Transcript Biochem. of anesthetics

PHM142 Fall 2016
Instructor: Dr. Jeffrey Henderson
Biochemistry of Anesthetics
Alex Seki
Shi Yao (William) Lu
Tom Wang
Anesthesia


Local

Produces temporary & reversible loss of sensation in a local area

No loss of consciousness
General

Produces temporary and reversible loss of sensation in whole body

Muscle paralysis & loss of reflexes

Loss of consciousness
Local Anesthetics
• Two classes: Ester and Amide
• Ester: Eg. Tetracaine
•
Hydrolyzed by plasma
pseudocholinesterases
• Amide: Eg. Lidocaine
•
N-dealkylated and
hydroxylated in liver by
CYP1A2 & CYP3A4
Local Anesthetics mechanism
of action

They are generally weak bases and can be ionized to
form cations

In uncharged state they can cross cell membranes of
neurons

Once inside the cell they form a cation that interact
with sodium channels
Review of electrochemical potential
In resting state, nongated K+ channels are
open, but the gated
Na+ channels are
closed. Outward flow
of K+ helps maintain
the negative cytosolic
face.
During depolarization,
Na+ channels open and
influx of sodium causes
the membrane
potential to reverse.
Fig. 7-32, Molecular Cell Biology 5th Edition – Lodish, Berk, Matsudaira,
Kaiser, Krieger, Scott, Zipursky, Darnell
Fig 5.4, Essentials of Regional Anesthesia – Kaye, Urman & Vadivelu
The local Anesthetic (B) crosses the cell membrane in its
uncharged form; becomes charged inside the cell
cytoplasm (BH+). The charged cation (BH+) enters the
open sodium channel, and binds, inhibiting Na+ entry,
and preventing depolarization.
Key properties to consider in Anesthetics
pKa
• needs to be neutral to get into cytosol, membranes & blood-brain barrier
• needs to be cation to have an effect on sodium channel (local anesthetics)
Lipid Solubility
• More lipophilic = more able to penetrate the cell membrane
• More lipophilic = greater sequestration in the myelin and cell membrane; serves as
depot for slow release of anesthetic = slower onset, but prolonged action
Fraction unbound
• Increase in molecular weight correlates to increase in plasma & tissue proteins
binding
• High protein binding  slow uptake and absorption & slow metabolism; again
serving as a depot = longer duration of action, slow onset
• Non-linear PK – some anesthetics bind to one uncommon blood protein
(e.g. a1 acid glycoprotein) with high affinity, and albumin with low affinity.
Initially, most anesthetic will bind the glycoprotein so no effect can be observed,
but increasing dose will saturate glycoprotein and so doubling dose may result in
disproportionate increase in unbound fraction
General Anesthetics

Administration


Inhaled or Intravenous
General properties

Typically small lipophilic molecules
General Anesthetic
mechanisms of action

Multiple Hypotheses

Unitary Lipid theory of anesthesia


Lipophilic anesthetics reside in the cell membrane, disrupting
the membrane fluidity  effects
The Ion Channel/Protein binding hypothesis

Lipophilic anesthetics bind to ion channels to disrupt ion flow
 effects
Strong Correlation
between solubility of a
anesthetic in a
hydrophobic solvent and
its potency.
Hypothesized that
anesthetics act in an
unitary mechanism
involving perturbing the
cell membrane to cause
changes in ion channel
conductance
Effective Concentration
Unitary lipid Theory
Olive Oil: Gas partition Coefficient
Modified from Fig 24.2, Anesthetic Pharmacology 2nd edition – Evers , Maze & Kharasch
Problems with the Unitary
lipid theory

Certain hydrophobic drugs do not obey the Meyer-Overton
correlation


Enantiomeric selectivity exists even though lipophilicity is
the same


CF4 causes anesthesia, but C2F6 and larger perfluoroalkanes
do not
Isoflurane (+) isomer is twice as effective as (-) isomer in
inducing anesthesia
Many drugs that should be predicted to function as
anesthetic instead triggered convulsions

Polyhalogenated and perfluorinated compounds

CCl2F2; CBrF2CF3, CBrF2CF2CF3
Ion Channel Theory
evidence suggest Anesthetics interact
with ligand gated ion channels and
voltage gated ion channels
Structure of NaChBac – Bacterial
homologue of Mammalian Sodium
Channel
Isoflurane binding
Fig. 25-7, Millers Anesthesia 8th Edition Vol. 1 – Miller, Cohen, Eriksson,
Fleisher, Wiener-Kronish & Young
Fig. 1 A & C, Raju et al. PLoS Comput Biol.:9(6) e1003090
Interactions with ligand gated
ion channels

Potentiates inhibitory GABAA and Glycine receptors on
postsynaptic neurons


This can come in the form of increasing the inhibitory
post-synaptic current
Inhibition of acetylcholine and glutamate receptors on
the postsynaptic neurons

This may come as reducing the excitatory post-synaptic
current
Halothane potentiates GABA
binding and Chloride Current
Fig 5-5, Clinical Anesthesia 7th edition – Barash, Stock, Cahalan, Stoelting, Cullen & Ortega,
Figures originally published by Wakamori et al. J. Neurophysiol: 66(6) 2014-21
A) Enflurane (Enf) and Halothane (Hal) increase the chloride current of activated
GABA receptors in Rat brainstem neurons (relative to control current)
B) Halothane decreases the concentration of GABA needed to generate a given
current strength
*Note that Hexafluorodiethyl Ether (HFE) is a convulsant, and has the reverse
effect on GABA mediated chloride currents.
Voltage gated ion channels

