Immunogenicity of Immunotoxins Containing Pseudomonas Exotoxin A: Causes, Consequences, and Mitigation

Our plain-English summary of Immunogenicity of Immunotoxins Containing Pseudomonas Exotoxin A: Causes, Consequences, and Mitigation, by R. Mazor, I. Pastan (Frontiers in Immunology, 2020).

The study

• Authors: R. Mazor, I. Pastan
• Journal: Frontiers in Immunology
• Published: 2020;11:1261
• Type of article: Review
• Cited by: 75 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Immunotoxins are cytolytic fusion proteins developed for cancer therapy, composed of an antibody fragment that binds to a cancer cell and a protein toxin fragment that kills the cell. Pseudomonas exotoxin A (PE) is a potent toxin that is used for the killing moiety in many immunotoxins. Moxetumomab Pasudotox (Lumoxiti) contains an anti-CD22 Fv and a 38 kDa portion of PE. Lumoxiti was discovered in the Laboratory of Molecular Biology at the U.S. National Cancer Institute and co-developed with Medimmune/AstraZeneca to treat hairy cell leukemia. In 2018 Lumoxiti was approved by the US Food and Drug Administration for the treatment of drug-resistant Hairy Cell Leukemia. Due to the bacterial origin of the killing moiety, immunotoxins containing PE are highly immunogenic in patients with normal immune systems, but less immunogenic in patients with hematologic malignancies, whose immune systems are often compromised. LMB-100 is a de-immunized variant of the toxin with a humanized antibody that targets mesothelin and a PE toxin that was rationally designed for diminished reactivity with antibodies and B cell receptors. It is now being evaluated in clinical trials for the treatment of mesothelioma and pancreatic cancer and is showing somewhat diminished immunogenicity compared to its un modified parental counterpart. Here we review the immunogenicity of the original and de-immunized PE immunotoxins in mice and patients, the development of anti-drug antibodies (ADAs), their impact on drug availability and their effect on clinical efficacy. Efforts to mitigate the immunogenicity of immunotoxins and its impact on immunogenicity will be described including rational design to identify, remove, or suppress B cell or T cell epitopes, and combination of immunotoxins with immune modulating drugs.

Copyright © 2020 Mazor and Pastan.

In plain English

This was a narrative review, by authors from the US Food and Drug Administration and the National Cancer Institute, of the immunogenicity of recombinant immunotoxins (RITs) built from Pseudomonas exotoxin A (PE), including moxetumomab pasudotox (Lumoxiti, anti-CD22), SS1P and LMB-100 (both targeting mesothelin and tested in mesothelioma and pancreatic cancer). The review summarised antibody responses reported in earlier trials: 16/16 patients given OVB3-PE made anti-PE antibodies within 14 days; 1/28 hairy cell leukaemia patients made neutralizing antibodies (Nabs) after the first Lumoxiti cycle and 10/28 during the phase 1 trial; 17% of patients with haematological malignancies but 92% of melanoma patients made Nabs after one cycle of LMB-2. The authors also analysed antibody and pharmacokinetic data from a trial of LMB-100 with nab-paclitaxel in pancreatic adenocarcinoma, where 9/20 patients had pre-existing antibodies and 9/10 patients evaluated after treatment were antibody positive. Mitigation approaches described included immune-suppressing drug combinations and removal of B- and T-cell epitopes. The authors concluded that de-immunization delayed, but did not eradicate, antibody formation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Narrative review of published clinical and preclinical work, including the authors' own analysis of antibody and pharmacokinetic data from a previously reported LMB-100 trial
• Who was studied: Patients treated with PE-based immunotoxins in published clinical trials (hairy cell leukaemia, CLL, T-cell leukaemia, mesothelioma, pancreatic and ovarian cancer, melanoma) and mice in preclinical studies; the authors additionally analysed 20 patients from an LMB-100 plus nab-paclitaxel pancreatic cancer trial
• Exposure: Recombinant and chemically conjugated immunotoxins containing Pseudomonas exotoxin A (PE, full length, PE38, PE24, LO10 and de-immunized variants) linked to antibody fragments targeting CD22, CD25, mesothelin, Lewis Y, EpCAM, EGFR, BCMA and GPC3
• Main results:
  • In the OVB3-PE trial, 16/16 patients developed anti-PE antibodies within 14 days, and human anti-mouse antibodies were detected in 75%
  • With Lumoxiti in hairy cell leukaemia, 1/28 patients made neutralizing antibodies after the first cycle and 10/28 during the whole phase 1 trial; in phase II/III trials monitored for binding ADA, 65% made ADA after two cycles and 75% had detectable ADA at the end of treatment in a larger trial
  • Patients with haematological malignancies given LMB-2 had 17% neutralizing antibody formation after the first cycle
  • Melanoma patients given LMB-2, with normal immune systems, had 92% neutralizing antibody formation
  • In the LMB-100 plus nab-paclitaxel pancreatic cancer trial analysed by the authors, 9/20 patients had pre-existing antibodies at a cut point of OD = 0.05, and after treatment 9/10 evaluated patients were ADA positive
  • The authors reported that more than half of patients given a second cycle of LMB-100 had detectable plasma drug levels, compared with more than 90% of SS1P patients having undetectable levels by the start of cycle 2
  • In 10 refractory mesothelioma patients, SS1P given with pentostatin and cyclophosphamide delayed neutralizing antibody formation; two made Nabs after the first cycle, 6 after the second, and one made none through six cycles
  • Pretreatment with rituximab removed 99.9% of circulating B cells in 5/5 patients, but all developed neutralizing anti-LMB-1 antibodies by day 21
  • A clinical trial combining LMB-100 with rapamycin-containing synthetic vaccine particles in mesothelioma patients was stopped because of unexpected lung toxicity
• The authors' conclusion: The authors concluded that LMB-100, the first humanized and B-cell de-immunized RIT tested in patients, showed lower immunogenicity than its parental SS1P but that antibody formation was only delayed, not eliminated, so further approaches (removing both B- and T-cell epitopes, or combining with immune suppressive agents) are needed
• Limitations the authors state:
  • The authors state that immunogenicity rates of LMB-100 cannot be compared directly with earlier trials because monitoring assays, doses, molecule size and structure and patient eligibility (pre-existing antibodies) differed
  • The authors state that T-cell de-immunized molecules have not yet been tested in patients
  • The authors state that HLA binding prediction algorithms missed experimentally identified T-cell epitopes and must be validated experimentally
  • The authors note that mouse-based screening for immunogenic regions is limited because mice and humans differ in self and non-self selection

Topics

Medical terms in this study, explained in our Lexicon: Mesothelin, Clinical Trial, Mesothelioma.

Related studies

• Immunogenicity of therapeutic recombinant immunotoxins — R. Mazor, M. Onda, et al. (2016)
• Anti-Mesothelin Recombinant Immunotoxin Therapy for Colorectal Cancer — A. Cerise, T.K. Bera, et al. (2019)
• Design and modification of EGF4KDEL 7Mut, a novel bispecific ligand-directed toxin, with decreased immunogenicity and potent anti-mesothelioma activity — B.J. Stish, S. Oh, et al. (2009)
• Recombinant Immunotoxin with T-cell Epitope Mutations That Greatly Reduce Immunogenicity for Treatment of Mesothelin-Expressing Tumors — R. Mazor, J. Zhang, et al. (2015)
• Dual B- and T-cell de-immunization of recombinant immunotoxin targeting mesothelin with high cytotoxic activity — R. Mazor, M. Onda, et al. (2016)
• Modulating mesothelin shedding to improve therapy — I. Pastan, Y. Zhang (2012)

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