Background: The granulocyte-macrophage colony-stimulating factor-transduced allogeneic prostate cancer cells vaccine (GVAX) has antitumour activity against prostate cancer; preclinical studies have shown potent synergy when combined with ipilimumab, an antibody that blocks cytotoxic T-lymphocyte antigen 4. We aimed to assess the safety of combined treatment with GVAX and ipilimumab in patients with metastatic castration-resistant prostate cancer (mCRPC). Methods: We did an open-labelled, single-centre, dose-escalation study of ipilimumab concurrent with a fixed dose of GVAX, with a subsequent expansion phase, both at the VU University Medical Centre (Amsterdam, Netherlands). Eligible patients had documented mCRPC and had not been previously treated with chemotherapy. All patients received a 5×10 8 cell priming dose of GVAX intradermally on day 1 with subsequent intradermal injections of 3×10 8 cells every 2 weeks for 24 weeks. The vaccinations were combined with intravenous ipilimumab every 4 weeks. We enrolled patients in cohorts of three; each cohort received an escalating dose of ipilimumab at 0·3, 1·0, 3·0, or 5·0 mg/kg. Our primary endpoint was safety. This study is registered with ClinicalTrials.gov, number NCT01510288. Findings: We enrolled 12 patients into our dose-escalation cohort. We did not record any severe immune-related adverse events at the first two dose levels. At the 3·0 mg/kg dose level, one patient had grade 2 and two patients grade 3 hypophysitis; at the 5·0 mg/kg dose level, two patients had grade 3 hypophysitis and one patient developed grade 4 sarcoid alveolitis (a dose-limiting toxic effect). Due to observed clinical activity and toxic events, we decided to expand the 3·0 mg/kg dose level, rather than enrol a further three patients at the 5·0 mg/kg level. 16 patients were enrolled in the expansion cohort, two of whom developed grade 2 hypophysitis, three colitis (one grade 1 and two grade 2), and one grade 3 hepatitis-all immune-related adverse events. The most common adverse events noted in all 28 patients were injection-site reactions (grade 1-2 events seen in all patients), fatigue (grade 1-2 in 20 patients, grade 3 in two), and pyrexia (grade 1-2 in 15 patients, grade 3 in one). 50% or greater declines in prostate-specific antigen from baseline was recorded in seven patients (25%); all had received 3·0 mg/kg or 5·0 mg/kg ipilimumab. Interpretation: GVAX combined with 3·0 mg/kg ipilimumab is tolerable and safe for patients with mCRPC. Further research on the combined treatment of patients with mCRPC with vaccination and ipilimumab is warranted. Funding: Cell Genesys Inc, Prostate Cancer Foundation, Dutch Cancer Society (KWF-VU 2006-3697), and Foundation Stichting VUmc Cancer Center Amsterdam.
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