**2. Radiotherapy of prostate cancer**

There are four major treatment approaches for localised prostate cancer: active surveillance, radical prostatectomy, external beam radiotherapy (EBRT) and low-dose rate (LDR) brachytherapy. RT is conventionally delivered with photons with delivery systems that have developed considerably over the past decade, leading to lower toxicity and allowing safe dose escalation. Higher doses have been demonstrated to improve tumour control outcomes in several large Phase III trials (Viani et al., 2009). Present trials are evaluating the role of intensity modulated radiotherapy (IMRT), hypofractionation (treatment in ~4 weeks) and improved imaging during treatment with image-guided radiotherapy (IGRT) (Khoo & Dearnaley, 2008). Further developments in EBRT delivery systems allow highly targeted treatment in 5-7 fractions, called stereotactic body radiotherapy (SBRT), although tumour control outcomes are not yet known (King et al., 2011; Madsen et al., 2007).

Combination of Immunotherapy & Radiotherapy for the Treatment of Prostate Cancer 197

indirect evidence about the immune system's role in tumour-control (Strauss & Thomas, 2010). Overcoming anti-tumour immune responses is described as an emerging hallmark of cancer (Hanahan & Weinberg, 2011). Further observations and experiments are accumulating in order to provide firm evidence about the role of anti-tumour immune

The specificity of tumour-infiltrating T cells reflects engagement with TAA. The presence of TAA-specific T cells in the TIL pool results in longer median survival compared to those patients whose TIL did not contain tumour-specific T cells, as observed in melanoma (22.5 months vs. 4.5 months) (Haanen et al., 2005). There is no such prognostic correlation for the frequency of TAA-specific T cells in the peripheral blood of patients. TAA-specific T cell

PCa-associated antigens include prostate-specific differentiation antigens, expressed both on healthy and malignant prostate epithelial cells, such as kallikrein-4, PAP (prostatic acid phosphatase) and PSA. Tumour antigens that are overexpressed on malignant cells (not all specific for PCa) compared to healthy epithelial cells are: PSMA (prostate specific membrane antigen), PSCA (prostate stem cell antigen), Her-2, MUC-1, survivin, STEAP (six transmembrane epithelial antigen of the prostate) and telomerase. Cancer-germline or cancer-testis oncofoetal antigens observed in PCa are not expressed on normal cells, but may be expressed by placental trophoblasts and testicular germ cells, such as NY-ESO, MAGE-

More recent additions to the list of potential TAAs in PCa are; AMACR (Honma et al., 2009), WT-1 (Nakatsuka et al., 2006), ADAM-17 (Sinnathamby et al., 2011), RHAMM (CD168) (Gust et al., 2009), SIM2 (Arredouani et al., 2009), TARP (Epel et al., 2008), SH3GLB2 (Fassò et al., 2008) and the androgen receptor (Olson & McNeel, 2011). T cells specific for some of these antigens have been identified in PCa patients and T cell clones or lines killed PCa cells,

Prostate cancer has a complex microenvironment which develops during the course of tumour development. Tumour cells are surrounded by endothelial cells of blood vessels, stromal fibroblasts, bone marrow-derived cells and lymphocytes. These cells produce growth factors and enzymes that enhance tumour growth and survival, aid stromaremodelling and recruit further immune cells into the tumour. The two main immune cell

The presence of activated T and/or natural killer (NK) cells in the tumour tissue is a positive prognostic factor in several solid cancers, including PCa (Kärjä et al., 2005; Gannon et al., 2009). CD8+ T cells are responsible for direct killing of target cells which express appropriate peptides on MHC Class I molecules, while NK cells play a role in killing tumour cells which downregulate MHC Class I molecules as an evasion mechanism from T cell recognition. Target cell killing occurs via delivering perforin and apoptosis-inducing granzyme complexes into the target cell (Thiery et al., 2011). CD4+ T cells, depending on their subtype: Th1, Th2, Th17 or T regulatory cells (Treg) produce cytokines which support pro- or anti-

C1, MAGE-C2 and 5T4 (Chen et al., 1997; Hudolin et al., 2006; Southall et al., 1990).

confirming the suitability of most of these TAA-antigens for targeted therapies.

types infiltrating the tumour are lymphocytes and myeloid cells.

responses in the control of cancer.

