JP-1302 is a 9-anilinoacridine, a planar tricyclic scaffold shared by classic DNA intercalators such as quinacrine, aminacrine and ethacridine, all of which are known to inhibit Pol I. Its asymmetric anilino tail group, however, places it structurally closer to amsacrine, a clinically used topoisomerase II poison developed for acute leukaemias. The compound was originally engineered as a highly selective alpha-2C-adrenergic receptor antagonist with a favourable lipophilicity profile (logP 4.88 and topological polar surface area of 31.4 square angstroms) that underlies its documented blood-brain barrier penetrance and in vivo activity at tolerated milligram-per-kilogram doses in rodents. A previous proteome-wide screen had flagged JP-1302 as a broad transcriptional perturbagen that suppresses RNA polymerase II phosphorylation at micromolar concentrations, but its effects on Pol I and nucleolar biology had never been examined.

The Karolinska team, led by Mikael Lindström together with Jiri Bartek and colleagues, first mined the DeepCover MOA proteomic dataset and noticed that JP-1302 markedly downregulates POLR1A, the catalytic subunit of Pol I, relative to most other profiled compounds. Resazurin-based profiling across a panel of cancer cell lines of varied origin and p53 status, including osteosarcoma, glioblastoma, colorectal carcinoma, breast cancer and lung adenocarcinoma, plus two non-cancerous lines, revealed growth-inhibitory IC50 values ranging from 0.75 to 4.28 micromolar at 24 hours and 0.13 to 1.63 micromolar at 48 hours. Strikingly, p53-wildtype and p53-knockout HCT116 cells converged to nearly identical IC50 values by 48 hours, suggesting that the growth-inhibitory effect operates largely independently of the tumour suppressor p53.

Immunoblotting then showed that JP-1302 triggers a concentration-dependent depletion of POLR1A protein, evident from around 500 nanomolar and most pronounced at 1 to 5 micromolar after 24 hours. The degradation proved selective: other Pol I subunits and pre-initiation complex components were largely spared, with only POLR1E showing a marginal reduction. Quantitative PCR confirmed that POLR1A messenger RNA was unaltered, pointing to post-transcriptional destruction, and co-treatment with the proteasome inhibitor MG-132 rescued POLR1A levels, confirming proteasome-mediated degradation. Crucially, knocking down the alpha-2C-adrenergic receptor did not affect POLR1A depletion, demonstrating that the compound’s anticancer activity here is entirely independent of the receptor it was designed to block. Notably, POLR2A, the catalytic subunit of Pol II, required higher concentrations of 5 micromolar before its levels declined, underscoring Pol I as the more sensitive target.

With the catalytic subunit under attack, the team measured the primary Pol I transcript, the 47S pre-ribosomal RNA. Quantitative PCR in U2OS osteosarcoma cells showed significant 47S downregulation starting from just 100 nanomolar after 3 hours. To confirm this reflected genuine transcriptional inhibition rather than altered RNA turnover, the researchers used a nascent RNA capture assay based on ethynyl uridine labelling. Nascent 47S rRNA was significantly reduced, while nascent Pol II transcripts such as GAPDH and POLR1A were largely unaffected, although the oncogene MYC and the Pol III transcript 5S rRNA were paradoxically increased at 5 micromolar, possibly reflecting compensatory stress responses. An rDNA promoter-driven luciferase reporter also showed reduced activity, though more modestly than at the endogenous locus, likely because the plasmid lacks native chromatin context.

Microscopy made the consequences visually unmistakable. Combining ethynyl uridine incorporation with staining for the nucleolar marker nucleolin, the team observed a significant reduction in nascent RNA synthesis from 500 nanomolar, and at 1 micromolar a prominent nucleolar ring formed by nucleolin, a classic hallmark of nucleolar stress caused by impaired Pol I transcription. Markers of nucleolar sub-compartments told a graded story: fibrillarin, marking the dense fibrillar component, showed altered distribution from 100 nanomolar, visible nucleolar caps appeared at 2 micromolar, and by 5 micromolar the nucleolus underwent complete architectural collapse. The progression from ring and cap formation to full disassembly mirrors the effects of the established Pol I inhibitor BMH-21, which the team used as a positive control.

