🧬 biology

Alternative Explanation for the Effect of myo-Inositol Trispyrophosphate on Normal Human Erythrocytes: 31P NMR Studies

Using 31P NMR spectroscopy, this study demonstrates that myo-inositol trispyrophosphate (ITPP) does not penetrate normal human red blood cells to bind intracellularly with hemoglobin, suggesting that its previously reported effects on oxygenation are likely caused by indirect perturbations of ionic equilibria rather than direct intracellular interaction.

Original authors: Philip W. Kuchel, Ken-Tye Yong, Biswaranjan Mohanty, Ian Luck

Published 2026-09-25

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CC BY 4.0

Original authors: Philip W. Kuchel, Ken-Tye Yong, Biswaranjan Mohanty, Ian Luck

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The human body relies on a constant, quiet delivery of oxygen to keep its tissues alive and functioning. This task falls to red blood cells, tiny, flexible discs that travel through the bloodstream carrying hemoglobin, the protein molecule that grabs onto oxygen in the lungs and releases it where it is needed. For decades, scientists have searched for ways to tweak this system, hoping to help patients suffering from conditions where tissues are starved of oxygen, such as in certain cancers or heart diseases. The idea is to find a substance that can enter these red blood cells and change how tightly they hold onto oxygen, effectively making them release more of it to struggling tissues. One such substance, a complex molecule called myo-inositol trispyrophosphate, or ITPP, was proposed as a potential solution. It was believed to be a master key, capable of slipping through the red blood cell's outer wall to reach the hemoglobin inside and alter its behavior directly. This concept was so compelling that it moved into clinical trials, with the assumption that the drug worked by entering the cell and binding to the oxygen carrier.

However, a team of researchers at the University of Sydney decided to look closer at this assumption using a technique that allows scientists to see molecules without touching them. They used a method called nuclear magnetic resonance spectroscopy, which acts like a highly sensitive camera for atoms containing phosphorus. Since ITPP is rich in phosphorus, the researchers could track exactly where it went when they added it to a suspension of fresh human red blood cells. They wanted to know if the molecule actually crossed the cell membrane to get inside, or if it stayed on the outside. To do this, they placed the blood cells in a machine that could distinguish between molecules floating in the liquid surrounding the cells and those trapped safely inside the cells. They added a special magnetic agent to the liquid outside the cells that would blur the signal of any molecule it could reach, effectively silencing anything that remained on the outside, while leaving the signals from molecules safely inside the cells clear and sharp.

The results were striking and contrary to the prevailing belief. Over a period of more than fourteen hours, the researchers watched the signals from the ITPP molecules. If the drug had entered the cells, they would have seen a second, distinct signal appear, representing the drug sitting inside the protective wall of the red blood cell. Instead, they saw nothing of the sort. The ITPP signal remained entirely outside the cells, and when they added the magnetic agent to the outside liquid, the signal vanished completely. This proved that the drug was not inside the cells at all; it was entirely exposed to the outside environment. The researchers also tested whether the drug might have entered and then been quickly broken down by the cell's internal machinery. To check this, they broke open the cells and mixed the drug directly with the cell's internal fluid. Even in this direct contact, the drug remained stable and did not disappear, proving that if it had entered the cells in the earlier experiments, it would have been detectable. The conclusion was clear: the red blood cell membrane acts as a strict barrier to this molecule, preventing it from entering in any significant amount.

This finding forces a re-evaluation of how ITPP works. If the drug cannot get inside the cell to touch the hemoglobin directly, then the changes in oxygen delivery observed in previous studies must be caused by something else. The researchers propose that the drug works from the outside, acting like a heavy weight placed on one side of a balance scale. By sitting on the outside of the cell, the highly charged drug alters the electrical and chemical balance between the inside and outside of the cell. This shift changes the acidity levels inside the red blood cell, which in turn influences how the hemoglobin behaves, even though the drug never actually touches it. It is a subtle, indirect effect, much like how a change in the weather outside a house can alter the temperature inside without anyone opening a window. The study suggests that the therapeutic effects attributed to this drug are likely the result of these external chemical shifts rather than a direct interaction with the oxygen-carrying protein inside the cell.

The implications of this discovery extend beyond just one molecule. The assumption that ITPP enters red blood cells has been the foundation for many studies in cancer research and cardiovascular medicine. If the drug does not enter the cells as previously thought, then the mechanisms behind its effects in patients need to be reconsidered. The researchers emphasize that while the drug may still have benefits, the story of how it works is different than the one told so far. They call for future studies to directly measure whether the drug enters cells in living patients, rather than assuming it does based on the biological changes it causes. Until such evidence is found, the idea that this molecule acts as a direct intracellular effector remains a hypothesis that has not been proven. The work serves as a reminder that in science, even well-established ideas must be tested against direct observation, and sometimes the most important discovery is realizing that a molecule stays exactly where it was put, outside the door.

