Lymphocyte targeted ricin as a potential therapy for lymphoid malignancy. I. Targeting efficiency.

Lymphocytes were studied as vehicles to target the plant toxin ricin, to lymphoid tissue in rats. Ricin-loaded thoracic duct lymphocyte (TDL) migrated normally into lymph nodes (LN) at 0.5 h, but this process was arrested by 3 h after injection. Ricin was successfully targeted to lymphoid tissue as evidenced by a 4-fold increase in ricin-associated radioactivity in LN, a 10-fold increase in the Peyers patches, a doubling in the spleen and a 35% reduction of radioactivity in the liver compared with free ricin. Nevertheless this represented a considerable shortfall in the expected targeting efficiency. The main problem was found to be high in vivo elution of ricin from TDL (70% within 0.5 h of i.v injection). This and other aspects relevant to maximising targeting efficiency are discussed.