Effect of vascular marker Hoechst 33342 on tumour perfusion and cardiovascular function in the mouse

The fluorescent stain Hoechst 33342 (H33342) has been employed extensively as an in vivo marker of functional tumour vasculature. We have found that H33342 causes a transient, dose-dependent decrease in tumour red blood cell (RBC) flow in SCCVII tumours as measured using laser Doppler flowmetry. After intravenous bolus injection of 15 mg kg-1 to anaesthetised mice, blood flow in subcutaneous back tumours declined to 19 +/- 11% of pretreatment values, returning to normal in less than 7 min. The effect was less pronounced in mice bearing foot tumours in which flow decreased to 52 +/- 14% of pretreatment values in unanaesthetised mice and to 50 +/- 15% in anaesthetised animals. RBC flow in foot tumours remained significantly depressed for only 2-3 min. A dose of 5 mg kg-1 was not significantly vasoactive in back tumours. H33342 also caused a transient 20 +/- 6 mmHg decline in mouse arterial blood pressure. Blood pH and haematocrit, and tumour cell oxygen consumption were unchanged by H33342. H33342-induced flow changes did not affect results obtained using an in vivo double staining protocol provided that the interval between stain injections was greater than 5 min. Due to its transient effects on tumour perfusion, the stain caused radiobiological tumour hypoxia if injected immediately prior to X-irradiation. Injection 20 min before irradiation had no influence on tumour radiation response. We conclude that the transient nature of H33342-induced perturbations in mouse cardiovascular physiology and tumour blood flow must always be considered but do not preclude the use of the stain as a vascular marker to detect spontaneous tumour blood flow fluctuations or acute hypoxia.