A phase 2 trial demonstrated ambroxol's safety and efficacy in penetrating cerebrospinal fluid (CSF) and engaging with its target in PD patients, including those with GBA1 variants.<h4>Methods</h4>We present the protocol of the ASPro-PD trial, a phase 3, multicentre, randomised, double-blind, placebo-controlled trial, aimed at evaluating whether high-dose ambroxol improves motor and non-motor function in PD patients. The gene discussed is GBA1; the disease is Parkinson disease.