Immunotherapeutic Targeting of Mesothelin Positive Pediatric AML Using Bispecific T Cell Engaging Antibodies

Simple Summary

Immunotherapy development in pediatric AML has been slow due to the paucity of validated AML-specific targets. We recently identified mesothelin (MSLN) as a therapeutic target in pediatric AML. Mice receiving T cell engaging bispecific antibodies (BsAbs) targeting MSLN and CD3 achieved complete remission and durable responses in two MSLN-positive patient-derived xenograft (PDX) models. This is a first report showing MSLN-targeting BsAbs are a viable immunotherapy for MSLN-positive pediatric AML.

Abstract

Advances in the treatment of pediatric AML have been modest over the past four decades. Despite maximally intensive therapy, approximately 40% of patients will relapse. Novel targeted therapies are needed to improve outcomes. We identified mesothelin (MSLN), a well-validated target overexpressed in some adult malignancies, to be highly expressed on the leukemic cell surface in a subset of pediatric AML patients. The lack of expression on normal bone marrow cells makes MSLN a viable target for immunotherapies such as T-cell engaging bispecific antibodies (BsAbs) that combine two distinct antibody-variable regions into a single molecule targeting a cancer-specific antigen and the T-cell co-receptor CD3. Using antibody single-chain variable region (scFv) sequences derived from amatuximab-recognizing MSLN, and from either blinatumomab or AMG330 targeting CD3, we engineered and expressed two MSLN/CD3-targeting BsAbs: MSLN^AMA-CD3^L2K and MSLN^AMA-CD3^AMG, respectively. Both BsAbs promoted T-cell activation and reduced leukemic burden in MV4;11:MSLN xenografted mice, but not in those transplanted with MSLN-negative parental MV4;11 cells. MSLN^AMA-CD3^AMG induced complete remission in NTPL-146 and DF-5 patient-derived xenograft models. These data validate the in vivo efficacy and specificity of MSLN-targeting BsAbs. Because prior MSLN-directed therapies appeared safe in humans, MSLN-targeting BsAbs could be ideal immunotherapies for MSLN-positive pediatric AML patients.

A sensitive luminescent assay for the histone methyltransferase NSD1 and other SAM-dependent enzymes

A major focus of our pediatric cancer research is the discovery of chemical probes to further our understanding of the biology of leukemia harboring fusion proteins arising from chromosomal rearrangements, and to develop novel specifically targeted therapies. The NUP98-NSD1 fusion protein occurs in a highly aggressive subtype of acute myeloid leukemia after rearrangement of the genes NUP98 and NSD1. The methyltransferase activity of NSD1 is retained in the fusion, and it gives rise to abnormally high levels of methylation at lysine 36 on histone 3, enforcing oncogene activation. Therefore, inhibition of the methyltransferase activity of NUP98-NSD1 may be considered a viable therapeutic strategy. Here, we report the development and validation of a highly sensitive and robust luminescence-based assay for NSD1 and other methyltransferases that use S-adenosylmethionine (SAM) as a methyl donor. The assay quantifies S-adenosylhomocysteine (SAH), which is produced during methyl transfer from SAM. SAH is converted enzymatically to adenosine monophosphate (AMP); in the process, adenosine triphosphate (ATP) is consumed and the amount of ATP remaining is measured using a luminescent assay kit. The assay was validated by pilot high-throughput screening (HTS), dose-response confirmation of hits, and elimination of artifacts through counterscreening against SAH detection in the absence of NSD1. The known methyltransferase inhibitor suramin was identified, and profiled for selectivity against the histone methyltransferases EZH2, SETD7, and PRMT1. HTS using the luminescent NSD1 assay described here has the potential to deliver selective NSD1 inhibitors that may serve as leads in the development of targeted therapies for NUP98-NSD1-driven leukemias.

Design and synthesis of compounds that extend yeast replicative lifespan

This past decade has seen the identification of numerous conserved genes that extend lifespan in diverse species, yet the number of compounds that extend lifespan is relatively small. A class of compounds called STACs, which were identified as activators of Sir2/SIRT1 NAD+-dependent deacetylases, extend the lifespans of multiple species in a Sir2-dependent manner and can delay the onset of age-related diseases such as cancer, diabetes and neurodegeneration in model organisms. Plant-derived STACs such as fisetin and resveratrol have several liabilities, including poor stability and relatively low potency as SIRT1 activators. To develop improved STACs, stilbene derivatives with modifications at the 4' position of the B ring were synthesized using a Horner-Emmons-based synthetic route or by hydrolyzing deoxyrhapontin. Here, we describe synthetic STACs with lower toxicity toward human cells, and higher potency with respect to SIRT1 activation and lifespan extension in Saccharomyces cerevisiae. These studies show that it is possible to improve upon naturally occurring STACs based on a number of criteria including lifespan extension.

