Pharmacokinetics and Pharmacodynamics

The pharmacokinetic profile of subcutaneous efalizu-mab was described by Gottlieb, Miller et al. (2003) in a Phase 1, open-label, escalating dose study, in which it was found that peak efalizumab plasma concentrations were achieved in approximately 1-2 days following injection, and that efalizumab was detectable in the serum for 3-5 weeks following the final injection. Patients receiving efalizumab 0.5 -1.0 mg/kg SC weekly had an average T1/2 of 4 days, whereas those receiving 1.0-2.0 mg/kg had a T1/2 of 6 days. Subcutaneous doses administered weekly gave peak plasma concentrations and efalizumab exposure of approximately one-third to one-half of the equivalent intravenous dose.

A study examining the pharmacodynamic profile of subcutaneous efalizumab found that CD11a expression on circulating lymphocytes rapidly decreased to 15-30% of pretreatment levels and remained at this suppressed level until efalizumab was cleared from the plasma (Wellington and Perry 2005). Within 7-10 days of efalizumab clearance, CD11a expression returned to baseline. Following subcutaneous efalizumab administration of 1 mg/kg/week, CD11a expression is reduced within 1-2 days and is maintained with weekly administration. In addition, phase 2 studies evaluating SC efalizumab have noted histologically that epidermal thickness was reduced in the 0.3 mg/kg group com pared to placebo. This treatment group, 0.3 mg/kg efa-lizumab, also noted a reduction in the number of CD3+ T cells in skin biopsy specimens with a concurrent increase in circulating lymphocytes.

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