(604d) Enzyme Triggered, Nanoparticle Controlled Fluorescence Emission for Sensitive and Specific Breast Cancer Detection

Nanoparticles with high plasmon field strength can be beneficially used to increase the sensitivity and specificity of fluorescent contrast agent. A good exemplary nanoparticle for this purpose, can be used biological system, is gold nanoparticle (GNP). Plasmon is the strongest at the particle surface and decrease with the distance from the surface. The level of fluorescence change depends on the plasmon field strength. When a fluorophore is placed in a very strong field, i.e., very close to the particle surface, the fluorescence is quenched. At a particular field strength, i.e., at a particular distance away from the particle (depending upon the GNP size and the fluorophore), the fluorescence can be enhanced. We have achieved fluorescence alterations from quenching to enhancement by conjugating fluorophore to GNP via molecular spacers at various lengths.

Utilizing this fluorescence manipulation, we have designed a nano-sized, optical contrast agent for breast cancer imaging. A cancer specific targeting molecule is conjugated onto the GNP to guide it to cancer cell. The fluorophore is conjugated to this GNP via a short molecular spacer. The spacer has a special molecular sequence that can be cleaved by an enzyme secreted by cancer cells. Initially, the entity does not emit fluorescence because the GNP quenches the fluorescence. When this nano-entity is delivered to the cancer cells, guided by the targeting molecule, the short spacer is then cleaved by the enzyme secreted by the cancer cell, and the fluorescence gets restored.

As our model system, Tamoxifen is used as cancer targeting molecule. Cypate is our fluorophore, and it is a derivative of Indocyanine Green (FDA approved, near-infrared). A short peptide with a G-G-R motif is used as the short spacer. This motif can be cleaved by urokinase-type plasminogen activator (uPA), secreted by breast cancer cell.

In the same contrast agent described above, fluorescence enhancing ability of GNP can also be incorporated. The fluorophore is conjugated to GNP via two spacers instead of one. One spacer is the short spacer with enzyme cleavable motif, and the other is a long spacer. When the contrast agent is delivered to cancer via targeting molecule, and short spacer is cleaved by the enzyme, the distance between the fluorophore and the GNP will be determined by the long spacer at a length for maximal fluorescence enhancement.

This contrast agent targets to the cancer cell and emits light only when the spacer is cleaved, providing dual specificity. It minimizes non-specific signals, enabling more accurate cancer detection. Also, the fluorescence is emitted at an enhanced level, giving high sensitivity. The enzyme specific fluorescence trigger mechanism can be used to characterize cancer type (diagnosis).

The authors acknowledge the financial support from U.S. Army Breast Cancer Program (BC074387).