Phase 1/2 study evaluating the safety and efficacy of DSP‐7888 dosing emulsion in myelodysplastic syndromes

Abstract DSP‐7888 is an immunotherapeutic cancer vaccine derived from the Wilms’ tumor gene 1 (WT1) protein. This phase 1/2 open‐label study evaluated the safety and efficacy of DSP‐7888 dosing emulsion in patients with myelodysplastic syndromes (MDS). DSP‐7888 was administered intradermally (3.5 or 10.5 mg) every 2 weeks for 6 months and then every 2‐4 weeks until lack of benefit. Twelve patients were treated in phase 1 (3.5 mg, n = 6; 10.5 mg, n = 6), with no dose‐limiting toxicities reported. Thus, the 10.5 mg dose was selected as the recommended phase 2 dose, and 35 patients were treated in phase 2. Forty‐seven patients received ≥1 dose of the study drug and comprised the safety analysis set. The most common adverse drug reaction (ADR) was injection site reactions (ISR; 91.5%). Grade 3 ISR were common (58.8%) in phase 1 but occurred less frequently in 2 (22.9%) following implementation of risk minimization strategies. Other common ADR were pyrexia (10.6%) and febrile neutropenia (8.5%). In the efficacy analysis set, comprising patients with higher‐risk MDS after azacitidine failure in phases 1 and 2 (n = 42), the disease control rate was 19.0%, and the median overall survival (OS) was 8.6 (90% confidence interval [CI], 6.8‐10.3) months. Median OS was 10.0 (90% CI, 7.6‐11.4) months in patients with a WT1‐specific immune response (IR; n = 33) versus 4.1 (90% CI, 2.3‐8.1) months in those without a WT1‐specific IR (n = 9; P = .0034). The acceptable safety and clinical activity findings observed support the continued development of DSP‐7888 dosing emulsion.


| INTRODUC TI ON
Myelodysplastic syndromes (MDS) encompass a heterogeneous group of closely related diseases of pluripotent hematopoietic stem cells, characterized by peripheral blood cytopenia due to ineffective hematopoiesis. 1 The clinical course of MDS is extremely variable and may be indolent or rapidly progressive, with marked symptom burden and transformation into acute myeloid leukemia (AML). 2,3 In Japan, the estimated incidence of MDS is 3.8 cases per 100 000 for men and 2.4 cases per 100 000 for women. 4 However, incidence increases sharply with age, particularly in those aged over 70 years. 4 Therapeutic options for MDS are limited and are largely based on a patient's age and prognosis, as determined by the International Prognostic Scoring System (IPSS) and the revised IPSS (IPSS-R). 5,6 Allogeneic hematopoietic stem cell transplantation (allo-HSCT) represents the only potentially curative option; however, most patients are not suitable candidates due to their advanced age. 7 For decades, the mainstay of treatment has been supportive care, including blood transfusions and hematopoietic factors, aimed at prolonging survival and improving quality of life. 8 The immunosuppressant agent lenalidomide and hypomethylating agents (HMA), including azacitidine (AZA) and decitabine, are approved for treatment of MDS globally. 9,10 Lenalidomide is approved for a small subset of MDS patients with chromosome 5q deletion. 9 In April 2020, luspatercept was approved by the US Food and Drug Administration for patients with very low to intermediate (Int) risk MDS. 11 AZA and decitabine have demonstrated improved response rates and prolonged time to AML transformation and survival compared to conventional care in randomized phase 3 trials, [12][13][14] and both are indicated in higherrisk patients, although decitabine is not approved in Japan. 15,16 Approximately 40% of patients fail to respond to HMA, 17 and most responders experience disease relapse within 2 years. 13 Prognosis after relapse is particularly poor, with limited treatment options and median overall survival (OS) of less than 6 months. 18,19 Treatment of MDS after failure of HMA represents a significant therapeutic challenge.
Preliminary clinical activity has been shown with our earlier generation WT1 peptide cancer vaccine, WT4869, in a phase 1/2 study in patients with HLA-A*24:02-positive MDS, including those with higher-risk (IPSS score ≥1.5) or lower-risk (score <1.5) disease who were red blood cell transfusion-dependent. 32 We hereby report the results of the phase 1/2 study evaluating the safety and efficacy of the peptide vaccine, DSP-7888, in patients with MDS.

