Inhibition of the enzymic degradation of suxamethonium and mivacurium increases the onset time of submaximal neuromuscular block

Simulation of the kinetics of neuromuscular block: implications for speed of onset

BACKGROUND

The onset time for paralysis varies 3-fold among nondepolarizing muscle relaxants. Possible explanations include: (a) pharmacokinetic differences among drugs and (b) buffering of drug molecules by acetylcholine receptors as they diffuse into the neuromuscular junction. Although some pharmacokinetic models consider buffered diffusion, these models do not account for either the high density of receptors or synapse geometry. Here, I used computer simulations to calculate the kinetics of buffered diffusion. The goal was to determine the conditions under which buffered diffusion could account for differences in onset time among nondepolarizing muscle relaxants.

METHODS

Monte Carlo simulation was used along with a realistic 3-dimensional model of the rat neuromuscular junction. Simulations determined the time dependence of the number of drug-bound receptors. A 1000-fold range of drug potency was examined. In some simulations, the drug concentration outside the junction was changed instantaneously. In other simulations, the concentration changed according to predictions of pharmacokinetic models assuming time-dependent changes in plasma drug concentration. The rate constant for equilibration of drug between plasma and muscle, keo, was varied between 0.15 and 0.6 min(-1). Twitch amplitude was calculated from receptor occupancy assuming a high safety margin for neuromuscular transmission. Some simulations used a synaptic model with an increased nerve-muscle contact width.

RESULTS

Simulations with instantaneous changes in drug concentration at the synapse, indicated that the time to 50% twitch depression (onset time) was 0.1 to 30 seconds and was proportional to drug potency. This corresponds to iontophoretic application of drug to isolated neuromuscular junctions, but is too fast to explain onset times in humans. When pharmacokinetic models were used to calculate the drug concentration outside the synapse, buffered diffusion increased onset times of potent drugs (drugs for which the effective concentration at 50% twitch height is <600 nM). Simulations using keo = 0.6 min(-1) and a model with a 2- to 3-fold wider nerve-muscle contact width indicated that buffered diffusion could account for the differences in clinical onset times among the nondepolarizing muscle relaxants.

CONCLUSION

Monte Carlo simulation provides a biophysically appropriate way to incorporate buffered diffusion into pharmacokinetic modeling. The simulations indicated that buffered diffusion could account for differences in onset time among drugs. However, a better understanding of the geometry of the human neuromuscular junction is needed before the magnitude of the effect of buffered diffusion can be quantified.

Is time to peak effect of neuromuscular blocking agents dependent on dose? Testing the concept of buffered diffusion

BACKGROUND AND OBJECTIVES

For neuromuscular blocking agents, an inverse relationship between potency and time to peak effect has been observed. To test the hypothesis that this relationship is due to buffered diffusion, we investigated the influence of dose on time to peak effect. Pharmacokinetic-pharmacodynamic simulations were performed to support the expected relationships between potency, dose, peak effect and time to peak effect.

METHODS

Pigs (20-28 kg body weight) were anaesthetized with ketamine and midazolam, followed by pentobarbital and fentanyl intravenously. Neuromuscular block was measured by stimulating the peroneal nerve supramaximally at 0.1 Hz and measuring the response of the tibialis anterior muscle mechanomyographically. After an initial dose to establish the individual ED90 of a neuromuscular blocking agent (rocuronium, vecuronium, pipecuronium or d-tubocurarine), five different doses of the same compound were administered to each animal, aiming at 20%, 40%, 60%, 75% or 90% block, in a random order. Doses were given 45 min after complete recovery of the twitch response.

RESULTS

For rocuronium and pipecuronium, time to peak effect increased with dose, whereas dose did not affect time to peak effect of vecuronium and d-tubocurarine. Simulations predict that time to peak effect decreases with dose if buffered diffusion is taken into account.

CONCLUSIONS

The results suggest that buffered diffusion does not play a dominant role in the time to peak effect of neuromuscular blocking agents. Therefore it is unlikely that the observed inverse relationship between potency and time to peak effect of neuromuscular blocking agents in the clinical range is due to buffered diffusion.

