In vitro efficacy of forodesine and nelarabine (ara-G) in pediatric leukemia

Abstract

Forodesine and nelarabine (the pro-drug of ara-G) are 2 nucleoside analogues with promising anti-leukemic activity. To better understand which pediatric patients might benefit from forodesine or nelarabine (ara-G) therapy, we investigated the in vitro sensitivity to these drugs in 96 diagnostic pediatric leukemia patient samples and the mRNA expression levels of different enzymes involved in nucleoside metabolism. Forodesine and ara-G cytotoxicities were higher in T-cell acute lymphoblastic leukemia (T-ALL) samples than in B-cell precursor (BCP)-ALL and acute myeloid leukemia (AML) samples. Resistance to forodesine did not preclude ara-G sensitivity and vice versa, indicating that both drugs rely on different resistance mechanisms. Differences in sensitivity could be partly explained by significantly higher accumulation of intracellular dGTP in forodesine-sensitive samples compared with resistant samples, and higher mRNA levels of dGK but not dCK. The mRNA levels of the transporters ENT1 and ENT2 were higher in ara-Gsensitive than -resistant samples. We conclude that especially T-ALL, but also BCP-ALL, pediatric patients may benefit from forodesine or nelarabine (ara- G IntroductionLeukemia is the most common childhood malignancy, and the general incidence in both adults and children of acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML) is approximately 1 per 100 000 and 2-3 per 100 000, respectively. Although overall cure rates have been improved over the last decades, approximately 20% of children with ALL and 40% of children with AML still eventually die from their disease. 1,2 In adults, the prognosis is worse with a survival below 60% in ALL 3 and 50% in AML, 4 indicating that there is still a great need for better therapy. Currently, purine nucleosides analogues are in clinical trials for different types of leukemia including clofarabine, forodesine (BCX-1777/Immucillin H), and nelarabine (506U78/Arranon/ Atriance) the latter being the pro-drug for 9-␤-D-arabinofuranosylguanine (ara-G).Forodesine is a noncleavable inosine analog developed to bind and inhibit the purine nucleoside phosphorylase (PNP) enzyme. 5 PNP normally degrades excess of intracellular deoxyguanosine (dGuo) into guanosine and deoxyribose-1-phosphate through phosphorylysis. dGuo is continuously produced in the body as the result of DNA degradation during cellular turnover. Inhibition of PNP by forodesine results in the intracellular accumulation of dGuo. DGuo is rapidly phosphorylated to dGTP in the purine salvage pathway leading to dGTP accumulation. 6,7 High intracellular levels of dGTP cause cell death through mechanisms that are still not fully understood, but which may likely involve imbalance in the deoxynucleotide pool and/or inhibition of ribonucleotide reductase 8 resulting in inhibition of DNA synthesis and/or by activation of a p53-induced cell-cycle arrest and apoptosis. 9 Whereas most nucleoside analogues depend on DNA incorporation to exert their toxic effect, this is not the case for forodesine. T cells seem...

Impact of pretransplant minimal residual disease after cord blood transplantation for childhood acute lymphoblastic leukemia in remission: an Eurocord, PDWP–EBMT analysis

“…These include pretransplantation treatment aimed at decreasing MRD levels with new drugs, such as clofarabine 35,36 and nelarabine 37 used in combination with other cytotoxic agents, use of the anti-CD19/antiCD3 bi-specific monoclonal antibody blinatunomab, 38 use of midostaurin for MLL rearranged ALL cases, or use of tyrosine kinase inhibitors for Ph-positive ALL. 39 Less intensive GvHD prophylaxis, rapid withdrawal of immunosuppression, 40 cell-based immunotherapy 41,42 (that is, leukemia-reactive cytotoxic T cells) or use of double UCBT 43,44 can also be useful in the perspective of increasing the GVL effect.…”

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

“…Preclinical investigations in primary T-ALL and B-ALL cells from pediatric patients have demonstrated activity of forodesine and dGuo in B lymphoblasts. 21 The lower activity in B-ALL was associated with less dGTP accumulation, which may result from a lower level of expression of nucleoside transporters in B cells and a higher level of 5’-nucleotidase.…”

“…21 Most of the PNP is in large body organs and red blood cells. At the physiologic dose in patients with B-ALL, there was more than 85% inhibition of this enzyme in red blood cells (Figure 4).…”

“…First, akin to T cells, inhibition of PNP promotes accumulation of dGTP, subsequent imbalance in the deoxyribonucleotide triphosphates (dNTPs), and eventually cell death in B cells. 21 Second, T-cell therapies such as nelarabine have demonstrated clinical activity in B-cell leukemias. 22 Third, B cells have high levels of deoxycytidine kinase (dCK), the enzyme that facilitates the accumulation of dGTP.…”

“…22 Third, B cells have high levels of deoxycytidine kinase (dCK), the enzyme that facilitates the accumulation of dGTP. 21 Fourth, the rate-limiting enzyme dCK that converts dGuo to dGTP is present in high levels in pediatric lymphoblasts. 23 Finally, forodesine has demonstrated some activity in adult patients with relapsed or refractory B-ALL.…”