A prophylactic monoclonal IgG infusion produces an initial serum concentration of 200 units of neutralizing activity. The protected threshold is 25 units. If the monoclonal IgG has an effective clinical half-life in the patient of 21 days, after how many days will the neutralizing activity decline to the threshold level? Show calculations.

• A. Approximately 126 days because monoclonal antibodies always require six half-lives to fall below clinically relevant thresholds regardless of the starting concentration.
• B. Approximately 21 days since each infusion is expected to lose all clinical activity after a single half-life, so once 200 units falls to 100 units at 21 days it effectively becomes nonprotective relative to a threshold of 25 units and is considered gone clinically.
• C. Approximately 9 days because antibody decay is linear rather than exponential after infusion, so dividing 200 by the threshold 25 gives 8 and multiplying by the half-life incorrectly yields a much shorter duration of protection.
• D. Approximately 63 days; three half-lives (3 × 21 days) reduce 200 units to 25 units.

Answer: (D)