Background: Ceramides are bioactive sphingolipids linked to Alzheimer’s disease (AD) and

Parkinson’s disease (PD). Plasma ceramide chain-length patterns, particularly very-long-

chain (VLC) versus long-chain (LC) fatty acids, have been associated with AD dementia risk.

Prior studies in PD suggest altered ceramide metabolism as being associated with cognitive

heterogeneity. Whether plasma VLC/LC ceramide ratios differ in PD with AD biomarker

positivity remains unknown. This study addresses this critical gap by exploring accessible

plasma biomarkers of PD/AD co-pathology.

Methods: PD participants from the PPMI database were included and restricted to only

individuals positive for cerebrospinal fluid (CSF) α-synuclein as determined by seed

amplification assay. Based on AD biomarker status (CSF p-tau181/Aβ42 ratio < 0.021),

participants were classified as PD+AD- (n = 220; age = 61.0 ± 9.3) or PD+AD+ (n = 36; age

= 66.0 ± 7.7). Four biologically relevant plasma ceramide ratios reflecting VLC/LC balance

were examined (C24:1/C16, C24/C16, C24:1/C18, and total VLC/total LC). Baseline group

di"erences were assessed using nonparametric ANCOVA adjusted for age, sex, BMI, and

study site with FDR correction. Sensitivity analyses were performed by MoCA (< 26) and

APOE-ε4 status. Spearman correlations assessed associations with MoCA and MDS-UPDRS III

within each group (FDR-controlled). Longitudinal ratio changes over three years were

analyzed using linear mixed-e"ects models with fixed e"ects for group, time, and group ×

time, as well as random intercepts and slopes.

Results: At baseline, PD+AD+ participants showed lower VLC/LC ceramide ratios compared

with PD+AD-, except for C24:1/C18; after FDR, only C24:C16 remained significant (p <0.05)

(Figure 1). Sensitivity analyses using MoCA and APOE-ε4 status revealed no significant group

di"erences. No significant correlations were observed between ceramide ratios and MoCA in

either group. In contrast, three ceramide ratios were positively correlated with MDS-UPDRS III

in the PD+AD+ group, with no significant correlations in PD+AD- (Figure 2). Longitudinal

analyses revealed no significant group × time interactions.

Conclusion: Plasma VLC/LC ceramide ratios in the PD+AD+ group tended to be lower and

were associated with motor severity, suggesting ceramide chain-length balance may reflect a

possible e"ect modification by AD biomarker status. These exploratory findings warrant

replication and investigation in larger cohorts.