Nucleolar stress is known to activate p53 through the impaired ribosome biogenesis checkpoint, in which free ribosomal proteins RPL5 and RPL11 bind 5S rRNA and inhibit the E3 ubiquitin ligase MDM2, thereby stabilizing p53. JP-1302 induced p53 accumulation from around 500 nanomolar, with p21 protein rising in parallel up to 2 micromolar. Silencing RPL5 and RPL11 reduced the p53 and p21 response, confirming that JP-1302 engages this canonical checkpoint. Yet at 5 micromolar the picture inverted: p21 protein and its messenger RNA both fell, even in p53-knockout cells, indicating a p53-independent mechanism of p21 depletion at high doses, consistent with the compound’s broader transcriptional suppression. Other p53 targets, ZMAT3 and MDM2, followed the same pattern.

Cell fate experiments revealed a sharp concentration threshold. At 1 micromolar, U2OS cells showed no significant death, and the effects were fully reversible after an 8-hour treatment followed by washout. At 5 micromolar, however, cells died extensively and irreversibly. EdU incorporation assays showed S-phase depletion and accumulation in G1 and G2 at low doses, and remarkably this cell-cycle arrest occurred equally in p53-wildtype and p53-knockout cells, meaning the cytostatic effect does not require p53 signalling. Mechanistically, the higher-dose regime proved far more destructive: a RADAR assay detected covalent trapping of topoisomerase II alpha and beta on DNA at 3 micromolar, chromatin fractionation showed enrichment of TOP2A, TOP2B, TOP1 and the FACT complex subunits SSRP1 and SPT16, and only at these higher concentrations did the DNA damage marker gamma-H2AX appear, delayed until 24 hours. Molecular docking and a fluorescence intercalator displacement assay supported direct DNA intercalation, though without the clear GC-content preference shown by actinomycin D and BMH-21.

The authors are candid about limitations: the non-cancerous BJ fibroblasts and RPE1 cells showed IC50 values in the same low-micromolar range as the most sensitive cancer lines, so a robust in vitro therapeutic window has not yet been demonstrated, and no reconstituted in vitro Pol I transcription assay was performed to prove direct enzyme inhibition. The upstream mechanism by which POLR1A is selectively routed to the proteasome also remains unresolved, though the E3 ligase FBXL14 is a strong candidate. Even so, the compound’s central nervous system pharmacokinetics make it especially intriguing for malignant brain tumours: all three glioblastoma lines tested, LN229, A172 and H4, showed low micromolar IC50 values, and alpha-2C-adrenergic receptors are expressed in specific brain structures and glioblastoma microenvironments including pericytes. The team suggests that chemical optimization of the acridine scaffold could yield more selective analogues, and that combining Pol I inhibition with autophagy inhibitors, a strategy known to counter pro-survival autophagy in solid tumours, may boost efficacy. For now, JP-1302 stands as a pharmacologically distinct addition to the growing arsenal of ribosome biogenesis inhibitors, one that couples rRNA synthesis shutdown with topoisomerase trapping in a single, brain-penetrant scaffold.

Subject of Research: Inhibition of RNA polymerase I transcription and nucleolar stress induction by the acridine derivative JP-1302 in cancer cell lines

Article Title: JP-1302 inhibits RNA polymerase I transcription and induces nucleolar stress in cancer cell lines

Article References: Saproo, S., Papadaki, S., Fernandez-Martinez, J., Sultana, P., Kanellis, D. C., Bartek, J., & Lindström, M. S. (2026). JP-1302 inhibits RNA polymerase I transcription and induces nucleolar stress in cancer cell lines. Cell Death Discovery, 12(1), Article 389. https://doi.org/10.1038/s41420-026-03381-8

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03381-8

Keywords: JP-1302, RNA polymerase I, nucleolar stress, ribosome biogenesis, POLR1A, cancer, glioblastoma, p53, topoisomerase II, DNA intercalation, FACT complex, acridine derivatives

