Reduction of intratumoral pH by the mitochondrial inhibitor m-iodobenzylguanidine and moderate hyperglycemia.

The interstitial pH of RIF-1 tumors was selectively lowered by i.p. administration of the mitochondrial inhibitor meta-iodobenzylguanidine (MIBG; 40-100 mg/kg), supported by sustained moderate hyperglycemia (plasma glucose concentration, 14 mM) in rats or by a single i.p. bolus injection of glucose (1.5 g/kg) in mice. Responses were evaluated in a multicenter study by pH measurements with semimicroelectrodes and 31P magnetic resonance spectroscopy, by biochemical analysis of tissue and plasma levels of glucose and lactate, and by positron emission tomography analysis of 2-[18F]fluoro-2-deoxy-D-glucose uptake. In both schedules, treatment with MIBG and glucose reduced the mean intratumoral pH as recorded with semimicroelectrodes to 6.2. In the mouse model, treatment with MIBG plus glucose was accompanied by a 2-3-fold stimulation of 2-[18F]fluoro-2-deoxy-D-glucose uptake and a corresponding increase in tumor glucose content. Responses were maximal in male mice with tumors of 0.2-0.8 g. 31P magnetic resonance spectroscopy analysis revealed no changes in intracellular pH or metabolic status, indicating that only extracellular pH was affected. MIBG was synergistic with bolus or continuous glucose administrations by a dual mechanism. The drug reduced by up to 5-fold the amount of glucose required for effective reduction of intratumoral pH and promoted the availability of (extra) glucose to tumor tissue in a stress-related, sympathomimetic response. Moreover, by converting oxic tumor cells into functionally hypoxic cells, combined treatment resulted in a more homogeneous decrease in intratumoral pH which included better perfused peripheral tumor areas. The effects of combined treatment on tumor glucose metabolism could be monitored noninvasively by 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography analysis.

Potentiation of DNA-adduct formation and cytotoxicity of platinum-containing drugs by low pH.

The low interstitial pH of tumor tissue is an important modulator of various anti-tumor modalities. In order to explore the optimal conditions for the potentiating action of low pH on the cytotoxic activities of cis- and carboplatin, we have investigated the temporal aspects of drug activity and pH modulation in L1210 murine leukemia cells in comparison with various other drugs. Extra- and intra-cellular pH of L1210 cells was modulated before, during and after drug exposure and survival of L1210 cells was determined. During short exposures, cytotoxicity of cisplatin and alkylating drugs was potentiated by conditions of low pH in the ranking order of: cisplatin, mitomycin C, melphalan and chlorambucil. Low pH had no effect on the cytotoxic activity of carboplatin and cytosine arabinoside and it inhibited the action of doxorubicin. During prolonged incubation at low pH, potentiation of cisplatin was increased and a more than 3-fold potentiation was induced in the case of carboplatin. Part of the latter effect was also manifested by 20 hr post-incubation in drug-free medium at low pH after a 4-hr exposure to carboplatin. Post-incubation did not increase the stimulating effect of low pH on the cytotoxic activity of melphalan and cisplatin. Acidification affected neither the uptake nor the extracellular hydrolysis of platinum-containing drugs. Under all circumstances, potentiation of platinum-containing drugs was accompanied by an increase in platinum-induced DNA modification, as detected by immunocytochemistry.

pH in human tumor xenografts and transplanted rat tumors: effect of insulin, inorganic phosphate, and m-iodobenzylguanidine.

Various strategies to improve the therapeutic index of anticancer agents aim at inducing, by stimulation of aerobic glycolysis, temporary pH differences between malignant and normal tissues which can be exploited to activate cytotoxic agents selectively in tumors. We have investigated whether the pH reduction induced by glucose, the "drug" commonly used to increase lactic acid production in malignant tissues, can be augmented by pharmacological manipulation of tumor cell glycolysis. At normal plasma glucose concentration (6 +/- 1 mM), inorganic phosphate, a modifier of hexokinase and phosphofructokinase activity, had no effect on pH in two transplanted rat tumors and a human tumor xenograft line (average pH, 6.80; range, 6.65-6.95). When plasma glucose concentration was raised to 30 +/- 3 mM by i.v. infusion of glucose, inorganic phosphate reduced the pH in those tumors which exhibited only a moderate pH response to glucose per se (mean pH, 6.60) to an average value of 6.20 (range, 6.05-6.35). In the same setting, insulin, continuously infused at dose rates up to 600 milliunits/kg body weight/min, did not result in acidification of tumor tissue exceeding that induced by glucose alone. However, the H+ ion activity in both transplanted rat tumors and human tumor xenografts was increased by m-iodobenzylguanidine (MIBG), an inhibitor of mitochondrial respiration. For example, at normoglycemia, MIBG reduced the mean pH in a human mesothelioma xenograft from 6.90 to 6.70. This pH value was further reduced to 6.20 by simultaneous low-dose i.v. glucose infusion (plasma glucose concentration, 14 +/- 3 mM). The acidosis induced by inorganic phosphate and MIBG was tumor specific. Normal tissues of tumor-bearing hosts were only marginally sensitive to hyperphosphatemia or MIBG administration. These results indicate that the known stimulatory effect of exogenous glucose on lactic acid production in malignant tumors in vivo can be further accentuated or, as in the case of MIBG, partially replaced by pharmacological manipulation of aerobic glycolysis using clinically established drugs.