Recent advances in immunology, tumor biology, and genomics have paved the way for the development of gene-based therapy. With the task of sequencing and identifying all human genes well under way, the first tentative steps toward clinical trials using gene-based treatment of urologic cancers, including those of the bladder, renal cell, and prostate, have begun. Gene therapy is a new form of therapy in which a functioning gene is inserted into a cell to correct a specific genetic defect or to target a specific molecular pathway that can alter disease at its most fundamental—molecular biologic—level. The prostate is an especially suitable target for gene therapy, since it is expendable after the reproductive years and is easily accessible through a transrectal, transperitoneal, or transurethral approach. Furthermore, using the regulatory sequences of prostate-specific proteins, such as prostate-specific antigen (PSA), prostatic acid phosphatase, prostate-specific membrane antigen, probasin, and other human glandular kallikreins to drive the expression of therapeutic genes, therapy potentially can be targeted to prostate cancer cells throughout the body using a systemic approach.
Numerous potentially therapeutic genes and delivery systems are currently being developed and evaluated. Overall, the different gene therapy modalities can be grouped into 3 main categories: immunomodulatory, corrective, and cytoreductive. Immunomodulation, simply stated, attempts to augment the body’s immune response to improve the immune system’s natural ability to seek out and destroy cancer cells. Corrective gene therapy, which is already being used by many investigators in the treatment of patients with prostate cancer, involves the replacement or inactivation of a defective gene, such as a mutated tumor suppressor gene, or a dominant oncogene that has been found to play a role in the pathogenesis or progression of prostate cancer. The replacement of a defective p53 tumor suppressor gene is an example of the type of corrective gene therapy that has produced the greatest interest (Table).
Table 1.
Human Gene Therapy Protocols for Prostate Cancer*
| Strategy | Gene | Vector | Title |
|---|---|---|---|
| Immunotherapy | GM-CSF | Retrovirus | Phase I/II study of autologous human GM-CSF gene-transduced |
| prostate cancer vaccines in patients with metastatic | |||
| prostate carcinoma | |||
| Immunotherapy | IL-2/ | Retrovirus | Phase I/II study of immunization with MHC class I matched |
| Interferon- γ | allogeneic human prostatic carcinoma cells engineered to | ||
| secrete IL-2 and interferon-γ | |||
| Antisense | c-myc | Retrovirus | Gene therapy for the treatment of advanced prostate cancer by |
| (gene replacement) | in vivo transduction with prostate-targeted retroviral vectors | ||
| expressing antisense c-myc RNA | |||
| Immunotherapy | PSA promoter | Vaccinia | Phase I study of recombinant vaccinia that expresses PSA in adult |
| patients with adenocarcinoma of the prostate | |||
| Immunotherapy | IL-2 | Liposome | Phase I study of autologous human IL-2 gene-modified tumor |
| cells in patients with locally advanced or metastatic | |||
| prostate cancer | |||
| Suicide gene | Thymidine | Adenovirus | Phase I study of adenoviral vector delivery of the HSV-tk gene |
| (cytoreductive) | kinase | and the intravenous administration of ganciclovir in men with | |
| local recurrence of prostate cancer after radiation therapy | |||
| Immunotherapy | PSA promoter | Vaccinia | Phase I trial of recombinant vaccinia virus that expresses PSA in |
| patients with adenocarcinoma of the prostate | |||
| Immunotherapy | PSA promoter | Vaccinia | Phase I/II clinical trial evaluating the safety and biologic activity |
| of recombinant vaccinia-PSA vaccine in patients with serologic | |||
| recurrence of prostate cancer following radical prostatectomy | |||
| Immunotherapy | IL-2 | Liposome | Phase I study evaluating the safety and efficacy of IL-2 gene |
| therapy delivered by lipid-mediated gene transfer (leuvectin) in | |||
| prostate cancer patients | |||
