PPV was demonstrated in previous phase II trials to improve OS in chemotherapy-naïve patients with CRPC or in patients with docetaxel-resistant CRPC. This phase III randomized trial investigated PPV as a second-line treatment after docetaxel chemotherapy in patients with progressing CRPC. To our knowledge, this is the first trial addressing this strategy in a relatively large number of patients for whom docetaxel induction chemotherapy failed. In this trial, no difference in OS or PFS between PPV and placebo was found.
There are several explanations for the lack of OS improvement in this study. First, target patients may have heterogeneous immune cell repertoires, a large tumor burden, and many immune suppressive elements, such as increased myeloid-derived suppressor cells (MDSC) or regulatory T cells (Treg) in the tumor microenvironment, and it is well known that tumor-associated immunosuppression is significantly involved in tumor progression and resistance to immunotherapy [17, 18]. Second, another contributory factor may be the availability of more effective salvage therapies that prolong OS after the study treatment, many of which were not widely available at the time of the previous phase II study. The current availability of such drugs (eg, cabazitaxel, abiraterone, enzalutamide, and radium-223) may have affected the disease course in patients receiving PPV or placebo. Indeed, 148 of 204 patients (72.5%) in the PPV arm and 71 of 102 patients (69.6%) in the placebo arm used such drugs after the study treatment. Third, the treatment discontinued early before sufficient doses of PPV were administered, thereby affecting the efficacy due to the lack of notable objective responses or PSA progression. The median dose of PPV was 12 (planned dose was up to 36), and only 21 of 204 patients (10.3%) completed the treatment in the PPV arm.
Peptide based vaccines have been designed to elicit CTL against antigens selectively expressed by tumor cells. PPV, in which a maximum of four HLA class IA-matched peptides are selected for vaccination from a pool of peptides on the basis of both HLA class IA type and the preexisting host immunity before vaccination, are designed to stimulate antigen-specific memory T cells. In a trial of neoadjuvant peptide vaccination before radical prostatectomy for patients with localized prostate cancer, we previously reported that PPV quickly induced infiltration of CD45RO + memory T cells, rather than naïve T cells or B cells, into cancer tissues [19]. We evaluated the preexisting immunity to vaccine candidate by peptide-specific IgG responses in pre-vaccination plasma, since our previous clinical trials demonstrated that the IgG-based selection is useful for predicting CTL boosting after vaccination [12, 20]. Our previous study also suggested the significance of peptide-specific IgG responses in patients with advanced cancer treated by PPV as a surrogate biomarker in monitoring vaccine-induced immune and clinical responses [21]. In this trial, there was no relationship among IgG changes, CTL changes, and OS in the PPV arm. However, IgG changes in the PPV arm of patients with < 64% neutrophils or ≥ 26% lymphocytes who showed a long OS, were significantly higher than those in patients with the complementary subgroup.
In general, the development of peptide-based vaccines has some issues to be solved. Peptides used in peptide-based vaccines are restricted by HLA class I or II molecule, and they have limitations when used in clinical trials. In addition, tumor cells, can downregulate the expression of these molecules. In this clinical trial, we used HLA-A24-restricted peptides, which are the most common in Japanese because they are targeted at Japanese. Another issue is that efficiently primed T cells often lose their responsiveness to tumor antigens. This may be explained by down-regulation or loss of tumor antigens, immunosuppression by Treg and MDSC cells, and T cell inhibition mediated by checkpoint molecules, such as CTLA-4, PD-1, and PDL-1 [22, 23]. Although we did not evaluate these mechanisms in this study, our previous analysis to evaluate effect of 20-mixed peptides for patients with CRPC showed that no obvious difference were observed in the frequency of Treg and MDSC and the level of serum CTLA-4 during the vaccination [24].
Further study of predictive biomarkers of PPV efficacy may be warranted to determine whether subgroups will improve the OS. The recent discovery that cancers deficient in DNA mismatch-repair function (dMMR) or with microsatellite instability (MSI-high) demonstrate high rates of objective tumor responses to immune checkpoint therapies [25] led to the FDA approval of pembrolizumab for the treatment of advanced dMMR/MSH-high cancers of any histologic type, among which mCRPC patients are a very small subset. We recently reported that the abnormal granulocytes present in the PBMC fraction at baseline may lead to the poor prognosis of advanced prostate cancer patients receiving PPV treatment using DNA microarray analysis [26]. We also demonstrated that the increase in granulocytic MDSC after PPV treatment was an unfavorable marker for the OS of mCRPC patients [27]. This suggested that the proportion of neutrophils, the majority of granulocytes, affects the efficacy of PPV treatment. The post hoc analysis in this trial demonstrated that patients with a low neutrophil proportion (< 64%) or a high lymphocyte proportion (≥ 26%) at baseline in the PPV arm had a significantly longer OS than their counterparts in the placebo arm; however, the proportions of eosinophils, basophils, and monocytes did not affect the efficacy of PPV treatment even though the neutrophil to lymphocyte ratio (NLR) was used. Although NLR was reported as a risk factor for the OS of patients with mCRPC as well as many other advanced cancers when it was higher than 2 [28–30], we found that the NLR was less sensitive than the proportion of neutrophils or lymphocytes as a biomarker to predict the efficacy of PPV with an interaction p = 0.09. This discrepancy in results may have occurred, in part, because the NLR value as a risk factor was based on a comparison of the lower and higher levels among patients in the same treatment arm in the previous studies rather than between patients in different treatment arms (i.e., PPV and placebo). The levels of PPV-induced IgG were significantly higher in patients with < 64% neutrophils or ≥ 26% lymphocytes treated by PPV than in their counterparts, and the median OS of these groups was significantly longer than that of the counter groups. These results suggested that patients with < 64% neutrophils or ≥ 26% lymphocytes can receive survival benefits from PPV treatment. Although further validation is required, this hypothesis is novel and helpful to prolong survival in patients with CRPC progressing after docetaxel chemotherapy.
The tolerability of PPV treatment was good overall, and the most common treatment-related AE in both arms was injection site reaction, which was mainly caused by incomplete Freund' adjuvant [31]. Dose interruptions or reductions were infrequent, and the overall safety profile was consistent with that observed in previous phase II trials. Treatment-related deaths were not increased with PPV, suggesting a lack of toxicity as the main contributing factor.
In conclusion, PPV did not prolong OS or PFS in HLA-A24-positive patients with CRPC progressing after docetaxel chemotherapy. Subgroup analyses demonstrated that patients with a low neutrophil proportion or a high lymphocyte proportion at baseline in the PPV arm had a significantly longer OS than their counterparts in the placebo arm in this setting. Additional validation of this finding is needed to better define subgroups of patients who can receive PPV treatment for progressing CRPC after docetaxel chemotherapy.