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Atgam (Equine Thymocyte Immune Globulin) - Description and Clinical Pharmacology

 
 



For Intravenous Use Only

DESCRIPTION

ATGAM Sterile Solution contains lymphocyte immune globulin, anti-thymocyte globulin [equine]. It is the purified, concentrated, and sterile gamma globulin, primarily monomeric IgG, from hyperimmune serum of horses immunized with human thymus lymphocytes. ATGAM is a transparent to slightly opalescent aqueous protein solution. It may appear colorless to faintly pink or brown and is nearly odorless. It may develop a slight granular or flaky deposit during storage. (For information about in-line filters, see Infusion Instructions in the DOSAGE AND ADMINISTRATION SECTION.)

Before release for clinical use, each lot of ATGAM is tested to assure its ability to inhibit rosette formation between human peripheral lymphocytes and sheep red blood cells in vitro. In each lot, antibody activity against human red blood cells and platelets is also measured and determined to be within acceptable limits. Only lots that test negative for antihuman serum protein antibody, antiglomerular basement membrane antibody and pyrogens are released.

Each milliliter of ATGAM contains 50 mg of horse gamma globulin stabilized in 0.3 molar glycine to a pH of approximately 6.8.

CLINICAL AND ANIMAL PHARMACOLOGY

ATGAM Sterile Solution is a lymphocyte-selective immunosuppressant as is demonstrated by its ability to reduce the number of circulating, thymus-dependent lymphocytes that form rosettes with sheep erythrocytes. This antilymphocytic effect is believed to reflect an alteration of the function of the T lymphocytes, which are responsible in part for cell-mediated immunity and are involved in humoral immunity. In addition to its antilymphocytic activity, ATGAM contains low concentrations of antibodies against other formed elements of the blood. In rhesus and cynomolgus monkeys, ATGAM reduces lymphocytes in the thymus-dependent areas of the spleen and lymph nodes. It also decreases the circulating sheep-erythrocyte-rosetting lymphocytes that can be detected, but ordinarily ATGAM does not cause severe lymphopenia.

In general, when ATGAM is given with other immunosuppressive therapy, such as antimetabolites and corticosteroids, the patient's own antibody response to horse gamma globulin is minimal. In a small clinical study, ATGAM administered with other immunosuppressive therapy and measured as horse IgG had a serum half-life of 5.73 days.

ANIMAL TOXICOLOGY

During the development of ATGAM Sterile Solution, aliquots of the various clinical lots were infused intravenously in either Macaca mulatta or Macaca irus monkeys. The dosage used was 100 mg/kg on day 0, 200 mg/kg on day 2, and 400 mg/kg on day 4. A 3-week observation period followed.

Many of the changes observed could have been anticipated on the basis of the antilymphocytic activity of ATGAM. They are decreased peripheral blood lymphocytes and increased total leukocyte and neutrophil counts occurring within 24 hours after infusion, decreased thymus size with involution or atrophy, or both, and decreased lymphocyte populations in the thymus-dependent areas of the spleen and lymph nodes. The atrophy was particularly common in the animals receiving the higher doses. In animals receiving either dosage regimen, packed cell volume, total erythrocyte counts, and hemoglobin concentrations have decreased and reticulocytes and nucleated erythrocytes have increased enough to be classified as anemia. An occasional animal death believed to have resulted from anemia has occurred. Transient decreases in blood platelet counts have also occurred. Thrombus formation occurred frequently along the routes of infusion, ie, the saphenous and femoral veins. However, the incidence of thrombi has dropped since in-line filters have been used during infusion. In these animals, definitive evidence of DIC (disseminated intravascular coagulation) has not been observed.

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