Taking aim: Mesothelin as a novel target for pediatric AML therapy

From the Meshinchi Lab, Clinical Research Division, and Cancer Consortium colleagues

Intensive chemotherapy is the standard of care for childhood acute myeloid leukemia (AML). Although the majority of patients show dramatic responses to these treatments, driving disease burden beneath the limits of detection, relapses are common and remain the major cause of mortality in this setting. Hematopoietic stem cell transplantation therapies, pioneered and refined by researchers at Fred Hutch, can be effective in management of AML, however, despite intensive therapy and stem cell transplantation, survival rates in AML remain unacceptably low. Targeted immunotherapies, consisting of antibody- or cell-based therapies designed to direct immune effectors against cancer cells, are a new area of investigation that holds potential for the treatment of pediatric AML. However, the success of these therapies is limited by the availability of appropriate tumor antigen targets.

The Meshinchi Laboratory in the Fred Hutch Clinical Research Division recently completed the largest ever target discovery effort in AML, identifying a library of new target candidates for immunotherapies against childhood and adult AML. One of their findings, which was recently published in Blood Advances in collaboration with Dr. Katherine Tarlock, faculty member in Pediatric Hematology/Oncology at Seattle Children’s Hospital, reveals that the cell surface protein mesothelin (MSLN) is highly expressed in a subset of high-risk pediatric and adult AMLs. Further, they generated several novel MSLN-directed antibody-drug conjugates (ADCs) and demonstrated that MSLN-specific ADCs successfully eliminate AML cells in culture and extend survival in preclinical mouse leukemia models, establishing the basis for future studies evaluating MSLN as a therapeutic target in AML.

Subsets of pediatric and adult AMLs overexpress MSLN at levels similar to MSLN+ solid tumors.
Subsets of pediatric and adult AMLs overexpress MSLN at levels similar to MSLN+ solid tumors. Image provided by Dr. Tarlock.

Tumor-targeted immunotherapies rely on the binding of antibodies or immune cell receptors to antigens presented on the surface of cancer cells, targeting them for destruction. The challenge lies in selecting an antigen that is expressed on tumor cells, but not in healthy tissues, to a sufficient degree as to allow effective killing of leukemic cells without causing toxicity. Because of similarities between leukemic cells and normal blood cells, unfortunately, such AML-specific therapies have been lacking. “The ability to target the AML cells and spare normal hematopoietic cells would have significant impact on how we treat AML,” said Dr. Tarlock. “The shared expression between cell surface antigens on AML and healthy blood cells has been one of the most significant limiting factors in the development of immunotherapeutics for AML.”

In their search for these elusive AML-specific antigen targets, Dr. Meshinchi spearheaded the largest ever discovery effort in AML by aiming to sequence over 2000 AML cases in infants, children and young adults. The effort, which began nearly 10 years ago, was originally funded by the National Cancer Institute/Children’s Oncology Group Therapeutically Applicable Research to Generate Effective Treatments (TARGET) AML initiative (phase I) and, more recently, was completed under funding support from Target Pediatric AML (phase 2), a grass-roots organization of parent advocates and private foundations. This unprecedented endeavor allowed the identification of a library of novel, AML-restricted targets that are currently under therapeutic development in the Meshinchi lab. Amongst these findings, their analyses revealed a familiar, but surprising, AML target candidate. “We found mesothelin to be expressed on the cell surface of approximately 1/3 of children, as well as adults, with AML,” said Dr Tarlock. “More importantly, MSLN is over-represented in a group of patients with rearrangements in the KMT2A gene which is associated with dismal outcomes with current therapies.”

MSLN is a cell surface protein that has attracted attention as a tumor-associated antigen overexpressed by many solid tumor types, and a variety of immunotherapeutic agents have already been developed for cancer treatment in these settings. Further, since MSLN protein can be cleaved from the surface of the cell, patients with MSLN-expressing tumors often exhibit elevated levels of soluble MSLN in the blood, lending itself as a diagnostic marker in some solid tumor settings. Importantly, though it is known to be present in some healthy cell types (including mesothelial cells lining the lungs, heart, and the abdominal cavity), immunotherapeutic agents directed against MSLN have not shown signs of off-tumor/on-target toxicity in the clinic.

