Figure 5. Correlation between the retention of SR-4554 bioreduction products in tumour cells detected by ¹⁹F magnetic resonance spectroscopy (MRS) indicating cellular hypoxia (3 h ¹⁹fluorine retention index) and tumour pO₂ measured by polarographic electrode (pO₂ values <2.5 mmHg) in groups of P22 tumour-bearing female SCID mice. Tumour hypoxia was increased by hydralazine (HDZ) and combretastatin A-4 phosphate (CA-4-P), decreased by carbogen and nicotinamide (C/N), or unmodulated (air). Error bars indicate standard error of the mean. (Modified from reference [102].)