A Study Comparing Siplizumab With Rabbit Anti-Thymocyte Globulin to Help the Body Accept a Kidney Transplant

The goal of this clinical trial is to learn if siplizumab can prevent rejection of a kidney transplant in adult participants with end stage kidney disease. The main questions it aims to answer are: How many adverse events do participants receiving two different doses of siplizumab have compared to rabbit anti-thymocyte globulin (rATG)? How many participants successfully keep their kidney transplant after receiving siplizumab or rATG? This will be calculated as those participants that did not: die; have their kidney fail to work properly (graft loss); their body rejects their transplant (tissue sample (biopsy)-proven acute rejection: BPAR); or who were lost to follow-up. How does the body respond to siplizumab after dosing at each of the 2 dose levels? How does siplizumab work in the body compared to rATG? Selected participants will be divided into 3 groups. In 2 of the groups, participants will be given siplizumab at one of the two study medicine doses. Participants in the third group will be given rATG. All participants will take the usual anti-rejection medicines given before, during, and after a kidney transplant. All participants will also be given medicines before the study drug to lower the risk of reactions, and medicines used to prevent or treat infections after the transplant. Participants will be asked to provide their medical history at the first visit at the study site where you will have your kidney transplant. At the first visit and other visits participants will be asked to provide their medication history, have a physical exam, check vital signs, have blood drawn for tests, and have non-invasive tests that record the electrical activity of your heart (ECG) and blood draws. Participants will be monitored for 12 months after transplant surgery.