Healthy males or females, 18–65 years old, with a body mass index (BMI) of 18–32 kg/m2 were included in the study. Subjects were excluded if they had a known allergy or hypersensitivity to any biologic therapy or vaccine; had a compromised immune system or had other comorbidities (such as having an active infectious disease, a history of Guillain–Barre Syndrome or lymphoma, leukemia, or any malignancy within the past 5 years); had received IXE or IL-17 antagonists; had a live vaccination within 1 year prior to screening; had received a tetanus toxoid–containing vaccine within the last 5 years; or had been immunized with the pneumococcal vaccine. Since the study was conducted in the USA, the baseline assumption was the tetanus vaccine would likely be a “booster” vaccine rather than the subjects’ first tetanus vaccination. While a tetanus booster is recommended at least once every 10 years , the restriction of a subject having no booster within the last 5 years was selected based on dose and schedule information in the Boostrix label  and published studies [5, 6, 9].
Study Design and Treatment
In this phase I, multicenter (three study centers in the USA), open-label, parallel-group study (NCT02543918) (Fig. 1), healthy subjects were randomized to IXE (N = 41) or control (N = 43). At randomization (week 0), IXE subjects received an SC 160-mg IXE starting dose, and at week 2, an 80-mg IXE dose. The 160-mg IXE dose at week 0 and the 80-mg IXE dose at week 2 have been administered extensively to subjects with psoriasis in phase III studies and are the start of the approved dosing regimen for moderate-to-severe plaque psoriasis. The dosing in this study is designed to achieve therapeutic levels by week 6, the time of the vaccine assessment.
Vaccinations for tetanus (Boostrix)  and pneumococcus (Pneumovax 23)  were administered 2 weeks after the IXE 160-mg dose (week 0) and on the same day as the 80-mg dose of IXE (week 2). The primary vaccine response endpoint measurement was at week 6 (4 weeks post-vaccination) and was based on the performing laboratory’s validated assay protocols for both vaccines. Published data also indicated that this time period includes the time from vaccination in which the greatest interference with response to the vaccine (2 weeks) is likely to occur .
Endpoints and Assessments
The primary endpoints are the percentages of patients with a response to the tetanus and pneumococcal vaccines. Response to tetanus vaccination was defined as anti-tetanus antibodies ≥ 1.0 IU and a ≥ 1.5-fold increase if baseline was ≤ 1.0 IU or a ≥ 2.5-fold increase if baseline was > 1.0 IU. Response to pneumococcal vaccination was defined as a ≥ 2-fold increase from baseline in anti-pneumococcal antibodies against > 50% of the 23 serotypes. These responses were based on validated laboratory assay protocols and published data [6, 9, 14]. Immune response to vaccinations, specifically antibody production to the tetanus and pneumococcal vaccines, was measured using validated quantitative multiplex bead-based immunoassays performed for routine clinical testing in a Clinical Laboratory Improvement Amendments (CLIA) certified laboratory (ARUP Laboratories, Salt Lake City, UT, USA). The serotypes for analysis of the pneumococcal vaccine were as follows: 1, 2, 3, 4, 5, 6B, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F, and 33F.
Additional exploratory endpoints were assessed based on various published criteria and post hoc assessments in response to a regulatory request to further understand the impact of IXE on the ability to produce an adequate antibody response [14,15,16]. These endpoints were (1) the percentage of subjects with either an increase to protective anti-tetanus antibody (ATAb) or anti-pneumococcal antibody (APAb) levels from non-protective baseline levels; (2) the change from baseline in geometric mean antibody levels at 2 and 4 weeks post-vaccination; (3) the number and percentage of subjects with a > 4-fold increase from baseline in ATAb level; and (4) the percentage of subjects with a ≥ 2-fold increase in APAb to at least 70% of serotypes for pneumococcal vaccine, those with a ≥ 4-fold increase from baseline in APAb to at least 50% of serotypes for pneumococcal vaccine, and those with a ≥ 4-fold increase from baseline in APAb to at least 70% of serotypes for the pneumococcal vaccine.
