X-Ray Vision Inside Cells: Rhenium Drug Candidates Defy Their Targeting Ligands

Chemists designing the next generation of metal-based anticancer drugs like to believe they can steer molecules to precise destinations inside a cell. Attach the right chemical tag, the reasoning goes, and the drug will home in on the mitochondria, the nucleus, or the cytosol. A new study puts that assumption to a stringent test — and the result is a surprise that could reshape how metallodrugs are designed. Using cryogenic X-ray fluorescence nanoanalysis at one of the world’s brightest synchrotron sources, researchers at Ruhr University Bochum and DESY in Hamburg tracked three rhenium complexes with deliberately different targeting ligands through single HeLa cancer cells, and found that all three ended up in remarkably similar places regardless of their chemical instructions.

The work, published in Frontiers in Chemical Biology, addressed a persistent gap in metallodrug research. While individual metal complexes have been imaged inside cells before, no systematic study had taken the same metal core and directed it to different cellular compartments by strategically modifying its ligand shell. The team built three rhenium(I) tricarbonyl complexes on a stable pyridine-triazole scaffold, installing the directing groups in a single-step copper-catalyzed azide-alkyne cycloaddition — the click chemistry reaction that makes the ligand framework so versatile. Sulfonamide derivatives on the first complex, Re1, were expected to localize in the cytosol. A pyrene moiety on Re2 was meant to target DNA. Triphenylphosphonium groups on Re3, a well-established mitochondrial targeting motif, should have steered that complex to the power plants of the cell.

All three complexes were synthesized in yields around 75 percent and fully characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis. HeLa cells grown on silicon nitride membranes were then incubated with 10 micromolar solutions of each compound for 45 minutes — a concentration and duration chosen through a range-finding study that balanced sufficient drug uptake against cell viability. What happened next is what sets this study apart from conventional imaging work: instead of chemically fixing the cells, which can redistribute metals and distort the very distributions researchers want to measure, the team plunge-froze them in a liquid ethane-propane mixture, vitrifying the cellular contents into amorphous ice within milliseconds.

That cryogenic preservation is widely regarded as the gold standard for spatially resolved nanoanalysis, because it locks cells close to their natural, biologically relevant state. The frozen samples traveled in liquid nitrogen to the P06 hard X-ray micro/nanoprobe beamline at PETRA III, where a 17-kiloelectronvolt synchrotron beam was focused to roughly 440 by 500 nanometers with a photon flux of 5.7 billion photons per second. Inside a cryogenic vacuum chamber held at about 120 kelvin and pressures between 10^-6 and 10^-8 millibar, each cell was raster-scanned in a continuous snake pattern at 400-nanometer steps. A cloverleaf-shaped Rococo 2 detector with four monolithically integrated silicon drift elements captured the fluorescence signal, allowing measurement times as short as 15 minutes per cell — fast enough to build a statistically meaningful dataset across an ensemble of 76 cells.

Two of the complexes, Re1 and Re2, fluoresce in the visible range, which enabled a correlative imaging workflow: the same cells were first mapped by fluorescence microscopy on the silicon nitride wafers, then vitrified and relocated on the X-ray nanoscope for element-specific imaging. The agreement between the two modalities was striking. Rhenium and bromine — the bromide ligand originally bound to the metal — appeared at the same intracellular spots in the X-ray maps, and those spots matched the fluorescence patterns. Because bromine almost certainly remained in the ligand sphere, the close overlap of Re and Br signals provides strong evidence that the complexes survived uptake largely intact. That verification also validates a marker-free approach for the third complex, Re3, which emits no visible fluorescence and could only be tracked through its rhenium signature.

Then came the surprise. Despite their chemically distinct ligand environments and the literature-backed expectations that each would go to a different compartment, all three complexes showed a similar intracellular distribution — accumulating in the cytosol and the perinuclear region, with nucleoli showing notably lower levels. Quantitatively, Re1 yielded average area concentrations of 38 plus or minus 26 nanograms per square centimeter, Re2 reached 34 plus or minus 7, and Re3 came in at 7 plus or minus 8, all statistically significantly above the untreated control level of less than 0.3. The researchers suggest that the large, relatively lipophilic metal core dominates over the targeting groups, and that such bulky hydrophobic complexes likely enter cells by endocytosis and initially lodge in lysosomes — the cell’s common defense compartment against foreign substances. The perinuclear rhenium pattern is consistent with that lysosomal route, and longer treatment times might allow redistribution to other organelles such as mitochondria.

The XRF maps did more than locate the drug. Because X-ray fluorescence simultaneously detects endogenous biometals, the team could quantify how treatment perturbed the cell’s native potassium and zinc inventories. Potassium levels rose dramatically: by roughly 450 percent after Re1 treatment, 370 percent for Re2, and about 350 percent for Re3, compared with untreated cells — a change the authors interpret as a global cellular stress response. Potassium homeostasis is a sensitive physiological marker; elevated intracellular potassium is known to suppress apoptosis-inducing enzymes such as caspase-3 and endonuclease, and altered potassium signaling has been implicated in cancer progression and drug resistance. The stress response appears to be triggered once a certain rhenium threshold is crossed, since potassium levels did not scale with the amount of rhenium in individual cells.

Zinc told a different and arguably more intriguing story. Average zinc concentrations rose modestly — about 40 percent for Re1 and 60 percent for Re2 — but the spatial analysis revealed something the bulk numbers could not: rhenium and zinc were colocalized. Pixel-by-pixel correlation within single cells produced Pearson coefficients of 0.62 for Re1 and 0.84 for Re2, with Re3 at 0.55, while potassium-rhenium correlations remained weak across the board (0.48, 0.19, and 0.40 respectively). Cells with higher rhenium content also tended to show elevated zinc levels, and within individual cells the rhenium hotspots overlapped with zinc-rich regions. One possible explanation is that rhenium complexes bind zinc-containing structures — zinc-finger proteins being an obvious candidate class, given their roles in DNA transcription, repair, and signal transduction — and the cell compensates by importing more zinc. The authors are careful to note that XRF colocalization provides evidence, not molecular proof, of such interactions.

For the field of metallodrug development, the implications cut both ways. On one hand, the finding that directing ligands exert only minor influence on localization under these conditions is a humbling reality check for rational targeting strategies; overall physicochemical properties such as size, lipophilicity, and charge may outweigh the contributions of individual targeting motifs. On the other hand, the demonstrated workflow — correlative fluorescence and cryogenic XRF nanoanalysis, rapid vitrification, and statistically robust pixel-by-pixel analysis across dozens of cells — offers a powerful, marker-free template for interrogating any non-fluorescent metal drug candidate. The observed rhenium-zinc association now provides a concrete starting point for hunting down the zinc-linked targets of emerging metallodrugs, a question the team says deserves follow-up at the molecular scale with complementary biochemical methods.

Subject of Research: Intracellular distribution of organometallic rhenium anticancer complexes imaged by cryogenic X-ray fluorescence nanoanalysis in HeLa cells

Article Title: Nanoprobe XRF analysis reveals the intracellular distribution of organometallic rhenium complexes with different directing ligand

Article References: Nanoprobe XRF analysis reveals the intracellular distribution of organometallic rhenium complexes with different directing ligand. (n.d.). https://doi.org/10.3389/fchbi.2026.1903385

Image Credits: AI Generated

DOI: 10.3389/fchbi.2026.1903385

Keywords: rhenium complexes, X-ray fluorescence, cryogenic nanoanalysis, metallodrugs, HeLa cells, anticancer agents, zinc colocalization, potassium homeostasis, synchrotron imaging, drug targeting, cellular stress response, click chemistry