A platform for the discovery of new macrolide antibiotics.

The chemical modification of structurally complex fermentation products, a process known as semisynthesis, has been an important tool in the discovery and manufacture of antibiotics for the treatment of various infectious diseases. However, many of the therapeutics obtained in this way are no longer effective, because bacterial resistance to these compounds has developed. Here we present a practical, fully synthetic route to macrolide antibiotics by the convergent assembly of simple chemical building blocks, enabling the synthesis of diverse structures not accessible by traditional semisynthetic approaches. More than 300 new macrolide antibiotic candidates, as well as the clinical candidate solithromycin, have been synthesized using our convergent approach. Evaluation of these compounds against a panel of pathogenic bacteria revealed that the majority of these structures had antibiotic activity, some efficacious against strains resistant to macrolides in current use. The chemistry we describe here provides a platform for the discovery of new macrolide antibiotics and may also serve as the basis for their manufacture.

Polyolefin-Polar Block Copolymers from Versatile New Macromonomers.

A new metallocene-based polymerization mechanism is elucidated in which a zirconium hydride center inserts α-methylstyrene at the start of a polymer chain. The hydride is then regenerated by hydrogenation to release a polyolefin containing a single terminal α-methylstyrenyl group. Through the use of the difunctional monomer 1,3-diisopropenylbenzene, this catalytic hydride insertion polymerization is applied to the production of linear polyethylene and ethylene-hexene copolymers containing an isopropenylbenzene end group. Conducting simple radical polymerizations in the presence of this new type of macromonomer leads to diblock copolymers containing a polyolefin attached to an acrylate, methacrylate, vinyl ester, or styrenic segments. The new materials are readily available and exhibit interfacial phenomena, including the mediation of the mixing of immiscible polymer blends.

Synthesis and Postpolymerization Modification of One-Pot ω-Pentadecalactone Block-like Copolymers.

We report the one-pot copolymerization of ω-pentadecalactone (PDL) to produce tri- and diblock-like copolymers with the ability to undergo postpolymerization modification. The ε-substituted ε-lactone (εSL), menthide (MI), was copolymerized with PDL to introduce side chain functionality into poly(ω-pentadecalactone) (PPDL) copolymers. The copolymerization was followed by quantitative 13C NMR spectroscopy, which revealed that the polymerization of MI occurred before the incorporation of PDL into the polymer chain to form a block-like copolymer. Transesterification side reactions were not found to occur interblock, although intrablock transesterification side reactions occurred only within the PPDL section. The same effect was demonstrated across a range of relative molar equivalents of monomers, and the generality of the approach was further demonstrated with the copolymerization of PDL with other εSL monomers. Finally, the copolymerization of PDL with an alkene-functionalized εSL was shown to produce one-pot PDL block-like copolymers that could undergo postpolymerization modification by thiol-ene addition to produce block copolymers with a range of characteristics in a simple procedure.

The evolving role of chemical synthesis in antibacterial drug discovery.

The discovery and implementation of antibiotics in the early twentieth century transformed human health and wellbeing. Chemical synthesis enabled the development of the first antibacterial substances, organoarsenicals and sulfa drugs, but these were soon outshone by a host of more powerful and vastly more complex antibiotics from nature: penicillin, streptomycin, tetracycline, and erythromycin, among others. These primary defences are now significantly less effective as an unavoidable consequence of rapid evolution of resistance within pathogenic bacteria, made worse by widespread misuse of antibiotics. For decades medicinal chemists replenished the arsenal of antibiotics by semisynthetic and to a lesser degree fully synthetic routes, but economic factors have led to a subsidence of this effort, which places society on the precipice of a disaster. We believe that the strategic application of modern chemical synthesis to antibacterial drug discovery must play a critical role if a crisis of global proportions is to be averted.

Methodological Advances Permit the Stereocontrolled Construction of Diverse Fully Synthetic Tetracyclines Containing an All-Carbon Quaternary Center at Position C5a.

Here we describe chemical innovations that enable the preparation of fully synthetic tetracyclines containing an all-carbon quaternary, stereogenic center at position C5a, a structurally novel class of compounds in this important family of therapeutic agents. In the key transformation and an important extension of the powerful Michael-Claisen cyclization (AB plus D) approach to the construction of fully synthetic tetracyclines, we show that the six-membered C ring comprising a C5a quaternary carbon center can be assembled by highly stereocontrolled coupling reactions of ß-substituted AB enones and o-toluate ester anion D-ring precursors. Novel and versatile ß-functionalization reaction sequences employing tris(methylthio)methyllithium and 2-lithio-1,3-dithiane have been developed to transform the AB enone 1 (the key precursor to fully synthetic tetracyclines) into a diverse range of ß-substituted AB enone products, including a highly efficient, single-operation method for the synthesis of a ß-methyl ester-substituted AB enone. A C5a-C11a-bridged cyclopropane tetracycline precursor was found to undergo efficient and regioselective ring-opening reactions with a range of nucleophiles in the presence of magnesium bromide, thus providing another avenue for the preparation of fully synthetic tetracyclines containing an all-carbon quaternary center at position C5a. Two compounds prepared from the bridged cyclopropane intermediate served as (further) diversifiable branch-points, allowing maximally expedient synthesis of C5a-substituted tetracyclines by final-step diversification.

