Placebo-Controlled Phase III Trial of Immunologic Therapy with Sipuleucel-T (APC8015) in Patients with Metastatic, Asymptomatic Hormone Refractory Prostate Cancer

This study involved 19 centers in the United States. Eligible patients had histologically confirmed adenocarcinoma of the prostate with radiologic evidence of metastases, serum testosterone less than 50 ng/dL, and an expected survival of at least 3 months. All patients had evidence of progressive disease, as defined by PSA Consensus Criteria.⁹ Other eligibility requirements included an Eastern Cooperative Oncology Group performance status of 0 or 1 and positive immunohistochemistry staining for PAP in at least 25% of cells assessed at a central laboratory. Negative serologic tests for human immunodeficiency virus, human T-cell leukemia virus type 1, hepatitis B, and hepatitis C were required, as were adequate hematologic, renal, and hepatic function. Prior investigational agents, other hormones, Saw Palmetto, PC-SPES, or other herbal preparations were allowed provided they were discontinued at least 1 month before treatment. Concurrent bisphosphonate therapy was permitted provided therapy was initiated at least 30 days before registration and was not discontinued (or initiated) during the study. Prior radiation therapy must have been completed at least 1 month before treatment, and radiopharmaceuticals could not have been administered within 1 year of treatment. Patients who required concurrent systemic corticosteroids or those who received prior immunotherapy were not eligible. Patients without prior bilateral orchiectomy continued on gonadal suppression with a luteinizing hormone-releasing hormone agonist throughout the trial. Prior chemotherapy was permitted provided at least 6 months had elapsed, or at least 3 months had elapsed and the CD4+ T-cell count was greater than 400. Patients with cancer-related bone pain, or the requirement of opioid analgesics for cancer pain, as well as patients with visceral metastases were not eligible. Local institutional review boards approved the trial at each study center and all patients signed institutional review board approved informed consent.

A centrally administered, block random assignment method encompassing all study centers was employed to assign patients to treatment in a 2:1 ratio (sipuleucel-T: placebo). Treatment was given on weeks 0, 2, and 4. Sipuleucel-T and placebo were prepared fresh for each treatment. Stratification variables were study center and bisphosphonate use (yes/no). Patients underwent a standard 1.5 to 2.0 blood volume mononuclear cell leukapheresis, which was transported to a sponsor-designated manufacturing facility. APCs were isolated from the leukapheresis product and sipuleucel-T was prepared as previously described.^5,6 The time from apheresis to infusion of final product was approximately 48 hours. For patients randomly assigned to placebo, APCs were similarly prepared and divided into two aliquots. One third of the cells were reinfused without being pulsed with PA2024, serving as placebo. The remaining two thirds were cryopreserved to later generate the open-label product, APC8015F. (Patients were unblinded at the time of disease progression and those randomly assigned to placebo were eligible for treatment with APC8015F.) For each of the three sipuleucel-T infusions, the number of cells infused was the maximum number of cells that could be prepared from the leukapheresis product; the median number of nucleated cells per infusion was 3.65 × 10⁹ and the median number of CD54+ bright cells per infusion was 7.45 × 10⁸. Patients were premedicated 30 minutes before each infusion with acetaminophen (650 mg) and diphenhydramine (50 mg). Sipuleucel-T or placebo was administered intravenously over 30 minutes, and patients were observed for 30 minutes after each infusion. After treatment, patients were observed every 8 to 12 weeks for safety (physical examinations, adverse event assessments, laboratory tests) and progression (radiographic imaging studies and pain assessment) until disease progression, at which point a crossover to open-label APC8015F was available for patients treated with placebo.

