A Dual-Targeted Lentiviral Vector Homing in on Prostate Bone Metastases
Authors: Yufang Hu, David Stout, Lily Wu
Journal: Molecular Therapy (2007)

The field of gene therapy has held great promise for the treatment of hereditary and acquired diseases. In the past decade, much effort has been focused on the construction of safe and efficient viral vectors.1Thomas CE Ehrhardt A Kay MA Progress and problems with the use of viral vectors for gene therapy.Nat Rev Genet. 2003; 4: 346-358Crossref PubMed Scopus (2003) Google Scholar,2Lavillette D Russell SJ Cosset FL Retargeting gene delivery using surface-engineered retroviral vector particles.Curr Opin Biotechnol. 2001; 12: 461-466Crossref PubMed Scopus (73) Google Scholar,3Sanders DA No false start for novel pseudotyped vectors.Curr Opin Biotechnol. 2002; 13: 437-442Crossref PubMed Scopus (63) Google Scholar,4Breckpot K Aerts JL Thielemans K Lentiviral vectors for cancer immunotherapy: transforming infectious particles into therapeutics.Gene Ther. 2007; 14: 847-862Crossref PubMed Scopus (97) Google Scholar However, the field has suffered major setbacks due to questions about the safety of the viral vectors used for transgene delivery.1Thomas CE Ehrhardt A Kay MA Progress and problems with the use of viral vectors for gene therapy.Nat Rev Genet. 2003; 4: 346-358Crossref PubMed Scopus (2003) Google Scholar In light of the safety concerns of retroviral vectors, much effort has been made toward improving the safety of lentiviral vectors via the incorporation of specific targeting mechanisms, with the aim of restricting transgene expression to only the cells of interest. Broadly defined, these targeting schemes fall into two classes: transductional and transcriptional. In this issue of Molecular Therapy, Pariente et al.5Pariente N Morizono K Virk MS Petrigliano FA Reiter RE Lieberman JR et al.A novel dual-targeted lentiviral vector leads to specific transduction of prostate cancer bone metastases in vivo after systemic administration.Mol Ther. 2007; 15: 1973-1981Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar report the construction of a dual-targeted lentiviral vector, combining both transductional and transcriptional taregeting, that they use to treat prostate bone metastases following systematic injection. This work marks a successful attempt at a dual-targeted lentiviral vector that may have significant impact on the development of in vivo lentiviral gene therapy. One of the primary goals of vector design is a vector that can home in on specific cell types to deliver transgenes efficiently while introducing few side effects. This goal is particularly relevant to cancer gene therapy, in which the delivery of transgenes should usually be restricted to tumor cells. A secondary goal is to create vectors that can be produced on a large scale at a sufficient titer for clinical applications. Among the choices of viral vector currently being used in gene therapy clinical trails worldwide, lentiviral vectors constitute about 0.7% of the total as reported in 2007 by the Journal of Gene Medicine database. The main advantages of lentiviral vectors are the ability to transduce both dividing and nondividing cells and the ability to be integrated into the host genome for sustained transgene expression. Transcriptional targeting usually involves the incorporation of a tissue-specific promoter into the lentiviral genome to restrict the transgene expression to only the selected cell type.6Follenzi A Sabatino G Lombardo A Boccaccio C Naldini L Efficient gene delivery and targeted expression to hepatocytes in vivo by improved lentiviral vectors.Hum Gene Ther. 2002; 13: 243-260Crossref PubMed Scopus (213) Google Scholar,7Yu D Chen D Chiu C Razmazma B Chow YH Pang S Prostate-specific targeting using PSA promoter-based lentiviral vectors.Cancer Gene Ther. 2001; 8: 628-635Crossref PubMed Scopus (69) Google Scholar Expansion of lentiviral tropism via transductional targeting is commonly achieved through modification of the viral envelope protein by pseudotyping of the viral vector with glycoproteins from other viruses (for an updated list, see Table 1 of ref. 4Breckpot K Aerts JL Thielemans K Lentiviral vectors for cancer immunotherapy: transforming infectious particles into therapeutics.Gene Ther. 2007; 14: 847-862Crossref PubMed Scopus (97) Google Scholar). Among the expanding list of proteins used for pseudotyping, the vesicular stomatitis virus G protein (VSV-G) has been the most widely used.8Naldini L Blomer U Gallay P Ory D Mulligan R Gage FH et al.In vivo gene delivery and stable transduction of nondividing cells by a lentiviral vector.Science. 1996; 272: 263-267Crossref PubMed Scopus (4018) Google Scholar However, VSV-G pseudotyped lentiviral vector suffers from potential shortcomings such as broad tissue tropism and neutralization by human serum.9Reynolds PN Nicklin SA Kaliberova L Boatman BG Grizzle WE Balyasnikova IV et al.Combined transductional and transcriptional targeting improves the specificity of transgene expression in vivo.Nat Biotechnol. 2001; 19: 838-842Crossref PubMed Scopus (213) Google Scholar In principle, a lentiviral vector that combines both transductional and transcriptional targeting mechanisms could offer improved tissue selectivity over a vector containing either transductional or transcriptional targeting alone.9Reynolds PN Nicklin SA Kaliberova L Boatman BG Grizzle WE Balyasnikova IV et al.Combined transductional and transcriptional targeting improves the specificity of transgene expression in vivo.Nat Biotechnol. 