Organotin compounds have long intrigued medicinal chemists because of their structural versatility and pronounced biological activity, but translating that raw potency into safe therapies has proven difficult. The type and number of organic groups attached to the tin atom strongly influence both cytotoxicity and selectivity, and diorganotin species of the form R2SnX2 generally offer the best balance between killing cancer cells and sparing healthy tissue. Methyl, butyl, and phenyl substituents each tune the Lewis acidity of the central tin and the spatial shape of the complex, which in turn governs how the molecule engages biological targets. The problem, the researchers note, is that the quantitative relationship between structure and activity remains stubbornly hard to predict, which is why systematic comparisons of closely related complexes matter for the field.

To build the new compounds, the team refluxed dimethyltin oxide with a series of substituted cinnamic acids in methanol under a strict 2:1 stoichiometry. The reaction proved remarkably reliable: five distinct complexes, C1 through C5, crystallized in yields of 80 to 86 percent after the filtrate was left to stand at room temperature for several days. The mild conditions and simple operation suggest the route is a general one for constructing this class of tetranuclear tin oxide clusters. Infrared spectroscopy provided the first confirmation that coordination had occurred, with the carboxylate stretching band shifting from the 1700 to 1725 wavenumber range typical of free carboxylic acids down to about 1640, a redshift that signals successful binding to the tin center.

Single-crystal X-ray diffraction revealed that all five complexes share a striking common architecture: a ladder-like Sn4O4 core built from three edge-sharing Sn2O2 rhomboids. The central ring features notably shorter tin-oxygen bonds, between 2.024 and 2.040 angstroms, than the terminal rings, which measure 2.217 to 2.254 angstroms. That contraction reflects the triple-bridging mode of the central oxygen atoms, which link three metal centers each and impose distinctive electronic and steric constraints. Perhaps the most intriguing structural detail is that the terminal, electrophilic tin atoms are capped exclusively by methoxy ligands generated in situ from the methanol solvent, rather than by the carboxylate groups of the cinnamate ligands, a clear example of site selectivity during the self-assembly process.

The two kinds of tin centers in each cluster adopt different geometries. Those in the central ring are five-coordinate with a trigonal bipyramidal arrangement, bound to two bridging oxygens, two methyl groups, and one methoxy ligand. The terminal tin atoms are six-coordinate and distorted octahedral, carrying a chelating carboxylate in addition to the other ligands. Nuclear magnetic resonance data corroborate this picture: the 119Sn spectra of all five complexes show two resonances, near minus 136 and minus 173 parts per million, consistent with coexisting five- and six-coordinate tin centers. Thermogravimetric analysis traced decomposition through loss of methoxy ligands near 100 degrees Celsius followed by pyrolysis of the organic framework, leaving a residue whose mass matched the theoretical yield of tin dioxide.

Before any biological claims could be made, the team needed to know whether the lead complex would survive in the body’s aqueous environment. Ultraviolet-visible spectroscopy showed that C1’s absorption profile is essentially identical in ethanol and in a dilute ethanol-Tris buffer, and the spectrum barely changes across pH values from 3 to 10, a range that spans the acidic microenvironment of tumors to physiological pH. Over 24 hours at pH 7.4, absorbance declined only slightly. Hirshfeld surface analysis of the crystal packing of C1, meanwhile, quantified the intermolecular forces holding the lattice together, finding that hydrogen-hydrogen van der Waals contacts dominate at 63.3 percent, followed by carbon-hydrogen contacts at 15.9 percent and bromine-hydrogen contacts at 8.9 percent.