K E Y W O R D S
DSP-7888, high-risk myelodysplastic syndrome, myelodysplastic syndromes, Wilms' tumor gene 1, WT1 peptide vaccine MDS and determine the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D). Preliminary activity and potential biomarkers were also explored. The objective of phase 2 was to evaluate the safety and efficacy of DSP-7888 in patients with higher-risk MDS (IPSS score ≥1.5 + IPSS score <1.5 disease with myeloblasts ≥5%) after AZA failure using the RP2D. For the purposes of this study, AZA failure was defined as non-response to therapy (ie primary failure), loss of response (ie secondary failure), or intolerance due to adverse events (AE) as per Prébet et al (2011). 19 The IPSS score was evaluated at enrollment (ie following AZA failure) in this study. Potential biomarkers of safety and efficacy were also explored.
The phase 1 dose-escalation part of the study involved a conventional 3 + 3 design, with sequential cohorts of three to six patients enrolled to receive DSP-7888 at two dose levels (3.5; 10.5 mg). After the DLT evaluation period in phase 1 or 2, dose reductions were permitted in patients experiencing an adverse drug reaction (ADR) that met the DLT criteria but did not meet discontinuation criteria (ie patient withdrawal, AE, and other reasons). Dose reductions were also permitted in patients experiencing an injection-site reaction (ISR) if there was a concern that the reaction may worsen.
The study was conducted in accordance with the ethical principles of the Declaration of Helsinki, the International Council for Harmonisation Harmonised Tripartite Guideline for Good Clinical Practice, and the Institutional Review Board (IRB) regulations and following the applicable local regulatory requirements. The clinical study protocol, the investigator's brochure, informed consent forms, and other study-related documents were reviewed and approved by the IRB of all study sites. All patients provided written informed consent to participate in the study.

| Patients
For phase 1 of the study, patients aged ≥20 years with a confirmed and high-risk (IPSS score ≥1.5) or low-risk (IPSS score <1.5) disease requiring treatment other than supportive treatment were eligible for inclusion irrespective of previous AZA treatment.
For phase 2, patients aged ≥20 years with a confirmed diagnosis of MDS (according to WHO 4th edition or FAB classification [including CMML and RAEB-t]) 33,34 and high-risk (IPSS score ≥1.5) or lowrisk (IPSS score <1.5) disease with myeloblasts ≥5% were eligible if they had ≥1 previous cycles of AZA treatment with no subsequent treatment after AZA.
Other major inclusion criteria for both parts were a white blood cell count ≤12 000/mm 3

| Assessments
The following data was collected at baseline, including demographics, MDS history (date of diagnosis, MDS classification, 33,34 and previous treatments), IPSS/IPSS-R score before treatment, concurrent diseases, clinical symptoms, peripheral blood and bone marrow findings, and ECOG PS. Data on blood transfusions was collected from 8 weeks before the initial DSP-7888 dose until completion of the final assessment.

| Dose-limiting toxicities and maximum tolerated dose
The MTD was defined as the highest dose at which no more than one out of six patients experienced a DLT, and the RP2D was determined by safety and biomarker activity. Dose-limiting toxicities were assessed within 29 days after the first DSP-7888 dose phase 1 and were defined as any ADR that met the following criteria: grade 4 febrile neutropenia (FN); grade 4 electrolyte abnormalities or grade 3 electrolyte abnormalities persisting for ≥7 days; grade 4 ISR or grade 3 ISR that the investigator deemed uncontrolled; grade 4 infections; or other grade ≥3 nonhematologic toxicities.

| Safety
Adverse events and ADR were monitored throughout the study, permitted if 3.5 mg was not tolerated. Vital signs, laboratory variables, and 12-lead electrocardiogram and chest X-ray findings were also monitored.

Primary efficacy endpoint
The primary efficacy endpoint was OS, defined as the period from the date of DSP-7888 initiation until the date of death from any cause. Patients alive at the time of analysis were censored at the last date known to be alive. As the median OS from historical data was 5.6 (95% CI, 5.0-7.2) months, 19 the prespecified survival threshold was set at 7.2 months. Based on this, it was concluded that DSP-7888 would be considered effective if the lower limit of the 90% CI for the median OS exceeded 7.2 months.

Secondary efficacy endpoints
Secondary efficacy endpoints were the hematological response rate and the disease control rate. Hematologic response was evaluated according to the International Working Group (IWG) 2006 response criteria. 35 The proportion of patients with hematologic improvement, cytogenetic response (both evaluated according to IWG 2006 response criteria), 35 transfusion independence, and AML transformation were also assessed. Transfusion independence was defined as ≥8 weeks without red blood cell and/or platelet transfusion. Time to AML transformation was calculated as the length of time from the date of DSP-7888 initiation to the date of a definite AML diagnosis. For patients without AML transformation at the time of analysis, data was censored at the date of last sampling. Responses were classified according to the difference from the control as: (a) (−) <2 mm; (b) (±) ≥2 mm but <5 mm; (c) (+) ≥5 mm but <10 mm; (d) (2+) ≥10 mm but <15 mm; and (e) (3+) ≥15 mm. Positive DTH responses were considered DTH ±, +, 2+, and 3+ reactions.