Tetracyclines promote survival and fitness in mitochondrial disease models.

Mitochondrial diseases (MDs) are a heterogeneous group of disorders resulting from mutations in nuclear or mitochondrial DNA genes encoding mitochondrial proteins1,2. MDs cause pathologies with severe tissue damage and ultimately death3,4. There are no cures for MDs and current treatments are only palliative5-7. Here we show that tetracyclines improve fitness of cultured MD cells and ameliorate disease in a mouse model of Leigh syndrome. To identify small molecules that prevent cellular damage and death under nutrient stress conditions, we conduct a chemical high-throughput screen with cells carrying human MD mutations and discover a series of antibiotics that maintain survival of various MD cells. We subsequently show that a sub-library of tetracycline analogues, including doxycycline, rescues cell death and inflammatory signatures in mutant cells through partial and selective inhibition of mitochondrial translation, resulting in an ATF4-independent mitohormetic response. Doxycycline treatment strongly promotes fitness and survival of Ndufs4-/- mice, a preclinical Leigh syndrome mouse model8. A proteomic analysis of brain tissue reveals that doxycycline treatment largely prevents neuronal death and the accumulation of neuroimmune and inflammatory proteins in Ndufs4-/- mice, indicating a potential causal role for these proteins in the brain pathology. Our findings suggest that tetracyclines deserve further evaluation as potential drugs for the treatment of MDs.

Relationship between normalized adductor pollicis train-of-four ratio and manifestations of residual neuromuscular block: a study using acceleromyography during near steady-state concentrations of mivacurium.

BACKGROUND: Baseline acceleromyographic adductor pollicis train-of-four (TOF) ratio varies significantly between individuals and is often greater than unity. Thus, normalization of acceleromyography data is necessary. The relationship between normalized acceleromyographic TOF ratio, lung volumes, and clinical signs of residual neuromuscular block was studied. METHODS: In 12 healthy volunteers, three steady-state levels of neuromuscular block were achieved with mivacurium infusions. TOF ratio was measured acceleromyographically at the adductor pollicis using a preload. Lung volume measurements and a series of clinical tests were made at each stable block and reconciled to the normalized TOF measures. RESULTS: None experienced airway obstruction or arterial oxygen desaturation, even at normalized TOF ratio less than 0.4. Functional residual capacity remained unchanged whereas vital capacity decreased linearly with decreasing TOF ratio. The ability to protrude the tongue was preserved at all times. The ability to clench the teeth was lost in one volunteer at normalized TOF ratio of 0.84 but retained in four at normalized TOF ratio less than 0.4. Four volunteers lost the ability both to raise the head more than 5 s and to swallow, with the most sensitive individual demonstrating these effects at normalized TOF ratio of 0.60. At mean normalized TOF ratio of 0.42, the mean handgrip strength was approximately 20% of baseline value. CONCLUSION: Lung vital capacity decreased linearly with decreasing TOF ratio. Responses to clinical tests of muscle function varied to a large extent among individuals at comparable TOF ratios. None of the volunteers had significant clinical effects of neuromuscular block at normalized acceleromyographic TOF ratio greater than 0.90.

A robust platform for the synthesis of new tetracycline antibiotics.

