Steady-State Serum IgG Trough Levels Are Adequate for Pharmacokinetic Assessment in Patients with Immunodeficiencies Receiving Subcutaneous Immune Globulin

Patients with primary immunodeficiency diseases often require lifelong immunoglobulin (IG) therapy. Most clinical trials investigating IG therapies characterize serum immunoglobulin G (IgG) pharmacokinetic (PK) profiles by serially assessing serum IgG levels. This retrospective analysis evaluated whether steady-state serum IgG trough level measurement alone is adequate for PK assessment. Based on individual patient serum IgG trough levels from two pivotal trials (phase 2/3 European [NCT01412385] and North American [NCT01218438]) of weekly 20% subcutaneous IG (SCIG; Cuvitru, Ig20Gly), trough level-predicted IgG AUC (AUC(τ,tp)) were calculated and compared with the reported AUC calculated from serum IgG concentration-time profiles (AUC(τ)). In both studies, mean AUC(τ,tp) values for Ig20Gly were essentially equivalent to AUC(τ) with point estimates of geometric mean ratio (GMR) of AUC(τ,tp)/AUC(τ) near 1.0 and 90% CIs within 0.80–1.25. In contrast, for IVIG, 10%, mean AUC(τ,tp) values were lower than AUC(τ) by >20%, (GMR [90% CI]: 0.74 [0.70–0.78] and 0.77 [0.73–0.81] for the two studies, respectively). Mean AUC(τ,tp) values calculated for 4 other SCIG products (based on mean IgG trough levels reported in the literature/labels) were also essentially equivalent to the reported AUC(τ) (differences <10% for all except HyQvia, a facilitated SCIG product), while differences for IVIG products were >20%. In conclusion, steady-state serum IgG levels following weekly SCIG remain stable, allowing for reliable prediction of AUC over the dosing interval using trough IgG levels. These findings indicate that measuring steady-state serum IgG trough levels alone may be adequate for PK assessment of weekly SCIG. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10875-021-00990-z.