Pharmacokinetics and pharmacodynamics of rapacuronium in patients with cirrhosis

Pharmacokinetic analysis of rapacuronium and its metabolite during liver transplantation: an assessment of its potential as a pharmacodynamic probe

The liver extracts aminosteroidal neuromuscular blocking drugs. We hypothesized that the duration of action of these drugs might provide a pharmacodynamic probe for assessing graft function during orthotopic liver transplantation. The pharmacokinetics of rapacuronium and its active metabolite, ORG 9488, were prospectively studied in 11 patients. Rapacuronium (1.5 mg/kg) was administered at induction of anesthesia, 2 minutes after clamping the portal vein, and 5 minutes after reperfusion of the new graft. Blood samples were drawn at intervals, and an independent laboratory analyzed plasma for both rapacuronium and ORG 9488. Rapacuronium's pharmacokinetics were characterized for 3 stages of the transplant using NONMEM software to construct mixed-effects compartmental models. Rapacuronium plasma clearance during the first stage of orthotopic liver transplantation was 7.25 mL/kg/min. Clearance decreased by only 44% during the anhepatic stage, to 3.91 mL/kg/min, and remained decreased after reperfusion. This effect suggests that an alternate clearance pathway exists. The clearance for ORG 9488 was 13.5 mL/kg/min during the paleohepatic and anhepatic stages, but it decreased 83% on reperfusion, suggesting accumulation after reperfusion. This pharmacokinetic analysis suggests that rapacuronium may not be suitable for use as a pharmacodynamic probe.

Conformation, action, and mechanism of action of neuromuscular blocking muscle relaxants

Since curare was introduced into clinical anaesthesia in 1942, efforts to create better neuromuscular blocking (NMB) muscle relaxants have continued. Today, muscle relaxation remains a mainstay of modern anaesthesia and intensive care. Through manipulation of the traditional structure-action relationships, many new and improved muscle relaxants have been created, and several have been brought to clinical use. However, structure-action relationship is inconsistent and has its limits. Using computer-aided molecular conformational analyses, the conformation-action relationships of NMB agents of various chemical classes have been explored. Conformation, no less than structure, of the NMB agents has shed new light on their mechanisms of action. By reflection, the conformations also suggest new details of the topology of the receptive sites of the nicotinic acetylcholine receptor modeled for the motor endplate of the skeletal muscle.

Pharmacokinetics and pharmacodynamics of rapacuronium bromide

Rapacuronium is an aminosteroidal nondepolarising neuromuscular blocking agent (NMBA). Its neuromuscular blocking effects have a different time course to those of most currently available agents. It also has lower potency than many of the other NMBAs. In doses consistent with short to medium duration of action, rapacuronium has rapid and complete onset. In some doses it gives tracheal intubating conditions that compare favourably with those produced by suxamethonium (succinylcholine) during rapid sequence induction of anaesthesia. Tracheal intubating conditions improve as dose increases, but adverse effects (including potentially severe bronchospasm) become more prominent. Rapacuronium has an active metabolite that is at least as potent as the parent compound and is eliminated much less efficiently. Consequently, the time course of action of rapacuronium is prolonged after multiple doses or an infusion. Its potency is similar across age ranges and its time course after single doses is little altered in patients with hepatic or renal insufficiency. At least in part because of its active metabolite, rapacuronium is highly cumulative in renal failure. In keeping with its rapid onset and short to medium duration of action, rapacuronium has a more rapid clearance than most other NMBAs. Values for clearance are in the range 0.26-0.67 L/h/kg, with most studies giving a value of approximately 0.45 L/h/kg. There is some evidence that clearance declines marginally with advanced age, and it is also reduced in children. A typical value for steady-state volume of distribution is 0.3 L/kg. This is similar to that of many other NMBAs, but is small compared with many other drugs, as expected with a highly polar compound. Pharmacokinetic parameters do not appear to differ markedly in hepatic insufficiency, but clearance is reduced by approximately 30% in renal failure. Rapacuronium equilibrates very rapidly between the plasma and the site of effect. This is the principal explanation behind its unusually rapid onset. It also appears to have a similar potency at the larynx compared with the adductor pollicis; most other NMBAs are less effective at the larynx. Because it gives rapid onset in a dose consistent with brief duration of action, it was hoped that rapacuronium might be a suitable alternative to suxamethonium. It does not have the problems associated with suxamethonium, but its use is associated with bronchospasm, the incidence of which is dose-related. Rapacuronium has been withdrawn from sale because of this adverse effect, and its future availability is uncertain.

Newer neuromuscular blocking agents: how do they compare with established agents?

