Synergism between metformin and analgesics/vitamin B12 in a model of painful diabetic neuropathy

Metformin (a widely used antidiabetic drug) has demonstrated efficacy in models of painful diabetic neuropathy (PDN), as well as certain clinical efficacy in relieving/preventing PDN. This study aimed to determine the type of interaction between metformin and duloxetine/oxycodone/eslicarbazepine acetate [ESL]/vitamin B12 in relieving diabetic pain hypersensitivity. Antihyperalgesic efficacy was determined using a Von Frey apparatus in mice with streptozotocin-induced PDN. We examined metformin’s efficacy following oral (acute and prolonged 7-day treatment) and local (spinal and peripheral) application. The examined analgesics were administered in a single oral dose, whereas vitamin B12 was intraperitoneally administered for 7 days. In combination experiments, metformin (prolonged treatment) and analgesics/vitamin B12 were co-administered in fixed-dose fractions of their ED50 values and the type of interaction was determined using isobolographic analysis. Metformin produced dose-dependent antihyperalgesic effects in diabetic mice after oral (acute and prolonged 7-day treatment) and local spinal/peripheral application. Two-drug metformin combinations with analgesics/vitamin B12 also dose- dependently reduced mechanical hyperalgesia. The isobolographic analysis revealed that metformin synergises with analgesics/vitamin B12, with a 6–7 fold dose reduction of both drugs in the examined combinations. In conclusion, metformin reduces hyperalgesia in diabetic animals, most likely by acting at the spinal and pe- ripheral level. Additionally, it synergizes with duloxetine/oxycodone/ESL/vitamin B12 in reducing hyperalgesia. Metformin co-treatment may increase analgesic efficacy and enable the use of lower (and potentially safer) analgesic doses for treating PDN. Combined metformin-vitamin B12 use may provide more effective pain relief and mitigate metformin-induced vitamin B12 deficiency.