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original article

Horse versus Rabbit Antithymocyte Globulin in Acquired Aplastic Anemia
Phillip Scheinberg, M.D., OlgaNunez, R.N., B.S.N., Barbara Weinstein, R.N., Priscila Scheinberg, M.S., Angélique Biancotto, Ph.D., Colin O. Wu, Ph.D., and Neal S. Young, M.D.

A BS T R AC T
Background
From the Hematology Branch(P.S., O.N., B.W., P.S., N.S.Y.) and Office of Biostatistics Research (C.O.W.), National Heart, Lung, and Blood Institute; and the TransNIH Center for Human Immunology, Autoimmunity, and Inflammation,National Institutes of Health (A.B., N.S.Y.) — both in Bethesda, MD. Address reprint requests to Dr. Scheinberg at 10 Center Dr., Bldg. 10 CRC, Rm. 3E-5140, MSC 1202, Bethesda, MD 20892-1202, or atscheinbp@mail .nih.gov. N Engl J Med 2011;365:430-8.
Copyright © 2011 Massachusetts Medical Society.

In severe acquired aplastic anemia, hematopoietic failure is the result of immunemediateddestruction of bone marrow stem and progenitor cells. Immunosuppressive therapy with antithymocyte globulin (ATG) plus cyclosporine is an effective alternative to stem-cell transplantation and improves bloodcounts and survival. Although horse ATG is the standard therapy, rabbit ATG is more potent in depleting peripheralblood lymphocytes and is preferred in other clinical circumstances.
Methods

FromDecember 2005 through July 2010, we performed a randomized trial comparing these two ATG formulations in conventional regimens. Patients were treated at a single facility. The primary outcome washematologic response at 6 months, as determined by blood counts. The study was designed to enroll 60 patients each for the rabbit-ATG and horse-ATG groups and was powered to detect a difference of 25percentage points in the response rate.
Results

A large, unexpected difference was observed in the rate of hematologic response at 6 months in favor of horse ATG (68%; 95% confidence interval [CI], 56...