Influence of pH on the cytotoxic activity of chlorambucil

Characterization of the Intracellular Acidity Regulation of Brain Tumor Cells and Consequences for Therapeutic Optimization of Temozolomide

A well-known feature of tumor cells is high glycolytic activity, leading to acidification of the tumor microenvironment through extensive lactate production. This acidosis promotes processes such as metastasis, aggressiveness, and invasiveness, which have been associated with a worse clinical prognosis. Moreover, the function and expression of transporters involved in regulation of intracellular pH might be altered. In this study, the capacity of tumor cells to regulate their intracellular pH when exposed to a range of pH from very acidic to basic was characterized in two glioma cell lines (F98 and U87) using a new recently published method of fluorescence imaging. Our results show that the regulation of acidity in tumors is not the same for the two investigated cell lines; U87 cells are able to reduce their intracellular acidity, whereas F98 cells do not exhibit this property. On the other hand, F98 cells show a higher level of resistance to acidity than U87 cells. Intracellular regulation of acidity appears to be highly cell-dependent, with different mechanisms activated to preserve cell integrity and function. This characterization was performed on 2D monolayer cultures and 3D spheroids. Spatial heterogeneities were exhibited in 3D, suggesting a spatially modulated regulation in this context. Based on the corpus of knowledge available in the literature, we propose plausible mechanisms to interpret our results, together with some new lines of investigation to validate our hypotheses. Our results might have implications on therapy, since the activity of temozolomide is highly pH-dependent. We show that the drug efficiency can be enhanced, depending on the cell type, by manipulating the extracellular pH. Therefore, personalized treatment involving a combination of temozolomide and pH-regulating agents can be considered.

Therapeutic targeting of the hypoxic tumour microenvironment

Hypoxia is prevalent in human tumours and contributes to microenvironments that shape cancer evolution and adversely affect therapeutic outcomes. Historically, two different tumour microenvironment (TME) research communities have been discernible. One has focused on physicochemical gradients of oxygen, pH and nutrients in the tumour interstitium, motivated in part by the barrier that hypoxia poses to effective radiotherapy. The other has focused on cellular interactions involving tumour and non-tumour cells within the TME. Over the past decade, strong links have been established between these two themes, providing new insights into fundamental aspects of tumour biology and presenting new strategies for addressing the effects of hypoxia and other microenvironmental features that arise from the inefficient microvascular system in solid tumours. This Review provides a perspective on advances at the interface between these two aspects of the TME, with a focus on translational therapeutic opportunities relating to the elimination and/or exploitation of tumour hypoxia.

Benzotriazine Di-Oxide Prodrugs for Exploiting Hypoxia and Low Extracellular pH in Tumors

Extracellular acidification is an important feature of tumor microenvironments but has yet to be successfully exploited in cancer therapy. The reversal of the pH gradient across the plasma membrane in cells that regulate intracellular pH (pHi) has potential to drive the selective uptake of weak acids at low extracellular pH (pHe). Here, we investigate the dual targeting of low pHe and hypoxia, another key feature of tumor microenvironments. We prepared eight bioreductive prodrugs based on the benzotriazine di-oxide (BTO) nucleus by appending alkanoic or aminoalkanoic acid sidechains. The BTO acids showed modest selectivity for both low pHe (pH 6.5 versus 7.4, ratios 2 to 5-fold) and anoxia (ratios 2 to 8-fold) in SiHa and FaDu cell cultures. Related neutral BTOs were not selective for acidosis, but had greater cytotoxic potency and hypoxic selectivity than the BTO acids. Investigation of the uptake and metabolism of representative BTO acids confirmed enhanced uptake at low pHe, but lower intracellular concentrations than expected for passive diffusion. Further, the modulation of intracellular reductase activity and competition by the cell-excluded electron acceptor WST-1 suggests that the majority of metabolic reductions of BTO acids occur at the cell surface, compromising the engagement of the resulting free radicals with intracellular targets. Thus, the present study provides support for designing bioreductive prodrugs that exploit pH-dependent partitioning, suggesting, however, that that the approach should be applied to prodrugs with obligate intracellular activation.

Reduction of intracellular pH as a strategy to enhance the pH-dependent cytotoxic effects of melphalan for human breast cancer cells

The microenvironment within solid tumors is slightly acidic, and manipulation of this extracellular acidity to cause intracellular acidification might be used to increase selective antitumor effects of some anticancer drugs. Potential mechanisms include inhibition of repair of DNA damage and inhibition of repopulation of tumor cells between successive courses of chemotherapy. Here, we evaluate the influence of extracellular pH (pHe) and of two agents that lead to intracellular acidification (cariporide and S3705) on toxicity of melphalan for two human breast cancer cell lines (MDA-MB231 and MCF7). Both the total number and number of colony-forming cells were evaluated during and after three sequential weekly drug treatments. Our results indicate the following: (a) Slow or absent repopulation after the first course of treatment that is influenced minimally by pHe. (b) Rapid repopulation after the second course of treatment that may be inhibited at low pHe. (c) Effects of low pHe following treatment with melphalan to increase cell kill. (d) Small effects of incubation in cariporide and S3705 at low pHe to increase the net cell kill after treatment with melphalan. Although these results add to evidence that manipulation of intracellular pH within the acidic environment of solid tumors can influence the effects of chemotherapy, they are too small and inconsistent to warrant clinical evaluation.

