Prostate cancer: advances in immunotherapy

A national multicenter phase 2 study of prostate-specific antigen (PSA) pox virus vaccine with sequential androgen ablation therapy in patients with PSA progression: ECOG 9802

BACKGROUND

E9802 was a phase 2 multi-institution study conducted to evaluate the safety and effectiveness of vaccinia and fowlpox prostate-specific antigen (PSA) vaccine (step 1) followed by combination with androgen ablation therapy (step 2) in patients with PSA progression without visible metastasis.

OBJECTIVE

To test the hypothesis that vaccine therapy in this early disease setting will be safe and have a biochemical effect that would support future studies of immunotherapy in patients with minimal disease burden.

DESIGN, SETTING, AND PARTICIPANTS

Patients who had PSA progression following local therapy were treated with PROSTVAC-V (vaccinia)/TRICOM on cycle 1 followed by PROSTVAC-F (fowlpox)/TRICOM for subsequent cycles in combination with granulocyte-macrophage colony-stimulating factor (step 1). Androgen ablation was added on progression (step 2).

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Step 1 primary end points included progression at 6 mo and characterization of change in PSA velocity pretreatment to post-treatment. Step 2 end points included PSA response with combined vaccine and androgen ablation.

RESULTS AND LIMITATIONS

In step 1, 25 of 40 eligible patients (63%) were progression free at 6 mo after registration (90% confidence interval [CI], 48-75). The median pretreatment PSA velocity was 0.13 log(PSA)/mo, in contrast to median postregistration velocity of 0.09 log(PSA)/mo (p=0.02), which is an increase in median PSA doubling time from 5.3 mo to 7.7 mo. No grade ≥4 treatment-related toxicity was observed. In the 27 patients eligible and treated for step 2, 20 patients achieved a complete response (CR) at 7 mo (CR rate: 74%; 90% CI, 57-87). Although supportive of larger studies in the cooperative group setting, this study is limited by the small number of patients and the absence of a control group as in a phase 3 study.

CONCLUSIONS

A viral PSA vaccine can be administered safely in the multi-institutional cooperative group setting to patients with minimal disease volume alone and combined with androgen ablation, supporting the feasibility of future phase 3 studies in this population.

PATIENT SUMMARY

These data support consideration of vaccine therapy earlier in the course of prostate cancer progression with minimal disease burden in future studies of vaccine approaches in earlier stages of disease.

Cancer immunotherapy: sipuleucel-T and beyond

In April 2010, sipuleucel-T became the first anticancer vaccine approved by the United States Food and Drug Administration. Different from the traditional chemotherapy agents that produce widespread cytotoxicity to kill tumor cells, anticancer vaccines and immunotherapies focus on empowering the immune system to overcome the tumor. The immune system consists of innate and adaptive components. The CD4(+) and CD8(+) T cells are the most crucial components of the adaptive arm of the immune system that act to mediate antitumor responses. However, T-cell responses are regulated by intrinsic and extrinsic mechanisms, which may interfere with effective antitumor responses. Many anticancer immunotherapies use tumor-associated antigens as vaccines in order to stimulate an immune response against tumor cells. Sipuleucel-T is composed of autologous mononuclear cells incubated with a fusion protein consisting of a common prostate cancer antigen (prostatic acid phosphatase) linked to an adjuvant (granulocyte-macrophage colony-stimulating factor). It is postulated that when the vaccine is infused into the patient, the activated antigen-presenting cells displaying the fusion protein will induce an immune response against the tumor antigen. In a recent randomized, double-blind, placebo-controlled, phase III clinical trial, sipuleucel-T significantly improved median overall survival by 4.1 months in men with metastatic castration-resistant prostate cancer compared with placebo. Although overall survival was improved, none of the three phase III clinical trials found a significant difference in time to disease progression. This, along with cost and logistic issues, has led to an active discussion. Although sipuleucel-T was studied in the metastatic setting, its ideal place in therapy is unknown, and clinical trials are being conducted in patients at different stages of disease and in combination with radiation therapy, antiandrogen therapy, and chemotherapy. Various other anticancer vaccines and immunotherapies for other tumor types are currently under investigation and in clinical trials. These immunotherapies were formulated to incorporate tumor-associated antigens aimed at stimulating effector T-cell responses or to block regulatory mechanisms that suppress the function of effector T cells. Additional studies will determine how these therapies can best improve clinical outcomes in patients with cancer.

Sipuleucel-T: a vaccine for metastatic, asymptomatic, androgen-independent prostate cancer

OBJECTIVE

To review the design, efficacy, safety, dosing, therapeutic, and pharmacoeconomic considerations of sipuleucel-T, an investigational, autologous, dendritic, cell-based prostate cancer vaccine.

DATA SOURCES

English-language literature searches of MEDLINE (1966-September 2007) and the Cochrane Database (2007, Issue 3) were performed using the terms sipuleucel-T, APC8015, and prostate cancer vaccine. Other data sources were identified from bibliographies of selected articles and from press releases.

STUDY SELECTION AND DATA EXTRACTION

All published articles or abstracts on human studies of sipuleucel-T for androgen-independent prostate cancer (AIPC) were reviewed for inclusion. Manufacturer Web sites, Food and Drug Administration (FDA) documents, and the clinical trials registry were used to obtain information regarding ongoing clinical trials.

