Pharmacology Of Hydrocodone

Indication	For relief of moderate to moderately severe pain. Also used for the symptomatic relief of nonproductive cough, alone or in combination with other antitussives or expectorants.
Pharmacodynamics	Hydrocodone, a semisynthetic opiate agonist and hydrogenated ketone derivative, is similar to other phenanthrene derivatives, such as codeine. Used as an analgesic, hydrocodone is combined with acetaminophen, ibuprofen, or aspirin to treat pain. Used as an antitussive, hydrocodone is combined with phenylephrine, pseudoephedrine, phenylpropanolamine, guaifenesin, pyrilamine, pheniramine, or chlorpheniramine. Opiate agonists exert their principal pharmacologic effect at specific receptor binding sites in the CNS and other tissues. There are several subtypes of opiate receptors including the mu receptor (localized in pain modulating regions of the CNS), the kappa receptor (localized in the deep layers of the cerebral cortex), the delta receptor (localized in the limbic regions of the CNS), and the sigma receptor (thought to mediate the dysphoric and psychotomimetic effects of some opiate partial agonists). Agonist activity at the mu or kappa receptor can result in analgesia, miosis, and/or decreased body temperature. Agonist activity at the mu receptor can also result in suppression of opiate withdrawal, whereas antagonist activity can result in precipitation of withdrawal. Opiate agonists act at several sites within the CNS involving several systems of neurotransmitters to produce analgesia, but the precise mechanism of action has not been fully determined. Opiate agonists do not alter the threshold or responsiveness of afferent nerve endings to noxious stimuli nor the conduction of impulses along peripheral nerves. Instead, they alter the perception of pain at the spinal cord and higher levels in the CNS and the person's emotional response to pain.
Mechanism of action	Hydrocodone acts as a weak agonist at OP1, OP2, and OP3 opiate receptors within the central nervous system (CNS). Hydrocodone primarily affects OP3 receptors, which are coupled with G-protein receptors and function as modulators, both positive and negative, of synaptic transmission via G-proteins that activate effector proteins. Binding of the opiate stimulates the exchange of GTP for GDP on the G-protein complex. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as substance P, GABA, dopamine, acetylcholine, and noradrenaline is inhibited. Opioids such as hydrocodone also inhibit the release of vasopressin, somatostatin, insulin, and glucagon. Opioids close N-type voltage-operated calcium channels (OP2-receptor agonist) and open calcium-dependent inwardly rectifying potassium channels (OP3 and OP1 receptor agonist). This results in hyperpolarization and reduced neuronal excitability.
Absorption	Well absorbed from the gastrointestinal tract.
Volume of distribution	Not Available
Protein binding	As most agents in the 5-ring morphinan group of semi-synthetic opioids bind plasma protein to a similar degree (range 19% [hydromorphone] to 45% [oxycodone]), hydrocodone is expected to fall within this range.
Metabolism	Hepatic and also in intestinal mucosa.
Route of elimination	Not Available
Half life	1.25-3 hours
Clearance	Not Available
Toxicity	Symptoms of overdose include respiratory depression (a decrease in respiratory rate and/or tidal volume, Cheyne-Stokes respiration, cyanosis), extreme somnolence progressing to stupor or coma, skeletal muscle flaccidity, dizziness, ringing in the ears, confusion, blurred vision, eye problems, cold and clammy skin, and sometimes bradycardia and hypotension. In severe overdose, apnea, circulatory collapse, cardiac arrest and death may occur. LD50=85.7mg/kg (subcutaneous, in mice).