Hemolytic uremic syndrome (HUS) is a group of diseases characterized by the triad of nonimmune microangiopathic hemolytic anemia, thrombocytopenia, and decreased renal function. HUS is the common clinical result of various pathological processes. Primary injury to the endothelium (mainly renal) causes cell swelling and detachment, formation of microthrombi, mechanical hemolytic anemia, and organ dysfunction, mainly of the kidneys. Extrarenal organ damage (i.e., neurological, cardiovascular, pulmonary, and gastrointestinal) sometimes also presents (Alfandary et al., 2020).

In its typical form, HUS is preceded by bloody diarrhea induced by a Shiga toxin (STX)-producing Escherichia coli (STEC), and transmitted by contaminated food or water (Monet-Didailler et al., 2020). Atypical HUS (aHUS) is a syndrome caused by inherited or acquired defects in regulation of the complement system’s alternative pathway, now described as “complement HUS”. Both dysregulation and excessive uncontrolled activation of the complement alternative pathway (Noris and Remuzzi, 2009; Zipfel et al., 2010; Zipfel and Skerka, 2009) result in persistent cleavage of complement protein C5 and formation of membrane attack complex (MAC). This leads to endothelial injury and systemic thrombotic microangiopathy (TMA) (Benz and Amann, 2010; Noris et al., 2010; Loirat and Frémeaux-Bacchi, 2011; Licht et al., 2015). Activation of the complement system is triggered by abnormal function of complement regulators: factor H, factor I, membrane cofactor protein, C3, and factor B. Genetic mutations have been identified in 50%–70% of patients with aHUS (Noris et al., 2010; Kavanagh and Goodship, 2011; Kavanagh and Goodship, 2010).

The various types of HUS are caused by different primary triggers; yet, their clinical features overlap (Alfandary et al., 2020). Several studies have demonstrated uncontrolled activation of the complement system in patients with STEC-HUS, as well as in patients after hematopoietic stem cell transplantation (HSCT), thus leading to transplant-associated TMA (TA-TMA) (Walsh and Johnson, 2019; Buelli et al., 2019; Poolpol et al., 2014; Orth et al., 2009; Dvorak et al., 2019; Cugno et al., 2021).

The classification of TA-TMA has evolved over recent years from traditionally categorized under secondary aHUS (Aklilu and Shirali, 2023; Goodship et al., 2017; Timmermans and van Paassen, 2021). Supported by literature and expert consensus, TA-TMA has increasingly classified as a distinct entity under the broader umbrella of TMA rather than as a subtype of secondary aHUS (Li and Sartain, 2024; Lazana, 2023). This shift reflects a growing understanding of its unique pathophysiology, particularly its strong association with endothelial injury rather than primary complement dysregulation.

Generally, aHUS has been treated with plasmapheresis, to induce stabilization of hematologic parameters, yet the efficacy has been limited. In observational studies, plasmapheresis was shown not to efficiently prevent the progression of tissue damage and substantial morbidity and mortality (Greenbaum et al., 2016; Fremeaux-Bacchi et al., 2013). This is due to the persistence of the underlying complement-mediated pathogenic mechanism of TMA (Noris et al., 2010; Fremeaux-Bacchi et al., 2013).

Eculizumab (Soliris^®, Alexion Pharmaceuticals), a terminal complement inhibitor, is a recombinant, humanized, monoclonal immunoglobulin G antibody that targets human C5 complement protein and inhibits the subsequent formation of terminal MAC. Consequently, eculizumab is expected to prevent host cell damage as well as TMA-characterized triad and multiorgan damage (Loirat and Frémeaux-Bacchi, 2011), as manifested in complement HUS (Licht et al., 2015). Likewise in TA-TMA, in which complement system overactivation has been identified (Jodele et al., 2014), a blockade with eculizumab might be a suitable therapeutic strategy. Additionally, the demonstration of hyperactivation of complement by Shiga toxin in STEC-HUS raises the possibility that eculizumab may also have beneficial effects in typical-HUS (Noris et al., 2012; Karpman and Tati, 2016).

