Centrally acting antitussive agents

SYNTHESIS AND DRUG PROFILE

Centrally acting antitussive agents

i. Dextromethorphan HBr

Synthesis

Properties and uses: Dextromethorphan hydrobromide (HBr) is a white crystalline powder, sparingly soluble in water, and soluble in alcohol. It is a substituted dextro isomer of the opioid levomethorphan.

It is not an analgesic or an addictive. Dextromethorphan HBr should not be given to patients who are on monoamine oxidase inhibitors therapy. It is an opioid receptor agonist used as a cough suppressant.

Assay: Dissolve the sample in a mixture of 0.01 M hydrochloric acid and alcohol (1:4) and titrate against 0.1 M sodium hydroxide. Determine the end point potentiometrically. Read the volume added between the two points of inflexion.

ii. Levo proproxyphene napsylate (Novrad)

Properties and uses: It is a white, bitter, crystalline powder, odourless, soluble in water, alcohol, or chloroform. It has little or no analgesic activity. It has been found to be relatively less potent than codeine in the treatment of cough reflexes. The (–)-isomer of the drug showed greater antitussive activity in comparison to either the (+)-isomer or the racemic mixture.

Dose: Usually 50–100 mg after every 4 h.

Synthesis

iii. Codeine phosphate (Zenodyl)

Synthesis

Properties and uses: Codeine phosphate is a white crystalline powder, soluble in water, and slightly soluble in ethanol. This drug acts by suppression of the cough centre in the brain stem.

Assay: Dissolve the sample in a mixture of anhydrous acetic acid and dioxan (1:2) and titrate against 0.1 M perchloric acid using crystal violet as indicator.

Dose: 10–20 mg every 4 to 6 h.

Dosage forms: Co-codamol tablets B.P., Effervescent co-codamol tablets B.P., Co-codaprin tablets B.P., Dispersible co-codaprin tablets B.P., Codeine linctus, paediatric Codeine linctus B.P., Codeine phosphate injection B.P., Codeine phosphate oral solution B.P., Codeine phosphate tablets B.P.

iv. Pholcodine (Ethnine, Simplex)

Synthesis

Properties and uses: Pholcodine is a white crystalline powder, sparingly soluble in water, soluble in acetone, alcohol, and dilute mineral acid. It is as effective as codeine. It is a cough suppressant with mild sedative, but practically negligible analgesic action. It is employed for the relief of unproductive cough.

Assay: Dissolve the sample in anhydrous acetic acid, warm gently and titrate against 0.1 M perchloric acid. Determine the end point potentiometrically at the second point of inflexion.

Dose: The dose for adults is 5–15 mg, for children over 2 years 5 mg, for children below 2 years 2.5 mg.

Dosage forms: Pholcodine linctus B.P., Strong pholcodine linctus B.P.