In vivo distribution of organophosphate antidotes: autoradiography of [14C]HI-6 in the rat.

In order to visualize the distribution of HI-6 in the rat after iv administration, autoradiographic experiments were carried out with [14C]HI-6, labeled at the carbon of the carboxamide moiety. Autoradiography clearly confirms penetration of HI-6 into the central nervous system. Considerable radioactivity was found in the cerebrum, the cerebellum, and the choroid plexus. No significant activity was detected in the pontomedullary region or the spinal cord. Peripherally, [14C]HI-6 is observed in large amounts in kidneys, heart, liver, nose, bladder, testes, and marrow-containing bone. The gastrointestinal tract was largely devoid of any radioactivity. The relative absence of HI-6 in the pontomedullary region renders centrally mediated influences of HI-6 on hemodynamic and respiratory parameters less likely.

Identification of two metabolites of the cholinesterase reactivator HI-6 isolated from rat urine.

Two metabolites, isolated from the urine of rats given the cholinesterase reactivator HI-6 intravenously, still contained quaternary nitrogen atoms and therefore could not be extracted from aqueous solutions by organic solvents. Both metabolites were isolated by preparative high performance liquid chromatography and were identified using mass spectrometry, gas chromatography, infrared spectrometry, ultraviolet spectrometry and proton nuclear magnetic resonance spectrometry. The structures were confirmed by in-vitro preparation of the compounds. both metabolites contained 2-pyridone moieties. One had an intact pyridinium-aldoxime moiety, and therefore could still be therapeutically active. The excretion of unchanged HI-6 together with the two identified metabolites does not provide for a 100% mass balance, indicating that in the rat, other, as yet unidentified, metabolites must be formed.

Stereospecific reactivation of human brain and erythrocyte acetylcholinesterase inhibited by 1,2,2-trimethylpropyl methylphosphonofluoridate (soman).

Human erythrocyte and brain acetylcholinesterase are preferentially inhibited by the P(-)-isomers of C(+/-)P(+/-)-soman. The enzymes inhibited by the P(-)-isomers behave similarly with respect to oxime-induced reactivation and aging. HI-6 is the best reactivator for C(+)P(-)-soman-inhibited acetylcholinesterases. Oxime-induced reactivation of the C(-)P(-)-soman-inhibited acetylcholinesterases is much more difficult to achieve.

Kinetic profile in blood and brain of the cholinesterase reactivating oxime HI-6 after intravenous administration to the rat.

The kinetic profile of the oxime HI-6, a potent cholinesterase reactivator, after iv administration of 0.1325 mmol/kg (50 mg/kg) to rats is described. The blood concentrations measured over a period of 300 min can be described by a two-compartment open model. Excretion occurred only by the kidney. Approximately 60% was excreted in unmetabolized form. Brain tissue concentrations were significantly above the detection limit of the HPLC analysis procedure even when a correction was made for the amount of HI-6 present in brain blood. The concentration in brain tissue is built up according to a rapid equilibration mechanism. Disappearance of HI-6 from brain occurred slowly compared to the elimination of HI-6 from blood. These findings are discussed in the light of the existing knowledge on the kinetic behavior of polar solutes in the central nervous system. Whether the concentrations of HI-6 built up in the brain are relevant for the therapy of organophosphate intoxications cannot be determined from the experiments described.

Determination of some pyridinium aldoxime compounds by means of ion-pair reversed-phase high-performance liquid chromatography: application in biological material.

Two reversed-phase high-performance liquid chromatographic systems are presented for the separation and assay of the pyridinium aldoximes benzyl-P2A, HI-6 and obidoxime in aqueous solutions and biological samples. The systems involve a 5-micrometer C18 silica gel stationary phase. The eluent consists of methanol, acetic acid buffer (pH 4.80), a counter ion (per-chlorate or n-octanesulphonate) and a surfactant. The compounds were detected spectrophotometrically at 304 nm. In the concentration range used, linear plots of concentration versus extinction were obtained, both in blood and in water. Detection limits plots of concentration versus extinction were obtained, both in blood and in water. Detection limits, even in blood are satisfactory (0.5-1 microM). Evidence of presented that, at least for HI-6, the addition of counter ions to the system does not lead to the formation of ion pairs to be retained by partition, but rather to a mechanism based on adsorption chromatography.