| Suicide gene | Thymidine | Adenovirus | Phase I trial of adenoviral-mediated HSV-tk gene transduction in |
| (cytoreductive) | kinase | conjunction with ganciclovir therapy as neoadjuvant treatment | |
| for patients with clinically localized (stage T1c and T2b&c) | |||
| prostate cancer prior to radical prostatectomy | |||
| Gene replacement | p53 | Adenovirus | Phase I study in patients with locally advanced or recurrent |
| adenocarcinoma of the prostate using SCH58500 (rAd/p53) | |||
| administered by intratumoral injection | |||
| Immunotherapy | GM-CSF | Retrovirus | Phase I/II study of allogeneic human GM-CSF gene-transduced |
| irradiated prostate cancer cell vaccines in patients with | |||
| prostate cancer | |||
| Principal Investigators | Institution | Status | FDA Protocol ID No. |
| J. Simons | Johns Hopkins Oncology Center, | Open | 9408-082 |
| Baltimore | |||
| B. Gansbacher | Memorial Sloan-Kettering Cancer | Open | 9503-102 |
| Center, New York | |||
| M. Steiner, J. Holt | Vanderbilt University Medical Center, | Open | 9509-123 |
| Nashville, Tenn | |||
| A. Chen | National Naval Medical Center, | Open | 9509-126 |
| Bethesda, Md | |||
| D. Paulson, K. Lyerly | Duke University Medical Center, | Open | 9510-132 |
| Durham, NC | |||
| P. Scardino, T. Thompson, | Baylor College of Medicine, | Open | 9601-44 |
| S. Woo | Houston | ||
| D. Kufe, J. Eder | Dana Farber Cancer Institute, | Open | 9609-160 |
| Boston | |||
| M. Sanda | University of Michigan Urology Clinics, | Open | 9702-176 |
| Ann Arbor | |||
| A. Belldegrun | UCLA School of Medicine, | Open | 9703-184 |
| Los Angeles | |||
| S. Hall, S. Woo | Mount Sinai School of Medicine, | Open | 9705-187 |
| New York | |||
| A. Belldegrun, R. Figlin | UCLA School of Medicine, | Open | 9706-192 |
| Los Angeles | |||
| J. Simons | Johns Hopkins Oncology Center, | Open | 9708-205 |
| Baltimore | |||
| Strategy | Gene | Vector | Title |
| Gene replacement | p53 | Adenovirus | Tolerance and efficacy study of intraprostatic INGN 201 followed |
| by pathologic staging and possible radical prostatectomy in | |||
| patients with locally advanced prostate cancer | |||
| Suicide gene | Thymidine | Adenovirus | Neoadjuvant preradical prostatectomy gene therapy (HSV-tk gene |
| (cytoreductive) | kinase | transduction followed by ganciclovir) in patients with poor | |
| prognostic indicators | |||
| Oncolytic virus | PSA promoter | Adenovirus | Phase I study of the intraprostatic injections of CN706, a PSA |
| (cytoreductive) | gene-regulated cytolytic adenovirus, in patients with locally | ||
| recurrent cancer following definitive radiotherapy | |||
| Immunotherapy | MUC-1/IL-2 | Vaccinia | Phase I/II trial of antigen-specific immunotherapy in MUC-1-positive |
| patients with adenocarcinoma of the prostate using | |||
| vaccinia virus-MUC1 -IL-2 (TG 1031) | |||
| Suicide gene | Thymidine | Adenovirus | Phase I study of Ad-OC-tk plus valacyclovir for the treatment |
| (cytoreductive) | kinase | of metastatic or recurrent prostate cancer | |
| Immunotherapy | PSA promoter | Vaccinia/ | Phase II randomized trial of recombinant fowlpox and |
| fowlpox | recombinant vaccinia virus-expressing PSA in patients with | ||
| adenocarcinoma of the prostate | |||
| Immunotherapy | GM-CSF | Retrovirus | Phase I/II study of a prime-boost schedule of human GM-CSF |
| gene-transduced irradiated prostate allogeneic cancer cell vaccines | |||
| (allogeneic prostate GVAX) in hormone-naive prostate cancer | |||
| patients | |||
| Immunotherapy | PSA promoter | Vaccinia/ | Phase II randomized study of vaccine treatment of advanced |
| fowlpox | prostate cancer | ||
| Immunotherapy | PSA | Safety and feasibility study of active immunotherapy in patients | |
| with metastatic prostatic carcinoma using autologous dendritic | |||
| cells pulsed with RNA-encoding PSA | |||
| Immunotherapy | IL-2 | Liposome | Phase II study evaluating the safety and efficacy of neoadjuvant |
| leuvectin immunotherapy for the treatment of prostate cancer | |||
| Suicide gene | Thymidine | Adenovirus | Phase I/II study evaluating HSV-tk + valacyclovir gene therapy |
