Neuroprotective effects of tempol acyl esters against retinal ganglion cell death in a rat partial optic nerve crush model

. Purpose: The aim of this study is to search for more effective derivatives of the superoxide dismutase mimetic tempol (4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl). Although tempol is neuroprotective in a rat partial optic nerve crush (PONC) model, relatively high doses are required to exert this effect. Methods: Tempol acyl esters with different‐length fatty acids (tempol‐C4, tempol‐C8, tempol‐C12 and tempol‐C16) were synthesized and the following properties were evaluated: water‐octanol partition coefficient, liposome‐liposome energy transfer, and electron paramagnetic resonance (EPR). Brown Norway rats underwent PONC and received tempol or acyl esters intraperitoneally once daily for 7 consecutive days. We then compared the effects of tempol and its four esters on retinal ganglion cell (RGC) damage using a retrograde labelling method. Results: The water‐octanol partition coefficient increased with increasing length of attached acyl chain. However, the energy of the liposome–liposome transfer seemed to be optimal for tempol‐C8 and tempol‐C12. The EPR signal was very similar for all tested compounds, suggesting similar efficiency of superoxide scavenging. Partial optic nerve crush in vehicle‐treated animals reduced RGC numbers by approx. 59% when compared with sham‐operated eyes. Tempol did not affect RGC loss at a dose of 1 mg/kg. In contrast, at molar doses equivalent to 1 mg/kg of tempol, tempol‐C8 showed a significant neuroprotective effect, whereas tempol‐C4, tempol‐C12 and tempol‐C16 did not act neuroprotectively. Conclusion: Manipulating the hydrophobicity of tempol seems to be a promising tool for developing more potent neuroprotectants in the PONC degeneration model. However, the resulting compounds need further pharmacological evaluation.