OPIOID PHARMACOLOGY PCTH 300/305 Andrew Horne, PhD andrew.horne@ubc.ca MEDC 309 THE OPIUM POPPY Papaver somniferum Sleep-bringing poppy Poppy Seeds Opiates Opium Poppy Straw 1
OPIATES VS. OPIOIDS Opiates: Any of the narcotic alkaloid isolates from P. somniferum Opioids Primarily morphine, codeine and thebaine Formerly, all non-opiate narcotic agents Now, generally includes opiates, and all semisynthetic and synthetic narcotics E.g. heroin, methadone, fentanyl, mepiridine ENDOGENOUS OPIOID PEPTIDES Exogenous opioid alkaloids aim to mimic this endogenous peptide system Modulation of painful stimuli, GI, endocrine and autonomic functions Endorphins Enkephalins Dynorphins Nociceptin/Orphanin FQ 5-31 amino acids in length Common N-terminal homology à opioid motif 2
THE RECEPTORS GPCRs, high homology 3 classical opioid receptors µ (Mu) δ (Delta) κ (Kappa) plus splice variants Primarily responsible for most clinical effects (particularly µ-ors) Nociceptin/orphanin FQ (N/OFQ) receptor Minimal affinity for most conventional opioid ligands Vary in expression throughout brain, spinal cord and periphery Granier et al. 2012. Nature 485 OPIOID AGONIST VARIETY Endogenous opioid agonists (EOPs): 5-31 amino acids long Exogenous opioid agonists: non-proteinaceouspolycyclic molecules Alkaloids yet one receptor handles both! http://thebrain.mcgill.ca/flash/i/i_0 3 /i_0 3 _m/i_0 3 _m_d ou /i_0 3 _m_d ou.h tml Common motif for both, with perhaps additional interactions for EOPs? http://www.fastol.com/~renkwitz/opiate_receptors_and_actions.html 3
OPIOID DRUGS Drug µ δ κ Morphine +++ + Methadone +++ Fentanyl +++ Sufentanil +++ + + Buprenorphine P -- Pentazocine P ++ Nalbuphine -- ++ Naloxone --- - -- Naltrexone --- - --- Source: Goodman & Gilman, 10 th Ed OPIOID RECEPTOR CASCADES Aimed at diminishing excitability and/or neurotransmission G i -mediated inhibition of adenylate cyclase Presynaptically: Suppression of Ca++ conductance Reduced vesicle fusion/nt release Postsynaptically: Activation of K+ conductance Hyperpolarization of membrane 4
EFFECTS (OVERVIEW) Analgesia CNS effects Gastrointestinal Effects ANALGESIA Primarily effective against nociceptive pain Intense, constant pain > sharp intermittent pain Acute or Chronic Moderate/Severe pain associated with Cancer Gradually gaining more acceptance in treatment of chronic pain Primary sensation of pain, as well the suffering All evoked reactions upon experiencing this sensation 5
THREE MODES OF ANALGESIA 1. Direct inhibition of ascending nociceptive transmission from spinal cord dorsal horn 2. Activation of pain control circuits descending to dorsal horn from midbrain 3. Decreases release of sensitizing/modulating agents (e.g. Substance P) ANALGESIA Treatment will vary depending on many factors, including: Severity of pain Concomitant conditions/disease Whether opioid-naïve or opioid-tolerant Coprescribe with other analgesics where possible Strive for dosing stability rather than on an as needed basis 6
CNS EFFECTS Respiratory Depression Far and away the most dangerous possible opioid-related effect Rarely relevant unless pre-existing pulmonary dysfunction and/or supratherapeutic doses Miosis Constrictor effect separates class from most other coma/rd-inducing agents Direct effects on relevant centers in brain to cause: Nausea/Emesis Cough Suppression (antitussive effects) Euphoria and/or Tranquility GI SYSTEM Delays intestinal peristalsis and decreases sensitivity to rectal distention à constipatory effect Side effect, or utile diarrhea treatment If side effect, can treat with stool softeners and/or laxatives Can delay gastric emptying à Reflux disease/heartburn 7
CHEMICAL CLASSES (AGONISTS) Phenanthrenes Morphine, Codeine Heroin, Oxycodone, Hydrocodone Most mixed agonist/antagonist drugs Phenylpiperidines Fentanyl, Sufentanil, Mepiridine Diphenylheptanes Methadone Morphine Fentanyl Natural Semisynthetic Synthetic Methadone MORPHINE Gold standard in opioid analgesia Effective via enteral and parental administration More Hydrophilic than most opioids Renal excretion 8
CODEINE Increased oral bioavailability Low affinity for opioid receptors Antitussive effects at sub-analgesic doses non-opioid receptor? Gasche et al. 2004. NEJM 351 HEROIN a.k.a. diacetylmorphine Very lipophilic Semisynthetic, ~3x more potent than morphine Increased [morphine] CNS à Rapid hydrolysis à Bencharit et al. 2003. Nature Structural Biology 10 9
FENTANYL AND SUFENTANIL Rapid onset (~5 minutes) Very lipophilic Termination via redistribution, unless already saturated Muscle rigidity common, especially in core trunk muscles Sufentanil 0.02 Not cross-tolerant with morphine/codeine METHADONE Long-acting morphine-like µ-op agonist Less euphoria, but longer duration Very lipophilic Reversal of tissue accumulation protracts plasma concentrations Reduces withdrawal symptoms Addiction/dependence treatment http://stonybrookcenter.com/heroin-vs-methadone/ 10
MIXED AGONIST/ANTAGONIST DRUGS Partial agonists of (some) opioid receptors In opioid-naïve à sub-maximal analgesia In opioid-tolerant à reduced efficacy compared to morphine (or other) precipitates withdrawal BUPRENORPHINE Antagonist κ-op receptors Partial agonist µ-op receptors Acts like morphine in naïve patients Available as a take-home oral detox option (+) Reduced risk of respiratory depression (-) Slower reversal of addiction 11
ANTAGONISTS Naloxone Competitive antagonist of all three classic receptors, highest affinity for µ-op Higher receptor affinity than most agonists Rapid relief of opioid-induced respiratory depression Approximate duration of 1-4 hours, risk of relapse if opioids still present Extensive first-pass metabolism Naltrexone Similar actions to naloxone, with a longer duration (up to 48 hours) Approved by US FDA for treatment of alcoholism, through unknown mechanism CONTRAINDICATIONS Head Injuries Pregnancy Impaired Pulmonary Function Impaired Hepatic/Renal Function 12
CHRONIC OPIOID USE LEADS TO: Tolerance Dependence Withdrawal Common features of opioids, to all degrees Extent may vary between different effects TOLERANCE Usually over 2-3 weeks of therapeutic dosing Remifentanil à tolerance within hours Onset and dissipation of tolerance varies Respiratory depression à days Emesis à months Does not occur equivalently for all effects Miosis, Constipation far less likely to show tolerance 13
WITHDRAWAL Lippincott s Illustrated Reviews TOLERANCE AND WITHDRAWAL Where their use is otherwise appropriate/indicated, opioids should never be avoided on the fear of tolerance or dependence/withdrawal Tolerance: Opioid rotation Progressive treatment plan involving regular rotation between equianalgesic (or equieffective) doses of opioids Dependence/Withdrawal: Controlled cessation regiment Usually limited by a 10-20% reduction in daily dose until drug-free 14