Siltuximab is a human-mouse chimeric monoclonal antibody that forms high affinity, stable complexes with soluble bioactive forms of human IL-6. Siltuximab prevents the binding of human IL-6 to both soluble and membrane-bound IL-6 receptors (IL-6R), thus inhibiting the formation of the hexameric signaling complex with gp130 on the cell surface. Interleukin-6 is a pleiotropic pro-inflammatory cytokine produced by a variety of cell types including T-cells and B-cells, lymphocytes, monocytes and fibroblasts, as well as malignant cells.
IL-6 has been shown to be involved in diverse normal physiologic processes such as induction of immunoglobulin secretion, initiation of hepatic acute phase protein synthesis, and stimulation of hematopoietic precursor cell proliferation and differentiation. Overproduction of IL-6, in chronic inflammatory diseases and malignancies has been linked to anaemia and cachexia and has been hypothesised to play a central role in driving plasma cell proliferation and systemic manifestations in patients with CD.
In vitro, siltuximab dose-dependently inhibited the growth of an IL-6-dependent murine plasmacytoma cell line in response to human IL-6. In cultures of human hepatoma cells, IL-6-stimulated production of the acute-phase protein serum amyloid A was dose-dependently inhibited by siltuximab. Similarly, in cultures of human Burkitt’s B-lymphoma cells, the production of immunoglobulin M protein in response to IL-6 was dose-dependently inhibited by siltuximab.
It is well established that IL-6 stimulates the acute-phase expression of C-reactive protein (CRP). The mechanism of action of siltuximab is neutralisation of IL-6 bioactivity, which can be measured indirectly by suppression of CRP. Siltuximab treatment in MCD results in rapid and sustained decreases in CRP serum concentrations. Measurement of IL-6 concentrations in serum or plasma during treatment should not be used as a pharmacodynamic marker, as siltuximab-neutralised antibody-IL-6 complexes interfere with current immunological-based IL-6 quantification methods.
Following the first administration of siltuximab (doses ranging from 0.9 to 15 mg/kg), the area under the concentration-time curve (AUC) and maximal serum concentration (Cmax) increased in a dose-proportional manner and clearance (CL) was independent of dose. Following the single dose administration at the recommended dose regimen (11 mg/kg given once every 3 weeks), the clearance was 3.54 ± 0.44 mL/kg/day and half-life was 16.3 ± 4.2 days. Following the repeat dose administration at the recommended dose, siltuximab clearance was found to be time-invariant, and systemic accumulation was moderate (accumulation index of 1.7). Consistent with half-life after the first dose, serum concentrations reached steady-state levels by the sixth infusion (intervals every 3 weeks) with mean (± SD) peak and trough concentrations of 332 ± 139 and 84 ± 66 mcg/mL, respectively.
As with all therapeutic proteins, there is potential for the generation of anti-medicine antibodies (immunogenicity). The immunogenicity of siltuximab has been evaluated using antigen-bridging enzyme immunoassay (EIA) and electrochemiluminescence (ECL)-based immunoassay (ECLIA) methods.
In clinical studies including monotherapy and combination studies, samples from a total of 432 patients were available for anti-siltuximab antibody testing with 189 patients having at least one sample tested with the high medicinal product-tolerant ECLIA assay. The incidence rate of detectable anti-siltuximab antibodies was 0.9% (4/432) overall and 2.1% (4/189) in patients with at least once sample tested with the high medicinal product tolerant ECLIA assay. Further immunogenicity analyses were conducted for all positive samples from the 4 patients with detectable anti-siltuximab antibodies. None of these patients had neutralising antibodies. No evidence of altered safety or efficacy was identified in the patients who developed antibodies to siltuximab.
Cross-study population PK analyses were performed using data from 378 patients with a variety of conditions who received single-agent siltuximab at doses ranging from 0.9 to 15 mg/kg. The effects of various covariates on siltuximab PK were assessed in the analyses.
Siltuximab clearance increased with increasing body weight; however, no dose adjustment is required for body weight since administration is on an mg/kg basis. The following factors had no clinical effect on the clearance of siltuximab: gender, age, and ethnicity. The effect of anti-siltuximab antibody status was not examined, as there were insufficient numbers of anti-siltuximab antibody positive patients.
The population PK of siltuximab were analysed to evaluate the effects of demographic characteristics. The results showed no significant difference in the PK of siltuximab in patients older than 65 years compared to patients age 65 years or younger.
No formal study of the effect of renal impairment on the pharmacokinetics of siltuximab has been conducted. For patients with baseline calculated creatinine clearance of 12 mL/min or greater, there was no meaningful effect on siltuximab PK. Four patients with severe renal impairment (creatinine clearance 12 to 30 mL/min) were included in the data set.
No formal study of the effect of hepatic impairment on the pharmacokinetics of siltuximab has been conducted. For patients with baseline alanine transaminase up to 3.7 times the upper limit of normal baseline albumin ranging from 15 to 58 g/L, and baseline bilirubin ranging from 1.7 to 42.8 mg/dL there was no meaningful effect on siltuximab PK.
The safety and efficacy of siltuximab have not been established in paediatric patients.
The repeat-dose toxicology studies conducted in young cynomolgus monkeys at doses of 9.2 and 46 mg/kg/week (up to 22-fold greater exposure than in patients receiving 11 mg/kg every 3 weeks) with siltuximab showed no signs indicative of toxicity. A slight reduction in T-cell dependent antibody response and a reduction in the size of the splenic germinal centers following Keyhole limpet hemocyanin (KLH) immunisation was observed which were considered to be pharmacological responses of IL-6 inhibition and not of toxicological significance.
Siltuximab (9.2 and 46 mg/kg/week) did not produce any toxicity of the reproductive tract in cynomolgus monkeys. In mice dosed subcutaneously with an anti-mouse IL-6 monoclonal antibody, no effects on male or female fertility were observed.
During an embryo-fetal development study where siltuximab was administered intravenously to pregnant cynomolgus monkeys (gestation day 20-118) at doses of 9.2 and 46 mg/kg/week, no maternal or fetal toxicity was observed. Siltuximab crossed the placenta during gestation whereby fetal serum concentrations of siltuximab at gestation day (GD) 140 were similar to maternal concentrations. Histopathological examination of lymphoid tissues from GD140 fetuses showed no morphological abnormalities in the development of the immune system.
Rodent carcinogenicity studies have not been conducted with siltuximab. Evidence from studies conducted with siltuximab and other IL-6 inhibitors suggest that the potential for siltuximab to cause carcinogenicity is low. However, there is also evidence to suggest that IL-6 inhibition may suppress immune responses, immune surveillance and lower defense against established tumours. Therefore, an increased susceptibility to specific tumours cannot be entirely ruled out.
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