Preclinical and Clinical Evaluation of Forodesine in Pediatric and Adult B-Cell Acute Lymphoblastic Leukemia

Abstract

The discovery that purine nucleoside phosphorylase (PNP) deficiency leads to T-cell lymphopenia was the basis for introducing PNP inhibitors for T-cell leukemias. Forodesine is an orally bioavailable PNP inhibitor with picomolar potency. Because T lymphoblasts and indolent chronic lymphocytic leukemia (CLL) B cells inherently elicit favorable pharmacokinetics to accumulate deoxyguanosine triphosphate (dGTP), forodesine demonstrated promising activity in preclinical and clinical settings for patients with T-cell acute lymphoblastic leukemia (T-ALL) and B-cell CLL (B-CLL). However, the use of forodesine in B-cell ALL (B-ALL) is unknown. Leukemic blasts obtained from pediatric patients with de novo B-ALL (n= 10) were incubated with forodesine and deoxyguanosine (dGuo), and the biological end points of apoptosis, intracellular dGTP accumulation, and inhibition of RNA and DNA synthesis were measured. Additionally, adult patients with B-ALL (n= 2) were intravenously infused with 80 mg/m(2)/d dailyfor 5 days. After therapy, clinical response, toxicity, laboratory biomarkers including PNP enzyme inhibition, and plasma forodesine, dGuo, and intracellular dGTP levels were analyzed. Our invitro investigations demonstrated that forodesine treatment inhibited proliferation and induced modest apoptosis in de novo B-ALL lymphoblasts. There was time-dependent accumulation of dGTP and inhibition of RNA and DNA synthesis. During therapy, neither patient achieved a complete response (CR), but there was disease stabilization for several weeks in both patients. There was significant maintained inhibition of PNP enzyme in red blood cells, accumulation of forodesine and dGuo in plasma, and intracellular dGTP accumulation in both patients. Our preclinical and clinical investigations suggest that forodesine has activity in B-ALL. However, it needs to be either infused with dGuo or combined with established chemotherapeutic agents based on mechanistic rationale.