CLASS: 11:00 – 12:00Scribe: Adam Baird

DATE: November 30, 2010Proof:

PROFESSOR: FalanyDRUG METABOLISMPage1 of 7

  1. DRUG METABOLISM [S1]
  2. Drug metabolism is a very big topic of focus (especially among pharmaceutical companies).
  3. MOVEMENT OF DRUGS IN THE BODY [S2]
  4. Even as an optometrist or a dentist, the drugs that you administer may affect the whole body.
  5. The goal is to get a drug across multiple membrane barriers (lipid, aqueous barriers, for example) into the blood.Once the drug is in the blood, it can distribute to its site of action (bound by the blood, distributed among tissue, etc.).
  6. 20 – 30% of drugs are excreted unchanged. The majority of drugs are metabolized and then excreted (and many times generate toxic compounds too).
  7. FUNCTIONS OF DRUG METABOLISM [S3]
  8. These are the four major functions of drug metabolism. This is exam material!
  9. Decrease biologic activity of drugs and xenobiotics.
  10. Xenobiotics are compounds that are foreign to the body.
  11. Anything that you eat or drink is a xenobiotics (food, Coke, etc.).
  12. Drugs, the, are xenobiotics.
  13. Metabolism is the same. There isn’t a unique metabolic system for drugs (separate from xenobiotics).
  14. Increase excretion by metabolism to water-soluble metabolites.
  15. Most of the drugs that you will likely administer are hydrophobic (lipid-soluble), so they will distribute into the body and get across membranes. To excrete these drugs, they must be converted into a charged (polar) and transportable form. It must also be in a form that can be concentrated in the urine and then remain in the urine.
  16. Metabolize prodrug to active drug; convert active drug to different active drug.
  17. Drug metabolite assistance converts prodrugs into active drugs, or to convert an active drug into another form of an active drug (sometimes this is desired, other times it is not desired because it can lead to other drug interactions).
  18. Generate toxic or mutagenic/carcinogenic metabolites.
  19. This is a very common process, happening constantly within the body.
  20. These compounds degenerate, but there are mechanisms to repair the damage done too.
  21. PHARMACOLOGIC INACTIVATION OF DRUGS [S4]
  22. Examples of the types of process that can result in inactivation:
  23. Deamination of amphetamine (a simple removal of the amine group)
  24. Evan a small chemical change in a drug (a simple oxidation, or reduction, or demethylation, or halogenation, for example, can have major effects on the biological activity of a drug and it’s distribution and ability to metabolize).
  25. Hydroxylation of phenobarbitol
  26. Hydroxylation is likely the most important drug metabolism reaction in the body.
  27. S-oxidation of chlorpromazine
  28. Small chemical changes can have a large affect on drugs.
  29. EXCRETION OF DRUG METABOLITES [S5]
  30. Many metabolites are charged and water-soluble.
  31. Glucuronides and sulfates (both of which are major end products) have pKa’s of 1 – 2. They will be charged in physiological solutions (especially urine).
  32. There are many anion and cation transport systems in the kidney (used for transporting drug metabolites into the urine). There are whole families of drug transporters.
  33. Efficient removal of charged metabolites from plasma.
  34. Excretion from liver into bile and GI tract. In humans, compounds that have a molecular mass of 700 have a very large tendency to be excreted into the bile (by the liver), which can then be excreted into the intestine and undergo metabolism (sometimes deglucuronidation or desulfation and be absorbed). This is prominent for bile acids, which prevents their excretion (because they are used over and over again in fat absorption). The same general process can be used for many drugs.