Tetracyclines and tetracycline analogues are prepared by a convergent, single-step Michael-Claisen condensation of AB precursor 1 or 2 with D-ring precursors of wide structural variability, followed by removal of protective groups (typically in two steps). A number of procedural variants of the key C-ring-forming reaction are illustrated in multiple examples. These include stepwise deprotonation of a D-ring precursor followed by addition of 1 or 2, in situ deprotonation of a D-ring precursor in mixture with 1 or 2, and in situ lithium-halogen exchange of a benzylic bromide D-ring precursor in the presence of 1 or 2, followed by warming. The AB plus D strategy for tetracycline synthesis by C-ring construction is shown to be robust across a range of different carbocyclic and heterocyclic D-ring precursors, proceeding reliably and with a high degree of stereochemical control. Evidence suggests that Michael addition of the benzylic anion derived from a given D-ring precursor to enones 1 or 2 is quite rapid at -78 degrees C, while Claisen cyclization of the enolate produced is rate-determining, typically occurring upon warming to 0 degrees C. The AB plus D coupling strategy is also shown to be useful for the construction of tetracycline precursors that are diversifiable by latter-stage transformations, subsequent to cyclization to form the C ring. Results of antibacterial assays and preliminary data obtained from a murine septicemia model show that many of the novel tetracyclines synthesized have potent antibiotic activities, both in bacterial cell culture and in vivo. The platform for tetracycline synthesis described gives access to a broad range of molecules that would be inaccessible by semisynthetic methods (presently the only means of tetracycline production) and provides a powerful engine for the discovery and, perhaps, development of new tetracycline antibiotics.

Tunable thermoresponsive water-dispersed multiwalled carbon nanotubes.

Polymers containing poly(ethylene glycol) methacrylate and 2-(2-methoxyethoxy)ethyl methacrylate have been synthesized by Cu(0)-mediated radical polymerisation for use as thermoresponsive water-dispersants for carbon nanotubes.

Diastereoselective Michael-Claisen Cyclizations of γ-Oxa-α,ß-unsaturated Ketones en Route to 5-Oxatetracyclines.

5-Oxatetracyclines were synthesized from d-arabinose using sequential Michael-Claisen cyclization reactions via a 5-oxa-AB enone substrate. The 5-oxatetracyclines were found to have poor stability in aqueous buffer (pH 7.4, 37 °C) and showed little to no inhibition of bacterial growth (S. aureus, E. coli).

Ischemic thresholds for gray and white matter: a diffusion and perfusion magnetic resonance study.

BACKGROUND AND PURPOSE: Although gray matter (GM) and white matter (WM) have differing neurochemical responses to ischemia in animal models, it is unclear whether this translates into differing thresholds for infarction. We studied this issue in ischemic stroke patients using magnetic resonance (MR) techniques. METHODS: MR studies were performed in patients with acute hemispheric ischemic stroke occurring within 24 hours and at 3 months. Cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and apparent diffusion coefficient (ADC) were calculated. After segmentation based on a probabilistic map of GM and WM, tissue-specific diffusion and perfusion thresholds for infarction were established. RESULTS: Twenty-one patients were studied. Infarction thresholds for CBF were significantly higher in GM (median 34.6 mL/100 g per minute, interquartile range 26.0 to 38.8) than in WM (20.8 mL/100 g per minute; interquartile range 18.0 to 25.9; P<0.0001). Thresholds were also significantly higher in GM than WM for CBV (GM: 1.67 mL/100 g; interquartile range 1.39 to 2.17; WM: 1.19 mL/100 g; interquartile range 0.94 to 1.53; P<0.0001), ADC (GM: 918x10(-6) mm2/s; 868 to 975x10(-6); WM: 805x10(-6); 747 to 870x10(-6); P<0.001), and there was a trend toward a shorter MTT in GM (GM 4.94 s, 4.44 to 5.38; WM 5.15, 4.11 to 5.68; P=0.11). CONCLUSIONS: GM has a higher infarction threshold for CBF, CBV, and ADC than WM in patients within 24 hours of ischemic stroke onset. Hence, when assessing patients for potential therapies, tissue-specific rather than whole-brain thresholds may be a more precise measure of predicting the likelihood of infarction.

A digital map of middle cerebral artery infarcts associated with middle cerebral artery trunk and branch occlusion.

BACKGROUND AND PURPOSE: Knowledge of the topographic distribution of infarcts of the middle cerebral artery (MCA) may give insight into the limits of the arterial territory and infarct mechanism and may influence the decision to use thrombolytic therapy. We describe the creation of a digital atlas of MCA (DA-MCA) infarction associated with MCA branch and trunk occlusion using magnetic resonance (MR) techniques. METHODS: Hemispheric infarcts, with evidence of MCA trunk or branch occlusion, were manually segmented into binary images, linearly registered into a common stereotaxic coordinate space, and averaged to yield the probability of involvement by infarction at each voxel. Comparisons were made with existing maps of the MCA territory. RESULTS: Twenty-eight patients with median age of 74 years (range, 26 to 87 years) were studied. On the DA-MCA, the highest frequency of infarction was within the striatocapsular region, centrum semiovale, and the insula. The mean and maximal MCA infarct volumes were 195.5 cm3 and 366.3 cm3, respectively. Comparison with published maps showed that the most common difference from the DA-MCA was in the superomedial extent of the MCA territory. Some maps showed the MCA territory reaching the interhemispheric fissure, whereas in the DA-MCA it did not. There was a lower variability in the anterior boundary of the MCA territory compared with its posterior counterpart. CONCLUSIONS: We have created a digital atlas of MCA infarction using MR imaging techniques. This approach may be useful to establish the distribution of the MCA and other arterial territories and the border zones between them with greater certainty.

