Engineering Glypican-3 Targeting Immunotoxins For The Treatment Of Liver Cancer

The expression of glypican-3 (GPC3) in hepatocellular carcinoma offers a target with high tissue specificity and cell signaling implications. A human single domain antibody discovered by phage display technology, HN3, was fused to domain II and domain III of Pseudomonas exotoxin A. This protein (HN3-PE38) showed a high level of cytotoxic activity with a IC50 of 0.4 nM on Hep3B cells, but a relatively low maximum tolerated dose of 0.8 mg/kg in mice. In order to produce an immunotoxin with reduced toxicity, a new version was constructed that removed domain II and seven B cell epitopes from the Pseudomonas toxin. This deimmunized immunotoxin (HN3-mPE24) was shown to have a similar IC50 of 0.2 nM on Hep3B cells. To determine if further deimmunization was possible, three new versions have been generated with T cell epitopes or a series of both B and T cell epitopes removed. These include the HN3-T20 immunotoxin which had six T cell epitopes removed, HN3-T19 with 4 B cell, 4 T cell and 2 shared epitopes, and HN3-M11 with 5 B cell, 4 T cell and 2 shared epitopes. A comparative analysis of these immunotoxins was made using in vitro cell proliferation assays using Hep3B. Both the HN3-T20 (IC50 = 0.6 nM) and HN3-T19 (IC50 = 0.8 nM) immunotoxins had similar activity to HN3-mPE24 (IC50 = 0.7 nM) in a side by side comparison. The HN3-M11 variant had poor cytotoxicity and was excluded from in vivo examination. A Hep3B subcutaneous xenograft model was generated in athymic nude mice and was followed by nine rounds of intravenous immunotoxin treatments. The mPE24, T20 and T19 immunotoxins all showed an increase in average survival rate 70 days (mPE24), 76 days (T20) and 72 days (T19) when compared to the 55 days for PE38 and the 41 days for PBS alone. Additionally, the T20 and the T19 showed a maximum tolerated dose that was similar to that of the mPE24 with dosages as high as 10 mg/kg being well tolerated. This data would suggest that the HN3-T19 immunotoxins has potential clinical applications because it represents the most deimmunized immunotoxin to date. Citation Format: Bryan D. Fleming, Brittany Nixon, Ira Pastan, Mitchell Ho. Engineering glypican-3 targeting immunotoxins for the treatment of liver cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 59. doi:10.1158/1538-7445.AM2017-59