Translational Science in Prostate Cancer: Exploring the Synergies Between Basic Research and Clinical Medicine

Translational medicine is currently the focus of much attention in the medical and research community and is the wave of the foreseeable future. “Translational medicine” is a catchphrase for the attempt to improve communication between basic science and clinical medicine so as to accelerate the development of drugs that attack key targets involved in diseases. This involves a multidisciplinary effort with continuous feedback, collaboration, and data-sharing.

Vast amounts of money are being spent in this effort by the federal government and pharmaceutical companies. The Translational Research Working Group (TRWG) was established to work with the cancer research community to develop recommendations about how the National Cancer Institute could best organize its efforts to further translational research. The TRWG defined a translational continuum and developed a “roadmap” to create a stronger research infrastructure to accelerate the clinical research enterprise. The field has moved forward with its own journal—the Journal of Translational Medicine.

Efforts in translational medicine have paid off in the development of cancer treatments like imatinib (Gleevec) for chronic myeloid leukemia and gastrointestinal stromal tumors (GISTs) and trastuzumab (Herceptin) for HER2/neu-positive breast cancer. Both imatinib and trastuzumab have in fact revolutionized treatment of these respective cancers and are lifesaving for patients who previously had no good treatment options.

According to researchers, prostate cancer poses a major challenge for those involved in translational medicine, mainly due to its heterogeneity. However, progress is being made on several fronts.

Characterizing Prostate Cancer

According to Robert DiPaola, MD, of The Cancer Institute of New Jersey at Robert Wood Johnson Medical School in New Brunswick, New Jersey, the field of prostate cancer is advancing due to a greater synergy between basic research and clinical science, aided by the discovery of molecular pathways, genetic alterations, mechanisms of drug resistance, and findings based on the clinical effectiveness of specific therapies.

DiPaola reviewed advances in characterization of targets and the pathways that drive prostate cancer in terms of discovering targets for drug development. He also discussed oncogenic targets, such as growth signaling pathways and apoptosis, and said that a more recent focus for drug development is nononcogenic stress response targets.

“Prostate cancer is a heterogeneous disease, making it difficult to personalize treatment,” he said. “The identification of new targets, made possible by sophisticated DNA microarrays from prostate cancer specimens, will move the field forward.” Some new targets include TMPRSS22- ERG, RAF, P13K, RAS/RAF, Rb, and SRC.

Fusions of the androgen-responsive gene TMPRSS22-ERG with transcription factors ERG or ETV1 (both members of the ETS family of transcription factors) have been identified in prostate tumors. Fusions of TMPRSS22-ERG occur in about 50% of invasive cancers, and fusions involving TMPRSS22 with other ETS family proteins occur in an additional 10% of prostate cancers. Genetic rearrangements have been found in 1% to 2% of prostate cancer specimens. In these tumors, DiPaola explained, RAF overexpression seems to be the driving molecular event. This suggests a potential role for small-molecule RAF inhibitors in this subset of patients.

Transcriptome and mutation analysis CNA data have identified P13 kinase (P13K), RAS/RAF, and Rb as oncogenic targets. Overexpression of these genes is predictive of biochemical recurrence following radical prostatectomy.

"The identification of new targets, made possible by sophisticated DNA microarrays from prostate cancer specimens, will move the field forward."

—Robert DiPaola, MD

Based on this research, DiPaola said that there is now a rationale for the combination of targeted therapies in the treatment of prostate cancer. During the past 10 years, there has been a 143% increase in options for treating prostate cancer, and there are now more than 800 agents in clinical development. In general, combination therapies fall into 3 strategies: identification and maximum inhibition of pathways (eg, synthetic lethal screens); inhibition of compensatory pathways; and targeting resistance of “successful” agents.

The hallmarks of targets for cancer therapy have been cell survival, proliferation, and immune evasion. New nononcogenic targets have been identified, including the following new stress phenotypes: mitotic stress, metabolic stress, proteotoxic stress, oxidative stress, and DNA damage stress.