Onset-potency relationship of nondepolarizing muscle relaxants: a reexamination using simulations

Nondepolarizing muscle relaxants (MRs) display an inverse onset-potency relationship, that is, less potent MRs display a more rapid onset. We have conducted the current investigation to estimate the impact of variable pharmacokinetic or pharmacodynamic properties of the MRs on potency and onset time, and on the onset-potency relationship. Using a model of neuromuscular transmission, we changed either the affinity of MRs for the postsynaptic receptors or the pharmacokinetic properties of the MRs. The elimination rate constant, k(10), which defines the systemic clearance, was assigned one of 9 values and the transport rate constant, k(12), one of 5 values. The transport rate constant into the effect compartment was constant (k(e1) = 0.2 min(-1)). Only one parameter was altered at a time. With constant pharmacokinetics, a 100-fold decrease in affinity caused a proportional decrease in potency, but little change (0.02 min) in onset time. With constant affinity, increasing the clearance from 1 to 250 mL x kg(-1) x min(-1) shortened the onset time from 7.2 to 0.7 min and decreased the potency 12-fold. In a double logarithmic plot, the onset-potency relationship was linear. Lesser affinities produce a nearly parallel rightward shift of the regression lines. The inverse onset-potency relationship may be explained by the pharmacokinetic factors producing changes in both the potency and onset times.

Development of ultra short-acting muscle relaxant agents: History, research strategies, and challenges

Author has reviewed the literature and his own work related to the chemistry, pharmacology, and clinical aspects of new muscle relaxants. Emphasis has been placed on the basic science concepts and technologies (e.g. structure-activity relationships, nicotinic receptor pharmacology, and investigation of side effects) behind the development of rapidly and short acting nondepolarizing muscle relaxants.

Physicochemical properties of neuromuscular blocking agents and their impact on the pharmacokinetic-pharmacodynamic relationship

BACKGROUND

Among the factors influencing the onset of action of neuromuscular blocking agents (NMBA), the potency (EC50) and the rate of equilibration between blood and the effect compartment (k(e0)) have been highlighted. Although these descriptors are intrinsically influenced by the physicochemical characteristics of the drug, the impact of lipid solubility, molecular weight and protein binding on pharmacokinetic-pharmacodynamic (PK-PD) descriptors has not been established for most NMBA.

METHODS

The octanol/phosphate buffer distribution coefficients (logD) of various NMBA (vecuronium, rocuronium, mivacurium isomers (cis-cis, cis-trans and trans-trans), doxacurium, cisatracurium, atracurium, succinylcholine) were determined. The free fraction for each drug was measured using an ultrafiltration technique. PK-PD descriptors were obtained from selected clinical studies. Correlations between physicochemical parameters (including molecular weight) and PK-PD descriptors were assessed by linear or multiple linear regression.

RESULTS

A wide range of log D (-4.15 for succinylcholine to 0.75 for vecuronium) and free fraction (from 31% for vecuronium to 80% for succinylcholine) is observed for NMBA. Molecular weight combined with either lipid solubility (r2=0.70; P=0.001) or free fraction (r2=0.84; P<0.001) were highly correlated with potency, while for k(e0) a greater degree of correlation was obtained when both lipid solubility and free fraction (r2=0.74; P=0.002) were included.

CONCLUSIONS

The basic characteristics of NMBAs, namely, molecular weight, lipid solubility and protein binding, are strongly associated with the kinetics of the drug response.

Pharmacodynamics of mivacurium chloride in 13- to 18-yr-old adolescents with thermal injury

BACKGROUND

Burned patients demonstrate resistance to the effects of non-depolarizing blocking drugs as a result of acetylcholine receptor changes. They also have decreased activity of plasma cholinesterase (PCHE), which metabolizes mivacurium. We hypothesized that decreased PCHE activity would decrease metabolism of mivacurium, and counteract the receptor-related resistance following burns.

METHODS

Thirteen burned patients and six controls, aged 13-18 yr were followed in 27 studies. The burned patients were sub-classified as having 10-30% or >30% body surface area burn and were studied whenever possible at < or =6 days, and at 1-12 weeks after the burn. Mivacurium pharmacodynamics were examined following a bolus (0.15 mg kg(-1)) dose, and during and after a continuous infusion.