| (cytoreductive) | kinase | in combination with radiotherapy for prostate cancer | |
| Principal Investigators | Institution | Status | FDA Protocol ID No. |
| C. Logothetis | University of Texas M. D. Anderson | Open | 9710-217 |
| Cancer Center, Houston | |||
| D. Kadmon, | Baylor College of Medicine, | Open | 9801-229 |
| E. Aguilar-Cordova | Houston | ||
| J. Simons | Johns Hopkins School of Medicine, | Open | 9802-236 |
| Baltimore | |||
| R. Figlin | UCLA School of Medicine, | Open | 9805-251 |
| Los Angeles | |||
| T. Gardner, L. Chung | University of Virginia Health Sciences Center, | Open | 9812-276 |
| Charlottesville | |||
| J. Eder | Dana Farber Cancer Institute, | Under review | 9901-282 |
| Boston | |||
| E. Small | University of California, | Open | 9901-283 |
| San Francisco | |||
| H. Kaufman | Albert Einstein College of Medicine, | Under review | 9902-293 |
| Bronx, NY | |||
| J. Vieweg | Duke University Medical Center, | Open | 9904-306 |
| Durham, NC | |||
| A. Belldegrun | UCLA School of Medicine, | Under review | 9905-312 |
| Los Angeles | |||
| B. Butler, E. Aguilar-Cordova | Baylor College of Medicine, Houston | Open | 9906-324 |
| Houston |
GM-CSF, granulocyte-macrophage colony-stimulating factor; IL-2, interleukin-2; MHC, major histocompatability complex; HSV-tk, herpes simplex virus thymidine kinase; PSA, prostate-specific antigen; BRCA1, breast cancer susceptibility gene 1.
As of 9/22/99.
To date, the most extensively studied approach for managing prostate cancer is the cytoreductive method. While several methods of cytoreductive therapy are feasible, “suicide” gene therapy—using a gene encoding an enzyme that can metabolize a nontoxic prodrug into its toxic metabolite—has undergone the most extensive development. The prototype of this system is the herpes simplex virus thymidine kinase (HSV-tk) gene, which converts the nontoxic prodrug ganciclovir (GCV) into a phosphorylated compound, ganciclovir triphosphate, that is toxic to cancer cells. Incorporation of ganciclovir triphosphate into the cancer’s cellular DNA leads to DNA chain termination, DNA fragmentation and, finally, cell death. This approach also exhibits a desirable secondary effect, the “bystander effect,” whereby nontransduced neighboring cancer cells are also killed. A group from Baylor College of Medicine has systematically developed adenovirus-mediated HSV-tk + GCV suicide gene therapy for the treatment of patients with prostate cancer. After extensive and favorable preclinical studies testing the efficacy and toxicity of this strategy in both in vitro and in vivo prostate cancer models, this group recently reported the results of the first gene therapy trial for human prostate cancer, in which they demonstrated the feasibility and safety of their strategy (Herman et al).
Although efficacy was not a primary end point of this phase I trial, the relatively limited potential of HSV-tk + GCV as monotherapy has prompted adjunctive studies using varying combinations of gene therapy with other strategies, such as standard hormonal therapy (Hall et al) as well as modulation of the antitumor cell immune response (Nasu et al). Indeed, dramatic response rates have not yet been definitively demonstrated in any single gene therapy protocol for cancer, suggesting that future efforts will focus on combining different types of gene-based strategies with more conventional therapies.
The keen interest in prostate cancer and its suitability for gene-based therapy is reflected by the plethora of centers around the country that are involved in phase I clinical trials evaluating different gene therapy strategies for the management of this disease (Table). The following recently published articles document the successes as well as the many technical and biologic challenges that we face in our effort to bring gene-based therapies into the clinic.
In Situ Gene Therapy for Adenocarcinoma of the Prostate: A Phase I Clinical Trial
Herman JR, Adler HL, Aguilar-Cordova E, et al.
Hum Gene Ther. 1999;10:1239–1249.
