Tumor pH is physiologically important since it influences a number of processes relevant to tumorigenesis and therapy. Hence, knowledge of localized pH within tumors would contribute to understanding these processes. The destructiveness, poor spatial resolution, and poor signal-to-noise ratio (SNR) of current technologies (e.g., microelectrodes, 31 P magnetic resonance spectroscopy) have limited such studies. An extrinsic chemical extracellular pH (pH e ) probe is described that is used in combination with 1 H magnetic resonance spectroscopic imaging to yield pH e maps with a spatial resolution of 1 ؋ 1 ؋ 4 mm 3 . Since the discovery of lactic acid production in tumors more than 50 years ago (1), it has generally been assumed that the pH of tumors is acidic. Indeed, numerous microelectrode measurements have shown that extracellular tumor pH (pH e ) is acidic (2). This acidic pH e of tumors has been confirmed with less invasive 31 P magnetic resonance spectroscopy (MRS) measurements (3). Although the intracellular pH (pH i ) of tumors remains neutral to alkaline (4,5), it is somewhat influenced by the pH e (6).An acidic pH e of tumors is physiologically important since it influences a number of processes relevant to carcinogenesis and therapy. Knowledge of localized pH within tumors, both intra-and extracellular, would allow more detailed study of these processes and relate them to intratumoral pH heterogeneity. For example, it has been found that low pH e in vitro causes tumorigenic transformation of primary Syrian hamster embryo cells (7) and can lead to chromosomal rearrangements in Chinese hamster embryo cells (8,9). Furthermore, culturing cells at low pH causes them to be more invasive in vitro (10) and metastatic in vivo (11). Finally, the orientation of the pH gradient across the cell membrane may influence cell drug resistance (6,12) Previously reported measurements of extracellular pH using either microelectrodes or 31 P MRS of 3-aminopropylphosphonate (3-APP) (3) have drawbacks. Microelectrodes are invasive and can destroy the membrane integrity, thereby disrupting the mechanism for maintaining the pH e . 31 P MRS does not suffer this drawback and has the additional advantage of permitting simultaneous measurements of intracellular pH. However, the limited sensitivity of 31 P MRS allows measurements of pH e only from relatively large tissue volumes. Hence, 31 P MRS provides measurements of pH ranges rather than different pH values for discrete spatial locations (13).The use of 1 H MRS, inherently more sensitive than 31 P MRS, would allow measurements of pH over smaller tissue volumes. For example, the imidazole protons of histidine have long been useful as intracellular pH indicators in NMR (14,15).Rabenstein and Isab (16) first proposed using imidazoles as extrinsic pH e indicators. Gil et al (17) suggested several modifications of the basic structure of the imidazole molecule to improve its performance as an extrinsic pH probe. To date, the most promising candidate for a 1 H nuclear magnetic resonance ...

The unique physiological environment of solid tumors, frequently characterized by areas of poor flow, hypoxia, high lactate and low extracellular pH (pHe), influences vascularization, invasion and metastasis. Thus, vascularization and the physiological and metabolic environment play permissive (and conversely preventive) roles in invasion and metastasis. By using a multi-parametric approach of combined vascular and spectroscopic imaging, we can begin to evaluate which combinations of vascular, metabolic and physiological regions in a solid tumor represent the highest 'metastatic threat'. Here, we present measurements of pHe, vascular volume and permeability from colocalized regions within a solid tumor. These studies were performed for a group of metastatic (MDA-MB-231) and non-metastatic (MCF-7) human breast cancer xenografts. In this study, we have demonstrated the feasibility of such an approach, and presented methods of analyses to detect differences in patterns of combined parameters obtained from spatially co-registered regions in a solid tumor.

Tumor pH is physiologically important since it influences a number of processes relevant to tumorigenesis and therapy. Hence, knowledge of localized pH within tumors would contribute to understanding these processes. The destructiveness, poor spatial resolution, and poor signal‐to‐noise ratio (SNR) of current technologies (e.g., microelectrodes, 31P magnetic resonance spectroscopy) have limited such studies. An extrinsic chemical extracellular pH (pHe) probe is described that is used in combination with 1H magnetic resonance spectroscopic imaging to yield pHe maps with a spatial resolution of 1 × 1 × 4 mm3. The principle of the technique is demonstrated on a phantom. Further data are shown to demonstrate its application in vivo, and results agree with previously reported pH values. The accuracy of the reported pH measurements is <0.1 pH units, as derived from a detailed analysis of the errors associated with the technique, the description of which is included. Magn Reson Med 41:743–750, 1999. © 1999 Wiley‐Liss, Inc.

The intracellular compartmentation of pyruvate in primary cultures of cortical neurons was investigated by high resolution (13)C NMR using mixtures of different pyruvate precursors conveniently labeled with (13)C or unlabeled. Cells were incubated with 1-5 mM (1-(13)C, 1,2-(13)C(2) or U-(13)C(6)) glucose only or with mixtures containing 1.5 mM (1-(13)C or U-(13)C(6)) glucose, 0.25-2.5 mM (2-(13)C or 3-(13)C) pyruvate and 1 mM malate. Extracts from cells and incubation media were analyzed by (13)C NMR to determine the relative contributions of the different precursors to the intracellular pyruvate pool. When ((13)C) glucose was used as the sole substrate fractional (13)C enrichments and (13)C isotopomer populations in lactate and glutamate carbons were compatible with a unique intracellular pool of pyruvate. When mixtures of ((13)C) glucose, ((13)C) pyruvate and malate were used, however, the fractional (13)C enrichments of the C2 and C3 carbons of lactate were higher than those of the C2 and C3 carbons of alanine and depicted a different (13)C isotopomer distribution. Moreover, neurons incubated with 1 mM (1,2-(13)C(2)) glucose and 0.25-5 mM (3-(13)C) pyruvate produced exclusively (3-(13)C) lactate, revealing that extracellular pyruvate is the unique precursor of lactate under these conditions. These results reveal the presence of two different pools of intracellular pyruvate; one derived from extracellular pyruvate, used mainly for lactate and alanine production and one derived from glucose used primarily for oxidation. A red-ox switch using the cytosolic NAD(+)/NADH ratio is proposed to modulate glycolytic flux, controlling which one of the two pyruvate pools is metabolized in the tricarboxylic acid cycle when substrates more oxidized or reduced than glucose are used.

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