Pharmacology of Opioid Analgesics

Transcription

1 Pharmacology of Opioid Analgesics Harvey Berman, PhD, MPH Department of Pharmacology and Toxicology (716) OPIOID_ANALGESIA_ November 17, 2018

2 NO SIGNIFICANT FINANCIAL, GENERAL, OR OBLIGATION INTERESTS TO REPORT OPIOID_ANALGESIA_ November 17, 2018

3 Objectives Receptor-based understanding of opioid action μ, κ, δ-opioid receptors Classification of opioids according to their efficacy Strong agonists Partial agonists Antagonists Predict physiological responses of opioid analgesics Understand indications, contra-indications, and adverse effects of opioid analgesia Utility of opioids in palliative care 3 01-OPIOID_ANALGESIA_ November 17, 2018

4 Analgesic agents NSAIDS Direct action at site of inflammation No direct effect on mood Opioids Inhibits propagation of pain signals Direct elevation of mood 4 01-OPIOID_ANALGESIA_ November 17, 2018

5 Opioid overdose epidemic, Erie County Why so many incidents of opioid overdose? 1.1 opioid deaths/day 5 01-OPIOID_ANALGESIA_ November 17, 2018

6 Drug Manufacturer Purdue (Oxycontin) Insys (Fentanyl) Physicians Distributors Pharmacies 6 01-OPIOID_ANALGESIA_ November 17, 2018

7 Anna Lembke TED TALK BOOK STORE OPIOID_ANALGESIA_ November 17, 2018

8 Pain An unpleasant sensory and emotional experience associated with actual or potential tissue damage Distinguish: Pain as a specific sensation Pain as suffering 8 01-OPIOID_ANALGESIA_ November 17, 2018

9 Pain is transmitted through Thin (1-5 μm), myelinated Aδ fibers v = m/sec Fast, 1st pain Very thin (< 1 μm), nonmyelinated C fibers v = m/sec Slow, 2nd pain Duller, aching, diffuse pain These fibers also carry sensation of itching 9 01-OPIOID_ANALGESIA_ November 17, 2018

10 Conduction of pain impulses from the periphery to the brain occurs through a multineuron pathway OPIOID_ANALGESIA_ November 17, 2018

11 Types of Pain Nociceptive pain results from injury to tissues Somatic pain Visceral pain Neuropathic pain results from injury to peripheral nerves. Each type respond differently to analgesic drugs OPIOID_ANALGESIA_ November 17, 2018

12 Nociceptive pain Somatic pain Results from injury to tissues Bones, joints, muscles Localized and sharp in quality Visceral pain Results from injury to visceral organs E.g., small intestine Vaguely localized with a diffuse aching quality OPIOID_ANALGESIA_ November 17, 2018

13 Papaver somniferum Opium poppy Resin from the poppy is collected and processed into opium OPIOID_ANALGESIA_ November 17, 2018

14 Opium alkaloids Morphine (9-17 % by weight opium) Chiefly responsible for pharmacological actions of opium Codeine (0.3-4 %) Methylmorphine, has similar but weaker actions Thebaine (0.2 %) Practically devoid of analgesic activity No medicinal use, but it provides a synthetic route of hydroxylated opioids OPIOID_ANALGESIA_ November 17, 2018

15 Morphine 17 N CH 3 HO 3 6 O OH OPIOID_ANALGESIA_ November 17, 2018

16 17 N CH 3 N CH 3 Structural relationships HO 3 6 O OH MORPHINE H 3 CO O CODEINE OH among N CH 3 some opiates N CH 3 H 3 CO O OCH 3 THEBAINE O H 3 CC O O HEROIN N O O CCH 3 CH 2 CH CH 2 OH N CH 3 OH O HO O NALOXONE O H 3 CO O OXYCODONE OPIOID_ANALGESIA_ November 17, 2018

