Self-inactivating helper virus for the production of high-capacity adenoviral vectors

Standard methods for producing high-capacity adenoviral vectors (HC-Ads) are based on co-infection with a helper adenovirus (HV). To avoid HV encapsidation, its packaging signal (Ψ) is flanked by recognition sequences for recombinases expressed in the producing cells. However, accumulation of HV and low yield of HC-Ad are frequently observed, due in part to insufficient recombinase expression. We describe here a novel HV (AdTetCre) in which Ψ is flanked by loxP sites that can be excised by a chimeric MerCreMer recombinase encoded in the same viral genome. Efficient modulation of cleavage was obtained by simultaneous control of MerCreMer expression using a tet-on inducible system, and translocation to the nucleus by 4-hydroxytamoxifen (TAM). Encapsidation of AdTetCre was strongly inhibited by TAM plus doxycicline. Using AdTetCre and 293Cre4 cells for the production of HC-Ads, we found that cellular and virus-encoded recombinases cooperate to minimize HV contamination. The method was highly reproducible and allowed the routine production of different HC-Ads in a medium-scale laboratory setting in adherent cells, with titers >10¹⁰ infectious units and <0.1% HV contamination. The residual HVs lacked Ψ and were highly attenuated. We conclude that self-inactivating HVs based on virally encoded recombinases are promising tools for the production of HC-Ads.

Deletion of the E3-6.7K/gp19K region reduces the persistence of wild-type adenovirus in a permissive tumor model in Syrian hamsters

A partial deletion of the adenovirus E3 region, comprising the overlapping 6.7K/gp19K genes, has been described for the incorporation of therapeutic genes in 'armed' oncolytic adenoviruses. This deletion allows the insertion of up to 2.5 kb genetic material into the virus and ensures strong expression of transgenes without reducing the replication and cytolytic potency of viruses in vitro. E3-gp19K and 6.7K proteins are involved in avoiding recognition and elimination of infected cells by the host immune system. Therefore, we have studied the effect of this deletion on the replication and transgene expression of the virus in immunocompetent models based on Syrian hamsters. Tumors were established by intrahepatic injection of pancreatic cancer cells with moderate (HaP-T1, HP-1) or low (H2T) permissivity for adenovirus replication. The wild-type human adenovirus 5 (Ad5) or a modified version containing the luciferase gene in the E3-6.7K/gp19K locus (Ad-WTLuc) were injected intratumorally. We found that elimination of Ad-WTLuc was faster than Ad5 in HaP-T1 and HP-1 tumors. In contrast, no differences were observed when the same tumor was established in severely immunocompromised NOD-scid IL2Rgamma(null) mice. In addition, virus-mediated luciferase expression was more stable in these animals. These results suggest that the lack of E3-6.7K/gp19K genes may accelerate the clearance of oncolytic adenoviruses in some immunocompetent tumor models.

Comparison of molecular strategies for breast cancer virotherapy using oncolytic adenovirus

Oncolytic viruses are regulated by the tumor phenotype to replicate and lyse cancer cells selectively. To identify optimal strategies for breast cancer we compared five adenoviruses with distinct regulatory mechanisms: Ad-dl922-947 (targets G1-S checkpoint); Ad-Onyx-015 and Ad-Onyx-017 (target p53/mRNA export); Ad-vKH1 (targets Wnt pathway), and AdEHE2F (targets estrogen receptor/G1-S checkpoint/hypoxic signaling). The quantity of virus required to kill 50% of breast cancer cells after 6 days (EC(50), plaque-forming units per cell) was measured. The most potent virus was Ad-dl922-947 (EC(50), 0.01-5.4 in SkBr3, MDA-231, MDA-468, MCF7, and ZR75.1 cells), followed by wild-type (Ad-WT; EC(50), 0.3-5.5) and AdEHE2F (EC(50), 1.4-3.9). Ad-vKH1 (EC(50), 7.2-72.1), Ad-Onyx-017 (EC(50), 8.4-167), and Ad-Onyx-015 (EC(50), 17.7-377) showed less activity. Most viruses showed limited cytotoxicity in normal human cells, including breast epithelium MCF10A (EC(50), >722) and fibroblasts (EC(50), >192) and only moderate cytotoxicity in normal microvascular endothelial cells (HMVECs; EC(50), 42.8-149), except Ad-dl922-947, which was active in HMVECs (EC(50), 1.6). After injection into MDA-231 xenografts, Ad-WT, AdEHE2F, and Ad-dl922-947 showed replication, assessed by hexon staining and quantitative polymerase chain reaction measurement of viral DNA, and significantly inhibited tumor growth, leading to extended survival. After intravenous injection Ad-dl922-947 showed DNA replication (233% of the injected dose was measured in liver after 3 days) whereas AdEHE2F did not. Overall, AdEHE2F showed the best combination of low toxicity in normal cells and high activity in breast cancer in vitro and in vivo, suggesting that molecular targeting using estrogen response elements, hypoxia response elements, and a dysregulated G1-S checkpoint is a promising strategy for virotherapy of breast cancer.

