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Anesthesiologists observe individual variations in clinical response to administered drugs every day. This variability can result from differences in pharmacokinetic processes such as absorption, distribution, metabolism, and excretion of a drug. Individual variation can also be caused by differences in pharmacodynamics, such as intrinsic differences in end-organ sensitivity, receptor upregulation and down-regulation, and alterations in physiology that affect drug action. The relationship between patient variability and a typical dose-response curve is shown in Figure 3-36 . Sources of pharmacokinetic and pharmacodynamic variability include genetics, patient physiology, and drug interactions.
The field of pharmacology that describes effects of genetic variation on drug action is called pharmacogenetics. Pharmacogenetics refers to the genetic diversity in the body's absorption, metabolism, and distribution of drugs (i.e., inborn pharmacokinetic variability) or in the body's response to the drug, as can be caused by differences in receptor structure or patient physiology (i.e., inborn pharmacodynamic variability). To understand the effect of genetic variation on pharmacokinetics and pharmacodynamics, it is important to first define genetic variants, or polymorphisms.
Genetic polymorphism refers to any type of variation in a gene. More precise language is used to describe each specific type of genetic variant. When one nucleotide is exchanged for another, the resulting variant is called a single nucleotide polymorphism (SNP). The rare substituted nucleotide is defined as the minor allele, whereas the more common wild-type nucleotide is defined the major allele. Most SNPs are biallelic, having only two possible nucleotides—the minor or the major allele. In rare cases, SNPs can be triallelic, with two possible minor alleles and three total possible nucleotides. With four total nucleotides
Figure 3-40
Response surfaces for potential pharmacodynamic interactions
of anesthetic drugs. A, Additive interaction between
two agonists that have the same mechanism of action (e.g., fentanyl and alfentanil).
B, Supra-additive interaction between two agonists
(e.g., isoflurane and fentanyl). C, Infra-additive
interaction between two agonists (i.e., reported for cyclopropane and nitrous oxide).
D, Partial agonist and full agonist (e.g., hypothetical
interaction of nalbuphine and fentanyl). E, Competitive
antagonist and full agonist (e.g., naloxone and fentanyl). F,
Inverse agonist and full agonist (e.g., R0 19-4063 and midazolam).
Allele frequency refers to the frequency of the minor allele, notated f(-). The frequency of the major allele is therefore 1−f(−). Because this is the minor allele, f(-) is by definition less than 0.5. Because individuals have two copies of each allele, there are three possible combinations of biallelic SNPs: AA, Aa, and aa (in which A designates the major allele, and a designates the minor allele). The Hardy-Weinberg equation provides the frequency of each of these combinations based on the generally correct assumption that pairing of alleles is random:
In addition to SNPs, other types of genetic variants include insertions and deletions, in which stretches of genomic DNA are missing or added, and microsatellite repeats, in which several nucleotides repeat (i.e., dinucleotide repeat GCGCGCGC [n = 4 GC repeats] or trinucleotide repeat TACTACTACTACTACTAC [n = 6 TAC repeats]). The number of repeats in a microsatellite correlates with some diseases and can be used in genome-wide scans to identify chromosome regions associated with disease.
Catastrophic DNA changes often include DNA deletions or insertions that are not multiples of three in the coding region. Such changes result in a frameshift in the triplet nucleotide sequence used to encode amino acids in proteins, with the likely result being a nonsense protein. SNPs that change an encoded amino acid to a stop codon may also be catastrophic because they may result in a truncated protein.
Not all genetic variants have biologic consequences. Just as variations in eye color or fingerprint pattern do not have medical consequences, many genetic variants provide "background" variation without overt biologic consequences. Genetic variants with biologic or medical effects represent the minority of all genetic variants known to exist.
Determining the association between genetic variants and disease, drug therapy, or patient outcome is not straightforward and ideally should involve the help of a statistical geneticist. Clinical covariants and even genetic population structure (loosely defined as genetic variation due to global population movement) must be carefully taken into account in pharmacogenetic studies. Depending on the number of genes to be examined, statistical testing for association ranges from straightforward, simple analyses (e.g., using the chi-square approach to test whether a variant or haplotype [a group of SNPs] is enriched in the target population) to very complicated analyses (e.g., complex modeling, permutation testing, prospective sequential testing). In clinical association studies, it is important to ensure that clinical end-point data are reproducible and ideally include intermediate end points such as protein levels or enzyme activity. As more clinically relevant human genetic variants become known, our knowledge of the role of genetics in drug response and patient outcome should be significantly enhanced. Pharmacogenetics is the wave of the future in clinical studies.
Naturally occurring genetic variants can affect the activity of enzymes responsible for drug metabolism, most often by altering a key amino acid in or near the site of enzymatic action. In practical terms, this enzymatic diversity is rarely evident in the absence of drug therapy. In the presence of drug therapy, pharmacogenetic variation in metabolism is evident as unexpected toxicity, duration of action, or lack of efficacy of administered drugs.