The tolerability and PK of IXE in healthy subjects were secondary and exploratory endpoints, respectively. Safety parameters assessed included adverse events (AEs), laboratory parameters, vital signs, and electrocardiogram parameters, and the Quick Inventory of Depressive Symptomatology–Self-Report (QIDS-SR16). Serum samples for IXE PK analysis were obtained during the study at the following times: prior to administration of the 160-mg IXE dose on day 1 (week 0), on days 3, 5, 8, and 11 following the 160-mg IXE dose, and prior to administration of the 80-mg IXE dose on day 15 (week 2) and then at weeks 4, 6, and 12. Serum samples were analyzed for IXE using a validated enzyme-linked immunosorbent assay (ELISA) (Intertek Pharmaceutical Services, San Diego, CA, USA). The lower limit of quantification was 7.5 ng/mL, and the upper limit of quantification was 300.0 ng/mL. A 1:5 minimum required dilution was applied to all samples. Samples above the limit of quantification were diluted to yield results within the calibrated range. The inter-assay precision (percentage relative standard deviation) during validation ranged from 11.8 to 17.3%.
Data analysis was performed using SAS® version 9.3. Antibody vaccine analyses included all randomized subjects receiving that vaccine who had a baseline and at least one evaluable post-baseline value. Safety analyses included all randomized subjects. PK analyses included all randomized subjects who received at least one dose of IXE and had sufficient evaluable PK data.
For the primary analysis, the difference between the two groups (IXE group minus control) in the proportion of responders to each vaccine at 4 weeks post-vaccination together with the 90% confidence interval (CI) of the difference was calculated for the tetanus and pneumococcal vaccines. Noninferiority of the IXE group to the control group for each vaccine was established if the lower limit of the 90% CI of the difference between the two groups (IXE minus control) in the proportion of responders to each vaccine at 4 weeks post-vaccination was greater than 40%. This noninferiority margin has been used previously . No multiplicity adjustment was used. CIs for the differences in proportions between the two groups were calculated using the Newcombe method based on the Wilson score [17, 18]. This method was also used to compare the proportions of patients between the two groups for post hoc analyses. A mixed-effect model was used to analyze the differences from baseline in natural log-transformed serotype levels with treatment arm (IXE or control), visit, and visit-by-treatment interaction as fixed effects and subject as a random effect. Geometric mean ratios (GMRs) for each serotype level to the baseline at 2 and 4 weeks post-vaccination (week 4 and week 6, respectively) with their 95% CI were reported for both the IXE and control groups. In addition, GMRs of serotype levels at 2 and 4 weeks post-vaccination (study week 4 and week 6, respectively) in the IXE group to the control group were provided with their 95% CI.
The proportion of subjects showing an increase from a non-protective level at baseline (≤ 1 IU) to a protective level (> 1 IU) of ATAb at 4 weeks post-vaccination was calculated. In addition, the proportion of subjects showing an increase from a non-protective level at baseline (≤ 1.3 µg/mL) to a protective level (> 1.3 µg/mL) of APAb to at least 50% of serotypes at 4 weeks post-vaccination was identified. CIs for the proportion of subjects that showed an increase from a non-protective level to a protective level for each group were calculated using the Wilson method . Additional exploratory, non-prespecified immune response statistical analyses were conducted to compare the proportion of responders between groups using the alternate responder definitions described previously in Sect. 2.3. The differences between the two groups (IXE group minus control) in the proportion of responders based on these criteria at 4 weeks post-vaccination together with the 95% CI of the difference were calculated.
The incidence of AEs for the IXE versus control group is presented. AEs reported during the study were not necessarily caused by the therapy. Therefore, the reported frequencies do not reflect causality as evaluated by the investigator.
PK parameters for IXE were calculated by standard non-compartmental methods of analysis using Phoenix WinNonlin version 6.2.1 and included the maximum concentration (C
max), time of maximum concentration (t
max), area under the concentration time curve from time zero to time t
last (AUC0–tlast), where t
last is the last time point with a measurable concentration after the 160-mg dose administered on day 1 (week 0), and the terminal elimination half-life (t
½) after administration of the 80-mg dose at week 2. The PK parameters were summarized using standard descriptive statistics.