Analysis of the antinociceptive interactions in two-drug combinations of gabapentin, oxcarbazepine and amitriptyline in streptozotocin-induced diabetic mice

Antiepileptic and antidepressant drugs are the primary treatments for pain relief in diabetic neuropathy. Combination therapy is a valid approach in pain treatment, where a reduction of doses could reduce side effects and still achieve optimal analgesia. We examined the effects of two-drug combinations of gabapentin, oxcarbazepine, and amitriptyline on nociception in diabetic mice and aimed to determine the type of interaction between components. The nociceptive responses in normal and diabetic mice were assessed by the tail-flick test. The testing was performed before and three weeks after the diabetes induction with streptozotocin (150 mg/kg; i.p.), when the antinociceptive effects of gabapentin, oxcarbazepine, amitriptyline and their two-drug combinations were examined. Gabapentin (10–40 mg/kg; p.o.) and oxcarbazepine (20–80 mg/kg; p.o.) produced a significant, dose-dependent antinociception in diabetic mice while amitriptyline (5–60 mg/kg; p.o.) produced weak antinociceptive effect. In normal mice, neither of the drugs produced antinociception. Gabapentin and oxcarbazepine, co-administered in fixed-dose fractions of the ED50 to diabetic mice, induced significant, dose-dependent antinociception. Isobolographic analysis revealed synergistic interaction. Oxcarbazepine (10–60 mg/kg; p.o.) + amitriptyline (5 mg/kg; p.o.) and gabapentin (10–30 mg/kg; p.o.) + amitriptyline (5 mg/kg; p.o.) combinations significantly and dose-dependently reduced nociception in diabetic mice. Analysis of the log dose–response curves for oxcarbazepine or gabapentin in a presence of amitriptyline and oxcarbazepine or gabapentin applied alone, revealed a synergism in oxcarbazepine–amitriptyline and additivity in gabapentin–amitriptyline combination. These findings provide new information about the combination therapy of painful diabetic neuropathy and should be explored further in patients with diabetic neuropathy.