Over the past two decades, molecular targeted diagnostic and therapeutic agents have dramatically improved cancer diagnosis and treatment. [1][2][3][4][5][6][7][8] Targeting allows the preferential delivery of therapeutic, diagnostic, or imaging agents to the intended site. Advances in nanotechnology have enabled the development of a variety of targeted nanoparticle platforms for diagnostic and therapeutic applications. [9][10][11] Preclinical data have shown that targeted nanoparticle systems accumulate preferentially in the target tissue, demonstrating the vast potential of targeted nanoparticles. [12][13][14] In addition, the development of multifunctional nanoparticle platforms, with both diagnostic and therapeutic capabilities, may allow in vivo monitoring of both biodistribution of the nanocarriers and tumor response to therapy. [11,[15][16][17] Therefore, research efforts have been focused on the further development of multifunctional molecular agents for the diagnosis and treatment of cancer.
One of the most promising diagnostic agents is superparamagnetic iron oxide nanoparticles (SPION). [18] SPION have several important advantages over traditional gadolinium-based magnetic resonance (MR) contrast agents: lower toxicity, stronger enhancement of proton relaxation, and lower detection limit. [19,20] Ferumoxtran-10 (Combidex), a dextran-coated SPION with a mean diameter of ~30 nm, is currently in phase III clinical trials for prostate cancer (PCa) imaging. [21] Combidex has a 90.5 % sensitivity and 97.8 % specificity for detecting PCa lymph node disease by passively accumulating in metastatic nodes. [22] The major shortcoming of Combidex is its inability to detect PCa disease outside of the lymph nodes.
Herein, we report the development of a novel, multifunctional, thermally cross-linked SPION (TCL-SPION) that can both detect PCa cells, and deliver targeted chemotherapeutic agents directly to the PCa cells. We previously reported the use of the A10 RNA aptamer (Apt), which binds the extracellular domain of the prostate-specific membrane antigen (PSMA), to engineer targeted nanoparticles for PCa therapy and imaging. [12,13,23] PSMA is a well-established marker for PCa cells, with relatively low levels of expression in normal prostate, kidney, brain, and small intestine tissue. [24] The percentage of PCa cells that express PSMA approaches 100 % with highest expression in androgen-independent PCa cells. [25,26] Additionally, we have shown that the A10 aptamer can be used to deliver doxorubicin (Dox), a chemotherapeutic agent, by intercalation of Dox into the CG sequence in the aptamer. [23,27,28] By combining the above concepts, we have formulated SPION-Apt bioconjugates for combined PCa imaging and therapy. The components of the nanoparticle include: a) N-terminated A10 aptamer, a 57bp nuclease-stabilized 2'-fluoropyrimidine RNA molecule modified with C 18 -amine at the 3' end, for targeting PSMA-expressing PCa cells, and acting as a carrier for Dox; b) TCL-SPION coated with a carboxylic acid-PEG-derived, anti-biofouling polymer, [29] which acts as both a MR contrast agent and as a carrier for Dox; and c) Dox, a chemotherapeutic agent that is intercalated in the aptamer and complexed with the TCL-SPION through charge interactions. The hydroxy and carbonyl groups on the surface of the TCL-SPION make them apt for the formulation of targeted nanoparticle platforms. The PEGylated surface prevents protein and cell adsorption, while the carboxyl groups allow conjugation of targeting moieties, like the A10 aptamer. TCL-SPIONs are also well suited for therapeutic delivery because of their low toxicity profiles. [30][31][32] Conjugation of the TCL-SPION with an A10 aptamer, using standard coupling chemistry, gave the TCL-SPION-Apt bioconjugate formulation (Figure 1 a); conjugation led to an increase in both size (60.8 AE 1.9 to 66.4 AE 1.5 nm), and z-potential