Metabolic Changes of Drugs and Related Organic Compounds

Metabolic Changes of Drugs and Related Organic Compounds Organic Pharmaceutical Chemistry I 3 rd Year Pharmacy 2018 -2019

Lecture 1: Introduction Metabolism plays a central role in the elimination of drugs and other foreign compounds (xenobiotics) from the body. Most organic compounds entering the body are relatively lipid soluble (lipophilic). To be absorbed, they must traverse the lipoprotein membranes of the lumen walls of the gastrointestinal (GI tract). Then, once in the bloodstream, these molecules can diffuse passively through other membranes and be distributed effectively to reach various target organs to exert their pharmacological actions.

Because of reabsorption in the renal tubules. lipophilic compounds are not excreted to any substantial extent in the urine. Xenobiotics then meet their metabolic fate through various enzyme systems that change the parent compound to render it more water soluble (hydrophilic). Once the metabolite is sufficiently water soluble, it may be excreted from the body.

Drug metabolism reactions have traditionally been regarded as detoxification processeshowever, drug metabolism reactions are not always detoxifying. Because many drugs are biotransformed to pharmacologically active metabolites, these metabolites may have significant activity that contributes substantially to the pharmacological or toxicological effect(s) ascribed to the parent drug. Occasionally, the parent compound is inactive when administered and must be metabolically converted to a biologically active drug (metabolite). These types of compounds are referred to as prodrugs.

It is clear that not all metbolites are nontoxic. Many adverse effects e. g. tissue necrosis , carcogensity and teratogenicity of drugs and environmental contaminants can be attributed drectly to the formation of chemically reactive metabolites that are harmful to the body, especiallyt when the patient has a disease state that inhibits metabolism or excretion. Metabolites are found in our sewage systems that may harm other animals or the environment.

GENERAL PATHWAYS OF DRUG METABOUSM Drug metabolism reactions have been divided into two categories: . Phase I (functionalization reactions), include oxidative. reductive, and hydrolytic biotransformations. The purpose of these reactions is to introduce a functional polar group(s) (e. g. . OH. COOH, NH 2. SH) into the xenobiotic molecule to produce a more water soluble compound. phase II reactions(conjugation reactions) is to attach small, polar and ionizable endogenous compounds such as glucuronic acid, sulfate. glycine. and other amino acids to the functional "handles" of phase I metabolites or parent compounds that already have suitable existing functional groups to form water-soluble conjugated products

Summary of Phase I and Phase II Metabolic Pathways Phase I 1. Oxidative Reactions 2. Reductive Reactions 3. Hydrolytic Reactions Phase II 1. Glucuronic acid conjugation 2. Sulphate conjugation 3. Glutothione or mercapturic acid conjugation 4. Acetylation 5. Methylation

Oxidation 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Aromatic moieties Olefins Benzylic & allylic C atoms and a-C of C=O and C=N At aliphatic and alicyclic C C-Heteroatom system C-N (N-dealkylation, N-oxide formation, Nhydroxylation) C-O (O-dealkylation) C-S (S-dealkylation, S-oxidation, desulfuration) Oxidation of alcohols and aldehydes Miscellaneous

Reduction 1. Aldehydes and ketones 2. Nitro and azo 3. Miscellaneous 4. Hydrolytic Reactions 5. Esters and amides 6. Epoxides and arene oxides by epoxide hydrase

Phase II – Conjugation 1. 2. 3. 4. 5. 6. Glucuronic acid conjugation Sulfate Conjugation Glycine and other AA Glutathione or mercapturic acid Acetylation Methylation

The Δ 9 - tetrahydrocannabinol, Δ 9 -THC metabolism example First oxidation at position 7, -CH 3 → -CH 2 OH → COOH then conjugation with glucuronic acid to –C OOR and or -OR where R= glucuronyl moiety. EX: Demonstrate the toxification of anisol, phenytoin , phenobarbitol and the NIH shift of anisol oxide to the corresponding hydroxyanisol.

SITES OF DRUG BIOTRANSFORMATION The liver is. by far, the most important organ in drug metabolism and detoxification of endogenous and exogenous compounds. Another important site, especially for orally administered drugs, is the intestinal mucosa. The latter contains the cytochrome P-450 (CYP) 3 A 4 isozyme and P-glycoprotein that can capture the drug and secrete it back into the intestinal tract. In contrast, the liver is rich in almost all of the drugmetabolizing enzymes

The First-Pass Effect Orally administered drugs that are absorbed into the bloodstream through the GI tract must pass through the liver before being further distributed into body compartments. Therefore, they are susceptible to hepatic metabolism known as the first-pass effect before reaching the systemic circulation. Depending on the drug , this metabolism can sometimes be quite significant and result in decreased oral bioavailability.

