Pilot Study of a Wilms Tumor Protein (WT1) Heteroclitic Peptide Vaccine in Patients with Myeloid and Thoracic Neoplasms.
Authors: Maslak Peter G. , Krug Lee , Chanel Suzanne , Dao Tao , James Leonard , Tyson Leslie , Bekele Sara , Scheinberg David A. 
Published: October 2019

WT1 is a transcription factor over-expressed in several types of leukemia and solid tumors which has the potential to serve as a target for immune based therapies. Previous studies have demonstrated the ability of WT1 peptides to induce specific HLA restricted T cell cytotoxic responses. In an effort to overcome immune tolerance associated with this self-antigen, we used computer prediction analysis to design heteroclitic peptides derived from WT1 protein sequences by substituting various amino acids at key HLA A0201 binding positions. A selected peptide (WT1-A1) was found to stabilize MHC class I molecules better than native sequences and was also able to cross react with native WT1 sequences to kill HLA matched tumor cells. A modified version of the heteroclitic peptide was also created by adding flanking amino acids (WT1-122A1) to the core sequence in an effort to extend reactivity of this peptide to also include a broad range of HLA class II molecules. These 2 peptides were then combined with 2 native WT1 sequences (WT1-427 & WT1-331) capable of recruiting CD4 T cell response, to form a polyvalent vaccine with the potential to react across a number of HLA-DR subtypes. A clinical pilot study was designed to test the safety, activity and immunogenicity of this vaccine in patients with Acute Myelogenous Leukemia (AML) in complete remission, but with molecular evidence of WT1 transcript as measured by quantitative polymerase chain reaction (RQ-PCR) or with advanced mesothelioma or non-small cell lung cancer (expressing WT-1 protein by immunohistochemistry). Each patient receives 6 vaccinations with the 4 WT1 peptides (200mcg each) plus the immune adjuvant Montanide over 12 weeks. In addition, GM-CSF injections are administered to local sites of inoculation 2 days before and on the day of vaccination. Study design called for 10 patients in each group (solid tumor; hematologic malignancy). Immune responses are evaluated by delayed-type hypersensitivity, CD4 T-cell proliferation, CD4 and CD8 T-cell interferon-gamma release, and WT1 peptide tetramer staining. In the AML patients, minimal residual disease was assessed by sequential RQ-PCR at baseline and after the 3rd and 6th vaccines. To date, 8 patients (4 AML, 3 mesothelioma, 1 non-small lung cancer) have been enrolled on study. Two of the mesothelioma patients discontinued the vaccines after 3 and 6 doses because of progression of disease. One other mesothelioma patient has stable disease following 10 vaccinations. Two of the 4 AML patients have relapsed while receiving the vaccine (one after 3rd dose, the other after the 6th). Each had evidence of increasing WT-1 transcript at the time of relapse. One other remains in CR and has a transcript level 50% of baseline. No significant adverse events have been observed to date in the patients undergoing vaccination. Three of 4 evaluable patients showed evidence of either a CD8 or CD4 immune response to the vaccinating peptides and native peptides in vitro. In summary, preliminary data suggest that this heteroclitic WT1 peptide vaccine can be administered safely to patients and that immune responses can be elicited.