Specific binding of sterically stabilized anti-B-cell immunoliposomes and cytotoxicity of entrapped doxorubicin

Administration of doxorubicin (DXR) formulated in sterically stabilized lip osomes, (SL) containig engrafted poly(ethylene glycol-modified phosphatidyl ethanolamine (PEG-PE) on their surface, has been shown to increase the ther apeutic index of the drug. A further improvement could be achieved through targeting of liposome-entrapped drug selectively to cancer cells. This pape r describes the conjugation of the anti-B-cell lymphoma monoclonal antibody LL2 to the surface of DXR-loaded liposomes by use of a PEG-based heterobif unctional coupling agent. Competitive-binding ELISA of the resulting immuno liposomes (SIL) against the monoclonal anti-idiotype antibody, WN, indicate d preserved immonological activity. The pi-I-sensitive probe. HPTS was used to study the binding of liposomes with target cells. The results showed a 3.8-fold increased cellular association of SIL compared to that of SL and a n apparent internalization of SIL into low pH compartments. Addition of an excess of unconjugated free LL? displaced about 72% of the HPTS-SIL associa tion with cells. Experiments with I-125-labeled free and SIL-bound LL2 show ed approximately 50% degradation for both preparations. In vitro MTT cytoto xicity tests against neoplastic B cells gave IC,, values of 1.6, 2.9 and 0. 35 mu M for DXR-SIL, DXR-SL and free DXR, respectively. Leakage of drug fro m the liposomes apparently reduced the specificity of the cytotoxic action of DXR-SIL. (C) 2000 Published by Elsevier Science B.V.