Purine nucleoside phosphorylase (
PNP) deficiency in humans produces a relatively selective depletion of T-cells. Inhibitors of PNP are therefore of interest as potential T-cell selective
immunosuppressive agents.
BCX-1777 is a potent inhibitor of PNP and in vitro T-cell proliferation. Inhibition of human T-cells by
BCX-1777 and
deoxyguanosine (dGuo) is accompanied by
deoxyguanosine triphosphate (
dGTP) accumulation. Unlike human T-cells, mouse, rat, dog and monkey T-cells are neither inhibited (IC50>100 microM) nor accumulate
dGTP in the presence of
BCX-1777 and dGuo. Cells pretreated with
BCX-1777 and dGuo for 24 h (to elevate
dGTP levels) prior to stimulation demonstrated 80% inhibition similar to the inhibition observed with cells treated with
BCX-1777 and dGuo during the stimulation and proliferation process. This further confirms that inhibition of T-cells is due to the accumulation of
dGTP in these cells. Deoxynucleotide (dNTP) analysis of the cells treated with
BCX-1777 and dGuo for 48 h showed no significant change in
deoxycytidine triphosphate (
dCTP) and
deoxyadenosine triphosphate (dATP) pools. However, a decrease (2-fold) in
thymidine triphosphate (
dTTP) pools, and a large increase in
dGTP pools (15-fold) were observed. Results from various groups have shown that alteration in the dNTP supply results in DNA fragmentation and cell death with characteristics of apoptosis. Indeed, apoptosis is observed in human T-lymphocytes treated with
BCX-1777 and dGuo. To compare the in vivo efficacy of
BCX-1777 with another potent T-cell inhibitor,
cyclosporin, these drugs were tested in a xenogeneic
graft-vs.-host disease model (XGVHD). In this model, human lymphocytes are engrafted into severe combined immunodeficient mice (SCID) mice inducing severe XGVHD. The efficacy of
BCX-1777 in the XGVHD model was comparable to
cyclosporin and a combination of
BCX-1777 and
cyclosporin treatment showed a trend towards increased efficacy compared to
cyclosporin alone. These results suggest that
BCX-1777 may be useful for the treatment of disease characterized by activated T-cell responses.