17 A-D-M-E In general: Opioids are absorbed well IM, SC, and oral and nasal mucosa Rapid distribution from blood into highly perfused tissues (lung, liver, kidney, spleen) [Brain] < other tissues, due to BBB First-pass metabolism in the liver inactivates many opioids (e.g., morphine) Polar metabolites are excreted in urine OPIOID_ANALGESIA_ November 17, 2018

18 Glucuronidation of opioids COOH COOH OH O + R OH OH O O R + UDP OH O OH UDP OH OH UDP-glucuronic acid OPIOID_ANALGESIA_ November 17, 2018

19 Pharmacological Actions of Morphine OPIOID_ANALGESIA_ November 17, 2018

20 Actions in the CNS Analgesia Euphoria Miosis Respiratory depression Cough suppression Nausea / vomiting Mood altering / rewarding properties OPIOID_ANALGESIA_ November 17, 2018

21 Analgesic Relief of moderate to severe acute pain renal colic biliary colic acute MI dyspnea associated with LVF and pulmonary edema extensive surgical procedures and postoperative pain severe burns serious wounding severe chronic pain (e.g., associated with malignancy) OPIOID_ANALGESIA_ November 17, 2018

22 Continuous dull pain is relieved more effectively than sharp intermittent pain. In patients suffering from pain, morphine induces a feeling of well being and tranquility (euphoria). In pain-free individuals, the response to morphine is not always pleasant: General malaise Sensations of anxiety or fear (dysphoria) OPIOID_ANALGESIA_ November 17, 2018

23 Miosis Due to excitation of the parasympathetic nerve innervating the pupil Antagonist muscles determine resting pupil size Sympathetic motor nerve fiber; Contraction caused by dim light or mydriatic agents Radially arranged dilator muscle fibers of the iris Sphincter muscle fibers of the iris Pupil Parasympathetic motor nerve fiber; Contraction caused by bright light or miotic agents OPIOID_ANALGESIA_ November 17, 2018

24 Respiration Morphine depresses respiration, even at normal therapeutic doses. Due to decreased sensitivity of brain stem to increases in PCO2 and [H + ]. The main stimulus to breathing at this point becomes hypoxia. This effect is increased by the concurrent use of other CNS-active drugs (alcohol, barbiturates, benzodiazepines) Caution: Pregnancy (neonate) OPIOID_ANALGESIA_ November 17, 2018

25 Respiration Suppression of respiratory drive contributes to usefulness of morphine in relieving difficult or labored breathing in certain conditions such as Left ventricular failure Pulmonary edema OPIOID_ANALGESIA_ November 17, 2018

26 Morphine increases intracranial pressure! Mechanism: indirect Respiration pco2 arteriolar dilatation CSF pressure But, if respiration is maintained at a normal rate, ICP will remain normal OPIOID_ANALGESIA_ November 17, 2018

27 Antitussive action Morphine suppresses the cough reflex that originates in the medulla oblongata. Unrelated to its analgesic action. Unrelated to its respiratory depressant action. Can modify the morphine molecule to select this single property Cough suppressant - dextromethorphan OPIOID_ANALGESIA_ November 17, 2018

28 Emesis Morphine stimulates the CTZ in the medulla to cause nausea and vomiting A vestibular component? OPIOID_ANALGESIA_ November 17, 2018

29 Cardiovascular System Blunts the baroreceptor reflex Causes peripheral arteriolar and venous dilatation Results, in part, from morphine-induced release of histamine Orthostatic hypotension and fainting In recumbent individuals, effect is mild In standing individuals, the effect is profound OPIOID_ANALGESIA_ November 17, 2018

30 GI Tract Morphine decreases propulsive peristalsis through GI tract Leads to constipation Morphine suppresses awareness of bowel stimuli Propulsive peristalsis through GI tract is decreased: Passage of GI contents is delayed. This leads to enhanced absorption of water. dehydration of feces. Tone of anal sphincter is increased OPIOID_ANALGESIA_ November 17, 2018

