DNA Vaccines

One approach with considerable promise for immunotherapy of prostate tumors involves using DNA-based constructs to activate a specific antitumor immune response. This approach was highlighted in a recent study by McNeel and colleagues. These investigators used a DNA vaccine directed against prostatic acid phosphatase (PAP) to treat 22 men with biochemically recurrent prostate cancer (so-called D0 disease). As is nearly always the case for immunotherapy, the treatment was well tolerated, and appeared to result in a T-cell and antibody response to the target antigen. Decreases in the rate of increase of PSA were noted, although the overall clinical significance of such changes is not without controversy. DNA-based immunotherapy vectors are straightforward to synthesize, and this trial therefore provides a foundation on which future trials evaluating additional target antigens may be based.

Viral Vectors: ProstVac VF

Another approach to immunotherapy that has been applied to prostate cancer and other tumor types involves viral vectors, in particular attenuated vaccinia strains. These vectors have the advantage of being capable of incorporating a large target payload, and straightforward synthesis and production. In the context of prostate cancer, a strategy targeting PSA has been systematically developed over the past decade. Because this platform has been extensively reviewed elsewhere, only a few points regarding this agent are discussed in this article. Early in the development of vaccinia-based vectors, it was noted that additional immunization did not seem to result in additional immunity directed against the targeted antigen, but rather at the viral components of the vector itself. Thus, a heterologous prime-boost strategy was designed, in which vaccinia and fowlpox vectors incorporating PSA were synthesized. In a randomized phase II trial, it was shown that vaccinia priming, followed by a series of fowlpox booster treatments, resulted in an optimal immune response as revealed by correlative laboratory studies. Long-term follow-up of that trial has suggested a trend toward increased progression-free survival in men treated with the optimal immunotherapy sequence. An additional refinement to the vaccinia-PSA approach was based on laboratory studies showing that the addition of a triad of costimulatory molecules (B7-1, LFA3, ICAM-1) to the immunizing agent resulted in a significant augmentation of immune responses. The resulting product, ProstVac VF, was partially developed by the Therion Corporation, which launched a 2:1 randomized trial comparing ProstVac VF with placebo in approximately 120 men with asymptomatic, metastatic, castrate-resistant prostate cancer (CRPC) (TBC-PRO-002). Data from this trial were originally presented in 2006, but at that time there was no statistical difference between the treatment groups in the primary end point (time to progression), or a secondary survival end point. However, long-term follow-up data from these patients were recently presented, and a statistically significant survival advantage was evident, with a median overall survival of 24.5 months in the immunotherapy group versus 16 months in the control group. Because overall survival was a secondary end point in this trial, those data should be considered hypothesis generating. Nevertheless, a phase III trial of this agent is in the planning stages for 2010 initiation ( http://www.bavarian-nordic.com/prostvac ). In addition to these trials, ProstVac VF has been combined with an impressive number of conventional therapies for prostate cancer, as discussed later. This developmental pathway illustrates several general principles surrounding immunotherapy for prostate cancer. First and perhaps foremost is the slow pace of disease; in the absence of reliable surrogate clinical end point trials take significant time to mature. Secondly, the vaccinia prime, fowlpox boost strategy exemplifies the notion of a heterologous prime-boost immunization scheme (heterologous strategies are rarely used in cancer immunotherapy).

Cell-Based Immunotherapy (GVAX Prostate and Onyvax-P)

