Morphine is a pure opioid agonist whose principal
therapeutic action is analgesia. Other members of the class known as opioid
agonists include substances such as oxycodone, hydromorphone, fentanyl,
codeine, and hydrocodone. Pharmacological effects of opioid agonists include
anxiolysis, euphoria, feelings of relaxation, respiratory depression,
constipation, miosis, cough suppression, and analgesia. Like all pure opioid
agonist analgesics, with increasing doses there is increasing analgesia, unlike
with mixed agonist/antagonists or non-opioid analgesics, where there is a limit
to the analgesic effect with increasing doses. With pure opioid agonist
analgesics, there is no defined maximum dose; the ceiling to analgesic
effectiveness is imposed only by side effects, the more serious which may
include somnolence and respiratory depression.
Central Nervous System
The principal actions of therapeutic value of morphine are
analgesia and sedation (i.e., sleepiness and anxiolysis).
The precise mechanism of the analgesic action is unknown.
However, specific CNS opiate receptors for endogenous compounds with
opioid-like activity have been identified throughout the brain and spinal cord
and are likely to play a role in the expression of analgesic effects.
Morphine produces respiratory depression by direct action on
brainstem respiratory centers. The mechanism of respiratory depression involves
a reduction in the responsiveness of the brainstem respiratory centers to
increases in carbon dioxide tension, and to electrical stimulation.
Morphine depresses the cough reflex by direct effect on the
cough center in the medulla. Antitussive effects may occur with doses lower
than those usually required for analgesia. Morphine causes miosis, even in
total darkness. Pinpoint pupils are a sign of narcotic overdose but are not
pathognomonic (e.g., pontine lesions of hemorrhagic or ischemic origins may
produce similar findings). Marked mydriasis rather than miosis may be seen with
worsening hypoxia.
Gastrointestinal Tract and Other Smooth Muscle
Morphine causes a reduction in motility associated with an
increase in smooth muscle tone in the antrum of the stomach and in the
duodenum. Digestion of food is delayed in the small intestine and propulsive
contractions are decreased. Propulsive peristaltic waves in the colon are
decreased, while tone may be increased to the point of spasm resulting in
constipation. Other opioid induced-effects may include a reduction in gastric,
biliary and pancreatic secretions, spasm of the sphincter of Oddi, and
transient elevations in serum amylase.
Cardiovascular System
Morphine produces peripheral vasodilation which may result
in orthostatic hypotension. Release of histamine can occur and may contribute
to opioid-induced hypotension. Manifestations of histamine release and/or
peripheral vasodilation may include pruritus, flushing, red eyes, and sweating.
Endocrine System
Opioids have been shown to have a variety of effects on the
secretion of hormones. Opioids inhibit the secretion of ACTH, cortisol, and
luteinizing hormone (LH) in humans. They also stimulate prolactin, growth hormone
(GH) secretion, and pancreatic secretion of insulin and glucagons in humans and
other species, rats and dogs. Thyroid stimulating hormone (TSH) has been shown
to be both inhibited and stimulated by opioids.
Immune System
Opioids have been shown to have a variety of effects on
components of the immune system in in vitro and animal models. The clinical
significance of these findings is unknown.
Pharmacodynamics
As with all opioids, the minimum effective plasma
concentration for analgesia varies widely among patients, especially among
patients who have been previously treated with potent agonist opioids. As a
result, patients must be treated with individualized titration of dosage to the
desired effect. The minimum effective analgesic concentration of morphine for
any individual patient may increase over time due to an increase in pain, the
development of new pain syndrome and/or the development of analgesic tolerance.
Plasma Level-Analgesia Relationships
In any particular patient, both analgesic effects and plasma
morphine concentrations are related to the morphine dose. In non-tolerant
individuals, plasma morphine concentration-efficacy relationships have been
demonstrated and suggest that opiate receptors occupy effector compartments,
leading to a lag-time, or hysteresis, between rapid changes in plasma morphine
concentrations and the effects of such changes. The most direct and predictable
concentration-effect relationships can, therefore, be expected at distribution
equilibrium and/or steady-state conditions.
While plasma morphine-efficacy relationships can be
demonstrated in non-tolerant individuals, they are influenced by a wide variety
of factors and are not generally useful as a guide to the clinical use of
morphine. The effective dose in opioid-tolerant patients may be significantly
greater than the appropriate dose for opioid-naive individuals. Dosages of
morphine should be chosen and must be titrated on the basis of clinical
evaluation of the patient and the balance between therapeutic and adverse
effects.
