Multiphoton microscopy provides in vivo
imaging with high spatial resolution, but concurrent imaging of multiple species is difficult with standard fluorophores. In this study, we have shown that the strengths of quantum dots match the demands inherent to intravital microscopy. The quantum dots are flexible fluorescent probes that can be excited concurrently with, and tuned away from, signals resulting from GFP and SHG. Unlike dextran conjugates that collect within the interstitium24
and impede clean demarcation of the vessel wall, the use of quantum dots as extrinsic fluorophores allows vessels to be both morphologically and spectrally distinct.
This work also shows the utility of the new, robust quantum dot–microsphere composites for optimizing delivery vehicles. As noted previously16
, the procedure used to prepare these materials enables preparation of a wide range of monodisperse submicron sphere sizes. The ability to tune not only the size but also the surface characteristics of these quantum dot composites through silica chemistry provides an additional handle for screening a large number of drug delivery parameters for targeting efficacy. Moreover, the use of multiphoton intravital microscopy with these fluorescent materials shows local inhomogeneities in both tumor components and delivery; information that is not provided by tumor-averaged techniques9
. Similarly, for cell trafficking studies, one may use different quantum dots to label subpopulations of bone marrow–derived cells (or progenitor cells isolated from different mutant animals) to investigate the degree to which the vascular and perivascular structures are formed or remodeled in response to cell homing.
Introducing this technology to a clinical setting will depend crucially upon biocompatibility and safety of the quantum dot formulation. The micelle quantum dot preparation used in this study was previously shown to be nontoxic to developing Xenopus
. Several studies indicate that other encapsulation chemistries render quantum dots biocompatible in cell culture25
. Of note, we infused nanomole quantities of quantum dot preparations in mice and noted no obvious adverse effects for up to 1 month. Thus, we may someday exploit the special synergy of multiphoton intravital microscopy and quantum dots as a clinical diagnostic and prognostic tool for cancer prevention and treatment.