Vascular effects dominate solid tumor response to treatment with combretastatin A‐4‐phosphate

Abstract

Vascular‐targeted therapeutics are increasingly used in the clinic. However, less is known about the direct response of tumor cells to these agents. We have developed a combretastatin‐A‐4‐phosphate (CA4P) resistant variant of SW1222 human colorectal carcinoma cells to examine the relative importance of vascular versus tumor cell targeting in the ultimate treatment response. SW1222Res cells were generated through exposure of wild‐type cells (SW1222WT) to increasing CA4P concentrations in vitro. Increased resistance was confirmed through analyses of cell viability, apoptosis and multidrug‐resistance (MDR) protein expression. In vivo, comparative studies examined tumor cell necrosis, apoptosis, vessel morphology and functional vascular end‐points following treatment with CA4P (single 100 mg/kg dose). Tumor response to repeated CA4P dosing (50 mg/kg/day, 5 days/week for 2 weeks) was examined through growth measurement, and ultimate tumor cell survival was studied by ex vivo clonogenic assay. In vitro, SW1222Res cells showed reduced CA4P sensitivity, enhanced MDR protein expression and a reduced apoptotic index. In vivo, CA4P induced significantly lower apoptotic cell death in SW1222Res versus SW1222WT tumors indicating maintenance of resistance characteristics. However, CA4P‐induced tumor necrosis was equivalent in both lines. Similarly, rapid CA4P‐mediated vessel disruption and blood flow shut‐down were observed in both lines. Cell surviving fraction was comparable in the two tumor types following single dose CA4P and SW1222Res tumors were at least as sensitive as SW1222WT tumors to repeated dosing. Despite tumor cell resistance to CA4P, SW1222Res response in vivo was not impaired, strongly supporting the view that vascular damage dominates the therapeutic response to this agent.

Structurally simplified biphenyl combretastatin A4 derivatives retain in vitro anti-cancer activity dependent on mitotic arrest

“…The anti-proliferative and pro-apoptotic properties displayed by these compounds are a proof-of-concept that the removal of CA4’s ethylene does not completely abolish in vitro anti-cancer activity. Importantly, the in vivo anti-cancer activity of CA4P has been shown to be largely independent of direct cancer cell targeting and instead is linked to its vascular damaging properties [ 50 ]. Thus, future characterization of MP5-G9 and MP5-59, as well as future biphenyl analogues, will need to involve animal models to evaluate the vascular damaging properties, bioavailability, and therapeutic potential.…”

An in vivo role for Rho kinase activation in the tumour vascular disrupting activity of combretastatin A‐4 3‐O‐phosphate

“…Staining for p-ERM showed a CA4P-induced expression in tumour cells, as well as in tumour vasculature, that was blocked by Y27632 treatment. However, by utilizing SW1222 tumour cells developed to be resistant to CA4P, we have previously shown that the anti-tumour activity of CA4P in vivo is mediated by its tumour vascular effects, rather than by direct toxicity towards the tumour cells themselves (Lunt et al ., 2011 ).…”

“…at 10 mL kg −1 in physiological saline. Mice were killed at 1, 3, 6 or 24 h after single-dose CA4P treatment (OxiGene Inc., San Francisco, CA, USA), at 100 mg kg −1 ; a previously determined effective dose for SW1222 tumours (Lunt et al ., 2011 ). Y27632 (Tocris Bioscience, Bristol, UK) was administered at the maximum tolerated dose in our mice of 50 mg kg −1 (unpublished data), either alone or 5 min before or 3 h after CA4P treatment.…”

“…Mice were treated as above and the tumour perfusion index calculated at 1, 3, 6 or 24 h after CA4P, as published previously (Lunt et al ., 2011 ). Briefly, 0.2 mg FITC-conjugated Lycopersicon esculentum tomato lectin (Vector Laboratories, Peterborough, UK) per mouse was injected 5 min before killing to detect perfused blood vessels at the time of lectin injection.…”

Dynamic Contrast Enhanced Fluorescent Molecular Imaging of Vascular Disruption Induced by Combretastatin-A4P in Tumor Xenografts

“…Combretastatin-A4 phosphate CA4P, also named (Zybrestat ™ or fosbretabulin) is a tubulin depolymerizing agent, which has been shown to cause acute vascular disruption in tumors and has been extensively evaluated in mice, and rats and human clinical trials. 5,6,18–23 Reported side effects have included hypertension, tumor pain and occasional cardiovascular toxicity. 24 It has been stated that CA4P is tumor selective rather than specific and the lack of specificity for tumor endothelium was highlighted by studies showing a CA4-induced inhibition of the neovascularization of hyperplastic thyroids and retina, suggesting that CA4P may be relevant to the treatment of some non-cancer diseases.…”