The objective of this review article is to summarize current knowledge of blood flow and perfusion-related parameters, which usually go hand in hand and in turn define the cellular metabolic microenvironment of human malignancies. A compilation of available data from the literature on blood flow, oxygen and nutrient supply, and tissue oxygen and pH distribution in human tumors is presented. Whenever possible, data obtained for human tumors are compared with the respective parameters in normal tissues, isotransplanted or spontaneous rodent tumors, and xenografted human tumors. Although data on human tumors in situ are scarce and there may be significant errors associated with the techniques used for measurements, experimental evidence is provided for the existence of a compromised and anisotropic blood supply to many tumors. As a result, O2-depleted areas develop in human malignancies which coincide with nutrient and energy deprivation and with a hostile metabolic microenvironment (e.g., existence of severe tissue acidosis). Significant variations in these relevant parameters must be expected between different locations within the same tumor, at the same location at different times, and between individual tumors of the same grading and staging. Furthermore, this synopsis will attempt to identify relevant pathophysiological parameters and other related areas future research of which might be most beneficial for designing individually tailored treatment protocols with the goal of predicting the acute and/or long-term response of tumors to therapy.

https://cancerres.aacrjournals.org/content/canres/49/23/6449.full.pdf

Blood Flow, Oxygen and Nutrient Supply, and Metabolic Microenvironment of Human Tumors: A Review

For more than half a century it has been generally believed that cancer cells are more acidic than normal cells. This dogma arose from the studies of Warburg and co-workers (Warburg, 1930) who showed that tumour cells preferentially convert glucose and other substrates to lactic acid, even under aerobic conditions. Since lactic acid has a pK of 3.7 it seemed obvious that the intracellular fluid would become acidic. Studies on cultured tumour cells or spheroids often showed low intracellular pH (pHi) values, but this could have been due to their highly artificial situation. What would be the pHi of cells in a solid tumour in a patient? The development of pH electrodes small enough to be inserted into living tissues led to the apparent confirmation of the prevailing wisdom. Numerous studies (reviewed by WikeHooley et al., 1984 and Vaupel et al., 1989; see also Figure lb) showed significantly more acidic pH (pHpoT) in tumours than that in normal tissues. Microelectrodes that can be used on solid tumours in vivo are usually quite large in comparison to a tumour cell (see Wike-Hooley et al., 1984, Table II), and they mainly measure the pH of the extracellular fluid (pHe) rather than pHi (Vaupel et al., 1989). For most purposes, the parameter of interest is pH1, the pH of the water in the cancer cell itself, but it was generally expected (and then tacitly assumed) that pHi would also be acidic. This supposed cellular acidity in tumours became part of the mental wallpaper of oncologists and cancer researchers, even though there was, for many years, no practical way to measure pHi of intact human tumours. It had clinical consequences, too, since anticancer treatments were often designed to take advantage of a low pH, (for a review, see WikeHooley et al., 1984). It was argued, for instance, that anticancer drugs would be more effective if they contained ionising groups that would cause them to be trapped in acidic environments (Wike-Hooley et al., 1984), or that radioresistant hypoxic cells would have a particularly low pHi and might therefore be especially sensitive to treatments such as hyperthermia which are known to act preferentially on isolated cells in acidic media (Freeman et al., 1981). There were also numerous attempts to lower pHi still further by administration of glucose, and thereby enhance the action of various pH-sensitive therapies (Ross, 1961, reviewed by Wike-Hooley et al., 1984). In general, these ideas have had little clinical success, but they are still the subject of active research (see, for instance, Tannock & Rotin, 1989). Within the last 10 years a non-invasive intracellular pH meter the Nuclear Magnetic Resonance Spectrometer has become widely available; it can be used on living tumours in situ, both in experimental animals and in man. The results obtained with these instruments have been surprising. Instead of having the expected acidic pHi, the cells of intact tumours turned out to be neutral, or a little alkaline, both in experimental animals (Griffiths et al., 1981; Iles et al., 1982) and man (Griffiths et al., 1983). Indeed, several studies by 31P

/pdf/are-cancer-cells-acidic-4g1naz01nv.pdf

Are cancer cells acidic

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.10181

MRI of the tumor microenvironment.

