TUMOUR PH

As reported in The Lancet nearly sixty years ago,(n1) a Nobel prize was awarded to otto Warburg in recognition of work(n2) that included the discovery of "the remarkable extent to which living tumour cells are able to convert carbohydrate into lactic acid". For many years afterwards it was assumed that tumours relied largely on the glycolytic pathway, produced large amounts of lactic acid, and consequently had an acidic intracellular pH (pHi). In the intervening years, microelectrode measurements of tumour pH seemed to confirm this assumption. However, we now know that these measurements largely reflect pHe, the interstitial (or extracellular) fluid pH (range 5.5-7.3).(n3-n5) With the advent of magnetic resonance spectroscopy (MRS), a non-invasive in-vivo measure of tissue pH became available, and in 1983 the pH of a human tumour was measured for the first time.(n6) The pHMRS measurement is based on the pH-dependent chemical shift difference between the 31Pi (inorganic phosphate) signal and an endogenous reference signal.(n7) At physiological pH, the Pi signal reflects the relative concentrations of the two phosphate species (H2PO4- and HPO42-) present.

The MRS measurement of tissue pH is a composite value of pHi and pHe. In normal tissues it is believed that pH measured in this way is intracellular. This assumption might not hold for tumours, since their extracellular volume can be much larger than in normal tissues. The proportion of Pi signal coming from the intracellular volume can be calculated if total tissue water content and the fractional volume of extracellular water are known. In animal tumours, if the extracellular volume does not exceed 50%, pHMRS largely represents intracellular pH.(n8)

Many human tumours, especially brain tumours and sarcomas (pH 7.01-7.35), have a similar or even slightly higher pHi than their respective normal tissues.(n3,n9) Positron emission tomography studies support the findings of high brain tumour pH.(n10)

These pHMRS values mean that human tumours are alkaline in comparison to their extracellular fluid--the exact opposite of normal tissues. Because it was widely assumed that microelectrode measurements of acidic PHe implied acidic pHi there have been many proposals(n11) for the development of drugs that would localise in these supposedly acidic tumour cells. Since it is now clear that pHi is more alkaline than pHe, drugs intended to partition preferentially across the cell membrane will actually partition into the acidic extracellular fluid. For some purposes this may not matter. Conjugates that release free drug at acid pH(n12) would benefit from being localised in the extracellular fluid. Ionising radiation and hyperthermia are more effective in cultured cell lines at low pH, although treatment of human tumours in vivo by these methods suggests that the converse may occur.(n13)

High lactate concentrations are observed concurrently with high pH, and this finding can be attributed to the fact that tumour cells readily extrude protons from the cell but retain the lactate ion. According to Spencer and Lehninger(n14) and Veech,(n15) this is to be expected, since lactate distributes across the cell membrane as a reciprocal of H+ distribution, which means that high pHi in comparison to pHe would be expected to be accompanied by high intracellular lactate.

Thus, after 60 years of "acidic tumours", we have to be more precise and be aware that tumours have a neutral to alkaline pHi in comparison with their extracellular environment, which is often acidic. It may be possible to exploit this difference to develop new approaches to cancer therapy.

(n1.) Anon. The Nobel prizeman. Lancet 1931; ii: 1035.

(n2.) Warburg O. The metabolism of tumours. English translation by F. Dickens. London: Constable, 1930.

(n3.) Vaupel P, Kallinowski F, Okunieff P. Blood flow, oxygen and nutrient supply, and metabolic microenvironment of human tumours: a review. Cancer Res 1989; 49: 6449-65.

(n4.) Wike-Hooley JL, Haveman J, Reinhold HS. The relevance of tumour pH to the treatment of malignant disease. Radiother Oncol 1984; 2: 343-66.

(n5.) Griffiths JR. Review: Are cancer cells acidic? Br J Cancer 1991; 64: 425-27.

(n6.) Griffiths JR, Cady E, Edwards RHT, McCready VR, Wilkie DR, Wiltshaw E. 31P-NMR studies of a human tumour in situ. Lancet 1983; i: 1435-36.

(n7.) Prichard JW, Alger JR, Behar KL, Petroff OAC, Shulman RG. Cerebral metabolic studies in vivo by 31P NMR. Proc Natl Acad Sci USA 1983; 80: 2748-51.

(n8.) Stubbs M, Bhujwalla ZM, Tozer GM, a al. An assessment of 31P MRS as a method of measuring pH in rat tumours. NMR Biomed (in press).

(n9.) Oberhaensli RD, Hilton-Jones D, Bore PJ, Hands LJ, Rampling RP, Radda GK. Biochemical investigation of human tumours in vivo with phosphorus-31 magnetic resonance spectroscopy. Lancet 1986; ii: 8-11.

(n10.) Rottenberg DA, Ginos JZ, Kearfott KJ, Junck L, Bigner D. In vivo measurements of regional brain tissue pH using positron emission tomography. Ann Neurol 1984; 15 (suppl): 98-102.

(n11.) Tannock IF, Rotin D. Acid pH in tumors arid its potential for therapeutic exploitation. Cancer Res 1989; 49: 4373-84: 12.

(n12.) Lavie E, Hirschberg DL, Schreiber G, et al. Monoclonal antibody L6-daunomycin conjugates constructed to release free drug at the lower pH of tumor tissue. Cancer Immunol Immunother 1991; 33: 223-30.

(n13.) Van Den Berg AP, Wike-Hooley JL, Broekmeyer-Reurink MA, Van der Zee J, Reinhold HS. The relationship between the unmodified initial tissue pH of human tumours and the response to combined radiotherapy and local hyperthermia treatment. Eur J Cancer Clin Oncol 1989; 25: 73-78.

(n14.) Spencer TL, Lehninger A. L-Lactate transport in Ehrlich ascites tumour cells. Biochem J 1976;154: 405-14.

(n15.) Veech RL. The metabolism of lactate. NMR Biomed 1991; 4: 53-58.

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