Technical Summary: Alternative Explanation for the Effect of myo-Inositol Trispyrophosphate on Normal Human Erythrocytes

Problem Statement
myo-Inositol trispyrophosphate (ITPP) has been proposed as a therapeutic agent for tissue hypoxia and cancer, based on the premise that it acts as a membrane-permeant allosteric effector of hemoglobin. The prevailing hypothesis suggests that ITPP rapidly crosses the red blood cell (RBC) membrane, enters the cytoplasm, and binds directly to hemoglobin to lower its oxygen affinity, thereby enhancing oxygen release to tissues. However, direct experimental evidence confirming the intracellular accumulation of ITPP in normal human RBCs has been absent, despite its proposed clinical applications. Furthermore, the permeability of polyvalent anions (>2-) across the RBC membrane is generally considered negligible on minute-to-hour timescales, making the claim of ITPP's "unique" permeability scientifically contentious.

Methodology
The authors employed non-invasive $^{31}$ P nuclear magnetic resonance (NMR) spectroscopy to directly monitor the transmembrane movement and intracellular fate of ITPP in fresh human RBC suspensions. The experimental design utilized several specific strategies to distinguish between intracellular and extracellular pools of phosphorus-containing compounds:

Spectral Verification: The identity of the commercial ITPP sample was confirmed via $^{31}$ P NMR, verifying its characteristic nine-line spin-system pattern consistent with the myo-inositol 1,6:2,3:4,5-tripyrophosphate structure.
Paramagnetic Quenching (Mn $^{2+}$ Test): The study utilized extracellular Mn $^{2+}$ ions as a paramagnetic relaxation agent. Mn $^{2+}$ cannot cross the intact RBC membrane; therefore, it broadens and suppresses NMR signals from extracellular species while leaving intracellular signals (protected by the membrane) unaffected.
Probe Molecules: The system was calibrated using known permeability markers:
- DMMP (Dimethyl methylphosphonate): A neutral molecule that exhibits slow exchange, showing distinct split peaks for intra- and extracellular pools.
- MeP (Methylphosphonate): A monoanion with slow permeation, used to verify the timescale of the experiment.
- TEP (Triethyl phosphate): A neutral molecule with rapid exchange, showing a single averaged peak.
Time-Course and Stability Assays:
- Intact RBCs: Suspensions were incubated with ITPP for over 14 hours to monitor for the emergence of intracellular ITPP signals or metabolic degradation.
- Hemolysates: RBCs were lysed to expose ITPP directly to the cytoplasmic enzymatic environment at 37°C to test for chemical stability and metabolic hydrolysis.

Key Results

Absence of Intracellular ITPP: In intact RBC suspensions exposed to ITPP (up to 0.9 mM extracellular concentration), no split peaks or separate intracellular resonances for ITPP were detected over a 14.2-hour period. The limit of detection (LOD) was established at approximately 30 µM, well below the concentrations required to modulate hemoglobin oxygen affinity.
Mn $^{2+}$ Suppression: Upon addition of extracellular Mn $^{2+}$ , all ITPP resonances were completely suppressed. In contrast, intracellular metabolites like 2,3-bisphosphoglycerate (2,3-BPG) and ATP remained unaffected. This confirms that the detectable ITPP resides entirely in the extracellular space and does not cross the membrane in measurable quantities.
Chemical Stability: In hemolysate experiments, ITPP remained chemically stable and was not hydrolyzed by cytoplasmic enzymes over 80 minutes, whereas endogenous ATP and ADP were rapidly metabolized. This rules out rapid intracellular degradation as the reason for the lack of detectable ITPP in intact cells.
Metabolic Perturbations: During the incubation of intact RBCs, a gradual acidification of the intracellular pH (from ~7.11 to 6.85) was observed. However, this shift was attributed to the ongoing hydrolysis of 2,3-BPG (accompanied by a stoichiometric increase in inorganic phosphate) and the general metabolic decline of the stored RBCs, rather than a direct effect of ITPP. The ITPP resonances themselves remained stable in both chemical shift and integrated intensity throughout the experiment.

Significance and Alternative Mechanism
The paper concludes that ITPP does not permeate the normal human RBC membrane in measurable quantities under the conditions examined. Consequently, the reported physiological effects of ITPP on RBCs are unlikely to result from direct intracellular binding to hemoglobin.

Instead, the authors propose an alternative mechanism based on Donnan electrochemical equilibrium. As a highly charged, impermeant polyanion, extracellular ITPP alters the distribution of permeant ions (Cl $^-$ and HCO $_3^-$ ) across the membrane to maintain electroneutrality. This redistribution perturbs the transmembrane potential and intracellular pH (pHi). The authors argue that the pH shifts and spectral changes previously attributed to ITPP entry (and often cited as evidence for Band 3 transporter involvement) are fully explainable by these extracellular Donnan effects. The inhibition of these effects by DIDS (an anion exchange inhibitor) is consistent with the disruption of the Cl $^-$ /HCO $_3^-$ exchange cycle required for the Donnan shift, rather than proof of ITPP transport.

Conclusion
The study challenges the prevailing assumption that ITPP acts via rapid uptake into RBCs. The authors assert that the observed effects are more plausibly ascribed to indirect effects arising from the perturbation of ionic equilibria and membrane potential by an extracellular polyanion. They call for a re-evaluation of mechanistic interpretations in oncology and cardiovascular studies involving ITPP, suggesting that future research must directly quantify intracellular ITPP concentrations rather than inferring transport from downstream biological responses.

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