BACKGROUND: Several studies have established the PR1 peptide (VLQELNVTV) as a human leukemia-associated antigen. PR1 is derived from proteinase 3, an aberrantly expressed protein in myeloid leukemia cells, and can be presented on HLA-A2 to cytotoxic T lymphocytes (CTL) that preferentially kill leukemia over normal hematopoietic progenitors.

METHODS: Thirty-five HLA-A2+ patients with AML, MDS and CML were vaccinated with the PR1 peptide in an incomplete Freund's adjuvant (Montanide ISA-51) and 75 mg of GM-CSF. Eligibility included AML with smoldering disease or ≥ 2^nd CR, CML not responding to upfront treatment or relapsed disease, and MDS with ≥ 5% blasts. Patients were assigned at random to receive 0.25 mg, 0.50 mg, or 1.0 mg PR1 SQ every 3 weeks for 3 total injections. Immune responses (IR) were assessed by PR1/HLA-A2 tetramer staining and intracellular IFN-γ production by CTL, and clinical responses were assessed by bone marrow biopsy before study entry and 3 weeks after the 3^rd vaccination.

RESULTS: Thirty-five patients with a median age of 50 (26–82) were treated at a median of 26 months from time of diagnosis, and follow up was 1 to 4 years. Toxicity was limited to grade 1 and 2 injection site reactions. Overall survival was 33% at 4 years. Immune responses (IR) were elicited in 20 of 33 evaluable patients (60%). Of 16 patients with relapsed or refractory AML, there were 4 (25%) clinical responses (3 CR, 1 PR). All 4 AML patients treated during CR remain in CR. Of 10 CML patients, there was 1 cytogentic CR. Three CML patients refractory to allogeneic transplant, interferon and imatinib had stable disease with some hematological improvement since these patients were able to discontinue hydroxyurea and anagrelide. Of 5 MDS patients, there was 1 PR (17% blasts reduced to 4%). Overall survival at 4 years was 14 of 20 patients with IR vs. 0 of 13 without IR (p < 0.0001). Progression-free survival for patients with or without IR was 6.4 months vs. 2.4 months, respectively (p = 0.003).

CONCLUSION: PR1 peptide vaccination is safe and can elicit both immunological and clinical responses in patients with refractory and relapsed myeloid leukemia, which improves progression-free survival of patients with IR to PR1. This is the first study to show complete molecular remission induced by peptide vaccination. These results warrant further study of immunization strategies in the treatment of leukemia.