Pro-drugs

Pro-drugs

1. Definitions

   • Drug metabolism
     • This is the modification or degradation through enzymes to convert lipophilic compounds into more polar compounds
       • This is important in excretion of drugs as polar molecules are excreted easily
   • Xenobiotic
     • This is a chemical which is not expected to be found in the body as it is not usually produced in the body and NOT REQUIRED
       • All drugs are xenobiotics
       • Ascorbic acid (Vitamin C) is not produced by our bodies but it is required to scavenge ROS so it is not a xenobiotic
   • Detoxification
     • Is the modification of the chemical, xenobiotic to remove the pharmacological effect
   • Bio transformation
     • This is the alteration of a drug in the body by the action of enzymes
       • Can result in detoxification or bioactivation
   • Bioactivation
     • Modification of chemicals with enzymes to produce xenobiotics which may have toxic/therapeutic effects
   • Pro-drug
     • An inactive drug when administered but through the action of endogenous enzymes will become pharmacologically active
   • Bioavailability
     • This is the proportion of drug which when administered reaches the systemic circulation

2. Why are pro-drugs important

   • They can increase the bioavailability
     • This is important as we don't want drugs to be metabolised right away by the liver resulting in an insufficient level of drug to deal with the cancer
   • Can increase the local delivery of drugs into the correct system
     • This is important as drugs given to kill cancer cells are not only toxic to them but also human cells
       • This means drugs will affect the body cells such as hepatocytes, hair growth resulting in side effects
     • This is also important in targetting the correct cancer type e.g. brain cancer (rabies virus vector) etc

3. Antibody directed enzyme pro-drug therapy (ADEPT)

   • This is a method which involves infusing an antibody to an enzyme to form an immunoconjugate
     • The antibody must be able to target an antigen found commonly on the tumour cells e.g. TSpecificAntigen or TAA
       • This allows specificity of the prodrug targetting
     • Once the antibody has bound the target antigen the pro-drug can be administered
       • These enzymes on the tumour cells will metabolise the prodrug which can be uptaken by the tumour cells exerting its toxic effects
     • The enzyme itself can be non human allowing modification and increased effects possibly

4. Enzyme reactions

   • There are six classes of enzyme reactions which will occur in the body. In terms of pro-drugs we want to utilise 4 of these
     • Oxidoreductases
       • Will catalyse oxidation and reduction reactions, commonly used to modify the properties of the compound making non-polar molecules polar or non-soluble molecules soluble
       • Examples
         • Aldehyde oxidase
           • Involved also in xanthine oxidase reactions.
           • Certain drugs with one oxygen will be inactive but the addition of molecular oxygen may give it the reactivity
       • Cytochrome P450
         • Involved with many conversion reactions of prodrug to drug reactions
         • It is found in smooth endoplasmic reticulum which carries out microsomal oxidation as part of the microsomal electron transport chain. CYP450 is the final checkpoint in this chain
         • Will increase the polarity of a substrate by hydroxylation (addition of OH)
           • This is an example of a phase 1 metabolism action
           • Phase 2 reaction will involve masking a polar group to reduce its reactivity or toxicity
             • Glucuronidation
             • Glutathione
         • CYP450 is also responsible for synthesis and metabolism of steroid hormones by increasing polarity
         • Enzymes can be found in the
           • Kidney
           • Liver
           • Lungs
           • Skin
           • Adrenal cortex
     • Transferase
       • Will transfer groups of atoms from one molecule to another e.g. phopshorylation
     • Hydrolases
       • Will catalyse reactions involved with hydrolysis
     • Liases
       • The attachment of functional groups to pi-bonds
     • Isomerase
       • Will catalyse reactions of isomerism. They are not used as usually isomers of drugs will not differ drastically in isomer forms
   • Ligases
     • These will catalyse biosynthesis reactions. This is not needed as prodrugs need to break down

5. Examples of pro-drug interactions

   • Codiene
     • Contains a methyl group which is difficult to remove
       • The attachment of an OH group makes the molecule electronegative and as a result the methyl group can be removed automatically
         • This forms morphine which can interact with its opioid receptors to produce an effect
   • Benzopyren
     • Usually non polar but CYP2A6 can hydoxylate it to result in hydroxy-benzopyren which is a highly carcinogenic compound
       • It will bind to the ayrl hydrocarbon receptors to cause dividing of cells even if they are not ready which result in HIF inducing growth.
   • Paclitaxel
     • Can have a glucuronic acid bound to it
       • Will be activated by b-glucuronidase by hydrolysis to allow the drug to be active. The drug will then bind to the microtubules to prevent cell division
         • b- glucuronidase is found in necrotic areas of a tumour
   • SeCys
     • Is metabolised by the amino acid oxidase pathway. Selenols work to induce apoptosis in non-functional p53 cells
       • Amino acid oxidase is found in the liver, kidneys and can be used as drug targetting
   • Prodrugs of 5-FU
     • Capecitabine
       • This is a 5-FU drug which can be metabolised by glutathione S-transferase
         • Glutathione S-transferase is found to be widespread in normal tissues however it is overexpressed in tumour cells
     • Ftorafur
       • Hydroxylated by CYP2A6
     • Works by inhibiting thymidylate synthase involved in pyramidine synthesis for DNA
       • Thymidylate synthase is also important in converting DMP into dTMP important in DNA repair and synthesis

6. Differences between cancer and normal cells

   • Cancer cells will produce more of enzymes which can be found around the body. These include glucuronidase and glutathione s transferase
     • Makes them good targets for paclitaxel and capecitabine respectively
   • Cancer cells are more weaker than differentiated, mature and healthy cells meaning that they can be killed by a lower dose of chemotherapeutic drugs as they are more sensitive