C1 esterase inhibitor (C1-INH)

Molecular Classification

Serine protease inhibitor (serpin superfamily), Glycoprotein, Two-domain structure (N-terminal domain, C-terminal serpin domain)

Other Names

C1-inhibitor, C1INH, C1 Inh, C1-inactivator, C1s-inhibitor, α2-neuramino-glycoprotein, Serpin G1, Plasma protease C1 inhibitor

Disease Roles

Hereditary angioedema (HAE)Autoimmune disease susceptibility (e.g., lupus erythematosus)Potential role in age-related macular degeneration

C1 esterase inhibitor Overview

C1-esterase inhibitor is a multifunctional plasma serine protease inhibitor (serpin) that regulates the classical complement pathway, the contact system, the coagulation cascade, and fibrinolysis by inhibiting proteases C1r, C1s, plasma kallikrein, Factor XIa, Factor XIIa, and plasmin. It is a highly glycosylated single-chain glycoprotein with a two-domain structure: a unique N-terminal domain and a C-terminal serpin domain responsible for protease inhibition. Deficiency or dysfunction leads to hereditary angioedema, autoimmune diseases, and increased risk for vascular permeability-triggered swelling, which is treatable by restoration of C1-INH activity. Therapies include plasma-derived or recombinant C1-INH, and drugs acting on downstream effectors. C1-INH's molecular activity is suicide inhibition of target proteases via a conformational trap in its reactive center loop, producing irreversible complexes and preventing excessive activation of its target cascades.

Mechanism of Action

Drugs work primarily by *replacing* deficient/dysfunctional C1-INH to normalize complement, contact, and coagulation system control. - Kallikrein inhibitors (ecallantide) block a downstream substrate (kallikrein) whose activation is normally controlled by C1-INH. - Bradykinin B2 receptor antagonists (icatibant) block effects of bradykinin, whose excess arises from C1-INH deficiency.

Biological Functions

Regulation of complement system (inhibition of C1r, C1s activation)

Regulation of the contact system (inhibition of plasma kallikrein, Factor XIIa, Factor XIa)

Regulation of coagulation pathway (inhibition of Factor XI, Factor XIIa)

Control of fibrinolysis (inhibition of plasmin)

Major anti-inflammatory activity in plasma

Regulation of vascular permeability

Disease Associations

Hereditary angioedema (HAE)

Autoimmune disease susceptibility (e.g., lupus erythematosus)

Potential role in age-related macular degeneration

Disorders of excessive inflammation and vascular leakage

Other (majo regulator in inflammatory/edematous diseases)

Safety Considerations

Hypersensitivity reactions and anaphylaxis (especially to plasma-derived therapies)
Thromboembolic events (rare, related to pro-coagulant activity of replacement therapies)
Infection risk from plasma-derived products (minimized in recombinant therapies)
Potential for inhibition of normal immune defenses if overdosed

Interacting Drugs

Recombinant and plasma-derived human C1-esterase inhibitor (Berinert, Cinryze, Ruconest)

Ecallantide (kallikrein inhibitor)

Icatibant (bradykinin B2 receptor antagonist; acts downstream)

Fresh frozen plasma (contains C1-INH)

Associated Biomarkers

Biomarker
Plasma C1-esterase inhibitor concentration and function (for diagnosis, treatment selection, and efficacy monitoring in HAE)
Complement C4 and C2 levels (may be decreased when C1-INH is deficient)
C1-INH autoantibody (for acquired deficiency)