Quantum dot-trastuzumab

Epidermal growth factor (EGF) is a 53-amino acid cytokine (6.2 kDa) secreted by ectodermic cells, monocytes, kidneys and duodenal glands (1). EGF stimulates growth of epidermal and epithelial cells. EGF with at least seven other growth factors and their transmembrane receptor kinases play important roles in cell proliferation, survival, adhesion, migration and differentiation. The EGF receptor (EGFR) family consists of four transmembrane receptors, including EGFR (HER1/erbB-1), HER2 (erbB-2/neu), HER3 (erbB-3) and HER4 (erbB-4) (2). HER1, HER3 and HER4 comprise three major functional domains: an extracellular ligand-binding domain, a hydrophobic transmembrane domain and a cytoplasmic tyrosine kinase domain. No ligand has been clearly identified for HER2. However, HER2 can be activated as a result of ligand binding to other HER receptors with the formation of receptor homodimers and/or heterodimers (3). HER1 as well as HER2 are overexpressed on many solid tumor cells such as breast, non-small-cell lung, head and neck, and colon cancer (4-6). The high levels of HER1 and HER2 expression on cancer cells are associated with a poor prognosis (7-10). Optical fluorescence imaging is increasingly used to monitor biological functions of specific targets in small animals (11-13). However, the intrinsic fluorescence of biomolecules poses a problem when fluorophores that absorb visible light (350-700 nm) are used. Near-infrared (NIR) fluorescence (700-1,000 nm) detection avoids the background fluorescence interference of natural biomolecules, providing a high contrast between target and background tissues. NIR fluorophores have a 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 fluorescence 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 (14, 15). Fluorescent semiconductor quantum dots (QDs) are nanocrystals made of CdSe/CdTe-ZnS with radii of 1-10 nm (16-18). 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 and (18-22). Although many cells have been labeled with QDs with little cytotoxicity, there are only limited studies of long-term toxicity of QDs in small animals (23-31). However, little is known about the toxicity and the mechanisms of clearance and metabolism of QDs in humans. Trastuzumab is a humanized IgG monoclonal antibody (mAb) against the extracellular domain of recombinant HER2 with an affinity constant () of 0.1 nM (32). Cardiotoxicity is the most serious complication of using trastuzumab in humans with breast cancer (33). One potential application of a radiolabeled anti-HER2 MAb is the pretreatment imaging of breast cancer patients to predict the therapeutic efficacy of trastuzumab. In-Trastuzumab, Cy5.5-trastuzumab, and Ga-trastuzumab -F(ab') have been developed for imaging of human breast cancer (34-38). Trastuzumab has also been successfully coupled with quantum dots for optical imaging of HER2 in tumors in mice (39).