QuantitativeIn VivoCharacterization of Intracellular and Extracellular pH Profiles in Heterogeneous Tumors: A Novel Method Enabling Multiparametric pH Analysis

Abstract

Acid production and transport are currently being studied to identify new targets for efficient cancer treatment, as subpopulations of tumor cells frequently escape conventional therapy owing to their particularly acidic tumor microenvironment. Heterogeneity in intracellular and extracellular tumor pH (pH i , pH e ) has been reported, but none of the methods currently available for measuring tissue pH provides quantitative parameters characterizing pH distribution profiles in tissues. To this intent, we present here a multiparametric, noninvasive approach based on in vivo 31 P nuclear magnetic resonance (NMR) spectroscopy and its application to mouse tumor xenografts. First, localized 31 P NMR spectrum signals of pH i and pH e reporter molecules [inorganic phosphate (P i ) and 3-aminopropylphosphonate (3-APP), respectively] were transformed into pH curves using established algorithms. Although P i is an endogenous compound, 3-APP had to be injected intraperitoneally. Then, we developed algorithms for the calculation of six to eight quantitative pH parameters from the digital points of each pH curve obtained. For this purpose, each pH distribution profile was approximated as a histogram, and intensities were corrected for the nonlinearity between chemical-shift and pH. Cancer Res; 73(15); 4616-28. Ó2013 AACR.

Monodispersed calcium carbonate nanoparticles modulate local pH and inhibit tumor growth in vivo

“…33 Our initial attempt to determine tumor pHe by the reported 31 P NMR spectroscopic technique using 3-aminopropylphosphonate 34 was unsuccessful due to technical issues. To determine whether nano- CaCO3 was capable of altering pH in vivo, we used a 5 mm diameter invasive in vivo pH electrode probe (Figure 5a), and putatively interpreted the resulting pH values as predominantly extracellular.…”

“…In addition, we putatively attributed the average pH values measured with the invasive pH electrode to predominantly the pHe of tumors. Validation of this assumption with a noninvasive method that measures the pHe of tumors more accurately 34 will be needed.…”

Multiparametric quantification of thermal heterogeneity within aqueous materials by water 1H NMR spectroscopy: Paradigms and algorithms

“…In addition, a well-known advantage of medians vs. means is that the former are more robust to outliers. Weighted-median temperature was determined according to an algorithm that is essentially equivalent to the general algorithm for median calculation from a frequency distribution [34, 42, 43]. …”

“…While kurtosis can be used as a measure of the peakedness or flatness of a heterogeneous temperature distribution, also the evenness (smoothness) of a temperature curve can be determined by employing a statistical function known as standard entropy, H S (discrete Shannon entropy [45–47]). Temperature entropy, based on the equation given in S2 File, is a direct measure of how even a temperature distribution is: low entropy indicates that there are significant sample volumes with particular temperature values that occur at much higher frequencies than other regions within the measured volume [42]. …”

Evaluating pH in the Extracellular Tumor Microenvironment Using CEST MRI and Other Imaging Methods

“…Furthermore, 3-APP has low toxicity and can measure pHe within a physiological range of pH 6 to 8. This method of analysis allows simultaneous pHe and pHi measurements with 3-APP and P i [43, 44] and has been used to study alkalosis and acidification of tumor models [45, 46]. This method has been a popular choice as a cross-reference when developing new methods to measure pHe [22, 47, 48].…”