An oncolytic adenovirus controlled by a modified telomerase promoter is attenuated in telomerase-negative cells, but shows reduced activity in cancer cells

The promoter for human telomerase reverse transcriptase (hTERTp) is preferentially active in malignant cells. It was recently used to control the expression of the adenoviral E1A gene for the development of oncolytic adenoviruses. To ensure maximal repression in normal cells, the inclusion of additional E-boxes in the proximal region of the core promoter was described. We found that the transcriptional activity of this artificial sequence (T-255-4DEB) is minimal in normal cells, but it is also reduced in all the cancer cell lines tested. The cancer specificity of a new oncolytic adenovirus based in this promoter (AdTE1) was evaluated by direct comparison with wild-type adenovirus type 5 (AdWT) in vitro and in vivo. In all the parameters tested, AdTE1 was attenuated in normal cells, but the efficacy in cancer cells showed a parallel reduction, suggesting a lack of specificity. However, the cytotoxicity of AdTE1 was repressed in senescent cells compared to AdWT. Therefore, we conclude that AdTE1 is preferentially attenuated only in cells that are permanently devoid of telomerase expression such as senescent cells. Further modifications in the telomerase-based promoters should be introduced in order to combine maximal attenuation of oncolytic adenoviruses in normal tissues and enhanced activity in tumors.

Evaluation of a new dual-specificity promoter for selective induction of apoptosis in breast cancer cells

The conditional expression of lethal genes in tumor cells is a promising gene therapy approach for the treatment of cancer. The identification of promoters that are preferentially active in cancer cells is the starting point for this strategy. The combination of tissue-specific and tumor-specific elements offers the possibility to artificially develop such promoters. We describe the construction and characterization of a hybrid promoter for transcriptional targeting of breast cancer. In many cases, breast cancer cells retain the expression of estrogen receptors, and most solid tumors suffer from hypoxia as a consequence of their aberrant vascularization. Estrogen response elements and hypoxia-responsive elements were combined to activate transcription in cells that present at least one of these characteristics. When a promoter containing these elements is used to control the expression of the pro-apoptotic gene harakiri, the induction of cell death can be activated by estrogens and hypoxia, and inhibited by antiestrogens such as tamoxifen. Finally, we show evidence that these properties are maintained in the context of an adenoviral vector (AdEHhrk). Therefore, infection with this virus preferentially kills estrogen receptor-positive breast cancer cells, or cells growing under hypoxic conditions. We propose the use of this promoter for transcriptional targeting of breast cancer.

A novel, conditionally replicative adenovirus for the treatment of breast cancer that allows controlled replication of E1a-deleted adenoviral vectors

The efficiency of gene therapy strategies against cancer is limited by the poor distribution of the vectors in the malignant tissues. To solve this problem, a new generation of tumor-specific, conditionally replicative adenoviruses is being developed. To direct the replication of the virus to breast cancer, we have considered one characteristic present in a great proportion of these cancers, which is the expression of estrogen receptors (ERs). On the basis of the wild-type adenovirus type 5, we have constructed a conditionally replicative adenovirus (Ad5ERE2) in which the E1a and E4 promoters have been replaced by a portion of the pS2 promoter containing two estrogen-responsive elements (EREs). This promoter induces transcriptional activation of the E1a and E4 units in response to estrogens in cells that express the ERs. Ad5ERE2 is able to kill ER(+) human breast cancer cell lines as efficiently as the wild-type virus, but has decreased capacity to affect ER(-) cells. By complementation of the E1a protein in trans, Ad5ERE2 allows restricted replication of a conventional E1a-deleted adenoviral vector. When a virus expressing the proapoptotic gene Bc1-xs (Clarke et al., Proc. Natl. Acad. Sci. U.S.A. 1995;92:11024-11028) is used in combination with Ad5ERE2, the ability of both viruses to induce cell death is dramatically increased, and the effect can be modulated by addition of the antiestrogen tamoxifen.