Antimalarial alkoxylated and hydroxylated chalcones [corrected]: structure-activity relationship analysis

Chalcones with 2',3',4'-trimethoxy, 2',4'-dimethoxy, 4'-methoxy, 4'-ethoxy, 2',4'-dihydroxy, and 4'-hydroxy groups on ring B were synthesized and evaluated in vitro against Plasmodium falciparum (K1) in a [3H] hypoxanthine uptake assay. The other ring A was quinoline, pyridine, naphthalene, or phenyl rings with electron-donating or electron-withdrawing substituents of varying lipophilicities. Trimethoxy 6 and 27, dimethoxy 7, 8, 29, and methoxy 31 analogues had good in vitro activities (IC(50) < 5 microM). 3-Quinolinyl ring A derivatives were well represented among the active compounds. Hydroxylated chalcones were less active than the corresponding alkoxylated analogues. When evaluated in vivo, 8 and 208 were comparable to chloroquine in extending the lifespan of infected mice. Multivariate data analysis showed that in vitro activity was mainly determined by the properties of ring B. Quantitative structure-activity relationship models with satisfactory predictive ability were obtained for various B ring chalcones using projections to latent structures. A model with good predictability was proposed for 19 active chalcones. Size and hydrophobicity were identified as critical parameters.

Novel combretastatin analogues effective against murine solid tumors: design and structure-activity relationships

A series of combretastatin A-4 (CA-4) analogues were synthesized, and their cytotoxic effects against murine Colon 26 adenocarcinoma and inhibitory activity on tubulin polymerization were evaluated. Since CA-4 has limited aqueous solubility, the target compounds were designed to improve solubility by introduction of a nitrogen-containing group. Among the compounds synthesized, those with an amino moiety in place of the phenolic OH of CA-4 showed potent antitubulin activity and cytotoxicity against murine Colon 26 adenocarcinoma in vitro. Some of the compounds which were potent in vitro were evaluated in the murine tumor model Colon 26 in vivo. Among these, 13bHCl, 21aHCl, and 21bHCl showed significant antitumor activity in the animal model, while CA-4 was ineffective. 13bHCl and 21aHCl were further evaluated in two murine tumor models (Colon 38 and 3LL) and human xenografts HCT-15. These compounds showed potent antitumor activity comparable or superior to that of CDDP. The structure-activity relationships of this series of compounds are also discussed.

Discovery of 7-hydroxy-6-methoxy-2-methyl-3-(3,4,5-trimethoxybenzoyl)benzo[b]furan (BNC105), a tubulin polymerization inhibitor with potent antiproliferative and tumor vascular disrupting properties

A structure-activity relationship (SAR) guided design of novel tubulin polymerization inhibitors has resulted in a series of benzo[b]furans with exceptional potency toward cancer cells and activated endothelial cells. The potency of early lead compounds has been substantially improved through the synergistic effect of introducing a conformational bias and additional hydrogen bond donor to the pharmacophore. Screening of a focused library of potent tubulin polymerization inhibitors for selectivity against cancer cells and activated endothelial cells over quiescent endothelial cells has afforded 7-hydroxy-6-methoxy-2-methyl-3-(3,4,5-trimethoxybenzoyl)benzo[b]furan (BNC105, 8) as a potent and selective antiproliferative. Because of poor solubility, 8 is administered as its disodium phosphate ester prodrug 9 (BNC105P), which is rapidly cleaved in vivo to return the active 8. 9 exhibits both superior vascular disrupting and tumor growth inhibitory properties compared with the benchmark agent combretastatin A-4 disodium phosphate 5 (CA4P).

Synthesis and antioxidant, anti-inflammatory and gastroprotector activities of anethole and related compounds

Some derivatives of trans-anethole [1-methoxy-4-(1-propenyl)-benzene] (1) were synthesized, by introducing hydroxyl groups in the double bond of the propenyl moiety. Two types of reactions were performed: (i) oxymercuration/demercuration that formed two products, the mono-hydroxyl derivative, 1-hydroxy-1-(4-methoxyphenyl)-propane (2) and in lesser extent the dihydroxyl derivative, 1,2-dihydroxy-1-(4-methoxyphenyl)-propane (3) and (ii) epoxidation with m-chloroperbenzoic acid that also led to the formation of two products, the dihydroxyl derivative (3) and the correspondent m-chloro-benzoic acid mono-ester, 1-hydroxy-1(4-methoxyphenyl)-2-m-chlorobenzoyl-propane (4). The structures of these compounds were confirmed mainly by mass, IR, 1H and 13C NMR spectral data. The activity of anethole and hydroxylated derivatives was evaluated using antioxidant, anti-inflammatory and gastroprotector tests. Compounds (2) and (3) were more active antioxidant agents than (1) and (4). In the anti-inflammatory assay, anethole showed lower activity than hydroxylated derivatives. Anethole and in lesser extent its derivatives 2 and 4 showed significant gastroprotector activity. All tested compounds do not alter significantly the total number of white blood cells.