Inhibits excitatory Na+ channels


Reducing the Sodium current
Inhibition of Ca2+ channels

Reducing the calcium current
Combine to reduce neurotransmitter release in presynaptic neurons
Figure 1A, Study Anesthesiology:81(1) 104-16
Ca2+ current in N-type Calcium Channel
in rat hippocampal pyramidal neurons
are depressed by isoflurane
Reduction in calcium can lead to reduced
neurotransmitter release in pre-synaptic
neuron
Na+ current in rat posterior pituitary
nerve terminals are depressed by
isoflurane
Figure 2A, Ouyang et al. Mol Pharm.:64(2) 373-81
Summary Page

Local anesthetics produces temporary & reversible loss of sensation in a local area

General anesthetics produces temporary and reversible loss of sensation in whole body

Two classes of local anesthetics: ester and amide

Esters metabolized by pseudocholinesterases

Amides are N-dealkylated and Hydroxylated by Cyp1A2 and Cyp3A4
Key Properties to consider

pKa



Neutral to cross membranes, Local anesthetics must be cation inside cell to have effect
Lipophilicity

↑ lipophilic = easy to cross membranes, stuck in membrane  slow onset, long duration

↓ lipophilic = hard to cross membranes, need more to have effect  toxicity? Faster onset, short duration
Fraction unbound

only unbound fraction will have be able to act on tissue; many don’t have linear PK
Mechanisms of Action

Local anesthetics


Cross cell as neutral compound, becomes cation inside cell, cation binds to sodium channel and blocks activity
General anesthetics
Lipid theory - anesthetic enters the lipid membrane and perturbs the ion channels reducing ion flow
Ion channel theory – Anesthetics bind to epitopes on ion channels – either voltage gated or ligand gated.

Potentiates inhibitory GABA and Glycine currents  increased IPSP

Inhibits excitatory ACh & Glutamate currents  decreased EPSP

Inhibits excitatory voltage-gated Na+, Ca2+ channels  reduced neurotransmitter release
Journal References:
REFERENCES
Franks, N., & Lieb, W. (1991). Stereospecific effects of inhalational general anesthetic optical isomers on nerve ion channels. Science,
254(5030), 427-430.
Kalow, W. (1952). Hydrolysis of local Anesthetics by Human Serum Cholinesterase. Journal of Pharmacology and Experimental Therapeutics,
104(2), 122-134.
Koblin, D. D., Chortkoff, B. S., Laster, M. J., Eger, E. I., II, Halsey, M. J., & Ionescu, P. (1994). Polyhalogenated and perfluorinated compounds
that disobey the Meyer-Overton hypothesis. Anesthesia & Analgesia, 79(6), 1043-1048.
Liu, J., Laster, M. J., Koblin, D. D., Eger, E. I., II, Halsey, M. J., Taheri, S., & Chortkoff, B. (1994). A Cutoff in Potency Exists in the Perfluoroalkanes.
Anesthesia & Analgesia, 79(2), 238-244.
Ouyang, W., Wang, G., & Hemmings, H., Jr. (2003). Isoflurane and Propofol Inhibit Voltage-Gated Sodium Channels in Isolated Rat
Neurohypophysial Nerve Terminals. Molecular Pharmacology, 64(2), 373-381.
Raju, S. G., Barber, A. F., Lebard, D. N., Klein, M. L., & Carnevale, V. (2013). Exploring Volatile General Anesthetic Binding to a Closed
Membrane-Bound Bacterial Voltage-Gated Sodium Channel via Computation. PLos Computational Biology, 9(6). e1003090.
Study, R. E. (1994). Isoflurane Inhibits Multiple Voltage-gated Calcium Currents in Hippocampal Pyramidal Neurons. Anesthesiology, 81(1), 104116.
Wakamori, M., Ikemoto, Y., & Akaike, N. (1991). Effects of two volatile anesthetics and a volatile convulsant on the excitatory and inhibitory
amino acid responses in dissociated CNS neurons of the rat. Journal of Neurophysiology, 66(6), 2014-2021.
Textbook References:
Barash, P. G., Stock, M. C., Cahalan, M. K., Stoelting, R. K., Cullen, B. F., & Ortega, R. (2013). Clinical Anesthesia (7th ed.). Chapter 5.
Evers, A. S., Maze, M., & Kharasch, E. D. (2011). Anesthetic Pharmacology Basic Principles and Clinical Practice (2nd ed.). Chapter 24, 36.
Kaye, A. D., Urman, R. D., & Vadivelu, N. (2012). Essentials of regional anesthesia. Chapter 5.
Lodish, H. F., Berk, A., Matsudaira, P., Kaiser, C. A., Krieger, M., Scott, M. P., Zipursky, L., & Darnell, J. (2003). Molecular cell biology (5th ed.).
Chapter 7.
Miller, R. D., Cohen, N. H., Eriksson, L. I., Fleisher, L. A., Wiener-Kronish, J. P., & Young, W. L. (2015). Miller's Anesthesia (8th ed.). Chapter 25, 36.