Retrospective and prospective clinical trials have established the efficacy and safety of eculizumab in adults with aHUS leading to its approval for pediatric patients with this condition (Rathbone et al., 2013; Legendre et al., 2013). Its beneficial effects in children with aHUS are supported by prospective and retrospective studies (Greenbaum et al., 2016; Ito et al., 2019; Davin et al., 2010; Simonetti, 2011) Beyond aHUS, eculizumab has been explored for other forms of thrombotic microangiopathy (TMA). Studies have reported favorable responses in adult and pediatric HSCT-recipients with complement-mediated TA-TMA (Dvorak et al., 2019; Zhang et al., 2021; Jodele et al., 2020; de Fontbrune et al., 2015; Dhakal et al., 2017). Growing evidence, including a prospective trial, supports eculizumab’s effectiveness in pediatric TA-TMA, improving survival and organ recovery (Jodele et al., 2024). Recent dosing strategies have further optimized its use, reinforcing its emerging role as a standard therapeutic option (Jodele et al., 2014; Jodele et al., 2020; Jodele et al., 2024). Additionally, retrospective studies have described both favorable and controversial outcomes in pediatric patients with STEC-HUS treated with eculizumab (Monet-Didailler et al., 2020; Walsh and Johnson, 2019; Percheron et al., 2018; Garnier et al., 2023). STEC-HUS patients should be considered eligible for eculizumab treatment based on clinical severity, TMA criteria, and the risk of multi-system involvement, as its use is reserved for cases where supportive care alone is insufficient. The studies evaluating the effectiveness and safety of eculizumab in aHUS (approved indication), as well as in STEC-HUS and TA-TMA (off-label use), have used different outcome measures, making direct comparisons challenging. Additionally, the small pediatric sample sizes in these studies further limit generalizability.

Due to the overlapping presentation and clinical features, patients diagnosed with complement HUS, STEC-HUS, or TA-TMA are currently treated by the same clinicians and medical specialists. To evaluate the effectiveness of eculizumab in these types of TMA, homogeneous clinical measurements are essential.

The objective of this study was to assess the efficacy and safety of eculizumab in pediatric patients diagnosed with aHUS, STEC-HUS, and TA-TMA, using the same outcome measures, at a tertiary center in a real-world setting.

Study design and patients: An observational retrospective study was conducted of all the patients diagnosed with aHUS, STEC-HUS, or TA-TMA at Schneider Children’s Medical Center during 2011–2020. Only the patients who received at least one dose of eculizumab for TMA treatment were included.

Demographic and baseline clinical data were collected, including HUS diagnosis and classification, the involvement of other organ systems, complement gene mutation status (if available), and prophylactic treatment for meningitis (vaccine and/or prophylactic amoxicillin) before eculizumab initiation.

Baseline and follow-up clinical parameters were documented. Hematologic parameters included platelet (PLT) count, lactate dehydrogenase (LDH) level and hemoglobin (Hb) concentration. Renal function was assessed according to the estimated glomerular filtration rate (eGFR), calculated by the bedside Schwartz equation, and the chronic kidney disease (CKD) stage, categorized according to the Kidney Disease: Improving Global Outcomes (KDIGO) 2021 clinical practice guideline (Kidney Disease: Improving Global Outcomes KDIGO Blood Pressure Work Group, 2021). Additionally, the need for dialysis or plasma infusion/plasma exchange (PI/PE) and CH-50 before or during follow-up was recorded. Data collection spanned a 28-week period following the first dose of eculizumab. For patients with a follow-up duration shorter than 28 weeks, data collection continued for 12 weeks after eculizumab discontinuation.

During the study period, eculizumab was administered according to body weight-based dosing recommendations provided by the manufacturer with adjustments required in administration frequency based on clinical and laboratory parameters and infused over 1–4 h (detailed in Supplementary Table S1).

Assessments of effectiveness: The primary endpoint was the proportion of patients who achieved a complete TMA response. This was defined as both sustained (≥4 weeks) normalization of hematologic parameters and improvement in renal function (detailed in Table 1) on two consecutive measurements.

