In vitro and in vivo studies with human carrier erythrocytes loaded with polyethylene glycol-conjugated and native adenosine deaminase

Polyethylene glycol-conjugated adenosine deaminase (pegademase) is used for enzyme replacement therapy for patients with severe combined immunodeficie ncy caused by adenosine deaminase deficiency. The entrapment of pegademase within human energy-replete carrier erythrocytes using a hypo-osmotic dialy sis procedure was investigated with the objective of prolonging the in vivo circulatory half-life of the enzyme and maintaining therapeutic blood leve ls, Native unmodified adenosine deaminase (ADA) was similarly studied. The efficiency of pegademase entrapment was low (9%) whereas the entrapment of native unmodified ADA was substantial (50%), suggesting that the polyethyle ne glycol side-chains were impeding intracellular entrapment. The biochemic al characteristics and the osmotic fragility of these carrier erythrocytes were not adversely affected by the entrapment of either pegademase or nativ e ADA. In vivo survival studies of pegademase-loaded Cr-51- labelled carrie r erythrocytes in an ADA-deficient adult patient showed a mean cell half-li fe of 16 d. Carrier erythrocyte-entrapped pegademase and native ADA had in vivo half-lives of 20 and 12.5 d, respectively demonstrating that entrapmen t prolongs the half-life over that of plasma pegademase, which has a circul ating half-life of 3-6 d. These results provide the basis for a more extens ive clinical evaluation of carrier erythrocyte-entrapped native adenosine d eaminase therapy.