SEVERE COMBINED IMMUNODEFICIENCY DISEASE ASSOCIATED WITH ADA DEFICIENCY
Severe combined immunodeficiency disease (SCID) associated with a deficiency of ADA is a rare, inherited, and often fatal disease. In the absence of the ADA enzyme, the purine substrates adenosine and 2'-deoxyadenosine accumulate, causing metabolic abnormalities that are directly toxic to lymphocytes.
The immune deficiency can be cured by bone marrow transplantation. When a suitable bone marrow donor is unavailable or when bone marrow transplantation fails, non-selective replacement of the ADA enzyme has been provided by periodic irradiated red blood cell transfusions. However, transmission of viral infections and iron overload are serious risks associated with irradiated red blood cell transfusions, and relatively few ADA deficient patients have benefitted from chronic transfusion therapy.
ADAGEN ® (pegademase bovine) Injection provides specific and direct replacement of the deficient enzyme, but will not benefit patients with immunodeficiency due to other causes.
In patients with ADA deficiency, rigorous adherence to a schedule of ADAGEN ® (pegademase bovine) Injection administration can eliminate the toxic metabolites of ADA deficiency and result in improved immune function. It is imperative that treatment with ADAGEN ® (pegademase bovine) Injection be carefully monitored by measurement of the level of ADA activity in plasma. Monitoring of the level of deoxyadenosine triphosphate (dATP) in erythrocytes is also helpful in determining that the dose of ADAGEN ® (pegademase bovine) Injection is adequate.
ADAGEN ® (pegademase bovine) Injection provides specific replacement of the deficient enzyme.
In the absence of the enzyme ADA, the purine substrates adenosine, 2'-deoxyadenosine and their metabolites are toxic to lymphocytes. The direct action of ADAGEN ® (pegademase bovine) Injection is the correction of these metabolic abnormalities. Improvement in immune function and diminished frequency of opportunistic infections compared with the natural history of combined immunodeficiency due to ADA deficiency only occurs after metabolic abnormalities are corrected. There is a lag between the correction of the metabolic abnormalities and improved immune function. This period of time is variable, and has been reported to be from a few weeks to as long as 6 months. In contrast to the natural history of combined immunodeficiency disease due to ADA deficiency, a trend toward diminished frequency of opportunistic infections and fewer complications of infections has occurred in patients receiving ADAGEN ® (pegademase bovine) Injection.
The pharmacokinetics and biochemical of ADAGEN ® (pegademase bovine) Injection have been studied in six children ranging in age from 6 weeks to 12 years with SCID associated with ADA deficiency.
After the intramuscular Injection of ADAGEN ® (pegademase bovine) Injection, peak plasma levels of ADA activity were reached 2 to 3 days following administration. The plasma elimination half-life of ADA following the administration of ADAGEN ® (pegademase bovine) Injection was variable, even for the same child. The range was 3 to > 6 days. Following weekly injections of ADAGEN ® (pegademase bovine) Injection at 15 U/kg, the average trough level of ADA activity in plasma was between 20 and 25 µmol/hr/mL.
The changes in red blood cell deoxyadenosine nucleotide (dATP) and S-adenosylhomocysteine hydrolase (SAHase) have been evaluated. In patients with ADA deficiency, inadequate elimination of 2'-deoxyadenosine caused a marked elevation in dATP and a decrease in SAHase level in red blood cells. Prior to treatment with ADAGEN ® (pegademase bovine) Injection, the levels of dATP in the red blood cells ranged from 0.056 to 0.899 µmol/mL of erythrocytes. After 2 months of maintenance treatment with ADAGEN ® (pegademase bovine) Injection, the levels decreased to 0.007 to 0.015 µmol/mL. The normal value of dATP is below 0.001 µmol/mL. In the same period of time, the levels of SAHase increased from the pretreatment range of 0.09 to 0.22 nmol/hr/mg protein to a range of 2.37 to 5.16 nmol/hr/mg protein. The normal value for SAHase is 4.18± 1.9 nmol/hr/mg protein.
The optimal dosage and schedule of administration of ADAGEN ® (pegademase bovine) Injection should be established for each patient, based on monitoring of plasma ADA activity levels (trough levels before maintenance injection), biochemical markers of ADA deficiency (primarily red cell dATP content), and parameters of immune function. Since improvement in immune function follows correction of metabolic abnormalities, maintenance dosage in individual patients should be aimed at achieving the following biochemical goals: 1) maintain plasma ADA activity (trough levels) in the range of 15-35 µmol/hr/mL (assayed at 37° C); and 2) decline in erythrocyte dATP to </=0.005-0.015 µmol/mL packed erythrocytes, or </=1% of the total erythrocyte adenine nucleotide (ATP + dATP) content, with a normal ATP level, as measured in a pre-injection sample.
immunologic data (lymphocyte response to mitogens and lymphocyte surface antigens) were obtained, but their clinical significance is unknown. Prior to treatment with ADAGEN ® (pegademase bovine) Injection, immune status was significantly below normal, as indicated by <10% of normal mitogen responses and circulating mononuclear cells bearing T-cell surface antigens. These parameters improved, though not always to normal, within 2 to 6 months of therapy.