Polyethylene glycol-modified liposome-encapsulated hemoglobin: a long circulating red cell substitute

A major obstacle in the development of red cell substitutes has been overcoming their short circulation persistence. In this study, distearoyl phosphoethanolamine polyethylene glycol 5000 (PEG-PE) (10 mol%) was added to the formulation of liposome-encapsulated hemoglobin (LEH) to decrease reticuloendothelial system uptake and prolong LEH circulation persistence. PEG-LEH was radiolabeled with technetium-99m, infused into rabbits (25% of blood pool at 1 ml/min) (n = 5), and monitored by scintigraphic imaging at various times out to 48 h. At 48 h, animals were sacrificed, and tissue samples were collected for counting in a scintillation well counter. Tissue distribution data at 48 h revealed that 51.3 +/- 3.4% of the technetium-99m-PEG-LEH remained in circulation, a greater than 3-fold increase in the circulation half-life compared with circulation half-lives previously reported for non-PEG-containing LEH formulations. The liver had the greatest accumulation at 48 h (12.7 +/- 0.7%), followed by bone marrow (6.2 +/- 0.1%), whereas the spleen had only 1.4 +/- 0.2%. The addition of PEG-PE to the LEH formulation greatly prolongs the circulation persistence of LEH and represents a significant step in the development of red cell substitutes with prolonged oxygen delivery.