The extracellular pH (pHex) of tumors is generally acidic. However, it is only recently that noninvasive magnetic resonance spectroscopic (MRS) measurements have determined that the intracellular pH (pHin) of tumor cells in situ is neutral or slightly alkaline compared with that of normal tissues. Thus cells in tumors maintain larger pH gradients than do cells in nontumor tissues. To date, measurements of pHex in tumors have been made using microelectrodes, which preclude measurement of pHex and pHin within the same preparation. In addition, microelectrodes are invasive and have the potential to alter the measured pH values. The present communication describes simultaneous measurement of pHex and pHin in vitro in bioreactor culture and in vivo using 31P-MRS analyses of 3-aminopropylphosphonate (3-APP) and inorganic phosphate. In vitro results indicate that 3-APP is not toxic and that its resonant frequency is sensitive to pH and not significantly affected by temperature or ionic strength. Bioreactor experiments indicate that this compound is neither internalized nor metabolized by cells. Experiments in vivo indicate that 3-APP can be administered intraperitoneally and that RIF-1 tumors maintain a steady-state pHin of 7.25 and a pHex of 6.66. These data have significance to basic tumor cell physiology and to the design of approaches to cancer chemotherapy and hyperthermic therapy, because both of these modalities exhibit pH sensitivity. It is also likely that these techniques will be applicable to localized MRS of other organ systems in vivo.

31P-MRS measurements of extracellular pH of tumors using 3-aminopropylphosphonate.

Neuroblastoma is a unique tumor of childhood that has a wide range of malignant expression. The prognoses range from excellent, with minimal treatment required, for patients with localized tumors o...

31P Nuclear magnetic resonance spectroscopic investigation of human neuroblastoma in situ

The hyperglycemia-induced in vivo metabolic changes produced in subcutaneous murine RIF-1 tumors, grown on female C3H/Anf mice, were examined with 31P surface-coil NMR. Serum glucose levels were elevated 4-fold by bolus intraperitoneal injection of 0.3 ml of an aqueous 50% glucose solution. Tumor pH was calculated from the chemical shift of Pi and relative phosphocreatine and ATP concentrations were determined by Simpson's rule integration of the peak areas. Tumor pH decreased by ca. 0.45 unit over 2 hr while phosphocreatine concentrations decreased by ca. 50% over the same time period (n = 9). Initial tumor pH correlated inversely with the initial peak intensity ratio of Pi:ATP (r = -0.77). In a significant number of tumors (n = 4), two pH populations were observed. In these tumors, one population was unaffected by hyperglycemia and the other showed a decrease in pH. In the other tumors (n = 5), the pH distribution broadened as the pH decreased. In these tumors, the observed decreased in phosphocreatine concentration correlated with that calculated from the effect of measured tumor pH on the intracellular creatine kinase equilibrium (n = 18, r = 0.91). This correlation and consideration of the Pi distribution in the tumor suggest that the pH measured by 31P NMR is weighted heavily by intracellular pH for the RIF-1 tumor. The presence of two distinct tumor pH populations or a broadened pH distribution likely reflects variations in tumor microcellular environment. Control experiments showed negligible changes in tumor pH and high energy phosphate concentrations after bolus intraperitoneal injection of 0.3 ml of isotonic saline. In addition, negligible changes in leg muscle pH and high energy phosphate concentrations were observed after glucose injection into mice with or without tumors. These results indicate that hyperglycemia induced by intraperitoneal glucose injection is effective in lowering the tumor pH of the murine RIF-1 tumor.

https://www.pnas.org/content/pnas/81/20/6496.full.pdf

David E. L. Jick

Papers

In vivo metabolic effects of hyperglycemia in murine radiation-induced fibrosarcoma: a 31P NMR investigation.