58 NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 6 1. OBJECTIVES The goal of the trial is to determine whether treatment with recombinant glycosylated human interleukin -7 (CYT107, RevImmune, Inc. ) after standard therapy with sipuleucel -T (PROVENGE®, Dendreon Corporation) can substantially increase and prolong immune responses to the sipuleucel -T fusion protein vaccine construct PAP -GM-CSF (PA2024) and the prostatic acid phosphatase (PAP) component and thereby provide a novel, nontoxic regimen to prolong the s urvival of patients with metastatic castration -resistant prostate cancer (mCRPC) with no or minimal symptoms. 1.1 Primary Objectives To determine whether CYT107 administration increases the vaccine -induced antigen -specific T-cell immune response to the sipuleucel -T fusion protein vaccine construct PAP -GM-CSF (PA2024) 1.2 Secondary Objectives 1. To determine whether CYT107 administratio n increases the vaccine -induced antigen - specific T -cell immune response to PAP. 2. To assess the character of the T -cell immune response to PAP and PA2024 3. To determine whether CYT107 administration increases the vaccine -induced antigen - specific antibody imm une responses to PAP and PA2024 4. To quantify the effects of CYT107 on T -cell repertoire diversity 5. To assess the effects of CYT107 on the immune competence of patients with advanced prostate cancer 6. To assess the clinical efficacy and tolerability of sipul eucel -T plus CYT107 compared with sipuleucel -T alone 2. BACKGROUND 2.1 Study Disease The proposed clinical trial is a phase II, open -label, multicenter, randomized study of the administration of CYT107 after the completion of standard FDA -approved therapy with sipuleucel -T for patients with asymptomatic or minimally symptomatic mCRPC. Prostate cancer is the second leading cause of death from cancer in men [ Cheever, 2011 ]. Localized prostate cancer may be cured with surgery or radiation therapy, but the disease recurs in approximately 20 to 30% of patients. Androgen -deprivation therapy, the most common treatment after recurrence, is effective, but the disease eventually progresses in most patients who receive such treatment [ FDA.gov, 2010 ], termed castration -resistant prostate NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 7 cancer ( mCRPC) . For men with mCRPC, the median survival ranges from 12.2 to 21.7 months [ Kantoff, 2010 ]. A number of drugs for mCRPC have been approved. Previous therapies like prednisone and mitoxantrone showed no additive impact on median overall survival beyond the expected survival of 10 -14 months seen in mCRPC. Current therapeutics such as docetaxel [ Tannock, 2004], abiraterone [ Ryan, 2013 ], and enza lutamide [Beer, 2014] increase overall survival by 2 to 8 months. Unfortunately, all patients eventually progress after receiving these therapies. Therefore, there is a critical need for improved therapies for mCRPC that can treat metastatic disease, improve quality of life, decrease morbidity, and prolong overall patient survival. Sipuleucel -T (PROVENGE®; Dendreon) is the first therapeutic cancer vaccine to be approved by the U.S. Food and Drug Administration. In men who have metastatic castration - resistant prostate cancer with no or minimal symptoms, sipuleucel -T prolongs median survival by 4.1 months compared with results in those treated with placebo [Kantoff, 2010 ]. IL-7 is a homeostatic growth factor for T cells and is capable of inducing proliferation, maintaining T -cell responsiveness, and prev enting and reversing T -cell anergy [ Mackall, 2011 ]. The hypothesis of the trial is that CYT107 will increase and prolong the sipuleucel -T induced immune response and thereby further prolong survival. 