Review article: angiogenesis, neovascular proliferation and vascular pathophysiology as targets for cancer therapy.

A body of evidence that vascular-mediated damage occurs in murine tumours after many existing forms of anti-tumour therapy is rapidly accumulating (see Gray Conference Proceedings edited by Moore & West, 1991). Rapid conventional screens of cells in vitro or using leukaemias of lymphomas will not detect this mode of action and such screens will therefore miss effective agents. A change in the approach to experimental cancer therapy is needed to ensure that this important new avenue is fully investigated. Solid tumours will need to be studied and the importance of specific tumour cell biochemistry (e.g. on tissue factor procoagulant activity), of endothelial status and the immunocompetence of the host are all likely to be important. It is a subject of considerable debate at present whether transplanted subcutaneous mouse tumours are adequate models and whether they will reflect the response of spontaneous tumours, or even of transplants into other sites. Xenografts are not likely to be appropriate if the immuno-suppressed hosts lack the cells needed for the cytokine component of the pathway. The strategy of design and screening of new agents, for scheduling of existing agents and particularly the sequencing of adjunctive therapies are likely to be completely different for the "direct" tumor cell or "indirect" vascular-mediated approaches. It may eventually be appropriate to combine vascular manipulation with direct cytotoxicity aimed at malignant cells but the two mechanisms must be recognized as distinct entities and considered separately before attempting to coordinate them. It is important therefore to identify the "hallmarks" of vascular mediated injury and the means by which this can be distinguished from direct cell kill. These may be detectable in the tumour response but clues can also be gained from the side effects that are seen in normal tissues both with existing and with novel therapies (Figure 7). The appeal of vascular-mediated ischaemic therapy is that it is systemic and will have the potential of being effective on any tumour with a newly evoked vascular network, i.e. of about 1 mm in diameter, but it will be even more effective on large tumours than on small. Thus it should affect both large primary tumours and disseminated small metastases. The studies with many different anti-cancer agents have illustrated the potential complexity of responses that can appear to cause tumour cell death by collapse or occlusion of the blood supply. They have also focused attention on features of disparate agents, e.g. TNF, FAA, PDT, which may share similar pathways. No single feature of neovasculature can be highlighted as the sole route by which such antivascular therapy should be targeted. Rapid proliferation of the endothelial cells may prove to be a target, but it also influences differentiation characteristics, so that the immature cells will function abnormally. The permeability of these poorly formed vessels may lead to extravasation of proteins leading to increase interstitial pressures and by this means to an imbalance between intravascular and extravascular pressures and hence to collapse of the thin-walled vessels. Changes in systemic blood pressure, cardiac output, viscosity or coagulation and especially a redistribution of regional perfusion would all have differential effects in tumours and normal vessels. Clearly both vascular patho-physiology and the complexity of endothelial cell function and its imbalance in neovasculature will be important in understanding the mechanism of action of antivascular strategies. This very challenging boundary between oncology and a number of other medical and biological fields promises to lead to altered attitudes to existing therapies and the discovery of completely new classes of anti-cancer agents. The next decade should translate into clinical benefit for patients if the progress in this field continues to be as rapid as it has been in the late eighties. We must now determine what characteristics make one tumour more sensitive than another to agents such as heat, PDT, cytokines and FAA, and learn how to extrapolate from those rodent tumours to the human.