A Sequentially Adaptive, Open Label, Dose-finding, Phase I/II Trial of Pentostatin in the Treatment of Steroid-refractory Acute Graft Versus Host Disease (aGvHD)

To determine a safe and effective dose of pentostatin in steroid-refractory aGvHD and to identify the minimal effective dose of pentostatin defined as the lowest dose that produces a response in 20% or more of patients while producing treatment failure (defines as death, grade 3/4 toxicity, or progressive disease) in 40% or less of patients.