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Characterization of RFB4-Pseudomonas exotoxin A immunotoxins targeted to CD22 on B-cell malignancies
E Mansfield1, I Pastan, D J FitzGerald
1Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Abstract:
To develop an immunotoxin for the treatment of B-cell malignancies, we constructed several candidate conjugates with RFB4, a B-cell specific anti-CD22 IgG1, and truncated forms of Pseudomonas exotoxin (PE). The four versions of PE included PE35 and PE35KDEL, which were linked to RFB4 via a disulfide bond, and PE38 and PE38KDEL, which were linked via a thioether bond. The PE35 truncated forms, which are fully active in ADP ribosylation and lack receptor binding sequences, do not require intracellular proteolytic cleavage in order to be active. PE35KDEL has the consensus endoplasmic reticulum retention signal, KDEL, replacing the wild type PE C-terminal sequence, REDLK. The PE38 forms retain all of domain II and therefore require cleavage to be active within cells. Cytotoxicity experiments on CD22-positive cell lines revealed that the PE35 conjugates were more active than the PE38 versions and the presence of the KDEL sequence generally enhanced toxicity by 5-10-fold compared to that of REDLK. The RFB4-PE35KDEL immunotoxin was most active in cytotoxicity assays against Burkitt's lymphoma cell lines such as Daudi and CA46 (IC50 = 0.2 ng/mL) and displayed little cytotoxicity toward human vascular endothelial cells (IC50 > 20 micrograms/mL). Results of experiments conducted in nude mice showed that both RFB4-PE35KDEL and RFB4-PE35 could inhibit the development of subcutaneous CA46 tumors.
Insights
Researchers developed a novel immunotoxin targeting B-cell malignancies by conjugating RFB4 antibody with Pseudomonas exotoxin (PE) variants. The RFB4-PE35KDEL immunotoxin demonstrated potent and specific cytotoxicity against B-cell lymphoma, offering a promising therapeutic strategy.
Area of Science:
- Oncology
- Immunology
- Molecular Biology
Background:
- B-cell malignancies represent a significant unmet medical need.
- Immunotoxins offer targeted therapy by combining a tumor-specific antibody with a potent toxin.
- CD22 is a promising target for B-cell specific therapies.
Purpose of the Study:
- To develop and evaluate novel immunotoxins targeting CD22 for B-cell malignancy treatment.
- To compare the efficacy of different truncated Pseudomonas exotoxin (PE) forms conjugated to the RFB4 antibody.
- To assess the impact of specific modifications (KDEL sequence) on immunotoxin activity and specificity.
Main Methods:
- Construction of RFB4-Pseudomonas exotoxin (PE) conjugates using truncated PE forms (PE35, PE38) and different linkers (disulfide, thioether).
- In vitro cytotoxicity assays on CD22-positive B-cell lines and non-target human vascular endothelial cells.
- In vivo efficacy studies in nude mice bearing human Burkitt's lymphoma xenografts.
Main Results:
- RFB4-PE35KDEL immunotoxin exhibited potent cytotoxicity against Burkitt's lymphoma cell lines (IC50 = 0.2 ng/mL) with minimal toxicity to normal cells.
- PE35 conjugates were more effective than PE38 versions, and the KDEL sequence enhanced toxicity 5-10 fold.
- In vivo studies demonstrated that RFB4-PE35KDEL and RFB4-PE35 inhibited tumor growth in mice.
Conclusions:
- The RFB4-PE35KDEL immunotoxin is a highly potent and specific agent for targeting CD22-positive B-cell malignancies.
- This engineered immunotoxin shows significant therapeutic potential for treating B-cell lymphomas.
- Further development of KDEL-modified immunotoxins is warranted for clinical application.