This is the first published report of a human clinical trial of gene therapy for prostate cancer. The group from Baylor College of Medicine conducted a phase I dose-escalation trial of intraprostatic injections of replication-deficient adenovirus construct (Ad5) containing the HSV-tk gene on a Rous sarcoma long-terminal repeat promoter, followed by intravenous administration of GCV. Eighteen patients were enrolled; all had biopsy-proven local recurrence after definitive radiation therapy and rising serum PSA levels but no clinical evidence of metastases. Patients were stratified into 4 groups receiving escalating doses of HSV-tk, which was increased in single-log fashion from 108 to 1011 infectious units (IU). With guidance provided by transrectal ultrasonography, all received a single, 1-mL injection of vector into the prostate region judged to have the greatest concentration of tumor-bearing tissue. Beginning 24 hours later, patients received parenteral GCV infusions (5 mg/kg over 1 hour every 12 hours) for 2 weeks. Post-treatment follow-up included digital rectal examinations (DREs), serum PSA measurements, and systematic needle biopsies.
Overall, toxicity was minimal for all treatment vector dose groups. Three patients experienced grade 1 toxic reactions and 2 patients, grade 2 toxic reactions (fever, shaking chills, cellulitis, and slightly abnormal liver function). All complications resolved either spontaneously or after administration of parenteral antibiotics. However, 1 patient at the highest dose (1011 IU) experienced substantial toxicity (grade 4), with thrombocytopenia and hepatotoxicity. The authors hypothesized that extravasation of the virus during the injections might have caused these complications. Discontinuation of treatment and administration of platelet transfusions led to the complete recovery of the patient. Daily viral cultures for adenovirus (ADV) in serum specimens and of matter obtained by ear and nasal swabs were consistently negative in all patients. Polymerase chain reactions of urine samples for the thymidine kinase gene indicated detectable dose-dependent vector shedding for a short period (up to 32 days) after intraprostatic injection.
Signs of efficacy were observed in 3 patients in the form of a suppression of serum PSA levels by more than 50% for a duration of 6 weeks to more than 1 year. In 1 of the 3 patients, ultrasonography revealed the development of a cystic cavity in the prostate at the site of injection. Serial DREs, transrectal sonograms, and systematic biopsies showed no consistent change in overall prostate size or histologic characteristics. While none of the patients was cured—and while, at the date of submission of the study, 5 patients were being treated for disease progression—this trial was a significant first step in proving the safety and feasibility of gene therapy for prostate cancer.
Cooperative Therapeutic Effects of Androgen Ablation and Adenovirus-Mediated Herpes Simplex Virus Thymidine Kinase Gene and Ganciclovir Therapy in Experimental Prostate Cancer
Hall SJ, Mutchnik SE, Yang G, et al.
Cancer Gene Ther. 1999;6:54–63.
While suicide gene therapy using ADV/HSV-tk + GCV has been shown to be of therapeutic potential in prostate cancer as well as in other diseases, such therapy has not led to complete cancer remission in animal models of prostate cancer. Because of the incomplete and unsustained therapeutic activity, tumor cells that survive the initial treatment course ultimately regrow, leading to the eventual death of the host. In a first attempt to further improve the efficacy of their strategy, Hall and colleagues reported on the combination of androgen ablation and HSV-tk + GCV. As in their previous studies, the authors used a replication-defective recombinant ADV under the transcriptional control of the Rous sarcoma virus long-terminal repeat promoter carrying the HSV-tk gene. They used an androgen-sensitive mouse prostate cancer cell line (RM-2) from a mouse prostate reconstitution model system to establish subcutaneous and orthotopic prostate tumors. Seven days before tumor inoculation, the animals were randomized to surgical castration or sham operation. Subcutaneous tumors were injected on day 2 and orthotopic tumors, on day 6 with 5 × 108 plaque-forming units (pfu) of HSV-tk, followed by intraperitoneal administration of GCV (10 mg/kg twice daily for 6 days).