RESULTS

Following a bolus, the onset time and the maximal effect were similar to controls. Recovery was prolonged in the 10-30% burn group at 1-12 weeks (P<0.008), with a similar trend in the >30% burn group at < or =6 days (P<0.082) compared with controls. The infusion requirements for mivacurium were not increased in the burned groups. The PCHE activity was decreased in all burn groups and was inversely related to recovery following the bolus (r=0.73, P<0.001) and the infusion (r=0.69, P<0.001).

CONCLUSION

In contrast to previous studies with non-depolarizers in burned patients, normal mivacurium doses can produce paralysis, at least as rapidly as in controls, but with a possibility of a prolonged recovery from block. The standard dose of mivacurium in the presence of decreased PCHE activity is in effect, a relative overdose that explains the above findings. Mivacurium is an effective drug for use in burns, irrespective of time after, or magnitude of burn injury.

Neuromuscular pharmacology of TAAC3, a new nondepolarizing muscle relaxant with rapid onset and ultrashort duration of action

We selected bis [N-(3,4-diacetoxybenzyl) tropanium-3alpha-yl] glutarate dibromide (TAAC3) from many new tropinyl diester derivatives to evaluate its neuromuscular blocking (NMB) and autonomic side effects on anesthetized rats, rabbits, guinea pigs, cats, pigs, dogs, and monkeys. NMB potency, onset, recovery index, and duration of action were determined. Comparisons of these pharmacologic variables were made between TAAC3 and rocuronium. In the cat, the degrees of train-of-four and tetanic fade, posttetanic potentiation, and pharmacologic antagonism were evaluated. For determination of the NMB maintenance dose, TAAC3 was also given to rabbits and pigs in the initial dose/maintenance infusion mode. Cardiac vagal block was evaluated in the rat, pig, cat, and guinea pig on the basis of the inhibition of the bradycardia to stimulation of the vagus nerve. Sympathetic ganglion block was studied on the superior cervical ganglion-nictitating membrane preparation of the cat. TAAC3 produced nondepolarizing NMB. Its NMB 90% effective doses ranged from 90 to 425 microg/kg, depending on the species. TAAC3 had a faster onset (0.8-1.0 min), shorter recovery index (0.6-1.1 min), and shorter duration of action (1.8-3.5 min) than rocuronium. It produced a slight cumulative effect on infusion, but not on repeated single-dose administration. Cardiac vagal block was present at doses exceeding the NMB 90% effective dose. In the cat and pig at equipotent NMB doses, the degree of cardiac vagal block was similar to that of rocuronium. There was no demonstrable sympathetic ganglion block in the cat. In view of its favorable NMB characteristics, TAAC3 is now undergoing detailed preclinical studies.

IMPLICATIONS

We developed a new nondepolarizing muscle relaxant, TAAC3, and investigated it in several animal models. TAAC3 has shown a very rapid onset and an ultrashort duration of neuromuscular blocking action. A minor degree of cardiac vagal block was observed. TAAC3 is promising for further studies.

[Acute neurotoxic organophosphate poisoning: insecticides and chemical weapons]

OBJECTIVE

To review clinical and therapeutic bases of an organophosphate poisoning, either with insecticide or nerve agent.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), from personal data (academic memoir, documents under approbation of the National Defense Office), from Internet's data.

