The anti‐CD20 antibody rituximab augments the immunospecific therapeutic effectiveness of an anti‐CD19 immunotoxin directed against human B‐cell lymphoma

Abstract

Summary The chimaeric anti‐CD20 antibody rituximab (Rituxan) sensitises lymphoma cells to small molecule cytotoxic drugs and to protein toxins. We have explored the augmentive effect of rituximab on the anti‐CD19 immunotoxin BU12‐SAPORIN in a model of human lymphoma. Intact rituximab and its F(ab)2 derivative both augmented the immunospecific protein synthesis inhibitory effects of BU12‐SAPORIN in a complement‐independent manner. A combination of rituximab + BU12‐SAPORIN completely abolished the proliferation of Ramos cells in vitro and also induced a significantly greater degree of apoptosis in these cells. Treatment with rituximab, BU12‐SAPORIN or a combination of both induced poly(ADPribose) polymerase and caspase 3 cleavage, although this was always consistently greater in combination‐treated cells. zVAD almost completely inhibited apoptosis in rituximab‐ or BU12‐SAPORIN‐treated cells but only partially in combination‐treated cells. In severe combined immunodeficient (SCID)‐Ramos mice the combination of rituximab + BU12‐SAPORIN was significantly better therapeutically than either single agent. The immunological fidelity of the therapeutic effect because of combination treatment was demonstrated through the failure of rituximab to augment an irrelevant anti‐CD7 immunotoxin. The therapeutic efficacy of rituximab and combination treatment was reduced in SCID‐Ramos mice depleted of serum complement while natural killer cell depletion failed to show any convincing role for antibody‐dependent cellular cytotoxicity. This study shows a clear therapeutic advantage from using rituximab to immunospecifically augment immunotoxin cytotoxicity warranting further investigation.

A CD19-specific single-chain immunotoxin mediates potent apoptosis of B-lineage leukemic cells

“…Interestingly, another CD19-directed saporin-based immunotoxin has been shown to synergize with rituximab. 41 This further demonstrates the feasibility of effectively combining CD19-directed immunotoxins even with other antibody-based therapeutics.…”

Immunotoxin therapy for hematologic malignancies: where are we heading?

“…A combination of Combotox and cytarabine (Ara-C) was used in an advanced ALL murine xenograft model using a NALM/6 cell line and exhibited longer median survival [9]. Anti-CD19 IT BU12-Saporin showed higher activity and, along with anti-CD20 Mab (rituximab) and its F(ab)2 derivative, induced apoptosis in Ramos cells and was effective in SCID-Ramos mice [10].…”

Immunotoxins and Other Conjugates Containing Saporin-S6 for Cancer Therapy

“…When the IT and Rituximab were used in combination, all animals survived and were disease free at day +120. The therapeutic efficacy was reduced in SCID-Ramos mice depleted of serum complement, whereas NK cell depletion failed to show any convincing role for ADCC [19]. …”