Study to Assess the Safety of MRx0029 or MRx0005 Compared to Placebo, in People With Parkinson's

This is a first-in-human study to evaluate the safety and tolerability of repeat oral administrations of MRx0029 (20 participants) or MRx0005 (20 participants) in participants diagnosed with idiopathic PD. Participants who are successfully screened will be randomized to 1 of 2 treatment sequences (TS) within their cohort (10 participants per sequence). Each treatment period will be separated by a washout period of 4 to 6 weeks. Cohort A Treatment Sequence 1: MRx0029 (1 capsule bid) for 4 weeks; 4-to 6-week washout period; placebo (1 capsule bid) for 4 weeks. Cohort A Treatment Sequence 2: Placebo (1 capsule bid) for 4 weeks; 4-to 6-week washout period; MRx0029 (1 capsule bid) for 4 weeks. Cohort B Treatment Sequence 1 MRx0005 (1 capsule bid) for 4 weeks; 4- to 6-week washout period; placebo (1 capsule bid) for 4 weeks. Cohort B Treatment Sequence 2: Placebo (1 capsule bid) for 4 weeks; 4-to 6-week washout period; MRx0005 (1 capsule bid) for 4 weeks Cohort A will be randomized first and when all participants have been randomized to Cohort A, Cohort B enrollment will begin.

There will be a total of 10 visits which will include a screening visit, 8 treatment period visits, and a Follow-up Visit. Three telephone calls will be made during the study to assess the participant's health status and confirm eDiary compliance. Participants will be screened for eligibility between Day -28 to Day -2 (Visit 1, screening). Informed consent will be obtained before performing any study-related procedures. Following randomization, participants will be provided with enough study medication to administer at home for 4 weeks (+/-2 days). The participants will discontinue treatment for 4 to 6 weeks. The participants will return to the site for the second period of study treatment and will again be provided with enough study medication to administer at home for 4 weeks (+/-2 days). Frequency of the AEs monitored throughout the study, and changes in laboratory values, vital signs, ECGs, and physical examinations will be assessed at the beginning and at the end of each treatment period. Gut permeability will be assessed with administration of a sugar solution and subsequent 24 hour urine collection. During the study, participants will report their daily bowel habits, including stool consistency, with the use of an electronic diary (the Bristol Stool Chart diary). The participants will also provide a stool sample on Day 1 and Day 29 of each treatment period and at the Follow-up Visit. PD symptoms will be assessed with the use of MDS-UPDRS and MDS-NMS to confirm that MRx0005 and MRx0029 do not adversely impact a participant's Parkinson's symptoms. Participants will report their dietary habits by filling out a Food Frequency Questionnaire at the clinic visits beginning (Day 1) and at the end of each treatment period, and at the Follow-up Visit. For the participants who are on stable immediate release formulations of levodopa, PK of levodopa will be assessed at the beginning (Day 1, prior to the first IMP dosing) and at the end (Day 29) of each treatment period. Fecal microbiome, as well as plasma/serum, fecal and urine biomarkers will be assessed, on Day 1 prior to the start of each treatment (baseline) period and at the end of each treatment period (Day 29), and at the Follow-up Visit. Participants will undergo a 4- to 6-week washout between treatment periods. Approximately 4 to 6 weeks after their second EOT visit, the participants will attend the clinic for their Follow-up Visit.