BASIC · EP 06 · PHARMACOLOGY
Before You Listen
Episode Setup
- Topic in one line: the pain-pharmacology spine of ABPMR Part 1, built around the three opioid receptors (mu, kappa, delta), the equianalgesic conversion table anchored on 10 milligrams of oral morphine, the 3-to-1 oral-to-intravenous morphine ratio, the 25-to-50 percent dose reduction required during opioid rotation for incomplete cross-tolerance, the 50 and 90 morphine milligram equivalent per day overdose thresholds, methadone’s non-linear 4-to-1 up to 20-to-1 conversion ratio with NMDA-receptor activity and a 15-to-60 hour half-life, the gabapentinoid alpha-2-delta calcium channel mechanism with gabapentin’s saturable absorption versus pregabalin’s linear kinetics, tricyclic antidepressants (amitriptyline, nortriptyline, desipramine) with their anticholinergic-antihistaminic-alpha-1-sodium-channel burden, serotonin-norepinephrine reuptake inhibitors (duloxetine, venlafaxine) for diabetic peripheral neuropathy and fibromyalgia, tramadol’s dual mu-agonist plus SNRI mechanism with the serotonin syndrome and seizure-threshold traps, topical lidocaine and capsaicin, the cyclooxygenase-1 versus cyclooxygenase-2 split with COX-1 protecting gastric mucosa and renal afferent dilation and platelet thromboxane A2 and COX-2 driving inflammation, the celecoxib paradox of preserved thromboxane A2 versus suppressed prostacyclin producing cardiovascular events, aspirin’s irreversible COX-1 acetylation lasting the 10-day platelet lifespan, clopidogrel’s irreversible P2Y12 receptor blockade, and acetaminophen’s central cyclooxygenase inhibition with a 3-to-4-gram daily ceiling and NAPQI hepatotoxicity.
- Prerequisites: comfort with first-pass hepatic metabolism, basic receptor pharmacology, the difference between competitive and irreversible enzyme inhibition, and the standard PM&R approach to acute and chronic pain.
- Runtime: approximately 30 minutes.
Vignette. A 62-year-old man with severe chronic low back pain from multilevel lumbar stenosis is taking morphine extended-release 100 milligrams orally every twelve hours (total 200 oral morphine milligram equivalents per day) plus oxycodone 10 milligrams orally every four hours as needed for breakthrough pain. He has poor pain control and developed myoclonic jerks at the higher end of his dosing. The team decides to rotate him to methadone for a unique receptor profile that may help his neuropathic component. The covering physician calculates the conversion using a standard 4-to-1 ratio, prescribes 50 milligrams of methadone daily, and the patient is found unresponsive with pinpoint pupils and a respiratory rate of 6 on hospital day 2.
Why did the standard 4-to-1 conversion ratio overdose this patient, what is the correct equianalgesic logic for methadone rotation at his daily oral morphine milligram equivalent dose, what additional pharmacokinetic property of methadone explains why the respiratory depression worsened on day 2 rather than appearing immediately, and what dose-reduction safety margin should have been applied during the rotation?
(Answer at the end of this chapter)
Section 1: Opioid Receptors and the Equianalgesic Conversion Map
Bottom line: opioids act through three G-protein-coupled receptors — mu, kappa, and delta. The mu receptor is the dominant analgesic target and produces profound pain relief along with euphoria, respiratory depression through brainstem suppression of the carbon-dioxide chemoreceptor response, miosis, severe constipation through enteric mu receptors that halt peristalsis, and physical dependence. Kappa receptor activation produces analgesia at the spinal level along with sedation and the classic dysphoria pattern, not euphoria, which is why kappa agonists have little abuse potential. Delta contributes to analgesia and mood modulation but is not a major board target. Equianalgesic dosing is anchored on 10 milligrams of oral morphine: 10 milligrams of oral morphine equals 3 milligrams of intravenous morphine (the 3-to-1 oral-to-IV ratio from first-pass hepatic metabolism), 7.5 milligrams of oral hydromorphone or 1.5 milligrams of intravenous hydromorphone (hydromorphone is roughly five times more potent than morphine on a milligram-for-milligram basis), 7 milligrams of oral oxycodone (oxycodone is roughly 1.5 times as potent as oral morphine), and a transdermal fentanyl patch delivering 25 micrograms per hour equals approximately 60 milligrams of oral morphine per day. During opioid rotation the calculated equianalgesic dose must then be reduced by 25 to 50 percent to account for incomplete cross-tolerance between agents, with subsequent titration to clinical response. Morphine milligram equivalents quantify daily opioid burden, with >50 morphine milligram equivalents per day marking the first overdose-risk threshold and >90 morphine milligram equivalents per day marking the high-risk threshold that should prompt naloxone co-prescription and reassessment of the pain plan.
Opioids are the foundational analgesic class for moderate-to-severe pain, and PM&R residents need both the receptor map and the conversion math cold for the boards. Three receptor subtypes carry the analgesic and adverse-effect signal: mu, kappa, and delta.
All three are G-protein-coupled receptors. When an opioid binds, it closes voltage-gated calcium channels on the presynaptic terminal (reducing release of pain neurotransmitters such as substance P and glutamate) and opens potassium channels on the postsynaptic membrane, hyperpolarizing the receiving neuron and silencing the nociceptive signal.
The mu receptor is the dominant analgesic target. Mu activation produces profound analgesia, euphoria (the addictive drive), respiratory depression, miosis, constipation, and physical dependence. The respiratory depression is the mechanism of fatal overdose: mu receptors in the brainstem medulla blunt the carbon-dioxide chemoreceptor response, so the brain stops sensing rising carbon dioxide and forgets to trigger the next breath. Constipation is driven by abundant mu receptors in the enteric nervous system that bring peristalsis to a near-halt. Miosis acts through the Edinger-Westphal nucleus and explains the pinpoint pupils of opioid overdose. The therapeutic and toxic effects share the same receptor, which is why the analgesic dose cannot be cleanly separated from the dose that depresses breathing.
The kappa receptor produces analgesia (largely at the spinal cord level) plus sedation and the classic dysphoria of kappa agonism — a state of unease, dissatisfaction, and sometimes hallucinations, not the euphoria of mu. Because dysphoria is not a feeling patients want to repeat, kappa agonists have very low abuse liability. The delta receptor contributes to analgesia and mood modulation and is a third-tier board focus behind mu and kappa.
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## Mnemonic — “Anchor at ten, drop by half”
Every conversion table anchors on 10 milligrams of oral morphine. From the anchor, multiply or divide to reach the other agent (oral-to-IV morphine divide by 3; morphine-to-hydromorphone divide by 4 oral, divide by 6.7 IV; morphine-to-oxycodone divide by 1.4; morphine-to-fentanyl patch use the 60-milligrams-per-day to 25-micrograms-per-hour map). Then drop by 25 to 50 percent for cross-tolerance and titrate up. The anchor and the drop are non-negotiable; the multipliers are memorization. :::
It really is a double-edged sword. The exact same receptor giving you life-altering pain relief is the one slowing down your breathing.
— BASIC-06-a podcast, ~2:55