**3.1 Tumour-associated antigens in PCa** 

infiltration is likely to be important in PCa too.

**3.2 Tumour-infiltrating immune cells in PCa** 

**3.2.1 Lymphocytes** 

Proton therapy is the delivery of EBRT using protons instead of photons. Protons have a different pattern of dose delivery within tissue, with energy deposited in a very tightly defined area as the protons slow. This results in less radiation being delivered beyond the target, and has become the radiotherapeutic modality of choice for childhood cancers and several other tumors. The evidence base for proton therapy for prostate cancer is less established, but its use in some countries has become widespread partly due to the results of a dose escalation trial using protons (Coen & Zietman, 2009). Proton therapy has not been compared to doseequivalent photon-RT. LDR brachytherapy, which uses multiple permanently planted radioactive seeds, can be used to deliver a very high radiation dose to a highly targeted volume in a single treatment with equivalent outcomes to EBRT and surgery.

Locally advanced disease is usually treated with a combination of EBRT and androgen deprivation therapy (ADT) (Shelley et al., 2009; Shelley et al., 2009; Warde et al., 2010). However, the outcome is still relatively poor. Recent and ongoing UK-based trials are currently exploring the potential advantage of dose escalation in either systemic therapies (James et al., 2009; Guerrero Urbano et al., 2010). High dose rate (HDR) brachytherapy, which uses a single high-intensity radiation source that is temporarily inserted into multiple positions in the prostate, may also have a role in locally advanced disease as a single agent or in combination with ADT and/or RT (Hoskin, 2008). EBRT has a proven role as adjuvant or salvage therapy after radical prostatectomy. In the adjuvant setting, it has been shown to reduce the rate of relapse in high risk patients by approximately 50% in three randomised trials (Bolla et al., 2007; Thompson et al., 2006; Wiegel et al., 2009).

The commonest site of metastases in castrate refractory metastatic PCa is bone, with 80% of patients dying with prostate cancer dying with bone metastases. They can cause one of several skeletal-related events, but pain is the predominant problem. Palliative EBRT is highly efficacious for single sites of disease. An alternative approach is the use of therapeutic bone-targeted radioisotopes. The interim results of a trial with a novel alphaemitting isotope, Radium-223, have reported a 3-month overall survival advantage, (http://www.algeta.com) suggesting that these drugs will be used more widely in the future. Radioimmunotherapy (RIT) refers to the use of antibody labelled with a therapeutic radionuclide, with the aim of delivering a cytotoxic radiation dose specifically to the tumour. The concept is equivalent to bone-targeted radioisotopes, but with the targeting of tumour-associated antigens (TAA) rather than osteoblastic metastases. The same principle can be used for imaging of micrometastatic disease if radionuclides of different properties (radiation type and energy) are used. There is much research in this field over recent years (Bouchelouche et al., 2011), partly due to the increasing number of PCa-specific TAA, as discussed later in this chapter.

#### **3. Immunological aspects of PCa**

PCa is an immunogenic cancer, as evidenced by a positive correlation between the frequency of CD8+ tumour-infiltrating T cells and prostate-specific antigen (PSA) recurrence-free survival (Kärjä et al., 2005). Immune cell behaviour towards tumour cells has been described by three stages: (1) elimination of tumour cells, (2) equilibrium between tumour, and immune cells – maintained by active immunological control of the tumour and (3) escape of tumour cells from immunological control. Apart from evidence from animal models underpinning this theory (Teng et al., 2008), clinical observations of donorderived melanoma developing in immunosuppressed organ transplant recipients provide indirect evidence about the immune system's role in tumour-control (Strauss & Thomas, 2010). Overcoming anti-tumour immune responses is described as an emerging hallmark of cancer (Hanahan & Weinberg, 2011). Further observations and experiments are accumulating in order to provide firm evidence about the role of anti-tumour immune responses in the control of cancer.
