Highly Sensitive Hill-Type Small-Molecule pH Probe That Recognizes the Reversed pH Gradient of Cancer Cells

Abstract

A hallmark of cancer cells is a reversed transmembrane pH gradient, which could be exploited for robust and convenient intraoperative histopathological analysis. However, pathologically relevant pH changes are not significant enough for sensitive detection by conventional Henderson-Hasselbalch-type pH probes, exhibiting an acid-base transition width of 2 pH units. This challenge could potentially be addressed by a pH probe with a reduced acid-base transition width (i.e., Hill-type probe), appropriate p K, and membrane permeability. Yet, a guideline to allow rational design of such small-molecule Hill-type pH probes is still lacking. We have devised a novel molecular mechanism, enabled sequential protonation with high positive homotropic cooperativity, and synthesized small-molecule pH probes (PHX1-3) with acid-base transition ranges of ca. 1 pH unit. Notably, PHX2 has a p K of 6.9, matching the extracellular pH of cancer cells. Also, PHX2 is readily permeable to cell membrane and allowed direct mapping of both intra- and extracellular pH, hence the transmembrane pH gradient. PHX2 was successfully used for rapid and high-contrast distinction of fresh unprocessed biopsies of cancer cells from normal cells and therefore has broad potentials for intraoperative analysis of cancer surgery.

Metabolic Reprogramming, Questioning, and Implications for Cancer

“…Finding a universal target for cancer metabolism would be a huge leap in the search for a therapeutic solution. However, for now, this universal target of metabolism, if it exists, is not yet known, although there may be promising leads, such as a reversed intra/extracellular pH gradient [19][20][21][22][23].…”

High‐Contrast Fluorescence Diagnosis of Cancer Cells/Tissues Based on β‐Lapachone‐Triggered ROS Amplification Specific in Cancer Cells

“…Thus, in recent years, some efforts have also been devoted to the deregulated tumor metabolism, i.e., aerobic glycolysis (also known as Warburg effect), [23, 24] which creates a cancer‐specific microenvironment regardless of cancer types, such as the decreased pH (acidic extracellular macroenvironment) and the enhanced levels of intracellular glutathione (GSH) and reactive oxygen species (ROS). However, since both pH and GSH levels between cancerous and normal tissues are not very different (pH 6.5–6.9 for tumor microenvironment and 7.4 for normal tissues; [25] intracellular GSH levels for cancer and normal cells are in 2–13 mM), [26, 27] the efficacy of the pH‐ or GSH‐based fluorescent probes still remains challenging, and indeed only a limited number of examples of success have been reported to date [28–35] …”

“…However,since both pH and GSH levels between cancerous and normal tissues are not very different (pH 6.5-6.9 for tumor microenvironment and 7.4 for normal tissues; [25] intracellular GSH levels for cancer and normal cells are in 2-13 mM), [26,27] the efficacy of the pHor GSH-based fluorescent probes still remains challenging, and indeed only alimited number of examples of success have been reported to date. [28][29][30][31][32][33][34][35] Conversely,c ancer cells have approximately ten times higher ROS(including HOC,O 2 C À ,H 2 O 2 ,ClO À ,ONOO À , 1 O 2 , etc. )l evel than normal cells, [36][37][38][39] and thus the ROS-responsive fluorescent probes,i np rinciple,s hould possess higher selectivity for cancer cells/tissues over normal ones.I nf act, the characteristic has been utilized to fabricate the ROSsensitive drug delivery systems for cancer therapy,s uch as those consisting of ROS-responsive polymers.…”

Allosteric Cooperativity and Template-Directed Synthesis with Stacked Ligands in Porphyrin Nanorings

“…We compare two types of 1:2 complexes (Figure 1): a 6-porphyrin nanoring binding two stacked tridentate ligands, c-P6•(T3) 2 and c-P6•(T3N) 2 , and a 12-porphyrin nanoring binding two stacked hexadentate ligands, c-P12•(T6e) 2 . The allosteric effect is strongly negative (anti-cooperative) for formation of c-P6•(T3) 2 and c-P6•(T3N) 2 , but strongly positive for formation of c-P12•(T6e) 2 . This difference in cooperativity is reflected by the fact that T3 and T3N do not act as templates for the formation of c-P6, whereas T6e is an effective template for the synthesis of c-P12.…”