A secondary endpoint was TMA event-free status, which was defined by the absence of a PLT decreases by >25% from baseline as measured on the day of eculizumab initiation, the absence of PE/PI, and no new dialysis for ≥12 weeks. Other secondary endpoints comprised additional hematologic and renal improvements, including normalization of PLT, LDH, Hb, eGFR, and CKD stage, sustained for ≥4 weeks on two consecutive measurements (detailed in Table 1). The time to normalization of these parameters was calculated from the first eculizumab dose to the day that the first normal measurement was obtained. To avoid biased positive results, analyses of individual parameters: PLT, LDH, Hb, eGFR, and CKD excluded patients with normal baseline values, for whom the drug is expected to be ineffective.

Assessments of safety: Data on adverse events, changes in medical conditions, and causality were obtained from electronic medical records.

Statistical analysis: The study was approved by the Institutional Helsinki Committee of Schneider Children’s Medical Center, with a waiver for informed consent. Baseline characteristics are presented as means, medians, standard deviations (SDs), and ranges (for continuous variables); and as frequencies and proportions (for categorical variables). Mean changes (with 95% confidence intervals) from baseline in eGFR, Hb, LDH, and PLT were calculated over 28 weeks. Two patients with end-stage kidney disease (ESKD) were excluded from the eGFR analysis. The data was assessed according to the last observation carried forward (LOCF). The analyses were performed with IBM-SPSS version 26. Locally estimated scatter plot smoothing (LOESS) was performed to generate smoothed plots over the follow-up period, using the geom, smooth function in the ggplot2 package in R.

Twenty-five children and adolescents were recruited for the study. One of them was excluded from the analysis due to the absence of baseline data.

In this retrospective study of 24 patients treated with eculizumab for HUS at one tertiary medical center, efficacy was demonstrated overall, and no severe adverse events were attributed to the drug. This study contributes to the existing literature by evaluating eculizumab across three TMA subtypes using consistent outcome measures.

The clinical outcomes we report for aHUS are consistent with the previously published results (Greenbaum et al., 2016; Ito et al., 2019), supporting the use of eculizumab as a first-line treatment in pediatric populations. Complete TMA response, the primary outcome, was achieved in 54% of aHUS patients, comparable to rates in pediatric reports of 64% by Greenbaum et al., (2016) and 36% by Ito et al., (2019) adult studies (Fakhouri et al., 2016). A significant proportion of patients also achieved TMA event-free status, hematologic normalization, and renal improvement, aligning with previously reported high success rates (Greenbaum et al., 2016; Ito et al., 2019).

Of our six patients with aHUS who did not achieve a complete response, two (15%) had pre-existing ESKD unrelated to HUS and had been on dialysis for several years. Despite their lack of improvement in renal function during the follow-up period, these patients were included in the analysis, unlike previous studies (Greenbaum et al., 2016). Additionally, this allowed us to assess eculizumab’s impact on hematologic and other parameters in ESKD patients, though including children with pre-existing ESKD from non-HUS glomerular diseases inevitably biased this outcome.

In our study, 61.5% of aHUS patients underwent genetic testing, which aligns with reported rates in retrospective studies (Ito et al., 2019) and is comparable to some prospective studies (Licht et al., 2015; Greenbaum et al., 2016) where systematic genetic screening was not mandated. While genetic testing provides valuable insights into complement dysregulation, it is not required for diagnosing aHUS or initiating eculizumab treatment (Licht et al., 2015; Zuber et al., 2012; Campistol et al., 2013). Given that results may take months to obtain, clinicians often rely on clinical presentation rather than genetic findings to guide immediate management (Noris et al., 2010; Licht et al., 2015; Fremeaux-Bacchi et al., 2013; Zuber et al., 2012). Our findings highlight that, in real-world settings, genetic testing, is not prioritized during acute conditions and performed selectively based on clinical judgment, rather than as a universal requirement.

STEC-HUS patients in our study were evaluated using the same criteria as aHUS patients, with 60% achieving complete TMA response and TMA event-free status, and all showing renal function improvement. Our study provides additional data on eculizumab use in pediatric STEC-HUS, including cases with and without neurological or systemic involvement (60% and 40%, respectively). Both subgroups showed clinical improvement, though not all met predefined outcomes due to rigorous criteria, the notable clinical advancements and highlight eculizumab potential benefit.

