Gablofen (Page 4 of 5)

7 DRUG INTERACTIONS

There is inadequate systematic experience with the use of intrathecal baclofen in combination with other medications to predict specific drug-drug interactions. Interactions attributed to the combined use of GABLOFEN and epidural morphine include hypotension and dyspnea.

8 USE IN SPECIFIC POPULATIONS

8.1 Pregnancy

Risk Summary

There are no adequate data on the developmental risk associated with the use of GABLOFEN in pregnant women.

In animal studies, oral administration of baclofen to pregnant rats produced an increase in fetal malformations (see Data). There are no animal data on developmental risk associated with baclofen administered via continuous intrathecal infusion.

The background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2 to 4% and 15 to 20%, respectively.

Data

Animal Data

Baclofen given orally to pregnant rats has been shown to increase the incidence of omphaloceles (ventral hernias) in fetuses at a dose associated with maternal toxicity. This abnormality was not seen in mice or rabbits.

8.2 Lactation

Risk Summary

There is insufficient information regarding levels of baclofen in milk of nursing mothers receiving GABLOFEN. There are no adequate data on the effects of GABLOFEN on the breastfed infant or on milk production. At recommended oral doses, baclofen is present in human milk and withdrawal symptoms can occur in breastfed infants when maternal administration of baclofen is stopped or when breastfeeding is stopped.

The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for GABLOFEN and any potential adverse effects on the breastfed infant from GABLOFEN or from the underlying maternal condition.

8.4 Pediatric Use

Children should be of sufficient body mass to accommodate the implantable pump for chronic infusion. Please consult pump manufacturer’s manual for specific recommendations.

Safety and effectiveness in pediatric patients below the age of 4 have not been established.

10 OVERDOSAGE

Special attention must be given to recognizing the signs and symptoms of overdosage, especially during the initial screening and dose-titration phase of treatment, but also during re-introduction of GABLOFEN after a period of interruption in therapy.

Symptoms of Intrathecal Baclofen Overdose

Drowsiness, lightheadedness, dizziness, somnolence, respiratory depression, seizures, rostral progression of hypotonia and loss of consciousness progressing to coma of up to 72 hours duration. In most cases reported, coma was reversible without sequelae after drug was discontinued. Symptoms of intrathecal baclofen overdose were reported in a sensitive adult patient after receiving a 25 mcg intrathecal bolus.

Treatment Suggestions for Overdose

There is no specific antidote for treating overdoses of GABLOFEN; however, the following steps should ordinarily be undertaken:

1) Residual intrathecal baclofen solution should be removed from the pump as soon as possible.

2) Patients with respiratory depression should be intubated if necessary, until the drug is eliminated.

Anecdotal reports suggest that intravenous physostigmine may reverse central side effects, notably drowsiness and respiratory depression. Caution in administering physostigmine is advised, however, because its use has been associated with the induction of seizures and bradycardia.

Physostigmine Doses for Adult Patients

Administer 2 mg of physostigmine intramuscularly or intravenously at a slow controlled rate of no more than 1 mg per minute. Dosage may be repeated if life-threatening signs, such as arrhythmia, convulsions or coma occur.

Physostigmine Doses for Pediatric Patients

Administer 0.02 mg/kg physostigmine intramuscularly or intravenously, do not give more than 0.5 mg per minute. The dosage may be repeated at 5 to 10 minute intervals until a therapeutic effect is obtained or a maximum dose of 2 mg is attained.

Physostigmine may not be effective in reversing large overdoses and patients may need to be maintained with respiratory support.

If lumbar puncture is not contraindicated, consideration should be given to withdrawing 30 to 40 mL of CSF to reduce CSF baclofen concentration.

11 DESCRIPTION

GABLOFEN (baclofen injection) is a muscle relaxant and antispastic. Baclofen’s pharmacological class is a gamma-aminobutyric acid (GABA) ergic agonist. Baclofen’s chemical name is 4-amino-3-(4-chlorophenyl) butanoic acid, and its structural formula is:

Baclofen

structure
(click image for full-size original)

C 10 H 12 ClNO 2

Baclofen is a white to off-white powder, with a molecular weight of 213.66. It is slightly soluble in water, very slightly soluble in methanol and ethanol, practically insoluble in acetone and ether, soluble in 0.1N hydrochloric acid, 0.1N sodium hydroxide, and insoluble in chloroform.

GABLOFEN is a sterile, pyrogen-free, isotonic solution free of antioxidants, preservatives or other potentially neurotoxic additives indicated only for intrathecal administration. The drug is stable in solution at 37°C and compatible with CSF. Each mL of GABLOFEN contains baclofen USP 50 mcg, 500 mcg, 1,000 mcg or 2,000 mcg and sodium chloride 9 mg in Water for Injection; pH range is 5.5 to 7.5.

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

The precise mechanism of action of baclofen as a muscle relaxant and antispasticity agent is not fully understood. Baclofen inhibits both monosynaptic and polysynaptic reflexes at the spinal level, possibly by decreasing excitatory neurotransmitter release from primary afferent terminals, although actions at supraspinal sites may also occur and contribute to its clinical effect. Baclofen is a structural analog of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and may exert its effects by stimulation of the GABAB receptor subtype. Baclofen when introduced directly into the intrathecal space permits effective CSF concentrations to be achieved with resultant plasma concentrations 100 times less than those occurring with oral administration. In people, as well as in animals, baclofen has been shown to have general CNS depressant properties as indicated by the production of sedation with tolerance, somnolence, ataxia, and respiratory and cardiovascular depression.

12.2 Pharmacodynamics

Intrathecal Bolus
Adult Patients
The onset of action is generally one-half hour to one hour after an intrathecal bolus. Peak spasmolytic effect is seen at approximately four hours after dosing and effects may last four to eight hours. Onset, peak response, and duration of action may vary with individual patients depending on the dose and severity of symptoms.
Pediatric Patients
The onset, peak response and duration of action are similar to those seen in adult patients.
Continuous Infusion
Adult Patients
Intrathecal baclofen’s antispastic action is first seen at 6 to 8 hours after initiation of continuous infusion. Maximum activity is observed in 24 to 48 hours.
Pediatric Patients No additional information on continuous infusions is available for pediatric patients.

12.3 Pharmacokinetics

The pharmacokinetics of cerebrospinal fluid (CSF) clearance of intrathecal baclofen calculated from intrathecal bolus or continuous infusion studies approximates CSF turnover, suggesting elimination is by bulk-flow removal of CSF.
Intrathecal Bolus
After a bolus lumbar injection of 50 mcg or 100 mcg intrathecal baclofen in seven patients, the average CSF elimination half-life was 1.51 hours over the first four hours and the average CSF clearance was approximately 30 mL/hour.
Continuous Infusion
The mean CSF clearance for intrathecal baclofen was approximately 30 mL/hour in a study involving ten patients on continuous intrathecal infusion. Concurrent plasma concentrations of baclofen during intrathecal administration are expected to be low (0 to 5 ng/mL). Limited pharmacokinetic data suggest that a lumbar-cisternal concentration gradient of about 4:1 is established along the neuroaxis during baclofen infusion. This is based upon simultaneous CSF sampling via cisternal and lumbar tap in 5 patients receiving continuous baclofen infusion at the lumbar level at doses associated with therapeutic efficacy; the interpatient variability was great. The gradient was not altered by position. Six pediatric patients (age 8 to 18 years) receiving continuous intrathecal baclofen infusion at doses of 77 to 400 mcg/day had plasma baclofen levels near or below 10 ng/mL.

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