A third approach to active immunotherapy for prostate cancer that has proceeded to phase III and phase II randomized clinical trials involves the use of allogeneic prostate cancer cells as immunotherapy vectors. Because cancer cells themselves are generally not sufficiently immunogeneic to mediate protection in murine models, for optimal immunity cancer cells must be either engineered to express a proinflammatory cytokine or administered along with a potent immune stimulator (ie, an adjuvant). The first approach is exemplified by GVAX prostate, which used the cell lines PC3 and LnCaP transfected to secrete GM-CSF. In clinical studies, these cells are irradiated to prevent ongoing division, then injected intradermally. Phase II studies of GVAX prostate were promising, and in 2004 a large, randomized phase III trial comparing GVAX prostate with standard docetaxel chemotherapy every 3 weeks in men with minimally symptomatic, metastatic CRPC was launched. This trial, VITAL-1, completed accrual in July, 2007; an interim safety analysis performed in January, 2008 supported trial continuation. A second randomized phase III trial of GVAX prostate was launched in July, 2005; in this trial GVAX prostate was combined with docetaxel to treat men with more advanced disease (ie, patients with symptomatic metastatic CRPC). Although docetaxel chemotherapy is generally administered along with prednisone at a daily dose of 10 mg, prednisone was omitted from the GVAX plus docetaxel treatment arm because of its immunosuppressive properties. The control arm was standard docetaxel combined with prednisone every 3 weeks. This trial (VITAL-2) was halted early because an interim analysis showed an imbalance of deaths in the immunotherapy arm (67 vs 47). The mechanism for this imbalance has yet to be explained, but it did not seem to result from a failure of randomization. Based on these data, an unplanned early futility analysis was designed and completed for the fully accrued VITAL-1 trial. At the time the analysis was conducted, only 371 deaths had occurred, fewer than the prespecified 400 required for 80% power, but the analysis suggested that VITAL-1 had less than a 30% chance of reaching its primary end point of a 20% improvement in survival, and further patient follow-up was halted. The Kaplan-Meier survival curves for VITAL-1 show a clear crossover at 21 months; after this time point patients on the immunotherapy arm show a superior survival. Thus, it remains theoretically possible that further follow-up of these study patients could reveal a late treatment effect, similar to that seen with ProstVac VF in the TBC-PRO-002 trial. Further clinical development of the GVAX prostate platform has been discontinued by the manufacturer (Cell Genesys Inc). Comparison of the GVAX prostate clinical experience with that of ProstVac VF (TBC-PRO-002) and with Sipuleucel-T (see later discussion) suggests that, in asymptomatic metastatic prostate cancer, a positive result in a randomized clinical trial is more likely when the trial design compares immunotherapy with placebo, rather than with docetaxel chemotherapy. In addition, the unexpected imbalance in deaths in VITAL-2 suggests that combining immunotherapy with other treatment modalities may prove complex, and that randomized phase II trials should be considered before initiating phase III studies.

Another cell-based immunotherapy approach for prostate cancer involves 3 allogeneic prostate cancer cell lines chosen to represent different disease states (Onyvax-P, Onyvax Inc). Specifically, the product includes an immortalized normal human prostate epithelial cell line (ONYCAP23), a transformed cell line from a well-differentiated prostate cancer (P4E6), and the androgen-dependent cell line LnCaP, which was derived from a lymph node metastasis. In clinical trials, the cell lines are first administered as 2 doses of irradiated cells at 2-week intervals along with bacillus Calmette-Guérin as an adjuvant, followed by monthly boosts of the cell lines alone. A phase II trial of Onyvax-P in men with CRPC showed the treatment to be well tolerated and showed a decrease in the PSA velocity in 11 of the 26 patients treated. The current status of Onyvax-P is uncertain, as previously ongoing phase II trials are now closed to accrual ( http://www.clincaltrials.gov ).

Active Cellular Immunotherapy (Sipuleucel-T)

The active immunotherapy that is most likely closest to approval by the US Food and Drug Administration (FDA) for prostate cancer involves an approach that is different from those described earlier. To manufacture Sipuleucel-T (Dendreon Inc, Seattle, WA, USA), patients undergo plasmapheresis, and a personalized immunotherapy product is produced by culturing a patient’s peripheral blood monocytes with a proprietary protein that couples GM-CSF with a target antigen (PAP). This approach has the theoretic advantage of removing antigen-presenting cells (and lymphocytes) from an immunosuppressive patient environment as the cells are activated. In addition, the discrete target antigen facilitates immune monitoring. Phase I and phase III trials of Sipuleucel-T have been reported, with encouraging results. FDA approval of this agent is pivotal on a large (>500 patients) randomized placebo-controlled phase III trial (IMPACT), which completed accrual in October, 2007, and which was reported to meet its primary (survival) end point in April, 2009 ( http://www.dendreon.com ). Thus, when considered along with the TBC-PRO-002 trial of ProstVac VF already described and an earlier phase III trial of Sipuleucel-T (D9901 ), 3 randomized trials of immunotherapy for metastatic prostate cancer have reported a survival benefit. In all 3 cases the comparator arm was a placebo treatment. Although a placebo comparator might seem unwarranted given that docetaxel chemotherapy every 3 weeks is approved by the FDA for metastatic CRPC in symptomatic and asymptomatic patients, both of these trials enrolled only asymptomatic men, and controversy exists regarding the optimal timing of chemotherapeutic intervention in metastatic prostate cancer.

