Knowledge of the proposed pharmacologic mechanisms of action of ILE in acute intoxication is important when considering clinical application. Despite extensive and ongoing study the precise mode of action for ILE in many poisonings remains to be fully elucidated. At present both pharmacokinetic and pharmacodynamic actions have been postulated to play a role in the action of ILE as antidote. Whilst synergy between proposed actions may exist, the relative contributions of each remain uncertain. It is quite possible that each mechanism may variably contribute to clinically observed improvement depending on unique combinations of patient factors, the mode and duration since poisoning, and the particular intoxicant.
Pharmacokinetic modulation/lipid sink theory
First forwarded by Weinberg in 1998 [
1], this hypothesis proposes that, following ILE injection, lipophilic intoxicant preferentially distributes into the newly created intravascular lipid compartment, and is held away from the site of toxic action. The hypothesis has been extended to include effects on subsequent enhanced redistribution of lipophilic intoxicants, with increased blood carriage thought to speed transport of toxin from target organ to peripheral adipose tissue depots. This so called ‘lipid sink’ thesis has served to guide much of the early development of ILE therapy, with the majority of agents studied thus far exhibiting high lipid solubility.
Evidence in support of the lipid sink has been derived from both pre-clinical models and human reports. Following their study with the prototypical lipophilic local anaesthetic bupivacaine in whole rat models of cardiotoxicity [
1], Weinberg and colleagues subsequently demonstrated enhanced myocardial washout of bupivacaine when cardiac perfusate was spiked with ILE in an isolated heart model [
11]. Both greater myocardial bupivacaine loss (
P < 0.0016) and increased coronary sinus effluent bupivacaine concentration (
P < 0.008) were recorded, in keeping with rapid tissue detoxification. Similarly, Chen and colleagues [
12] have shown lesser myocardial bupivacaine concentrations, and associated dose-dependent increases in rate-pressure product for isolated rat hearts subjected to lipid emulsion.
Subsequent authors investigating the effect of ILE injection in whole rats have demonstrated evidence of enhanced drug redistribution with reduced bupivacaine concentration in heart, brain, lung, kidney and splenic tissue observed following ILE treatment [
13]. The hepatic bupivacaine concentration was increased. In this model ILE use was associated with greater bupivacaine clearance, and prolonged half-life (T1/2) compared with control animals, in keeping with favourable modulation to bupivacaine pharmacokinetics.
Support for a lipid sink effect is not confined to lipophilic local anaesthetics, with animal models and human cases demonstrating an effect with a range of disparate intoxicants, including calcium channel blockers [
25]-[
27], lipophilic beta-blockers [
28],[
29], tricyclic antidepressants [
15]-[
17], antipsychotics [
30],[
31], and antiarrhythmics [
32]. Despite lacking commonality in their toxicodynamic mechanisms, agents demonstrating response to ILE treatment typically exhibit high lipid solubility (log D (octanol:water partition coefficient at physiologic pH) >2; bupivacaine logD = 3.65), consistent with the proposed pharmacokinetic mechanism. Indeed, drug partitioning constants and volumes of distribution have been investigated as predictors of ILE responsiveness. French and co-workers in 2011 [
33] demonstrated
in vitro sequestration of lipophilic drugs in human plasma spiked with lipid emulsion, and subsequently assembled a compendium of predicted lipid extraction efficiency on the basis of the intrinsic pharmacologic parameters for over 50 commonly used drugs.
In vivo studies using non-local anaesthetic drugs have confirmed sink effects in intact animal models and human subjects. In a rabbit model of intravenous clomipramine toxicity, increased total blood concentrations of clomipramine were seen in concert with improved blood pressure following ILE injection, consistent with sequestration of toxin into an intravascular lipid phase [
34]. Litonius and colleagues [
35] similarly demonstrated an increased total amitriptyline concentration with a decrease in free amitriptyline fraction in amitriptyline toxic pigs when ILE was given after amitriptyline infusion. Notably, however, there was no improvement seen in haemodynamic metrics nor survival in this model. The same group furthermore demonstrated that pretreatment with ILE improved haemodynamics during an amiodarone infusion, and resulted in increased amiodarone total concentration and decreased free drug concentration [
32]. ILE has also been shown to reduce brain concentrations of amitriptyline when given following intravenous infusion and tissue distribution time in swine [
36]. In the same experiment a non-statistically significant reduction in cardiac amitriptyline concentrations was additionally reported (
P = 0.07).
