Replicating murine leukemia virus (RMLV)-based vectors can propagate specifically within actively dividing cancer cells and achieve highly efficient and tumor-selective gene delivery. However, in view of the known potential for retroviral integration-mediated genotoxicity, additional mechanisms to target the virus specifically to cancer cells are desirable.

To further improve their tumor specificity and safety profile, we have developed RMLV vectors regulated by two different tissue-specific promoters.

We generated GFP-expressing RMLV vectors ACE-emd (wild type), ACE-At-emd (regulated by ARR2PB promoter, a synthetic variant of the probasin promoter which exhibits high specificity for androgen receptor (AR)-positive prostate cancer cells) and ACE-PSES-emd (regulated by PSES promoter, an androgen-independent promoter which is highly active in PSA/PSMA-positive prostate cancer cells, both in the presence and absence of androgen). We also developed ACE-yCD, ACE-At-yCD and ACE-PSES-yCD carrying the yeast cytosine deaminase (yCD) prodrug-activator gene, which converts the nontoxic prodrug 5-fluorocytosine (5FC) into the chemotoxin 5-fluorouracil.

These vectors were first in vitro tested in both prostate (LNCaP, MDA PCa 2b and PC-3) and non-prostate (293T) cell lines in the presence and absence of androgen. RMLV vector ACE-At-emd exhibited high specificity and robust replication in AR-positive prostate cancer cell lines, but only in the presence of androgen. RMLV vector ACE-PSES-emd exhibited high specificity for PSA/PSMA-positive prostate cancer cell line both in the presence and absence of androgen.

For in vivo studies, male nude mice were inoculated subcutaneously with LNCaP cells (AR-positive, PSA/PSMA-positive) in Matrigel. RMLV vectors expressing GFP were injected intratumorally, and tumors were analyzed by flow cytometry 2 and 4 weeks after vector injection. The results showed that both ARR2PB- and PSES-regulated RMLV vectors achieve efficient gene delivery to LNCaP s.c. tumors in vivo.

Cytotoxic activity of RMLV-yCD vectors was examined in vitro in LNCaP cells in the presence and absence of 5FC after virus infection. Cell-specific expression of each vector correlated with reduced cell viability compared to uninfected cells after 5FC treatment. We are now conducting studies to evaluate therapeutic efficacy vs. potential genotoxicity of these vectors for prodrug-activator gene therapy in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5393. doi:10.1158/1538-7445.AM2011-5393