Androgen Ablation

One of the more interesting combination regimens for prostate cancer involves the administration of immunotherapy along with androgen ablation. The clinical rationale for this hypothesis stems from a demonstration of an increased T-cell infiltrate in the prostate glands of men who had undergone androgen ablation before radical prostatectomy. The infiltrating CD4 T cells showed evidence of oligoclonality, suggestive of an antigen-driven immune response. The author obtained similar results in an animal model of prostate cancer, and showed that androgen ablation appeared to mitigate systemic CD4 T-cell tolerance significantly. Further animal studies explored the relative timing of immunotherapy and androgen ablation, and showed that an optimal combinatorial effect is obtained when immunotherapy is administered before androgen ablation. In addition to mitigating T-cell tolerance and promoting T-cell infiltration into the prostate gland, androgen ablation also mediates regrowth of the thymus, the organ in which T cells are produced, and thymic regrowth is accompanied by an increase in T-cell production. Given these considerable data, it is perhaps surprising that more clinical trials have not been performed with this combination. An initial trial was performed by Sanda and colleagues several years ago; ProstVac (without TRICOM or the heterologous prime-boost developed later), was administered to a small number of patients with biochemically relapsed disease. Using a more developed version of ProstVac VF, Madan and colleagues studied sequencing the androgen-receptor antagonist nilutamide in a phase II trial. These results (although derived from only a few patients) mirrored the results obtained by Kast and colleagues in animal studies; patients who received the ProstVac VF vector before secondary androgen manipulation with nilutamide appeared to have an increased survival compared with those who were treated with immunotherapy after androgen manipulation. In the Dendreon P-11 study, men with biochemically relapsed prostate cancer received Sipuleucel-T after androgen ablation. Although final study results have yet to be reported, an initial presentation of these data suggested that this particular combination did not seem to affect the time to PSA recurrence, which was the primary trial end point. Additional clinical trials are warranted, but the results obtained thus far strongly suggest that the precise sequence of the 2 modalities may be of critical importance.

Chemotherapy

Two large phase III randomized controlled trials demonstrated that docetaxel chemotherapy significantly prolongs survival in men with metastatic CRPC, and several trials have combined docetaxel chemotherapy with an additional agent in an effort to improve on this documented survival benefit. Agents tested in this manner include antiangiogenesis agents, mTOR inhibitors, endothelin inhibitors, and others ; it followed that immunotherapy might be tested in a similar manner. As discussed earlier, the Cell Genesys VITAL-2 trial compared the combination of GVAX prostate plus docetaxel (without prednisone) with docetaxel plus prednisone in men with symptomatic CRPC, and interim analysis suggested a potential adverse outcome in the men treated with combination therapy. In contrast, a phase II trial comparing docetaxel plus ProstVac VF with ProstVac VF alone showed a prolongation of progression-free survival in the group who received combination treatment, and found that docetaxel did not seem to impair an immunotherapy-mediated immune response. In addition to combination with standard docetaxel, preclinical and clinical data suggest that low-dose cyclophosphamide may enhance an immunotherapy-induced antitumor immune response, most likely through transient depletion of Treg. Recent studies by the author’s group in an autochronous murine model of prostate cancer confirm these results and reinforce the notion that chemotherapy and immunotherapy must be delivered in a precise sequence for optimal efficacy.

Radiotherapy

Radiotherapy is a conventional methodology for treating localized and recurrent prostate cancer, and several studies showed that radiotherapy has significant proinflammatory effects: upregulation of the proapoptotic molecules Fas and FasL on tumor cells and increasing levels of class I MHC (important for T-cell-mediated tumor lysis). Several preclinical studies directly support the notion of combining radiotherapy for prostate cancer with immunotherapy. The combination of active immunotherapy and radiotherapy has been evaluated clinically, once again using the ProstVac VF platform. Thirteen of the 17 patients randomized to the combination radiotherapy/immunotherapy arm in this trial showed an increased T-cell response to the target antigen (PSA), supporting the notion of combining radiotherapy with immunotherapy for prostate cancer. In addition, an ongoing phase III trial of the immune modulator ipilimumab (see later discussion) in patients who have failed docetaxel chemotherapy includes a low dose of radiotherapy in an effort to enhance tumor antigen presentation.