  35. CONVERSION OF AN ACTIVE COMPOUND TO A DIFFERENT ACTIVE COMPOUND [S6]
  36. Examples of conversions of an active compound to a different active compound:
  37. N-demethylation or hydroxylation of diazepam (tranquilizer) to oxazepam (anti-convulsant)
  38. This conversion will depend upon the metabolic institution of each individual (everyone is different).
  39. N-demethylation of imipramine (serotonin uptake blocker) to desmethylipramine (norepinephrine uptake blocker)
  40. PRODRUG CONVERSION: PROTOSIL TO SULFANILAMIDE [S7]
  41. Example of a prodrug conversion:
  42. Azo-reduction of prontosil to sulfanilamide (which was the first antibacterial agent and what all “sulfa” drugs are based upon).
  43. The bacterial enzyme that converts p-arminobenzoic acid to folic acid is about 100x more sensitive than the human enzyme (so they are more sensitive to “sulfa” drugs).
  44. BIOACTIVATION OF HYDROXYMETHYL-PAH BY SULFATION [S8]
  45. Example of bioactivation:
  46. Hydroxymethyl-PAH by sulfation
  47. Prominent reaction that occurs within the body
  48. Hydroxymethyl-PAH will be formed anywhere organic material is burned (or partially burned). It is commonly formed by plants (especially as they fight of insects as their defense mechanism).
  49. Once hydroxymethyl-PAH is sulfated, it becomes a very unstable compound (as it spontaneously rearranges) and produces a carbonium ion (reactive electrophile), which can combine with neucoleophiles in the cell (like DNA, as it forms DNA adducts, which can become mutagenic or carcinogenic if not repaired).
  50. NO TITLE [S9]
  51. Everyone is different. Different age groups respond differently.
  52. Children have more body water (than adults). As you get older, body water is lost and fat increases (meaning that drugs are distributed differently).
  53. Metabolism varies with age. Generally, it is low in children and highest in young adults, and then decreases again with higher age. It is dependent upon the amount of enyzme activity expressed in the tissues.
  54. Because people may respond to drugs differently (since metabolism is controlling the active concentration).
  55. PLASMA HALF-LIVES OF SELECTED DRUGS IN YOUNG ADULTS AND ELDERLY PATIENTS [S10]
  56. The half-lives of these drugs depends on their metabolism.
  57. Some of these drugs vary in age while others remain constant.
  58. REACTIONS CLASSED AS PHASE I OR PHASE II [S11]
  59. Reactions can be classified as Phase I or Phase II.
  60. NO TITLE [S12]
  61. Phase I – Small changes in a compound (reduction, oxidation, hydrolysis, hydration, etc.) that affect the chemistry of the drug (a little bit) and affect the biology of the drug (quite a lot).
  62. Only slightly modify the chemistry of the compound, but may have a larger effect on the activity of the drug or the toxicity of the drug. This prepares them for Phase II.
  63. Phase II – Bulky charged side group is added to a drug to block its activity so that it can be readily charged (and so that it can be effectively excreted).
  64. This is not a simple process.
  65. Metabolism occurs in every tissue in the body, so every tissue can metabolize drugs. The ability of the tissue to metabolize a drug depends on the enzymes involved in the metabolism process (and this may vary from tissue to tissue).
  66. The liver is the most important tissue.
  67. Most drugs are administered orally and metabolize in the intestine and are immediately directed towards to the liver, where the drug is further metabolized and distributed to the body.
  68. Every tissue that is exposed to the environment (skin, lungs, etc.) has a large abundance of drug metabolite enzymes.
  69. NO TITLE [S13]
  70. Here are two benzene rings that can be hydroxylated. It could also be sulfonated. Both are conjugation reactions.
  71. MUTIOPLE PHASE I METABOLIC PATHWAYS FOR CHLORPOMAZINE [S14]
  72. There are 20 – 30 metabolites of chlorpromazine in your body.
  73. For Phase I reactions, there is ring hydroxylation, sulfur oxidation, ring hydroxylation (multiple times), N-demethylation, or N-oxidation. All of this can occur in the metabolism of chlorpromazine.
  74. Again, this is not a simple process. As you can see, it can be very complex. It is a “web” of reactions that depends on the individual enzymes systems within the body (which may vary from person to person).
