Sickle cell disease (SCD) is characterized by frequent and unpredictable vaso-occlusive complications (VOCs); however, there are no objective measures for VOC as a clinical endpoint. Vaso-occlusion results from processes that reduce blood flow in the microvasculature, including red cell stickiness and erythrocyte sickling. These processes lead to pain, chronic organ damage, and decreased life expectancy.
The decision to seek medical contact varies amongst patients. When VOCs are managed at home valuable information remains unknown. We designed a longitudinal, observational study to capture adhesion data at home and in a hospital setting. The objective of this study was to determine whether a standardized, flow-based adhesion bioassay is capable of identifying VOCs occurring in SCD patients with varying degrees of medical contact. SCD patients (n=33) were evaluated over a 6-month period. Blood samples were collected every 3 weeks; when patients report a VOC corresponding blood samples are collected and steady state samples are resumed. During 6 months of evaluation, longitudinal measures of pain and clinical laboratory and biomarker samples from SCD patients (+/- hydroxyurea therapy) in steady state to VOC were studied.
Blood samples were collected in sodium citrate from SCD subjects at steady state and during VOCs. Blood samples were perfused through VCAM-1-coated microchannels at standard physiologic flow conditions (1dyne/cm2, 1.67Hz). An adhesion index was established by quantifying adherent cells within a standard viewing area (cells/mm2), and could be obtained within 6-9 min.
Adhesion indices varied from sample-to-sample at baseline (n=289; mean = 355 ± 235; median = 297 cells/mm2) and during VOC (n=59, mean=416±233, median=390). Repeated measures of adhesion over 6 months reveals significant intra-patient associations with C-reactive protein (CRP, n=335, r=0.16; p=0.006), lactose dehydrogenase (LDH, n=336, r=0.12; p=0.032), white blood cells (WBC, n=341, r=0.13; p=0.019), and reticulocyte percent (n=336, r=0.37, p<0.0001). The results also show significant inter-patient (n=35) correlations with CRP (r=0.34, p=0.047), fetal hemoglobin (HbF, r=-0.61, p=0.0001), reticulocyte percent (r=0.63, p<0.0001), reticulocyte (r=0.77, p<0.0001), and uric acid (r=0.37, p=0.028). At-home VOC adhesion indices (n=33; mean=482±255) were significantly higher than both ER-based VOC (n=8; mean=322± 153; p=0.031) and hospital-based VOC (n=18, mean=336±182; p=0.018) adhesion indices. The difference between at home VOC adhesion indices and baseline adhesion indices approached significance (482 ± 255 vs 355 ± 235, p=0.088).
This study represents the largest longitudinal study of adhesion indices using a standardized clinical assay. These data confirm the normal range and longitudinal variability of SCD adhesion indices at baseline and during VOC. Adhesion increased during patient-reported VOCs in a subpopulation of individuals with SCD, suggesting there may be a subphenotype who are more predisposed to adhesion-mediated VOCs. At-home VOCs are likely higher because ER-VOC indices are influenced by fluid boluses, blood transfusions, or anti-inflammatory therapy. Further studies are underway to determine if a clinical adhesion index can effectively monitor response to SCD-modifying therapies and prospectively predict disease progression.
White:functional fluidics: Equity Ownership. Gao:Functional Fluidics: Equity Ownership. Liu:Functional Fluidics: Equity Ownership. Callaghan:Bioverativ: Honoraria; Alnylam Pharmaceuticals: Equity Ownership; Bayer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Octapharma: Honoraria; Sancilio Pharmaceuticals Company: Employment; Novo Nordisk: Employment, Membership on an entity's Board of Directors or advisory committees; Shire: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Hema Pharmaceuticals: Honoraria; Grifols: Honoraria; Pfizer: Employment, Honoraria, Research Funding; Roche/Genentech: Employment, Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Employment; Global Blood Therepeutics: Employment. Hines:functional fluidics: Equity Ownership.
Asterisk with author names denotes non-ASH members.