T-cell proliferation response to sipuleucel-T was evaluated in those patients for whom samples were submitted at baseline (preinfusion) and at week 8 (postinfusion) and for whom the samples could be processed within 24 hours of collection. All immunologic assays were undertaken without the knowledge of treatment assignment. Peripheral blood mononuclear cells (PBMCs) were isolated from peripheral blood using CPT tubes. The T-cell proliferation assay was set up in triplicate in 96-well round-bottom plates with PBMCs acting as both APCs and responding cells. PA2024 was used at a concentration of 50 μg/mL, while PBMCs without antigen were used as the negative control. Cells were incubated for 6 days, and [³H]thymidine was added for the final 18 hours to assess T-cell proliferation. Plates were harvested onto filter mats and radioactivity was measured by a scintillation counter. Data were reported as counts per minute (cpm). For each assay, the T-cell stimulation index was defined as median cpm with antigen/median cpm without antigen.

Data from a small phase II study of sipuleucel-T demonstrated a trend in delaying TTP correlating with T-cell immune response to PAP.⁵ Because of the confounding nature of subsequent treatments, including crossover to open-label APC8015F, TTP was chosen as a primary end point and was defined by any of the following: progressive disease on serial radiographic imaging tests; new cancer-related pain associated with a radiographic anatomic correlation or; other clinical events consistent with progression such as spinal cord compression, nerve root compression, or pathologic fracture. Radiographic imaging studies were specified in the protocol to occur every 8 weeks until week 32 then every 12 weeks thereafter, and progression was confirmed by central review of scans. PSA was not used to determine disease progression or to trigger radiographic evaluations. Progressive disease was defined as an increase greater than 50% in measurable disease, clear worsening of nonmeasurable disease, the appearance of at least two new lesions on a bone scan, the development of cancer-related pain that correlated with a site of metastasis, or the development of other clinical events consistent with progression (such as spinal cord compression or pathologic fracture).

Published data suggest that the median TTP in patients with symptomatic HRPC is approximately 3 months.¹⁰ The assumption was made that the TTP for asymptomatic patients would be somewhat longer (4 months; null hypothesis). Sample size calculations were based on attaining a power of 80% with a two sided .05 significance level and the assumption that sipuleucel-T would result in an increase in median TTP to 7.7 months, translating into a hazard ratio (HR) of 1.925. Factoring in nonuniform patient entry and a loss to follow-up rate of 5%, enrollment of approximately 120 patients was targeted to achieve the required 80 events.

While this study was not powered to detect a survival difference between the two treatment arms, there was a protocol-specified requirement to follow each patient for survival (and treatment-related adverse events) up to 36 months after random assignment. The median TTP and survival time were estimated with the Kaplan-Meier method.¹¹

The log-rank test was performed to compare distributions of TTP and survival times. All tests were two sided and performed at the .05 level. Risk ratios with 95% CIs based on the Cox proportional hazards model were also provided. For the analysis of T-cell proliferative responses, the ratio of stimulation index at 8 weeks was compared with the ratio of stimulation index at baseline for different subsets using the Wilcoxon rank sum method.¹²

To test the robustness of the survival findings, the individual effect of 20 potentially important prognostic factors that have been described in the literature was evaluated. Using exploratory Cox regression models, each one having treatment and a single covariate as main effects, the treatment effect remained significant. Covariates that were significant at the .05 level were then selected to build a predictive multicovariate Cox regression survival model. A backwards stepwise selection method (P = .05 for entry and P = .10 for removal, likelihood ratio test) was used to identify significant prognostic factors and their impact on treatment effect.

Between January 2000 and October 2001, 127 patients were enrolled at 19 study centers. Eighty-two patients were randomly assigned to receive sipuleucel-T; 45 patients were randomly assigned to receive placebo. Table 1 demonstrates that the treatment groups were similar with regard to median age, race, Eastern Cooperative Oncology Group performance status, PSA, PAP, alkaline phosphatase, hemoglobin, lactate dehydrogenase, and Gleason score. While not statistically significant, there were apparent differences in the treatment arms with respect to bone only disease (42.7% in sipuleucel-T v 26.7% in placebo) and the percentage of patients with more than 10 bone metastases (40.2% in sipuleucel-T v 26.7 in placebo). Overall, however, bone involvement was similar (93.9% in sipuleucel-T arm and 91.3% in the placebo arm.). All 127 patients received at least one infusion, and all are included in the intent-to-treat (ITT) population.