Is white matter involved in patients entered into typical trials of neuroprotection?

BACKGROUND AND PURPOSE: One of the reasons for the failure of trials of neuroprotection in stroke may be the lack of white matter (WM) protection. However, whether patients entered into typical neuroprotection trials have WM involved in the ischemic process is unknown. We studied patients who were enrolled in neuroprotection trials at our center and used a neuroimaging coregistration approach to determine whether final infarcts involved WM and, if so, in what proportion. We also aimed to provide the first in vivo volume distribution of gray matter (GM) and WM in normal stroke-aged brains. METHODS: Patients enrolled in trials of neuroprotection had late computed tomography or magnetic resonance scans coregistered in standard stereotaxic coordinate space after segmentation of symptomatic cerebral infarcts. These were then superimposed on a probabilistic map of GM and WM, which was developed from age-matched normal controls in whom GM and WM volumes were assessed. RESULTS: Forty-two patients (mean age, 73.7+/-10.5 years) were studied from 6 trials of neuroprotection. WM formed 41.7% of the brain volume in 37 control subjects (mean age, 73.5+/-8.4 years). In the segmented infarcts, WM comprised a median of 49% (interquartile range, 36.5 to 77.9) of the infarct volume. Ninety-five percent of infarcts had some involvement of WM tracts. CONCLUSIONS: WM occupies approximately 42% by volume of the normal stroke-aged brain. Patients entered into typical trials of neuroprotection may have significant WM volumes involved in the ischemic process, thus providing a rationale for the development of neuroprotectants for this compartment.

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.

Influence of chronic phenytoin administration on the pharmacokinetics and pharmacodynamics of vecuronium.

BACKGROUND: The duration of action of vecuronium is reduced in patients receiving phenytoin. In this study, the authors examined, simultaneously, the influence of phenytoin on both the pharmacokinetics and the pharmacodynamics of vecuronium. METHODS: This study was approved by the institutional review board of the University of California, San Francisco, and patients gave written informed consent. Twenty-two patients, 11 taking phenytoin and all scheduled to undergo prolonged neurosurgical procedures with general anesthesia, participated in the study. In 12 patients (6 phenytoin, 6 control), vecuronium was infused at 7.5 microg x kg(-1) x min(-1) until the first response (T1) of each train-of-four decreased by 50%; in the remaining 10 patients (5 phenytoin, 5 control), 200 microg/kg vecuronium was infused over 10 min. Arterial blood samples were drawn at intervals over the next 5-7 h. Plasma concentrations of vecuronium and 3-desacetylvecuronium were measured by capillary gas chromatography. Pharmacokinetic and pharmacodynamic modeling was used to characterize the disposition of vecuronium and patient responses to it in the two groups. RESULTS: Clearance was typically increased by 138% (95% confidence interval, 93-183%) in patients taking phenytoin. The effect of vecuronium was well described using a sigmoid Emax model. The concentration of vecuronium giving 50% twitch depression was increased 124% (45-202%) in patients taking phenytoin. CONCLUSIONS: Chronic phenytoin therapy reduces the effect of vecuronium by mechanisms that include both increased vecuronium metabolism and reduced sensitivity of the patient to circulating concentrations of vecuronium.

Distinguishing animal subsets in toxicokinetic studies: comparison of non-linear mixed effects modelling with non-compartmental methods.

The purpose of this study was to compare the ability of non-compartmental analysis and compartmental mixed effects modelling (MEM) to determine the existence and magnitude of exposure differences (i.e. exposure ratio estimates) between subsets of animals during destructive toxicokinetic studies. Data from five toxicokinetic studies of an experimental compound were analysed using a linear trapezoidal calculation of the area under the curve (non-compartmental analysis) or modelled using MEM. With the non-compartmental method the Bailer-Satterthwaite approximation was used to construct confidence intervals around the exposure estimates of each subset of animals and these were used to determine if exposure differed between the subsets. The MEM analyses were performed on the full datasets and on datasets with arbitrary reductions in the number of animal replicates. With MEM, additional model parameters were used to differentiate between subsets of animals, and were incorporated only if they were justified statistically. Estimates of the existence and magnitude of exposure differences between animal subsets were similar with the two techniques. The MEM analyses were influenced only marginally by substantial reductions in the number of animals studied and were less compromised by extremely limited or unbalanced data. These analyses show that MEM and non-compartmental methods are similarly effective at detecting exposure differences between animal subsets in toxicokinetic studies. Estimates provided by both methods were influenced by the degree of variance in the data. These results support the proposition that it may be possible to reduce the number of animals employed in toxicokinetic studies if MEM is used.

