A Phase II Randomised Trial of Three Regimens of GX301 Vaccination in Castration-resistant Prostate Cancer

GX301, an experimental therapeutic (anti-cancer) vaccine, is composed of four immunogenic peptides from human telomerase and two complementary adjuvants, Montanide ISA-51 VG and imiquimod. An earlier Phase 1 study of GX301 has provided evidence of vaccine-specific immune response in a small sample of stage 4 prostate cancer patients given eight GX301 administrations over 9 weeks. The present Phase 2, randomised, parallel-group, multicentre trial is aimed at comparing three different GX301 administration regimens in patients with progressive, castration-resistant prostate cancer who have completed a first-line docetaxel treatment and have achieved response to chemotherapy or disease stability. Primary comparisons will include regimen efficacy in inducing vaccine-specific immunological responses over a period of 6 months following randomisation; and treatment safety and tolerability over the same period. A further study aim is to investigate whether achievement of immunological response, irrespective of the assigned GX301 regimen, is related to progression-free and/or overall survival. Eligible patients will be randomly assigned to receive one of three GX301 vaccination regimens consisting of two, four or eight administrations, respectively, each regimen being given over a fixed 9-week period. Randomisation ratio will be 1:1:1. Randomisation will be stratified by previous cumulative exposure to docetaxel. Following randomisation, immunological responses to GX301 will be determined over a 6-month period. However, on-study patient observation will be continued until the occurrence of one of the following end-points, whichever the earliest: (a) disease progression; (b) death; (c) completion of an 18-month observation period; or (d) patient's decision to terminate his participation in the study. All patients discharged from the trial for reasons (a) or (c) will undergo a follow-up to ascertain survival until 24 months from randomisation. Data analysis will be carried out in two sequential steps. The first step will focus on co-primary outcomes and will therefore take place upon completion of the study dataset up to the 6-month time-point. The second step will incorporate secondary outcomes and will therefore be conducted upon completion of the full study dataset.