Propoxyphene and Acetaminophen

PROPOXYPHENE AND ACETAMINOPHEN — propoxyphene napsylate and acetaminophen tablet, film coated
Altura Pharmaceuticals, Inc.

WARNINGS

  • There have been numerous cases of accidental and intentional overdose with propoxyphene products either alone or in combination with other CNS depressants, including alcohol. Fatalities within the first hour of overdosage are not uncommon. Many of the propoxyphene-related deaths have occurred in patients with previous histories of emotional disturbances or suicidal ideation/attempts and/or concomitant administration of sedatives, tranquilizers, muscle relaxants, antidepressants, or other CNS-depressant drugs. Do not prescribe propoxyphene for patients who are suicidal or have a history of suicidal ideation.
  • The metabolism of propoxyphene may be altered by strong CYP3A4 inhibitors (such as ritonavir, ketoconazole, itraconazole, troleandomycin, clarithromycin, nelfinavir, nefazadone, amiodarone, amprenavir, aprepitant, diltiazem, erythromycin, fluconazole, fosamprenavir, grapefruit juice, and verapamil) leading to enhanced propoxyphene plasma levels. Patients receiving propoxyphene and any CYP3A4 inhibitor should be carefully monitored for an extended period of time and dosage adjustments should be made if warranted (see CLINICAL PHARMACOLOGY – Drug Interactions, and WARNINGS, PRECAUTIONS and DOSAGE AND ADMINISTRATIONfor further information).

DESCRIPTION

Propoxyphene Napsylate, USP is an odorless, white crystalline powder with a bitter taste. It is very slightly soluble in water and soluble in methanol, ethanol, chloroform, and acetone. Chemically, it is (αS ,1R)-α-[2-(Dimethylamino)-1-methylethyl]-α-phenylphenethyl propionate compound with 2-napthalenesulfonic acid (1:1) monohydrate, which can be represented by the accompanying structural formula.

This is an image of the structural formula for Propoxyphene Napsylate.
(click image for full-size original)

Propoxyphene napsylate differs from propoxyphene hydrochloride in that it allows more stable liquid dosage forms and tablet formulations. Because of differences in molecular weight, a dose of 100 mg (176.8 μmol) of propoxyphene napsylate is required to supply an amount of propoxyphene equivalent to that present in 65 mg (172.9 μmol) of propoxyphene hydrochloride.

Acetaminophen, 4′-hydroxyacetanilide, is a non-opiate, non-salicylate analgesic and antipyretic which occurs as a white, odorless, crystalline powder, possessing a slightly bitter taste. The molecular formula for acetaminophen is C8 H9 NO2 and the molecular weight is 151.16. It may be represented by the following structural formula.

This is an image of the structural formula of Acetaminophen.
(click image for full-size original)

Propoxyphene Napsylate and Acetaminophen Tablets, USP 50 mg/325 mg
Each tablet contains:
Propoxyphene Napsylate ……………………….50 mg (88.4 μmol)
Acetaminophen …………………………………..325 mg (2150 μmol)
In addition each tablet contains the following inactive ingredients: carnauba wax, D&C Yellow #10 Aluminum Lake, FD&C Red #40 Aluminum Lake, hypromellose, lactose monohydrate, magnesium stearate, polyethylene glycol, polysorbate 80, sodium starch glycolate and titanium dioxide.

Propoxyphene Napsylate and Acetaminophen Tablets, USP 100 mg/650 mg
Each tablet contains:
Propoxyphene Napsylate ………………………100 mg (176.8 μmol)
Acetaminophen ……………………………………650 mg (4300 μmol)
In addition each tablet contains the following inactive ingredients: carnauba wax, hypromellose, lactose monohydrate, magnesium stearate, polyethylene glycol, polysorbate 80, sodium starch glycolate and titanium dioxide. The orange tablets also contain D&C Yellow #10 Aluminum Lake and FD&C Red #40 Aluminum Lake. The pink tablets also contain FD&C Red #40 Aluminum Lake.