Disposition specifics are presented in Figure 1. Treatment with chemotherapy or other anticancer therapy was prohibited until the primary end point of TTP was met. After progression, 55.7% of sipuleucel-T patients and 62.8% of placebo patients received some form of chemotherapy, 43.6% of sipuleucel-T and 52.4% of placebo patients received a taxane-based chemotherapy, and 35.9% of sipuleucel-T and 47.6% of placebo patients received docetaxel-based therapy (Table 1).

A total of 115 patients (90.5%) contributed a progression event to the primary analysis of TTP (Fig 1). All patients except one progressed before death. The one exception was a man treated with sipuleucel-T who developed and succumbed to complications of a glioblastoma 6 months after treatment. The median TTP was 11.7 weeks (95% CI, 9.1 to 16.6) in sipuleucel-T-treated patients and 10.0 weeks (95% CI, 8.7 to 13.1) in placebo-treated patients (P = .052, log-rank; HR, 1.45; 95% CI, 0.99 to 2.11; Fig 2.)

Complete follow-up to death or to the prespecified 36 month end point after random assignment was obtained for all 127 patients. In an ITT analysis, the median overall survival was 25.9 months (95% CI, 20.0 to 31.9) in sipuleucel-T-treated patients compared with 21.4 months (95% CI, 12.3 to 25.8) in placebo-treated patients (P = .01, log-rank; HR, 1.70; 95% CI, 1.13 to 2.56; Fig 3.) The estimated survival rate at the last assessment before censoring at 36 months was 34% in the sipuleucel-T group compared with 11% in the placebo group (P = .005, χ² based on the number of survivors at 36 months). The placebo group for these analyses included the 34 patients (75.5%) subsequently treated with open-label APC8015F.

While overall survival at 36 months was prespecified in the analysis plan, it was not specified as the primary efficacy end point, so the P value obtained for this analysis should be interpreted with caution. Therefore, in order to assess the robustness of the survival benefit observed, an exploratory Cox regression analysis identified five clinical variables (lactate dehydrogenase, PSA, number of bone metastases, body weight, and localization of disease) that were highly predictive of overall survival in this study cohort (supra vide). To correct for any potential imbalances, the treatment effect of sipuleucel-T was adjusted using these variables and remained statistically significant (P < .002, Wald test; HR, 2.12; 95% CI, 1.31 to 3.44).

Adequate sample collection for immunologic testing was available for 49 patients: 18 treated with placebo (40% of total); 31 treated with sipuleucel-T (38% of total). The PA2024 T-cell stimulation index is a measure of specific T-cell responsiveness against the target antigen. This exploratory analysis was performed in a subset of patients for whom cells could be processed within 24 hours of collection, thus precluding the need to freeze the cells before analysis. All analyses were performed before study unblinding. The median ratio of the T-cell stimulation index at 8 weeks versus baseline (preinfusion) was approximately eight-fold higher in sipuleucel-T- versus placebo-treated patients (16.91 Wilcoxon v 1.99; Wilcoxon rank sum P < .001).

Therapy was generally well tolerated. Toxicities that were significantly more common (P ≤ .05) in sipuleucel-T treated patients included rigors (59.8% v 8.9%), pyrexia (29.3% v 2.2%), tremor (9.8% v 0%), and feeling cold (8.5% v 0%; Table 2.) Rigors and pyrexia were seen primarily as infusion reactions. The majority of toxicities from each treatment group (70.7% in sipuleucel-T; 68.9% in placebo) were grade 1 or 2. Ninety-five percent of patients received all three planned infusions. No patient discontinued the trial because of toxicity.