Manipulation of gene expression by an ecdysone-inducible gene switch in tumor xenografts

BACKGROUND

Rapid, robust and reversible induction of transgene expression would significantly facilitate cancer gene therapy as well as allow the in vivo functional study of newly discovered genes in tumor formation and progression. The popularity of the ecdysone inducible gene switch system has led us to investigate whether such a system can successfully regulate gene expression in a syngeneic tumor system in vivo.

RESULTS

MBT-2 and Panc02 carcinoma cells were transfected with components of a modification of the ecdysone switch system driving firefly luciferase (F-Luc). In vitro luciferase expression +/- ecdysone analog GS-E indicated a robust induction with minimal baseline activity and complete decay after 24 hours without drug. In vitro selection of MBT-2 transfected cell clones which had complete absence of F-Luc expression in the absence of stimulation but which expressed this gene at high levels in response to GS-E were chosen for in vivo evaluation. Tumors from engineered MBT-2 cells were grown to 5 mm in diameter prior to GS-E administration, animals euthanized and tumors removed at 6, 12 and 24 hours after GS-E administration and assayed for F-Luc activity. GS-E resulted in a maximal induction of F-Luc activity at 6 hours in tumor tissue with almost complete reversion to control levels by 12 hours.

CONCLUSIONS

This study is the first demonstration that robust and reversible transgene expression in tumors is feasible using the ecdysone system, allowing future rapid in vivo functional characterization of gene function or gene therapy applications.

Small molecule activators of sirtuins extend Saccharomyces cerevisiae lifespan

In diverse organisms, calorie restriction slows the pace of ageing and increases maximum lifespan. In the budding yeast Saccharomyces cerevisiae, calorie restriction extends lifespan by increasing the activity of Sir2 (ref. 1), a member of the conserved sirtuin family of NAD(+)-dependent protein deacetylases. Included in this family are SIR-2.1, a Caenorhabditis elegans enzyme that regulates lifespan, and SIRT1, a human deacetylase that promotes cell survival by negatively regulating the p53 tumour suppressor. Here we report the discovery of three classes of small molecules that activate sirtuins. We show that the potent activator resveratrol, a polyphenol found in red wine, lowers the Michaelis constant of SIRT1 for both the acetylated substrate and NAD(+), and increases cell survival by stimulating SIRT1-dependent deacetylation of p53. In yeast, resveratrol mimics calorie restriction by stimulating Sir2, increasing DNA stability and extending lifespan by 70%. We discuss possible evolutionary origins of this phenomenon and suggest new lines of research into the therapeutic use of sirtuin activators.

Similar incidence of K-ras mutations in lung carcinomas of FVB/N mice and FVB/N mice carrying a mutant p53 transgene

Mutated p53 genes are capable of complementing activated ras genes in the transformation of primary rat embryo fibroblasts in vitro. Mutations in both genes have also been found in several human cancers, including lung carcinomas. We generated transgenic mice containing a p53 construct with a missense mutation in exon 5 (ala135val) to study the role of p53 mutations in lung tumorigenesis, and to facilitate identification of other genetic events that might complement p53 mutations in mouse lung carcinogenesis. The p53 transgenic lines exhibited a higher frequency of lethal lung tumors than the parental FVB/N strain. We examined the spontaneously-arising lung carcinomas from mice expressing the mutated p53 transgene for K-ras mutations using single-stranded conformation polymorphism (SSCP) and/or direct sequencing approaches. Fifteen of 29 (52%) carcinomas contained mutations in the K-ras oncogene. Six of 15 of the K-ras mutations were in codon 61 and 9/15 were in codon 12. Subsequent analysis of spontaneous lung carcinomas from mice of the FVB/N parental strain showed that 9/12 (75%) carcinomas examined contained K-ras mutations. Two of these were in codon 12, one in codon 13, and 6 were in codon 61. These results demonstrate that the frequency of ras mutations does not differ between the p53 FVB/N transgenic mice and their parental FVB/N strain but suggest that a high frequency of mutations K-ras can be correlated with lung tumorigenesis in both groups of mice.