Real World Applications of Clinical Whole Blood Adhesion (VCAM and P-selectin) Biomarkers in a Pediatric SCD Population

Background: Vaso-occlusive (VOC) pain is the most common complication in sickle cell disease (SCD) and is associated with increased morbidity and early mortality. VOC is the result of intravascular sickling, decreased red blood cell deformability with increased inflammation and adhesion in the vessel bed. Recent data has demonstrated the ability of whole blood adhesion to both vascular cell adhesion molecule (VCAM) and P-selectin (Psel) to stratify individuals with SCD according to their risk of developing a self-reported VOC. Literature regarding the clinical utility of whole blood adhesion biomarkers in pediatric patients with SCD is limited. We describe our real-world application of the flow adhesion of whole blood to VCAM-1 (FA-WB-VCAM) and flow adhesion of whole blood to P-selectin (FA-WB-Psel) assays in pediatric patients with SCD.

Methods: This was a cross sectional analysis of baseline flow adhesion of whole blood to VCAM-1 and flow adhesion of whole blood to P-selectin indices obtained in children (<18 y/o) and young adult patients (18-21 y/o) with SCD at Children's National Hospital in Washington, D.C. between January 2020 and March 2022. Both FA-WB-VCAM and FA-WB-Psel were performed by the Functional Fluidic laboratory in Detroit, MI following overnight shipping at 4-8^oC. Change in disease severity included increased VOC pain rate (home, emergency department or hospital management), acute chest syndrome, priapism, and nephropathy. Change in disease modifying therapy (DMT) included new SCD-specific pharmacologic agent or dose increases for effect (not including minor adjustments for weight gain). Statistical analysis included descriptive statistics, Pearson's correlation, and categorical analysis by t-test (GraphPad, v9, 2021). P value <0.05 was statistically significant.

Results: We identified 109 unique pediatric patients with FA-WB-VCAM and FA-WB-Psel assays performed as part of the standard of care. Eighty-seven percent were children (N=95), and 13% were young adults (N=14). The median age was 11 years (IQR 7-15 years), with 47% males (N=51) and 53% females (N=58). Sixty-two percent of patients were on hydroxyurea (N=68), 4.5% on crizanlizumab (N=5), 5.5% on voxelotor (N=6), and 3% on Endari (N=3). FA-WB-VCAM was similar in children vs young adults [median (IQR): 242 (142-500) cells/m2 vs. 239 (134.8-324.8) cells/m2, p=0.0856] while FA-WB-Psel was higher in the young adult population [median (IQR): 32 (11-52) cells/m2 vs. 45 (20-76.5) cells/m2, p=0.8681]. Seventy-four percent of patients were HgbSS, 22% Hgb SC, 2% Hgb SBeta Thalassemia Zero, 1% Hgb SBeta Thalassemia Plus, and 1% Hgb SE. Baseline FA-WB-P-Sel and FA-WB-VCAM was performed as part of our standard of care. The most common indication for testing was for baseline evaluation (N=76, 70%), followed by a change in disease severity (N=18, 16%), pre-initiation of DMT/change in DMT (N=5, 4.5%), and a combination of change in disease severity and DMT (N=8, 7%). There were two patients (2%) that had an acute clinical event, including splenic sequestration and crizanlizumab-infusion related VOC. There were FA-WB-Psel values in patients with a change in disease severity vs. without (55.8 vs. 37.28 cells/m2, p=0.036) but no difference in FA-WB-VCAM (299.7 vs. 329.2 cells/m2, p=0.729). Figure 1 represents a comparison of average FA-WB-VCAM (Fig 1A)(362.4±271 vs 216.8±138 cells/m2, p<0.001) and FA-WB-PSEL (Fig 1B)(46.4±49 vs 27.83±24 cells/m2, p=0.0138) values between HgbSS (N=81) and HgbSC (N=24) genotypes. There was an inverse correlation between FA-WB-VCAM-1 and HgbF levels (Pearson's R = 0.483, p<0.001).

Conclusions: We established preliminary mean values of FA-WB-Psel and FA-WB-VCAM adhesion indices within all SCD genotypes. FA-WB-VCAM was higher in children while FA-WB-Psel was higher in young adults. Our data represents the expected differences in HgbSS and HgbSC phenotypes with patients with HgbSS reflecting higher adhesion values. There was also an inverse correlation to VCAM-1 and Hgb F levels, consistent with an improvement in RBC health in response to HgbF induction. FA-WB-Psel values were most reflective of a recent change in disease severity and may represent an attractive target to monitor and predict risk of changes in SCD severity. Future studies will measure serially pre vs. post therapeutic intervention and from VOC pain hospitalization to outpatient follow-up clinic visits in pediatric SCD.

Darbari:Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Hines:Functional Fluidics: Current Employment, Current holder of stock options in a privately-held company. Campbell:Novartis: Consultancy, Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; Agios: Consultancy, Research Funding; Forma: Consultancy, Research Funding; Vertex: Consultancy.