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.

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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.

Pharmacokinetics

The pharmacokinetics and biochemical effects 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.

Biochemical Effects

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.

In vitro 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.

INDICATIONS AND USAGE

General

Any laboratory or clinical indication of a decrease in potency of ADAGEN® (pegademase bovine) Injection should be reported immediately by telephone to Sigma-Tau Pharmaceuticals, Inc. Telephone 866-792-5172.

There have been no reports of hypersensitivity reactions in patients who have been treated with ADAGEN® (pegademase bovine) Injection.

One of 12 patients showed an enhanced rate of clearance of plasma ADA activity after 5 months of therapy at 15 U/kg/week. Enhanced clearance was correlated with the appearance of an antibody that directly inhibited both unmodified ADA and ADAGEN® (pegademase bovine) Injection. Subsequently, the patient was treated with twice weekly intramuscular injections at an increased dose of 20 U/kg, or a total weekly dose of 40 U/kg. No adverse effects were observed at the higher dose and effective levels of plasma ADA were restored. After 4 months, the patient returned to a weekly dosage schedule of 20 U/kg and effective plasma levels have been maintained.

Appropriate care to protect immune deficient patients should be maintained until improvement in immune function has been documented. The degree of immune function improvement may vary from patient to patient and, therefore, each patient will require appropriate care consistent with immunologic status.

Laboratory Tests

The treatment of SCID associated with ADA deficiency with ADAGEN® (pegademase bovine) Injection should be monitored by measuring plasma ADA activity and red blood cell dATP levels.

Plasma ADA activity and red cell dATP should be determined prior to treatment. Once treatment with ADAGEN® (pegademase bovine) Injection has been initiated, a desirable range of plasma ADA activity (trough level before maintenance injection) should be 15–35 μmol/hr/mL. This minimum trough level will ensure that plasma ADA activity from injection to injection is maintained above the level of total erythrocyte ADA activity in the blood of normal individuals.

Plasma ADA activity (pre-injection) should be determined every 1-2 weeks during the first 8-12 weeks of treatment in order to establish an effective dose of ADAGEN® (pegademase bovine) Injection. After 2 months of maintenance treatment with ADAGEN® (pegademase bovine) Injection, red cell dATP levels should decrease to a range of ≤ 0.005 to 0.015 μmol/mL. The normal value of dATP is below 0.001 μmol/mL. Once the level of dATP has fallen adequately, it should be measured 2-4 times a year during the remainder of the first year and 2-3 times a year thereafter, assuming no interruption in therapy.

Between 3 and 9 months, plasma ADA should be determined twice a month, then monthly until after 18-24 months of treatment with ADAGEN® (pegademase bovine) Injection.

Patients who have successfully been maintained on therapy for two years should continue to have plasma ADA measured every 2-4 months and red cell dATP measured twice yearly. More frequent monitoring would be necessary if therapy were interrupted or if an enhanced rate of clearance of plasma ADA activity develops.

Once effective ADA plasma levels have been established, should a patient’s plasma ADA activity level fall below 10 μmol/hr/mL (which cannot be attributed to improper dosing, sample handling or antibody development) then the patients receiving this lot of ADAGEN® (pegademase bovine) Injection should be requested to have a blood sample for plasma ADA determination taken prior to their next injection of ADAGEN® (pegademase bovine) Injection.

Immune function, including the ability to produce antibodies, generally improves after 2-6 months of therapy, and matures over a longer period. Compared with 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. However, the lag between the correction of the metabolic abnormalities and improved immune function with a trend toward diminished frequency of infections and complications of infection is variable, and has ranged from a few weeks to approximately 6 months. Improvement in the general clinical status of the patient may be gradual (as evidenced by improvement in various clinical parameters) but should be apparent by the end of the first year of therapy. Antibody to ADAGEN® (pegademase bovine) Injection may develop in patients and may result in more rapid clearance of ADAGEN® (pegademase bovine) Injection. Antibody to ADAGEN® (pegademase bovine) Injection should be suspected if a persistent fall in pre-injection levels of plasma ADA to < 10 μmol/hr/mL occurs. If other causes for a decline in plasma ADA levels can be ruled out [such as improper storage of ADAGEN® (pegademase bovine) Injection vials (freezing or prolonged storage at temperatures above 8°C), or improper handling of plasma samples (e.g., repeated freezing and thawing during transport to laboratory)], then a specific assay for antibody to ADA and ADAGEN® (pegademase bovine) Injection (ELISA, enzyme inhibition) should be performed.

In patients undergoing treatment with ADAGEN® (pegademase bovine) Injection, a decline in immune function, with increased risk of opportunistic infections and complications of infection, will result from failure to maintain adequate levels of plasma ADA activity [whether due to the development of antibody to ADAGEN® (pegademase bovine) Injection, to improper calculation of ADAGEN® (pegademase bovine) Injection dosage, to interruption of treatment or to improper storage of ADAGEN® (pegademase bovine) Injection with subsequent loss of activity]. If a persistent decline in plasma ADA activity occurs, immune function and clinical status should be monitored closely and precautions should be taken to minimize the risk of infection. If antibody to ADA or ADAGEN® (pegademase bovine) Injection is found to be the cause of a persistent fall in plasma ADA activity, then adjustment in the dosage of ADAGEN® (pegademase bovine) Injection and other measures may be taken to induce tolerance and restore adequate ADA activity.

Drug Interactions

There are no known drug interactions with ADAGEN® (pegademase bovine) Injection. However, Vidarabine is a substrate for ADA and 2′-deoxycoformycin is a potent inhibitor of ADA. Thus, the activities of these drugs and ADAGEN® (pegademase bovine) Injection could be substantially altered if they are used in combination with one another.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term carcinogenic studies in animals have not been performed with ADAGEN® (pegademase bovine) Injection nor have studies been performed on impairment of fertility.

ADAGEN® (pegademase bovine) Injection did not exhibit a mutagenic effect when tested against Salmonella typhimurium strains in the Ames assay.

Pregnancy

Pregnancy Category C. Animal reproduction studies have not been conducted with ADAGEN® (pegademase bovine) Injection. It is also not known whether ADAGEN® (pegademase bovine) Injection can cause fetal harm when administered to a pregnant woman or can affect reproduction capacity. ADAGEN® (pegademase bovine) Injection should be given to a pregnant woman only if clearly needed.

Nursing Mothers

It is not known whether ADAGEN® (pegademase bovine) Injection is excreted in human milk. Because many drugs are excreted in human milk, caution should be exercised when ADAGEN® (pegademase bovine) Injection is administered to a nursing woman.

ADVERSE REACTIONS