For any fixed dose and dosing interval, MS CONTIN® will have
at steady-state, a lower Cmax and a higher Cmin than conventional morphine.
Concentration - Adverse Experience Relationships
MS CONTIN® Tablets are associated with typical opioid-related
adverse experiences. There is a general relationship between increasing
morphine plasma concentration and increasing frequency of dose-related opioid
adverse experiences such as nausea, vomiting, CNS effects, and respiratory
depression. In opioid-tolerant patients, the situation is altered by the
development of tolerance to opioid-related side effects, and the relationship
is not clinically relevant.
As with all opioids, the dose must be individualized (see DOSAGE
AND ADMINISTRATION), because the effective analgesic dose for some patients
will be too high to be tolerated by other patients.
Pharmacokinetics And Metabolism
MS CONTIN is a controlled-release tablet containing morphine
sulfate. Morphine is released from MS CONTIN somewhat more slowly than from
immediate-release oral preparations. Following oral administration of a given
dose of morphine, the amount ultimately absorbed is essentially the same
whether the source is MS CONTIN or an immediate-release formulation. Because of
pre-systemic elimination (i.e., metabolism in the gut wall and liver) only
about 40% of the administered dose reaches the central compartment.
Variation in the physical/mechanical properties of a
formulation of an oral morphine drug product can affect both its absolute
bioavailability and its absorption rate constant (ka). The formulation employed
in MS CONTIN has not been shown to affect morphine's oral bioavailability, but
does decrease its apparent ka. Other basic pharmacokinetic parameters (e.g.,
volume of distribution [Vd], elimination rate constant [ke], clearance [Cl]),
are unchanged as they are fundamental properties of morphine in the organism.
However, in chronic use, the possibility that shifts in metabolite to parent
drug ratios may occur cannot be excluded.
When immediate-release oral morphine or MS CONTIN is given
on a fixed dosing regimen, steady-state is achieved in about a day.
For a given dose and dosing interval, the AUC and average
blood concentration of morphine at steady-state (Css) will be independent of
the specific type of oral formulation administered so long as the formulations
have the same absolute bioavailability. The absorption rate of a formulation
will, however, affect the maximum (Cmax) and minimum (Cmin) blood levels and
the times of their occurrence.
Absorption
Following the administration of immediate-release oral
morphine products, approximately fifty percent of the morphine that will reach
the central compartment intact reaches it within 30 minutes. Following the
administration of an equal amount of MS CONTIN to normal volunteers, however,
this extent of absorption occurs, on average, after 1.5 hours.
Food Effects
The possible effect of food upon the systemic
bioavailability of MS CONTIN® has not been systematically evaluated for all
strengths. One study, conducted with the 30 mg MS CONTIN Tablets, showed no
significant differences in Cmax and AUC (0-24h) values, whether the tablet was
taken while fasting or with a high-fat breakfast.
Distribution
The volume of distribution (Vd) for morphine is
approximately 4 liters per kilogram. Once absorbed, morphine is distributed to
skeletal muscle, kidneys, liver, intestinal tract, lungs, spleen, and brain.
Morphine also crosses the placental membranes and has been found in breast
milk.
Metabolism
Although a small fraction (less than 5%) of morphine is
demethylated, for all practical purposes, virtually all morphine is converted
to the 3- and 6- (M3G and M6G) glucuronide metabolites. M3G is present in the
highest plasma concentration following oral administration and possesses no
significant analgesic activity. M6G, while possessing analgesic activity, is
present in the plasma in low concentrations.
Excretion
The elimination of morphine occurs primarily as renal
excretion of morphine-3- glucuronide and its terminal elimination half-life
after intravenous administration is normally 2 to 4 hours. In some studies
involving longer periods of plasma sampling, a longer terminal half-life of
about 15 hours was reported. A small amount of the glucuronide conjugate is
excreted in the bile, and there is some minor enterohepatic recycling. As with
any drug, caution should be taken to guard against unanticipated accumulation
if renal and/or hepatic function is seriously impaired.
Special Populations
Renal Impairment
Morphine pharmacokinetics are altered in patients with renal
failure. Clearance is decreased and the metabolites, M3G and M6G, may
accumulate to much higher plasma levels in these patients as compared to
patients with normal renal function.
Drug-Drug Interactions
Known drug-drug interactions involving morphine are
pharmacodynamic not pharmacokinetic.
Last reviewed on RxList: 6/8/2010