We appreciate the comments from Saccà and Pelto 1 on our article “Inhibition of classical and alternative complement pathway by ravulizumab and eculizumab.” 2 We agree that our data represented an initial step toward improving the understanding of therapeutic drug monitoring for C5 inhibitors. However, Saccà and Pelto state that CH50 and AH50 assays have not been analytically or clinically validated for monitoring C5 inhibitor therapy. In contrast, therapeutic drug monitoring for eculizumab has long incorporated the assessment of complement activity (e.g., CH50 or AH50) together with drug levels across several clinical indications, including paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), and myasthenia gravis (MG) 3-6. Hemolytic complement activity assays were also employed in pivotal clinical trials of eculizumab in aHUS and PNH 7, 8. One of the cited publications (Schönfelder et al.) explicitly states that the retrospective analysis did not include pharmacokinetic or pharmacodynamic data on eculizumab or ravulizumab, nor measurements of free C5 or complement activity (e.g., CH50, AH50, or soluble C5b-9). Another cited report (Cataland et al.), available only as a poster abstract, employs heterogeneous measurement methods and does not report CH50 values. Consequently, these studies do not allow conclusions regarding the suitability of these biomarkers for therapeutic drug monitoring. Previous studies have demonstrated marked interindividual variability in complement activity and drug levels, underscoring the potential relevance of therapeutic drug monitoring for more individualized treatment strategies 3, 4, 9. We agree that the choice of assay system is important. In addition to the classical hemolytic assay based on erythrocyte lysis used in our study, several alternative approaches to measure CH50 exist, including ELISA- and liposome-based assays. However, some of these provide only a threshold readout (e.g., < 10% or < 10 U/mL) without further differentiation 10, 11. Although such assays may be useful for monitoring patient adherence, particularly with self-administered formulations, they do not allow a precise assessment of the degree of complement inhibition or the detection of potential differences between complement inhibitors. For ravulizumab, the manufacturer has introduced measurement of free C5 as a surrogate marker of terminal complement blockade, with levels of < 0.5 μg/mL considered indicative of complete inhibition. Notably, in a recent phase 3 trial, CH50 analysis—alongside measurement of free C5—was defined as a primary pharmacodynamic endpoint for assessing C5 inhibition by ravulizumab, with hemolytic activity of < 20% considered consistent with complete blockage 12. Our study suggests that the AH50 functional complement assay (reflecting alternative pathway activity) may represent a promising biomarker in patients with MG treated with ravulizumab and could support more precise monitoring and optimization of therapy, including the detection of suboptimal complement inhibition or earlier wearing-off of therapeutic effects. Whether free C5 measurements provide similar or complementary information in this context remains to be determined. Our colleagues further highlight the importance of assay methodology for measuring drug levels. Although ELISA-based measurements may differ from those obtained by liquid chromatography–tandem mass spectrometry, the key finding in our cohort was the marked interindividual variability in ravulizumab pharmacokinetics rather than the proportion of patients below the proposed threshold. Saccà and Pelto also underscore the importance of continued clinical assessment using validated scores. We agree; however, instruments such as the MG-ADL capture only part of the patients' experience 13, 14. Current recommendations therefore also advocate the inclusion of subjective and holistic patient-reported measures to complement clinical scores when evaluating treatment response and patient satisfaction 15. At the same time, there remains a clear need for objective biomarkers to support therapy monitoring. In conclusion, we believe that recommendations for therapeutic drug monitoring will likely be incorporated into future guidelines and treatment algorithms. Until then, further biomarker studies, including assessments of complement activity, are needed to improve our understanding. Ultimately, the goal should be to identify biomarkers that predict treatment responses either before therapy initiation or early during the course of treatment. L.G. and A.M.: conceptualization. All authors: drafting/revision of the manuscript for content. The authors have nothing to report. L.G. received speaker's honoraria, served as an advisory board member and/or received travel and congress fees from Alnylam, Alexion, Johnson & Johnson, Roche and UCB and is a shareholder of RareLink digital health GmbH. J.S.-B. has received honoraria for attendance at advisory boards from Novartis. A.M. received speaker's honoraria, served as an advisory board/DSMB member and consultant, and has received research grants (paid to his institution) from Alexion AstraZeneca Rare Disease, Amgen, Argenx, Axunio, Desitin, Genpharm, Grifols, Hormosan, Immunovant, Janssen, Merck, Neopharm, Novartis, Octapharma, Regeneron, Sanofi and UCB. He serves as chairman of the Association for Research into Myasthenic Syndroms in Germany and is member of the medical advisory board of the German Myasthenia Gravis Society. G.W. reports no conflicts of interest. Data sharing is not applicable to this article as no datasets were generated or analyzed for this reply.
Gerischer et al. (Tue,) studied this question.