Current Status and Optimal Management of Eculizumab Poor-Responders Due to C5 Polymorphisms

(Introduction) Paroxysmal Nocturnal Hemoglobinuria (PNH) is a rare but life-threatening stem cell disease caused by expansion of PIGA mutated clone(s). PIGA mutation abolishes the expression of GPI-anchored proteins on the cell surface including CD55 and CD59 which protect red blood cells from complement attack, resulting in complement-mediated destruction of PNH erythrocytes. Eculizumab (Ecu) effectively ameliorates the intravascular hemolysis in PNH patients by blocking complement C5 in the terminal pathway. We previously reported that 3-4 % of the Japanese patients did not respond to Ecu due to C5 polymorphism c.2654G>A (predicting p.Arg885His) and one Argentina patient with similar but different polymorphism c.2653C>T (predicting p.Arg885Cys) (N Engl J Med 370:632, 2014). Since then, we have received various consults regarding poor response cases and requests for analysis from around the world. Here, we summarize the latest series of analyses in C5 polymorphisms and propose optimal management based on these findings. (Method) Once poor response to Ecu, defined as sustained high serum LDH, was suspected with sustained high serum LDH level, peripheral blood samples with clinical data were sent to our institute after obtaining patients' informed consent. DNA was extracted from the samples, and the hot spot of C5 polymorphisms at exon 21 was sequenced by Sanger method. If no polymorphism was identified, all 41 exons of C5 were sequenced. (Results) At the initial publication (2014), 11 cases of c.2654G>A were identified out of 345 PNH patients (3.2%). As of July, 2018, a total of 22 patients were identified among roughly 600 patients treated with Ecu in Japan (3.7%). To determine the distribution of the polymorphism, a DNA panel containing 120 Han Chinese persons were previously screened, and one had the polymorphism. The same C5 polymorphism was newly identified in one among 89 patients treated with Ecu in Korea. A similar polymorphism c.2653C>T (predicting p.Arg885Cys) was also previously identified in an Argentina patient. Another polymorphism c.2653C>A (predicting p.Arg885Ser) was identified in a Dutch patient. Furthermore, a novel mutation c.2422 G>A (predicting p.Val 808 Ile) was found in a poor responder in Israel, and currently under functional analyses. (Discussion) In PNH patients treated with Ecu, serum LDH level usually drops importantly after the first loading dose, and mostly comes to upper limit of normal range after second or third loading dose. If the serum LDH level remains high, poor response should be considered. CH50 is usually not detected in Ecu responsive patients, so monitoring of CH50 level is critical to evaluate the responsiveness to Ecu. Once poor response is suspected, hot spot of exon 21 should initially be sequenced, and then the whole 41 exons of C5 may need to be sequenced. c.2654G>A was found among Han　Chinese in our previous study, so it is reasonable that a Korean case had the same polymorphism considering the geography. Surprisingly, a Caucasian case with the same polymorphism has been reported (Blood Advances 1:1254, 2017) in addition to the Dutch case, underlining the importance of poor responsiveness due to polymorphisms (p.Arg885) even outside of Asia. We previously reported that Coversin, a C5 inhibitor derived from tick saliva protein, blocked hemolysis in vitro using the serum from the patients with c.2654G>A, and it was reported that post-transplant thrombotic microangiopathy was successfully treated with Coversin for the patient with the polymorphism (Blood Advances 1:1254, 2017). C5 inhibitors targeting a different epitope or having different mechanisms from Ecu as well as other upcoming complement inhibitors targeting Factor D or C3 are expected to benefit patients with C5 polymorphism and resistance to Ecu. Further analyses and clinical trials may pave the way to a second generation anti-complement drug to treat PNH patients.

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