A Phase II Study of the Purine Nucleoside Phosphorylase (PNP) Inhibitor RO5092888 (BCX-4208) in Patients with Moderate to Severe Chronic Plaque Psoriasis: Safety, Tolerability and Lymphocyte Effects

Introduction: PNP is a purine-metabolizing enzyme that catalyzes the phosphorolysis of 2′-deoxyguanosine [dGuo] to guanine and deoxyribose-1-phosphate. In T-cells, PNP inhibition leads to accumulation of deoxyguanosine triphosphate (dGTP), triggering apoptotic cell death. Chronic-plaque type psoriasis (psoriasis) is an autoimmune disorder, in which T-cells contribute, at least in part, to the manifestations and maintenance of the disease. Therefore, targeting T-cells may be a beneficial treatment strategy. Since for chronic inflammatory diseases the safety of potential immunosuppressive drugs is a major consideration, we conducted this study to investigate safety and tolerability of oral RO5092888 (BCX-4208) in patients with moderate to severe chronic plaque psoriasis.

Methods: This was a randomized, double-blind, placebo-controlled, dose-ranging study. Sixty-six patients 18 to 70 years old were randomized into one of three groups: placebo, 20mg/d or 120mg/d. The study was conducted from August 2007 to June 2008. Patients received study drug over six weeks and were observed over an additional 4 weeks. Assessments for safety included tracking of adverse events (AEs) including infections; vital signs; ECGs; chemistry panel; LFTs; hematologic parameters including peripheral blood (PB) lymphocyte subsets CD3⁺, CD4⁺, CD8⁺, CD56⁺, CD20⁺; and urinalysis.

Results: 65 of the 66 enrolled patients were analyzed. One serious AE was observed in the 20mg/d group, a deep vein thrombosis (DVT) in a patient with a history of DVTs, and was considered unrelated to study drug. The percentage of patients experiencing at least one adverse event (AE) of any grade was placebo: 33% (7/21), 20mg: 41% (9/22), and 120mg: 59% (13/22). During treatment, two infections occurred in the placebo group (influenza and sinusitis), one in the 20mg group (nasopharyngitis), and 4 in the 120mg group (2-upper respiratory tract infections, 1 bronchitis, and 1 otitis externa). Reductions in PB lymphocytes and subsets were observed (Table 1). Nine patients showed decreased levels of CD4+ lymphocytes, below 350 cells/μL (20mg/d, 3; 120mg/d, 6).

Conclusion: Daily oral administration of 20 mg or 120 mg of RO5092888 for up to 6 weeks demonstrated adequate safety and tolerability. Reductions in PB T cells, T cell subsets and B cells were observed. Further investigation of RO5092888 is warranted in both T-cell and B-cell diseases.

Table 1