Minireview: Does in-vitro testing of oximes help predict their in-vivo action after paraoxon exposure?

K-oximes have recently been developed in the search for efficacious broad-band reactivators of acetylcholinesterase (AChE) inhibited by organophosphorus compounds (OPC). Before clinical use, their toxicity and efficacy need to be assessed, and there is clear demand for simple in vitro tests that can predict in vivo performance. This article summarizes our in vitro data obtained for conventional and experimental oximes in human and rat blood exposed to the OPC paraoxon and correlates them with our in vivo results. The intrinsic AChE inhibitory activity of oximes, as reflected by their in vitro IC₅₀, is strongly correlated with their LD₅₀ (rat): oximes with a high IC₅₀ (K-27, K-48, pralidoxime and obidoxime) also show a high LD₅₀ and are thus relatively non-toxic, whereas oximes K-105, K-108 and K-113 have a low IC₅₀, a low LD₅₀ and are far more toxic. The IC₅₀ is also correlated with the in vivo capacity to protect from paraoxon-induced mortality: oximes with a higher IC₅₀ reduce the relative risk of death more. In contrast, the protective ability as assessed in vitro by the slope of the IC₅₀ shift (tana), is not correlated with in vivo protection from paraoxon-induced mortality: the best in vivo protectors (K-27 and K-48) show a much lower tana value (around 2) than K-110 and K-113 (tana around 10), which hardly reduce the relative risk of death after paraoxon exposure. The partition coefficient logP of the individual oximes is inversely correlated with their IC₅₀ and with their LD ₅₀ and is therefore an indicator of toxicity: strongly hydrophilic oximes tend to be less toxic than less hydrophilic ones. These data highlight the good predictive value of in vitro IC₅₀ testing for in vivo toxicity and the limited practical significance of in vitro assessment of protective potency.