P1433: positron emission tomography imaging of vla-4 as a new biomarker of vaso-occlusion in sickle cell disease

Topic: 26. Sickle cell disease Background: Sickle cell disease (SCD) is characterized by vaso-occlusive crises (VOC) – episodes of acute pain in areas of microvascular occlusion. Hyperadhesion of blood cells to the endothelium and to one another is critical to VOC. Very late antigen-4 (VLA-4 or integrin α4β1) mediates the adhesion of white blood cells (WBC) and reticulocytes to the endothelium, and could therefore be harnessed as an imaging biomarker of VOC. Previous work in our lab has shown that the VLA-4 peptidomimetic PET tracer 64Cu-CB-TE1A1P-LLP2A (64Cu-LLP2A) can quantitatively and in real time image VOC induced by lipopolysaccharide (LPS) in a SCD mouse model. However, VOC is frequently triggered by hypoxia in humans with SCD. Aims: To further validate our imaging strategy, we investigated whether 64Cu-LLP2A could image hypoxia-induced VOC in Townes sickle mice. We also conducted a first-in-SCD study in patients. Methods:64Cu-LLP2A was injected via the tail vein in mice (Townes sickle and non-sickle litter mates; n=10/group) at the dose of 200 µCi per animal. Baseline PET/CT images were acquired at 1h, 4h, and 24h post-injection (p.i.). After one week, all mice were challenged with 1 hour of hypoxia (FiO2=0.07) followed by 1 hour of normoxia (FiO2=0.21) in a small animal hypoxia chamber, a treatment known to cause vaso-occlusion. PET/CT images were acquired at 1h, 4h and 24 h p.i. by using a PET/CT scanner. PET image standard uptake values (SUVs) for the humerus and femur, typical sites of vaso-occlusive pain in humans, were measured using IRW software. Complete blood count analysis was conducted at baseline and after hypoxia challenge and bones were harvested and preserved. Results: The SCD mice showed uptake of 64Cu-LLP2A in the humerus and femur at both baseline and after hypoxia challenge. SUV ratios at 1 h p.i in the humerus and entire femur compared to muscle in SCD mice were increased over baseline, whereas the control mice did not show any significant change in uptake. The 1h timepoint showed the most pronounced uptake, which indicates that the effect of ischemia/reoxygenation on the expression of VLA-4 wanes after 1h. CBC data showed SCD mice had significantly higher neutrophil counts at baseline and post hypoxia. Summary/Conclusion: Our data show that 64Cu-LLP2A can image hypoxia-induced VOC in SCD mice. Increased VLA-4 uptake identifies areas of hyperadhesion and vaso-occlusion. The ongoing PET Imaging of Vaso-Occlusive Crisis (VOC) in SCD clinical study (ClinicalTrials.gov Identifier: NCT04925492, conducted under the FDA-approved IND 150142) at our institution aims to develop 64Cu-LLP2A as a quantitative PET imaging biomarker of VOC in patients with SCD. An early goal of the study is to identify an ideal post-injection time for imaging. Steady-state subjects are scanned at two points post injection which vary by subject and span a range of times, not to exceed 48 hours. Preliminary results show robust uptake in the hematopoietic marrow (Figure 1).Figure 1 – Example of 64Cu-LLP2A distribution in a human in steady-state two hours after injection of 236 MBq. A sagittal slice through the right side of the body is shown and includes a portion of the right hip and femur. Keywords: Sickle cell disease, Positron emission tomography (PET), Sickle cell adhesion