Delayed ADAMTS13 normalization in caplacizumab/PEX-treated patients associated with anti-ADAMTS13 IgG antibody...

Delayed ADAMTS13 normalization was previously reported in a subgroup of caplacizumab/plasma exchange (PEX) cases. To investigate this, anti-ADAMTS13 immunoglobulin G (IgG) levels were analysed at multiple timepointsduring an acute thrombotic thrombocytopenic purpura episode in 59 caplacizumab/PEX and 50 non-caplacizumab/PEXcases. Caplacizumab/PEX cases required fewer days of PEX, taking longer to achieve ADAMTS13 activity of >20%compared to non-caplacizumab/PEX cases (median, 33 days vs 17.5 days; P< .0001). More caplacizumab/PEX casescompared with non-caplacizumab/PEX showed increased anti-ADAMTS13 IgG levels after PEX (40% vs 25.5%; P =.14). Cases requiring >30 days of caplacizumab had higher presenting anti-ADAMTS13 IgG levels compared with <30days (63.5% vs 25%; P = .02). Half the cases requiring >30 days of caplacizumab increased anti-ADAMTS13 IgG levelsafter PEX; median 101 days to ADAMTS13 activity of >20% compared to 28 days (P< .0001) in cases without anantibody increase. 54% of caplacizumab cases had presenting anti-ADAMTS13 IgG levels of >40%; 47% had increasedantibody levels after PEX, taking longer to achieve ADAMTS13 activity of >20%. 33% of patients with presenting anti-ADAMTS13 IgG levels of <40% increased their antibody levels after PEX and took 97.5 days to achieve ADAMTS13activity of >20%. Cases with presenting anti-ADAMTS13 IgG levels <40% and no increase in antibody levels after PEXtook a median of 14 days (P = .0005). Therefore, the subgroup of caplacizumab/PEX cases with delayed ADAMTS13normalization was associated with antibody boosting, more common with a higher presenting anti-ADAMTS13 IgG and,subsequently, longer caplacizumab use. These findings were subsequently confirmed in 93 cases that were treated withcaplacizumab/PEX over 3 years.