2.2 CTEP IND Agent: N/A 2.3 Other Agents Sipuleucel -T (PROVENGE®, Dendreon Corporation): Sipuleucel -T is not a direct component of this protocol. However, successful completion of standard sipuleucel -T therapy within 3 -7 days is a mandatory entry criterion for the trial. Sipuleucel -T is a therapeutic vaccine formulated to stimulate an immune response to prostate cancer cells [ Cheever, 2011 ] by generating PAP -specific T cells capable of recognizing and killing prostate cancer cells that expres s PAP. In sipuleucel -T, the PAP vaccine immunogen is formulated as PA2024, a fusion protein combining recombinant PAP with recombinant granulocyte -macrophage colony -stimulating factor (GM -CSF) [ FDA.gov, 2010 ], [Kantoff, 2010 ], [Cheever, 2011 ]. Sipuleucel -T is constructed as an autologous cellular immunotherapy product consisting of peripheral blood mononuclear cells (PBMC), including antigen presenting cells (APC). To manufacture sipuleucel -T, a subject undergoes leukapheresis to collect PBMC. Mononuclear cells, including quiescent APC, are cultured ex vivo (serum and cytokine free conditions) at 37oC in the presence of PAP -GM- CSF (PA2024) antigen. GM -CSF is used to activ ate APCs within the autologous PBMC product [ FDA.gov 2010 ]. These activated APC are proficient at activating and inducing the replication of PAP -specific T cells. Three days after leukapheresis and 40 hours of subsequent incubation, the cells are washed to remove the fusion protein and suspended in Lactated Ringer’s Injection, USP, shipped to the patient care site, and infused as the sipuleucel -T product into the patient. The standard FDA approved therapy is three doses. After infusi ng the first dose, increases in APC and T -cell activation NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 8 markers in PBMCs are detectable in treated patients as increased ex -vivo production of cytokines associated with T -cell activation. Thus, with each dose, increasing levels of activated APCs and T cells are infused into patients [ Sheikh, 2010 ]. Prior Sipuleucel -T Trials : The phase III randomized IMPACT trial led to FDA approval because of improvements in overall survival (OS) for patients with mCRPC [ Kantoff, 2010 ]. IMPACT was a double -blind, placebo -controlled study involving 512 patients with asymptomatic or minimally symptomatic mCRPC randomly assigned 2:1 to receive sipuleucel -T (n = 341) or placebo (n = 171). Sipuleucel -T was administe red every 2 weeks for a total of 3 doses. Of the 330 patients who received sipuleucel -T, 92% received all 3 infusions. Median OS was 25.8 months in the treatment arm and 21.7 months in the control arm ( P = 0.032). The adjusted hazard ratio for death was 0. 78 (95% confidence interval, 0.61–0.98). The Kaplan -Meier estimate for OS for each group at 36 months was 31.7% for sipuleucel -T vs. 23.0% for placebo , indicating a 38% increase in survival at 3 years for patients in the sipuleucel -T arm [ Kantoff, 2010 ]. The median OS predicted by the Halabi prognostic model was 20.3 months for the treatment arm vs. 21.2 months for the control arm [FDA.gov, 2010 ], supporting the efficacy of sipuleucel -T. Vaccine -induced therapeutic responses sometimes appear to continue to function after the median time to progression. In the IMPACT trial, PSA reductions of at least 50% compared with baseline were confirmed at 4 weeks in 2.6% of treated patients vs. 1.3% of control patie nts [Kantoff, 2010 ]. However, in another double -blind, randomized, placebo -controlled study (P -11) of 176 men with rising PSA after prostatectomy receiving hormonal therapy followed by vaccine or control [ Beer, 2007 ], sipuleucel -T did not significantly delay cancer progression (defined as PSA reaching 3.0 ng/mL); however, the rate of the PSA increase was slower in patients who received the vaccine. The FDA -approved regimen is thre e vaccinations administered on days 0, 14, and 28. The initial IMPACT trial publication reported that at week 6 after the first vaccination (2 weeks after the third vaccination), T -cell proliferation responses (stimulation index >5) to PA2024 were observed in 73.0% of treated patients and 12.1% of control patients. In addition T -cell proliferation responses to PAP (stimulation index >5) were observed in 27.3% of treated patients compared with 8.0% of control patients [ Kantoff, 20 10]. Of note, the PA2024 fusion protein contains a joining -region segment that is not expressed by normal PAP or GM -CSF, which patient immune systems recognized as foreign. Eliciting an immune response to this foreign joining -region segment signals immu ne competence and is likely to facilitate epitope spreading to the PAP portion of the molecule. All patients with a T cell response to PAP also had a T cell response to PA2024. In a prespecified analyses, patients in the sipuleucel -T group who had an anti body titer of more than 400 against PA2024 or PAP at any time after baseline lived longer than did those who had an antibody titer of 400 or less ( P < 0.001 