Immune Responses in AML Patients Following Vaccination with GRNVAC1, Autologous RNA Transfected Dendritic Cells Expressing Telomerase Catalytic Subunit hTERT.

Telomerase activity of leukemic cells from AML patients is frequently increased, especially among cases with the most aggressive clinical features. The major protein catalytic subunit of telomerase (hTERT) is known to include immunogenic epitopes which can stimulate generation of hTERT-specific cytotoxic T cells.

This study was designed to evaluate the feasibility, safety, and immunologic effects of telomerase catalytic subunit (hTERT)-expressing autologous dendritic cells (DC) in 20 or more evaluable adult AML patients in CR1 or CR2. GRNVAC1 (VAC1) was produced from patient-specific leukapheresis harvests. Monocyte-enriched PBMCs were cultured with cytokines at a central facility to the immature DC stage, then electroporated with an RNA construct encoding hTERT and a lysosomal targeting sequence (LAMP-1). Further culture with cytokines induced a mature DC phenotype. The RNA construct enables expression of multiple HLA appropriate hTERT epitopes. CD83⁺CD14^low phenotype, hTERT protein expression, and sterility were required for release of the cryopreserved doses. CR1 patients were eligible with intermediate or high risk cytogenetics; all CR2 patients were eligible. Patients would have PBMCs harvested after induction, but could complete up to 4 chemotherapy consolidation cycles prior to start of study vaccination, at which point they were re-evaluated for remission status. Patients in ongoing CR or early relapse were vaccinated. Early relapse was defined as marrow blasts < 20%, ANC > 400/ μL with no new infection, and plts > 30,000/ μL. VAC1 in doses of 1 × 10⁷ cells was administered by intra-dermal injections weekly X 6, followed by 4 weeks off, then boost vaccinations every other week X 6. Injections could then continue on a q28 day extended boost schedule. Immune response was evaluated by delayed type hypersensitivity (DTH) and cell based and peptide pool ELISPOT assays. Levels of WT-1 in blood samples were monitored (qPCR, normalized to abl) as an exploratory MRD marker. Bone marrows were performed at specified intervals to evaluate remission status.

VAC1 cells having mature DC immunophenoptype and that were functional in mixed lymphocyte cultures and in vitro migration assays could be produced from leukapheresis product. 18 patients have now commenced vaccination (median age 55 yrs; range 34 to 75) and leukapheresis harvests from 2 additional patients are currently in manufacture. Sixteen patients who have started vaccination were in CR1 and 2 in CR2. Two patients were in early relapse, one of whom progressed following 3 vaccinations, while the other continues on study following 3 vaccinations. Five of the patients who started vaccination had leukemias with cytogenetic abnormalities.

VAC1 was well tolerated, with the exception of one patient who presented with ITP 2.4 months following start of VAC1 treatment. This patient had initial immunophenotype consistent with minimally differentiated subtype, and continued in AML remission for 7.3 months from treatment start. Three additional patients who started vaccination in CR relapsed at 1.2 to 2.3 months, while 12 continue in CR at 1.4 to 17.5 months following vaccination start. The patient who has continued in CR the longest had flt-3 ITD+ leukemia with normal cytogenetics. This patient had elevated WT-1 levels at the time of leukapheresis, persisting unchanged to treatment week 7, but then gradually declining to within the normal range by week 21. Three patients who relapsed within 2.3 months had elevated and increasing WT-1 levels at and following start of vaccination.

Seven of 13 patients on whom data is currently available had ELISPOT immune response, as defined by ≥ 2 fold increases over baseline. Five of 13 had evidence of DTH reactions. To date, there is no clear correlation between measured immune response and clinical outcome in this small number of patients.

We have produced and administered GRNVAC1 to 18 AML patients in complete remission (16) or early relapse (2). Treatment has been generally well tolerated, with the exception of 1 patient who developed ITP. Twelve of 16 CR patients continue in remission. There was evidence of hTERT specific immune response by ELISPOT in over half the evaluated patients. The study is ongoing, now focused on patients with AML with poor risk cytogenetic or molecular features or in CR2.

DiPersio:Genzyme: Honoraria. Blum:Celgene: Research Funding. Elias:Geron Corporation: Employment, Equity Ownership. Reddy:Geron Corporation: Employment, Equity Ownership. Smith:Geron Corporation: Employment, Equity Ownership.