DATA SYNTHESIS

Generally, organophosphate poisoning occurs during accidental exposure with agricultural insecticide or suicide. The effects of organophosphate compounds are due to the inhibition of the enzyme acetylcholinesterase. The intoxication symptoms can be divided into muscarine-like, nicotine-like effects, effects on the central nervous system and symptoms related to the dysfunction of the neuromuscular junction. The interest of biological acetylcholinesterase's measuring is minimal because it is weakly specific or sensitive. The immediate severity is due to hypoxia. Respiratory failure results from the lack of central drive inflated with excessive bronchial secretions, bronchospasm and respiratory muscles paralysis. The secondary complications are early myopathies whose gravity is correlated with the decrease of acetylcholinesterases, or later neuropathies induced by a different mechanism. Beside the symptomatic measures, atropine is the specific anticholinergic treatment. When promptly used, oximes can regenerate cholinesterases. The attempted effects of the treatment are mouth dryness, pupilar dilatation and flushing of the skin. Nerve agents are lethal toxics which have a short onset time and produce severe neurological pathology. In a terrorist incident, it is as important to identify rapidly the toxic agent and provide emergency decontamination as to manage medical care. An effective response must be multidisciplinary, involving clinicians, toxicologists, Emergency Medical Service and public's health personnel.

[Pharmacodynamics and safety of mivacurium in infants and children under halothane-nitrous oxide anesthesia]

OBJECTIVES

To determine pharmacodynamic effects and safety of mivacurium in paediatric patients.

STUDY DESIGN

Multicentric, prospective, open, non-randomized study.

PATIENTS

Forty-eight three-month-old to eight-year-old physical class ASA I or II children.

METHOD

Anaesthesia was induced and maintained with halothane and nitrous oxide. Tracheal intubation was performed without a neuromuscular blocking agent. Neuromuscular blockade was measured with a strain force transducer after train-of-four stimulation of the ulnar nerve at the wrist every ten seconds. A single bolus dose of mivacurium (0.2 mg.kg-1) was injected during 15 seconds in patients allocated into three groups. Group 1: three to 12-month-old infants (n = 15), group 2: one- to three-year-old children (n = 16) and group 3: three- to eight-year-old children (n = 17). Onset and recovery parameters were measured in each patient. Heart rate and noninvasive arterial blood pressure were recorded every minute for five minutes after mivacurium injection.

RESULTS

Following halothane administration for 29 and 32 min, and a FEThalothane = 1 vol%, mivacurium (0.2 mg.kg-1) determined a 100% neuromusmcular blockade in all patients. The onset time was 71 +/- 34 s (mean +/- SD) in all patients and did not differ between groups. Time to 25% and 95% recovery of the first twitch and recovery index for all the patients were 12 +/- 3 min, 19 +/- 5 min and 4 +/- 2 min respectively and did not differ between groups. No prolonged paralysis was observed. No significant changes of HR and BP occurred.

CONCLUSIONS

Following 0.2 mg.kg-1 of mivacurium in patients aged between three months to eight years, a complete blockade occurs with a rapid onset time and a short duration of action, without significant cardiovascular effect.

Organophosphorus poisoning and anaesthesia

Organophosphorus compounds, used as insecticides and agents of chemical warfare, are a major global cause of health problems. These irreversible inhibitors of cholinesterase produce three well-recognised clinical entities: the initial cholinergic phase, which is a medical emergency often requiring management in an intensive care unit; the intermediate syndrome, during which prolonged ventilatory care is necessary; and delayed polyneuropathy. In addition, disturbances of body temperature and endocrine function, electrolyte imbalances, immunological dysfunction and disorders of reproduction have been reported in animals and man. Vocal cord paralysis, pancreatitis, cardiac arrhythmias and a wide range of neuropsychiatric disorders are known to follow acute and chronic exposure to organophosphorus compounds. As a result of the inhibition of plasma cholinesterase, there can be increased sensitivity to drugs hydrolysed by this enzyme, e.g. suxamethonium and mivacurium. The inhibition of acetylcholinesterase causes dysfunction at the neuromuscular junction which can produce altered responses to nondepolarizing neuromuscular blockers. Anaesthetists may encounter patients exposed to organophosphorus compounds either following acute poisoning, trauma (warfare) or as patients with a wide range of nonspecific disorders presenting for surgery. The traditional use of oximes and atropine in treatment has failed to reduce the morbidity and mortality associated with poisoning. The roles of agents that have reduced the toxicity of organophosphorus compounds in animal experiments are discussed as potential therapeutic agents. There is an urgent need for accurate information on the problems associated with exposure to organophosphorus compounds. This would best be achieved by collaborative research between technologically advanced countries and developing countries, where organophosphorus compounds are a leading cause of ill health.