For example, in humans, several drugs are metabolized extensively by the first pass effect. The following list includes some of those drugs: Isoproterenol Morphine Propoxyphene Lidocaine Nitroglycerin Propranolol Meperidine Pentazocine Salicylamide

The First-Pass Effect………. . cont Some drugs eg. Lidocaine are removed so effectively by first pass metabolism that they are ineffective when given orally while nitroglycerin is administered buccally to bypass the liver. Bacterial flora present in the intestine and colon appear to play an important role in the reduction of many aromatic azo and nitro drugs (e. g. . sulfasalasine). Intestinal β-glucuronidase enzymes can hydrolyze glucuronide conjugates excreted in the bile, thereby liberating the free drug or its metabolite for possible reabsurption (enterohepatic circulation or recycling).

ROLE OF CYTOCHROME P-450 MONOOXYGENASES IN OXIDATIVE BIOTRANSFORMATIONS Of the various phase I reactions that are considered in this chapter. oxidative biotransformation processes are, by far. the most common and important in drug metabolism. The general equation that describes the oxidation of many xenobiotics (R-H) to their corresponding oxidized metabolites(R-OH) is given by the following equation RH + NADPH + O 2 ROH + NADP+ + H 2 O The enzyme systems carrying out this biotransformation are referred to as mixed—function oxidases or monoxygenases. There is a large family that carry out the same basic chemical reactions, their nomenclature is based on amino acid homology. The reaction requires both molecular oxygen and the reducing agent NADPH (reduced form of nicotinamide adenosine dinucleotide phosphate).

Cytochrome P 450 Enzyme Nomenclature There are four components to the name. CYP refers to the cytochrome system. This is followed by the number that specifies the cytoehrome family (CYP 1. CYP 2. CYP 3. etc. ). Next is a capital letter that represents the subfamily (CYP 1 A. CYP 1 B. CYP 2 A, CYP 2 B. CYP 3 A. CYP 3 B. etc. ). Finally the cytochrome name ends with another arabic number that specifies the specific enzyme responsible for a particular reaction (CYP 1 A 2. CYP 2 C 9. CYP 2 C 19. CYP 3 A 4. etc. ). 1. Cyp: Cytochrome P 450 enzymes 2. Family: (CYP 1, CYP 2, Cyp 3, etc. ) 3. Capital letter: Subfamily (CYP 1 A, CYP 2 C, Cyp 3 A, etc. ) 4. Individual enzyme in a subfamily:

During this oxidative process. one atom of molecular oxygen (O 2) is introduced into the substrate R-H to form R-OH and the other oxygen atom is incorporated into water. The mixed-function oxidase system is actually made up of several components, the most important being the super family of cytochromc P-450 enzymes which are responsible for transferring an oxygen atom to the substrate R-H. Other important components of this system include the NADPH-dependent cytochrome P-450 reductase and the NADH-Iinked cytochrome b 5.

CYP 450 CYP 450, Hepatic microsomal flavin containing monooxygenases (MFMO or FMO) Monoamine Oxidase (MAO) and Hydrolases The Cytochrome P-450 enzymes are heme proteins. The heme portion is an iron-containing porphyrin called protoporphyrin IX and the protein portion is called the apoprotein.

Cytochrome P 450 system: localized in the smooth endoplasmic reticulum. Cytochrome P 450 is a Pigment that, with CO bound to the reduced form, absorbs maximally at 450 nm Cytochromes are hemoproteins (heme-thiolate) that function to pass electrons by reversibly changing the oxidation state of the Fe in heme between the 2+ and 3+ state and serves as an electron acceptor–donor P 450 is not a singular hemoprotein but rather a family of related hemoproteins. Over 1000 have been identified in nature with ~50 functionally active in humans with broad substrate specificity

Cytochrome P-450 is found in high concentrations in the liver, the major organ involved in the metabolism of xenobiotics. The presence of this enzyme in many other tissues (e. g. . lung, kidney, intestine, skin. placenta, adrenal cortex) shows that these tissues have drugoxidizing capability too. The name cytochrome P-450 is derived from the fact that the reduced Fe+2 form of this enzyme binds with carbon monoxide to form a complex that has a distinguishing spectroscopic absorption maximum at 450 nm. One important feature of the hepatic Cytochrome P 450 mixed-function oxidase system is its ability to metabolize an almost unlimited number of diverse substrates by a variety of oxidative transformations.

This versatility is believed to be due to the substrate non specificity of cytochrome P- 450 as well as to the presence of multiple forms of the enzymes. Some of these P-450 enzymes are selectively inducible by chemicals (e. g. . Phenobarbital , benzo[α]pyrene). The cylochrome P 450 monooxygenase are located in the endoplasmic reticulum, a highly organized and complex network of intracellular membranes that is particularly abundant in tissues such as the liver. When these tissues are disrupted by homogenization , the endoplasmic reticulum loses its structure and is converted into small vesicular bodies known as , microsomes. Mitochondria house many of the Cytochrome enzymes that are responsible for the biosynthesis of steroidal hormones and metabolism of certain vitamins.