Cyclization-activated prodrugs. Basic carbamates of 4-hydroxyanisole

A series of basic carbamates of 4-hydroxyanisole was prepared and evaluated as progenitors of this melanocytotoxic phenol. All of the carbamates were relatively stable at low pH but released 4-hydroxyanisole cleanly at pH 7.4 at rates that were structure dependent. A detailed study of the N-methyl-N-[2-(methylamino)ethyl]carbamate showed that generation of the parent phenol followed first-order kinetics with t1/2 = 36.3 min at pH 7.4, 37 degrees C, and was accompanied by formation of N,N'-dimethylimidazolidinone. These basic carbamates are examples of cyclization-activated prodrugs in which generation of the active drug is not linked to enzymatic cleavage but rather depends solely upon a predictable, intramolecular cyclization-elimination reaction.

Conversion of p-methoxyphenyl glycosides into the corresponding glycosyl chlorides and bromides, and into thiophenyl glycosides

p-Methoxyphenyl (pMP) beta-D-glycopyranosides (Glc, Gal, GlcNPhth, GalNPhth, GlcNTroc, Gal beta 4Glc, Gal alpha 4Gal) were prepared from the corresponding 1-O-acetyl sugars in 79-90% yield, using boron trifluoride etherate as promoter. Treatment of the pMP glycosides with acyl chlorides or bromides in the presence of various Lewis acids gave the corresponding glycosyl chlorides and bromides in 81-98% yield. Treatment of the acyl-protected pMP glycosides with thiophenol and boron trifluoride etherate gave the corresponding thioglycosides in 80-100% yield and high (> 20:1) beta/alpha selectivity. The stability of pMP glycosides was investigated against a series of reagents.

Preparation and photoactivation of caged fluorophores and caged proteins using a new class of heterobifunctional, photocleavable cross-linking reagents

The design, synthesis, and spectroscopic and chemical properties of four members of a new class of heterobifunctional photocleavable (caged) cross-linking reagents were described. One of the two reactive groups of the cross-linker reacted with amino groups to form the corresponding photolabile carbamates. Amino group containing compounds or proteins caged with these reagents can be coupled through the thiol reactive oxirane group of the cross-linker to a different biomolecule or to a thiol-derivatized surface. The 3,4-dimethoxy-6-nitrophenyl photoisomerization group of the reagent was physically and chemically isolated from the cross-linking functionality, and the high extinction coefficient and red-shifted action spectrum of this chromophore make it suitable for photoactivation applications of caged compounds on surfaces or in living cells. The bifunctional, photocleavable cross-linking reagents were used to prepare a thiol reactive caged rhodamine 110. The new reagents and conjugation procedures described may be used as part of a general procedure to cage the activity of proteins by physically masking binding sites.

Novel 2,7-Dialkyl-Substituted 5(S)-Amino-4(S)-hydroxy-8-phenyl-octanecarboxamide Transition State Peptidomimetics Are Potent and Orally Active Inhibitors of Human Renin

The action of renin is the rate-limiting step of the renin-angiotensin system (RAS), a key regulator of blood pressure. Effective renin inhibitors directly block the RAS entirely at source and, thus, may provide a vital weapon for hypertension therapy. Our efforts toward identifying novel small-molecule peptidomimetic renin inhibitors have resulted in the design of transition-state isosteres such as 1 bearing an all-carbon 8-phenyl-octanecarboxamide framework. Optimization of the extended P3 portion of 1 and extensive P2' modifications provided analogues with improved in vitro potencies in the presence of plasma. X-ray resolution of rh-renin/38a in the course of SAR work surprisingly unveiled the exploitation of a previously unexplored pocket (S3sp) important for strong binding affinities. Several inhibitors demonstrated oral efficacy in sodium-depleted marmosets. The most potent, 38a, induced dose-dependently a pronounced reduction in mean arterial blood pressure, paralleled by complete blockade of active plasma renin, up to 8 h post-dose. Oral bioavailability of 38a was 16% in marmosets.

