Background: PR1 is a nonomeric HLA-A2-restricted peptide derived from the myeloid leukemia-associated antigens, proteinase 3 and neutrophil elastase. PR1-specific cytotoxic T lymphocytes (PR1-CTL) selectively kill MDS, AML and CML and contribute to complete cytogenetic remission. We conducted a phase I/randomized phase II trial to evaluate the safety and efficacy of a PR1 peptide vaccine to elicit PR1-CTL in leukemia patients.

Methods: Sixty-six HLA-A2+ patients with acute myeloid leukemia (42), chronic myeloid leukemia (13) or myelodysplastic syndrome (11) were treated with PR1 peptide vaccine. The first 54 patient received three vaccinations, and the last 12 patients received 6 vaccinations. PR1 was injected with incomplete Freud’s adjuvant (Montanide ISA) subcutaneously, at 3 week intervals at one of three dose levels: 0.25, 0.5 or 1.0 mg per vaccination. GM-CSF at a dose of 75 mg was injected subcutaneously into the same site. Immune response to the vaccine was defined as a ≥ 2-fold increase in peripheral blood PR1-CTL. Fifty-three patients had measurable disease and 13 were in complete remission at enrollment.

Results: The vaccine was well tolerated, with toxicity limited to grade I and II injection site reactions. PR1-specific immune response was observed in 25/53 (47%) patients with measurable disease. Clinical responses were observed in 9/25 (36%) immune responders versus 3/28 (10%) immune non-responders (p=0.03). The PR1 vaccine-induced immune response was associated with a longer event-free survival, 8.7 months vs. 2.4 months (p = 0.03), and a trend towards longer overall survival. Among the 13 patients treated in complete remission, 4 have remained in remission for a median of 30.5 months (range 11–59 months). This includes continued complete remissions in 3/10 who were IRV+, and 1/3 who were IRV-. PR1-specific immune response lasted for up to 4 years in some patients. In a multivariate model, low bone marrow blast count (<10%) emerged as a significant predictor of an immune response and a longer event-free survival (p=0.001).

Conclusion: PR1 peptide vaccine-induced immune response is associated with a clinical response and better event-free survival in patients with myeloid leukemia, who had measurable disease at vaccination. Patients with ≤10% bone marrow blasts are likely to obtain the maximum benefit.

Author notes

Disclosure:Research Funding: Jeffrey J Molldrem MD: The Vaccine Company. Honoraria Information: Muzaffar H. Qazilbash: Speaker fees, The Vaccine Company.