Of particular interest to anesthesiologists is the activity of cytochrome P450 (CYP) 3A4. This is the most abundant cytochrome in the liver and intestines, and it is responsible for the metabolism of almost one half of all drugs, including many important drugs in anesthesia: opioids (e.g., fentanyl, alfentanil, sufentanil, methadone) (see Chapter 11 ), benzodiazepines (e.g., diazepam, midazolam, alprazolam, triazolam) (see Chapter 10 ), local anesthetics (e.g., cocaine, lidocaine, ropivacaine) (see Chapter 14 ), steroids, calcium channel blockers, haloperidol, and halothane. Only two of the many described genetic mutants of the CYP3A4 enzyme alter enzyme activity, with CYP3A4*18 decreasing and CYP3A4*19 increasing drug turnover.[19] [20] Most of these CYP3A4 studies have examined only heterozygous (Aa genotype) individuals.
In contrast to the lack of causative CYP3A4 variants, a different story exists for CYP2C19, the enzyme system responsible for metabolizing mephenytoin, omeprazole, diazepam, proguanil, propranolol, and certain antidepressants. Newly described SNPs in this gene, present predominantly in African Americans, appear to decrease metabolism of these drugs in vitro.[21] Effects on drug concentrations in plasma need further testing in prospective human clinical trials.
One of the best-studied pharmacogenetic variants is CYP2D6, also called debrisoquine hydroxylase or sparteine oxygenase. About 7% to 10% of Caucasians are homozygous for an inactive variant of CYP2D6. This causes altered metabolism of at least 40 drugs. For example, codeine is a prodrug with no intrinsic analgesic efficacy. Codeine, oxycodone, and hydrocodone (Vicodin) all undergo O-demethylation to morphine, oxymorphone, and hydromorphone, their more pharmacologically active metabolites, by means of CYP2D6. Individuals who are homozygous for inactive CYP2D6 have little to no analgesic efficacy from codeine.[22] Such individuals are occasionally accused of malingering after surgery or injury when in fact they have no pain relief and need to be rescued by another analgesic agent. CYP2D6 polymorphisms have also been shown to impair dextromethorphan and metoprolol metabolism. When patients can be assayed for CYP2D6 activity, it will become a useful screen before initiating pain therapy.
Anesthesiologists are familiar with the role of genetics in determining butyrylcholinesterase (pseudocholinesterase) activity. Subjects with abnormal butyrylcholinesterase are at risk for prolonged paralysis from succinylcholine and exaggerated systemic effects from ester-based local anesthetics. Not surprisingly, there are ethnic differences in cholinesterase activity. For example, individuals of Middle Eastern descent are likely to have less butyrylcholinesterase activity than those of European descent. Molecular approaches have permitted extensive characterization of cholinesterase variants in plasma. Most of these variants are not "all or none" types but rather reflect a spectrum of enzymatic activity.
There may be an evolutionary explanation for pharmacogenetic variability in metabolism. It is probable that variability in drug metabolism is a natural adaptive response to an environment that can present a huge variety of toxins. Enzymatic diversity probably is maintained through natural selection to ensure survival when we are confronted with novel environmental toxins.
In addition to variants that affect drug pharmacokinetics, genetic variants can alter drug activity by pharmacodynamic mechanisms. Biologically active genetic polymorphisms have been found in numerous receptors, second messenger systems, and ion channels. One of the most intensively studied receptors in this regard is the β2 -adrenergic receptor (β2 AR) (see Chapter 16 ). Several β2 AR SNPs have particular relevance to this discussion ( Fig. 3-41 ). [23] One variant, located at -47(T/C) in the 5'-untranslated sequence (referred to as the 5'-leader cistron [5LC]), increases levels of normal (nonvariant) β2 AR mRNA and protein by 72%. This increase in airway wild-type β2 AR expression is protective against methacholine challenge. Another β2 AR SNP present in the amino terminus, Arg16Gly (convention = major allele, amino acid number, minor allele), causes the receptor to have markedly enhanced agonist-promoted downregulation.[24] Because β2 AR stimulation in vessels is important in mediating vasodilation, an SNP such as Arg16 that enhances desensitization (i.e., dampens receptor responsiveness) is not surprisingly associated with a more frequent occurrence of higher arterial blood pressure or hypertension, or both. In contrast, β2 ARs containing Gly16 appear completely resistant to desensitization, and the Glu27 variant has
Figure 3-41
Location of some clinically important single nucleotide
polymorphisms in the β2
-adrenergic receptor. Amino acid residues
identified with a boxed circle have the clinical
relevance described. BP, blood pressure; CHF, congestive heart failure; HTN, hypertension.
Malignant hyperthermia represents the most striking example of genetic variability in response to commonly used anesthetic drugs (see Chapter 29 ). Malignant hyperthermia manifests in approximately 1 of 20,000 anesthetic regimens. Various anesthetics, especially halothane, seem to trigger abnormal hypermetabolic responses that result in uncontrolled rise in body temperature, muscle rigidity, metabolic acidosis, hyperkalemia, and hypercapnia. More than 50% of cases of malignant hyperthermia have been positively linked to a series of mutations in the ryanodine receptor protein gene RYR1, which leads to altered calcium regulation in the sarcoplasm of muscle tissue. There is evidence of the involvement of the voltage-gated dihydropyridine receptor, which is also involved in sarcoplasmic regulation of calcium, in some cases of malignant hyperthermia not linked to the ryanodine receptor protein.
It is likely that many forms of genetically based variability in response to drugs reflect the interaction of many polymorphisms that subtly affect receptor structure, cellular function, and physiologic response. Understanding the interaction of multiple genes on the variability of drug response presents daunting technical challenges in pharmacogenetic research.
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