Oncogenic Targets

Most of the drugs studied in clinical trials of castration-resistant prostate cancer (CRPC) are aimed at oncogenic targets. Some examples of targets and drugs aimed at those targets are P13K (everolimus and deforolimus); VEGF/ VEGFR (sorafenib, sunitinib, and bevacizumab); Bcl-2 (oblimersen, ABT-263); insulin growth factor (IMC A-12); and androgen receptor (AR)/ CYP17 (abiraterone, MDV3100, and immune targets [sipuleucel-T, PROST-VAC, ipilimumab, MDX-1106]).

Novel agents targeted to self-sufficiency in growth signal and evading apoptosis—both oncogenic targets—are under development. These include abiraterone and MDV3100. The PTEN gene is lost or mutated in 0% to 80% of prostate cancers, activating P13K/AKT, which DiPaola said supports further studies of P13K and AKT inhibitors in prostate cancer. Other novel agents are aimed at evading apoptosis. BH3 mimetics in early stages of clinical development include ABT- 264, AT-101, PARP inhibitors, and GX-15070.

DiPaola said that this type of research was showcased at the recent ASCO Genitourinary Cancers Symposium (ASCO GU) held in Orlando, Florida, in February, where 3 different poster presentations focused on early studies of AT-101. A phase II trial evaluated AT-101 to abrogate Bcl- 2-mediated resistance in patients with newly diagnosed androgen-dependent prostate cancer (Abstract 173). A randomized, double-blind, phase II study evaluated docetaxel and prednisone combined with AT-101 or placebo as first-line therapy for CRPC (Abstract 125). A phase I trial looked at the combination of AT-101 plus paclitaxel plus carboplatin in solid tumors and prostate cancer (Abstract 169). A different recent study looked at Bcl-2 inhibition with ABT 263/737 in prostate cancer, DiPaola said.

Nononcogenic Stress Response Targets

Regarding nononcogenic stress response targets, DiPaola said that they can be identified by synthetic lethal screens. Examples of targets identified by synthetic lethal screens include the use of PARP inhibitors targeted to tumors with BRCA1 and BRCA2 mutations, mitotic stress, mitochondrial metabolism, and reactive oxygen species generation.

A more definitive example of a nononcogenic target is autophagy, DiPaola noted. “We were focused on apoptosis for a long time, and now it turns out that autophagy is an important survival mechanism in prostate cancer,” he said. Autophagy refers to how a cell deals with oxidative, metabolic, and proteotoxic stress. If autophagy is present, cells can withstand a great degree of stress. There are not many drugs to act on this pathway, but hydroxychloroquinolone is being studied in this regard.

A poster presented at ASCO GU focused on patients with a rising PSA after primary therapy for prostate cancer with no evidence of metastasis who were treated with hydroxychloroquinolone (Abstract 164), with the rationale that blocking autophagy would be beneficial. The authors said that the drug appeared to have some activity in PSA progression, including increasing PSA doubling time and reducing PSA, but further study using higher doses is needed.

The novel agent ABT-737 induces autophagy, and preclinical data suggest that when this drug is combined with hydroxychloroquinolone the effect is strong. There are at least 4 other ongoing trials that are targeting autophagy.

“The bottom line is that we have moved the field forward by characterizing prostate cancer and discovering new oncogenic and nononcogenic targets for drug development,” said DiPaola. “Now the research needs to be done.”

Lessons from the Vaccine Trials

Schlom listed the following takeaway points from the vaccine trials:

• Vaccines as monotherapy do not appear to influence time to progression, but they do improve survival. Vaccines appear to change the growth rate of the tumor, Schlom noted. “We propose that if you get to the patient early on [with a vaccine], you can change the growth rate of the tumor and improve survival,” he said.
• Vaccines have little toxicity, according to the 2 phase III trials of sipuleucel-T and the 2 phase II trials of PSA-TRICOM. Schlom said, “This is important and often overlooked.”
• It takes time for the vaccine to exert its effect. Unlike other therapies, vaccines do not act directly on tumor cells; they activate the host immune system. Thus, unlike other prostate cancer therapies, continued boosting by vaccine should enhance therapeutic efficacy without enhanced toxicity. “This may well be the reason delayed efficacy of vaccines are observed,” he noted. “At times, vaccine therapy may be withdrawn prematurely due to disease progression, not allowing the additional immune stimulation of booster vaccines to take effect.”
• Unlike chemotherapy, after vaccination the vaccine-activated immune response endures. This may in turn slow the growth rate of the tumor, but may not be strong enough to eliminate tumor mass. This fits in with the observed lack of difference between vaccine and placebo in time to progression but with meaningful and statistically significant differences in OS.
• The long-lived immune response induced by vaccines can have a robust effect on concomitant or subsequent therapies for prostate cancer.

