Tiapride pre-treatment in acute exposure to paraoxon: Comparison of effects of administration at different points-in-time in rats

Introduction: Accidental and suicidal exposures to organophosphorus compounds (OPC) are frequent. The inhibition of esterases by OPC leads to an endogenous ACh poisoning. Recently, the FDA approved, based on animal experiments, for military combat medical use oral pyridostigmine (PSTG) for pre-exposure treatment of soman; the concept is to block the cholinesterase reversibly using the carbamate pyridostigmine in order to deny access to the active site of the enzyme to the irreversible inhibitor (OPC) on subsequent exposure. We have shown previously that tiapride (TIA) is in vitro a weak inhibitor of AChE. We also have shown recently that in rats coadministration of TIA with the organophosphate paraoxon significantly decreases mortality without having an impact on red blood cell cholinesterase (RBC-AChE) activity. Purpose of the study: To establish in a prospective, non-blinded study in a rat model of acute high dose OPC (paraoxon; POX) exposure the ideal point in time for TIA pre-treatment administration and to correlate it with measured TIA plasma levels. Material and Methods: There were six groups of rats in each cycle of the experiment and each group contained six rats. The procedure was repeated twelve times (cycles) (n= 72 for each arm; half male and half female). All substances were applied ip. All groups (1-6) received 1 μMol POX (≈LD₇₅); groups 1-5 also received 50 μMol TIA at different points in time. Group 1 (G₁): TIA 120 min before POX Group 2 (G₂): TIA 90 min before POX, Group 3 (G₃): TIA 60 min before POX, Group 4 (G₄): TIA 30 min before POX, Group 5 (G₅): TIA & POX simultaneously, Group 6 (G₆): POX only. The animals were monitored for 48 hours and mortality/survival times were recorded at 30 min, 1, 2, 3, 4, 24 and 48 h. AChE activities were determined at 30 min, 24 and 48 h in surviving animals. Statistical analysis was performed on the mortality data, cumulative survival times and enzyme activity data. Mortality data was compared using Kaplan-Meier plots. Cumulative survival times and enzyme activites were compared using the Mann-Whitney rank order test. No Bonferroni correction for multiple comparisons was applied and an α ≤ 0.05 was considered significant. Results: Mortality is statistically significantly reduced by TIA pre-treatment at all points-in-time. Highest protection is achieved if TIA is given 90 to 0 min before OPC exposure. The reduction in mortality is not correlated to TIA plasma levels (C_max ≈ 120 min post ip-administration). TIA pre-treatment is not affecting AChE activity regardless of the timing of administration. Conclusion: The lack of correlation between TIA plasma levels and degree of mortality reduction as well as the lack of protective effect on enzyme activity seem to indicate that the site of action of TIA is not the blood. While our hypothesis that TIA would protect AChE in a pyridostigmine-like manner (via protection of the enzyme) could not be confirmed, the reduction in mortality with TIA pre-treatment is nevertheless of potential interest.