipopolysaccharide) sequestrants. Despite their polycationic nature, the aqueous solubilites are limited owing to the considerable overall hydrophobicity contributed by the long-chain aliphatic substituent, but solubilization is readily achieved in the presence of human serum albumin (HSA). We desired first to delineate the structural basis of lipopolyamine-albumin interactions and, second, to explore possible structure-activity correlates in a well-defined, congeneric series of N-alkyl and -acyl homospermine lead compounds. Fluorescence spectroscopic and isothermal titration calorimetry (ITC) results indicate that these compounds appear to bind to HSA via occupancy of the fatty-acid binding sites on the protein. The acyl and carbamate compounds bind HSA the strongest; the ureido and N-alkyl analogues are significantly weaker, and the branched alkyl compound is weaker still. ITC-derived dissociation constants are weighted almost in their entirety by enthalpic deltaH terms, which is suggestive that the polarizability of the carbonyl groups facilitate, at least in large part, their interactions with HSA. The relative affinities of these lipopolyamines toward HSA is reflected in discernible differences in apparent potencies of LPS-sequestering activity under experimental conditions requiring physiological concentrations of HSA, and also of in vivo pharmacodynamic behavior. These results are likely to be useful in designing analogues with varying pharmacokinetic profiles.

Effect of heme oxygenase-1 on the kidney during septic shock in rats

The aim of this study was to clarify the effect of heme oxygenase (HO)-1 on the kidney during septic shock in rats. Eighty healthy and clean Sprague-Dawley rats were randomly divided into 4 groups: control group (group C) , septic shock group (group SS) , lipopolysaccharide plus ZnPP-IX group (group LZ) , and ZnPP-IX (group Z) . The plasma levels of COHb, Cr, and BUN; the urine levels of gamma-GTP, alpha1-MG, RBP, and NAG; and MDA content, SOD activity; HO-1mRNA; HO-2mRNA; HO-1 protein; and HO-2 protein from kidney were measured. The plasma levels of Cr and BUN; the urine levels of gamma-GTP, alpha1-MG, RBP, and NAG; and MDA content from kidney in group SS were obviously higher than in group C and Z (both P < 0.05), respectively, but lower than in group LZ (both P < 0.05), respectively. In contrast, the plasma levels of COHb and SOD activity from kidney in group SS were obviously lower than those in group C and Z (both P < 0.05), respectively, but higher than in group LZ (both P < 0.05), respectively. The HO-1mRNA and HO-1 protein of nephridial tissue in group LZ were obviously higher than in group C and Z, respectively (both P < 0.05), but lower than in group SS, respectively (both P < 0.05). There were no significant differences of HO-2mRNA and HO-2 protein among these 4 groups (both P > 0.05). It is concluded that upregulation of the HO-1 protein might contribute to the protection of kidney during septic shock in rats.

Structure-activity relationships of lipopolysaccharide sequestration in N-alkylpolyamines

We have previously shown that simple N-acyl or N-alkyl polyamines bind to and sequester Gram-negative bacterial lipopolysaccharide, affording protection against lethality in animal models of endotoxicosis. Several iterative design-and-test cycles of SAR studies, including high-throughput screens, had converged on compounds with polyamine scaffolds which have been investigated extensively with reference to the number, position, and length of acyl or alkyl appendages. However, the polyamine backbone itself had not been explored sufficiently, and it was not known if incremental variations on the polymethylene spacing would affect LPS-binding and neutralization properties. We have now systematically explored the relationship between variously elongated spermidine [NH(2)-(CH(2))(3)-NH-(CH(2))(4)-NH(2)] and norspermidine [NH(2)-(CH(2))(3)-NH-(CH(2))(3)-NH(2)] backbones, with the N-alkyl group being held constant at C(16) in order to examine if changing the spacing between the inner secondary amines may yield additional SAR information. We find that the norspermine-type compounds consistently showed higher activity compared to corresponding spermine homologues.

Pharmacokinetics of DS-96, an alkylpolyamine lipopolysaccharide sequestrant, in rodents

The pharmacokinetics of DS-96, an N-alkylhomospermine analog designed to sequester bacterial lipopolysaccharides, has been determined in rodent species. The elimination half-life in mice and rats are about 400 and 500 min, respectively, with other PK parameters being quite similar in the two rodent species. Interestingly, the mouse intravenous plasma concentration time curves exhibit an apparent absorption phase. While the rat intravenous data did not exhibit a pronounced apparent absorption phase immediately following injection, plasma levels did increase between 10 and 30 min following an expected drop from time 0 to 5 min. The data are consistent with first-pass uptake, possibly by the lung, with back diffusion as a function of time. The observed C(max) values of 1.36 microg/mL in the mouse intraperitoneal model suggest that a plasma concentration of 0.5-1 microg/mL corresponds to complete protection for a 200 ng/animal dose of intraperitoneally administered LPS in the D-galactosamine-primed model of endotoxin-induced lethality.

