Survival of human carrier erythrocytes in vivo

Erythrocytes offer the exciting opportunity of being used as carriers of th erapeutic agents. Encapsulation within erythrocytes will give the therapeut ic agent a clearance equivalent to the normal life of the erythrocyte there fore maintaining therapeutic brood levels over prolonged periods and also g iving a sustained delivery to the monocyte-macrophage system (reticuloendot helial system). Both the dose and frequency of therapeutic interventions co uld thus be reduced. Ensuring a near-physiological survival time of carrier erythrocytes is essential to their successful use as a sustained drug deli very system, and this has not been demonstrated in man. In this study we as sessed the survival in vivo of autologous unloaded energy-replete carrier e rythrocytes in nine volunteers, using a standard Cr-51 erythrocyte-labellin g technique. Within 144 h after infusion there was a 3 to 49% fall in circu lating labelled cells, followed thereafter by an almost complete return to initial circulating levels; surface counting demonstrated an initial seques tration of erythrocytes by the spleen and subsequent release. Mean cell lif e and cell half-life of the carrier erythrocytes were within the normal ran ge of 89 to 131 days and 19 to 29 days respectively. These results demonstr ate the viability of carrier erythrocytes as a sustained drug delivery syst em.