and P = 0.08, respectively, by log -rank test). PA2024 -specific antibody responses that exceeded the designated low value of 400 at any time after baseline were observed in 66.2% of treated patients and only 2.9% of control patients. By contrast, titers of antibodies against PAP that exceeded 400 at any time after baseline were observed in 28.5% of treat ed patients compared with 1.4% of control patients. Although the IgG antibody responses correlated with survival and cognate recognition of antigen by CD4+ T cells is required for such antibody responses, the initial publication NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 9 [Kantoff, 2010 ] reported that no survival difference could be detected between patients in the sipuleucel -T group who had T -cell proliferation responses to PA2024 or PAP at week 6 and those who did not [ Kantoff, 2010 ]. Th e presumption is that the responses were present, but below the level of detection by the assay employed. A subsequent analysis of immune function ( Figure 1 ) [Sheikh, 2012 ] provided additional credence to the correlation between immune response and overall survival (OS). Antigen - specific peripheral cellular and humoral responses were assessed in a 237 patient subset of subjects enrolled in the IMPACT study. The IMPACT subset provided blood for immun e response determination at baseline (week 0) as well as 6, 14, and 26 weeks following the first infusion. Antigen -specific immune responses were observed in 78.8% of monitored subjects and correlated with OS (p=0.003). Overall survival correlated significantly with the development of at least one post -baseline peripheral immune response to PA2024 or PAP (HR=0.47 [95% CI: 0.29, 0.78] P=0.003; Figure 1D ), to PA2024 (HR=0.46 [95% CI: 0.28, 0.76] P =0.002; Figure 1E ), or to PAP (HR=0.53 [95% CI: 0.31, 0.90] P=0.019; Figure 1F ). The strongest correlation between survival and the development of a post -baseline immune response to PA2024 at any timepo int was observed with antibody responses (HR=0.42 [95% CI: 0.26, 0.67] P<0.001), while the correlation with memory IFNγ ELISPOT at any time point approached statistical significance (HR=0.55 [95% CI: 0.28, 1.08] P=0.08); T cell proliferation response did n ot significantly correlate with overall survival. Overall Survival is depicted in Figure 1 from Sheikh et al [ Sheikh, 2012 ]. Data is presented as Kaplan -Meier survival plots for antigen -specific im mune response for sipuleucel -T responders and non -responders for a subset of IMPACT subjects. Figure 1 [Sheikh, 2012] Figure 1 . The hazard ratio and p -value are based on a Cox regression analysis with response NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 10 (0 = non responder, 1 = responder) in the model. Median -based analysis of cumulative product parameters from a Cox regression model, stratified by study, and adjusted for baseline PSA and LDH. P -values in the table are from analyses of each parameter as a continuous measure. Hazard ratios and 95% confidence intervals from a Cox regression model, adjusted for baseline PSA and LDH. D) Response to PA2024 or PAP in at least one of the three immune response assays (HR=0.47 [95% CI: 0.29, 0.78] P=0.003) E) Respo nse to PA2024 in at least one of the three immune response assays (HR=0.46 [95% CI: 0.28, 0.76] P=0.002) F) Response to PAP in at least one of the three immune response assays (HR=0.53 [95% CI: 0.31, 0.90] P=0.02). The peripheral immune response paramete rs examined were log -transformed SI medians, ranked IFNγ ELISPOT medians, and ranked ELISA titers. Responder status was analyzed using a Cox regression model, with and without adjustment for baseline PSA and LDH [Sheikh, 2012 ]. All p -values reported are two -tailed. No adjustment for multiplicity of endpoints or time points was made. These correlations remained after adjusting for baseline prognostic factors (PSA and LDH) that are independently correlated with survival. Positi ve thresholds for treatment -related immune responses were selected in order to ensure <5% of subjects would exceed the value at baseline. Thresholds were proliferation: SI > 12 for PA2024, > 8 for PAP; IFNγ ELISPOT (per 3x10 5 PBMC): >10 spots for PA2024, > 40 spots for PAP; ELISA titer: >400 for both anti -PA2024 and anti -PAP antibodies. Serum samples that gave a positive response to an initial ELISA were subsequently