Rapacuronium bromide (rapacuronium; ORG-9487) is a nondepolarising muscle relaxant (NMBA) with a low potency [90% effective dose (ED90) 1 mg/kg], which to some extent is responsible for its rapid onset of action. Because of the high plasma clearance (5.3 to 11.1 mg/kg/min) of rapacuronium, its clinical duration of action following single bolus doses up to 2 mg/kg in adults is short (i.e. <20 minutes). Rapacuronium forms a pharmacologically active 3-desacetyl metabolite, ORG-9488, which may contribute to a delay in spontaneous recovery after repeat bolus doses or infusions. After rapacuronium 1.5 mg/kg clinically acceptable intubating conditions are achieved within 60 to 90 seconds in the majority of adult and elderly patients undergoing elective anaesthesia. However, in a rapid-sequence setting. intubating conditions are less favourable after rapacuronium 1.5 to 2.5 mg/kg than after succinylcholine. The most prominent adverse effects of rapacuronium (tachycardia, hypotension and bronchospasm) are dose-related, and in particular pulmonary adverse effects are observed more frequently under conditions of a rapid-sequence induction in adults. Therefore, it seems worthwhile to consider only doses of rapacuronium < or = 1.5 mg/kg to facilitate rapid tracheal intubation, and to use succinylcholine or rocuronium rather than rapacuronium in a rapid-sequence setting. Rapacuronium, however, is a suitable alternative to mivacurium chloride (mivacurium) and succinylcholine for short procedures (e.g. ambulatory anaesthesia). Rocuronium bromide (rocuronium) is a relatively low-potent, intermediateacting NMBA. Its main advantage is the rapid onset of neuromuscular block whereby good or excellent intubating conditions are achieved within 60 to 90 seconds after rocuronium 0.6 mg/kg (2 x ED95), and within 60 to 180 seconds after smaller doses (1 to 1.5 x ED95). Larger doses of rocuronium (> or = 1 mg/kg) seem to be suitable for rapid-sequence induction under relatively light anaesthesia. However, it is still a matter of controversy whether, in the case of an unanticipated difficult intubation, the long duration of rocuronium administered in such large doses outweighs the many adverse effects of succinylcholine. Rocuronium has mild vagolytic effects and does not release histamine, even when administered in large doses. Rocuronium is primarily eliminated via the liver and its pharmacokinetic profile is similar to that of vecuronium bromide (vecuronium). Unlike vecuronium, rocuronium has no metabolite. Cisatracurium besilate (cisatracurium), the IR-cis, 1'R-cis isomer of atracurium besilate (atracurium) is approximately 4 times more potent than atracurium. The onset time of cisatracurium is significantly slower than after equipotent doses of atracurium. The recommended intubating dose is 0.15 to 0.2 mg/kg (3 to 4 times ED95). Over a wide range of clinically relevant doses the recovery properties of cisatracurium are affected by neither the size of the bolus dose nor by the duration of infusion. Unlike atracurium, cisatracurium does not trigger histamine release. Like atracurium, cisatracurium undergoes Hofmann elimination. In contrast to atracurium, cisatracurium does not undergo hydrolysis by nonspecific plasma esterases. Moreover, about 77% of the drug is cleared by organ-dependent mechanisms.

The new neuromuscular blocking agents: do they offer any advantages?

The pharmacodynamics and pharmacokinetics of the two most recent aminosteroid neuromuscular blocking drugs to become available, rapacuronium bromide (Org 9487) and rocuronium bromide are reviewed. Two new classes of drug with neuromuscular blocking properties, the bis-tetrahydroisoquinolinium chlorofumarates and the tropinyl diester derivatives are introduced. Comparisons between these drugs and mivacurium and cisatracurium are made. Rapacuronium 1.5 mg kg(-1) (ED95 1 mg kg(-1)), produces maximal neuromuscular block in 54 s. Time to recovery of the train-of-four ratio to 0.7 is achieved within 20 min after neostigmiine 0.05 mg kg(-1) given at 2 min. The plasma clearance of rapacuronium is 7-8 ml kg(-1) min(-1). Rapacuronium undergoes hepatic metabolism: no prolongation of effect has been reported after a single bolus or a short infusion in patients with hepatic or renal failure. Org 9488 is the 3-desacetyl metabolite of rapacuronium, which has neuromuscular blocking properties. Its much lower clearance (1.28 ml kg(-1) min(-1)) and plasma equilibration constant (0.105 min(-1)) may limit the prolonged use of rapacuronium. Rocuronium given at 2xED95 produces maximal neuromuscular block in 1 min. Spontaneous recovery of the train-of-four ratio to 0.7 takes over 40 min. Rocuronium has a plasma clearance of 4 ml kg(-1) min(-1). Its pharmacodynamics are altered in hepatic and renal disease. A number of anaphylactoid reactions to rocuronium have been reported recently. The bis-tetrahydroisoquinolinium chlorofumarate GW280430A has an ED95 of 0.19 mg kg(-1). Given at three times this dose, onset of neuromuscular block occurs within 100 s; the duration of block is 8-9 min. Following a 2 h infusion, the recovery index does not seem to be increased. Early studies suggest that this drug has no adverse cardiovascular or respiratory side-effects. The tropinyl diester derivative G-1-64 will produce 80-90% neuromuscular block in less than 2 min using 3xED80. Ninety per cent recovery of the first twitch of the train-of-four occurs after 5-7 min using one ED80. A recovery index of less than 2 min has been reported in rats. All the tropinyl diesters appear to produce vagal block.

Rapacuronium

Rapacuronium (Org 9487; ED90 = 1 mg x kg(-1)) is a new, low potency, short-acting, non-depolarizing neuromuscular blocking agent. An intubating dose of 1.5xED90 or 1.5 mg x kg(-1) of rapacuronium offers acceptable intubating conditions within 60-90 s in most patients. A complete block from this dose can be reversed immediately with satisfactory recovery in 12-16 min. Side-effects are dose-related and are mainly haemodynamic, i.e. an increase in heart rate and a decrease in blood pressure.