In vitro metabolism studies showed that CYP1A2 was the major enzyme responsible for the metabolism of ropinirole. There is thus the potential for substrates or inhibitors of this enzyme when coadministered with ropinirole to alter its clearance. Therefore, if therapy with a drug known to be a potent inhibitor of CYP1A2 is stopped or started during treatment with REQUIP, adjustment of the dose of REQUIP may be required.
Coadministration of carbidopa + l-dopa (SINEMET® 10/100 mg twice daily) with ropinirole (2 mg 3 times daily) had no effect on the steady-state pharmacokinetics of ropinirole (n = 28 patients). Oral administration of REQUIP 2 mg 3 times daily increased mean steady state Cmax of l-dopa by 20%, but its AUC was unaffected (n = 23 patients).
Coadministration of REQUIP (2 mg 3 times daily) with digoxin (0.125 to 0.25 mg once daily) did not alter the steady-state pharmacokinetics of digoxin in 10 patients.
Administration of theophylline (300 mg twice daily, a substrate of CYP1A2) did not alter the steady-state pharmacokinetics of ropinirole (2 mg 3 times daily) in 12 patients with Parkinson’s disease. Ropinirole (2 mg 3 times daily) did not alter the pharmacokinetics of theophylline (5 mg/kg IV) in 12 patients with Parkinson’s disease.
Coadministration of ciprofloxacin (500 mg twice daily), an inhibitor of CYP1A2, with ropinirole (2 mg 3 times daily) increased ropinirole AUC by 84% on average and Cmax by 60% (n = 12 patients).
Population pharmacokinetic analysis revealed that estrogens (mainly ethinylestradiol: intake 0.6 to 3 mg over 4-month to 23-year period) reduced the oral clearance of ropinirole by 36% in 16 patients. Dosage adjustment may not be needed for REQUIP in patients on estrogen therapy because patients must be carefully titrated with ropinirole to tolerance or adequate effect. However, if estrogen therapy is stopped or started during treatment with REQUIP, then adjustment of the dose of REQUIP may be required.
Since ropinirole is a dopamine agonist, it is possible that dopamine antagonists such as neuroleptics (phenothiazines, butyrophenones, thioxanthenes) or metoclopramide may diminish the effectiveness of REQUIP. Patients with major psychotic disorders treated with neuroleptics should only be treated with dopamine agonists if the potential benefits outweigh the risks.
Population analysis showed that commonly administered drugs, e.g., selegiline, amantadine, tricyclic antidepressants, benzodiazepines, ibuprofen, thiazides, antihistamines, and anticholinergics, did not affect the oral clearance of ropinirole.