Preclinical assessment of anti-cancer therapeutic strategies using in vivovideomicroscopy

Preclinical in vivo studies of agents targeted against metastasis have to d ate been based primarily on end-point assays. Such assays can determine whe ther a treatment affects the number or size of metastases in an organ at a given time, but are poorly suited to determining how and at what stage in t he process the treatment affected the end point. High resolution in vivo vi deomicroscopy permits direct observation of the process of metastasis as it occurs in living animals over time. Studies based on this technique and a cell accounting procedure we have devised, have shown that early steps in t he metastatic process (survival in the circulation, extravasation) contribu te relatively little to cell loss and metastatic inefficiency. Steps that o ccur after extravasation appear to be primarily responsible for the signifi cant losses that result in metastatic inefficiency, and these steps may rep resent good targets for the design of new antimetastatic therapies. Matrix metalloproteinases have been implicated functionally in metastasis, and are viewed as an appropriate target in the development of inhibitors of metast asis. Using both endogenous and synthetic exogenous metalloproteinase inhib itors, we have shown that the inhibition of metastasis which these agents p roduce is not due to inhibition of cell extravasation from the circulation into the tissue, but to reduction of angiogenesis within metastases. A simi lar conclusion was reached concerning the mechanism of action, on metastasi s, of carboxyamidotriazole, an inhibitor of calcium-mediated signal transdu ction which is currently in Phase II single agent clinical trials. In vivo videomicroscopy of sequential steps in metastasis, coupled with methods tha t allow precise quantification of cell loss at specific steps in the metast atic process, as well as standard histological assessment at stages identif ied as crucial, allow characterization of the details of metastasis as an o ngoing process. This provides a powerful complement to end-point assays, fo r it allows mechanistic information to be obtained from in vivo experiments , an approach which provides better understanding of how and when a drug ma y function in vivo to inhibit metastasis.