  75. CYTOCHROMES P450 [S15]
  76. The most important family: cytochrome P450, because they are the major drug metabolizing system in the body.
  77. Cytochrome P450 is a family of heme containing mono-oxygenases (the most powerful in vivo oxidizing system known).
  78. The basic function: to transfer one oxygen atom (from molecular oxygen) to a substrate. This is an evolutionarily old system and is found in all plant and animal families. In animals, the P450s involved in xenobiotic metabolism evolved. In humans, there are 57 CYP genes and 33 CYP pseudogenes (divided into 18 families and 42 subfamilies). Some plants have 350 – 500 CYP genes.
  79. EXAMPLE OF REACTIONS CATALYZED BY CYTOCHROMES P450… [S16]
  80. P450 genes catalyze about 60 different kinds of reactions; only a few of them have substrates.
  81. NADPH CYTOCHROME P450… [S17]
  82. There are two different cytochrome P450 mixed function oxidase systems.
  83. Liver Microsomal MFO System (mainly associated with xenobiotic metabolism)
  84. NADPH Cytochrome P450 Reductase
  85. It’s goal: to transfer electrons from NADPH to P450
  86. Cytochrome P450
  87. There are over 40 forms of P450 in the liver (out of the total 57 forms)
  88. Found in all animal phyla
  89. Found in all human tissues (except skeletal muscle and RBCs, because they contain heme)
  90. Adrenal Mitochondrial MFO System
  91. Involved in steroid biosythesis
  92. Adrenodoxin reductase and adrenodoxic (replaced NADPH CYP450 reducatase)
  93. Limited number CYPT isoforms (P450scc, P450c1)
  94. Structurally stable complexes
  95. MIXED FUNCTION OXIDASE REACTION [S18]
  96. 2 electrons from NADPH transferred (by reductase) to the P450, and then a substrate enters oxygen. In the classic reaction, a hydroxylated compound and water will be generated.
  97. Many things can happen when the oxygen is inserted (structural rearrangement, loss of metabolites, etc.).
  98. The major reaction: hydroxylation. There are an abundance of other reactions too though.
  99. CYP450 HYDROXYLATION REACTIONS [S19]
  100. The classic CYP450 hydroxylation reaction: aromatic ring hydroxylation.
  101. Other reactions: aliphatic ring hydroxylation, aliphatic hydroxylation, etc.
  102. CYTOCHROME P450 REACTIONS [S20]
  103. Additional reaction: epoxidation of benzo(a)pyrene to 4,5-epoxide. (All epoxides are reactive; some are very potent, some are carcinogens, etc.)
  104. Another reaction: oxidative deaminiation of amphetamine (rearrangement). Once the hydroxyl group is inserted on the beta carbon (the alpha carbon relative to the amine), it can rearrange and release the amine group.
  105. All you need to know is that this can occur (you don’t necessarily need to know how it occurs).
  106. SUBSTRATES AND FUNCTIONS OF HUMAN CYP GENE FAMILIES [S21]
  107. The first three families (CYP1, CYP2, CYP3) are associated with drug metabolism (xenobiotic metabolism, absorption from compounds that you receive from your diet, compounds that you breath, etc.)
  108. The other families are more involved with endogenous compound metabolism synthesis (especially steroids, sterols, cholesterol, and other hydrophobic type compounds). As designer drugs are made, these enzyme systems are targeted by making molecule mimic drugs (of biological compounds). This is very important with selective metabolism of drugs.
  109. MAJOR CYP450 CONTENT IN HUMAN LIVER [S22]
  110. There are 40 isoforms of P450 in the liver.
  111. The major isoform in the liver (and the major enzyme in human drug metabolism) is CYP3A4.
  112. The other major isoforms: CYP2C8, CYP2D6 (which is actually very important).
  113. About half of these isoforms are metabolized by CYP3A4 and CYP3A5, which are two very similar genes (the only difference is in an amino acid). It’s difficult to separate them so many times they are clumped together into one category.
