longsurr is an R package to (1) assess the proportion of treatment effect explained by a longitudinal surrogate marker, and (2) estimate the treatment effect on a longitudinal surrogate marker
All methods are described in detail in Agniel and Parast (2021). Evaluation of Longitudinal Surrogate Markers, Biometrics, 77(2): 477-489, doi:10.1111/biom.13310 and Wang et al (2025) "Semiparametric Joint Modeling to Estimate the Treatment Effect on a Longitudinal Surrogate with Application to Chronic Kidney Disease Trials," Biometrics, 81(3): ujaf104, doi: 10.1093/biomtc/ujaf104.
Go here to view a tutorial for this package: longsurr Tutorial.