Baclofen Stability up to One Year in In Vivo Intrathecal Infusion Pumps

Commercial intrathecal baclofen treatment (ITBT) infusion pumps are recommended to be refilled within a maximum of 180 days, thus necessitating at least twice-yearly outpatient visits and refill injections. In particular, pumps with 40-mL reservoir volumes would allow much longer refill intervals. We investigated baclofen stability in active implanted ITBT infusion pumps in vivo with refill intervals up to 367 days to study the feasibility of lengthening refill intervals beyond six months.

We obtained 25 baclofen samples from 19 patients receiving ITBT with varying pump refill intervals. All patients had a baclofen infusion system delivering undiluted 2 mg/mL baclofen at continuous rates of 96.1 to 673.7 μg/d with a concentration of 2.002 mg/mL. Baclofen concentrations of the infusate samples acquired during the refill procedures were analyzed using a validated high-performance liquid chromatography with diode-array detection (HPLC-DAD) assay, later complemented with repeat assay with pH and physical measurements. We also present the validation data of the HPLC-DAD assay.

During the mean refill interval of 247 days (SD 90, range 54-367 days), the mean change in baclofen concentration was -0.0156 mg/mL (-0.8%, SD 0.14, range -0.30 to 0.32 mg/mL, paired t-test p = 0.57, t₂₄ = 0.57). Only a low negative correlation was found between the baclofen concentration and the refill interval (Pearson's r = -0.32, p = 0.12).

We could not show a significant change in baclofen concentration over the time studied; 2 mg/mL baclofen ITBT refill intervals could be lengthened to up to one year-the theoretical maximum refill interval in our cohort would have been 489 days. Further studies with larger sample sizes and other baclofen brands are warranted.