In-Vitro Efficacy of the Deoxyguanoside Analogs Forodesine (BCX- 1777) and ARA-G in Pediatric Acute Lymphoblastic Leukemia Cells.

Purine nucleotide phosphorylase (PNP) deficiency in humans is associated with elevated dGuo plasma levels. This results in the intra-cellular conversion of dGuo into dGTP, following 3 consecutive kinase steps and depletion of T-cells resulting in immune deficiency. This T-cell toxicity provided the rationale for the development of deoxyguanosine analogues as potential therapeutic compounds for T-cell malignancies. Forodesine (BCX-1777; BioCryst-Mundipharma) is an efficient blocker of PNP activity. Forodesine facilitates the conversion of dGuo into dGTP raising the intracellular dGTP pool. AraG (9-b-D-arabinofuranosyl-guanine) is a compound that is resistant to PNP-mediated degradation resulting in phosphorylation of AraG into AraGTP. AraGTP becomes incorporated in the DNA and blocks DNA synthesis resulting in apoptosis. In a phase II clinical trial, the AraG prodrug Nelarabine enforced a complete remission rate of 55% for pediatric T-ALL patients at 1^st relapse. (Berg, JCO 2005). Clinical data of forodesine treatment in pediatric ALL patients are not yet available.

The cytotoxic effect of Forodesine was investigated on primary leukemia cells from newly diagnosed pediatric acute lymphoblastic leukemia (ALL) patients in-vitro. Cells were incubated with a fixed concentration of Forodesine (1μM) in the presence of increasing concentrations of dGuo (0.001–50μM). The dGTP levels under conditions where PNPactivity was completely blocked was monitored.

Incubation of primary leukemic cells obtained from 6 pediatric ALL patients (4 T-ALL, 2 B-ALL) with 10μM dGuo results in rapid dGuo degradation (t½<4hrs) by the PNP enzyme that is completely abolished by the addition of 1μM of Forodesine. Cells consequently accumulate dGTP levels upon Forodesine treatment to a median 7.9 (range 0.5–378 fold) that is comparable between T-ALL (n=31) and B-ALL (n=11) patient samples. This reflects equal intrinsic ability of de-novo nucleotide synthesis for both T-ALL and B-ALL cells. In accordance with T-cell selective toxicity, T-ALL cells were more sensitive to Forodesine/ dGuo treatment (median T-ALL LC50 value: 1.1μM dGuo/1μM Forodesine, n=27, p=0.001) compared to B-ALL cells, which had a median LC50 value of 8.8μM dGuo/1μM Forodesine (n=30). All patients that responded demonstrated dGTP accumulation (1.5– 222.1 fold), but the magnitude of dGTP accumulation did not relate to Forodesine/dGuo toxicity.

Studying in-vitro responsiveness to AraG, T-ALL cells were more sensitive compared to B-ALL cells (p=0.0002) with a median AraG LC50 value of 20.5μM for T-ALL samples (n=24) versus 48.3μM for B-ALL samples (n=20). However, TELAML1 negative B-ALL cases were sensitive to AraG where as TELAML1 positive B-ALL cases were remarkable insensitive to AraG treatment (median LC50 value >50μM, n=9). No correlation was identified between in-vitro Forodesine/dGuo and AraG cytotoxicities. Most patient samples that displayed AraG resistance still responded to Forodesine/dGuo treatment.

In contrast, AraG cytotoxicity strongly correlated with AraC cytotoxicity (r²=0.71, p<0.0001). In conclusion, T-ALL cells are sensitive to Forodesine/dGuo treatment in-vitro in contrast to B-ALL cells that have nearly 8 fold higher LC50 values. In-vitro Forodesine mediated cytotoxicity seems more potent in pediatric ALL than AraG treatment. Resistance to AraG treatment does not preclude responsiveness to Forodesine treatment and vice versa, indicating that Forodesine and AraG rely on different cellular mechanisms for cytotoxicity.