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Safety of Two Doses of Siplizumab Compared with Rabbit Anti‑Human Thymocyte Immunoglobulin in Kidney Transplant Recipients Receiving Standard Immunosuppression

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What is this trial about?

A plain-language summary of the goals, design and what participants do

The study focuses on people who have received a kidney transplant, a procedure done when a failing kidney is replaced with a healthy one. After transplantation, patients must take medicines that lower the activity of the immune system so the body does not reject the new organ. In this trial, participants receive the standard medicines tacrolimus, mycophenolic acid and corticosteroids, together with either the experimental drug Siplizumab or the established drug rabbit anti-human thymocyte immunoglobulin given through a vein (infusion).

The purpose of the study is to evaluate the safety of two different dose levels of the experimental medicine compared with the standard treatment. Participants are randomly assigned without knowing which medication they receive (double‑blind) and are followed for about one year after the transplant. During this time, doctors monitor for any side effects, serious side effects, or events of special interest, and they also track how well the transplanted kidney works. The study involves a series of clinic visits where blood samples are taken and routine health checks are performed, but no complex procedures are required.

The research process

The trial runs in 7 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Enrollment and randomization

    After joining the study, you will be assigned by the study system to receive either siplizumab or the standard treatment called rabbit anti‑human thymocyte immunoglobulin. the assignment is done without you knowing which medication you will receive.

  2. Step 2

    Induction infusion on transplant day

    On the day of your kidney transplant you will receive a single infusion (medication delivered through a vein).

    If you are assigned to the siplizumab group, the dose will be 1.2 mg per kilogram of body weight, given as a solution for infusion.

    If you are assigned to the comparator group, the dose will be 1.5 mg per kilogram of body weight of rabbit anti‑human thymocyte immunoglobulin, also given by infusion.

  3. Step 3

    Start of standard immunosuppression

    After the infusion you will begin the usual medicines that help the body accept the new kidney: tacrolimus, mycophenolic acid, and corticosteroids.

    These medicines are taken daily as directed by the transplant team and are continued for at least twelve months.

  4. Step 4

    Routine follow‑up visits and safety monitoring

    You will attend clinic visits to check your health, kidney function, and any side effects.

    The first visits occur frequently during the first week after transplant, then usually monthly until the twelve‑month end point.

    At each visit blood will be drawn to monitor kidney function, blood counts, and the levels of the medicines.

  5. Step 5

    Blood sampling for drug level assessment

    If you receive siplizumab, additional blood samples will be collected over the first twelve weeks to measure the highest concentration (cmax) and overall exposure (auc) of the drug in your bloodstream.

    These samples help determine how the medication is processed in your body.

  6. Step 6

    Possible kidney biopsy for clinical reasons

    If your doctors suspect a problem with the transplanted kidney, they may recommend a small tissue sample called a kidney biopsy.

    The biopsy is performed only when it is medically necessary and helps evaluate the health of the transplant.

  7. Step 7

    Final assessment at twelve months

    At twelve months after the transplant you will have a comprehensive evaluation that includes kidney function tests, blood counts, and a review of any adverse events that occurred during the study.

    The results will be used to compare the safety of the two dose levels of siplizumab with the standard treatment.

Who can join the trial?

7 criteria

  • Be an adult between 18 and 70 years old.
  • Have received a renal allograft (kidney transplant) from a living or deceased donor who is not an exact HLA match (the donor’s tissue type is not identical).
  • Had the transplanted kidney kept cold for less than 30 hours before surgery; using a cooling machine that circulates fluid (hypothermic machine perfusion) during that time is also allowed.
  • If you are a woman who could become pregnant (women of childbearing potential), you must agree to use a highly reliable form of birth control during the study medication and for 24 weeks after stopping it.
  • Women who are post‑menopausal must not have had any menstrual bleeding for at least one year before the first dose.
  • Women who have had a surgical sterilization procedure (such as tubal ligation) must have had that surgery at least six months before the first dose, with proof of the procedure available.
  • All women who could become pregnant must have a negative result on a pregnancy test at the screening visit.

Who cannot join the trial?

