Regadenoson, An Adenosine 2A Receptor Agonist, Is Safe and Inhibits Invariant NKT Cells in Sickle Cell Disease

Abstract 849^FN2

Adenosine 2A receptor (A2AR) agonists decrease pulmonary inflammation and injury in a murine model of sickle cell disease (SCD) by interrupting invariant NKT (iNKT) cell activation that occurs as a result of vaso-occlusion (Clin Immunol 2011). iNKT cells represent a subpopulation of T lymphocytes that rapidly generates pro-inflammatory cytokines upon activation and thereby may play a role in sustaining or propagating vaso-occlusion in human SCD. To evaluate the effects of A2AR activation in human SCD, we are conducting a dose-seeking and safety study of the FDA-approved, highly selective A2AR agonist regadenoson in patients with SCD ≥ 14 years of age. This is a multi-center, phase I clinical trial of infusional regadenoson that utilizes a 3+3 design and is comprised of four stages to determine maximum tolerated dose and safety during a 12 hour infusion in adults at baseline (defined as absence of increased pain, ED or hospital visit in the past 2 weeks) (stage 1), a 24 hour infusion in adults at baseline (stage 2), a 24 hour infusion in adults during a painful vaso-occlusive crisis (VOC) (stage 3), and a 24 hour infusion in children (≥ 14 years) during a painful VOC (stage 4). Three dose levels of infusional regadenoson have been evaluated thus far: 0.24 (level 0), 0.6 (level 1) and 1.44 mcg/kg/hour (level 2). Samples for plasma levels and iNKT cell activation are obtained at 0, 6, 12, 18 and 30 hours after start of infusion. Two methods are used to identify iNKT cells: 1) CD1d tetramers, loaded with the α-GalCer-like glycolipid PBS57 (defined as tetramer⁺CD3⁺), and 2) 6B11 antibody to the invariant T cell receptor (defined as CD45⁺6B11 high). NF-kB activation is measured using phospho-Ser536-p65 antibody (phospho-NF-kB). To date, 15 subjects have received infusional regadenoson. Regadenoson appears to be safe (devoid of cardiovascular or other side effects) and biologically active in reducing iNKT cell activation markers during a 12 hour infusion at dose levels 1 and 2, and we are currently evaluating the safety of a 24 hour infusion at dose level 2. No dose limiting toxicities occurred in the 15 subjects at dose levels 0, 1 or 2. Pharmacokinetic analyses showed mean peak plasma concentrations of regadenoson for dose levels 0, 1 and 2 were 0.37 ng/ml, 1.16 ng/ml and 2.19 ng/ml, respectively. Based on animal models, we suspect that biologically active plasma concentrations are approximately 1 ng/ml. At baseline, iNKT cells were activated in patients with SCD as defined by elevated levels of phospho-NF-kB expression and A2AR expression as assessed by anti-A2AR immunoreactivity. In particular, CD4+ iNKT cells showed increased phospho-NF-kB expression whereas CD4- iNKT cells were not activated. At dose levels 1 and 2, regadenoson decreased iNKT cell activation in patients with SCD. For example, when pre- and post-12 hour regadenoson infusion measurements were compared at dose level 2 (N=5), mean percent of cells expressing phospho-NF-kB in the activation gate was 40% (90% CI 23–58) at baseline and decreased by 53% at the end of infusion to 19% (90% CI 6–33). Similar results were found in measurements of A2ARs. We conclude that infusional regadenoson administered at 0.6 and 1.44 mcg/kg/hour is safe during a 12 hour infusion and decreases activation of iNKT cells. Our next steps are to determine whether infusions of a longer duration (24 and 48 hours) are safe and similarly biologically effective during a VOC. We then intend to conduct a clinical trial to measure the clinical efficacy of regadenoson infusion during VOC and acute chest syndrome episodes.