Pharmacokinetics and tissue distribution. After subcutaneous administration, secukinumab reaches systemic circulation with an absolute bioavailability of approximately 73%, and absorption follows a first-order process with an estimated absorption rate constant of 0.18/day. Consistent with the behaviour of a large IgG1-kappa antibody, the drug exhibits limited extravascular distribution: in a typical 90-kg patient, serum clearance is approximately 0.19 L/day with a central compartment volume of 3.61 L and peripheral volume of 2.87 L. Steady-state peak exposure occurs at a Tmax of approximately 6 days after dosing, and the terminal half-life averages approximately 27 days, supporting target engagement across monthly dosing intervals. Like other IgG antibodies, secukinumab is cleared primarily through FcRn-mediated recycling and proteolytic catabolism rather than renal elimination. Distribution into the clinical target tissue is rapid: dermal interstitial fluid concentrations reach 28–39% of contemporaneous serum levels within 7–14 days of a single 300 mg dose, providing a pharmacokinetic rationale for early cutaneous responses.

Primary target and initial binding event. Secukinumab is a fully human IgG1-kappa monoclonal antibody that selectively binds and neutralizes IL-17A, preventing its interaction with the IL-17 receptor. IL-17A signals through a heterodimeric receptor complex composed of IL-17RA and IL-17RC; by sequestering free IL-17A in the extracellular space, secukinumab eliminates the ligand-dependent trigger for receptor complex activation and the downstream signalling cascade that follows.

Receptor-proximal signalling disruption. In the intact pathway, receptor ligation drives recruitment of the adaptor protein Act1 to the IL-17 receptor via homotypic SEFIR–SEFIR domain interactions. This Act1–receptor engagement enables downstream wiring: Act1 recruits TRAF6 through its TRAF-binding motifs, initiating canonical inflammatory signalling. TRAF6 activation leads to TAK1/IKK-mediated phosphorylation and degradation of IκBα, releasing NF-κB dimers for nuclear translocation. In parallel, the pathway activates mitogen-activated protein kinase cascades including ERK, p38, and JNK, as well as C/EBP family transcription factors, broadening transcriptional output. IL-17 signal transduction additionally branches into TRAF6- and TRAF5-dependent cytokine and chemokine programmes: TRAF6 primarily drives representative cytokine outputs such as IL-6, while TRAF5 drives chemokine outputs such as CXCL1. By neutralising IL-17A upstream, secukinumab prevents Act1 recruitment and simultaneously blunts both the cytokine and chemokine arms. A cell-intrinsic modulator of this cascade is intracellular cholesterol availability: cholesterol availability modulates IL-17A-induced NF-κB nuclear translocation in keratinocytes, indicating that lipid context can tune downstream signal amplitude even when receptor signalling is intact.

Gene expression consequences. Downstream of NF-κB and MAPK activation, a key transcriptional node in keratinocytes is IκBζ (encoded by NFKBIZ), whose mRNA rises within hours of IL-17A stimulation and is required for subsequent induction of beta-defensins, IL-19, and CSF3. IL-17A also controls gene expression post-transcriptionally: signalling through an Act1–TRAF2–TRAF5 complex stabilises target mRNAs via HuR and related factors, prolonging chemokine and cytokine protein production beyond the initial transcriptional wave. Counter-regulation is provided by Regnase-1, which degrades actively translated IL-17 target transcripts including Il6 and Nfkbiz, limiting the amplitude of inflammation.

Feed-forward loop and clinical translation. In vivo, Act1/CIKS signalling in keratinocytes drives neutrophilic microabscess formation and sustains a feed-forward loop that amplifies IL-17A and IL-17F production by dermal γδ T cells. Airway epithelial studies show that IKKi is required for IL-17-induced neutrophilia and pulmonary inflammation, illustrating how this kinase connects IL-17 receptor engagement to the recruitment of innate effectors in barrier tissues. Blocking IL-17A with secukinumab breaks this amplification circuit, reducing keratinocyte activation, epidermal hyperproliferation, neutrophil infiltration, and the sustained Th17-driven cytokine environment that characterises plaque psoriasis, psoriatic arthritis, and ankylosing spondylitis.