Resistance to atracurium in rats with experimental inflammation: role of protein binding

Plasma protein binding: from discovery to development

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug-drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

Alpha‐1‐acid glycoprotein directly affects the pharmacokinetics and the analgesic effect of methadone in the rat beyond protein binding

Methadone is a basic drug highly bound to alpha1-acid glycoprotein (AGP), a plasma protein that increases in several pathological situations. Our aims were to evaluate the processes (pharmacokinetics-PK and/or pharmacodynamics-PD) associated with changes of methadone analgesia under conditions of increased AGP, and whether these changes are dependent on binding, secondary to a pathology, or directly attributable to AGP. AGP levels, in rats, were increased by two different methods: (a) experimental inflammation with turpentine oil (TP), and (b) by directly infusing the protein (exo-AGP). Both had a corresponding control group. Tail-flick analgesia and PK were evaluated after methadone dose (0.35 mg/kg i.v.). Bicompartmental PK parameters as well as interanimal and assay variabilities were estimated using NONMEM. The relationship between Cp and analgesic effect (PD) was analyzed with WINNONLIN. AGP levels in both pretreated groups (TP and exo-AGP) were significantly increased, and the unbound fraction (fu) was decreased, compared to controls. All PK parameters were lower in the pretreated groups, but in exo-AGP the difference was maintained even when corrected by fu. Paradoxically, also in exo-AGP the analgesic effect was practically nonexistent, although the unbound Cp remained high, possibly associated to a change in the PD. AGP appears responsible for alterations in both PK and PD, beyond protein binding and inflammatory processes.

Muscle relaxants in neurosurgical anaesthesia: a critical appraisal

The use of muscle relaxants, considered until recently as common practice in current neurosurgical anaesthesia protocols, becomes increasingly more questionable today. The reasons rely on the evolution of neurosurgery including the advent of new surgical techniques, the evolution of anaesthesia having the benefit of new drugs and devices, and the rationale for using muscle relaxants balanced against their potential side-effects and possible pharmacodynamic alterations in neurosurgical patients.

Resistance to vecuronium in a term neonate

We report a case of resistance to curarization in a term neonate undergoing surgery. The dosage of vecuronium required to obtain satisfactory muscular blockade and cessation of spontaneous breathing efforts was more than 10-fold the normal one. The operation was delayed for 90 min The mechanism of this marked resistance is unknown, but some possible hypotheses are presented, focusing on the neonate's poor intrauterine growth and an abnormality in pharmacodynamics.

Atracurium during thoracic surgery: impaired efficiency in septic processes

OBJECTIVE

The aim of the study was to examine whether the neuromuscular blocking potency of atracurium changes in patients with a septic intrathoracic process.

DESIGN

Prospective clinical study.

SETTING

Community hospital.

PARTICIPANTS

Thirty patients who underwent thoracic surgery for resection of a pulmonary carcinoma were examined. Fifteen patients showed typical signs of a concomitant bacterial superinfection (infection group), 15 age-matched patients without infection served as the control (no-infection) group.

INTERVENTIONS

Relaxation was induced with atracurium, 0.6 mg/kg intravenously for intubation, followed by a continuous infusion to maintain a 90% neuromuscular blockade. Relaxometry was performed electromyographically using the Datex Relaxograph by stimulating the ulnar nerve next to the wrist.

MEASUREMENTS AND MAIN RESULTS

The onset time was significantly longer (5.3 +/- 2.9 v 3.3 +/- 1.2 minutes; p < 0.05), and the recovery phase (DUR 10%) was significantly shorter (23.5 +/- 8.6 v 36.9 +/- 7.3 minutes; p < 0.001) in the infection group compared with the controls. The infusion rate within the first hour of continuous application was 77.4% higher in the infection group than in the control group (11.0 +/- 2.9 v 6.2 +/- 1.0 microg/kg/min; p < 0.001).

CONCLUSION

The study showed that septic intrathoracic processes cause a clear reduction of the neuromuscular blocking potency of atracurium. To guarantee adequate muscle relaxation in such cases, precise neuromuscular monitoring is highly advisable.

Pharmacokinetic-pharmacodynamic modeling of mivacurium in rats

The pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of the neuromuscular blocking agent mivacurium were evaluated separately in two groups of rats receiving 0.6 mg kg-1 of mivacurium in a 2.5-min intravenous continuous (iv) infusion. The PK parameters for mivacurium were determined in the first group. A two-compartment model describes the kinetics of mivacurium in plasma. The estimates of the apparent volume of distribution at steady-state and plasma clearance [mean(SE)] were 650 (123) mL kg-1 and 9.9 (0.75) mL min-1 kg-1, respectively. In the second group, the evoked tibialis anterior muscle tension was monitored. The PK parameters derived from the first group were used to compute mivacurium plasma concentrations (C) at the times the PD measurements were recorded in the second group. The concentration-neuromuscular effect [% depression of initial twitch tension (E)] relationship was analyzed by two approaches. (1) The relationship of estimated effect site concentrations versus E; a sigmoidal Emax model described the effect compartment concentrations versus E relationship. The estimate [mean(SE)] of Cess50 (steady-state plasma concentration eliciting half of maximum E) was 0.65 (0.01) microgram mL-1. The value [mean-(SE)] of Keo (rate constant of equilibration between plasma and effect site) was estimated at 0.32 (0.03) min-1. (2) The relationship of descending limb C versus E; a sigmoidal Emax model described such relationship. The estimate [mean(SE)] of C50 (post-infusion C eliciting half of maximum E) was 0.57(0.03) microgram mL-1. The PD properties of mivacurium were also evaluated in another two groups of animals receiving either 5- or 10-min continuous iv infusion; PK and PD parameters obtained from the 2.5-min infusion experiments were used to predict the time course of E in the groups receiving 0.6 mg kg-1 of mivacurium in 5- and 10-min infusions; simulations using the estimated parameters adequately describe the time course of E in those groups. The effect of mivacurium on the mean arterial blood pressure (MAP) was also investigated; a 10% nonsignificant decrease (p > 0.05) in MAP was found in all groups.