evaluated for IgM and IgG antibody isotypes. Figure 1 presents the impact of any positive immune response parameter to OS survival. Below are the hazard ratios and associated 95% confidence intervals, as well as the p -values, for comparison of the survival curves in the IMPACT study for responders versus non - responde rs for the different immune response assays. For the primary endpoint of the current trial, IFN ELISpot to PA2024, the hazard ratio estimate of 0.53 (0.27, 1.04) for responders to IFN ELISpot to PA2024 compared to non - responders. Table 1 (Data from Dendreon) Antigen Assay HR (95% CI) p-value PA2024 Humoral 0.45 (0.28, 0.71) p=0.0007 PA2024 Proliferation 0.87 (0.46, 1.67) p=0.169 PA2024 IFN ELISpot 0.53 (0.27, 1.04) p=0.065 PAP Humoral 0.60 (0.33, 1.08) p=0.087 PAP Proliferation 0.66 (0.27, 1.61) p=0.364 PAP IFN ELISpot 0.29 (0.04, 2.15) p=0.227 The hazard ratio and p -value are based on a Cox regression analysis with response (0 = non responder, 1 = responder) in the model. Similar results are obtained with models adjusting for baseline PS A and LDH. NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 11 Data from a neoadjuvant trial demonstrate that sipuleucel -T can modulate the presence of lymphocytes at the prostate tumor site giving further credence to the influence of the anti - tumor immune response to the effect of sipuleucel -T on OS. In the neoadjuvant trial [NeoACT (P07 -1; NCT00715104)] [ Fong, 2012 ] sipuleucel -T treatment has been shown to be associated with an increased frequency of T cells in prostate cancer tissue at the interface of the benign and malignan t glands. Patients received the standard sipuleucel -T regimen of 3 infusions of sipuleucel -T at approximately 2 -week intervals. At 6 -7 weeks after start, patients underwent radical prostatectomy. By IHC there was a significant observed increase in CD3+ a nd CD4+ T cell populations (≥3 fold) at the tumor interface (where benign and malignant glands interface), compared with the pre -treatment biopsy, benign radical prostatectomy tissue, and tumor tissue (ANOVA post hoc Newman -Keuls test: p<0.0001 for each co mparison). FoxP3+ CD4+ T cells were also increased (p=0.0005) at the tumor interface, but represented a small fraction of the observed CD4+ T cells. Recombinant Glycosylated Human Interleukin -7) (CYT107) NSC #767713 , RevImmune, Inc. : IL-7 is a homeosta tic growth factor for T cells and is capable of inducing proliferation, maintaining T -cell responsiveness, and preventing and reversing T -cell anergy [ Mackall, 2011 ]. IL-7 signals through the IL -7 receptor (IL -7R) heterodimer composed of the common γ-chain (CD132) cytokine receptor and a unique IL -7Rα (CD127). During normal conditions, IL-7R expression is maintained on resting T cells [ Mackall, 2011 ] and IL -7 is continuously available in secondar y lymphoid organs as a result of stromal cell IL -7 production [ Takada, 2009 ]. In general, IL -7 has more potent effects on conventional CD8+ T cells and CD4+ cells than it does on CD4+ regulatory T cells (Tregs). IL -7Rα express ion is notably low on FOXP3+ Treg cells compared with non -regulatory T -cell subsets [ Seddiki, 2006 ], [Liu, 2006 ]. Its continuous signaling induces antiapoptotic and costimulatory responses that are essential for the survival of naïve T cells. After either T -cell activation, IL -7 signaling, or both, IL -7Rα is down -regulated on terminally differentiated, senescent T cells [ Fry, 2003 ], [Park, 2004 ] but becomes re -expressed after several days and remains expressed during the later contraction phase. The effects of IL -7 are most potent on recent thymic emigrants in which IL -7–mediated signaling can induce prolifera tion in the absence of T -cell receptor signaling [ Swainson, 2007 ]. IL-7Rα is also selectively expressed on a small minority of effector T cells that are destined to enter the central memory T -cell pool, thus implicating IL - 7 as a modulator of the effector to memory cell transition [ Kaech, 2003 ]. Prior Trials : Preclinical and clinical results support the rationale for developing trials testing IL-7 administration after sipuleucel -T therapy to incr ease or prolong the sipuleucel -T– induced T -cell response to PAP. In animal models and in humans, IL -7 has been shown to increase by several -fold the number of peripheral blood T cells with little toxicity and to increase