Anna J. Strachan

A New Small Molecule C5a Receptor Antagonist Inhibits the Reverse-Passive Arthus Reaction and Endotoxic Shock in Rats

C5a is implicated as a pathogenic factor in a wide range of immunoinflammatory diseases, including sepsis and immune complex disease. Agents that antagonize the effects of C5a could be useful in these diseases. We have developed some novel C5a antagonists and have determined the acute anti-inflammatory properties of a new small molecule C5a receptor antagonist against C5a- and LPS-induced neutrophil adhesion and cytokine expression, as well as against some hallmarks of the reverse Arthus reaction in rats. We found that a single i.v. dose (1 mg/kg) of this antagonist inhibited both C5a- and LPS-induced neutropenia and elevated levels of circulating TNF-α, as well as polymorphonuclear leukocyte migration, increased TNF-α levels and vascular leakage at the site of immune complex deposition. These results indicate potent anti-inflammatory activities of a new C5a receptor antagonist and provide more evidence for a key early role for C5a in sepsis and the reverse Arthus reaction. The results support a role for antagonists of C5a receptors in the therapeutic intervention of immunoinflammatory disease states such as sepsis and immune complex disease.

Species dependence for binding of small molecule agonist and antagonists to the C5a receptor on polymorphonuclear leukocytes.

This study investigated the receptor binding affinities of a C5a agonist and cyclic antagonists for polymorphonuclear leukocytes (PMNs) isolated from human, sheep, pig, dog, rabbit, guinea pig, rat and mouse. The affinities of the two small molecule antagonists, F-[OPdChaWR] and AcF-[OPdChaWR], and the agonist, YSFKPMPLaR, revealed large differences in C5a receptor (C5aR) affinities between species. The antagonists bound to human, rat and dog PMNs with similar high affinities, but with lower affinities to PMNs from all other species. The C5a agonist also bound with varying affinities between species, but showed a different affinity profile to the antagonists. In contrast, recombinant human C5a had similar affinity for PMNs of all species investigated. The low correlation between the affinities of the antagonists and the agonist between species either suggests that different receptor residues are important for distinguishing between agonist/antagonist binding, or that the agonist and antagonist peptides bind to two distinct sites within the C5aR.

Inhibition of immune‐complex mediated dermal inflammation in rats following either oral or topical administration of a small molecule C5a receptor antagonist

Initiation of a peritoneal Arthus reaction by deposition of immune‐complexes results in vascular leakage, polymorphonuclear leukocyte (PMN) infiltration, and tumour necrosis factor α (TNFα) and interleukin‐6 (IL‐6) production. We now demonstrate in rats that oral administration of the C5a receptor antagonist AcPhe[Orn‐Pro‐D‐Cyclohexylalanine‐Trp‐Arg] (AcF‐[OPdChaWR]; 1 – 10 mg kg−1 30 min prior to immune‐complex deposition) inhibits these inflammatory markers in the peritoneal Arthus reaction. Initiation of a dermal Arthus reaction resulted in a significant increase in vascular leakage, PMN infiltration, systemic production of TNFα and pathological changes in the dermis. Pretreatment of rats with AcF‐[OPdChaWR] either intravenously (1 mg kg−1 10 min prior to immune‐complex deposition) or orally (1 – 10 mg kg−1 30 min prior to immune‐complex deposition) significantly inhibited immune‐complex mediated dermal vascular leakage and systemic cytokine production. Topical pretreatment with AcF‐[OPdChaWR] (400 μg site−1 in 10% dimethyl sulphoxide 10 min prior to immune‐complex deposition) also inhibited vascular leakage, as well as histopathological changes associated with a dermal Arthus reaction. Oral administration of 3 mg kg−1 AcF‐[OPdChaWR] resulted in the appearance of the drug in plasma within 5 min, with peak blood levels ∼0.3 μM reached within 20 min. The plasma elimination half‐life was ∼70 min. The oral activity and bioavailability of AcF‐[OPdChaWR], its activity when applied topically to the skin, suggest that small molecule C5a receptor antagonists may have therapeutic utility in dermal inflammatory disorders involving complement activation. This is the first demonstration for either an orally or topically active C5a receptor antagonist, and suggests that small molecule C5a antagonists may have therapeutic utility when given by multiple routes of application.

Inhibition of immune-complex mediated dermal inflammation in rats following either oral or topical administration of a small molecule C5a receptor antagonist (vol 134, pg 1778, 2001)

1. Initiation of a peritoneal Arthus reaction by deposition of immune-complexes results in vascular leakage, polymorphonuclear leukocyte (PMN) infiltration, and tumour necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) production. We now demonstrate in rats that oral administration of the C5a receptor antagonist AcPhe[Orn-Pro-D-Cyclohexylalanine-Trp-Arg] (AcF-[OPdChaWR]; 1 - 10 mg kg(-1) 30 min prior to immune-complex deposition) inhibits these inflammatory markers in the peritoneal Arthus reaction. 2. Initiation of a dermal Arthus reaction resulted in a significant increase in vascular leakage, PMN infiltration, systemic production of TNFalpha and pathological changes in the dermis. 3. Pretreatment of rats with AcF-[OPdChaWR] either intravenously (1 mg kg(-1) 10 min prior to immune-complex deposition) or orally (1 - 10 mg kg(-1) 30 min prior to immune-complex deposition) significantly inhibited immune-complex mediated dermal vascular leakage and systemic cytokine production. Topical pretreatment with AcF-[OPdChaWR] (400 microg site(-1) in 10% dimethyl sulphoxide 10 min prior to immune-complex deposition) also inhibited vascular leakage, as well as histopathological changes associated with a dermal Arthus reaction. 4. Oral administration of 3 mg kg(-1) AcF-[OPdChaWR] resulted in the appearance of the drug in plasma within 5 min, with peak blood levels approximately 0.3 microM reached within 20 min. The plasma elimination half-life was approximately 70 min. The oral activity and bioavailability of AcF-[OPdChaWR], its activity when applied topically to the skin, suggest that small molecule C5a receptor antagonists may have therapeutic utility in dermal inflammatory disorders involving complement activation. 5. This is the first demonstration for either an orally or topically active C5a receptor antagonist, and suggests that small molecule C5a antagonists may have therapeutic utility when given by multiple routes of application.