Human pharmacokinetic data are limited to two clinical studies and sporadic case reports. Minton and colleagues [
37] demonstrated a statistically non-significant increase in plasma amitriptyline concentrations in subjects in pharmacokinetic steady state subjected to infused lipid emulsion. Free drug concentrations were not measured. Litonius and colleagues [
38] furthermore found a decreased half-life for a nontoxic dose of bupivacaine in blood after administration of ILE in eight normal subjects, consistent with an effect on tissue redistribution. A decrease in free verapamil concentration was reported after treatment with ILE for intoxication [
39], commensurate with clinical improvement. In another report, increased total blood amitriptyline concentrations were seen following ILE therapy in a case of mixed overdose with prominent tricyclic toxidrome [
40].
While attractive in its simplicity, emerging evidence suggests application of the sink theory must be approached with some caution in all but a few overdose scenarios. A pure sink effect may be important in cardiac arrest due to direct intravenous bolus administration of intoxicant - the centralised circulation meaning that rapid equilibration of heart and brain concentrations with the introduced, centrally confined lipid phase may be possible. Subsequent redistribution of intoxicant to peripheral fat depots on return of effective circulation would conceivably ensue - such a situation could be hypothesized to occur in most forms of LAST.
Any pharmacokinetic effect for ILE in the non-arrested patient likely depends on both the pharmacokinetic phase and circulatory status at the time of intoxication. Increased plasma carriage for lipophilic toxins with ILE injection will increase drug transport between organ systems according to a complex interplay of tissue drug affinity, relative concentration, and perfusion. As such, net movement of intoxicant may not prove universally beneficial (that is, from target organ to peripheral depot). For example, in one study investigating the effect of ILE on thiopental anaesthesia in rabbits, early lipid administration appeared to increase the depth, but not overall duration, of anaesthesia. This could be explained by the action of lipid emulsion as a high affinity conduit - maintaining blood concentrations able to interact with the brain during early redistribution to hydrophilic tissues, before augmenting later redistribution from rapidly perfused to lesser perfused lipophilic tissues [
41].
Perhaps most vexing is the issue of ILE administration following enteric overdose. Data suggesting that ILE injection may actually augment absorption of lipophilic toxins from the gastrointestinal lumen have emerged, with increased mortality reported when ILE was administered early in the course of oral amitriptyline and verapamil toxicity [
42], and earlier death following ILE in rectal thiopentone overdose [
43] in rodent models. Documented increases in plasma intoxicant concentrations were reported in both studies. Effects on enhanced gastrointestinal uptake are likely to be limited to the absorptive phase of intoxication. For example, when ILE was administered 5 minutes after oral poisoning with the lipophilic organophosphate parathion in rats, no difference was observed in time to respiratory arrest. When given at 20 minutes (at the time of anticipated peak intravenous parathion concentration), ILE effected a significant increase in survival times, consistent with beneficial augmentation of drug redistribution [
44].
The sum of what is known on the pharmacokinetic effects of ILE as antidote seems to be that there are demonstrable effects on absorption, distribution and redistribution for lipophilic intoxicants. Early ILE use in enteric poisoning may be contraindicated because of potential for enhancing gastrointestinal absorption, albeit any such adverse effect is likely dependent on adequate intestinal perfusion, and mitigated somewhat by development of shock-like states secondary to intoxication. The clinical corollary of this would be that effects on blood concentrations following ILE administration likely depend on the timing of administration relative to both the pharmacokinetic phase and the mode of intoxication. It would also seem prudent to ensure actions necessary to mitigate gastrointestinal absorption, such as activated charcoal, were taken before or contemporaneous to lipid emulsion.
