Effect of fentanyl and its three novel analogues on biochemical, oxidative, histological, and neuroadaptive markers after sub-acute exposure in mice

Aims: Comparative sub-acute toxicity, including tolerance and dependence potential of fentanyl and its three novel and potent analogues was determined in mice. Main methods: Comparative sub-acute (21 d, intraperitoneal; 1/10 LD50) toxicity of fentanyl and its three novel analogues viz., N-(1-(2-phenoxyethyl)-4-piperidinyl) propionanilide (2), N-isopropyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (5), and N-t-butyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (6) was determined in mice. Animals were observed for additional seven days to assess the recovery. The brain, liver and kidney toxicity was assessed on the basis of various biochemical, oxidative, histological, and neuroadaptive markers. The expression levels of key neuronal markers associated with drug tolerance and dependence were investigated by western blot and immunohistochemistry. Key findings: Fentanyl and its analogues caused abnormal levels of liver and kidney specific biomarkers in plasma. Brain malondialdehyde (MDA) levels were raised by all the treatments and kidney MDA level by analogue 6 (21 d). Reduced glutathione levels in brain, liver, and kidney were diminished by all the treatments (21 & 28 d) and a significant change in the levels of antioxidant enzymes was also produced mainly after 21 d. The deleterious effects of fentanyl and its analogues were further substantiated by corresponding histopathological changes in brain, liver and kidney (21 & 28 d). These compounds were also found to produce neuroadaptive changes as evidenced by the increased expression levels of c-Fos, glucocorticoid receptor, N-methyl-D-aspartate receptorl and mu-opioid receptor (21 & 28 d). Significance: Three novel analogues of fentanyl were envisaged to have alternative therapeutic potentials. However, their comparative sub-acute toxicity revealed undesirable side effects, thereby masking their therapeutic ability.