  114. CYP2D6 is important because of what it metabolizes (namely, neuroactive drugs, like anti-depressants, for example).
  115. HUMAN CYP2D6 PHARMACOGENETICS [S23]
  116. Pharmacogenetics: the differences between how people respond to drugs (based on genetic expression).
  117. CYP2D6 is a good example.
  118. There are about 30 identified defective CYP26 alleles in humans. About 4 – 5 of these are the most common for 95 – 99% of the difference in CYP2D6 activity.
  119. This is measured with test compounds, and then the amount of the drug is measured by urine samples.
  120. Rapid metabolizers:
  121. Defined by metabolic ratio of debrisoquine/4-OH debrisoquine.
  122. Most individuals contain at least one of the wild-type alleles: CYP2D6*1 or CYP2D6*2.
  123. Poor metabolizers:
  124. Caucasians – About 8% of the population will be poor metabolizers of CYP2D6*4 and CYP2D6*5, meaning that rates will be significantly lower for drugs. You may, as the doctor, have to adjust the dosage, for example, in cases like this.
  125. Orientals – There is a different allele that isn’t present in Caucasians or African-Americans called CYP2D6*10 (Pro34Ser), meaning that there is a decreased rate of metabolism. It is not as low as “poor” metabolizers, but it is not as fast as “rapid” metabolizers. This is seen very frequently (in about half of the Oriental population).
  126. African-Americans – There is a different allele that isn’t present in Orientals or Caucasians called CYP2D6*17 (thr107Ile in CYP2D6*2), which changes the affinity of the enzyme for many drugs, meaning that there many be a different response than expected.
  127. There are racial differences in how people will respond to drugs.
  128. Student question (inaudible). Answer: In a sense, yes, because some people will response better. Some people won’t respond to certain drugs, in other cases, they will respond too well. It may be related to the time course (how long the drug is in the body). This is a very complex issue though; a minor issue.
  129. Ultrarapid metabolizers:
  130. Recall: genes are replicated in the body. They may have, then, up to 13 copies of CYP2D6 (instead of just the 2 CYP2D6 copies that normal people have). This is especially prevalent in Semitic populations (about 33% of the population), but much less prevalent in other populations.
  131. The main point: everyone is different. This is something you have to take into consideration when prescribing drugs.
  132. DEMETHYLATION OF CODEINE TO MORPHINE BY CYP2D6 [S24]
  133. Codeine is an analgesic prodrug. It is demethylated to morphine by CYP2D6. Poor metabolizers, then, won’t get any pain relief from codeine (because it is not metabolized morphine rapidly enough). For dentists, this is a very important factor in drug therapy.
  134. FLAVIN-CONTAINING MONOOXYGENASES [S25]
  135. There are other enzymes involved (other than the P450s), like the flavin-containing monooxygenases, for example.
  136. They can catalyze many of the same reactions that the P450s can. There aren’t many isoforms involved.
  137. They are microsomal. They are membrane bound enzymes.
  138. They don’t react with negatively charged compounds (so their specificity is much less than the P450s).
  139. Individuals who lack MFO3 has fish-order syndrome (they can’t metabolize cysteamine properly in the body, so they produce trimethylamine, which smells like fish).
  140. REACTIONS OF THE FLAVIN-CONTAINING MONO-OXYGENASES (FMO [S26]
  141. The types of reactions of the flavin-containing mono-oxygenases:
  142. N-oxidation
  143. S-oxidation
  144. P-oxidation
  145. ETHANOL OXIDATION TO ACETIC ACID [S27]
  146. Ethanol is metabolized by alcohol dehydrogenase (which is a cytosolic enzyme) to acetaldehyde, which is then metabolized back again (aldehyde dehydrogenase to acetic acid).
  147. Ethanol can also be metabolized through catalase (in the peroxisomes), which is a protective mechanism for getting rid of hydrogen peroxide (which is reactive oxygen species in the body that can lead to oxygen toxicity).