The combination of castration and HSV-tk + GCV resulted in significantly higher growth suppression than either castration alone or HSV-tk + GCV alone. On day 14, tumors were significantly smaller (tumor volume) in the subcutaneous model and lighter (wet weight) in the orthotopic model of the combination therapy group than in groups with either treatment alone. Kaplan-Meier survival analysis showed a greater improvement in survival of animals treated with combination therapy than in animals treated with either therapy alone, although they, in turn, experienced a statistically significant increase in survival when compared with controls. Apoptotic activity was significantly increased in tumors managed with HSV-tk + GCV alone (2-fold, P = .013) or with combination therapy (3.5-fold, P = .006). However, the necrotic activity in tumors managed with HSV-tk + GCV was 5-fold higher than in controls, regardless of androgen status. Similarly, the enhanced antimetastatic activity observed in the HSV-tk + GCV groups was independent of androgen status. These findings suggest that the cooperative therapeutic effect of HSV-tk + GCV and androgen ablation is achieved through synergy in apoptosis induction and not through the necrosis or metastasis suppression pathway. Although the mechanism of action of the combination treatment is not understood, there undoubtedly is a beneficial effect. This study suggests that gene therapy used in combination with standard treatment options has the potential to enhance the overall therapeutic effect. It would also be ethically more correct—and make more sense—to give gene therapy as adjuvant therapy rather than as monotherapy.
Adenovirus-Mediated Interleukin-12 Gene Therapy for Prostate Cancer: Suppression of Orthotopic Tumor Growth and Pre-Established Lung Metastases in an Orthotopic Model
Nasu Y, Bangma CH, Hull GW, et al.
Gene Ther. 1999;6:338–349.
In their previous preclinical and clinical studies with HSV-tk + GCV gene therapy for prostate cancer, the group from Baylor (as well as others) observed a type-1 T-helper-mediated antitumor immune response. Indeed, multiple patients in the authors’ phase I human clinical trial exhibited a transient but significant T-cell activation. Hoping to boost this antitumor immune response directly through enhancing the immunogenicity of the tumors by an ADV that coexpresses the immunomodulatory cytokine interleukin-12 (IL-12), they evaluated the efficacy of direct immune modulation with an ADV expressing human cytomegalovirus promoter-driven recombinant murine IL-12 (AdmIL-12).
In vitro infection of RM-9 cells (a poorly immunogenic cell line derived from the mouse prostate reconstitution system) at increasing multiplicity of infections (MOIs) (from 0 to 200) generated a time- and dose-dependent production of AdmIL-12, without any significant direct cytotoxicity. Escalating viral doses (5 × 107 to 3 × 108 pfu) of AdmIL-12 were then directly injected into orthotopically established prostate tumors. Significant growth suppression (more than 50% reduction of tumor weight, all P < .05) on day 14 after vector injection as well as increased mean survival time (28.9 ± 1.2 days versus 23.4 ± 0.8 days, P < .0001) were observed in the AdmIL-12 group as compared with controls. Toxicity was limited to dose-related splenomegaly as well as ascites in some of the mice treated with the highest viral dose (3 × 108 pfu). Histologic examination and cytokeratin staining of the regional lymph nodes revealed a reduction in the number of spontaneous metastases of AdmIL-12-treated animals as compared with control animals. Suppression of preestablished lung metastases was not statistically significant (P = .056) in a low-metastasis protocol but was highly significant (P = .017) in a high-metastasis protocol (which differed from the low-metastasis protocol by an increased number of preestablished lung metastases). Serum IL-12 levels peaked at day 1 and decreased thereafter, until becoming undetectable on day 14 after vector injection. Cytolytic natural killer (NK) cell activity was increased within splenocytes shortly after virus injection. In addition, the intratumoral infiltration of CD4+ and CD8+ T cells, as well as nitric oxide synthase-positive macrophages, was increased in AdmIL-12-treated animals. Systemic blockage of NK cells with anti-asialo-GM1 serum led to significantly increased numbers of lung metastases in AdmIL-12-treated tumors (P = .014) but did not affect local tumor growth, demonstrating that some—but not all—of the AdmIL-12 activity is mediated by NK cells.
In their comprehensive study, the investigators not only examined the efficacy and toxicity of IL-12 gene therapy in an orthotopic mouse prostate cancer model but also investigated the mechanism of action of IL-12 in prostate cancer. They demonstrated that AdmIL-12 has systemic antimetastatic activity that suppresses the development of distant metastases, mainly through NK cells and a non-NK component. This study has set the stage for future, 2-pronged combination therapy with HSV-tk + GCV.