Morphine, a pure opioid agonist, is relatively selective for the mu receptor,
although it can interact with other opioid receptors at higher doses. In addition
to analgesia, the widely diverse effects of morphine include drowsiness, changes
in mood, respiratory depression, decreased gastrointestinal motility, nausea,
vomiting and alterations of the endocrine and autonomic nervous system.
Effects on the Central Nervous System (CNS)
The principal therapeutic action of morphine is analgesia. Other therapeutic
effects of morphine include anxiolysis, euphoria and feelings of relaxation.
Although the precise mechanism of the analgesic action is unknown, specific
CNS opiate receptors and endogenous compounds with morphine-like activity have
been identified throughout the brain and spinal cord and are likely to play
a role in the expression and perception of analgesic effects. In common with
other opi-oids, morphine causes respiratory depression, in part by a direct
effect on the brainstem respiratory centers. Morphine and related opioids depress
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
opioid overdose; however, when asphyxia is present during opioid overdose, marked
mydriasis occurs.
Effects on the Gastrointestinal Tract and on Other Smooth Muscle
Gastric, biliary and pancreatic secretions are decreased by morphine. Morphine
causes a reduction in motility and is associated with an increase in tone in
the antrum of the stomach and duodenum. Digestion of food in the small intestine
is delayed and propulsive contractions are decreased. Propulsive peristaltic
waves in the colon are decreased, while tone is increased to the point of spasm.
The end result may be constipation. Morphine can cause a marked increase in
biliary tract pressure as a result of spasm of the sphincter of Oddi. Morphine
may also cause spasm of the sphincter of the urinary bladder.
Effects on the Cardiovascular System
In therapeutic doses, morphine does not usually exert major effects on the
cardiovascular system. Morphine produces peripheral vasodilation which may result
in orthostatic hypotension and fainting. Release of histamine can occur, which
may play a role in opioid-induced hypotension. Manifestations of histamine release
and/or peripheral vasodilation may include pruritus, flushing, red eyes and
sweating.
Pharmacodynamics
Morphine concentrations are not predictive of analgesic response, especially
in patients previously treated with opioids. The minimum effective concentration
varies widely and is influenced by a variety of factors, including the extent
of previous opioid use, age, and general medical condition. Effective doses
in tolerant patients may be significantly higher than in opioid-naïve patients.
In all patients, the dose of morphine should be titrated on the basis of clinical
evaluation of the patient and to achieve a balance between therapeutic and adverse
effects.
Pharmacokinetics
AVINZA consists of two components, an immediate release component that rapidly
achieves plateau morphine plasma concentrations and an extended release component
that maintains plasma concentrations throughout the 24-hour dosing interval.
The amount of morphine absorbed from AVINZA following oral administration is
similar to that absorbed from other oral morphine formulations.
The oral bioavailability of morphine is less than 40% and shows large inter-individual
variability due to extensive pre-systemic metabolism.
Absorption
Following single-dose oral administration of a 60 mg dose of AVINZA under fasting
conditions, morphine concentrations of approximately 3 to 6 ng/ml were achieved
within 30 minutes after dosing and maintained for the 24-hour dosing interval.
The pharmacokinet-ics of AVINZA were shown to be dose-proportional over a single
oral dose range of 30 to 120 mg in healthy volunteers and a multiple oral dose
range of at least 30 to 180 mg in patients with chronic moderate to severe pain.
Food Effects
When a 60 mg dose of AVINZA was administered immediately following a high fat
meal, peak morphine concentrations and AUC values were similar to those observed
when the dose of AVINZA was administered in a fasting state, although achievement
of initial concentrations was delayed by approximately 1 hour under fed conditions.
Therefore, AVINZA can be administered without regard to food. When the contents
of AVINZA were administered by sprinkling on applesauce, the rate and extent
of morphine absorption were found to be bioequivalent to the same dose when
administered as an intact capsule.
Steady State
When dosed once-daily, AVINZA steady-state pharmacokinetics are characterized
by a plateau-like plasma concentration profile. Steady-state plasma concentrations
of morphine are achieved 2 to 3 days after initiation of once-daily administration
of AVINZA.
AVINZA 60 mg Capsules (once-daily) and 10 mg morphine oral solution (6 times
daily) were equally bioavailable.
Graph 1: Mean Steady-State Plasma Morphine Concentrations
Following Once – Daily Administration of AVINZA Capsules or 6-Times Daily Administration
of Morphine Solution
A once-daily dose of AVINZA provided similar Cmax, Cmin, and AUC values and
peak-trough fluctuations (% FL, Cmax-Cmin/Cav) compared to 6-times daily administration
of the same total daily dose of morphine oral solution (Table 1).
Table 1 Pharmacokinetic Data Mean
| Parameter |
AVINZA Capsules
One-Daily |
Morphine Oral Solution
6-Times Daily |
| AUC (ng/ml.h) |
273.25 ± 81.24 |
279.11 ± 63.00 |
| Cmax (ng/ml) |
18.65 ± 7.13 |
19.96 ± 4.82 |
| Cmin (ng/ml) |
6.98 ± 2.44 |
6.61 ± 2.15 |
| % FL |
106.38 ± 78.14 |
116.22 ± 26.67 |
Distribution
Once absorbed, morphine is distributed to skeletal muscle, kidneys, liver,
intestinal tract, lungs, spleen and brain. Although the primary site of action
is the CNS, only small quantities cross the blood-brain barrier. Morphine also
crosses the placental membranes and has been found in breast milk. The volume
of distribution of morphine is approximately 1 to 6 L/kg, and morphine is 20
to 35% reversibly bound to plasma proteins.
