Figures & data
Figure 1 Structure of widely used toxins and immunotoxins based on them.
Notes: Native PE A contains 3 functional domains: domain Ia (binding domain), domain II (mediates translocation of the toxin), and III (catalytic domain). BL22, which is also called CAT3888 or RFB4(dsFv)-PE38, is produced by replacing the domain I with a single-chain Fv target CD22. VL and VH are linked by disulfide bond. In its second generation of mutant, HA22-LR, most of domain II was deleted and PE amino acids 251–394 were replaced by 274–284. DT also contains 3 domains: A (enzymatic domain), B (binding domain), and T (transmembrane domain). In ONTAK, a recombinant human IL-2 was fused to the C-terminus of the toxin. Ricin is a plant toxin that belongs to holotoxins and has both enzymatic domain (A) and binding domain (B). RFT5-dgA is formed by a monoclonal antibody target CD25 and a deglycosylated ricin A chain. A hindered heterobifunctional cross-linker links the antibody and toxin. Gelonin only has enzymatic domain (A). HUM-195/Rgel contains a recombinant gelonin conjugated to a humanized antibody target CD33.
Abbreviations: PE, Pseudomonas exotoxin; DT, Diphtheria toxin; IL-2, interleukin-2; Rgel, recombinant gelonin; dgA, deglycosylated ricin A chain; SMPT, N-succimidyl-oxycarbonyl-α-methyl-α-(2-pyridyldithio)-toluene.
Abbreviations: PE, Pseudomonas exotoxin; DT, Diphtheria toxin; IL-2, interleukin-2; Rgel, recombinant gelonin; dgA, deglycosylated ricin A chain; SMPT, N-succimidyl-oxycarbonyl-α-methyl-α-(2-pyridyldithio)-toluene.
Figure 2 Mechanism of ADCs and immunotoxins based on PE A and DT.
Notes: ADCs and immunotoxins are internalized into an endocytic compartment after binding on the cell surface. The ADCs travel to lysosomes, where the drug is released from the antibody, inducing drug penetration in the cytosol, disruption of microtubule dynamics, and cell death. Modified PE toxin is cleaved from immunotoxin by the furin protease and transported to the ER through the Golgi. The toxin catalyzes ADP ribosylation of eEF2, inducing inhibition of protein synthesis and cell death. The T domain of DT forms a pore in the membrane of the endosome, allowing transit of DT in the cytosome. DT also catalyzes inhibitory modification of eEF2.
Abbreviations: ADC, antibody–drug conjugate; ER, endoplasmic reticulum; PE, Pseudomonas exotoxin A; DT, Diphtheria toxin; ADP-ribose, adenosine diphosphate ribose; eEF2, eukaryotic elongation factor 2; RIT, recombinant immunotoxins.
Abbreviations: ADC, antibody–drug conjugate; ER, endoplasmic reticulum; PE, Pseudomonas exotoxin A; DT, Diphtheria toxin; ADP-ribose, adenosine diphosphate ribose; eEF2, eukaryotic elongation factor 2; RIT, recombinant immunotoxins.
Table 1 RITs targeting hematologic malignancies
Table 2 RITs targeting solid tumors
Figure 3 Development of the PE-based immunotoxins to increase the toxicity and to decrease the immunogenicity.
Notes: (A) RITs targeting CD22. (B) RITs targeting MSLN. BL22, also named RFB4(dsFv)-PE38 and CAT-3888, contained a single-chain Fv of the anti-CD22 antibody fused to truncated PE toxin. HA22 was mutated from BL22 by replacing the residues at positions 100, 100a, and 100b of VH, represented here by horizontal red bars. In HA22-LR, the deletion mutant, most of domain II of PE toxin was deleted. HA22-LR-8M, a mutant of HA22-LR, was reported to contain 8 mutations, D406A, R432G, R467A, R490A, R513A, E548A, K590S, and Q592A. SS1P consists of the Fv fragment from anti-MSLN monoclonal antibody coupled to the same PE fragment with BL22. SS1-LR-GGS was developed by deleting the domain II of PE toxin with GGS linker between the antibody and the domain III. RG7787 used a humanized Fab fragment of anti-MSLN antibody and deleted PE domain II. In RG7787, there were 7-point mutations in domain III of PE toxin at B-cell epitopes to eliminate binding to B-cell receptor. LMB-T20 consisted of the Fv fragment coupled to the similar PE fragment with RG7787, but with 6-point mutations in domain III at T-cell epitopes.
Abbreviations: PE, Pseudomonas exotoxin A; RIT, recombinant immunotoxins; MSLN, mesothelin.
Abbreviations: PE, Pseudomonas exotoxin A; RIT, recombinant immunotoxins; MSLN, mesothelin.