T -cell receptor diversity (i.e., na ïve T cells and conventional CD4+ and CD8+ T cells), with the expansion of conventional T cells at a greater proportion than the expansion of Tregs [ Levy, 2012 ]. In animal models, IL -7 administration augments vaccine -induced cel l responses (e.g., CD8+ memory T -cell responses) and immune -mediated control of both tumors and viruses. In addition, IL -7 can maintain effector T -cell function and prevent T -cell exhaustion. Consistent with the preclinical work, human trials of IL -7 in ch ronic viral infections induce increased T -cell number and demonstrate beneficial effects. IL -7 has been NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 12 well tolerated in early phase clinical trials and can be administered on an outpatient basis. Furthermore, other animal studies show that IL -7 administr ation can augment immune reconstitution after lympholytic stem cell transplantation and chemotherapy [ Mackall, 2001 ], [Morrissey, 1991 ], [Perales, 2011 ]. Importa nt for the current protocol, IL -7 administration can augment vaccine -induced antigen -specific T -cell responses [ Colombetti, 2009 ], [Melchionda, 2005 ]. In mice, transient administration of IL-7 around the time of vaccination or initial viral infection increases the number of antigen -specific effector and memory T -cell populations and has long -lasting effects on the memory T -cell pool [ Colombetti, 2009 ], [Nanjappa, 2008 ], [Pellegrini, 2009 ], [Melchionda, 2005 ]. Therefore, administration of IL -7 after a vaccine such as sipuleucel -T may have enhanced efficacy and substantial improvements in vaccine -induced responses. The effectiveness of IL -7 as a vaccine adjuvant depends on the timing of administration. Durable effects on the memory T -cell pool have been observed only when IL -7 was administered during the contra ction phase of the CD8+ T-cell response [ Nanjappa, 2008 ]. IL- 7 expanded the magnitude of the secondary CD8+ response when administered during the contraction phase of the primary or the secondary response, which is relevant to this proposal. Two IL -7 formulations have been successively developed by Cytheris. The technology for the secon d formulation, CYT107, as well as vialed material has been taken over by RevImmune, Inc. and is the agent used in this current protocol. Both had the same primary sequence similar to the sequence of the native protein. The IL -7 for this proposal (CYT107) is produced from recombined Chinese hamster ovary (CHO) mammalian cells and is highly glycosylated. Seven phase I/IIa dose escalation studies and two phase II studies have been initiated with CYT107 for the treatment of cancer and chronic viral infections (e.g., HIV, hepatitis B, hepatitis C) and for use after hematopoietic stem cell transplantation [ Mackall, 2011 ]. In those studies, CYT107 has been administered subcutaneously in a dose range of 3 – 60 μg/kg with a schedule regimen of one weekly subcutaneous administration for 3 or 4 weeks (corresponding to one cycle). In general, CYT107 has been well tolerated as an outpatient regimen with mild to moderate constitutional symptoms and reversible spleen and lymph node enlargement. In addition, marked dose -dependent increases in peripheral CD3+, CD4+, and CD8+ lymphocytes were seen, resulting in a rejuvenated circulating T-cell profile, resembling that seen earlier in life [ Sportes, 2008 ]. The most common side effects have been low-grade fever, malaise, transient increases in liver enzyme levels, erythema and induration at the site of admini stration, and rash or allergic reaction with repeated cycles in patients with HIV. In marked contrast to IL -2 and IL -15, no significant capillary leak or acute toxicity have been observed following IL -7 therapy. The trial regimen administers CYT107 subcut aneously once a week on days 0, 7, 14 and 21 and is expected to maintain increased T -cell number throughout this trial to at least week 12, although a sustained effect up to 1 year has been observed in an HIV trial (CLI -107-06) [Levy, 2012 ]. Although the measured half -life of CYT107 is 10 –15 hours, the biologic effects persist well after circulating levels of IL -7 return to baseline. The biologic half -life is much longer possibly due to a depot effect from IL -7 binding to extrace llular matrix and the slow release at lower than detectable levels [ Mackall, 