No matter how specific a drug is and how effective it is at killing cancer cells in vitro, delivering it to a real tumour is an entirely different challenge. The drug must make its way through the vasculature, cross the vascular wall, and then struggle through the tumour's extracellular matrix (ECM). Rakesh Jain and colleagues have adapted and optimized a new technology that allows them to determine the effects of these barriers on diffusion.

Two-photon fluorescence-correlation microscopy (TPFCM) is a three-dimensional microscopy technique that allows in vivo measurements of transport parameters in tumours to be made. TPFCM enables researchers to measure the concentration and diffusion of fluorescently labelled molecules within heterogeneous samples, such as tumours, or even within cells.

Using TPFCM, Jain's group made in vivo measurements of the diffusion coefficients of fluorescently labelled macromolecules and liposomes. They found that these tracers underwent both a slow and a fast component of diffusion. The tumour interstitial matrix is thought to be composed of two phases — viscous and aqueous. This is the first direct evidence that these two phases affect the transport of molecules within the tumour matrix.

The hyaluronan and collagen components of the ECM are thought to be the main barriers to drug delivery, so some researchers have proposed treating tumours with enzymes that degrade these structures. Jain and colleagues exposed tumours to hyaluronidase and collagenase and found that, predictably, collagenase increased the fraction of the fast-diffusing component. Conversely, hyaluronidase treatment reduced the percentage of fast-diffusing molecules. The authors propose that as hyaluronan forms a cage-like structure that contains water-filled spaces, through which molecules diffuse quickly, collapsing these structures is likely to increase viscous hindrance. Hyaluronidase would, therefore, not facilitate drug delivery.

Jain and colleagues hope to use this system to investigate other barriers to drug delivery in vivo.