Quantum dot-prostate-specific membrane antigen antibody J591

Excerpt

Optical fluorescence imaging is increasingly used to monitor biological functions of specific targets in small animals (1-3). However, the intrinsic fluorescence of biomolecules poses a problem when visible light (350-700 nm) absorbing fluorophores are used. Near-infrared (NIR) fluorescence (700-1000 nm) detection avoids the background fluorescence interference of natural biomolecules, providing a high contrast between target and background tissues. NIR fluorophores have wider dynamic range and minimal background as a result of reduced scattering compared with visible fluorescence detection. They also have high sensitivity, resulting from low infrared background, and high extinction coefficients, which provide high quantum yields. The NIR region is also compatible with solid-state optical components, such as diode lasers and silicon detectors. NIR fluorescence imaging is becoming a non-invasive alternative to radionuclide imaging in small animals (4).

Fluorescent semiconductor quantum dots (QDs) are nanocrystals made of CdSe/CdTe-ZnS with radii of 1-10 nm (5-7). They can be tuned to emit in a range of wavelengths by changing their sizes and composition, thus providing broad excitation profiles and high absorption coefficients. They have narrow and symmetric emission spectra with long, excited-state lifetimes, 20-50 ns, as compared with 1-10 ns of fluorescent dyes. They process good quantum yields of 40-90% and high extinction coefficients. They are more photo-stable than conventional organic dyes. They can be coated and capped with hydrophilic materials for additional conjugations with biomolecules, such as peptides, antibodies, nucleic acids, and small organic compounds, which were tested in vitro and in vivo (7-11). Although many cells have been labeled with QDs in vitro with little cytotoxicity, there are only limited studies of long-term toxicity of QDs in small animals (12-20). However, little is known about the toxicity and the mechanisms of clearance and metabolism of QDs in humans.

Prostate-specific membrane antigen (PSMA) is a cell-surface glycoprotein with a molecular weight of ~100 kDa. It is a unique, type II, transmembrane-bound glycoprotein that is overexpressed on prostate tumor cells and in the neovasculature of most solid prostate tumors, but not in the vasculature of normal tissues (21, 22). This unique expression of PSMA makes it an important biomarker as well as a large extracellular target of imaging agents (23, 24). PSMA has also been detected in other tissues such as the kidneys, the proximal small intestine, and the salivary glands (2). PSMA was found to have N-acetyl α-linked acidic dipeptidase (NAALADase) or glutamate carboxypeptidase II activity (3). PSMA may play an important role in the progression of prostate cancer and glutamatergic neurotransmission, as well as in the absorption of folate (4). In the central nervous system, PSMA metabolizes N-acetyl-aspartyl-glutamate, and in the proximal small intestine it removes γ-linked glutamates from poly-γ-glutamate folate and folate hydrolase (2). PSMA can be used as a marker for the detection of metastatic cancers with imaging agents. Although a commercially available monoclonal antibody (¹¹¹In-labeled Capromomab pendetide (¹¹¹In-CYT-356)) is in clinical use for the detection of prostate cancer, the results obtained with this antibody are not entirely reliable (5). In addition, this antibody has limited access to tumors and may produce low signal/noise ratios because the target is the intracellular domain of PSMA (6, 7). J591, a monoclonal antibody against the extracellular domain of PSMA (25), was conjugated to QD655 (QD655-J591) and accumulated in a human prostate cancer cell line in vitro and in nude mice (15).