If it does not help, it might hurt: Pharmacodynamics of a second IVIg course in Guillain-Barré syndrome.

Objectives: Intravenous immunoglobulin (IVIg) is an effective treatment for Guillain-Barré syndrome (GBS), but recovery varies between patients. This study aims to evaluate the pharmacokinetics (PK) and pharmacodynamics (PD) of a single and a second IVIg dose (SID) in patients with GBS.

Methods: Data were analyzed from the SID-GBS trial, a double-blind, randomized, placebo-controlled study. Patients with poor prognosis (modified Erasmus GBS Outcome Score, mEGOS ≥6) after a standard course of IVIg (0.4 g/kg for 5 days) were randomized to receive either SID or placebo. Serum IgG levels were measured at standard serial time points and clinical outcomes were assessed using the GBS disability score and Medical Research Council sum score. PK modeling was performed to predict IVIg exposure and its association with clinical outcomes.

Results: Serum IgG concentration after a single and double course of IVIg was variable, but accurately described by the current PK model. Lower ΔIgG and IVIg exposure were associated with poorer clinical outcomes. SID increased the IgG concentration, but did not result in an improvement in clinical outcome. Serious adverse events, including thromboembolic events, occurred more frequently in the SID group and were associated with lower IVIg exposure.

Interpretation: SID increases serum IgG levels in GBS patients as predicted by the current PK model, but does not improve clinical outcomes and increases the risk of serious adverse events. Model-informed precision dosing may guide individualization of treatment.