Combination Studies: The Next Frontier

Several combination studies of vaccines plus other therapies are ongoing or are in the planning stages. One completed study has utilized PSA-TRICOM alone or with ipilimumab. An ongoing study is comparing the effect of Quadramet (153 Sm chelate) alone versus Quadramet plus vaccine in men with prostate cancer and bone metastases. A multicenter trial has just been initiated in patients with metastatic prostate cancer that is comparing docetaxel/prednisone alone versus 3 months of PSA-TRICOM vaccine followed by docetaxel/prednisone. Moreover, an ongoing trial in advanced prostate cancer patients is comparing flutamide plus or minus vaccine.

In summary, combination therapy with vaccine and other agents that either stimulate the immune system or suppress inhibitory factors, or the eventual use of combinations of vaccines, suggests that vaccines may have a role both as monotherapy in early-stage disease and in numerous types of combination therapies for patients with later-stage disease.

Vaccines in Prostate Cancer

Jeffrey Schlom, PhD, of the NCI’s Center for Cancer Research in Bethesda, Maryland, is involved in research on vaccines for prostate cancer. He reviewed phase III studies of sipuleucel-T (Provenge), the first FDA-approved vaccine for prostate cancer, which is made from the patient’s own antigen-presenting cells and a tumor antigen, and phase II trials of an off-the-shelf vaccine under development by NCI called PSA-TRICOM.

“At NCI, we are focusing on developing an off-the-shelf vaccine to be used early in the course of prostate cancer in patients with low tumor burden,” Schlom said.

Why is immunization an attractive strategy for prostate cancer? He said, “Immune targeting of prostate cancer has reached a tipping point in terms of agents and strategies leading to patient benefit, with great potential for the use of vaccines in combination with chemotherapy, radiation, hormone therapy, and/or small molecule-targeted therapeutic agents.”

Sipuleucel-T was recently approved by the FDA for treatment of metastatic castrationresistant prostate cancer (CRPC), based on phase III trials. In 1 randomized phase III trial, median overall survival (OS) was 25.8 months for men treated with the vaccine versus 21.7 months for placebo, representing a survival difference of 4.1 months (P = .032). Schlom pointed out that there was no difference between the 2 arms in time to progression, which gives an indication of how vaccines exert their effects.

“The results of this phase III trial were virtually identical to a previous phase III trial of sipuleucel-T where the primary endpoint was time to progression and was not reached, but the vaccine demonstrated a clear statistically significant difference from placebo in median overall survival,” Schlom said.

Sipuleucel-T is a unique vaccine platform in which the patient’s own leukapheresed product is sent to a central laboratory for processing. There the patient’s own antigen-presenting cells are incubated with a fusion protein. The vaccine is then shipped back to the treating site where it is reinfused. This process happens every 2 weeks for a total of 3 treatments. Schlom emphasized that the interval between processing of the vaccine and administration of therapy is short and the treatment has little toxicity.

PROSTVAC is a second vaccine that is undergoing evaluation in the setting of metastatic CRPC. This off-the-shelf vector vaccine consists of a prime-boost regimen of 2 different viruses (ie, recombinant vaccinia prime and usually 5 to 6 recombinant fowlpox boosters), each containing transgenes for PSA and 3 costimulatory molecules (TRICOM) aimed at boosting the immune system.

"At times, vaccine therapy may be withdrawn prematurely due to disease progression, not allowing the additional immune stimulation of booster vaccines to take effect."

—Jeffrey Schlom, PhD

In 1 randomized, placebo-controlled phase II trial, PSA-TRICOM (ie, PROSTVAC) achieved 3-year OS of 30% versus 17% of controls. Median OS was 8.1 months longer in those treated with PSA-TRICOM (25.1 months vs 16.6 months, respectively). That trial was conducted at 43 centers and included 125 patients with metastatic CRPC.

“An important point is that there was no difference between the treatment arms in time to progression—similar to the sipuleucel-T trials—and the vaccine had little toxicity,” Schlom stated.

