Two Saporin-Containing Immunotoxins Specific for CD20 and CD22 Show Different Behavior in Killing Lymphoma Cells

Abstract

Immunotoxins (ITs) are hybrid proteins combining the binding specificity of antibodies with the cytocidal properties of toxins. They represent a promising approach to lymphoma therapy. The cytotoxicity of two immunotoxins obtained by chemical conjugation of the plant toxin saporin-S6 with the anti-CD20 chimeric antibody rituximab and the anti-CD22 murine antibody OM124 were evaluated on the CD20-/CD22-positive cell line Raji. Both ITs showed strong cytotoxicity for Raji cells, but the anti-CD22 IT was two logs more efficient in killing, probably because of its faster internalization. The anti-CD22 IT gave slower but greater caspase activation than the anti-CD20 IT. The cytotoxic effect of both immunotoxins can be partially prevented by either the pan-caspase inhibitor Z-VAD or the necroptosis inhibitor necrostatin-1. Oxidative stress seems to be involved in the cell killing activity of anti-CD20 IT, as demonstrated by the protective role of the H2O2 scavenger catalase, but not in that of anti-CD22 IT. Moreover, the IT toxicity can be augmented by the contemporary administration of other chemotherapeutic drugs, such as PS-341, MG-132, and fludarabine. These results contribute to the understanding of the immunotoxin mechanism of action that is required for their clinical use, either alone or in combination with other drugs.

Strategies to Improve the Clinical Utility of Saporin-Based Targeted Toxins

“…Rituximab/Saporin and OM124/Saporin are respectively directed against CD20 and CD22, both antigens highly expressed on Non-Hodgkin’s lymphomas (NHLs) and found particularly expressed at high levels on normal mature B-cells and on a large population of B-lymphoma cells but absent in the normal tissues and hematopoietic stem cells. A way to increase the therapeutic efficacy of these two Saporin-based ITxs was obtained by the simultaneous administration of chemotherapeutic agents or cell inhibitors, such as the proteasome inhibitors PS-341, MG-132, or the purine analogue fludarabine [ 111 ].…”

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

“…Cytotoxicity of both ITs could be in part prevented by pan-caspase inhibitor Z-VAD or necrostatin-1 a necroptosis inhibitor. Oxidative stress was involved in the cell-killing activity of Rituximab IT, as demonstrated by the protective role of the hydrogen peroxide scavenger catalase, but not in that of anti-CD22 IT [ 93 ]. Thus, depending on the targeting moiety, different pathways may be found activated.…”

Ricin: An Ancient Story for a Timeless Plant Toxin

“…In conclusion, ricin is a highly cytotoxic plant protein and has been of great utility to develop a number of anti-cancer immunotoxins. The ability of ricin, and of some other RIPs, to act on multiple molecular target inside the cell, thus triggering different death pathways, makes it more attractive for cancer treatment than conventional chemotherapy, in which one of the major problems is the rise of resistant cells [67,124]. However, ricin-containing ITs also exhibited many limitations like unspecific toxicity, organ toxicity (mainly liver, kidney and vasculature), immunogenicity, fast…”