CLINICAL PHARMACOLOGY

Pharmacology

Propoxyphene is a centrally acting opiate analgesic. In vitro studies demonstrated propoxyphene and the metabolite norpropoxyphene inhibit sodium channels (local anesthetic effect) with norpropoxyphene being approximately 2-fold more potent than propoxyphene and propoxyphene approximately 10-fold more potent than lidocaine. Propoxyphene and norpropoxyphene inhibit the voltage-gated potassium current carried by cardiac rapidly activating delayed rectifier (hERG) channels with approximately equal potency. It is unclear if the effects on ion channels occur within therapeutic dose range.

Acetaminophen is a non-opiate, non-salicylate analgesic and antipyretic. The site and mechanism for the analgesic effect of acetaminophen has not been determined. The antipyretic effect of acetaminophen is mediated through activity in the hypothalamic heat-regulating centers. Acetaminophen inhibits prostaglandin synthetase. Therapeutic doses of acetaminophen have negligible effects on the cardiovascular or respiratory systems; however, toxic doses may cause circulatory failure and rapid, shallow breathing.

Pharmacokinetics

Absorption

Peak plasma concentrations of propoxyphene are reached in 2 to 2.5 h. After a 65-mg oral dose of propoxyphene hydrochloride, peak plasma levels of 0.05 to 0.1 μg/mL for propoxyphene and 0.1 to 0.2 μg/mL for norpropoxyphene (major metabolite) are achieved. Repeated doses of propoxyphene at 6 h intervals lead to increasing plasma concentrations, with a plateau after the ninth dose at 48 h. Propoxyphene has a half-life of 6 to 12 h, whereas that of norpropoxyphene is 30 to 36 h.

Acetaminophen is absorbed from the gastrointestinal tract and has a plasma half-life of 1.25 to 3 h, which may be increased by liver damage and following overdosage.

Distribution

Propoxyphene is about 80% bound to proteins and has a large volume of distribution, 16 L/kg.

Acetaminophen is relatively uniformly distributed throughout most body fluids. Binding of the drug to plasma proteins is variable; only 20% to 50% may be bound at the concentrations encountered during acute intoxication.

Metabolism

Propoxyphene undergoes extensive first-pass metabolism by intestinal and hepatic enzymes. The major route of metabolism is cytochrome CYP3A4 mediated N-demethylation to norpropoxyphene, which is excreted by the kidneys. Ring hydroxylation and glucuronide formation are minor metabolic pathways.

Acetaminophen is extensively metabolized in the liver. Less than 5% of acetaminophen dose is excreted unchanged in the kidney. About 85% of an acetaminophen dose is metabolized by conjugation, mainly glucuronidation via UDP-glucuronosyltransferase (mainly UGT1A6) and to a lesser extent sulfation via sulfotransferase (mainly SLT1A1 and SLT1A3). The glucuronide and sulfate conjugates are nontoxic and are largely excreted in the urine and bile. About 8-10% of an acetaminophen dose is oxidized by cytochrome CYP2E1 to form the toxic reactive intermediate, N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is further metabolized via glutathione (GSH) conjugation, yielding non-toxic thiol metabolites including cysteine, mercapturate, methylthioacetaminophen, and methanesulfinylacetaminophen that are excreted in the urine. Acetaminophen is also oxidized at a low percentage by cytochrome CYP2A6 to form inert catechols (e.g., methoxyacetaminophen).

Excretion

In 48 h, approximately 20 to 25% of the administered dose of propoxyphene is excreted via the urine, most of which is free or conjugated norpropoxyphene. The renal clearance rate of propoxyphene is 2.6 L/min.

Elimination of acetaminophen is principally by liver metabolism (conjugation) and subsequent renal excretion of metabolites. Approximately 85% of an oral dose appears in the urine within 24 hours of administration, most as the glucuronide conjugate, with small amounts of other conjugates and unchanged drug.

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