#074 · Drug of the Day Morphinan · C6-keto µ full agonist (Dilaudid) Schedule II 2026-09-08

Hydromorphone

Morphine's C6-ketone, several times over

4,5α-epoxy-3-hydroxy-17-methylmorphinan-6-one · C17H19NO3 · MW 285.34 g/mol · CAS 466-99-9 · Dilaudid · ChEMBL CHEMBL398707

Hydromorphone (Dilaudid). A hydrogenated, C6-oxidized relative of morphine (#009) — the 6-ketone raises potency roughly 5× oral / ~8× IV versus morphine. Full µ-opioid agonist, highly water-soluble, the house IV opioid of many hospitals, and every bit as capable of stopping a brainstem as fentanyl is, just with more milligrams on the syringe.

Primary targetµOR (MOR)
MechanismFull agonist
vs morphine (IV)~8× more potent
MOR Ki~0.4 – 1.8 nM
Active metaboliteNone like M6G
H3GNeuroexcitatory
ReversalNaloxone
01 · Mechanism of Action

Same Receptor, Harder Push, No M6G Crutch

Hydromorphone is a full MOR agonist: Gi/o coupling, cAMP down, GIRK open, CaV closed, β-arrestin recruited. The C6 ketone (versus morphine's C6-OH) and 7,8-saturation shift potency upward without inventing a new mechanism. Unlike morphine, there is no 6-glucuronide active metabolite carrying extra µ-agonism; hydromorphone-3-glucuronide (H3G) is instead a neuroexcitant that accumulates in renal failure.

High aqueous solubility made it the default PCA/IV opioid. That is a formulation fact, not a safety fact. Respiratory depression is naloxone-reversible and fatally synergistic with benzodiazepines and alcohol.

① MOR full agonism

Orthosteric MOR, no efficacy ceiling. Pre-Bötzinger suppression is dose, not mystery.

② Gi/o cascade

↓ cAMP, ↑ GIRK, ↓ CaV — analgesia, euphoria, constipation, miosis, cough suppression.

③ Potency vs morphine

Typical equianalgesic: 1 mg IV hydromorphone ≈ 7–8 mg IV morphine. Oral ~5×. Do not 'same-number' the milligrams.

④ No M6G

Morphine's active 6-glucuronide has no hydromorphone twin. Renal failure still matters because of H3G.

⑤ H3-glucuronide

Neuroexcitatory (myoclonus, agitation, seizures) when GFR collapses. Opioid toxicity that naloxone will not fully fix.

⑥ β-arrestin / reward

Classical full-agonist arrestin recruitment and VTA disinhibition — dependence is not optional with daily use.

Hydromorphone → MOR full agonist → Gi + β-arr2 → analgesia · reward · no respiratory ceiling
UGT → H3G (renal) → neuroexcitation if kidneys fail
02 · Pharmacokinetics

Fast IV, Honest Glucuronide

IV onset minutes; oral IR Tmax ~1 h; t½ ~2–3 h (shorter than morphine's active metabolites). UGT1A3/2B7 to H3G; renal clearance of the glucuronide.

IV onsetminutes
Oral Tmax~1 h
T½~2 – 3 h
Oral bioavailability~30 – 50% (variable)
MetabolismUGT → H3G
Active µ metaboliteNo (unlike M6G)
H3GNeuroexcitatory, renal
SolubilityHigh (IV/PCA)
03 · Equianalgesic Arithmetic Kills

Milligrams Are Not Interchangeable Across Morphinans

Hydromorphone errors are classic: writing "4" because morphine 4 mg IV was the previous order, or confusing oral and IV. Exalgo-type ER tablets are a different animal again. Treat it as a high-potency full agonist beside oxycodone #018 and fentanyl #007, not as "hospital morphine."

04 · FlexAIDΔS · Shannon Entropy Analysis

A Pre-Organized Morphinan in an Active-State Pocket

FlexAIDΔS · Entropy Commentary

PDB 5C1M is active-state MOR + BU72 — no hydromorphone co-crystal exists. The rigid epoxymorphinan cage arrives pre-organized (ΔS_conf modest); the C6 ketone tweaks hydrogen-bond geometry versus morphine's C6-OH, a small enthalpic bump that reads out as several-fold potency. Shannon collapse of the orthosteric pocket is the same full-agonist story as morphine, harder driven.

05 · Harm Reduction

Full Agonist Rules Apply

Naloxone reverses the µ effects. It does not reverse H3G seizures. Benzos and alcohol remain the usual co-conspirators.

Respiratory depression

  • Dose-dependent apnea; synergistic with benzos, alcohol, gabapentinoids
  • Naloxone reverses µ-tone — may need repeat dosing given hydromorphone's kinetics vs ultra-short naloxone

Renal / H3G

  • Myoclonus, delirium, seizures in renal failure — lower dose or switch
  • Naloxone will not clear the glucuronide

Conversion errors

  • IV hydromorphone is several-fold morphine — never copy the milligram number
  • Oral ≠ IV; ER ≠ IR

Practice

  • Carry naloxone; don't use alone
  • See Morphine #009 for the parent scaffold
FATAL COMBINATIONS: Hydromorphone + benzodiazepines / alcohol / gabapentinoids → lethal respiratory depression. Equianalgesic conversion errors. Renal failure + usual doses → H3G neurotoxicity plus opioid apnea.
3D · Active-state MOR (BU72 surrogate) PDB: 5C1M
Loading structure from RCSB…
Receptor (refined cartoon)
Contact residues
Ligand (ball-and-stick)
Structure: 5C1M — active-state µ-opioid receptor + morphinan agonist BU72 (Huang et al., Nature 2015). No hydromorphone co-crystal exists; BU72 stands in for a high-affinity morphinan in the same pocket. Rotate · scroll to zoom · right-drag to translate.
View on RCSB →

Receptor Binding Affinities

hydromorphone
Target Affinity Rel. Action
µ / MOR
mu-opioid receptor (OPRM1)
Ki ≈ 0.4 – 1.8 nM
full agonist · ChEMBL
Full agonist
κ / KOR
kappa-opioid receptor
weaker than MOR
not the clinical driver
Weak
δ / DOR
delta-opioid receptor
weaker than MOR
secondary
Weak
H3G
neuroexcitant metabolite
not a µ agonist
renal accumulation
Neurotoxic
Human MOR Ki for hydromorphone is sub- to low-nanomolar across ChEMBL binding assays (CHEMBL398707). PDB 5C1M is an active-state MOR + BU72 surrogate — no hydromorphone co-crystal.