The group validated increased expression of MSLN transcripts by quantitative reverse transcription PCR (qRT-PCR) performed on a large cohort of pediatric and adult AML samples, compared to a group of healthy bone marrow samples. Further, developing a new diagnostic tool, they performed multidimensional flow cytometry to assess surface MSLN protein using samples collected from 158 childhood and 43 adult AML patients. The two measures confirmed MSLN overexpression in ~29% of both pediatric and adult AML cases, and showed that MSLN expression was absent in normal hematopoietic cells. Finally, the investigators evaluated whether soluble MSLN could be detected in the context of AML, assaying serum from 122 pediatric patients with matched transcriptional data. Excitingly, they observed a direct correlation between MSLN transcript and soluble protein levels, with 97% specificity and 75% sensitivity, indicating possible diagnostic applications for this assay in AML.

Having established MSLN as a target antigen candidate in AML, the investigators next pursued preclinical studies to assess the efficacy of MSLN-targeted therapies against AML. They tested two different MSLN-targeted ADC-based compounds, one made by Bayer Pharma (anetumab ravtansine - AR) and a second novel ADC developed by the Meshinchi lab, anti-MSLN-DGN462, carrying microtubule inhibitor and DNA alkylator payloads, respectively. Encouragingly, both agents showed in vitro target-dependent cytotoxicity against leukemia cell lines engineered to express MSLN, and DNA-damaging ADCs were effective against AML cells that naturally overexpress MSLN. Finally, using patient-tumor derived xenograft models, AR ADC therapy yielded significantly longer survival in a MSLN-dependent manner compared to controls.

This work establishes MSLN as a promising target candidate for pediatric and adult AMLs, with important potential implications for patients. “We are excited about developing therapies against these novel, AML-restricted targets,” said Dr. Meshinchi.  “For decades we have treated AML in a non-targeted manner given lack of more directed therapies. With emerging targeted therapies, especially immunotherapies, the main roadblock is lack of appropriate targets. The holy grail of cancer is finding targets that are expressed on the cancer cells, but not on normal tissues, allowing precise and effective treatment with no side effects. We believe that we have identified such targets, with MSLN being one of a library of many targets in AML.”

Moving forward, the authors are pursuing several MSLN-specific therapeutic platforms for the treatment of AML. “The presence of mesothelin on the cell surface means it is a target for immunotherapeutic intervention and we have shown successful targeting with antibody drug conjugates as well as other strategies including MSLN-specific CAR T cells,” said Dr. Tarlock. With a nod to the role of the UW/Fred Hutch Cancer Consortium in helping unite researchers and clinicians working towards better patient outcomes, Drs. Tarlock and Meshinchi described their ongoing efforts. “Dr. Meshinchi and I are leading the development of a clinical trial through the Children's Oncology Group consortium with a MSLN-targeted ADC for children with relapsed and refractory AML”, said Dr. Tarlock. “In addition, Dr. Quy Le, a Fred Hutch senior scientist, is leading the development of anti-mesothelin CAR T cells in the Meshinchi lab,” she added. “We are teaming up with Dr. Mary-Beth Percival to conduct the first MSLN-directed CAR T cell trial through SCCA in adults and children with AML,” continued Dr. Meshinchi. “Armed with deep knowledge of the disease and with new targets, we are highly optimistic about the future of immunotherapy in AML.”

This work was funded by the St. Baldrick’s Foundation, TARGET Pediatric AML, the Leukemia and Lymphoma Society, the National Institutes of Health, the National Cancer Institute , the Department of Health and Human Services, the Andrew McDonough B+ Foundation, the American Society of Clinical Oncology Conquer Cancer Foundation, the Children’s Oncology Group Foundation, Hyundai Hope on Wheels, Project Stella, the Rally Foundation for Childhood Cancer Research, The Truth 365, and the Leukemia Research Foundation of Delaware.

UW/Fred Hutch Cancer Consortium members Katherine Tarlock and Soheil Meshinchi contributed to this work.

Kaeding AJ, Barwe SP, Gopalakrishnapillai A, Ries RE, Alonzo TA, Gerbing RB, Correnti C, Loken MR, Broderson LE, Pardo L, Le QH, Tang T, Leonti AR, Smith JL, Chou CK, Xu M, Triche T, Kornblau SM, Kolb EA, Tarlock K, Meshinchi S. Mesothelin is a novel cell surface disease marker and potential therapeutic target in acute myeloid leukemia. Blood Adv. 2021 May 11;5(9):2350-2361. doi: 10.1182/bloodadvances.2021004424. PMID: 33938941; PMCID: PMC8114558.