Mepolizumab is a fully humanized monoclonal antibody (IgG1/κ) targeting human interleukin-5 (IL-5), a key haematopoietin needed for eosinophil development and function. Mepolizumab blocks human IL-5 from binding to the α-chain of the IL-5 receptor complex on the eosinophil cell surface, thereby inhibiting IL-5 signalling. The pharmacokinetics of mepolizumab have been evaluated in clinical studies at doses of 0.05-10 mg/kg and at 250 mg, 750 mg and 1500 mg. Mepolizumab was eliminated slowly, with mean initial and terminal phase half-life values of approximately 2 and 20 days, respectively. Plasma clearance ranged from 0.064 to 0.163 mL/h/kg and steady-state volume of distribution ranged from 49 to 93 mL/kg. Pharmacokinetics were dose proportional and time independent. Estimates based on a two-compartment intravenous infusion model from patients with asthma or healthy subjects following single doses predicted mepolizumab plasma concentrations in multiple-dose studies involving patients with hypereosinophilic syndrome (HES), asthma or eosinophilic oesophagitis. The absolute bioavailability of mepolizumab was 64-75% following subcutaneous injection and 81% following intramuscular injection. Peripheral blood eosinophil levels decreased in healthy subjects and patients with HES, asthma, eosinophilic oesophagitis or atopic dermatitis after intravenous mepolizumab infusion and subcutaneous injection. Reductions in eosinophil counts in oesophagus, sputum, skin, bone marrow, nasal lavage fluid and/or bronchial mucosa after treatment with mepolizumab were observed in placebo-controlled studies in various indications. The relationship between percentage change from baseline in blood eosinophils and mepolizumab plasma concentrations was described by an indirect pharmacological response model. The estimated maximal decrease in eosinophil count was approximately 85% from baseline and the half-maximal inhibitory concentration (IC50) was approximately 0.45 μg/mL.