Purine nucleoside phosphorylase (
PNP) deficiency in humans results in
T lymphocytopenia.
Forodesine, a potent inhibitor of PNP, was designed based on the transition-state structure stabilized by the
enzyme. Previous studies established that
forodesine in the presence of
deoxyguanosine (dGuo) inhibits the proliferation of T lymphocytes. A phase 1 clinical trial of
forodesine in T-cell
malignancies demonstrated significant antileukemic activity with an increase in intracellular dGuo
triphosphate (
dGTP). High accumulation of
dGTP in T cells may be dependent on the levels of
deoxynucleoside kinases. Because
B-cell chronic lymphocytic leukemia (B-CLL) cells have high activity of
deoxycytidine kinase (dCK), we hypothesized that these lymphocytes would respond to
forodesine. This postulate was tested in primary lymphocytes during in vitro investigations. Lymphocytes from 12 patients with CLL were incubated with
forodesine and dGuo. These CLL cells showed a wide variation in the accumulation of intracellular
dGTP without any effect on other deoxynucleotides. This was associated with DNA damage-induced p53 stabilization, phosphorylation of p53 at Ser15, and activation of p21. The
dGTP accumulation was related to induction of apoptosis measured by
caspase activation, changes in mitochondrial membrane potential, and PARP cleavage. Based on these data, a phase 2 clinical trial of
forodesine has been initiated for CLL patients.