  148. CYP2E1 metabolizes ethanol, releasing water to acetic acetaldehyde. CYP2E1 is not highly expressed. However, in response to ethanol abuse (people to drink too much), CYP2E1 can be induced to become responsible for most ethanol metabolism in the body. One of the problems in this process: CYP2E1 generates too much reactive oxygen species (producing a super-oxide anion, producing hydrogen peroxide, etc., leading to cirrhosis of the liver). Many of the problems that are associated with cirrhosis of the liver in alcoholics are related to the induction of CYP2E1 in the body. And so, this is an individual response that you must take into account and contend with.
  149. EPOXIDE HYDROLASE REACTIONS [S28]
  150. One of the major reactions of the P450 system is epoxidation (and epoxides are reactive). The body, then, has enzyme systems (including 3 epoxide hydrolases) that hydrolyze these (so that they get rid of them). So, reactive compounds are made, and then those reactive compounds are gotten rid of.
  151. PHASE II: CONJUGATION REACTIONS [S29]
  152. Many conjugation reactions have different functions.
  153. These are all families of enzymes.
  154. GLUCURONIDATION [S30]
  155. Functionally, the most important of these families is the glucuronidation (which is transfer of a glucuronic acid group from UDP-glucuronic acid to many other different compounds, like oxygen, nitrogen, sulfur, carbon atoms, etc.). It is a very promiscuous reaction.
  156. The pKa of glucuronic acid is 2 (so the conjugates are charged at physiological pHs.
  157. There is a super family. Quantitatively, it is the most important of the Phase II reactions.
  158. B-ESTRADIOL GLUCURONIDATION [S31]
  159. This is one of the major examples of glucuronidation. Steroid are extremely good substrates the drug metabolizing enzymes.
  160. The UDPGA is transported to the 3 group to form the charged glucuronide.
  161. If this is placed on the 17 position (which is a hydroxyl group), it can result in cholestasis of pregnancy (which can be fatal to both the fetus and the mother because of the unique estrogen metabolism). Humans make tremendous amounts of estrogen during pregnancy (only a few higher primates do this). If the wrong end of estrogen is glucuronidized (especially of estradiol), bile acid excretion can be inhibited, producing cholestasis.
  162. GLUCURONIDATION REACTIONS [S32]
  163. Notice how many glucuronidation reactions there are.
  164. SULFOTRANSFERASE REACTIONS [S33]
  165. Another major reaction in humans (especially in steroid metabolism, but in drug metabolism too): sulfation, the transfer of a sulfonate group from PAPS to an acceptor compound; a charged compound is simply being generated.
  166. One of the main points: notice what happens in different tissues.
  167. PROPOSED ROLE OF hP-PST-1 IN AROMATIC AMINE METABOLISM [S34]
  168. 2-Naphthylamine (common environmental pollutant) can be hydroxylated to the P450 system, producing a hydroxy-aromatic amine. These are carcinogens. (Any aromatic amine can be a carcinogen.) It can be sulfated, producing N-sulfoxy-naphthylamine, which can then adduct DNA and protein.
  169. However, if the same enzyme (the same enzyme that N-hydroxy) can be sulfated, producing sulfamate (where the sulfate group is directly linked to nitrogen). This is not toxic.
  170. The metabolism and the fate of this compound depends on whether it is hydroxylated first, where it is metabolized, and the interaction between the two enzyme systems (which may lead to toxicity in some tissue, but not others).
  171. ACETYLATION OF SMALL AMINES AND SULFONAMINES [S35]
  172. Acetylation of compounds is also very common.
  173. One main difference between acetylation and glucuronidation and sulfation: the acetyl group is not charged. A simple side group is just being added. The mass and the chemistry change, but the charge is unchanged.
  174. BIPHASIC DISTRIBUTION OF ISONIAZID ACETYLATION OF NAT2 [S36]
  175. Sulfa drugs, for example, are acetylated in metabolism.