31 Urinary retention Morphine causes urinary hesitancy and urinary urgency Increased tone of bladder smooth muscle Increased tone of sphincter muscle Morphine suppresses awareness of bladder stimuli OPIOID_ANALGESIA_ November 17, 2018

32 Biliary colic Morphine can cause spasm of the lower end of the common bile duct (The Sphincter of Oddi) pressure within the biliary tract. In patients with pre-existing biliary colic, morphine can intensify the pain. Meperidine a better choice? Causes less smooth muscle spasm? OPIOID_ANALGESIA_ November 17, 2018

33 33 01-OPIOID_ANALGESIA_ November 17, 2018

34 Chronic Actions of Morphine Tolerance Tolerance develops to Analgesia Respiratory depression Tolerance does not develop to: Constipation Pupillary miosis Dependence OPIOID_ANALGESIA_ November 17, 2018

35 Opioid Poisoning Triad of 3 signs strongly suggests opioid poisoning. Coma Pinpoint pupils Depressed respiration OPIOID_ANALGESIA_ November 17, 2018

36 Treatment of Opioid Poisoning Maintain patent airway and ventilate the patient Treat with antagonist to reverse CNS effects Naloxone (NARCAN) is drug of choice OPIOID_ANALGESIA_ November 17, 2018

37 Mechanism of Action The Morphine Receptor Morphine binds to a specific receptor Binding is stereospecific Only the l-isomer is active Implies that binding involves at least three binding sites Regional distribution The regions of highest binding are associated with the limbic system Three principal receptors: μ, κ, δ OPIOID_ANALGESIA_ November 17, 2018

38 Opiate binding sites (green dots) within the brain and spinal cord Morphine binds to opiate receptors concentrated within the reward pathway and the pain pathway OPIOID_ANALGESIA_ November 17, 2018

39 The μ-morphine Receptor The k-morphine Receptor Spinal and supra-spinal analgesia Miosis Respiratory depression Euphoria Physical dependence GI motility Spinal analgesia Miosis Less intense than µ Respiratory depression Less intense than µ Sedation Dysphoria OPIOID_ANALGESIA_ November 17, 2018

40 The δ-morphine Receptor Much less is known about this receptor Alters affective behaviour? Main barrier to clinical utility is that most of the δ -receptor agonists are peptides and do not cross the BBB OPIOID_ANALGESIA_ November 17, 2018

41 Endogenous Opioids Opioid drugs produce analgesia by activating the same receptors that are activated on by the endogenous painsuppressing system The enkephalins The endorphins The dynorphins OPIOID_ANALGESIA_ November 17, 2018

42 The enkephalins Met-enkephalin: H-Tyr-Gly-Gly-Phe-Met-OH Leu-enkephalin: H-Tyr-Gly-Gly-Phe-Leu-OH They mimic the actions of morphine on tissues that contain morphine receptors The endorphins The dynorphins OPIOID_ANALGESIA_ November 17, 2018

43 Mechanisms of Action Opioid receptors respond to opioids and to endogenous peptides (suppress painful stimuli) Opioids work both presynaptically and post-synaptically In general, opioid receptors are G-protein coupled receptors Activation reduces Ca ++ influx during the action potential, leading to less neurotransmitter release OPIOID_ANALGESIA_ November 17, 2018

44 Spinal sites of opioid action μ, κ, δ agonists reduce transmitter release from presynaptic terminals of nociceptive primary afferents. µ agonists also hyperpolarize second-order neurons by K + conductance ipsp OPIOID_ANALGESIA_ November 17, 2018

45 Opioid antagonist Naloxegol (Movantik) PEGylated derivative of naloxone MW = 742 Does not penetrate BBB Treatment of OIC Permits antagonism of μ-opioid effects in GI tract OPIOID_ANALGESIA_ November 17, 2018

46 Next Lecture: Opioids Other Than Morphine OPIOID_ANALGESIA_ November 17, 2018