Differential heat sensitivity of tumour microvasculature

Enhanced Effectiveness of Radiochemotherapy with Tirapazamine by Local Application of Electric Pulses to Tumors

Tumor hypoxia is associated with resistance to radiotherapy and anticancer chemotherapy. However, it can be exploited to therapeutic advantage by concomitantly using hypoxic cytotoxins, such as tirapazamine (TPZ). Tumor electroporation offers the means to further increase tumor hypoxia by temporarily reducing tumor blood flow and therefore increase the cytotoxicity of TPZ. The primary objective of this work was to determine whether electric pulses combined with TPZ and radiotherapy (electroradiochemotherapy) was more efficacious than radiochemotherapy (TPZ + radiation). In these studies using the SCCVII tumor model in C3H mice, electroradiochemotherapy produced up to sixfold more tumor growth delay (TGD) than TPZ + radiation. In these studies, (1) large tumors (280 +/- 15 mm3) responded better to electroradiochemotherapy than small tumors (110 +/- 10 mm3), (2) TGD correlated linearly with tumor volume at the time of electroradiochemotherapy, (3) electric pulses induced a rapid but reversible reduction in O2 saturation, and (4) the electric field was highest near the periphery of the tumor in a 3D computer model. The findings suggested that electroradiochemotherapy gained its therapeutic advantage over TPZ + radiation by enhancing the cytotoxic action of TPZ through reduced tumor oxygenation. The greater antitumor effect achieved in large tumors may be related to tumor morphology and the electric-field distribution. These results suggest that electro-pulsation of large solid tumors may be of benefit to patients treated with radiation in combination with agents that kill hypoxic cells.

Vascular targeting of solid tumours: a major 'inverse' volume-response relationship following combretastatin A-4 phosphate treatment of rat rhabdomyosarcomas

Tumour-specific vascularisation may be therapeutically approached in two different ways: by antiangiogenic treatments specifically directed to dividing and migrating endothelial cells, or by agents that target principally the inadequate and ill-structured tumour vasculature. Combretastatin A-4 phosphate (combreAp), a recently synthesised prodrug (OXiGENE, Lund, Sweden), is a vascular targeting agent of the latter kind. We evaluated the effect of a single intraperitoneal (i.p.) combreAp injection on the growth of rhabdomyosarcomas syngeneic in WAG/Rij rats. Different tumour volume groups, ranging between 0.1 and 27 cm(3), were selected to assess the relationship between the size at treatment time and the response to combreAp. A double combreAp treatment (2x25 mg/kg) was investigated within the same overall aim: the relationship between growth delay and tumour size. Our results show that the systemic administration of combreAp induces a clear-cut differential growth delay in the solid rat rhabdomyosarcomas: with very large tumours (>/= 14 cm(3)), a 17.6-fold stronger effect was measured than with very small tumours (<1 cm(3)). This is the 'inverse' of the volume-response seen with the conventional therapeutic approaches (radiotherapy, chemotherapy or surgery). These combreAp antitumour responses were observed without treatment limiting systemic toxicity in the rats. With clinical digital subtraction angiography, using microsurgical cannulation of a major tumour draining vessel, and with histopathology, we demonstrate that growth delay is related to an early (within 3-6 h) and extensive breakdown of tumour blood vessels. The experiments involving a second injection also indicate a volume-dependent effect of combreAp in reducing the regrowth rate of small or large rhabdomyosarcomas. This significant differential volume-response obtained with 'selective' vascular targeting, stronger in larger tumours than smaller ones, suggests the potential of broadening the therapeutic window.

Thermal dose and secondary tumour cell death

Both primary and secondary tumour cell death may occur in clinical hyperthermia. The equation usually used for calculation of thermal dose takes only the primary cell death into consideration. We propose that the thermal dose equation should be reassessed; contributions from secondary cell death should also be included. The secondary cell death is governed mainly by the temperature distribution during treatment, the arteriolar density distribution in the tumour and the heat sensitivity of the arterioles. Increased thermal dose and hence increased tumour treatment response may result if hyperthermic treatments are designed to maximize the secondary cell death. Massive secondary cell death may be achieved by inducing hot spots in tumour areas with high arteriolar density, identified in pretreatment tumour angiograms.

Monoclonal antibody accumulation in experimental and spontaneous lung metastases of human malignant melanoma in rodents

Monoclonal antibody (mAb) uptake in metastatic lung lesions was depicted and evaluated by digital autoradiography. The models examined were experimental metastases of a human melanoma in nude mice and spontaneous metastases of human melanoma in immunocompromised young rats. By comparing uptake patterns in local (s.c.) tumours and in lung processes of various sizes it was found that patterns were essentially similar in both types of malignant tissue. From the point of view of visualization, however, the high blood content of lung tissue resulted in high background and low contrast. This could be overcome by the use of rapidly cleared antibody fragments.

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.