Abstract

The profound suppression of T-cell immunity seen in patients with an inherited PNP deficiency supports the potential application of inhibitors of this purine salvage enzyme in the therapy of T-cell malignancies and T-cell mediated autoimmune diseases. About thirty percent of PNP deficient patients also show evidence of B-cell dysfunction. BCX-4208 is a novel potent transition state analog inhibitor of PNP (IC50 ~ 0.0005 μM) and in the presence of 10 μM deoxyguanosine (dGuo), inhibits human lymphocyte proliferation induced by MLR, IL-2 or Con-A with IC50s of 0.159, 0.26 and 0.73 μM, respectively. The IC50 for dGuo in the same assays in the presence of 1 μM BCX-4208 ranges from 1–3 μM. Neither BCX-4208 alone nor dGuo alone inhibits proliferation of lymphocytes. In the presence of PNP inhibitor, dGuo is converted to dGMP and then to dGTP. Accumulation of dGTP results in the alteration of deoxynucleotide (dNTP) pools, causing death of cells via a mechanism characteristic of apoptosis. In vitro data demonstrates that following exposure to BCX-4208 and dGuo, dGTP in human lymphocytes is elevated and a 5–8 fold increase in dGTP results in 50% inhibition of lymphocyte proliferation. Flow cytometric analyses of human lymphocytes using annexin-V staining reveal that BCX-4208 in the presence of dGuo induces cellular apoptosis not only in T cells (CD3+), but also in B cells (CD20+; CD19+) (Table 1). BCX-4208 is orally bioavailable in mice, can achieve maximal inhibition of PNP, and elevates plasma dGuo levels to 3–5 μM (predose levels < 0.004 μM), which is similar to levels seen in PNP deficient patients and to levels needed to cause apoptosis in T and B-cells. These data support the evaluation of BCX-4208 in the treatment of not only T-cell mediated diseases but also B-cell mediated diseases. BCX- 4208 is currently undergoing early clinical investigation in patients with psoriasis.

Table 1.

Cell subsets % Apoptotic cells mean ± SEM (n = 4–7) 
 Vehicle Treatment group 
*p ≤ 0.01 compared to vehicle 
CD3+ 7.2 ± 0.9 18.3 ± 3.7* 
CD20+ 24.0 ± 3.6 41.8 ± 6.2* 
CD19+ 25.0 ± 3.1 51.6 ± 7.4* 
Cell subsets % Apoptotic cells mean ± SEM (n = 4–7) 
 Vehicle Treatment group 
*p ≤ 0.01 compared to vehicle 
CD3+ 7.2 ± 0.9 18.3 ± 3.7* 
CD20+ 24.0 ± 3.6 41.8 ± 6.2* 
CD19+ 25.0 ± 3.1 51.6 ± 7.4* 

Disclosures: No relevant conflicts of interest to declare.

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