In vitro screening of anticancer drug penetration into tumor tissue: taxanes

5936 The failure of many anticancer drugs to control the growth of solid cancers may stem in part from inadequate delivery to tumour regions distant from vasculature. In this study we use a novel, effect-based assay that employs multilayered cell culture (MCC), a tissue-engineered disc grown from tumour cells on a permeable membrane. MCCs are similar to multicellular spheroids but their planar nature permits flux through the cultures to be more easily measured. Like spheroids they exhibit a gradient in proliferation but since MCCs grow as discs, this gradient forms as a mirror image from either side towards the middle. In this study, we exploit this symmetry by exposing MCCs to drugs from one side and then comparing their effect on the exposed side versus the far side of the cultures. This approach circumvents issues that normally arise from the biochemical gradients that occur with distance into tissue (e.g. changing intrinsic sensitivity of cells to drugs with depth into tissue). In this study we have compared the tissue penetration of the microtubule targeting agents paclitaxel and docetaxel. The distribution of drug effect within the cultures was assessed 2 days after exposure via immunodetection of S-phase cells using bromodeoxyuridine. Using an automated computer analysis routine, the effect of the drugs in the first three cell layers located on either edge of the cultures relative to controls was then assessed. Paclitaxel exhibited better tissue penetration, exerting a more equal effect within the cultures, than docetaxel. Over the concentration range studied, 0.03 - 3 μM, paclitaxel exhibited a ∼3-fold gradient in drug concentration across the MCCs while docetaxel was greater than 10-fold. As expected, docetaxel exhibited a greater effect on the side directly exposed to drug as compared with paclitaxel, but showed less effect on the far sides. This was consistent with its higher rate of cellular uptake relative to paclitaxel leading to increased consumption by the first layers of cells and resulting in less drug reaching further into the tissue. Only at concentrations greater than 1 μM was docetaxel able to exert an equal effect on both sides. Interestingly, MCC growth-delay, as measured by thickness, was similar for both drugs. Results obtained with the MCCs will be compared with xenograft experiments in which S-phase cells are mapped in relation to tumour vasculature. This model could be applied as a screening system for the discovery of biologically active drugs, which exhibit desirable penetration properties. This research is supported by the Canadian Institutes for Health Research and the Michael Smith Foundation for Health Research.

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