2011 ]. In a study of single -dose rhIL -7 (CYT 99 007) administered to patients with HIV infection, CYT 99 007 induced early NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 13 lymphopenia, followed by a significant increase in the rate of T -cell cycling on day 4 with increases in circulating CD4+ and CD8+ T-cell counts persisting for at least 14 days [ Sereti, 2009 ]. The administration of CYT107 is also likely to transiently increase the size of the spleen and lymph nodes as well as peripheral blood T cells. Specificities of IL -7/IL-7Rα pathway regulation and current data on CYT107 in humans favor injection once a week and short therapeutic cycles. According to monkey and hu man data, a 1 -week interval between each injection of CYT107 should lead to full recovery of IL - 7Rα expression (CD127) at the surface of T cells that responded to a prior dose of IL -7. Furthermore, a sustained response of T cells to IL -7 should be fully re stored, as measured 1 week later by Ki-67 expression. Short cycles, limited to 3 doses, should avoid exhaustion of T-cell responsiveness to high doses of IL -7, which could occur despite the recovery of CD127 expression. T-cell proliferation rates in respo nse to IL -7 are correlated with IL -7Rα expression. At present, the kinetics of IL -7Rα after other cancer vaccines in general and sipuleucel -T specifically is not known, but critical. T -cell activation and IL -7 itself can down -regulate IL - 7Rα expression. IL -7Rα expression is expected to be re -expressed at about the time of CYT107 administration and persist during the period of function of the administered IL -7. The once -weekly regimen beginning after sipuleucel -T is expected to increase proliferation and sur vival of PAP -specific T cells, possibly for an extended period, and increase the number of PAP -specific memory cells without equivalent expansion of Tregs to dampen the antitumor effect. CYT107 will be instituted 3 to 7 days after the third and final administration of sipuleucel -T. IL-7 function requires upregulated IL -7 receptors and upregulation will occur after completion of PAP vaccination and during the contraction phase. Because sipu leucel -T is manufactured from leukapheresed patient PBMC, provisions of CYT107 before leukapheresis would change the character of cells placed into culture and thus change the manufacturing scheme and would be disallowed by the FDA. Many insurance programs , including Medicare, will not pay for unapproved agents or regimens in clinical trials. Sipuleucel -T would be considered an experimental agent by some insurance programs if administered concurrent with CYT107. Therefore, testing of CYT107 will begin only after completing standard therapy with sipuleucel -T. 2.4 Rationale Sipuleucel -T is the first FDA -approved cancer vaccine, designed to elicit therapeutic immune responses to PA2024 and subsequently PAP. Sipuleucel -T prolongs patient survival by 4.1 month s on average. IL -7 is a homeostatic growth factor for T cells. IL -7 induces substantial proliferation of antigen -specific T cells that express a unique IL -7 receptor, IL -7Rα with very little toxicity. IL -7 can maintain T -cell responsiveness and prevent and reverse T -cell anergy. IL-7 is also thymopoïetic and can expand recent thymic immigrants and thus the T -cell repertoire, which is known to be low in individuals the age of most patients with advanced prostate cancer. NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 14 We expect that CYT107 will increase and prolong T -cell and antibody immune responses to PA2024 and PAP and provide a novel, nontoxic regimen to prolong survival. We also expect that CYT107 will expand T -cell repertoire and reset the homeostatic total T -cell level potentially mitigating the i mmune compromise observed in patients with mCRPC because of age and prior treatment. Because clinical development of sipuleucel -T for FDA approval began in 1997, the formulation and regimen of sipuleucel -T have remained unchanged for almost 15 years. Given the advances made in immunology and immunotherapy in the last decade and a half, it is axiomatic that regimens based on current immunologic science would provide higher and longer lasting immune responses (i.e., greater areas under the curve) and, in t he case of sipuleucel -T, enhanced patient benefit. The question remains “Which strategies