A Pilot Phase II Trial of a Synthetic Tumor-Specific Breakpoint Peptide Vaccine in Patients with Chronic Myeloid Leukemia (CML) and Minimal Residual Disease.

Peptides derived from BCR-ABL have been used to trigger immune responses in CML pts. The native peptides may have low immunogenicity because they bind with low affinity to the major histocompatibility complex (MHC) molecules. Synthetic peptides containing mutations in the sequences of these peptides can bind with higher affinity than parental peptides and may generate a cross-reactive T cell response to the native sequence (known as a heteroclitic response). We used a mixture of heteroclitic and native peptides derived from both b3a2 and b2a2 sequences in a pilot study to vaccinate pts with CML in complete cytogenetic remission on imatinib (IM) therapy with stable bcr-abl transcript levels. Montanide ISA51 and GM-CSF were used as immunological adjuvants. Pts were required to have received IM for ≥12 mo with no change in dose for at least 6 mo and were not allowed any dose increases after registration. Pts were vaccinated with the peptides corresponding to their BCR-ABL breakpoint as follows: every 2 wks x4, then once 3 wks later, followed by 10 monthly vaccinations for a total of 15 vaccinations in 12 mo. After registration, pts had 3 additional measures of bcr-abl transcripts to better define baseline values. 8 pts have been vaccinated. At enrollment, pts had a median age of 45 yrs (range 29–63) and had been receiving IM for a median of 63 mo (range 35–68 mo) on a median dose of 600mg (range 300–800mg). Pts have been followed for a median of 15 wks (range 2–20) and have received a median of 7 vaccinations (range 1 to 8). To determine the effect of vaccination on immune leukocytes, we measured peripheral blood T-cell subsets, B, NK, and dendritic cells by FACS at baseline and at 3-month intervals thereafter. The percentage of myeloid DC (DC1) and plasmacytoid DC (DC2) cells were identified by the mutually exclusive expression of CD11c and CD123; Treg cells by the surface co-expression of CD3, CD4 and CD25 together with cytoplasmic expression of CTLA-4 (cytotoxic T lymphocyte-associated antigen 4) and FoxP3 (forkhead/winged helix transcription factor). Vaccinations have been well tolerated. The only adverse event attributable to the vaccine is local site reactions grade 1 or 2 in 5 pts. Among the 5 pts who have had an evaluation at 3 mo from the 1^st vaccination, 1 has achieved a major molecular response (bcr-abl/abl <0.05%). The Wilcoxon signed ranks test was used to assess changes in the immunophenotype of peripheral leukocytes of 5 pts evaluated for leukocyte FACS analyses at baseline and after 3 mo on study. Compared with baseline, there were significant decreases in the percentages of B (p = 0.042) and NKT (p = 0.043) cells, and significant increases in the percentages of DC2 (p = 0.043) and Treg (p = 0.043) cells. All pts continue receiving vaccination as planned. We conclude that vaccination with b2a2 and b3a2 synthetic peptides is well tolerated. Further follow-up is required to determine the immunogenecity and efficacy of the vaccine in this setting.