Eculizumab treatment was initiated based on clinical severity, including TMA diagnostic criteria and the risk mentioned of multi-system involvement, particularly neurological or cardiovascular complications. Since eculizumab is not ultimately indicated for STEC-HUS, its use was reserved for cases where supportive care alone was insufficient. This selection approach aligns with current evidence, which suggests that while complement activation plays a role in STEC-HUS pathogenesis, its clinical significance varies (Garnier et al., 2023), necessitating individualized treatment decisions.

Several studies have investigated the effectiveness of eculizumab in treating STEC-HUS, highlighting both benefits and limitations. Retrospective studies by Giordano et al., (2019) and Percheron et al., (2018) underscored the importance of early treatment, particularly when the presentation is severe and with neurological involvement, showing improvement in hematologic and neurological outcomes. Percheron et al. also suggested that sustained complement inhibition with eculizumab might correlate with favorable outcomes.

However, findings on eculizumab’s effectiveness in milder STEC-HUS cases are mixed. Monet-Didailler et al. (2020) reported no significant differences in blood pressure, proteinuria, or renal function between eculizumab-treated and untreated patients at 1- and 12-months follow-ups, suggesting limited efficacy in mild cases. Garnier et al. (2023) randomized controlled trial showed no difference in acute disease progression between eculizumab-treated and control groups, but observed less renal damage after 1 year in the treatment group. These findings indicate that eculizumab may be effective in severe presentations, as supported by our study’s results, but its benefits in milder presentations remain uncertain, reinforcing the need for careful patient selection.

This study adds to the growing evidence supporting eculizumab’s effectiveness in treating TA-TMA in children post-bone marrow transplant. Consistency across studies strengthens our understanding of eculizumab’s efficacy in managing TA-TMA. Jodele et al. (2020) and Jodele et al. (2020), Dhakal et al., 2017) reported improved clinical outcomes in 66% and 92% of surviving children, respectively, 1 year after adding eculizumab to other treatments. More recently, a prospective study by Jodele et al. (2024) in children with multi-organ injury demonstrated a 71% survival rate at 6 months and 62% at 1 year, further reinforcing eculizumab’s role in high-risk TA-TMA (Jodele et al., 2024). In comparison, our study observed a 67% survival rate, which aligns with these previous findings. Additionally, 50% of our TA-TMA patients achieved TMA event-free status and hematological normalization, while 33% attained complete TMA response. Our study employed stricter outcome definitions, requiring multiple hematologic and renal criteria for response, in contrast to prior studies that primarily assessed survival. This difference in outcome measures may account for variations in reported response rates. Renal function improved in the three relevant children, further supporting eculizumab as a promising treatment for children with TA-TMA and preserved kidney function, though larger studies are needed. Notably, during our study period, and still today, a standardized, consensus-based regimen for TA-TMA remains under investigation with precision-guided dosing strategies only recently emerging to further optimize eculizumab’s effectiveness (Jodele et al., 2024; Mizuno et al., 2022). Recognizing the need for intensified dosing regimen in TA-TMA, as supported by recent studies (Jodele et al., 2024; Mizuno et al., 2022), an individualized dosing approach was adapted with intensified frequent initial dosing, guided by clinical and laboratory biomarkers to optimize outcomes.

Like prior research, our study focused on children with severe TA-TMA manifestations. Notably, all six TA-TMA patients in our cohort had pre-eculizumab GVHD, a factor associated with increased non-relapse mortality and high-risk TA-TMA, as described in previous studies (de Fontbrune et al., 2015; Schoettler et al., 2023). The high severity of GVHD and the two infection-related deaths in our cohort further underscore the strong temporal and clinical link between severe GVHD and TA-TMA, with GVHD typically preceding TA-TMA by several weeks. However, prognosis is multifactorial, with complement activation markers, multi-organ dysfunction, and infections also play a critical role (Schoettler et al., 2023). Further research is needed to assess eculizumab’s efficacy in milder disease and across a broader spectrum of TA-TMA severity.