The mechanistic core of the study concerns how C1 interacts with DNA. Ultraviolet titration showed the complex’s 300-nanometer absorption band shifting about 5 nanometers to the red and diminishing in intensity as DNA concentration rose, behavior characteristic of a planar molecule slipping between stacked base pairs through pi-pi interactions. The calculated binding constant of 1.22 times 10 to the fourth liters per mole falls squarely in the range of established intercalators. A competitive assay using ethidium bromide, a fluorescent dye that fluoresces only when intercalated into DNA, showed dose-dependent quenching as C1 displaced the dye from the helix, with a Stern-Volmer constant of 2.04 times 10 to the fourth. Viscosity measurements sealed the case: DNA viscosity rose sharply with increasing C1 concentration, far more than with the free cinnamic acid ligand or dimethyltin oxide alone, because intercalation lengthens the helix and increases its hydrodynamic volume.

Molecular docking simulations added atomic-level texture to the picture. The optimized binding pose shows the planar cinnamate moiety of C1 slotting coplanar between DNA base pairs, while the tin-bound methyl groups split their duties: some project into the minor groove to make van der Waals contacts, and others point outward to avoid clashing with the phosphate backbone. In cell-based assays against MCF-7 breast cancer, HepG2 liver cancer, and NCI-H460 lung cancer lines, every complex beat cisplatin by a wide margin. Substituent effects were decisive. The 2-bromo derivative C1 led with IC50 values of 3.04, 1.86, and 4.78 micromolar across the three lines, likely because bromine’s electron-withdrawing character and halogen-bonding capacity improve target affinity. The 2,6-dichloro complex C4 followed closely, while the alpha-methyl derivative C5 was weakest, its electron-donating group apparently disrupting favorable interactions.

Flow cytometry revealed how the cells actually die. Using Annexin V-FITC and propidium iodide dual staining, the researchers showed that C1 triggers apoptosis in a dose-dependent fashion in HepG2 cells: 21.26 percent of the population after 48 hours at 1 micromolar, 35.6 percent at 2 micromolar, and 53.6 percent at 4 micromolar. Cell-cycle analysis showed a corresponding concentration-dependent accumulation of cells in the G2/M phase, with depletion of the G0/G1 and S populations. That arrest pattern suggests C1 interferes with mitotic machinery or introduces DNA lesions that activate the cell’s damage response; when the damage cannot be repaired, the intrinsic apoptotic pathway takes over. This coupling of cycle disruption to programmed death offers a coherent mechanistic account of the compound’s antiproliferative power.

The authors position C1 as a high-activity lead for the design of next-generation metallodrugs, emphasizing that its cytotoxicity arises from a synergistic interplay of mechanisms rather than a single target. The work also fills a gap in the organotin literature: while dibutyltin and tricyclohexyltin cinnamates have been systematically studied, dimethyltin analogues remained scarce. The Hunan-based team, funded by the Hunan Provincial Natural Science Foundation, argues that systematically mapping how substituent type and position modulate activity is the surest path from laboratory curiosity to clinical candidate. Considerable distance remains between potent in vitro activity and an approved medicine, and organotin compounds carry well-known toxicity concerns that selectivity data alone cannot resolve. Still, a molecule that is stable across physiological pH, intercalates DNA by a classical mechanism, arrests cancer cells in mitosis, and spares normal liver cells at therapeutic concentrations gives medicinal chemists a concrete scaffold to optimize.

Subject of Research: Synthesis, crystal structures, and anticancer activity of tetranuclear dimethyltin(IV)-cinnamate complexes with ladder-type Sn4O4 clusters

Article Title: Tetranuclear dimethyltin(IV)-cinnamate complexes with a ladder-type Sn4O4 cluster: synthesis, crystal structures, and anticancer activity

Article References: Zhang, Z., Jiang, Z., Tan, Y., Tan, Y., & Jiang, W. (2026). Tetranuclear dimethyltin(IV)-cinnamate complexes with a ladder-type Sn4O4 cluster: synthesis, crystal structures, and anticancer activity. Journal of Saudi Chemical Society, 30(5), Article 75. https://doi.org/10.1007/s44442-026-00129-y

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00129-y

Keywords: organotin, dimethyltin, cinnamic acid, anticancer, Sn4O4 cluster, DNA intercalation, cisplatin, HepG2, apoptosis, cell cycle arrest, crystal structure, metallodrugs