IN-VIVO PH MEASUREMENT AND IMAGING OF TUMOR-TISSUE USING A PH-SENSITIVE FLUORESCENT-PROBE (5,6-CARBOXYFLUORESCEIN) - INSTRUMENTAL AND EXPERIMENTAL STUDIES

This study evaluated the effectiveness of dual-wavelength ratio fluorescence imaging using a pH-dependent indicator (5,6-carboxyfluorescein, 5,6-CF) for in vivo pH mapping of tissue. A prototype version of a highly sensitive fluorescence imaging device consisting of a modified xenon lamp, an image-intensified camera and a digital image-processing system has been developed. 5,6-Carboxyfluorescein was used because its fluorescence emission increases as a function of pH in the physiological (6.0-7.4) pH range. The ratio of fluorescence intensities obtained with the imaging system has been calibrated using aqueous 5,6-CF standards at various pH values. Because the pH of interstitial fluid of malignant tumors tends to be lower than that of normal tissue and can be depressed by glucose administration, experiments were performed on 10 CDF mice bearing lymphoid leukemia P388 grafted subcutaneously. The range of linearity of the calibration curve was obtained between 5.3 and 6.7 with a measured pK(a) value of 5.93. Consequently the maximum sensitivity was observed in this range. The calculated pH from ratio images was 6.21 +/- 0.12 in tumorous tissue. This value was equivalent to those obtained at the same time using microelectrodes (6.2 +/- 0.3). These experiments showed that a dose of 5 mg/kg 5,6-CF and an excitation power density of 2.5 mW/cm(2) are sufficient to give a fluorescent pH image of tumors. The limitation of 5,6-CF for the in vivo mapping of tissue results from its low pK(a) and consequent range of sensitivity. The advantages of this imaging technique compared to microelectrodes are that it (1) is noninvasive, (2) displays a two-dimensional pH image with high resolution (profile distribution of pH in tissue) and (3) can be used to monitor pH over a few hours.