Midterm 1: P450 Enzymes and Drug Metabolism Flashcards Preview

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Flashcards in Midterm 1: P450 Enzymes and Drug Metabolism Deck (19):
1

  • The P450 enzymes provide the major line of defense against the accumulation of lipophilic compounds in the body. The defense strategy honed by evolution appears to be relatively simple. 

  • Disperse, each invading xenobiotic (drug) via metabolism into multiple metabolites, This means that the concentration of any given metabolite will be lower than the parent drug. Also of benefit is the fact that the metabolites are polar which facilitates excretion and also allows for further conjugation reactions and excretion of the conjugates in urine or bile. 
  • Metabolic dispersion of the drug dose usually involves metabolism of a single compound to multiple metabolites by multiple enzymes. 
  • All other things being equal, conversion of a drug to multiple metabolites by multiple enzymes is a desirable property for drugs and is a major goal for the development of new drugs. 

2

  • Drug metabolism is a major determinant of drug clearance from the body and drug half-life in the blood. We rely on drug metabolism when we dose and would like to treat every individual the same way. Some complications of drug therapy that are due to variability in metabolism: 

  • Phenotypic variability (variable levels of enzyme expression in the population at large
  • Genotypic variability (polymorphic forms of P450 enzymes
  • Enzyme induction (many drugs can cause significant increases in the amounts of P450 enzymes causing drug-drug interactions where the object drug levels fall to possibly subtherapeutic concentrations) 
  • Enzyme inhibition (many drugs can inhibit P450 enzymes causing levels of object drug to rise to potentially toxic concentrations.) 
  • Properties of metabolites (metabolites of drugs may have pharmacological or toxic effects) 

3

Desirable properties of a given drug from the perspective of metabolism. 

  • Don’t cause DDIs The drug should have minimal inhibitory and inductive effects on drug metabolism enzymes at therapeutic concentrations. 
  • Metabolic clearance is constant in the population: The ideal drug should be metabolized significantly by multiple enzymes to decrease (1) interindividual variability in metabolic clearance and (2) susceptibility to drug-drug interactions caused by other drugs or ingested compound (grapefruit juice) 
  • Metabolites are benign: Cleared to multiple metabolites to decrease the potential for toxicity due to the toxic effect of circulating metabolites. 
  • Dosing should be simple in the population: Drug pharmacokinetic profile (half-life; peak blood levels) should be compatible with desired pharmacological effect (e.g. short acting-long acting). 

4

The P450 nomenclature 

  •  classifies P450's as CYP_ _ _ (example CYP2D6) based on primary amino acid sequences of the enzymes which reflects relatedness. 

    • The first number after the CYP stands for the family (1,2,3,4 etc). Enzymes in the same family are > 40% homologous in their amino acid sequences.

    • The first letter stands for the subfamily. Enzymes with the same first number and first letter are > 55% homologous. 

  • Most P450 dependent drug metabolism is carried out by the CYP1, CYP2, and CYP3 families of enzymes. 

  • The different P450 enzymes are sometimes called isoforms or isoenzymes. 

  • P450's of the same designation (say CYP1A2) in different species (rat, rabbit, man) do not have identical amino acid sequences nor do they exhibit identical substrate and metabolite profiles. This species difference in the structures and activity of the P450 enzymes somewhat limits the usefulness of metabolic studies in animals for predicting human metabolism 

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Variability in drug metabolism is significant and therapeutically relevant (dosing to effect). 

  • A person's ability to process a particular drug and clear it from the body is largely controlled by the amounts of the relevant P450 enzymes contained in that individual's liver.
  • What this means is that the clearance of drug from the body and half-life of a given drug in the body is highly dependent on the amount of the P450 enzymes that process that drug that exists in that persons liver at that time. You should know the following simple relationships for enzyme levels in drug metabolism and pharmacokinetics. 

