The behavioral effects of a mixed efficacy antinociceptive peptide, VRP26, following chronic administration in mice

These results suggest that chronic treatment with VRP26 may produce less tolerance or physical dependence than chronic treatment with clinically available mu opioid analgesics such as fentanyl. Additionally, VRP26 produces less rewarding effects than fentanyl. This desirable in vivo profile may be due to the mixed efficacy nature of VRP26 and could provide the framework for safer opioid analgesics.

The antinociceptive effects of VRP26 and fentanyl were assessed using the mouse warm water tail withdrawal (WWTW) assay. Measurement of antinociceptive tolerance and physical dependence occurred after 7 days of continuous administration of either fentanyl (0.3 mg/kg/day) or VRP26 (10 mg/kg/day); tolerance was measured by a shift in the antinociceptive dose response curve in the WWTW assay. Physical dependence was determined by observation of withdrawal symptoms after precipitated withdrawal. Rewarding effects were measured by the ability of VRP26 or fentanyl to produce conditioned place preference.

The aim of this study is to explore the development of tolerance, dependence, and conditioned place preference of VRP26 as compared with the traditional opioid analgesic fentanyl.

Fentanyl produced significant tolerance and dependence, as well as significant conditioned place preference. VRP26 produced neither tolerance nor physical dependence, nor did it produce significant conditioned place preference. Conclusions: These results suggest that chronic treatment with VRP26 may produce less tolerance or physical dependence than chronic treatment with clinically available mu opioid analgesics such as fentanyl. Additionally, VRP26 produces less rewarding effects than fentanyl. This desirable in vivo profile may be due to the mixed efficacy nature of VRP26 and could provide the framework for safer opioid analgesics.