DATA SYNTHESIS

AIPC is an incurable disease with a median survival rate of 18-20 months. Docetaxel-based chemotherapy is currently the only FDA-approved treatment for AIPC with a survival benefit (2.4 mo). Sipuleucel-T is a novel active cellular immunotherapy under investigation for the treatment of metastatic, asymptomatic AIPC. In clinical trials, the primary endpoint of time to disease progression was not met; however, an underpowered analysis of data suggests that sipuleucel-T prolongs survival by a median of 4.5 months compared with placebo. Sipuleucel-T has been relatively well tolerated, although a possible increased risk of cerebrovascular events may exist. In May 2007, the FDA did not approve the biologics license application for sipuleucel-T since the primary endpoint of the Phase 3 trials was not met. However, its approval will be reconsidered by the FDA once interim survival results from an ongoing Phase 3 trial, IMPACT, are determined. These data are anticipated to be released in the fourth quarter of 2008.

CONCLUSIONS

Metastatic AIPC is an incurable disease that currently has limited treatment options. Approval of sipuleucel-T hinges on results from the IMPACT trial. If improved survival is shown, sipuleucel-T may become the first approved active cellular immunotherapy for treating metastatic, asymptomatic AIPC.

From "Hellstrom Paradox" to anti-adenosinergic cancer immunotherapy

Cancer therapy by endogenous or adoptively transferred anti-tumor T cells is considered complementary to conventional cancer treatment by surgery, radiotherapy or chemotherapy. However, the scope of promising immunotherapeutic protocols is currently limited because tumors can create a ‘hostile–immunosuppressive microenvironment that prevents their destruction by anti-tumor T cells. There is a possibility to develop better and more effective immunotherapies by inactivating mechanisms that inhibit anti-tumor T cells in the tumor microenvironment and thereby protect cancerous tissues from immune damage. This may be now possible because of the recent demonstration that genetic deletion of immunosuppressive A2A and A2B adenosine receptors (A2AR and A2BR) or their pharmacological inactivation can prevent the inhibition of anti-tumor T cells by the hypoxic tumor microenvironment and as a result facilitate full tumor rejection [Ohta A, Gorelik E, Prasad SJ et al (2006) Proc Natl Acad Sci USA 103(35):13132–3137]. This approach is based on in vivo genetic evidence that A2AR play a critical role in the protection of normal tissues from overactive immune cells in acutely inflamed and hypoxic areas. The observations of much improved T-cell-mediated rejection of tumors in mice with inactivated A2AR strongly suggest that A2AR also protects hypoxic cancerous tissues and that A2AR should be inactivated in order to improve tumor rejection by anti-tumor T cells.

Inhibition of decay-accelerating factor (CD55) attenuates prostate cancer growth and survival in vivo

Decay-accelerating factor (CD55) is a member of membrane-bound complement-regulatory proteins. CD55 expression correlates with poor survival in patients with colorectal cancer and has been implicated in the survival and tumorigenesis of blood-borne malignancies. Histologic analysis of clinical specimens from patients with advanced prostate cancer revealed an increase in CD55 expression in prostate tumor epithelial cells. CD55 was shown to be functionally active and to inhibit complement-mediated lysis in PC-3 and DU145 cells. The percentage of lysis was correlative with the CD55 expression profile observed in these prostate cancer cell lines. These data suggest that CD55 is an important regulator of prostate cancer cell survival. As a result, we have hypothesized that CD55 expression on prostate cancer cells promotes cell survival and contributes to the metastatic potential of prostate cancer cells. To determine the role of CD55 in prostate cancer tumorigenesis and metastasis, we generated PC-3(Luc) prostate cancer cells with CD55 siRNA-targeted disruption. We found that PC-3(Luc)/CD55 siRNA constructs in SCID mice resulted in a significant attenuation of overall tumor burden. Further investigation into the mechanisms of CD55-mediated tumor cell/microenvironment interaction is necessary to understand the role of CD55 in tumor cell survival and metastatic lesion formation.

Tissue-specificity of prostate specific antigens: Comparative analysis of transcript levels in prostate and non-prostatic tissues

Activation of immune defense mechanisms against tumor antigens appears to be a promising therapeutic option for advanced prostate cancer (PCa). Specific immunotherapy critically depends on target antigens that are selectively expressed in the tumorous and optional in the normal prostate tissue in sufficient amounts. Although several prostate antigens have been described and some have already been used in clinical trials, a detailed comparative evaluation of their tissue-specificity and expression levels is still lacking. We determined the transcript levels of eight prostate targets (PSA, PAP, PSCA, PSGR, Prostein, PSMA, AIbZIP, trp-p8) in 16 different tissues by quantitative PCR and calculated a tissue-specificity index (TSI) for each molecule. Besides a preferential expression in prostate for all targets, striking differences in the expression levels and TSI were revealed which may be important for the selection of appropriate antigens for immunotherapy of PCa.

The ABCs of artificial antigen presentation

Artificial antigen presentation aims to accelerate the establishment of therapeutic cellular immunity. Artificial antigen-presenting cells (AAPCs) and their cell-free substitutes are designed to stimulate the expansion and acquisition of optimal therapeutic features of T cells before therapeutic infusion, without the need for autologous antigen-presenting cells. Compelling recent advances include fibroblast AAPCs that process antigens, magnetic beads that are antigen specific, novel T-cell costimulatory combinations, the augmentation of therapeutic potency of adoptively transferred T lymphocytes by interleukin-15, and the safe use of dendritic cell-derived exosomes pulsed with tumor antigen. Whereas the safety and potency of the various systems warrant further preclinical and clinical studies, these emerging technologies are poised to have a major impact on adoptive T-cell therapy and the investigation of T cell-mediated immunity.