Metabolism
The major pathway of morphine detoxification is conjugation, either with D-glucuronic
acid to produce glucuronides or with sulfuric acid to produce morphine-3-etheral
sulfate. While a small fraction (less than 5%) of morphine is demethylated,
virtually all morphine is converted by hepatic metabolism to the 3- and 6-glucuronide
metabolites (M3G and M6G; about 50% and 15%, respectively). M6G has been shown
to have analgesic activity but crosses the blood-brain barrier poorly, while
M3G has no significant analgesic activity.
Excretion
Most of a dose of morphine is excreted in urine as M3G and M6G, with elimination
of morphine occurring primarily as renal excretion of M3G. Approximately 10%
of the dose is excreted unchanged in urine. A small amount of the glucuronide
conjugates are excreted in bile, with minor enterohepatic recycling. Seven to
10% of administered morphine is excreted in the feces.
The mean adult plasma clearance is approximately 20 to 30 ml/min/kg. The effective
terminal half-life of morphine after IV administration is reported to be approximately
2 hours. In some studies involving longer periods of plasma sampling, a longer
terminal half-life of morphine of about 15 hours was reported.
In Vitro AVINZA-Alcohol Interaction
In vitro studies performed by the FDA demonstrated that when AVINZA
30 mg was mixed with 900 mL of buffer solutions containing ethanol (20% and
40%), the dose of morphine that was released was alcohol concentration-dependent,
leading to a more rapid release of morphine. While the relevance of in vitro
lab tests regarding AVINZA to the clinical setting remains to be determined,
this acceleration of release may correlate with in vivo rapid release of the
total morphine dose, which could result in the absorption of a potentially fatal
dose of morphine.
Special Populations
Geriatric
Elderly patients (aged 65 years or older) may have increased sensitivity to
morphine. AVINZA pharmacokinetics have not been studied specifically in elderly
patients.
Nursing Mothers
Low levels of morphine sulfate have been detected in maternal milk. The milk:
plasma morphine AUC ratio is about 2.5:1. The amount of morphine delivered to
the infant depends on the plasma concentration of the mother, the amount of
milk ingested by the infant, and the extent of first-pass metabolism.
Pediatric
The pharmacokinetics of AVINZA have not been studied in pediatric patients
below the age of 18.The range of dose strengths available may not be appropriate
for treatment of very young pediatric patients. Sprinkling on applesauce is
NOT a suitable alternative for these patients.
Gender
A gender analysis of pharmacokinetic data from healthy subjects taking AVINZA
indicated that morphine concentrations were similar in males and females.
Race
There may be some pharmacokinetic differences associated with race. In one
published study, Chinese subjects given intravenous morphine had a higher clearance
when compared to Caucasian subjects (1852 +/- 116 ml/min compared to 1495 +/-
80 ml/min).
Hepatic Failure
Morphine pharmacokinetics have been reported to be significantly altered in
patients with cirrhosis. Clearance was found to decrease with a corresponding
increase in half-life. The M3G and M6G to morphine plasma AUC ratios also decreased
in these subjects, indicating diminished metabolic activity.
Renal Insufficiency
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 patients with renal failure as compared to patients with normal
renal function.
Drug-Drug Interactions
Known drug-drug interactions involving morphine are pharma-codynamic, not pharmacokinetic.
(see PRECAUTIONS: DRUG INTERACTIONS)
Clinical Studies
AVINZA was studied in over 140 healthy volunteers and 560 patients with chronic,
moderate to severe pain who participated in 6 pharmacokinetic studies, 4 clinical
studies and 3 studies which provided both pharmacokinetic and clinical data.
The patient population included those who were either receiving chronic opioid
therapy or had a prior sub-optimal response to acetaminophen and/or NSAID therapy,
as well as patients who previously received intermittent opioid analgesic therapy.
In the controlled clinical studies, patients were followed from 7 days to up
to 4 weeks, and in the open label studies, patients were followed for up to
6 to 12 months.
AVINZA was studied in a double-blind, placebo-controlled, fixed-dose, parallel
group trial in 295 patients with moderate to severe pain due to osteoarthritis.
These patients had either a prior sub-optimal response to acetaminophen, NSAID
therapy, or previously received intermittent opioid analgesic therapy. Thirty-milligrams
AVINZA capsules administered once-daily, either in the morning or the evening,
were more effective than placebo in reducing pain.
Table 2 : Change from Baseline in WOMAC OA Index Pain VAS
Subscale Score
| Overall |
Placebo |
AVINZA QAM |
AVINZA QPM |
| LS Mean |
-36.23 |
-75.26* |
-75.39* |
| Std. Error |
11.482 |
11.305 |
11.747 |
| *P < 0.05; REPEATED MEASURES ANALYSIS |
This study was not designed to assess the effects of AVINZA on the course of
the osteoarthritis.
Last updated on RxList: 2/26/2009