Synthesis and hypolipidemic and antiplatelet activities of alpha-asarone isomers in humans (in vitro), mice (in vivo), and rats (in vivo)

A series of alpha-asarone isomers was synthesized and investigated for their hypolipidemic and antiplatelet activity. Considering the hypolipidemic activity in rats at a dose of 80 mg/kg/day, some isomers were more potent than clofibrate at 150 mg/kg. Compound 3 was one of the most active agents elevating the HDL cholesterol level by 56% and lowering the LDL cholesterol level by 46.8% in rats after 7 days of administration. The activities of the platelet aggregation test in vitro were significant but lower than those of the reference substances (indomethacine and acetylsalicylic acid). In the pulmonary thromboembolic in vivo test in mice, two compounds (alpha-asarone (6) and compound 4) produced significant antithrombotic effects at 100 mg/kg, namely 44% and 52% protection against lung microembolia, respectively. alpha-Asarone derivatives form a new group of potential hypolipidemic and/or antithrombotic agents. The compounds 3, 4, and 6 may serve as lead substances whose structural modifications may result in original drugs.

Synthesis and Cytotoxic Evaluation of Combretafurans, Potential Scaffolds for Dual-Action Antitumoral Agents

We have synthesized rigid analogues of combretastatin bearing a furan ring in place of the olefinic bridge. These compounds are cytotoxic at nanomolar concentrations in neuroblastoma cells, display a similar structure-activity relationship compared to combretastatin A4, and inhibit tubulin polymerization. We also show that the furan ring can be further functionalized. Thus, it is possible that combretafurans could act as scaffolds for the development of dual-action antitumoral agents.

Design, synthesis, and in vitro activity of catamphiphilic reverters of multidrug resistance: discovery of a selective, highly efficacious chemosensitizer with potency in the nanomolar range

On the basis of the results obtained in previous research, three series of compounds (A-C), derived from verapamil, were designed and synthesized to obtain drugs able to revert multidrug resistance (MDR), an acquired resistance that frequently impairs cancer chemotherapy. The ability of the obtained compounds to revert MDR was evaluated on anthracycline-resistant erythroleukemia K 562 cells, measuring the uptake of THP-adriamycin (pirarubicin) by continuous spectrofluorometric monitoring of the decrease of the fluorescence signal of the anthracycline at 590 nm (lambdaex = 480 nm), after incubation with cells. Cardiovascular activity, which is responsible for unwanted side effects, was also evaluated. The results obtained show that many of the compounds studied are potent reverters of MDR and are endowed with reduced cardiovascular activity. One of the compounds (7, MM36) presents a pharmacological profile (unprecedented nanomolar potency, high reversal of MDR, low cardiovascular activity) that makes it a promising drug candidate to treat MDR and a useful tool for studying P-glycoprotein.

Streamlined, Asymmetric Synthesis of 8,4‘-Oxyneolignans

Highly direct, modular syntheses of several natural 8,4'-oxyneolignans [(-)-1, (+)-1, (-)-2, and (-)-3] and some related variants [(-)-26, (+)-26, (+)-27, and (-)-28] are reported. Utilizing (S)- or (R)-methyl lactate as the chiral sources, two complementary syn- or anti-oriented routes were designed, encompassing nine and five steps, which were carried out to deliver the targets in an enantiomerically pure form. The embodiment of the two independent aryl and aryloxy moieties onto the lactate frame was performed according to a diversity-oriented protocol from the common precursors, aldehydes 6 and ent-6 for the syn-oriented routes and mesyl esters 19 and ent-19 for the anti-oriented routes. These syntheses set the stage for the generation of a wide and diverse repertoire of 8,4'-oxyneolignan compounds and the broad biological interrogation of its members.

