Inability of Poly-ADP-Ribosylation Inhibitors to Protect Peripheral Blood Lymphocytes from the Toxic Effects of ADA Inhibition

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Abstract
We have evaluated the effectiveness of two inhibitors of poly-ADP-ribosylation, nicotinamide and 3-aminobenzamide as rescue agents in resting and PHA-stimulated lymphocytes damaged by the combination of deoxycoformycin (dCF) plus deoxyadenosine (dAdo). Incubation with dCF (10(-5)M) and dAdo (10(-4)M) for 18 hours, inhibited protein and RNA synthesis in unstimulated lymphocytes and impaired the ability of the cells to respond to PHA stimulation or to give rise to T-cell colonies in methyl-cellulose. Predominantly dead cells using trypan blue exclusion were observed at day 4, in both unstimulated and PHA-stimulated lymphocytes, whether or not the drugs were removed at 18 hours. The number of viable cells at day 4 increased from 13.7% to 41.1% with the addition of 5 mM nicotinamide, and to 28.8% with 5 mM 3-aminobenzamide added with dCF and dAdo. Although nicotinamide was able to prevent a fall in NAD concentration for 24h (but not for 48h) and to reduce the fall of cell ATP concentration, the inhibition by dCF and dAdo of protein synthesis, RNA synthesis, ability of cells to form colonies or to respond to PHA was not reversed. We conclude that inhibition of NAD utilisation by inhibiting ADP-ribosylation with nicotinamide or 3-aminobenzamide does not protect cells in vitro from deoxyadenosine toxicity with ADA inhibition and is not likely to give significant clinical benefit in ADA deficiency.