are most likely to be effective and the most ripe for testing?” In answer to this question, a strong rationale supports therapy with IL -7. IL-7 is a homeostatic T -cell growth factor responsible for the expansion of T cells during lymphopenia [Mackall, 2011 ]. In human clinical trials, IL -7 induces dramatic expansions in peripheral T - cell numbers and in total body CD4+ and CD8+ T cells wit hout substantial toxicity. Yet, if an immunotherapy is to enhance the effects of an approved regimen, the agent must address many of the reasons an immunotherapy fails. Such reasons include (1) insufficient T -cell number, (2) insufficient CD8+ or CD4+ T-cell subsets, (3) a preponderance of regulatory T cells (Treg), (4) short -lived T cells unable to provide long -term tumor control, (5) a narrow repertoire of response (sometimes monoclonal), (6) little or no physical access of T cells to the tumor, and (7) strong local immunosuppressive effects of TGF ß, one of the most potent tumor immunosuppressors . IL-7 can mitigate many of these problems. Regarding insufficient numbers and types of T cells, IL-7 administration increases the number of CD4+ and CD8+ T cells, an effect that is both potent and sustained [ Mackall, 2011 ], [Fry, 2003 ], [Levy, 2009 ], [Levy, 2012 ]. This boost of CD4+ T cells provi des efficient help to CD8+ T cells and warrants their functionality. IL -7 favors the homing of T cells to lymph nodes, gut, mucosa, and tumors [Kaech, 2003 ], [Beq, 2009 ], [Sereti, 2012 ] and thus potentially the ability of T cells to infiltrate tumors. With IL -7, the expansion of Treg is proportionally less than conventional CD8+ and CD4+ T cells and the produced T cells are long -lived because of various antiapoptotic effec ts and the boosting of central memory T cells [ Kaech, 2003 ], [Seddiki, 2006 ], [Liu, 2006 ]. In addition, IL -7 broadens the repertoire of T cells because IL -7 supports thymopoïetic activity that produces naïve T cells [ Colombetti, 2009 ], [Nanjappa, 2008 ], [Levy 2012 ]. Finally, IL -7 antagonizes the production and signaling of TGF ß [Pellegrini, 2009 ]. 2.5 Correlative Studies Background The primary aim is to determine whether CYT107 administration increases the vaccine - induced antigen -specific T -cell immune response to PA2024, as immune response to PA2024 best correlates with survival. The level and character of T -cell immune response to PA2024 as well as PAP will be assessed at day 0 (week 1) [before CYT107 administration], at day 35 (week 6) [two week s after last CYT107 to assess the presumed peak immune response], at NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 15 day 70 (week 11) [to assess persistence of immune response and to comp are to Dendreon data] and, at day 154 (week 23), and day 365 (week 53) [to assess persistence of immune response] . Both T cell responses and antibody responses will be assessed. The primary endpoint will be quantification of T -cell responses to PA202 4 meas ured at day 70 (week 11), 7 weeks after the last CYT107 injection. T cell responses will be measured by interferon - gamma ELISPOT as the most quantifiable and reproducible assay. In the IMPACT study, 2 weeks after last sipuleucel -T administration, the mean PA2024 ELISPOT was 40 with a standard deviation of 69 based on n = 63. At 10 weeks after the last administration the mean PA2024 ELISPOT was 23 with a standard deviation of 36 based on n = 42. This data is based on Dendreon performing the ELISPOT assay on PBMC. This assay has been performed in the UCSF neo -adjuvant program study and was shown to be robust. Dendreon is supplying reagents to the UW Central lab (Immune Monitoring Lab) for the development of assays needed to test for response to specific protei ns. The UW Central Lab is validating and establishing standards for the assay. Longitudinal responses to PA2024 and PAP will be considered in the primary analysis; however power calculations are based on estimations of the magnitude of T cell responses a t week 11 . To meet the primary aim and determine whether CYT107 administration increases the vaccine -induced antigen -specific T -cell immune response to PA2024, the level and character of T -cell immune response to PA2024 will be determined at day 0 (week 1) , day 35 (week 6), day 70 (week 11), day 154 (week 23), and day 365 (week 53) evaluating T cell responses. T -cell responses to PA2024 