Complement: a host defense mechanism ready for pharmacological manipulation?

The Activity of Complement in the Presence of N-(2-hydroxypropyl)methacrylamide Copolymers

This study indicates that N-(2-hydroxypropyl)methacrylamide homopolymers and copolymers containing oligopeptide sequences terminated in carboxylic acid groups, amine groups, aromatic units, or puromycin have no prominent effect on the porcine complement system in vitro. Inhibition of both pathways of the complement system occurred at concentrations highly exceeding the dose suitable for therapeutic purposes.

Prevention of complement-mediated immune hemolysis by a small molecule compound

Complement sensitization of red blood cells (RBCs) can result in transfusion reactions and hemolytic anemias. We hypothesized that manipulating the complement system using small organic molecules might prevent RBC destruction, thereby prolonging RBC survival in patients. Using a simple, rapid, large-scale hemolytic assay, we screened a 10,000 compound library, enriched in anti-inflammatory compounds at a final concentration of 25 microM, and identified a 549Da compound (C(34)H(24)N(6)O(2)) with a symmetrical structure containing two benzimidazole rings that, as compared to a known anti-complement molecule FUT-175, was more effective in reducing hemolysis by the classical pathway and had comparable anti-hemolytic activity against the alternative pathway. Furthermore, in a xenotransfusion mouse model, treatment of mice with 1.2mg/kg of the compound significantly prolonged the survival of transfused RBCs, reducing C3 deposition, but not the deposition of control IgG or IgM, for the first hour post-transfusion. These data suggest that further studies are warranted to determine if this compound has usefulness in a transfusion setting.

Structure/function of C5 convertases of complement

C5 convertases are serine proteases that cleave both C3 and C5. Alternative pathway C3/C5 convertases formed with monomeric C3b (C3b,Bb) because of their weak interaction with C5 primarily cleave C3 thereby opsonizing the cell surface with C3b. In contrast, C3/C5 convertases formed with a high density of C3b/cell exhibit higher affinities for C5 as indicated by Km values well below the physiological concentration of C5 in blood. These C3/C5 convertases bind C5 efficiently and cleave it at a velocity approaching Vmax thereby switching the enzyme from C3 cleavage to production of the cytolytic C5b-9 complex. Studies of the structure of C3/C5 convertases have postulated that C4b-C3b and C3b-C3b dimers from high affinity C5 binding sites while indel studies have shown two binding sites in C5 for the convertase in addition to the C5 cleavage site. Together, these studies indicate that with increasing deposition of C3b on the surface, C3b complexes are formed which through multivalent attachment bind the substrate C5 with higher affinities, thereby converting the low affinity C3/C5 convertases to high affinity C5 convertases. The process underlying the formation of high affinity C5 convertases during complement activation is discussed.

Complement inhibitors: a resurgent concept in anti-inflammatory therapeutics

In addition to its essential role in immune defense, the complement system contributes to tissue damage in many clinical conditions. Thus, there is a pressing need to develop therapeutically effective complement inhibitors to prevent these adverse effects. This concept, though old, received little scientific attention until recently. Data from animal models of diseases that have been produced using complement-deficient, knockout, and transgenic animals, as well as data demonstrating that complement proteins are produced in many important tissue sites (including the brain) have attracted the interest of many basic research scientists and applied scientists from the biotechnology field and larger pharmaceutical firms. This resurgence of interest has generated a wealth of new information in the field of complement inhibition. In this article, we comprehensively review up-to-date information in the field of complement inhibitors.

Solution structure of Compstatin, a potent complement inhibitor

The third component of complement, C3, plays a central role in activation of the classical, alternative, and lectin pathways of complement activation. Recently, we have identified a 13-residue cyclic peptide (named Compstatin) that specifically binds to C3 and inhibits complement activation. To investigate the topology and the contribution of each critical residue to the binding of Compstatin to C3, we have now determined the solution structure using 2D NMR techniques; we have also synthesized substitution analogues and used these to study the structure-function relationships involved. Finally, we have generated an ensemble of a family of solution structures of the peptide with a hybrid distance geometry-restrained simulated-annealing methodology, using distance, dihedral angle, and 3J(NH-Halpha)-coupling constant restraints. The Compstatin structure contained a type I beta-turn comprising the segment Gln5-Asp6-Trp7-Gly8. Preference for packing of the hydrophobic side chains of Val3, Val4, and Trp7 was observed. The generated structure was also analyzed for consistency using NMR parameters such as NOE connectivity patterns, 3J(NH-Halpha)-coupling constants, and chemical shifts. Analysis of Ala substitution analogues suggested that Val3, Gln5, Asp6, Trp7, and Gly8 contribute significantly to the inhibitory activity of the peptide. Substitution of Gly8 caused a 100-fold decrease in inhibitory potency. In contrast, substitution of Val4, His9, His10, and Arg11 resulted in minimal change in the activity. These findings indicate that specific side-chain interactions and the beta-turn are critical for preservation of the conformational stability of Compstatin and they might be significant for maintaining the functional activity of Compstatin.

[Drugs inhibiting complement activation]

Complement may be activated by anaesthetic agents as well as contrast media, and so could anaphylactoid reactions be induced. Efficient and harmless complement blocking agents were still lacking. The only agents actually used by anaesthetists were steroids and/or antifibrinolytic drugs. Other agents were not indicated in surgical patients because of their adverse effects and the impossibility of giving them by the intravenous route.

Differential effects of pharmacologic agents on the reverse passive Arthus reaction in guinea pigs

Five different pharmacologic agents were examined for their effects upon edema, hemorrhage, and vascular infiltration by neutrophils in the reverse passive Arthus reaction (RPAR) in guinea pigs. Two agents, colchicine (3.0 mg/kg p.o.) and ibuprofen (100 mg/kg p.o.) significantly inhibited all three parameters of RPAR. Cobra venom factor (100 units/kg i.p.) inhibited edema and hemorrhage but it did not inhibit neutrophil infiltration. Aminophylline and sulfinpyrazone (100 mg/kg p.o.) inhibited only hemorrhage; they did not inhibit edema or neutrophil infiltration. The results from these studies with five chemically or biologically unrelated pharmacologic agents suggest that the RPAR in guinea pigs can be separated into its basic components (edema, hemorrhage, and neutrophilic infiltration) by selective inhibitors. Inhibition of edema and hemorrhage, or hemorrhage alone of the two-hour RPAR in guinea pigs is not dependent upon inhibition of neutrophilic infiltration.

New inhibitors of complement fixation

A search for compounds that inhibit complement fixation revealed a number of new inhibitors. The presence of EDTA in the phosphate buffer containing complement and compound aided in the detection of compounds that inhibit complement fixation. Compounds were identified as inhibitors that previously could not be shown experimentally to inhibit complement fixation.