A second single-arm trial that included 32 patients with metastatic CRPC found that, similar to the randomized phase II trial, PSA-TRICOM achieved median OS of 26.6 months, which was 9.2 months longer than survival of these patients predicted by the Halabi nomogram, a tool used to predict the aggressiveness of metastatic prostate cancer. Investigators from the same institution treated patients with docetaxel alone and reported median OS similar to that predicted by the Halabi nomogram.

Schlom pointed out that patients treated with PSA-TRICOM in the single-arm phase II trial had a median OS similar to that predicted by Halabi if the Halabi Predictive Score (HPS) was ≤18 months, while those with more indolent disease and tumor burden (ie, HPS of >18 months) had a median OS far better than that predicted by HPS.

The Androgen Receptor in Prostate Cancer

Prostate cancers of all stages are exquisitely dependent on the activity of the AR, explained Karen E. Knudsen, PhD, of the Kimmel Cancer Center at Thomas Jefferson University in Philadelphia, Pennsylvania. Androgen ablation is considered first-line therapy for cancers that have spread beyond the prostate. Knudsen said that despite available therapies, CRPC can develop, and there is no effective therapy for it.

Experts agree that despite hormonal therapy, a proportion of CRPCs emerge due to recurrent activation of the AR. One of the events that drives CRPC is AR deregulation. Tumors with a high level of AR are associated with an increase in mortality and disease progression. Mechanisms responsible for activation of the AR receptor fall into 3 different categories: those that directly affect AR modulation, those that involve alterations in AR cofactors, or those that promote intratumor ligand synthesis.

“The most frequently observed mechanism [of AR reactivation] to date involves upregulation of AR levels,” Knudsen said. But she added that the mechanisms that underlie AR deregulation are not well understood.

Knudsen and colleagues have identified retinoblastoma tumor suppressor (Rb) as a mechanism that controls AR levels, AR signaling, and the transition to CRPC.

“Retinoblastoma tumor suppressor constrains the transition to CRPC by supporting Rb function,” she said. “Perturbation of Rb function is a key driver of human CRPC development. Further analysis suggests that the Rb gene locus may be a major mechanism involved in Rb disruption.” Clinical studies also suggest that loss of Rb function is associated with poor outcome in men with prostate cancer.

“Taken together, these observations suggest that a new paradigm is emerging for the role of Rb in prostate cancer progression,” she said.

"The present studies support the hypothesis that pharmacologic awakening of Rb activity in Rb-positive tumors could be clinically beneficial."

—Karen E. Knudsen, PhD

Knudsen posed 3 questions: (1) How relevant/frequent is Rb loss? (2) How can it be detected? and (3) Is this a new opportunity for therapeutic intervention?

She said that the 159 gene signature for Rb loss can be detected by immunohistochemistry, and it is frequently seen in CRPC. Rb loss of function is enriched in CRPC. Patients that scored as having Rb loss of function had reduced progression-free survival. Knudsen believes that Rb loss presents a new opportunity for therapeutic interventions.

Rb status should be considered in the management of prostate cancer, she said. Studies are needed to determine whether Rb status should be developed as a measure for predicting poor response to androgendeprivation therapy and AR-directed therapeutics.

“The present studies support the hypothesis that pharmacologic awakening of Rb activity in Rb-positive tumors could be clinically beneficial,” Knudsen explained. “Human tissue explants are being used to study this hypothesis using a Pfizer CDK4 inhibitor: PD 0332991. These studies show that Rb-enriched tumors respond to CDK4 inhibitors.”

Knudsen and her colleagues are also testing the hypothesis that Rb-deficient tumors are hypersensitive to selected chemotherapy agents.

“Collectively, these studies indicate that the Rb pathway serves a novel function in prostate cancer by controlling AR expression, AR function, and related transition to CRPC,” she concluded. “Rb status may be an important means to direct personalized medicine.”

The research discussed by DiPaola, Schlom, and Knudsen focus on one specific area of oncology research—prostate cancer—and represent only a small fraction of the work being done in the field of translational medicine. Current research is exploring the cellular and molecular biology of tumors in all types of cancers. Translational medicine facilitates this transfer of knowledge from the laboratory to clinical practice and holds the promise of uncovering new breakthrough therapies in cancer treatment.