A Phase 1b, Randomized, Double-Blind, Placebo-Controlled Study of PF-04447943 in Patients with Stable Sickle Cell Disease: Changes in Exploratory Biomarkers

Background: Sickle cell disease (SCD), caused by a single amino acid change in the β-globin gene, is characterized by a complex and heterogeneous pathophysiology. Currently, no validated biomarkers for SCD exist. PF-04447943 is a selective inhibitor of the cyclic guanosine monophosphate (cGMP)−specific phosphodiesterase-9A enzyme in clinical development for the prophylactic treatment of patients with SCD. In nonclinical studies in Townes sickle cell mouse models, PF-04447943 reduced soluble E-selectin and leukocyte-platelet aggregates (Jasuja et al. Blood 2016;128:1293). In this phase 1b study, we sought to explore biomarkers that may be informative in demonstrating the pharmacologic effect of PF-04447943 in patients with SCD to serve as a bridge between nonclinical and clinical data. This study was the first evaluation of PF-04447943 in patients with SCD.

Methods: This multinational, multicenter, double-blind, placebo-controlled study enrolled adults aged 18−65 years with stable SCD (hemoglobin [Hb] SS or HbS-β⁰ thalassemia) with or without the coadministration of hydroxyurea. Patients were randomized in a 3:1 active to placebo ratio to receive twice-daily (BID) PF-04447943 5 mg, PF-04447943 25 mg, or placebo. Exploratory end points included change from baseline at day 29 in markers associated with cellular adhesion; monocyte- and neutrophil-platelet aggregates percent and size; macrophage-1 antigen expression on monocytes and neutrophils; markers associated with coagulation; circulating endothelial microparticles; and HbF. Changes from baseline in each dose group were analyzed using repeated-measures analysis of covariance models, adjusting for baseline and hydroxyurea use. A statistically significant biomarker change from baseline was predefined as a Holm step-down adjusted P value was <0.15.

Results: In total, 28 patients received PF-04447943 or placebo and had available biomarker assessments. In the PF-04447943 25-mg BID cohort (n=14), significant reductions from baseline to day 29 were observed for soluble E-selectin (-11%; P=.064), neutrophil-platelet aggregate numbers (-46%; P=.095) and size (-34%; P=.018), and monocyte-platelet aggregate numbers (-52%; P=.025) and size (-36%; P=.140). Similar trends were observed in the PF-04447943 5-mg BID cohort (n=7), but only the change in neutrophil-platelet aggregate size (-43%; P=.020) was significant. No significant change from baseline was noted at day 29 in HbF levels in either of the PF-04447943 cohorts; treatment duration (29 days) may not have been a sufficient amount of time to demonstrate reductions in HbF. No significant change in any biomarker was observed in the placebo cohort (n=7). Patients with and without hydroxyurea co-administration showed similar trends in biomarker activity.

Conclusions: In this phase 1b study in patients with stable SCD, changes in the exploratory biomarker panel (soluble E-selectin, neutrophil-platelet aggregates, and monocyte-platelet aggregates) are similar to those previously observed in preclinical SCD mouse models and with rivipansel (an E-selectin inhibitor; previously known as GMI 1070) administration in a pilot study of patients with SCD (Wun et al. PLoS ONE 2014;9:e101301). A cGMP pathway working model is proposed for patients with SCD in vaso-occlusive crisis in which PF-04447943 results in an increase in cGMP and, therefore, a decrease in cellular aggregates and minimal endothelial activation as well. The findings of this study bridge previous preclinical and clinical data and provide guidance for relevant biomarkers in future clinical studies of PF-04447943.