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Alfentanil (Alfentanil Hydrochloride) - Description and Clinical Pharmacology


INJECTION, USP                           


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Alfentanil Injection, USP is an opioid analgesic chemically designated as N-[1-[2-(4-ethyl-4,5-dihydro -5-oxo-1H-tetrazol-1-yl) ethyl]-4-(methoxymethyl) -4-piperidinyl] -N- phenylpropanamide monohydrochloride (1:1) with a molecular weight of 452.98 and an n-octanol:water partition coefficient of 128:1 at pH 7.4. The structural formula of Alfentanil hydrochloride is:

Alfentanil Injection, USP is a sterile, non-pyrogenic, preservative free aqueous solution containing alfentanil hydrochloride equivalent to 500 mcg per mL of alfentanil base for intravenous injection. The solution, which contains sodium chloride for isotonicity, has a pH range of 4.0 to 6.0.


Alfentanil is an opioid analgesic with a rapid onset of action.

At doses of 8-40 mcg/kg for surgical procedures lasting up to 30 minutes, alfentanil provides analgesic protection against hemodynamic responses to surgical stress with recovery times generally comparable to those seen with equipotent fentanyl dosages.

For longer procedures, doses of up to 75 mcg/kg attenuate hemodynamic responses to laryngoscopy, intubation and incision, with recovery time comparable to fentanyl. At doses of 50-75 mcg/kg followed by a continuous infusion of 0.5-3 mcg/kg/min, alfentanil attenuates the catecholamine response with more rapid recovery and reduced need for postoperative analgesics as compared to patients administered enflurane. At doses of 5 mcg/kg, alfentanil provides analgesia for the conscious but sedated patient. Based on patient response, doses higher than 5 mcg/kg may be needed. Elderly or debilitated patients may require lower doses. High intrasubject and intersubject variability in the pharmacokinetic disposition of alfentanil has been reported.

The pharmacokinetics of alfentanil can be described as a three-compartment model with sequential distribution half-lives of 1 and 14 minutes; and a terminal elimination half-life of 90‑111 minutes (as compared to a terminal elimination half-life of approximately 475 minutes for fentanyl and approximately 265 minutes for sufentanil at doses of 250 mcg). The liver is the major site of biotransformation.

Alfentanil has an apparent volume of distribution of 0.4-1 L/kg, which is approximately one-fourth to one-tenth that of fentanyl, with an average plasma clearance of 5 mL/kg/min as compared to approximately 8 mL/kg/min for fentanyl.

Only 1% of the dose is excreted as unchanged drug; urinary excretion is the major route of elimination of metabolites. Plasma protein binding of alfentanil is approximately 92%.

In one study involving 15 patients administered alfentanil with nitrous oxide/oxygen, a narrow range of plasma alfentanil concentrations, approximately 310-340 ng/mL, was shown to provide adequate anesthesia for intra-abdominal surgery, while lower concentrations, approximately 190 ng/mL, blocked responses to skin closure. Plasma concentrations between 100-200 ng/mL provided adequate anesthesia for superficial surgery.

Alfentanil has an immediate onset of action. At dosages of approximately 105 mcg/kg, alfentanil produces hypnosis as determined by EEG patterns; an anesthetic ED90 of 182 mcg/kg for alfentanil in unpremedicated patients has been determined, based upon the ability to block response to placement of a nasopharyngeal airway. Based on clinical trials, induction dosage requirements range from 130-245 mcg/kg. For procedures lasting 30-60 minutes, loading dosages of up to 50 mcg/kg produce the hemodynamic response to endotracheal intubation and skin incision as comparable to those from fentanyl. A pre-intubation loading dose of 50-75 mcg/kg prior to a continuous infusion attenuates the response to laryngoscopy, intubation and incision. Subsequent administration of alfentanil infusion administered at a rate of 0.5-3 mcg/kg/min with nitrous oxide/oxygen attenuates sympathetic responses to surgical stress with more rapid recovery than enflurane.

Requirements for volatile inhalation anesthetics were reduced by thirty to fifty percent during the first 60 minutes of maintenance in patients administered anesthetic doses (above 130 mcg/kg) of alfentanil as compared to patients given doses of 4-5 mg/kg thiopental for anesthetic induction. At anesthetic induction dosages, alfentanil provides a deep level of anesthesia during the first hour of anesthetic maintenance and provides attenuation of the hemodynamic response during intubation and incision.

Following an anesthetic induction dose of alfentanil, requirements for alfentanil infusion are reduced by 30 to 50% for the first hour of maintenance.

Patients with compromised liver function and those over 65 years of age have been found to have reduced plasma clearance and extended terminal elimination for alfentanil, which may prolong postoperative recovery. Repeated or continuous administration of alfentanil produces increasing plasma concentrations and an accumulation of the drug, particularly in patients with reduced plasma clearance.

Bradycardia may be seen in patients administered alfentanil. The incidence and degree of bradycardia may be more pronounced when alfentanil is administered in conjunction with non-vagolytic neuromuscular blocking agents or in the absence of anticholinergic agents such as atropine.

Administration of intravenous diazepam immediately prior to or following high doses of alfentanil has been shown to produce decreases in blood pressure that may be secondary to vasodilation; recovery may also be prolonged.

Patients administered doses up to 200 mcg/kg of alfentanil have shown no significant increase in histamine levels and no clinical evidence of histamine release.

Skeletal muscle rigidity is related to the dose and speed of administration of alfentanil. Muscular rigidity will occur with an immediate onset following anesthetic induction dosages. Preventative measures (see WARNINGS) may reduce the rate and severity.

The duration and degree of respiratory depression and increased airway resistance usually increase with dose, but have also been observed at lower doses. Although higher doses may produce apnea and a longer duration of respiratory depression, apnea may also occur at low doses.

During monitored anesthesia care (MAC), attention must be given to the respiratory effects of alfentanil. Decreased oxygen saturation, apnea, decreased respiratory rate, and upper airway obstruction can occur. (See WARNINGS)

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