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Pentazocine and Acetaminophen (Pentazocine Hydrochloride / Acetaminophen) - Description and Clinical Pharmacology

 
 



DESCRIPTION

Pentazocine and acetaminophen tablets are a combination of pentazocine hydrochloride, USP, equivalent to 25 mg base and acetaminophen, USP, 650 mg.

Pentazocine is a member of the benzazocine series (also known as the benzomorphan series). Chemically, pentazocine is (2R*,6R*,11R* )1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(3-methyl-2-butenyl)-2,6-methano-3-benzazocin-8-ol, hydrochloride, a white, crystalline substance soluble in acidic aqueous solutions, and it has the following structural formula:

C19H27NO HCl            Molecular Weight: 321.88

Chemically, acetaminophen is acetamide, N-(4-hydroxypheynyl)-, and it has the following structural formula:

C8H9NO2             Molecular Weight: 151.16

Pentazocine is an analgesic and acetaminophen is an analgesic and antipyretic.

Each tablet for oral administration contains 25 mg of pentazocine as the hydrochloride and 650 mg of acetaminophen. In addition, each tablet contains the following inactive ingredients: colloidal silicon dioxide, crospovidone, D&C Yellow No. 10 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake, microcrystalline cellulose, povidone, pregelatinized starch, sodium starch glycolate, and stearic acid.

CLINICAL PHARMACOLOGY

Pentazocine is a Schedule IV opioid analgesic with agonist/antagonist action which when administered orally is approximately equivalent on a mg for mg basis in analgesic effect to codeine.

Acetaminophen is an analgesic and antipyretic.

Pentazocine weakly antagonizes the analgesic effects of morphine, meperidine, and phenazocine; in addition, it produces incomplete reversal of cardiovascular, respiratory, and behavioral depression induced by morphine and meperidine. Pentazocine has about 1/50 the antagonistic activity of nalorphine. It also has a sedative activity.

Onset of significant analgesia with pentazocine usually occurs between 15 and 30 minutes after oral administration, and duration of action is usually three hours or longer.

Pentazocine is well absorbed from the gastrointestinal tract. Plasma levels closely correspond to the onset, duration, and intensity of analgesia. The time to mean peak concentration in 24 normal volunteers was 1.7 hours (range 0.5 to 4 hours) after oral administration and the mean plasma elimination half-life was 3.6 hours (range 1.5 to 10 hours).

The action of pentazocine is terminated for the most part by biotransformation in the liver with some free pentazocine excreted in the urine. The products of the oxidation of the terminal methyl groups and glucuronide conjugates are excreted by the kidney. Elimination of approximately 60% of the total dose occurs within 24 hours. Pentazocine passes into fetal circulation.

Onset of significant analgesic and antipyretic activity of acetaminophen when administered orally occurs within 30 minutes and is maximal at approximately 2 1/2 hours. The pharmacological mode of action of acetaminophen is unknown at this time.

Acetaminophen is rapidly and almost completely absorbed from the gastrointestinal tract. In 24 normal volunteers the mean time to peak plasma concentration was 1 hour (range 0.25 to 3 hours) after oral administration and the mean plasma elimination half-life was 2.8 hours (range 2 to 4 hours).

The effect of pentazocine on acetaminophen plasma protein binding or vice versa has not been established. For acetaminophen there is little or no plasma protein binding at normal therapeutic doses. When toxic doses of acetaminophen are ingested and drug plasma levels exceed 90 mcg/mL, plasma binding may vary from 8% to 43%.

Acetaminophen is conjugated in the liver with glucuronic acid and to a lesser extent with sulfuric acid. Approximately 80% of acetaminophen is excreted in the urine after conjugation and about 3% is excreted unchanged. The drug is also conjugated to a lesser extent with cysteine and additionally metabolized by hydroxylation.

If pentazocine and acetaminophen tablets are taken every 4 hours over an extended period of time, accumulation of pentazocine and to a lesser extent, acetaminophen, may occur.

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