Noninvasive measurement of interstitial pH profiles in normal and neoplastic tissue using fluorescence ratio imaging microscopy.

The tumor interstitial pH and its modification play a significant role in cancer treatment. Current in vivo pH measurement techniques are invasive and/or provide poor spatial resolution. Therefore, there are no data on perivascular interstitial pH gradients in normal or tumor tissue. We have optically measured interstitial pH gradients with high resolution in normal and tumor (VX2 carcinoma) tissue in vivo by combining a fluorescence ratio imaging microscopy technique and the rabbit ear chamber preparation. The strengths of our approach include the ability to follow pH in the same location for several weeks and to relate these measurements to local blood flow and vascular architecture. Our results show: (a) tumor interstitial pH (6.75 units; N = 6 animals, n = 324 measurements) is significantly (P < 0.001) less than normal interstitial pH (7.23; N = 5, n = 274). This increased acidity in the tumor interstitium is in agreement with the previously reported data on this tumor; (b) with respect to pH spatial gradients in normal tissue, the interstitial pH decreased by approximately 0.32 pH units over a distance of 50 microns away from the blood vessel, while in tumor tissue, interstitial pH decreased by approximately 0.13 units over the same distance. Although the pH gradient near the vessel wall was steeper in normal tissue compared to tumor, the proton concentration gradient in normal tissue was less than that in the tumor. The approximate increase in proton concentration from 0-50 microns from the vessel was 4.5 x 10(-8)M in normal versus 5.7 x 10(-8)M in tumor tissue; (c) a simple one-dimensional diffusion-reaction model suggested that tumor tissue was producing protons at a rat 65-100% greater than normal tissue; (d) feasibility studies of temporal dynamics resulting from hyperglycemia (6 g/kg) or hypercapnia (10% CO2) led to significant (P < 0.05) interstitial pH reductions. During hyperglycemia, pH dropped by more than 0.2 pH units in about 90 min in tumor tissue but remained constant in normal tissue. Hypercapnia dramatically reduced pH by approximately 0.3 pH units in tumor tissue. Our limited studies on hyperglycemia and hypercapnia are in agreement with the previously published studies and demonstrate the capability of fluorescence ratio imaging microscopy to measure spatial as well as temporal changes in interstitial pH. Fluorescence ratio imaging microscopy should permit noninvasive evaluation of new pH-modifying agents and offer unique mechanistic information about tumor pathophysiology in tissue preparations where the surface of the tissue can be observed.