The pH partition theory predicts the accumulation and toxicity of doxorubicin in normal and low-pH-adapted cells

The accumulation and toxicity of the weak base doxorubicin has been investigated as a function of extracellular pH, intracellular pH and the cellular pH gradient in cells previously cultured under normal (pH 7.4) and low-pH (6.8) conditions. Low-pH-adapted cells exhibit transmembrane pH gradients which substantially differ from normal cells at the same extracellular pH. No relationship was obtained between intracellular pH and the uptake or toxicity of doxorubicin in the two cell types. In contrast, doxorubicin accumulation and toxicity increased with increasing extracellular pH in both normal and low-pH-adapted cells. However, at the same extracellular pH, drug cytotoxicity was more pronounced in normal than in low-pH-adapted cells. The difference in doxorubicin accumulation and cytotoxicity at the same extracellular pH was found to be dependent on the difference in the transmembrane pH gradient of the two cell types. As the cellular pH gradient differs between tumour and normal tissue, this observation suggests a basis for enhancing cellular drug uptake in either tissue type.

Cytotoxicity of weak electrolytes after the adaptation of cells to low pH: role of the transmembrane pH gradient

Theory suggests that the transmembrane pH gradient may be a major determinant of the distribution of lipophilic weak electrolytes across the cell membrane. The present study evaluates the extent to which this factor contributes to pH-dependent changes in the cytotoxicity of two such chemotherapeutic drugs: chlorambucil and mitoxantrone. Experiments were performed with two cell types of the same origin but exhibiting different pH gradients at the same extracellular pH (pHe): CHO cells cultured under normal physiological conditions (pH 7.4) and acid-adapted cells obtained by culturing under low pH conditions (6.8). Over the pHe range examined (6.0-7.6), the difference between intracellular pH (pHi) and pHe increased with decreasing pHe. Acid-adapted cells were more resistant to acute changes in pHi than normal cells, resulting in substantially larger gradients in these cells. Drug cell survival curves were performed at pHe values of 6.4, 6.8 and 7.4. The cytotoxicity of chlorambucil, a weak acid, increased with decreasing pHe, and low pH-adapted cells were more sensitive than normal cells at the same pHe. In contrast, for the weak base, mitoxantrone, cytotoxicity increased with pHe and was more pronounced in normal cells. As predicted by the theory, the cytotoxicity of both drugs changed exponentially as a function of the pH gradient, regardless of cell type. For mitoxantrone, the rate of such change in cytotoxicity with the gradient was approximately two times greater than for chlorambucil. This difference is probably due to the presence of two equally ionizable crucial groups on mitoxantrone vs one group on chlorambucil. It is concluded that the cellular pH gradient plays a major role in the pH-dependent modulation of cytotoxicity in these weak electrolytes. The data obtained also suggest that a pronounced differential cytotoxicity may be expected in vivo in tumour vs normal tissue. In comparison with normal cells at a pHe of 7.4 (a model of cells in normal tissues), acid-adapted cells at a pHe of 6.8 (a model of cells distal from supplying blood vessels in tumours) were more sensitive to chlorambucil, with a dose-modifying factor of approximately 6, and were more resistant to mitoxantrone by a factor of 14.

Bioreducible mustards: a paradigm for hypoxia-selective prodrugs of diffusible cytotoxins (HPDCs)

Existing hypoxia-selective cytotoxins (HSCs) are designed to kill only the hypoxic subpopulation in tumours, and to be used in conjunction with other therapies (e.g., radiation). A new class of drugs, hypoxia-activated prodrugs of diffusible cytotoxins (HPDCs) are proposed. These are designed to exploit, rather than merely deal with, tumour hypoxia, by releasing diffusible cytotoxins on bioreduction in hypoxic regions. Such diffusible cytotoxins are required to be much more cytotoxic than the parent prodrug, to be sufficiently stable (half lives from 0.1 to 10 min) to allow them to diffuse up to 200 microns from the hypoxic regions, and to be equally effective against all major tumour cell subpopulations, including non-cycling cells. Nitrogen mustards, which show little cell cycle specificity, which kill cells by a well-understood mechanism (DNA cross-links), and which have stabilities and reactivities able to be predictably controlled by structural variations, are proposed as suitable candidates fur such diffusible cytotoxins. Design parameters for two classes of potential HPDCs are discussed; nitro-deactivated aromatic mustards, and cobalt (III) complex-deactivated aliphatic mustards. Examples of both classes show greater cell-killing activity against intact compared with dissociated multi-cellular spheroids. This suggests they may indeed function as HPDCs, by penetrating to the hypoxic core of the spheroid and there releasing potent cytotoxins which diffuse out to kill surrounding cells at lower oxygen tensions.