The power of visual imagery in synthesis planning. Stereocontrolled approaches to CGP-60536B, a potent renin inhibitor

Two strategies were developed toward the stereocontrolled synthesis of 8-aryl-3-hydroxy-4-amino-2,7-diisopropyloctanoic acids with predetermined stereogenic centers. This is a generic motif in a new class of potent inhibitors of the enzyme renin, exemplified by CGP-60536B. The synthesis relies on the utilization of L-pyroglutamic acid as chiron, and proceeds through the incorporation of required functionality by exploiting internal induction. One of the strategies shows the power of visual imagery in synthesis planning, akin to a Dali-like representation of objects that can be viewed in more than one way. Thus, the entire carbon skeleton of the target molecule is encompassed in a partially functionalized bicyclic indolizidinone precursor. In a second strategy, an intermediate common to the first approach is elaborated into an appended gamma-lactone which is alkylated through enolate chemistry and ultimately transformed into the intended target compound. X-ray crystallography was used to corroborate the structures and stereochemistries of several intermediates.

Naturally occurring 5-lipoxygenase inhibitor. II. Structures and syntheses of ardisianones A and B, and maesanin, alkenyl-1,4-benzoquinones from the rhizome of Ardisia japonica

New alkenyl-1,4-benzoquinones, ardisianones A (1) and B (2), and the known maesanin (3) as 5-lipoxygenase inhibitors have been isolated from the rhizome of Ardisia japonica. Their structures have been elucidated as 2-methoxy-6-[(Z)-10'-pentadecenyl]-1,4-benzoquinone and 5-hydroxy-2-methoxy-6-[(Z)-8'-tridecenyl]-1,4-benzoquinone, respectively, on the basis of spectroscopic data and chemical degradation. Ardisianone A (1), maesanin (3) and belamcandol A (7) have been synthesized starting from belamcandol B (6), readily prepared by Wittig reaction between 9-(2-tetrahydropyranyloxy)nonanal and 3,5-dimethoxybenzyltriphenylphsophonium bromide followed by selective demethylation with sodium thioethoxide.

Ultrasound assisted green synthesis of bis(indol-3-yl)methanes catalyzed by 1-hexenesulphonic acid sodium salt

1-Hexenesulphonic acid sodium salt as catalyst for green synthesis of bis(indol-3-yl)methanes was described. The reaction of indole with various aldehydes in water using ultrasound irradiation at ambient temperature for appropriate time using 1-hexenesulphonic acid sodium salt furnish the desired product in good to excellent yield. Utilization of aqueous medium, simple reaction conditions, isolation, and purification makes this manipulation very interesting from an economic and environmental perspective.

Synthesis and preliminary evaluation of [11C]NE-100 labeled in two different positions as a PET sigma receptor ligand

N,N-Dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine (NE-100) was labeled with 11C in two different positions by the alkylation of an N-despropyl precursor with [11C]propyl iodide and of an O-desmethyl precursor with [11C]methyl iodide and was evaluated for the potential as a tracer for mapping sigma 1 receptors in the CNS and peripheral organs by PET. Following i.v. injection of [N-propyl-11C]NE-100 or [O-methyl-11C]NE-100 into mice, the two tracers showed similar tissue distribution patterns except for the liver and brain. With the coinjected carrier NE-100 or haloperidol, the uptake of [N-propyl-11C]NE-100 by the liver, pancreas and spleen was significantly decreased at 15 min after injection, whereas the effect was not significant for [O-methyl-11C]NE-100. The coinjection of NE-100 enhanced the brain uptake of the two tracers. Haloperidol also enhanced the brain uptake of [N-propyl-11C]NE-100, but not that of [O-methyl-11C]NE-100. The regional brain distribution assessed with [O-methyl-3H]NE-100 was consistent with the distribution pattern of the sigma receptors. Four sigma drugs reduced the regional brain uptake of [O-methyl-3H]NE-100 to 70%-90% of the control. In an ex vivo autoradiographic study of the rat brain, the uptake of [O-methyl-11C]NE-100 was blocked by carrier NE-100 or haloperidol (53%-59% of the control in the cortex), which suggests a receptor-specific distribution. These results show that [O-methyl-11C]NE-100 has limited potential as a PET ligand for mapping sigma 1 receptors in the peripheral organs and the CNS because of high nonspecific binding.

