Anesth Analg 2008; 107:365-366
© 2008 International Anesthesia Research Society
doi: 10.1213/ane.0b013e31817e0e5b
EDITORIAL
Does It Add Up?
Max B. Kelz, MD, PhD, and
Roderic G. Eckenhoff, MD
From the Department of Anesthesiology and Critical Care, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
Address correspondence to Roderic G. Eckenhoff, MD, 311A John Morgan Building, 3620 Hamilton Walk, Philadelphia, PA 19104-6112. Address e-mail to roderic.eckenhoff{at}uphs.upenn.edu.
A few years ago, we raised the unpopular notion that the state of anesthesia might be conveyed by small effects of anesthetics on many molecular targets.1 Although not entirely novel, (Ray Fink invoked such a concept in his metaphor "grit in well-oiled machinery"2), it was unpopular primarily because it is so difficult to test, and was therefore considered unscientific in this era of hypothesis-driven, reductionist biology.3 A group of manuscripts in this issue of Anesthesia & Analgesia4–7 now tackles this question, starting with the reasonable assumption that a multiple target mechanism should produce other than additive effects when disparate drugs are combined. The approaches, assumptions, drugs, models, and datasets differ among the papers, and thus it should not be surprising that a single harmonious answer was lacking. Nonetheless, the quest is lofty, worthwhile, and certainly thought-provoking.
If a multiple target model should produce non-additivity, should a single target produce additivity? Jenkins et al.4 examine this question for anesthetics presumed to act via the family of ligand-gated ion channels. They conclude that allosteric modulation of these channels by anesthetics is entirely additive, even for drugs strongly suspected as using different binding sites within the single receptor. Although not mentioned, the obvious case of nonadditive behavior between agonist ( aminobutyric acid, glycine, acetylcholine) and anesthetics on these receptors certainly raises the possibility of allosteric synergy between divergent sites on the same receptor. Perhaps the "anesthetics" used in their study are physicochemically too similar to produce the necessary site selection. Regardless, this study is important support for the idea that additive behavior implies single targets.
But single receptors do not well represent the behavior of a cell, let alone a network or an organism. Thus, to accommodate the additional complexity, Eger et al.5 extend the question to intact animals. Here again, the group of inhaled anesthetics demonstrate additivity in the minimum alveolar anesthetic concentration (MAC) response, with the exception of nitrous oxide, which showed a small degree of antagonism. It is of interest that a 10% degree of deviation from the line of strict additivity was tolerated, presumably because this is the typical level of confidence in MAC studies. Of course, an alternate arbitrary value or improved signal-to-noise ratio could change the conclusions, as well illustrated by Shafer et al.7 Nevertheless, this study was interpreted by these authors as being consistent with immobilization arising from interactions with a single target, but one should keep in mind that there is little precedent for the extraction of molecular mechanisms from behavioral measurements.
Hendrickx et al.6 open things even further, by including any drug that can produce hypnosis or immobility, and using a retrospective review of the literature as the source of in vivo data. In a sense, this is the "positive control" for the assumption that synergy arises from actions on multiple targets. As might be expected for such a diverse collection of compounds (e.g., neurotransmitters, opioids, and inhaled anesthetics), there is diversity in the results. Consistent with the above studies, the inhaled anesthetics are generally additive with each other, but synergistic with other classes of drugs. Other drugs are synergistic with each other. Although at first glance the choice of drugs seems arbitrary and undisciplined, the point here is well taken, if the inhaled anesthetics truly use diverse sites to produce immobility and hypnosis, then synergy would be expected. But how diverse is diverse? Is a two- or threefold difference in potency at a given target site sufficient? Or is a higher degree of selectivity (e.g., dopamine vs fentanyl) necessary? And what if the effect at the diverse sites is only small to begin with?
Shafer et al.7 begin to address these questions from a theoretical perspective, and now consider the binding event as distinct from the effect on activity. After a lot of sophisticated modeling that most will be happy to find sequestered in the appendices, the authors conclude that it depends. If receptor site occupancy is low relative to the EC50 (KD>EC50) then interactions with diverse receptors might indeed show additive behavior whereas if it is high, then synergy may emerge. And this is the extreme example of where the drug binding to the disparate molecular effectors is truly selective. For the anesthetics, this is unlikely to be the case. Not a single example of an inhaled anesthetic binding site that excludes another has been found, whereas several examples that bind many anesthetics have been described.8–10 Differences in effect at different targets might be due to only subtle differences in occupancy, as emphasized by Shafer et al. So now all we need to know is the relative position of KD and EC50 for some of the targets. Unfortunately, as Shafer et al. correctly point out, there is a paucity of binding data, especially in targets considered to be "relevant." Nevertheless, for anesthetics interacting with proteins like the nAChR,11,12 rhodopsin,13 serum albumin,8 calmodulin,14 SR CaATPase,15 PSD-95,16 and SNARE/SNAP-25/syntaxin,17 the KD tends to exceed MAC by two to fivefold. Given the probable degeneracy of anesthetic binding sites (features are similar in many targets), it seems unlikely that binding sites in other plausible targets will be much different, but the sole example of a protein where KD<MAC, ferritin,10 should provide hope for the unitary people among us.
Perhaps the most interesting feature of this group of manuscripts is the authorship. Many of the same authors appear on most or all of the papers, apparently taking (somewhat) opposing sides of the synergy/additivity argument. Like Freud’s ego and id, the two selfs struggle for, but never quite reach dominance. Either the data are simply not yet of high enough fidelity to resolve the question, we do not yet have the necessary data, or we have not yet asked the right question.
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Footnotes
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Accepted for publication April 25, 2008.
Reprints: Not available.
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REFERENCES
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- Fink BR. Molecular mechanism of anesthesia: another voice. Anesth Analg 1994;78:407–8[Medline]
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