Cytotoxicity of chlorambucil and chlorambucil-fatty acid conjugates against human lymphomas and normal human peripheral blood lymphocytes

The cytotoxic activity of chlorambucil (Chl) and of chlorambucil-fatty acid conjugates of different degree of unsaturation have been assayed in vitro upon two human lymphoma cell lines and comparatively, upon quiescent and mitogen-activated lymphocytes from healthy blood donors. The cell toxicity observed with Chl-arachidonic acid and Chl-docosahexaenoic acid against lymphoma cells was, at any experimental condition used, equal or higher than the individual toxic potential of either chlorambucil or fatty acids. The two conjugates, like chlorambucil alone, were toxic against mitogen-activated lymphocytes. Contrary to chlorambucil, Chl-arachidonic at any concentration tested, lacked of toxicity towards normal non-activated lymphocytes. Chl-oleic acid conjugate was, whatever the cell species tested, much less toxic than Chl alone. In conclusion, the coupling of chlorambucil with polyunsaturated fatty acids increases: (a) the selectivity against neoplastic versus quiescent lymphocytes and (b) the toxicity for B-lymphoma cells. The selective effect of Chl-fatty acid conjugates is discussed in relation with the expression of an AFP/AFP-receptor autocrine system in malignant lymphoblastoid cells and in mitogen-activated lymphocytes.

Epidemiologic and toxicologic evidence for chronic health effects and the underlying biologic mechanisms involved in sub-lethal exposures to acidic pollutants

Since the 1880s, a disparate and extensive literature has evolved examining the biologic effects of acidification on cells. More recently, effects on the health of human and other species of acidic agents contained, for example, in pollutants have been suggested, particularly relating to long-term exposures. This paper provides a review of the epidemiologic and toxicologic evidence concerning health effects--particularly carcinogenicity--attributable to sub-lethal acid exposure. Underlying biologic mechanisms that explain adverse health outcomes include pH modulation of toxicity for a number of xenobiotics (including carcinogens, genotoxins, and teratogens), and low-pH-induced changes of cells involving, for example, alterations in mitotic and enzyme regulation. More focused research is recommended to test the relationship between long-term exposures to acidic agents (with a consequent lowered cellular pH) and various health effects.

Extracellular pH, transmembrane distribution and cytotoxicity of chlorambucil

The effects of extracellular pH (6.2 to 7.3) on uptake and cytotoxicity of the weak acid anti-tumor drug chlorambucil were investigated. Decreasing extracellular pH from 7.3 to 6.5 had a negligible effect on the intracellular pH of Chinese hamster V79 fibroblasts, thus resulting in the formation of a transmembrane pH gradient (intracellular alkaline). Addition of high concentrations of acetate or bicarbonate partially collapsed the pH gradient. Chlorambucil (pKa = 5.8) behaved as a weak acid with enhanced accumulation and cytotoxicity at extracellular pH less than 7.0. As predicted for a weak acid, partial collapse of the transmembrane pH gradient decreased both uptake and cell killing. Since the interstitial pH of micrometastases and solid tumors of many cancers is low relative to normal tissues, these results have potential implications for both in vitro drug testing and in vivo therapy.

Stability of solutions of antineoplastic agents during preparation and storage for in vitro assays. General considerations, the nitrosoureas and alkylating agents

In vitro drug sensitivity of tumour biopsies is currently being determined using a variety of methods. For these chemosensitivity assays many drugs are required at short notice, and this in turn means that the drugs must generally be stored in solution. There are, however, a number of potential problems associated with dissolving and storing drugs for in vitro use, which include (a) drug adsorption; (b) effects of freezing; (c) drug stability under the normal conditions of dilution and setting up of an in vitro assay; and (d) insolubility of drugs in normal saline (NS) or phosphate-buffered saline (PBS). These problems are considered in general, and some recommendations for use of solutions of drugs in in vitro assays are suggested. The nitrosoureas and alkylating agents are also investigated in greater detail in this respect. The nitrosoureas are found to be very labile in PBS at pH 7, with 5% degradation (t0.95) occurring in 10-50 min at room temperature. These values are increased about 10-fold on refrigeration and about 5- to 10-fold on reduction of the pH of the medium to pH 4-5. At pH 7 and room temperature, t0.95 is observed in under 1 h with the alkylating agents nitrogen mustard, chlorambucil, melphalan, 2,5-diaziridinyl-3,6-bis(2-hydroxyethylamino)-1,4-benzoquinone (BZQ), dibromodulcitol, dibromomannitol, treosulphan, and procarbazine. Of the other alkylating agents, 4-hydroperoxycylophosphamide (sometimes used in vitro in place of cyclophosphamide), busulphan, dianhydrogalactitol, aziridinylbenzoquinone (AZQ), and dacarbazine have a t0.95 of between 2 and 24 h, while ifosfamide and pentamethylmelamine are both stable in aqueous solution for greater than 7 days. About half the drugs studied in detail have been stored frozen in solution for in vitro use, although very little is known about their stability under these conditions.