Gene therapy for prostate cancer. New perspectives on an old problem

Terapia génica en el cáncer de próstata. ¿Es posible una vacuna?

BACKGROUND

New approaches for prostate cancer are needed due to limitations of current therapies for the treatment in advanced stages of the disease. In fact, there is no effective treatment for these patients. Development in molecular biology research on prostate cancer has improved the knowledge of common alterations encoded in DNA sequence, which may be useful as targets for prostate cancer approach. In this review we give an overview of current gene therapy concepts, the most common gene alterations in prostate cancer and the gene therapy treatment strategies.

Highly specific expression of luciferase gene in lungs of naïve nude mice directed by prostate-specific antigen promoter

PSA promoter has been demonstrated the utility for tissue-specific toxic gene therapy in prostate cancer models. Characterization of foreign gene overexpression in normal animals elicited by PSA promoter should help evaluate therapy safety. Here we constructed an adenovirus vector (AdPSA-Luc), containing firefly luciferase gene under the control of the 5837 bp long prostate-specific antigen promoter. A charge coupled device video camera was used to non-invasively image expression of firefly luciferase in nude mice on days 3, 7, 11 after injection of 2 x 10(9)PFU of AdPSA-Luc virus via tail vein. The result showed highly specific expression of the luciferase gene in lungs of mice from day 7. The finding indicates the potential limitations of the suicide gene therapy of prostate cancer based on selectivity of PSA promoter. By contrary, it has encouraging implications for further development of vectors via PSA promoter to enable gene therapy for pulmonary diseases.

Purine nucleoside phosphorylase and fludarabine phosphate gene-directed enzyme prodrug therapy suppresses primary tumour growth and pseudo-metastases in a mouse model of prostate cancer

Gene-directed enzyme prodrug therapy based on the E. coli purine nucleoside phosphorylase (PNP) gene produces efficient tumour cell killing. PNP converts adenosine analogs into toxic metabolites that diffuse across cell membranes to kill neighbouring untransduced cells (PNP-GDEPT). Interference with DNA, RNA and protein synthesis kills dividing and non-dividing cells, an important consideration for slow-growing prostate tumours. This study examined the impact of administering PNP-GDEPT into orthotopically grown RM1 prostate cancers (PCas) on the growth of lung pseudo-metastases of immunocompetent mice. C57BL/6 mice bearing orthotopic RM1 PCas received a single intraprostatic injection of OAdV220 (10(10) particles), a recombinant ovine atadenovirus containing the PNP gene controlled by the Rous Sarcoma virus promoter, followed by fludarabine phosphate (approximately 600 mg/m(2)/day) administered intraperitoneally (ip) once daily for 5 days. Pseudo-metastases were induced 2 days after intraprostatic vector administration by tail-vein injection of untransduced RM1 cells. Mice given PNP-GDEPT showed a significant reduction both in prostate volume (approximately 50%) and in lung colony counts (approximately 60%). Apoptosis was increased two-fold in GDEPT-treated prostates compared with controls (P < 0.01), but was absent in the lungs. Staining for proliferating cell nuclear antigen (PCNA) indicated that proliferation of both RM1 prostate tumours (P < 0.01) and lung colonies (P < 0.01) was significantly suppressed after GDEPT. Although prostate tumour immune cell infiltration did not differ significantly between treatments, immunostaining for Thy-1.2 (CD90) showed that GDEPT promoted Thy-1.2(+) cell infiltration into the prostate tumour site. This study showed that a single course of PNP-GDEPT significantly suppressed local PCa growth and reduced lung colony formation in the aggressive RM1 tumour model.

Prostate cancer transfection by acoustic energy using pEGFP-N1 as reporter gene in the solid Dunning R-3327-MatLu tumor

BACKGROUND

Gene therapy is expected to play a major role in medical treatment in the future. Several different methods for DNA transfection exist. We evaluated the efficacy and effects of transfection by acoustic energy in a standardized prostate cancer model.

METHODS

Subcutaneous implantation of Dunning tumors was followed by injection of pEGFP-DNA plasmid. Tumors were treated by acoustic energy with different parameter settings and the transfection rate was assessed by FACScan.

RESULTS

Standardized experimental conditions resulted in minor intragroup deviations. Intratumoral injection resulted in a transfection rate of 0.3% (control group). The statistically significant increase of 4.6% in cell transfection rate was gained by applying acoustic energy.

CONCLUSIONS

DNA transfection of solid tumors can be mediated by the application of acoustic energy. Achieved transfection rates are encouraging and imply therapeutical levels. Prodrug activation or suicide gene therapy are possible fields of investigation in the treatment of prostate cancer.

Immunotherapy and prostate cancer

Prostate cancer is the second leading cause of cancer death in the US, largely because of the limitations of our current therapeutic options, especially once the cancer has metastasized. Investigators have long sought new therapeutic modalities such as angiogenesis inhibitors, vaccines, and gene therapy, among others. It appears that a combination approach will be required to cure the majority of malignancies. Immunotherapy for prostate cancer appears feasible and a likely therapeutic modality in the armamentarium. Unfortunately, further research in basic immunology and the interaction of the immune system with other forms of therapy is needed. Many obstacles exist in immunotherapy, including vector design, tumouricidal specificity, and tumor evasion, which will have to be overcome in order to realize the maximum therapeutic benefit from this treatment modality.