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Four major factors that can affect active P450 levels or activities of the hepatic CYPs in the population 

  1. Baseline interindividual variability in the constitutive levels of P450 enzymes.
  2. Genetic polymorphisms (inherited) where an enzyme is (a) expressed in an altered, less active, form (different amino acid sequence), (b) not expressed at all or multiple gene copies exist on the genome. CYP2D6, CYP2C19, CYP2C9 as well as others less important.
  3. The presence of drugs or dietary constituents that induce the levels of P450 enzymes.
  4. The presence of drugs or dietary constituents that inhibit the activity of P450 enzymes. 

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Factor 1: inter-individual variability in content 

  • CYP 3A4, 2C9, 2C19 and 2D6 are the major drug metabolizing enzymes in terms of numbers of drugs metabolized in the liver. CYP2E1 processes and clears the inhalation anesthesisa 
  • Reported interindividual variability in activity and content of each P450 enzyme in liver microsomes is significant. Thus we expect variablity in the dose required to achieve a given plasma level in the population. Fortunately we rarely have to vary dose level to match basedline metabolic capacity from individual to individual. Most people cluster around the mean and therapeutic indexes are wide 
  • Drugs with Narrow Therapeutic Indexes are more difficult to dose effectively due to interindividual variability in metabolism as well other factors that change enzyme activity during therapy (polytherapy; inhibition, induction) 

8

Drugs with Narrow Therapeutic Indexes 

  • When a drug has a narrow therapeutic index the patient is at relatively higher risk of underdose or overdose. It is often necessary to titrate the dose of the drug in order to match the drug metabolizing capacity in that particular patient. Dosing algorithms can be used to systematically arrive at the best dose while minimizing the risk to the patient. Warfarin, theophylline, phenytoin and the tricyclic antidepressants are examples of these kinds of drugs. 
  • Conversely, when the toxic threshold of a drug is much higher than therapeutic concentrations (wide therapeutic index; desirable) dosing to desired effect is less problematic since a wide range of drug concentrations can be achieved the desired pharmacological effect without causing toxicity 

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Factor 2: Genetic polymorphisms create sub-populations of patients that may require individual therapy strategies 

 

  • When a variant of the wild type enzyme P450 sequence with normal, reduced or no activity is expressed due to inherited differences in gene sequences (common)
  • When the altered DNA sequence does not produce enzyme (sometimes).
  • When an upstream site of a mutation in the DNA exists that changes how much native enzyme is expressed (rare) or duplicate copies of the active gene exist (also rare).
  • The important polymorphisms in drug metabolism are for CYP2D6, CYP2C9 and CYP2C19. Many of these polymorphisms are too rare (low frequency) to worry about. 

 

10

most important P450 polymorphism in drug metabolism 

  • CYP2D6 polymorphism 
  • The important null mutant is CYP2D6*4 is prevalent (10-20%) in Caucasians 
  • many cardioactive and neuroactive drugs are significantly metabolized by this enzyme (narrow Therapeutic Index). 

11

  • EM: Extensive Metabolizers 
  • IM: Intermediate metabolizers 
  • PM: Poor Metabolizers 
  • Extremely-rapid Metabolizers  

  • EM
    • Normal, two copies of the wild type gene (normal drug metabolism). 
  • IM
    • One copy of the wild type and one copy of the mutant gene are present and usually expressed. Impaired metabolism is observed in population studies but is normally not a problem for drug metabolism so practically speaking this group can be lumped with Extensive Metabolizers (normal group) 
  • PM 
    • About 7-10% of the Caucasian population and 2% of the Asian population do not express any functional enzyme as a result of having two copies of “defective” genes. 
    • As far as we can tell the poor metabolizer status has no effect on an individual’s health except when it comes to drug therapy. 
  • ​Extremely-rapid Metabolizers  
    • multiple copies of the gene for the active enzyme (up to 13). These individuals have high levels of active enzyme and are called ultra-rapid metabolizers. They are relatively rare. 