2001; 19: 838-842Crossref PubMed Scopus (213) Google Scholar,10Muller OJ Leuchs B Pleger ST Grimm D Franz WM Katus HA et al.Improved cardiac gene transfer by transcriptional and transductional targeting of adeno-associated viral vectors.Cardiovasc Res. 2006; 70: 70-78Crossref PubMed Scopus (131) Google Scholar In the new work reported in this issue of Molecular Therapy, transcriptional targeting was achieved by cloning of an enhanced prostate-specific promoter, PSE-BC,11Wu L Matherly J Smallwood A Adams JY Billick E Belldegrun A et al.Chimeric PSA enhancers exhibit augmented activity in prostate cancer gene therapy vectors.Gene Ther. 2001; 8: 1416-1426Crossref PubMed Scopus (90) Google Scholar into a third-generation self-inactivating lentiviral vector that drives the expression of a humanized luciferase gene. The vector was pseudotyped with a modified Sindbis virus envelope, which was created by the insertion of the IgG-binding region of protein A from Staphylococcus aureus into the Sindbis envelope protein E1 at its receptor-binding domain.12Morizono K Bristol G Xie YM Kung SK Chen IS Antibody-directed targeting of retroviral vectors via cell surface antigens.J Virol. 2001; 75: 8016-8020Crossref PubMed Scopus (108) Google Scholar,13Morizono K Xie Y Ringpis GE Johnson M Nassanian H Lee B et al.Lentiviral vector retargeting to P-glycoprotein on metastatic melanoma through intravenous injection.Nat Med. 2005; 11: 346-352Crossref PubMed Scopus (184) Google Scholar Modification of viral tropism was achieved by incubation of the lentiviral vector with a monoclonal antibody, 1G8, that binds prostate stem cell antigen (PSCA).14Lam JS Yamashiro J Shintaku IP Vessella RL Jenkins RB Horvath S et al.Prostate stem cell antigen is overexpressed in prostate cancer metastases.Clin Cancer Res. 2005; 11: 2591-2596Crossref PubMed Scopus (107) Google Scholar PSCA is a cell-surface antigen that has been found to be overexpressed in 87% of clinical samples of bone metastases.14Lam JS Yamashiro J Shintaku IP Vessella RL Jenkins RB Horvath S et al.Prostate stem cell antigen is overexpressed in prostate cancer metastases.Clin Cancer Res. 2005; 11: 2591-2596Crossref PubMed Scopus (107) Google Scholar A clear advantage of the reported approach is the relative ease and flexibility of using the engineered protein A binding domain in the Sindbis glycoprotein to pseudotype via incubation with an antibody. Hence, it is easy to envision that the versatility of the two-tier targeting mechanism should allow targeting of a wide variety of tissue types by pairing appropriate antibodies with tissue-specific promoters. Of particular interest to cancer gene therapy, the authors found that luciferase transgene expression using either the VSV-G pseudotyped, transcriptionally targeted vector PSE-BC/VSV-G or the dual-targeted vector PSE-BC/1G8 was largely confined to the grafted tumor cells after systematic injection. However, upon closer inspection, vector DNA was found in virtually all organs except the brain with the VSV-G singly targeted lentivirus. In comparison, when using the dual-targeted vector, the vector DNA was found predominantly in the grafted tumor, with diminished levels in the liver and spleen and negligible amounts in all other organs. These data highlight the advantage of applying transductional targeting in selective gene transfer into targeted tumor that can lead to a reduction in the risk of insertional mutagenesis in normal, nontargeted organs. Of significant impact is the system in which the authors demonstrated, as a proof of principle, targeting of prostate bone metastases. Prostate cancer is the second leading cause of cancer-related death among U.S. men, and bony metastases are developed at the end stage of the disease that contribute to serious clinical complications and morbidity.15Logothetis CJ Lin SH Osteoblasts in prostate cancer metastasis to bone.Nat Rev Cancer. 2005; 5: 21-28Crossref PubMed Scopus (456) Google Scholar Figure 1 illustrates some of the aggressive features of prostate cancer bone lesion. Bone metastases are extremely hard to treat because of variables such as the structural barrier of bone, the unique microenvironment of tumor cells and bone interphase, and the existence of multiple metastatic lesions. Hence, the dual-targeting technology developed by Pariente et al. that can achieve gene delivery to the bony lesions holds promising prospects to improve the clinical outcome.16Bagi CM Targeting of therapeutic agents to bone to treat metastatic cancer.Adv Drug Deliv Rev. 2005; 57: 995-1010Crossref PubMed Scopus (57) Google Scholar Eventual implementation of this technology in the clinic requires further improvement of the vector in several areas. For example, the envelope modifications of the dual-targeted vector may have slightly diminished its infectivity in general, resulting in a lower luciferase expression level as compared with the VSV-G pseudotyped control vector. As the authors noted, boosting the activity of the tissue-specific promoter could be one way to compensate for the lowered infectivity. Additionally, the transductional modification scheme utilized in this model relies heavily on the incorporation of a single antibody; therefore, caution should be applied in the selection of the antibody candidate. One must take into account the heterogeneous nature of the diseased cells as well as the specificity of the antibody. This is particularly relevant to cancer treatment because many types of cancer lack a universally expressed surface biomarker.17Weinberg RA The Biology of Cancer. Garland Science, New York2007Google Scholar Perhaps a viral vector mixture containing a cocktail of several surface and genetic modifications is needed to address the heterogeneity of the disease. A third issue that should be addressed in future studies is the impact of the host immune system on the dual-targeted vector. Additional experimentation in immune-competent animal models will be critical for further advancement of this technology. Collectively, the work by Pariente et al. marks an important step in the right direction toward improving the safety of lentiviral vectors and their application in targeted gene medicine.