Adenovirus mediated prostate specific enzyme prodrug gene therapy using prostate specific antigen promoter enhanced by the Cre-loxP system

PURPOSE

Tissue or tumor specific gene delivery is crucial for achieving successful results in suicide gene therapy. Prostate specific antigen (PSA) promoter is known to be highly specific in prostate tissue but its promoter activity is much weaker than that of constitutive viral promoters. In the current study we enhanced PSA promoter activity by combining it with the Cre-loxP system. We also applied this system to adenovirus mediated suicide gene therapy with the cytosine deaminase (CD) gene.

MATERIALS AND METHODS

The Cre-loxP DNA recombination system was used to enhance PSA promoter. A plasmid with the PSA promoter-enhancer combination was constructed to drive Cre recombinase. Another plasmid contained the cytomegalovirus promoter-loxP-flanked stop signal-luciferase gene. LNCaP human prostate cancer cells were co-transfected with these 2 plasmids and luciferase activity was measured to assess promoter activities. Adenoviral vectors with the CD suicide gene were constructed in similar fashion and tested in LNCaP cells in in vitro/in vivo prostate cancer models.

RESULTS

Promoter activity of the combined PSA promoter/enhancer and Cre-loxP system was 3 times stronger than that of PSA promoter/enhancer alone. It was further enhanced 7-fold in the presence of testosterone. Application of this system to CD suicide gene therapy by adenoviral vectors inhibited subcutaneous LNCaP tumor growth in nude mice.

CONCLUSIONS

Combining the Cre-loxP system with PSA promoter/enhancer amplified promoter activity and was found to inhibit the growth of PSA producing prostate cancer cells in vivo.

Gene therapy for prostate cancer delivered by ovine adenovirus and mediated by purine nucleoside phosphorylase and fludarabine in mouse models

A gene-directed enzyme pro-drug therapy (GDEPT) based on purine nucleoside phosphorylase (PNP), that converts the prodrug, fludarabine to 2-fluoroadenine, has been described, but studies are limited compared with other GDEPTs. We investigated the in vitro and in vivo efficacies of PNP-GDEPT for treating androgen-independent (AI) prostate cancer. The PNP gene controlled by Rous sarcoma virus (RSV) constitutive promoter was delivered using a recombinant ovine adenovirus vector (OAdV220) that uses a different receptor from human adenovirus type 5. In vitro, OAdV220 provided increased transgene expression over a comparable human Ad5 vector in infected AI, murine RM1 prostate cancer cells. Subsequent in vivo testing was therefore confined to OAdV220. Transduction of RM1 cells with OAdV220 before implantation in immunocompetent mice dramatically inhibited subcutaneous (s.c.) tumor growth when fludarabine phosphate was administered systemically and increased mouse survival in a dose-dependent manner. In tumor-bearing C57BL/6 mice, a single intratumoral injection of OAdV220 produced detectable PNP activity for at least 6 days and with prodrug, retarded the growth of aggressive RM1 s.c. tumors by 35% at day 14. There was a consistent trend to reduction of pre-established intraprostatic RM1 tumors. A similar regimen induced significant therapeutic efficacy in human PC3 xenografts. Thus, ovine adenovirus-mediated GDEPT using the PNP system was effective in vivo against AI prostate cancers, the aggressive murine RM1, and the human PC3 lines. Methods that improve viral dissemination and stimulate the immune system in vivo may further improve efficacy.

Transcription-targeted gene therapy for androgen-independent prostate cancer

The Escherichia coli enzyme (purine nucleoside phosphorylase, PNP) gene is delivered directly into PC3 tumors by one injection of replication-deficient human type-5 adenovirus (Ad5). Expressed PNP converts the systemically administered prodrug, 6MPDR, to a toxic purine, 6MP, causing cell death. We sought to increase the specificity of recombinant Ad vectors by controlling PNP expression with the promoter region from the androgen-dependent, prostate-specific rat probasin (Pb) gene. To increase its activity, the promoter was combined with the SV40 enhancer (SVPb). Cell lines were transfected with plasmids containing both a reporter gene, under SVPb control, and a reference gene cassette to allow normalization of expression levels. Plasmids expressed approximately 20-fold more reporter in prostate cancer than in other cells, but surprisingly, the SVPb element was both androgen-independent and retained substantial prostate specificity. Killing by Ad5-SVPb-PNP vector of cell lines cultured with 6MPDR for 6 days was 5- to 10-fold greater in prostate cancer than in liver or lung cells. In vivo, a single intratumoral injection of Ad5-SVPb-PNP (4 x 10(8) pfu), followed by 6MPDR administration twice daily for 6 days, significantly suppressed the growth of human prostate tumors in nude mice and increased their survival compared to control animals. Thus, the androgen-independent, prostate-targeting Ad5 vector reduces human prostate cancer growth significantly in vitro and in vivo. This first example of an androgen-independent vector points the way toward treatment of emerging androgen-independent prostate cancer in conjunction with hormone ablation therapy at a time when the tumor burden is low.

Prostate cancer gene therapy and the role of radiation

Even though prostate cancer is detected earlier than in the pre-PSA era, prostate cancer is the second leading cause of cancer mortality in the American male. Prostate cancer therapy is not ideal, especially for high-risk localized and metastatic cancer; therefore, investigators have sought new therapeutic modalities such as angiogenesis inhibitors, inhibitors of the cell signaling pathway, vaccines, and gene therapy. Gene therapy has emerged as potential therapy for both localized and systemic prostate cancer. Gene therapy has been shown to work supra-additively with radiation in controlling prostate cancer in vivo. With further technological advances in radiation therapy, gene therapy, and the understanding of prostate cancer biology, gene therapy will potentially have an important role in prostate cancer therapy.