Fentanyl citrate is a potent narcotic analgesic. Each milliliter of solution contains fentanyl (as the citrate) 50 mcg (0.05 mg), adjusted to pH 4.0 to 7.5 with sodium hydroxide. Fentanyl citrate is chemically identified as N -(1-Phenethyl-4-piperidyl) propionanilide citrate (1:1) with a molecular weight of 528.61. The structural formula of fentanyl citrate is:
Fentanyl citrate injection is a sterile, nonpyrogenic, preservative free aqueous solution for intravenous or intramuscular injection.
Fentanyl citrate is a narcotic analgesic. A dose of 100 mcg (0.1 mg) (2 mL) is approximately equivalent in analgesic activity to 10 mg of morphine or 75 mg of meperidine. The principal actions of therapeutic value are analgesia and sedation. Alterations in respiratory rate and alveolar ventilation, associated with narcotic analgesics, may last longer than the analgesic effect. As the dose of narcotic is increased, the decrease in pulmonary exchange becomes greater. Large doses may produce apnea. Fentanyl appears to have less emetic activity than either morphine or meperidine. Histamine assays and skin wheal testing in man indicate that clinically significant histamine release rarely occurs with fentanyl. Recent assays in man show no clinically significant histamine release in dosages up to 50 mcg/kg (0.05 mg/kg) (1 mL/kg). Fentanyl preserves cardiac stability, and blunts stress-related hormonal changes at higher doses.
The pharmacokinetics of fentanyl can be described as a three-compartment model, with a distribution time of 1.7 minutes, redistribution of 13 minutes, and a terminal elimination half-life of 219 minutes. The volume of distribution for fentanyl is 4 L/kg.
Fentanyl plasma protein binding capacity decreases with increasing ionization of the drug. Alterations in pH may affect its distribution between plasma and the central nervous system. It accumulates in skeletal muscle and fat, and is released slowly into the blood. Fentanyl, which is primarily transformed in the liver, demonstrates a high first pass clearance and releases approximately 75% of an intravenous dose in urine, mostly as metabolites with less than 10% representing the unchanged drug. Approximately 9% of the dose is recovered in the feces, primarily as metabolites.
The onset of action of fentanyl is almost immediate when the drug is given intravenously; however, the maximal analgesic and respiratory depressant effect may not be noted for several minutes. The usual duration of action of the analgesic effect is 30 to 60 minutes after a single intravenous dose of up to 100 mcg (0.1 mg) (2 mL). Following intramuscular administration, the onset of action is from seven to eight minutes, and the duration of action is one to two hours. As with longer acting narcotic analgesics, the duration of the respiratory depressant effect of fentanyl may be longer than the analgesic effect. The following observations have been reported concerning altered respiratory response to CO2 stimulation following administration of fentanyl citrate to man.
DIMINISHED SENSITIVITY TO CO2 STIMULATION MAY PERSIST LONGER THAN DEPRESSION OF RESPIRATORY RATE. (Altered sensitivity to CO2 stimulation has been demonstrated for up to four hours following a single dose of 600 mcg (0.6 mg) (12 mL) fentanyl to healthy volunteers.) Fentanyl frequently slows the respiratory rate, duration, and degree of respiratory depression being dose related.
The peak respiratory depressant effect of a single intravenous dose of fentanyl citrate is noted 5 to 15 minutes following injection. See also WARNINGS and PRECAUTIONS concerning respiratory depression.