Structure-activity relationships of acetylcholinesterase noncovalent inhibitors based on a polyamine backbone. 2. Role of the substituents on the phenyl ring and nitrogen atoms of caproctamine

Continuing our studies on polyamine-based compounds of potential interest in the field of Alzheimer's disease therapeutics, we investigated the structure-activity relationships (SAR) of a lead compound (caproctamine, 3) identified in a previous work. In particular, we varied the substituents on the phenyl ring and on the nitrogen functions (both the amine and the amide), and studied the effects of such modifications on the inhibitory potency against isolated acetyl- and butyryl-cholinesterase (AChE and BChE). Moreover, the ability of selected compounds to reverse the d-tubocurarine-induced neuromuscular blockade and their antagonism toward muscarinic M(2) receptors in guinea pig left atrium were assayed. The most interesting SAR result was the identification of a relationship between the electronic characteristics of 2-substituents (measured by pK(a)) and the AChE inhibitory potency (pIC(50)) of tertiary amine compounds 6-12, which was confirmed by the invariance of the pIC(50) values of the corresponding methiodide derivatives 14-20. With regard to the biological profile, the most interesting compound was the N-ethyl-analogue of caproctamine (9), that showed pIC(50) values of 7.73 (+/-0.02) and 5.65 (+/-0.03) against AChE and BChE, respectively. The ability to increase the acetylcholine level was maintained in the functional assay (pAI(50) for reversing the neuromuscular blockade was 6.45 (+/-0.07)), as well as the ability to antagonize the M(2) receptors (pK(b) = 5.65 (+/-0.06)). Moreover, 9 showed a long duration of action as AChE inhibitor, an useful property in view of a possible development of this compound as a therapeutic agent.

Synthesis and hypolipidaemic evaluation of a series of alpha-asarone analogues related to clofibrate in mice

A series of alpha-asarone analogues related to clofibrate, containing an acetic acid group at C-2 of the aromatic ring, has been prepared as the acids or as the ethyl and methyl esters. The corresponding alcohols were also synthesized by reduction of the ethyl esters. The compounds were examined in hyperlipidaemic male mice to evaluate their ability to modify serum lipoprotein cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol and triglycerides after oral administration of 40 and 80 mg kg(-1) for 6 days. Except for methyl 2-methoxy-5-nitro-4-(2-propenyl)phenoxyacetate at either dose, these clofibrate-related phenoxyacetic acid derivatives were found to have significant hypocholesterolaemic activity. Levels of low-density lipoprotein cholesterol and triglycerides were significantly reduced and those of high-density lipoprotein cholesterol were elevated. 2-Methoxy-5-nitro-4-(2-propenyl)phenoxyacetic acid was active at both doses in all the tests. Clofibrate (150 mg kg(-1)) was more potent at reducing low-density lipoprotein cholesterol. No activity was detected for the alcohol derivatives. These preliminary results suggest that this class of compound might have more promise as potential hypolipidaemic agents than other alpha-asarone derivatives. Further investigation and characterization should be performed to determine the mode of action of these agents on lipid metabolism.

Synthesis of radioiodinated aryl iodides via boronate precursors

Arylboronate esters are converted to iodine-123 labeled aryl iodides using no-carrier-added iodine-123 labeled sodium iodide in the presence of chloramine-T. High yields of radiochemically pure products are obtained.

Synthesis and chemical carcinogen inhibitory activity of 2-tert-butyl-4-hydroxyanisole

The title compound 1 was selectively synthesized in its pure isomeric form by means of the hydroxyl-protecting reagent dimethyl-tert-butylchlorosilane. Exclusive silylation occurred at the less hindered hydroxyl group of 3. Dimethyl sulfate methylation of 4 gave 5 in excellent yield. Compound 1 was then obtained by acid hydrolysis of 5. The two BHA isomers, 1 and 2, were tested on their inhibitory effects toward benzo[alpha]pyrene-induced neoplasia in the forestomach of the ICR/Ha mouse. Both isomers, when added to the diet, reduced the number of mice with tumors and the number of tumors per mouse. Isomer 1, which has the less hindered free hydroxyl group, showed higher inhibitory effect in the present experimental model.

Design, synthesis, structure-activity relationships, and biological characterization of novel arylalkoxyphenylalkylamine sigma ligands as potential antipsychotic drugs

