ICAM-1 recycling in endothelial cells: a novel pathway for sustained intracellular delivery and prolonged effects of drugs

Intercellular adhesion molecule-1 (ICAM-1) is a target for drug delivery to endothelial cells (ECs), which internalize multivalent anti-ICAM nanocarriers (anti-ICAM/NCs) within 15 to 30 minutes. The concomitant ICAM-1 disappearance from the EC surface transiently inhibited subsequent binding and uptake of anti-ICAM/NCs. Within 1 hour, internalized ICAM-1 diverged from anti-ICAM/NCs into prelysosomal vesicles, resurfaced, and enabled uptake of a subsequent anti-ICAM/NC dose. Thus, internalized ICAM-1 was able to recycle back to the plasma membrane. In vivo pulmonary targeting of a second anti-ICAM/NC dose injected 15 minutes after the first dose was decreased by 50% but recovered between 30 minutes and 2.5 hours, comparable to cultured ECs. Anti-ICAM/NCs affected neither EC viability nor fluid-phase endocytosis and traffic to lysosomes. However, lysosomal trafficking of the second dose of anti-ICAM/NCs was decelerated at least 2-fold versus the first dose; hence the major fraction of anti-ICAM/NCs resided in prelysosomal vesicles for at least 5 hours without degradation. Two successive