Clinical Implications of Baseline-Corrected Total Immunoglobulin G Levels: Pharmacokinetic Data from a Phase 3, Open-Label Study of Patients with Primary Immunodeficiency Disease Treated with fSCIG 10%

Dosing immunoglobulin G (IgG) replacement therapy in primary immunodeficiency disease (PIDD) depends on IgG pharmacokinetics (PK) and patients’ clinical status. Conventionally, PK data used to determine dosing were based on total IgG levels, comprising both endogenous and exogenous IgG. However, this may overestimate IgG half-life (t1/2), which is a key parameter in determining steady-state IgG PK that informs dosing. We assessed the PK and clinical implications of baseline correction of IgG in patients with PIDD receiving facilitated subcutaneous immunoglobulin (fSCIG) 10%, a hyaluronidase-facilitated subcutaneous IgG approved for PIDD. PK data were analyzed for a subgroup of patients with PIDD (aged ≥12 years) receiving fSCIG 10% in a phase 3, open-label study (NCT00814320). Endogenous IgG levels were estimated from baseline values (before IgG infusion). Exogenous IgG levels were calculated by subtracting individual baseline IgG concentrations from measured IgG concentrations at each time point. Noncompartmental analysis (Phoenix WinNonlin version 8.5) was used for PK assessment and PK parameter calculation. Baseline correction produced marked changes in mean (±SD) IgG PK parameters compared with uncorrected values: maximum plasma concentration (469 [±269] vs. 1,607 [±382] mg/dL) and area under the curve/week (13.4 [±12.0] vs. 89.7 [±22.9] g·days/L) were substantially reduced, reflecting exclusion of endogenous IgG. Mean (±SD) estimated IgG t1/2 showed clinically relevant reductions (baseline corrected, 8.9 [±7.3] days; uncorrected, 55.0 [±30.3] days). While baseline correction remains an approximation owing to individual fluctuations in endogenous IgG levels, it provides a more specific indication of exogenous IgG elimination. Given the shorter t1/2, steady-state IgG after fSCIG 10% initiation would be expected to be achieved within ∼3 months, conservatively, significantly earlier than the currently assumed 6-month period and consistent with other IgGs. Baseline-corrected PK offers a practical estimation of exogenous IgG elimination for fSCIG 10%. Used alongside uncorrected PK, it provides better characterization of IgG PK. Baseline-corrected PK suggests steady-state IgG would be achieved within ∼3 months. Endogenous IgG variability and limitations of baseline correction should be acknowledged in clinical decision-making. Comprehensive dosing strategies should integrate PK approaches with patient-specific factors. This study was funded by Takeda Development Center Americas, Inc.