sigma Receptor antagonists may be effective antipsychotic drugs that do not induce motor side effects caused by ingestion of classical drugs such as haloperidol. We obtained evidence that 1-(2-dipropylaminoethyl)-4-methoxy-6H-dibenzo[b,d]pyran hydrochloride 2a had selective affinity for sigma receptor over dopamine D2 receptor. This compound was designed to eliminate two bonds of apomorphine 1 to produce structural flexibility for the nitrogen atom and to bridge two benzene rings with a -CH2O- bond to maintain the planar structure. In light of the evidence, N, N-dipropyl-2-(4-methoxy-3-benzyloxylphenyl)ethylamine hydrochloride 10b was designed. Since compound 10b had eliminated a biphenyl bond of 6H-dibenzo[b,d]pyran derivative 2a, it might be more released from the rigid structure of apomorphine 1 than compound 2a. The chemical modification of compound 10b led to the discovery that N, N-dipropyl-2- [4-methoxy-3-(2-phenylethoxyl)phenyl]ethylamine hydrochloride 10g (NE- 100), the best compound among arylalkoxyphenylalkylamine derivatives 3, had a high and selective affinity for sigma receptor and had a potent activity in an animal model when the drug was given orally. We report here the design, synthesis, structure-activity relationships, and biological characterization of novel arylalkoxyphenylalkylamine derivatives 3.

Towards NR2B receptor selective imaging agents for PET-synthesis and evaluation of N-[11C]-(2-methoxy)benzyl (E)-styrene-, 2-naphthyl- and 4-trifluoromethoxyphenylamidine

Three potent and selective 11C-labelled NR2B antagonists have been synthesized and evaluated as PET ligands. The brain uptake of the compounds in mice varied substantially and was dominated by metabolism. One compound was found to have favourable uptake and retention in the brain, as well as a binding pattern consistent with the expression of the target receptor as measured by in vitro autoradiography. However, the metabolism of the compounds tested was too rapid to allow for in vivo imaging.

Asymmetric synthesis and anti-protozoal activity of the 8,4'-oxyneolignans virolin, surinamensin and analogues

The asymmetric synthesis of 8,4'-oxyneolignans (-)-virolin, (-)-surinamensin and a number of analogues has been achieved. A divergent synthesis was used, with all compounds being elaborated from a single chiral aldehyde derived from ethyl lactate. In the 15 compounds that were tested, the level of substitution on the A-ring was found to directly influence the activity against Leishmania donovani whilst the activity against Plasmodium falciparum was influenced by numerous substitution and stereochemical factors.

Neuromuscular blocking activity and therapeutic potential of mixed-tetrahydroisoquinolinium halofumarates and halosuccinates in rhesus monkeys

Structure-activity relationships in rhesus monkeys for a novel mixed-onium class of ultra-short-acting nondepolarizing tetrahydroisoquinolinium neuromuscular blockers (NMBs) are described. Bis-onium chlorofumarate 20a with (1R,2S)-benzyltetrahydroisoquinolinium groups was a potent lead compound (ED(95) = 0.079 mg/kg) with an ultra-short duration of NMB effect (7.1 min) and a selectivity index (SI: defined as a ratio of the cardiovascular threshold dose to the ED(95)) similar to that of mivacurium (3). The mean threshold dose for cardiovascular effects with 20a was ca. 20 times its ED(95) value (SI = 20). A novel mixed-onium analogue of 20a was prepared by replacing the benzyltetrahydroisoquinolinium group distal to the fumarate chlorine atom with a (1S,2R)-phenyltetrahydroisoquinolinium moiety. The resulting mixed-onium chlorofumarate 24a displayed good NMB potency (ED(95) = 0.063 mg/kg), ultra-short duration of action (5.6 min) and an improved selectivity index (SI = 57). Several other mixed-onium derivatives containing octanedioate (25a; ED(95) = 0.103 mg/kg), difluorosuccinate (27c; ED(95) = 0.056 mg/kg), and fluorofumarate (28a; ED(95) = 0.137 mg/kg) linkers were also potent, ultra-short-acting NMBs with good to excellent selectivity index values (SI = 37-96). Octanedioate 25a was longer acting at higher doses compared to difluorosuccinate 27c and chlorofumarate 24a. Durations of NMB effect following a 0.4 mg/kg bolus dose (100% block) of 25a, 27c, and 24a were 16.9, 13.0, and 10.0 min, respectively. Recovery time for mixed-onium chlorofumarate 24a following a 1 h continuous infusion at 10-20 microg/kg/min (95-100% block) was ca. 5 min which is similar to that observed following a 0.2 mg/kg bolus dose of this compound and indicates a lack of cummulative effects. Preliminary studies with chlorofumarate 24a in whole human blood revealed that mixed-onium thiazolidine 29 was the major metabolite and that plasma cholinesterases do not play the primary role in duration of NMB effect. The NMB properties of 24a in rhesus monkeys led to its clinical evaluation as a possible alternative to succinylcholine.