Five Years On, Roche's Anti-Synuclein Antibody Still Tracks Slower Parkinson's Progression

New open-label data presented at AD/PD 2026 extend a four-year Nature Medicine finding on prasinezumab, but the comparisons rest on an external cohort, not a placebo group, as the drug moves into phase III testing.

Roche has presented five-year data on its investigational antibody prasinezumab showing sustained slowing of motor and functional decline in people with early Parkinson's disease, compared with an external comparator group, according to a company presentation at the AD/PD 2026 conference and confirmed in a Prothena Corporation regulatory filing dated May 7, 2026. Prothena, which partners with Roche on the drug, said the AD/PD analysis found "approximately two years in delay of disease progression over a five year period" using a new time-based method Roche calls a "time saved" analysis, drawn from the open-label extension of the phase II PASADENA trial (NCT03100149).

PASADENA enrolled patients with early Parkinson's disease and, in its randomized, double-blind first year, missed its primary endpoint on the MDS-UPDRS motor rating scale, a result Roche disclosed in 2020. Everyone who continued into the trial's open-label extension received the antibody, so there is no placebo group left in the later years; instead, the five-year outcomes were measured against an external cohort drawn from the Parkinson's Progression Markers Initiative (PPMI), an observational study of people with Parkinson's not enrolled in PASADENA. A peer-reviewed analysis of the same extension study, published in Nature Medicine on October 8, 2024, reported slower motor progression at four years in the prasinezumab group compared with the same type of external PPMI comparator, using MDS-UPDRS Part II and Part III scores in the on- and off-medication states.

Roche's five-year AD/PD 2026 presentation, reported by Clinical Trials Arena, described relative differences in MDS-UPDRS Part III scores of a 41% smaller decline off medication and a 95% smaller decline on medication in the prasinezumab group versus the PPMI comparator at year five; the presentation also noted that average levodopa-equivalent daily dose was roughly 100 milligrams higher in the comparator group, a difference in symptomatic treatment that could itself affect scores independent of any effect on underlying disease. Roche has separately begun a phase III program for prasinezumab in early-stage Parkinson's disease, announced in June 2025, after a related phase IIb trial called PADOVA also missed its primary endpoint in December 2024 while showing signals on some secondary measures.

Prothena's chief executive, Gene Kinney, said in the same May 2026 filing that Roche also presented "longer-term data from the PADOVA open-label extension study" and biomarker analyses suggesting the antibody "may impact the underlying disease biology," though the filing did not report a placebo-controlled result confirming a slowing of disease at five years.

Whether prasinezumab changes the course of Parkinson's disease for patients, rather than for a statistical model, is now a question for the phase III trials Roche has begun; families weighing the five-year figures should know they compare a treated group with an outside cohort, not with a matched group who did not receive the drug over that same period.