Chemical stability of admixtures combining ziconotide with baclofen during simulated intrathecal administration

Objective.  To determine the stability of admixtures combining ziconotide with commercially formulated or powdered baclofen during simulated intrathecal infusion under laboratory conditions at 37°. Materials and Methods.  Admixtures of ziconotide (25 µg/mL) with commercially formulated (1.5 mg/mL) or powdered (2.0 mg/mL) baclofen were stored in implantable intrathecal pumps at 37°. Drug concentrations were determined with high-performance liquid chromatography, and the length of time that the concentrations of both drugs remained ≥90% and ≥80% of initial (ie, the 90% and 80% stability, respectively) was estimated based on lower 95% confidence bounds obtained via linear regression. Results.  Baclofen was stable in both admixtures. In the commercially formulated baclofen admixture, the mean ziconotide concentration declined to 82.2% of initial in 30 days; the estimates for 90% and 80% stability were 12 and 29 days, respectively. In the powdered baclofen admixture, the mean ziconotide concentration declined to 87.4% of initial in 30 days; the estimates for 90% and 80% stability were 20 and 41 days, respectively. Conclusion.  Ziconotide-baclofen admixtures were more stable when prepared using powdered baclofen rather than a commercial baclofen formulation.