Abstract 5481: Theranostic imaging of metastatic prostate cancer

There is a compelling need to find effective treatments for metastatic disease. The ideal therapy would target cancer cells while sparing normal tissue. With prodrug therapy, where a drug-activating enzyme is delivered to the tumor followed by the administration of a non-toxic prodrug administered systemically, side effects can be minimized. We previously synthesized a prototype agent consisting of the enzyme bacterial cytosine deaminase (bCD), that converts prodrug 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU), labeled with MR and optical imaging reporters. Here we have developed a prototype targeted nanoplex that delivers bCD together with siRNA for theranostic imaging of metastatic prostate cancer. Prostate-specific membrane antigen (PSMA) has an abundant expression on the surface of prostate carcinomas. We have therefore incorporated a low molecular weight PSMA binding agent to target the nanoplex to prostate cancer cells. Since choline kinase (Chk) is significantly upregulated in aggressive prostate cancer cells we have used siRNA against Chk in the nanoplex. Our nanoplex was synthesized by conjugating three compartments: (i) the prodrug-activating enzyme bCD, (ii) the poly-L-lysine (PLL) that carries [ 111 In]DOTA for SPECT or [Gd 3+ ]DOTA for MRI and a near-infrared fluorescent probe Cy5.5, (iii) the siRNA delivery vector: PEI (polyethyleneimine)-PEG (polythethyeneglycol) co-grafted-polymer. These compartments are covalently conjugated and siRNA-Chk is associated with the PEI-PEG co-grafted polymer through electrostatic affiliation. For the targeted nanoplexes, a low molecular weight urea-based PSMA targeting moiety (2-(3-[1-carboxy-5-[7-(2,5-dioxo-pyrrolidin-1-yloxycarbonyl)-heptanoylamino]-pentyl]-ureido)-pentanedioic acid was used. Imaging studies with PSMA targeted nanoplexes were performed with PC-3 human prostate cancer xenografts genetically engineered to overexpress PSMA (PC-3 PIP) and with LNCaP human prostate cancer xenografts. Non-PSMA expressing PC-3 xenografts were used as controls. MR experiments were performed on a Bruker horizontal bore 9.4T animal MR scanner. SPECT/CT images were acquired on a Gamma Medica X-SPECT scanner. Initial studies with a nontargeted nanoplex carrying bCD and Chk siRNA established the ability of the nanoplex to effectively downregulate Chk and total choline and to convert the nontoxic prodrug 5-FC to 5-FU. By using SPECT and MRI, we found that the addition of a PSMA targeting moiety allowed specific targeting of the nanoplex to PSMA expressing prostate cancer cells in vivo. The targeted nanoplexes we develop will allow us to deliver siRNA together with a prodrug enzyme, under image guidance for developing theranostic imaging of metastatic prostate cancer. The strategies developed here can be extended, in the future, to down-regulate multi-drug resistance pathways, or repair enzymes to increase the efficiency of therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5481.