The influence of mild hypothermia on the pharmacokinetics and time course of action of neostigmine in anesthetized volunteers.

BACKGROUND: The pharmacokinetics, maximum effect, and time course of action of neostigmine were studied in seven human volunteers. METHODS: Each volunteer was studied twice, during both normothermia and hypothermia. Anesthesia was induced with 30 microg/kg alfentanil and 3 mg/kg propofol, and was maintained with 60-70% nitrous oxide and 0.7-0.9% isoflurane. The mechanical response of the adductor pollicis to train-of-four stimulation of the ulnar nerve was recorded, and central body temperature maintained stable at either less than 34.5 degrees C or greater than 36.5 degrees C by surface cooling or warming. Before neostigmine administration, a stable 5% twitch height was obtained by an infusion of vecuronium, and the infusion rate remained unchanged thereafter. Neostigmine, 70 microg/kg, was then infused over 2 min, and blood samples for estimation of neostigmine concentrations were collected at intervals for 240 min. RESULTS: With hypothermia, the central volume of distribution of neostigmine decreased by 38%, and onset time of maximum effect increased (4.6 vs. 5.6 min). Hypothermia did not change the clearance (696 ml/min), maximum effect, or duration of action of neostigmine. CONCLUSIONS: The efficacy of neostigmine as an antagonist of vecuronium-induced neuromuscular block is not altered by mild hypothermia.

Salvaging the ischaemic penumbra: more than just reperfusion?

1. The ischaemic penumbra is defined as a moderately hypoperfused region that retains structural integrity but has lost function. In animal models of ischaemic stroke, this region is prone to recurrent anoxic depolarization and will become infarcted if reperfusion does not occur. In the macaque model, an ischaemic penumbra has been identified for up to 3 h after ischaemic stroke onset, whereas in selected human patients it may exist for up to 48 h. 2. Although most definitions of the ischaemic penumbra stress a time-brain volume concept, few incorporate the idea that selective and delayed neuronal injury plays an important role. Thus, in addition to necrotic cell death caused by acute injury, it is important to also consider delayed death mediated by caspase-dependent and -independent apoptotic pathways. 3. Salvage of penumbral tissue is possible if reperfusion (e.g. after thrombolysis) occurs. However, neurons within this salvaged region may be still at risk of further delayed neuronal injury. 4. In the present review, we aim to revisit the concept of the ischaemic penumbra and explore the role of selective and delayed neuronal injury in enlargement of the volume of infarction, as well as pathogenic mechanisms of white matter ischaemia. Both animal and human models of cerebral ischaemia imaged using magnetic resonance and positron emission tomography techniques will be discussed.

Pharmacokinetics of propofol infusions in critically ill neonates, infants, and children in an intensive care unit.

BACKGROUND: Propofol is a commonly used anesthetic induction agent in pediatric anesthesia that, until recently, was used with caution as an intravenous infusion agent for sedation in pediatric intensive care. Few data have described propofol kinetics in critically ill children. METHODS: Twenty-one critically ill ventilated children aged 1 week to 12 yr were sedated with 4-6 mg. kg(-1).h(-1) of 2% propofol for up to 28 h, combined with a constant morphine infusion. Whole blood concentration of propofol was measured at steady state and for 24 h after infusion using high-performance liquid chromatography. RESULTS: A propofol infusion rate of 4 mg. kg(-1).h(-1) achieved adequate sedation scores in 17 of 20 patients. In 2 patients the dose was reduced because of hypotension, and 1 patient was withdrawn from the study because of a increasing metabolic acidosis. Mixed-effects population models were fitted to the blood propofol concentration data. The pharmacokinetics were best described by a three-compartment model. Weight was a significant covariate for all structural model parameters; Cl, Q2, Q3, V1, and V2 were proportional to weight. Estimates for these parameters were 30.2, 16.0, and 13.3 ml. kg(-1).min(-1) and 0.584 and 1.36 l/kg, respectively. The volume of the remaining peripheral compartment, V3, had a constant component (103 l) plus an additional weight-related component (5.67 l/kg). Values for Cl were reduced (typically by 26%) in children who had undergone cardiac surgery. CONCLUSIONS: Propofol kinetics are altered in very small babies and in children recovering from cardiac surgery. Increased peripheral distribution volume and reduced metabolic clearance following surgery causes prolonged elimination.

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.