We recently observed that dysregulation of the complement system may be involved in the pathogenesis of hematopoietic stem cell transplantation–associated thrombotic microangiopathy (HSCT-TMA). These findings suggest that the complement inhibitor eculizumab could be a therapeutic option for this severe HSCT complication with high mortality. However, the efficacy of eculizumab in children with HSCT-TMA and its dosing requirements are not known. We treated 6 children with severe HSCT-TMA using eculizumab and adjusted the dose to achieve a therapeutic level >99 μg/mL. HSCT-TMA resolved over time in 4 of 6 children after achieving therapeutic eculizumab levels and complete complement blockade, as measured by low total hemolytic complement activity (CH50). To achieve therapeutic drug levels and a clinical response, children with HSCT-TMA required higher doses or more frequent eculizumab infusions than currently recommended for children with atypical hemolytic uremic syndrome. Two critically ill patients failed to reach therapeutic eculizumab levels, even after dose escalation, and subsequently died. Our data indicate that eculizumab may be a therapeutic option for HSCT-TMA, but HSCT patients appear to require higher medication dosing than recommended for other conditions. We also observed that a CH50 level ≤ 4 complement activity enzyme units correlated with therapeutic eculizumab levels and clinical response, and therefore CH50 may be useful to guide eculizumab dosing in HSCT patients as drug level monitoring is not readily available.

Overactivated complement is a high-risk feature in HSCT recipients with transplant associated thrombotic microangiopathy (TA-TMA), and untreated patients have dismal outcomes. We present our experience of 64 pediatric HSCT recipients with high risk TA-TMA and multi-organ injury treated with the complement blocker eculizumab. We demonstrate significant improvement in 1y post-HSCT survival to 66% in treated patients from our previously reported untreated cohort with same high-risk TA-TMA features that had 1y post-HSCT survival of 16.7%. Responding patients benefited from a brief but intensive eculizumab therapy course using PK/PD guided dosing, requiring a median of 11 doses of eculizumab (IQR 7-20). Therapy was discontinued due to resolution of TA-TMA at a median of 66 days (IQR 41-110). Subjects with higher complement activation measured by elevated blood sC5b-9 at the start of therapy were less likely to respond to treatment (OR =0.15, p-value 0.0014), and required more doses of eculizumab [r = 0.43, p-value = 0.0004]. Patients with intestinal bleeding had the fastest eculizumab clearance, required the highest number of eculizumab doses (20 vs 9, p=0.0015), and had lower 1y survival (44% vs 78%, p=0.01). Over 70% of survivors had proteinuria on long term follow up. The best GFR recovery in survivors was a median 20% lower (IQR 7.3-40.3%) than their pre-HSCT GFR. In summary, complement blockade with eculizumab is an effective therapeutic strategy for high risk TA-TMA, but some patients with severe disease lack a complete response, prompting us to propose early intervention strategies and search for additional targetable endothelial injury pathways.

Thrombotic microangiopathy (TMA) after hematopoietic stem cell transplant (HSCT) associated with terminal complement activation, as measured by elevated plasma terminal complement (sC5b-9) concentrations, has a very high mortality. The complement inhibitor eculizumab may be a therapeutic option for HSCT-associated TMA. We examined the pharmacokinetics and pharmacodynamics (PK/PD) of eculizumab in children and young adult HSCT recipients with TMA and activated complement to determine drug dosing requirements for future efficacy trials. We analyzed prospectively collected laboratory samples and clinical data from 18 HSCT recipients with high-risk TMA presenting with complement activation who were treated with eculizumab. We measured eculizumab serum concentrations, total hemolytic complement activity (CH50), and plasma sC5b-9 concentrations. Population PK/PD analyses correlated eculizumab concentrations with complement blockade and clinical response and determined inter-individual differences in PK parameters. We also compared transplant survival in patients treated with eculizumab (n=18) to patients with the same high-risk TMA features who did not receive any targeted therapy during a separate prospective observational study (n=11). In the PK analysis, we found significant inter-patient variability in eculizumab clearance, ranging from 16 to 237 mL/hr/70kg in the induction phase. The degree of complement activation measured by sC5b-9 concentrations at the start of therapy, in addition to actual body weight, were significant determinants of eculizumab clearance and disease response. Sixty one percent of treated patients had complete resolution of TMA and were able to safely discontinue eculizumab without disease recurrence. Overall survival was significantly higher in treated subjects compared to untreated patients (56% versus 9%, p=0.003). Complement blocking therapy is associated with improved survival in HSCT patients with high-risk TMA who historically have dismal outcomes, but eculizumab pharmacokinetics in HSCT recipients differ significantly from reports in other diseases like atypical hemolytic uremic syndrome and paroxysmal nocturnal hemoglobinurina. Our eculizumab dosing algorithm, including pre-treatment plasma sC5b-9 concentrations, patient’s actual body weight, and the first eculizumab dose (mg), accurately determined eculizumab concentration-time profiles for HSCT recipients with high-risk TMA. This algorithm may guide eculizumab treatment and ensure that future efficacy studies use the most clinically appropriate and cost-efficient dosing schedules.

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Sarcoidosis is a systemic disease of unknown etiology characterized by noncaseating granulomas, consisting mainly of epithelioid cells and multinucleated giant cells derived from monocyte-macrophage lineage cells. Monocytes fall into subpopulations comprising CD14++ CD16-, and CD14+ CD16+ cells, and expansion of the later monocytes has been reported under some pathological conditions. In this study, we examined the immunophenotype of blood monocytes in patients with sarcoidosis using two-color immunofluorescence flow cytometry. In healthy controls CD14+ CD16+ monocytes account for 5.8 +/- 2.8% of monocytes. The percentage of CD14+ CD16+ monocytes was significantly higher (p <0.02) in the sarcoidosis patients (11.8 +/- 4.9%) compared with those in healthy control subjects. The serum ACE levels were significantly correlated with the percentage of CD14+ CD16+ monocytes (p <0.05). In contrast, the percentage was not correlated with purinergic receptor expression of monocytes as estimated by LDH release from BzATP-stimulated monocytes. These findings suggest that CD14+ CD16+ monocytes represent a sensitive marker for the disease activity of sarcoidosis.