This is a UK only clinical trial in patients with Parkinson's disease (PD) of a drug called ambroxol hydrochloride, which is an already licensed drug for the treatment of respiratory conditions (such as a common cold) in many European countries. The aim of this trial is to find out whether ambroxol hydrochloride can slow down the progression of Parkinson's disease and to evaluate it's safety and tolerability.

This is a 104-week, randomized, double-blind, multi-centre, parallel group, placebo-controlled clinical trial of ambroxol hydrochloride in patients with PD, with a 26-week open-label extension phase. Participants will undergo screening to evaluate their eligibility to participate in the trial. All eligible participants will be randomised to receive either ambroxol hydrochloride (420mg) or it's matching placebo in a 1:1 ratio three times a day for 104 weeks, including a 2-week dose escalation period. Once the end of the blinded treatment has been reached, all participants will enter the open-label extension phase and will receive ambroxol hydrochloride (420mg) three times a day for 26 weeks, including a 2-week dose escalation period. All clinical staff, study investigators, and participants will be blinded to study assignments throughout the entirety of the trial.

There will be an optional sub-study including 106 participants in which a cerebrospinal fluid (CSF) sample will be taken on two occasions via a Lumbar Puncture procedure to measure ambroxol drug levels, assess whether the glucocerebrosidase enzyme has been stimulated and the levels of other substances thought to be associated with the development of PD and confirm whether the study drug has penetrated the cerebrospinal fluid and Central Nervous System.

Inclusion Criteria:

1. A diagnosis of Parkinson's disease (in accordance with the MDS diagnostic criteria) within 7 years of the screening visit confirmed by year of diagnosis.
2. Adults aged ≥ 35 and ≤ 75 years.
3. Hoehn and Yahr stage between 1-2.5, inclusive (in ON stage) at screening visit.
4. Known glucocerebrosidase gene (GBA1) status, positive or negative (status MUST be confirmed prior to screening).
5. On stable dopaminergic treatment for at least 3 months before enrolment.
6. Able and willing to provide informed consent prior to any study related assessments and/or procedures.
7. Able and willing to attend trial visits and comply with all study procedures for the duration of the trial.
8. Willing and able to self-administer oral ambroxol medication or placebo.

Exclusion Criteria:

1. Participation in another interventional clinical trial of an Investigational Medicinal Product (IMP) and use of an Investigational Medicinal Product (IMP) within 90 days prior to the first dose of trial treatment.

2. Use of an Investigational Medicinal Product (IMP) within 90 days prior to the first dose of trial treatment.

3. Participation in another clinical trial of an Investigational New Drug being tested for PD disease modifying potential within 12 months prior to the first dose of trial treatment.

4. Past surgical history of deep brain stimulation.

5. Use of ambroxol in the past 12 months.

6. Exposure to Exenatide within 12 months prior to the first dose in this current trial.

7. Concomitant medications that in the opinion of the Investigator would preclude participation in the study e.g., exenatide or other GLP1 agonist for diabetes.

8. Confirmed dysphagia that would preclude self-administration of ambroxol.

9. History of known sensitivity to the study medication, ambroxol or its excipients (lactose monohydrate, granulated microcrystalline cellulose, copovidone and magnesium stearate) in the opinion of the investigator that contraindicates their participation.

10. History of known rare hereditary disorders of galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption.

11. Presence of the LRRK2 G2019S mutation (status to be confirmed prior to screening).

12. History of drug abuse or alcoholism in the opinion of the Investigator that would preclude participation in the trial.

13. Pregnant (or planned pregnancy during the trial) and/or breastfeeding.

14. Women of childbearing potential (WOCBP) and male participants with a partner of childbearing potential not willing to use highly effective contraception or abstinence for the duration of the trial treatment and for 2 weeks following the last dose of the study drug.

15. Any clinically significant or unstable medical or surgical condition that in the opinion of the Investigator may; put the participant at risk when participating in the study, influence the results of the study or affect the participants ability to take part in the study, as determined by medical history, physical examinations, electrocardiogram (ECG) or laboratory tests. Such conditions may include:

    A. Impaired renal function with creatinine clearance <50ml/min at screening visit.

    B. Moderate/Severe hepatic impairment.

    C. A major cardiovascular event (e.g., myocardial infarction, acute coronary syndrome, compensated congestive heart failure, pulmonary embolism, coronary revascularisation) that occurred within 6 months prior to the screening visit.

16. Severe depression defined by a score >20 on the Beck Depression Inventory-II (BDI-II) at screening.

17. Significant cognitive impairment defined by a score <20 on the Montreal Cognitive Assessment (MoCA) at screening.

18. Use of trihexyphenidyl or benztropine within 30 days prior to the first dose of trial treatment.

19. Only applicable for those patients consenting to the optional CSF sub-study: Evidence or history of hypersensitivity to lidocaine or its derivatives.

20. Only applicable for those patients consenting to the optional CSF sub-study: current treatment with anti-coagulants (e.g., warfarin) that might preclude safe completion of the lumbar puncture in the opinion of the Investigator. Aspirin will be permitted.

21. Only applicable for those patients consenting to the optional CSF sub-study: Significant known lower spinal malformations or other spinal abnormalities that would preclude a lumbar puncture.

Double Blind