GDNF and Parkinson disease: Although there are several possible reasons why compounds such as GDNF, neurturin, CoQ10, CEP-1347, GPI-1485, pioglitazone, exenatide, and others that showed promising neuroprotective effects in MPTP models failed to demonstrate clear disease-modifying effects in clinical trials34, the relevance and translatability of the MPTP (and other toxin-based) models of PD have been called into question35, based on the assumption that the toxin models do not faithfully replicate key aspects of the disease35.