| Biomarkers
Wilms' tumor gene 1-specific CD8 + T cell induction in peripheral blood was assessed to confirm the mechanism of action of DSP-7888 and examine the relationship with safety and efficacy.
The percentage of WT1-specific CD8 + T cells was measured using an HLA tetramer assay, with assessments performed within 28 days (c) HLA-A*02:01 WT1 tetramer-VLDFAPPGA-PE, which was customized at the request of the sponsor. WT1-specific CD8 + T cell induction was confirmed if the WT1 CD8 + tetramer + T cell/Total CD8 + T cell ratio to baseline was ≥2 at any time and the WT1 CD8 + tetramer + T cell was ≥10 at that time. If the WT1 tetramer + CD8 + T cell at baseline was 0, WT1-specific CD8 + T cell induction was confirmed when the WT1 tetramer + CD8 + T cell was ≥10 at any time. If either a positive DTH-K response or WT1-specific CD8 + T cell induction (confirmed by HLA tetramer assay) was confirmed, the diagnosis was "WT1-specific IR positive." WT1 messenger RNA (mRNA) expression levels in the bone marrow and peripheral blood were also measured prior to treatment (≤28 days prior to first administration); on Day 15 before doses 2, 6, and 12; and following every six cycles thereafter.
With the exception of DTH response to WT1 peptides, biomarkers were measured at a central laboratory.

| Statistical analysis
The full analysis set comprised all patients who received ≥1 dose of DSP-7888 and the safety analysis set comprised all patients who received ≥1 dose of DSP-7888, which contained a DTH solution. The efficacy analysis set included patients with higher-risk MDS after AZA failure in phases 1 and 2 (ie some patients in the phase 1 part of the study who met the eligibility criteria for the phase 2 part and all patients in the phase 2 part of the study). For phase 1, a maximum of 12 patients was planned based on the 3 + 3 design. For patients with higher-risk MDS who had failed prior AZA therapy, a maximum of 42 subjects was planned. This calculation was based on historical data reporting a median OS of 5.6 (95% CI, 5.0-7.2) months in this patient population. 19 In the subgroup analysis of the phase 1/2 trial using our earlier vaccine, WT4869, a median OS of 13.0 (95% CI, 7.3-NA) months was reported in 11 patients with higher-risk MDS after AZA failure. 32 As DSP-7888 was expected to be equivalent or more efficacious than WT4869, a median OS of 14.0 months was assumed, with the lower bound of the two-sided 90% CI set at 80% above the upper threshold of 7.2 months based on historical data.
The Kaplan-Meier method was used to calculate the median survival and time to AML transformation.
Subgroup analyses of efficacy were performed in patients with higher-risk MDS by WT1-specific IR.

| Patient characteristics
Baseline demographic and clinical characteristics of patients in the safety analysis set, and those with higher-risk and other MDS, are presented in Table 1. The median (min-max) age at study enrollment was 74.0 (52-93) years, and most patients (n = 35, 74.5%) were male and had the HLA-A*24:02 allele (n = 28, 59.6%). The majority of patients had IPSS Int-2 (n = 28, 59.6%) and high-risk (n = 9,

| Safety
No DLT were observed in either the 3.5 or 10.5 mg dose cohorts in phase 1; therefore, the MTD was not determined. There was no significant difference in safety profile, including ISR, between 3.5 and 10.5 mg dose cohorts.
The most common AE (≥20% incidence) during DSP-7888 treatment are presented in The relationship between the ISR and WT1-specific IR are presented in Figure 2. The proportion of patients who had a positive WT1-specific IR increased with increasing ISR grade.  TA B L E 2 Occurrence of common adverse events (with an incidence of ≥20%) a overall and by study cohort

| Efficacy
Clinical activity in the efficacy analysis set is summarized in Table 5.
Median OS for patients in the efficacy analysis set was 8. Disease control (95% CI) was achieved in eight patients (19.0%) in the efficacy analysis set, and clinical efficacy (complete remission, partial remission, bone marrow complete remission, and hematologic improvement) was achieved in four patients (9.5%; Table 5).
The best hematological response was SD in eight patients (

| Biomarkers
In phase 1, WT1-specific CD8 + T cell induction was relatively higher in Cohort 2 (10.5 mg) than in Cohort 1 (3.5 mg; Figure 8). Figure S1 presents the results of the WT1 tetramer staining at baseline and on Day 15 of Cycle 2 in a patient who was WT1-specific CD8 + T cell induction positive.

Occurrence of ADR, SOC (PT)
Blood and lymphatic system disorders

| DISCUSS ION
This phase 1/2 study showed the safety and clinical activity of DSP-7888 in patients with higher-risk MDS who had failed prior AZA treatment. No DLT were observed at either the 3.5 or 10.5 mg dose, and there was no significant difference in safety profile, including ISR, between dose cohorts. In addition, WT1-specific CD8 + T cell induction was relatively higher in Cohort 2 (10.5 mg) than in Cohort 1 ADR, adverse drug reaction; PT, preferred term; SOC, system organ class. a Rows that contain any event with a frequency of ≥5% are presented.
b An instance of Grade 4 pancytopenia occurred in one patient (2.1%) during the study.

TA B L E 4
Occurrence of common adverse drug reactions (with an incidence of ≥5%) a by severity   37 which was more pronounced in the "very good/good," "intermediate" and "poor" karyotype groups compared with the "very poor" karyotype ( Figure 4).