Organic Pharmaceutical Chemistry II

Organic Pharmaceutical Chemistry II

• Topic: Medicinal Chemistry
• Lecturer: Dr. Ahmed Ali Al-Karmalawy, Associate Professor of Medicinal Chemistry

Narcotic Analgesic

• Types of Epoxymorphinans:

  • 4,5-Epoxymorphinans
  • Morphinans
  • Benzomorphans
  • 4-Phenylpiperidines and 4-Anilidopiperidines
  • Diphenylheptanes
  • Miscellaneous

• Morphine:

  • Prototype ligand for the µ-receptor
  • Contains 5 chiral centers and 16 optical isomers
  • Opium contains over 40 alkaloids, with morphine (8%-17%), codeine (0.7%-5%), thebaine (0.1%-2.5%), papaverine (0.5%-1.5%), and noscapine (1%-10%)
  • Poorly soluble in water, thus sulfate salt is formed for oral, IV, and suppository use.
  • Primary use is analgesia.

• Codeine:

  • Prepared from morphine by methylating the phenolic OH group.
  • Monoacidic base that forms salts (sulfate and phosphate).
  • Similar to morphine but with lower analgesic potency and addiction potential.
  • Side effects include respiratory depression, miosis, constipation, nausea, itching, dry mouth, and drowsiness.
  • Role as an antitussive agent is questioned.
  • Available in combination with aspirin, ibuprofen, or acetaminophen for moderate pain treatment.

• Heroin:

  • 3,6 diacetylated form of morphine.
  • Passes through blood-brain barrier quicker than morphine, causing euphoria and addiction.
  • Not available as a prescription drug in the US, but available in some countries for cancer and myocardial infarction pain.

• Hydromorphone:

  • Synthetic derivative of morphine.
  • Oxidation of the 6-OH group decreases potency.
  • Increasing flexibility of 7,8-double bond results in a compound with slightly enhanced binding.
  • Approximately 5 times more potent than morphine.

• Oxymorphone:

  • 14 beta-hydroxyl version of hydromorphone
  • Analogous to hydrocodone.
  • Lower oral bioavailability than hydromorphone.
  • Injectable form is more potent.

• Hydrocodone:

  • 3-methoxy version of hydromorphone.
  • Loss of the 3-OH group results in a weaker opioid (about equal to morphine).
  • Better brain penetration and increased binding to the µ-receptor.
  • Marketed as an antitussive agent.
  • Available in combination with anticholinergic agents and acetaminophen/aspirin.

• Oxycodone:

  • 14 beta-hydroxyl version of hydrocodone.
  • Synthesized from thebaine. 1.5 times more potent than hydrocodone orally due to increased receptor affinity.
  • Marketed with acetaminophen, aspirin, and ibuprofen.

Morphinans

• Morphinans were created by removing the E ring of morphine, to simplify the structure.

Levorphanol

• Levorotatory form of methorphan.
• Approximately 7.5 times more potent orally than morphine.
• Available as tablet and injection.
• Alternative for patients without adequate pain relief from morphine.

Dextromethorphan

• Dextrorotatory form of levorphanol with a methoxy group on the 3-position.
• OTC cough and cold formulations.
• Antagonizes the NMDA receptor.
• Used to treat phantom pain, diabetic neuropathy, and postoperative acute pain.

Benzomorphans

• Removing the C ring of the morphinan structure yields benzomorphans (benzazocines).
• Pentazocine is the only benzomorphan in clinical use.
• Racemic mixture; partial agonist at the µ-receptor and agonist at the K-receptor; contributes to dysphoria.

4-Phenylpiperidines and 4-Anilidopiperidines

• Removal of the B ring from benzomorphans yields 4-substituted piperidines.
• Structures are flexible, with the A ring existing in either axial or equatorial position.

Meperidine

• Discovered in 1939 during serendipitous screening.
• Low potency at the opioid receptor, but higher brain penetration.
• Metabolized to normeperidine, causing potential CNS excitation, including tremors, twitching, and myoclonus.

Diphenoxylate

• Weak opioid agonist, combined with atropine (Lomotil).
• Dissuades abuse and used as an antidiarrheal agent

Loperamide

• Acts as an antidiarrheal via direct binding to opiate receptors in the gut wall.
• Decreases peristalsis and fluid secretion to increase GI transit time and reduce fecal matter volume.
• Effectively crosses the blood-brain barrier, has no significant CNS opioid effects.
• Eliminated via P-glycoprotein pump (MDR1).
• Available as capsule form.

Fentanyl

• 4-aniline substituent of meperidine increases potency (approximately 500x stronger).
• Marketed as an analgesic and adjunct anesthetic.
• Available in injectable and transdermal formulations (Duragesic).

SAR studies of 4-Phenylpiperidine Analgesics

• Propionamide is the optimal chain length.
• Adding polar groups to the 4-piperidine carbon increases potency (e.g. CH2OCH3, COOCH3).

Diphenylheptanes (Methadone)

• Synthetic opioid approved for analgesic therapy and opioid maintenance.
• Racemic mixture is marketed, but opioid activity resides in the R-enantiomer.
• 7-50 times more potent than the S-enantiomer.

Miscellaneous

• Tramadol:
  • Analgesic agent with several mechanisms of action
  • Weak μ-agonist
  • Minimal effect on respiratory rate, heart rate, blood pressure, or GI transit time.
  • Similar structure to codeine (B, D, and E rings removed)
  • Available as a racemic mixture; (+) enantiomer being much stronger.
• Codeine, tramadol, and other drugs:
  • Some drugs, like tramadol are metabolised to a more potent version by enzymes (e.g. CYP2D6)
• Buprenorphine:
  • Semisynthetic opiate derived from thebaine
  • Mixed µ-agonist/antagonist (partial agonist) and weak κ-antagonist.
  • High affinity for µ-receptors and slow dissociation means longer duration (6-8 hours).
  • Primarily used for treatment of opioid dependence, but with lower oral bioavailability due to metabolism in the intestines and liver.
  • Sublingual and transdermal formulations are available outside the US.

Opioid Antagonists

• Naltrexone:
  • Pure opioid antagonist with highest affinity at μ-receptor.
  • Orally bioavailable, blocks opiate agonist effects (approximately 24 hours).
  • Developed into extended-release injectable microsphere formulation (Vivitrol) for better compliance with opioid/alcohol dependence treatment.
• Naloxone:
  • Pure opioid antagonist structurally similar to oxymorphone, but with an allyl group instead of a methyl substituent.
  • Used to reverse respiratory depression effects of opioid overdoses.
• Nalmefene:
  • 6-methylene analog of naltrexone with similar structural characteristics compared to naloxone.
  • Higher oral bioavailability compared to naltrexone and naloxone.
  • Treatment for pathological gambling and alcohol abuse, available in oral form.
• Methylnaltrexone:
  • Methylated, quaternary form of naltrexone.
  • Acts as an antagonist on peripheral opioid receptors, preventing drug from crossing the blood-brain barrier (BBB).
  • Approved in 2008 to treat opioid-induced constipation in palliative care patients via SC injection .

Test your knowledge on narcotic analgesics and their chemistry in Organic Pharmaceutical Chemistry II. This quiz covers the types of epoxymorphinans, the structure and use of morphine, and other relevant medicinal chemistry concepts. Enhance your understanding of drug formulations and pharmacology.