SurrogateSeq is an R package to implement group sequential procedures that allow for early stopping to declare efficacy using a surrogate marker and the possibility of futility stopping.
More details on the method are available in Parast L and Bartroff J (2024). Group Sequential Testing of a Treatment Effect Using a Surrogate Marker. Biometrics, 80(4): ujae108. doi:10.1093/biomtc/ujae108.
Go here to view a tutorial for this package: Surrogate Seq Tutorial