12

 clearance of the tricylic antidepressants  

  • highly dependent on CYP2D6
  • CYP2D6 tends to metabolize at locations about 8 angstroms from an amine functional group
  • Here we see the effect of CYP2D6 metabolizer status on the dose requirements of nortryptyline. 
  • Poor metabolizer=no CYP2D6. 
    If have to give lots of smaller doses, know that another process getting rid of the drug (PM). Pathway in PM that clears drug also in the EM. 2D6 pathway operative in EM. In EM, 90% of the dose is cleared by 2D6 pathway (10% by other pathway).

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13

Poor Metabolizers do not receive analgesic benefit from codeine: 

  • O-dealkylation. Codeine itself is a cough suppressant.

  • Codeine is also used for analgesia however it's pain-reducing effects are largely due to a metabolite morphine. Codeine itself does not produce clinically significant analgesia.

  • CYP2D6 converts approximately 10% of a dose of codeine to morphine via an O- demethylation reaction. Thus CYP2D6 poor metabolizers receive little analgesic benefit from codeine since they cannot convert it to morphine. Cough suppression (a different receptor) is still observed. 

  • Since only 10% of the codeine dose is cleared by CYP2D6 toxic levels of codeine are not observed in poor metabolizers. 

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Testing for the CYP2D6 polymorphisms: The following are true

  • Once a poor metabolizer, always a poor metabolizer.

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Testing For Phenotype

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  • single doses of test drugs and looking for the ratio of parent drug to CYP2D6 metabolite in urine. 
  • Or do pharmacokinetics study and look at the AUC of a population and distinguish IM from PM 

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Testing for Genotype 

DNA analysis (bucal swab or blood sample and PCR test) provides genotype information. 

17

Personalized Medicine: 

  • developing methods to reduce the error rates in drug therapy.
  • The anticancer agent tamoxifen binds to the estrogen receptor. Metabolites, particularly afimoxifene (4-hydroxy tamoxifen) are equal or more active than parent. CYP2D6 makes the metabolites. There is much interest in determining whether genotype can be used to guide therapy. 

18

CYP2C19 polymorphism 

  • Major polymorphism is CYP2C19*2 which is inactive. 2-5% of caucasians and 15-20% of asians are poor metabolisers) . 
  • controversial black box warning that has been recently put on Plavix which must be metabolized to an active metabolite to have an anticoagulant effect. CYP2C19 poor metabolizers are at risk of clots after surgery. 
  • omeprazole (Prilosec) is potent inhibitor of CYP2C19 and causes a significant DDI when coadministered with Plavix. 

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19

CYP2C9 polymorphism 

  •  due to single nucleotide change (SNP) in the gene.
  • still metabolically active however it is a much poorer catalyst.
  • The metabolism of (S)-warfarin to the 6 and 7 hydroxymetabolites by CYP2C9 is responsible for 85% of the metabolic clearance.
  • Dose reductions of as much as 10-fold may be required for CYP2C9 PM’s although they are rare.
  • clearance of (S)-warfarin is lower in individuals that express one copy of the wild type enzyme (CYP2C9*1) and one copy of the leucine to isoleucine mutant (CYP2C9*3). The dose of warfarin in these IM individuals must be reduced to avoid toxic effects because the clearance of warfarin is reduced.  
  • dosed to effect (INR) 
  • IM’s require on average a warfarin dose that is 1/3 to 1/2 of the dose in the wild type (EM) population (2 mg/day vs 6 mg/day).
  • The dose requirement of Phenytoin , another CYP2C9 substrate, also is lower in CYP2C9 IM than EM’s. 
  • The CYP2C9*1/CYP2C9*3 genotype confers intermediate metabolizer status (IM) and is reasonably rare <3%. Two small studies suggest that pre-knowledge of genotype does not improve the time required to normalize an individual on warfarin. However a large multi-center trial is currently ongoing to test this hypothesis. 

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