14 criteria

  • If you do not have antibodies against Epstein‑Barr virus (EBV) (meaning you are seronegative for EBV), you cannot join the study.
  • If you have received a medication that blocks the complement system (a part of the immune system) within the past 6 months, or you are likely to need such medication during the study, you cannot join.
  • If you are already taking immunosuppressive therapies (drugs that lower the activity of your immune system) for other conditions and cannot stop them to start the study’s required medication plan (which includes drugs like cyclosporine, azathioprine, sirolimus or tacrolimus (TAC)), you cannot join.
  • If you have ever had cancer in any organ, except for a small skin cancer that was completely removed or an early‑stage cervical cancer that was fully surgically removed with clean margins, you cannot join.
  • If you have active tuberculosis (TB) infection or a suspected TB infection that has not been fully checked and treated, you cannot join.
  • If you have a serious infection throughout your body at the time of screening or within two weeks before you might be assigned to a treatment group, you cannot join.
  • If any of the following blood test results are low: hemoglobin less than 8 g/dL (a measure of red blood cells), white blood cell count 2,000 per microliter or fewer, or platelet count 50,000 per microliter or fewer, you cannot join.
  • If you test positive for human immunodeficiency virus (HIV) or for hepatitis B surface antigen, or if you test positive for hepatitis C virus (HCV) without proof that the virus has been cleared after treatment, you cannot join.
  • If the kidney you receive comes from a donor who tests positive for HIV or hepatitis B, or from a donor who is hepatitis C positive without proof that the donor’s virus has been cleared, you cannot join.
  • If the kidney you receive is from a non‑heart beating donor (DCD) and the donor was either uncontrolled or older than 55 years, you cannot join.
  • If you received more than one organ at the same time (such as kidney‑pancreas), more than one kidney transplant, or a stem‑cell transplant, you cannot join.
  • If you have donor‑specific antibodies (DSA) detected by a special lab test (single‑antigen bead assay) within the last three months, you cannot join.
  • If your crossmatch test (a test that checks if your blood reacts against the donor’s tissue) is positive, you cannot join. (A positive B‑cell crossmatch alone is not a reason to be excluded.)
  • If you are receiving a kidney from a donor with a different blood type that is not compatible (ABO‑incompatible), you cannot join.
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Investigated drugs

Siplizumab
Siplizumab is a laboratory‑made protein that is given through an IV infusion. In this study it is being tested as a new drug to help prevent the body’s immune system from attacking a newly transplanted kidney. It works by targeting specific immune cells that can cause rejection, aiming to keep the transplant healthy while patients also receive standard medicines that suppress the immune system. Rabbit Anti‑Human Thymocyte Immunoglobulin (rATG)
Rabbit anti‑human thymocyte immunoglobulin, often called rATG, is a medication made from rabbit antibodies. It is given by IV infusion and is used to lower the activity of the immune system right after a kidney transplant. In the trial it serves as the active control, meaning it is the established treatment that the new drug (siplizumab) is being compared against. Tacrolimus
Tacrolimus is an oral pill that blocks a key part of the immune response. After a kidney transplant, it helps prevent the body from rejecting the new organ. All participants in the study continue to take tacrolimus as part of the standard care regimen alongside the study drug or the comparator. Mycophenolic Acid
Mycophenolic acid is another oral medication that suppresses the immune system. It works by stopping certain immune cells from multiplying. In this trial, every participant receives mycophenolic acid together with tacrolimus and steroids to provide a strong, combined protection against transplant rejection. Corticosteroids
Corticosteroids are anti‑inflammatory drugs that are given by mouth or IV to further calm the immune system after transplantation. They are a routine part of the standard immunosuppressive therapy in this study, helping to reduce the risk of early rejection while the other medicines take effect.

What is already known about the treatment

  • Siplizumab

    Given as an intravenous infusion of a clear solution at a dose of 1.2 mg per kilogram, siplizumab is still an experimental drug being studied in a Phase 2b trial and is not yet approved for general use. It is being tested to help prevent the immune system from rejecting a newly transplanted kidney. The medicine works by attaching to a protein called CD2 on immune cells, which blocks the cells from becoming active and attacking the transplant. It belongs to the class of immunosuppressive monoclonal antibodies.

  • Rabbit Anti‑Human Thymocyte Immunoglobulin

    Administered by intravenous infusion at about 1.5 mg per kilogram, rabbit anti‑human thymocyte immunoglobulin (rATG) is an approved drug that has been used for many years in organ‑transplant patients. It is used to lower the risk of early rejection after a kidney transplant. The drug contains a mixture of antibodies that bind to T‑cells, leading to their removal from the blood and reducing the immune attack on the new organ. It is classified as a polyclonal anti‑thymocyte globulin, a type of immunosuppressive therapy.

Investigated diseases

Renal transplantation - Renal transplantation is the surgical placement of a donor kidney into a person whose own kidneys no longer work. After the operation, the new kidney begins to filter blood and produce urine. Over time, the kidney is monitored for signs of the body’s immune response that can affect its function. The transplanted kidney can continue to work for many years if it remains healthy. The condition involves the ongoing adaptation of the recipient’s body to the new organ.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IITrial ID2026-525646-29-00Protocol codeTCD601B205Estimated enrolment122 patients

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