Structure-activity relationships of lipopolysaccharide sequestration in guanylhydrazone-bearing lipopolyamines

The toxicity of gram-negative bacterial endotoxin (lipopolysaccharide, LPS) resides in its structurally highly conserved glycolipid component called lipid A. Our major goal has been to develop small-molecules that would sequester LPS by binding to the lipid A moiety, so that it could be useful for the prophylaxis or adjunctive therapy of gram-negative sepsis. We had previously identified in rapid-throughput screens several guanylhydrazones as potent LPS binders. We were desirous of examining if the presence of the guanylhydrazone (rather than an amine) functionality would afford greater LPS sequestration potency. In evaluating a congeneric set of guanylhydrazone analogues, we find that C(16) alkyl substitution is optimal in the N-alkylguanylhydrazone series; a homospermine analogue with the terminal amine N-alkylated with a C(16) chain with the other terminus of the molecule bearing an unsubstituted guanylhydrazone moiety is marginally more active, suggesting very slight, if any, steric effects. Neither C(16) analogue is significantly more active than the N-C(16)-alkyl or N-C(16)-acyl compounds that we had characterized earlier, indicating that basicity of the phosphate-recognizing cationic group, is not a determinant of LPS sequestration activity.

Structure-activity relationships of antimicrobial and lipoteichoic acid-sequestering properties in polyamine sulfonamides

We have recently confirmed that lipoteichoic acid (LTA), a major constituent of the gram-positive bacterial surface, is the endotoxin of gram-positive bacteria that induces proinflammatory molecules in a Toll-like receptor 2 (TLR2)-dependent manner. LTA is an anionic amphipath whose physicochemical properties are similar to those of lipopolysaccharide (LPS), which is found on the outer leaflet of the outer membranes of gram-negative organisms. Hypothesizing that compounds that sequester LPS could also bind to and inhibit LTA-induced cellular activation, we screened congeneric series of polyamine sulfonamides which we had previously shown effectively neutralized LPS both in vitro and in animal models of endotoxemia. We observed that these compounds do bind to and neutralize LTA, as reflected by the inhibition of TLR2-mediated NF-kappaB induction in reporter gene assays. Structure-activity studies showed a clear dependence of the acyl chain length on activity against LTA in compounds with spermine and homospermine scaffolds. We then sought to examine possible correlations between the neutralizing potency toward LTA and antimicrobial activity in Staphylococcus aureus. A linear relationship between LTA sequestration activity and antimicrobial activity for compounds with a spermine backbone was observed, while all compounds with a homospermine backbone were equally active against S. aureus, regardless of their neutralizing potency toward LTA. These results suggest that the number of protonatable charges is a key determinant of the activity toward the membranes of gram-positive bacteria. The development of resistance to membrane-active antibiotics has been relatively slower than that to conventional antibiotics, and it is possible that compounds such as the acylpolyamines may be useful clinically, provided that they have an acceptable safety profile and margin of safety. A more detailed understanding of the mechanisms of interactions of these compounds with LPS and LTA, as well as the gram-negative and -positive bacterial cell surfaces, will be instructive and should allow the rational design of analogues which combine antisepsis and antibacterial properties.

Comparison of the immunostimulatory and proinflammatory activities of candidate Gram-positive endotoxins, lipoteichoic acid, peptidoglycan, and lipopeptides, in murine and human cells

The role of lipopolysaccharide (LPS) in the pathogenesis of Gram-negative septic shock is well established. The corresponding proinflammatory and immunostimulatory molecule(s) on the Gram-positive bacteria is less well understood, and its identification and characterization would be a key prerequisite in designing specific sequestrants of the Gram-positive endotoxin(s). We report in this paper the comparison of NF-kappaB-, cytokine- and chemokine-inducing activities of the TLR2 ligands, lipoteichoic acid (LTA), peptidoglycan (PGN), and lipopeptides, to LPS, a prototype TLR4 agonist, in murine macrophage cell-lines as well as in human blood. In murine cells, di- and triacyl liopopeptides are equipotent in their NF-kappaB inducing activity relative to LPS, but elicit much lower proinflammatory cytokines. However, both LPS and the lipopeptides potently induce the secretion of a pattern of chemokines that is suggestive of the engagement of a TLR4-independent TRIF pathway. In human blood, although the lipopeptides induce p38 MAP kinase phosphorylation and CD11b upregulation in granulocytes at ng/ml concentrations, they do not elicit proinflammatory cytokine production even at very high doses; LTA, however, activates neutrophils and induces cytokine secretion, although its potency is considerably lower than that of LPS, presumably due to its binding to plasma proteins. We conclude that, in human blood, the pattern of immunostimulation and proinflammatory mediator production elicited by LTA parallels that of LPS.