Direct cardiotonic effect
While clinical and experimental effects spanning agents and organ systems favour a pharmacokinetic mechanism as important in the action of ILE, they are not a complete explanation. Recent data have emerged that suggest lipid emulsions at the doses used have direct cardiotonic effects. Experimentally, rats infused with lipid emulsion showed increased blood pressure and aortic flow rates relative to the same volume of saline. Additionally, isolated rat hearts in isovolumetric contraction were seen to contract more forcefully, with increased oxygen demand, when perfusate was augmented with lipid emulsion [
45]. Stehr and colleagues [
46] additionally demonstrated a positive effect on bupivacaine toxic cardiac myocytes at levels below those that could have effected any 'sink' phenomenon. The same investigators furthermore modelled the relative contributions of volume, sink and cardiotonic effects in an
in silico model for the observed effect on recovery of rats from bupivacaine-induced shock. The model of best fit included all three factors, with the direct cardiotonic effect being the most prominent factor responsible for observed improvement [
47].
It seems likely that direct cardiotonic effects play a significant role as one of the mechanisms responsible for resuscitation from drug-induced cardiotoxicity. This may be particularly so in the arrested or critically compromised circulation in order to offer immediate support before any distributive effects are likely to occur.
There is evidence that metabolic pathways influenced by lipid emulsion are important in the experimental mechanism of action of ILE. Partownavid and colleagues [
48] demonstrated failure of ILE rescue in rat bupivacaine toxicity in the presence of the inhibitor of fatty acid metabolism CTV. A dose-dependent relationship for CTV on both resuscitation outcome and measures of cardiac function post-resuscitation was additionally observed. In an associated laboratory experiment, bupivacaine toxic mitochondria were more resistant to external calcium pulses, thought to be a mechanism effecting cell death, when treated with ILE prior to extraction. Conversely, Bania and colleagues [
49] reported no adverse effect for oxfenicine, a blocker of fatty acid transport into mitochondria, on survival time during verapamil infusion in whole rats.
While experimental data showing reduction in myocardial bupivacaine concentration post-ILE demonstrate that pharmacokinetic mechanisms play a role in ILE resuscitation, these metabolic experiments are compelling that metabolic and cardiotonic considerations are at least a necessary, if not sufficient, mechanism of action in experimental bupivacaine toxicity. This, in concert with the data on direct cardiotonic effects, is particularly important in the clinical development of ILE as antidote, as it suggests that pharmacokinetic testing alone will tend to incompletely evaluate the potential for antidotal action of ILE. Metabolic factors may furthermore hold differential importance depending on the toxic compound.
Ion channel modulation theory
Free fatty acids, the levels of which increase with ILE, are known to have effects on both sodium and calcium ion channel function. Arachidonic, linoleic and linolenic acid all increased calcium currents through activated calcium channels in isolated guinea pig cardiac myocytes [
50]. This effect appears to be directly on the channel, not modulated by any cellular second messenger system. Such an effect could be particularly important in ameliorating toxicity from calcium channel blockers, and could potentially be responsible for the short, direct cardiotonic effect seen with ILE.
Mottram and colleagues [
51] demonstrated that stearic acid partially antagonises cultured human voltage-gated sodium channels. This antagonist effect was shown experimentally to partially reverse the sodium channel antagonism seen with bupivacaine. Nadrowitz and colleagues [
52] have also demonstrated partial reversal of bupivacaine-induced sodium channel antagonism on the Nav15 sodium channel, known for particular tropism with bupivacaine, with lipid application in cultured human kidney cells. Notably, in cases where lipid emulsion has been reported as having a positive outcome as an antidote, sodium channel antagonists are prominent intoxicants [
18]-[
22], and very rapid resolution of QRS duration is often reported.