HLö 7 dimethanesulfonate, a potent bispyridinium-dioxime against anticholinesterases

Abstract

HLö 7 dimethanesulfonate (1-[[[4-(aminocarbonyl)pyridinio] methoxy] methyl] -2,4-bis [(hydroxyimino) methyl]pyridinium dimethanesulfonate) is a broad-spectrum reactivator against highly toxic organophosphorus compounds. The compound was synthesized by a new route with the carcinogenic bis(chloromethyl)ether being substituted by the non-mutagenic bis(methylsulfonoxymethyl)ether. The very soluble dimethanesulfonate of obidoxime was also prepared by this way. HLö 7 dimethanesulfonate is the first water-soluble salt of HLö 7 that should be suitable for the wet/dry autoinjector technology, because aqueous solutions of HLö 7 are not very stable (calculated shelf-life 0.2 years when stored at 8°C, 1 M solution, pH 2.5). The crystalline preparation contains 96% of thesyn/syn-isomer, less than 2% of thesyn/anti-isomer and some minor identified by-products. HLö 7 was very efficient in reactivating acetylcholinesterase (AChE) blocked by organophosphates as long as ageing did not prevent dephosphylation. HLö 7 was superior to HI 6 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2-[(hydroxyimino)methyl]pyridinium dichloride) in reactivating soman and sarin-inhibited AChE from erythrocytes, and literature data indicate that HLö 7 exceeds HI 6 by far in reactivating tabun-inhibited AChE. In atropine-protected, soman-poisoned mice HLö 7 was three times more potent than HI 6 (protective ratio 5 versus 2.5), and in sarin-poisoned mice HLö 7 was 10 times more potent than HI 6 (protective ratio 8 for both oximes). In atropine-protected guinea-pigs HLö 7 was less effective than HI 6 (protective ratio: 2.3 versus 5.2 for soman; 5.2 versus 6.8 for sarin; 4.3 versus 3.8 for tabun). The mean survival time of anaesthetized guinea-pigs exposed to 5 LD₅₀ soman (6.3 min) was increased by atropine (27 min) and atropine + HLö 7 (57 min). HLö 7 alone did not prolong the survival. The most impressive effect of HLö 7 was on respiration: 3 min after i.v. injection of HLö 7 and atropine, the depressed respiration increased rapidly to 60% of control and remained at that level during the observation period (60 min). With atropine alone, respiration recovered only slowly. Behavioural and physiologic parameters were determined in atropine-protected mice exposed to a sublethal soman dose. The running performance was significantly improved by HLö 7. Even central symptoms, e.g. hypothermia and convulsions, were decreased markedly by HLö 7 (evaluation 60 min after poisoning). The pharmacokinetic data for HLö 7 in male beagle dogs are similar to those of HI 6. After i.v. injection: t_1/2α = 5 min; t_1/2ß = 46 min; V_D = 0.24 1/kg; Cl_p1 = 3.7 ml x min⁻¹ x kg⁻¹; Cl_ren= 3.2 ml x min⁻¹ x kg⁻¹; renal excretion of unchanged HLö 7 = 86%. After i. m. injection: t_1/2abs = 14 min; t_1/2ß = 48 min; Vd = 0.27 1/kg; Cl_p1= 3.9 ml x min⁻¹ x kg⁻¹; Cl_ren= 2.7 ml x min⁻¹ x kg⁻¹; renal excretion of unchanged HLö 7 = 76%; bioavailability >95%. Plasma protein binding was <5%; HLö 7 did not permeate into red cells. A dose of 20 μmol/kg was well tolerated both after i.v. and i.m. administration. In anaesthetized dogs (chloralose) HLö 7 i.v. (20 (imol/kg) showed marginal hypotensive effects, whereas 50 μmol/kg resulted in decreased mean blood pressure (−15%) and blood flow (−30%) without reflex tachycardia. One out of four dogs developed a circulatory shock syndrome with anuria. Respiration varied only transiently. Blood gases and pH were not influenced. Similar cardiovascular effects were observed in anaesthetized (urethane) guinea-pigs. In isolated guinea-pig hearts (Langendorff) sinus and ventricular heart rate were not influenced by HLö 7 <500 μM. HLö 7 antagonized both carbachol and nicotine effects. Red cell AChE was inhibited by HLö 7 by up to 50%; C₅₀ about 100 μM. Previously, HLö 7 was shown to block ganglionic transmission (IC₅₀= 500 μM), probably due to ion-channel blockade. These data indicate that HLö 7 combines ganglion blocking, anticholinergic and indirect cholinergic properties like other bispyridinium compounds. The results suggest that HLö 7 may be tolerated by man at a dose of 10 μmol/kg. Vital functions are not expected to be impaired. At such a dose (250–500 mg), which can be injected by an autoinjector, HLö 7 is expected to be superior to HI 6.