Gabapentin enhances the antinociceptive effect of intrathecal morphine in refractory cancer pain patients

Morphine infusion through Intrathecal Drug Delivery System (IDDS) is widely used to relieve refractory cancer pain. However, continuous escalation of morphine dose caused by opioid tolerance and/or progress of cancer was commonly observed. Combining morphine with medications of different analgesic mechanisms is applied to blunt the rate of morphine increase. The purpose of this study was to determine the analgesic efficacy and safety of combining gabapentin with morphine after IDDS implantation. This study compared patients that received IDDS implantation from January 1, 2017 to November 10, 2018 in our institute. Key outcomes included change in mean pain score, dose of morphine used in patients, percentage of patients with 30% and 50% reduction in mean pain score, Patient Global Impression of Change scores, breakthrough pain characters and side effects. 34 patients in the combination group (morphine + gabapentin) and 40 patients in the monotherapy group(morphine)were analyzed. The results showed that both therapy groups achieved similar analgesic efficacy, demonstrated by Numerical rating scale (2.42 ± 0.88 vs 2.57 ± 0.85; Combination vs Monotherapy), PGIC and responder status. Mean daily dose of morphine was significantly lower in combination group compared to monotherapy group (3.54 ± 1.29 mg vs 4.64 ± 1.28 mg, P = 0.007). More patients experienced dizziness and somnolence after receiving combination therapy compared to morphine-alone treatment although no statistical significance was found (P = 0.49). Addition of gabapentin achieved similar analgesic efficacy with lower dose of morphine compared to morphine alone accompanying with higher incidence of dizziness and somnolence.