will be measured by interferon -gamma ELISPOT and T-cell proliferative responses to PAP as a confirmatory assay. The effect on antigen -specific T-cell immune responses to PAP will be measured using the same assays and time points. The immune response to other ongoing and nascent antitumor responses (e.g., PRAME, NY - ESO -1 and p53), additional tumor antigens specific to prostate cancer (e.g., PSA and PSMA), memory viral responses (influenza A), and chronic viral responses (cytomegalovirus) will also be assessed. Assays will be performed in the Cancer Immunotherapy Trials Network (CITN) Central Laboratory, IML (Immunologic Monitor ing Laboratory), of the Tumor Vaccine Group led by Nora Disis, MD. A number of other (secondary) correlative studies will be conducted to evaluate the effects of IL-7 on T and B cell immunity. Certain blood and serum for secondary correlative studies will be collected and stored for future analysis pending funding availability. The effect on T lymphocyte number and phenotype (i.e., CD4+ and CD8+ subsets of naïve, effector memory, central memory cells, CD4+ Treg and activated T cells) will be assessed. The frequency and percentage of these and other PBMC subsets (i.e., T cells, NK cells, NKT cells, B cells, monocytes and dendritic cells) will be determined using multiparameter flow cytometric analysis on whole blood. IgM and IgG responses to PA2024 and PAP will be measured by standard ELISA. The T -cell repertoire will be characterized by T -cell receptor (TCR) deep sequencing and the analysis will be done with the Immunoseq Analyzer software (Harlan Robins, PhD, with Adaptive Biotechnologies, Seattle). NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 16 Thymic function will be assessed by measuring levels of T -cell receptor rearrangement DNA excision circles (TRECs). Adverse events will be graded and reported using the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) Version 4.0. Therapeutic response will be assessed by consensus PSA response criteria for PSA blood tests, immune - related response criteria for computed tomography and bone scan (only as clinically indicated ), circulating tumor cells, and patient survival. 3. PATIENT SELECTION 3.1 Eligibility Criteria 3.1.1 Asymptomatic or minimally symptomatic metastatic castration -resistant prostate cancer (mCRPC) 3.1.2 Patients must have successfully completed therapy with sipuleucel -T within 3 -7 days of planned CYT107 study drug treatment 3.1.3 Assessable disease with a positive bone scan and/or measurable disease on CT scan and/or MRI of the abdomen and pelvis 3.1.4 Prior orchiectomy or must be on ongoing LHRH agonist or antagonist (e.g., degarelix) therapy 3.1.5 No ongoing anti -androgen therapy  Patients must be off anti -androgen therapy for at least 30 days 3.1.6 Patients receiving any other hormonal therapy, including any dose of megestrol acetate (Megace), Proscar (finasteride), any herbal product known to decrease PSA levels (e.g. Saw Palmetto, PC -SPES), or any systemic corticosteroid, must discontinue the agent for at least 30 days prior to study treatment 3.1.7 Laboratory requirements (assays within 7 days of first CYT107 injection)  Absolute neutrophil count (ANC) ≥ 1500/µL  Bilirubin < 1.5 x ULN  Hemoglobin ≥ 10 g/dL  Platelets > 100,000/mcL  AST and ALT < 2.5 x ULN  Creatinine clearance ≥ 60mL/min by the Cockcroft -Gault equation (see Appendix C) NCI Protocol #: CITN12 -03 Version Date: July 26, 2017 17 3.1.8 Testosterone ≤ 50 ng/dL (documented at any time while on LHRH agonist or antagonists or s/p orchiectomy) 3.1.9 ECOG performance status of 0 -1 or a Karnofsky performance status of ≥ 80% 3.1.10 Life expectancy of at least 6 months 3.1.11 At least 18 years of age or older 3.1.12 Prior local radiation therapy must be completed at least 30 days prior to enrollment and the patient must have recovered from all toxicity 3.1.13 Prior “systemic” radiopharmaceuticals (strontium, samarium , radium 223 dichloride ) must be completed ≥ 8 weeks prior to enrollment 3.1.14 Patients must agree to use 2 methods of adequate contraception for the duration of study participation, and for four months after discontinuing therapy, because of the unknown potent ial risk to a gamete and/or developing embryo from this investigational therapy. 3.1.15 Ability to understand and the willingness to sign a written informed consent document.