Extreme thrombocytosis in response to PEG-ADA: early therapeutic and risk indicator

Treatment with Elapegademase Restores Immunity in Infants with Adenosine Deaminase Deficient Severe Combined Immunodeficiency

PURPOSE

Patients with adenosine deaminase 1 deficient severe combined immunodeficiency (ADA-SCID) are initially treated with enzyme replacement therapy (ERT) with polyethylene glycol-modified (PEGylated) ADA while awaiting definitive treatment with hematopoietic stem cell transplant (HSCT) or gene therapy. Beginning in 1990, ERT was performed with PEGylated bovine intestinal ADA (ADAGEN®). In 2019, a PEGylated recombinant bovine ADA (Revcovi®) replaced ADAGEN following studies in older patients previously treated with ADAGEN for many years. There are limited longitudinal data on ERT-naïve newborns treated with Revcovi.

METHODS

We report our clinical experience with Revcovi as initial bridge therapy in three newly diagnosed infants with ADA-SCID, along with comprehensive biochemical and immunologic data.

RESULTS

Revcovi was initiated at twice weekly dosing (0.2 mg/kg intramuscularly), and monitored by following plasma ADA activity and the concentration of total deoxyadenosine nucleotides (dAXP) in erythrocytes. All patients rapidly achieved a biochemically effective level of plasma ADA activity, and red cell dAXP were eliminated within 2-3 months. Two patients reconstituted B-cells and NK-cells within the first month of ERT, followed by naive T-cells one month later. The third patient reconstituted all lymphocyte subsets within the first month of ERT. One patient experienced declining lymphocyte counts with improvement following Revcovi dose escalation. Two patients developed early, self-resolving thrombocytosis, but no thromboembolic events occurred.

CONCLUSION

Revcovi was safe and effective as initial therapy to restore immune function in these newly diagnosed infants with ADA-SCID, however, time course and degree of reconstitution varied. Revcovi dose may need to be optimized based on immune reconstitution, clinical status, and biochemical data.

Enzymes approved for human therapy: indications, mechanisms and adverse effects

Research and drug developments fostered under orphan drug product development programs have greatly assisted the introduction of efficient and safe enzyme-based therapies for a range of rare disorders. The introduction and regulatory approval of 20 different recombinant enzymes has enabled, often for the first time, effective enzyme-replacement therapy for some lysosomal storage disorders, including Gaucher (imiglucerase, taliglucerase, and velaglucerase), Fabry (agalsidase alfa and beta), and Pompe (alglucosidase alfa) diseases and mucopolysaccharidoses I (laronidase), II (idursulfase), IVA (elosulfase), and VI (galsulfase). Approved recombinant enzymes are also now used as therapy for myocardial infarction (alteplase, reteplase, and tenecteplase), cystic fibrosis (dornase alfa), chronic gout (pegloticase), tumor lysis syndrome (rasburicase), leukemia (L-asparaginase), some collagen-based disorders such as Dupuytren's contracture (collagenase), severe combined immunodeficiency disease (pegademase bovine), detoxification of methotrexate (glucarpidase), and vitreomacular adhesion (ocriplasmin). The development of these efficacious and safe enzyme-based therapies has occurred hand in hand with some remarkable advances in the preparation of the often specifically designed recombinant enzymes; the manufacturing expertise necessary for commercial production; our understanding of underlying mechanisms operative in the different diseases; and the mechanisms of action of the relevant recombinant enzymes. Together with information on these mechanisms, safety findings recorded so far on the various adverse events and problems of immunogenicity of the recombinant enzymes used for therapy are presented.

Polyethylene glycol-conjugated adenosine phosphorylase: development of alternative enzyme therapy for adenosine deaminase deficiency

Purine nucleoside phosphorylase had previously been engineered to accept 6-amino substituted purine nucleosides by two active site substitutions, Asn243Asp; Lys244Gln. In the present study, recombinant adenosine phosphorylase (AP) has been conjugated to branched polyethylene glycol (PEG) polymers of approximately 42.5 kDa. Matrix-assisted laser desorption/ionization analysis and SDS acrylamide electrophoresis analysis indicated a subunit composition of greater than 205 kDa consistent with the conjugation of as many as four PEG molecules per AP subunit. The PEG-conjugated enzyme retained greater than 90% of the native catalytic activity. Administration of the enzyme to mice demonstrated the PEG-AP to have a 67-fold increased plasma half-life compared to the native enzyme, 65.1+/-2.9 h versus 57.8+/-1.1 min, respectively. PEG-AP was principally confined to the plasma with minimal activity detected in tissues and of these spleen had the greatest activity and essentially no activity was found in urine. PEG-AP has retained activity with inosine and its injection into PNP-deficient mice resulted in a 2.7-fold increase in urine urate. AP was also shown to protect human CEM cells in culture from the toxic effects of 2'-deoxyadenosine. These studies provide evidence for consideration of PEG-AP as an alternative enzyme therapy for the inherited deficiency of adenosine deaminase.