The recently introduced TA-TMA risk stratification system (Schoettler et al., 2023; Carabante et al., 2024) classifies all six patients in our cohort as ‘High-Risk TMA’. This was based on severe GVHD (Grade 3–4) and peak LDH >2 times ULN in all patients, in addition to systemic infections, random urine protein-to-creatinine ratio (rUPCR) ≥1 mg/mg, and organ dysfunction (excluding acute kidney injury) in some of them. Given the poorer prognosis associated with high-risk TA-TMA, this classification helps contextualize our cohort’s response rates.

Diagnosing TA-TMA remains challenging due to overlapping of non-specific symptoms with common post-HSCT complications such as GVHD, infections, and drug toxicities (Schoettler et al., 2023) and lacks specific biomarkers. These share clinical features such as anemia, thrombocytopenia, hypertension, and proteinuria that often delay recognition and, consequently, eculizumab initiation. Additionally, the lack of standardized TA-TMA screening and treatment protocols—which have only recently emerged in clinical practice—has further contributed to treatment delays (Mizuno et al., 2022). Earlier initiation of eculizumab has been suggested to improve clinical outcomes (de Fontbrune et al., 2015), emphasizing the importance of timely diagnosis.

The delayed initiation of eculizumab in four TA-TMA patients was attributed to these diagnostic challenges. Beyond TA-TMA cases, two additional aHUS patients faced delays in eculizumab initiation due to unique challenges. One patient, with a history of FSGS leading to chronic dialysis and hypertension, had overlapping symptoms, making it difficult to confirm aHUS as the cause of TMA and delaying treatment. Another patient had been diagnosed with aHUS for several years before eculizumab was approved, treated with plasmapheresis and received treatment through a compassionate-use program. Remarkably, only one aHUS patient in our cohort initiated treatment more than 30 days from diagnosis, and no TA-TMA patient experienced such a delay.

Notably, some of our patients with both aHUS and STEC-HUS had favorable outcomes after prompt administration of eculizumab. Our findings collaborate those (Goodship et al., 2017; Dhakal et al., 2017; Giordano et al., 2019; Loirat et al., 2016) who claimed that the timing of eculizumab initiation relative to symptom onset and the involvement of additional organ systems are key factors influencing treatment efficacy in children.

Compared to single-timepoint measurements, our longitudinal monitoring of PLT, LDH, Hb, and eGFR over 28 weeks provided a more comprehensive evaluation of treatment outcomes and better informing of clinical decision-making. The use of the LOCF method helped overcome the challenges due to variations in follow-up durations and measurement frequency. The most significant changes occurred within the first 5 weeks of treatment. Statistically significant improvement was observed in all the parameters from baseline onwards, except for eGFR, for which a trend of improvement was exhibited. This observation can be attributed to the inclusion of two children with ESKD requiring dialysis (2/24) and three with normal baseline eGFR (3/24). Given their pre-existing renal status, these five children, representing 21% of the cohort, were not expected to show any improvement in their renal function. Consequently, their inclusion may have masked the overall improvement in eGFR. However, the remaining 19 children displayed notable improvements in eGFR and other renal parameters, underscoring the positive impact of eculizumab treatment on renal function in the majority of the cohort.

As expected, CH-50 assay values decreased significantly after eculizumab initiation, potentially reaching complete absence of activity of the complement system, thus confirming the drug’s effect. As a measure of complement system activity and an indication of the extent of MAC blockade by eculizumab, the CH-50 assay reflects treatment efficacy and aids in dosage adjustments, especially in patients with TA-TMA and high complement activation (Jodele et al., 2020).

Using the same standardized parameters for evaluating pediatric patients with aHUS, STEC-HUS, and TA-TMA, eculizumab was shown to be efficacious and safe. Overall, hematological and renal parameters improved, and the need for plasmapheresis and dialysis decreased. TMA progression was prevented in children with complement-mediated HUS. Moreover, early intervention was more effective across all HUS indications. The unique inclusion of patients with ESKD highlights the drug’s beneficial effects beyond renal improvement, and its potential to impact additional organ systems. The diversity in symptom severity and system involvement, including the neurological system, provide a comprehensive analysis of eculizumab’s efficacy. Well-designed, prospective, randomized controlled trials are needed, encompassing a larger, more diverse patient population, with varying disease severities.