Great expectations for in vivo vasculature imaging

Optical-based in vivo imaging of vasculature is an emerging modality for studying vascular structure, function and angiogenesis. Changes in vascularization and vascular dysfunction have been implicated in many diseases, such as cardiovascular and kidney disease, as well as in many cancers, making this an area of intense interest. Vascular imaging provides information about the number and spacing of vessels, permeability of the vasculature and functional abnormalities of the vessels. Vascular imaging is of particular importance in tumor biology where imaging can be used to assess the efficacy of anti-tumor and anti-angiogenesis drugs. Imaging agents that fluoresce in the Near IR are particularly useful as vascular contrast agents as they avoid the signal interference due to hemoglobin absorbance. Vascular contrast agents are transient imaging reagents with imaging time windows ranging from a few minutes to several hours, depending on the properties of the probe. The choice of a contrast agent is dictated by the experimental model and the imaging protocol.

Vascular contrast reagents to fit your experimental needs

Each of the vascular contrast agents shows distinctly different behaviors when imaged in murine vasculature; this difference is emphasized in xenograft bearing mice. The extravasation rate for Alexa Fluor® conjugated BSA appears to the quickest followed by Alexa Fluor® conjugated transferrin. It is of interest to note that the Qtracker Non-Targeted nanocrystals show little or no extravasation from the vasculature up to two hours post introduction into the animal model. It is, therefore, prudent to investigate these vascular contrast agents individually in each specific animal model to determine their best experimental fit. These reagents can be used separately, or in combination  if spectral deconvolution is available on the in vivo imaging instrument.

Conjugates to measure capillary leakage

SAIVI™ Alexa Fluor® 680 and Alexa Fluor® 750 Injectable Contrast Agents Serum proteins albumin and transferrin are common tumor markers that also play a role of transport of cytostatic agents into tumors. By conjugating BSA and transferrin to Molecular Probes™ near infrared (NIR) dyes, these are an effective tool for cancer research.

These contrast agents will accumulate in tumors, making visualization of tumors in small animals easy. Within 60 minutes of injection, these agents will clearly delineate the blood vessels in tumors. Over time the BSA and transferrin conjugates will readily leak from the capillaries and diffuse into the interstitial space, and thus more generally highlight the tumor. This can be particularly useful in studies to look at the effects of a cancer drug on the vasculature or to compare normal and the more highly leaky tumor vasculature.

In most cases the BSA contrast agent is recommended due to the speed of the labeling. However, if using a human product is important to you transferrin is a good alternative agent. Also, some tumors may have a high level of transferrin receptors and this will enhance the labeling, and thus the transferrin reagent can be used as a tumor marker in transferrin positive cancers.

SAIVI™ In Action

SAIVI™ In Action

View a short video that demonstrates the vascular labeling and capillary leakage in a human tumor xenograft in a living mouse with the SAIVI™ Alexa Fluor® 750 injectable Contrast Agent *bovine serum albumin* as imaged using the CRi Maestro™ imaging system.

Distinguish vasculature using non-targeted Qdot® nanocrystals

 Ideal for vascular imaging, the Qtracker® non-targeted quantum dots provide a novel in vivo solution for viewing normal vasculature down to the capillaries. In contrast to the BSA and transferrin conjugated vasculature agents, the Qdot® nanocrystals have minimal leakage from the vasculature, and allow imaging for at least 3 hours.

The Qtracker® non-targeted quantum dots will specifically label just the vessels, and are retained within the vessels, as shown in Figure 1.

Distinguish vasculature using non-targeted Qdot® nanocrystals  

Figure 1.
  Qtracker® non-targeted quantum dots 800 labeling mouse vasculature  in vivo. Image was obtained using the CRi Maestro™ imaging system.

Get more information by multiplexing

It has been demonstrated that using non-targeted quantum dots as a vascular contrast agent along with Alexa Fluor® 680 labeled anti-CEA antibody (Zymed Invitrogen) to label a tumor mass, that a composite image can be generated to show tumor vascularization in a mouse xenograft model. (Figure 2)
Tumor cells and vasculature
Figure 2. Multiplex imaging of tumor cells and vasculature

Tumor cells are labeled with Anti-CEA Ab conjugated to Alexa Fluor® 680. The vasculature is labeled with Qtracker® 655 non-targeted quantum dots. Images were obtained using the CRi Maestro™ imaging system.