New Thienopyridazine-Thiazole Hybrids Show Potent Anticancer Activity in Early Screening

A team of chemists in Saudi Arabia has synthesized a new family of hybrid molecules that combine two biologically active ring systems, thienopyridazine and thiazole, and found that one of them kills colorectal and breast cancer cells in laboratory tests with potency approaching that of the widely used chemotherapy drug doxorubicin, while sparing normal lung fibroblast cells. The study, published in the Journal of the Saudi Chemical Society, blends synthetic organic chemistry with quantum-chemical modelling, molecular docking and in silico pharmacokinetic prediction to build a full picture of how these molecules behave both electronically and biologically.

The rationale behind the work rests on molecular hybridization, a drug-design strategy in which two pharmacophores are fused into a single molecular entity to achieve synergistic effects. Pyridazine-based compounds have long attracted attention in medicinal chemistry because they can modulate key oncogenic targets; clinically successful kinase inhibitors such as Vatalanib, Talazoparib and Simmiparib all contain the pyridazine heterocycle. Adding a thiophene ring to form the thienopyridazine scaffold increases electron density and enhances the pharmacological profile of the pyridazine system. Thiazole rings, meanwhile, appear in established anticancer drugs including Dasatinib and Tiazofurin, and are known to improve pi-pi stacking, hydrogen bonding and lipophilicity. The researchers reasoned that merging the two frameworks could amplify target binding, cellular uptake and pharmacokinetic behaviour simultaneously.

The synthesis began from 3-chloro-4-cyano-5,6-dimethylpyridazine, which was converted through an addition-elimination reaction with thiourea into a mercapto intermediate and then cyclized to the key building block, 6-acetyl-5-amino-3,4-dimethylthieno[2,3-c]pyridazine. This bifunctional compound served as the branching point for two synthetic routes. In the first, condensation with thiosemicarbazide produced a thiosemicarbazone derivative in 74.4 percent yield, which was then reacted with alpha-halo-carbonyl reagents in Hantzsch-type cyclizations to yield thiazole-bearing hydrazones, or with chloroacetic acid to form a thiazolin-4-one ring that could subsequently undergo Knoevenagel condensations with substituted benzaldehydes. The second route converted the amino group of the building block into a chloroacetamide, which reacted with ammonium thiocyanate to close a thiazolidin-4-one ring, again followed by benzylidene elaboration. All products were verified by infrared, nuclear magnetic resonance and mass spectrometry, together with elemental analysis.

Quantum-chemical calculations at the DFT/B3LYP level with a 6-311++G(d,p) basis set revealed how structural modifications reshape the electronic landscape of the molecules. The optimized geometries were largely non-planar, with torsion angles showing that phenyl substituents introduce significant twisting while thiazolidinone ring closure produces a more rigid, near-planar architecture. Frontier molecular orbital analysis showed that in the parent aminothienopyridazine the highest occupied and lowest unoccupied molecular orbitals both sit on the fused ring system, corresponding to pi-pi* transitions with limited charge separation. In the nitro-substituted benzylidene analogues, by contrast, the LUMO localizes strongly on the benzylidene phenyl group, creating a pronounced donor-acceptor separation. HOMO-LUMO gaps ranged from 2.81 to 3.80 electron volts, with the nitro compounds showing the smallest gaps, the highest softness and the greatest electrophilicity, all hallmarks of strong intramolecular charge-transfer capability. Computed dipole moments, polarizabilities and first hyperpolarizabilities exceeded those of urea by factors of up to 5.25 and 22.80 respectively, underlining the strong charge separation in these conjugated systems.

The biological screening used the MTT assay against three cancer cell lines, HepG2 liver carcinoma, HT-29 colorectal adenocarcinoma and MCF-7 breast cancer, alongside normal WI-38 fibroblasts, with doxorubicin as the reference drug. The results traced a clear structure-activity relationship. The parent compounds, lacking extended heterocyclic conjugation, were weakly active. Introducing a methyl-substituted thiazole boosted activity markedly against HT-29 and MCF-7, while the phenyl-substituted regioisomer acted selectively on HepG2, showing that subtle changes in heteroatom orientation can redirect cellular sensitivity. Benzylidene substitution on the thiazolidinone rings further enhanced potency, particularly with electron-donating methoxy or halogen substituents, whereas the strongly electron-withdrawing nitro group diminished activity across the series.

The standout compound was the chloroacetamide analogue 8, which achieved IC50 values of 12.57 micromolar against HT-29 cells and 17.66 micromolar against MCF-7 cells, close to or better than doxorubicin at the same cell lines, while showing the highest tolerance in normal WI-38 fibroblasts at 88.21 micromolar. The authors attribute this performance to the synergistic effect of the electrophilic chloroacetamide linkage with an optimal electronic distribution, adequate molecular size and enhanced hydrogen-bonding capability. A related thiazolidinone imine analogue showed the strongest inhibition of HepG2 cells at 19.40 micromolar, suggesting that scaffold flexibility can confer organ-specific sensitivity. Most analogues displayed high selectivity indices against the normal cell line, a critical safety signal for any prospective anticancer lead.

To rationalize these findings computationally, the team docked the molecules into the N-terminal ATP-binding domain of Hsp90alpha, a molecular chaperone that stabilizes numerous oncogenic proteins and is an established cancer drug target. Binding scores ranged from -4.9488 to -7.2492 kilocalories per mole, with the lead chloroacetamide compound achieving the strongest score, driven by a hydrogen-acceptor bond between its chlorine atom and Lys58. Derivatives carrying thiazole and thiazolidinone rings consistently bound better than their simpler precursors because those rings increase hydrophobic and pi-stacking interactions within the binding cavity. Docking of doxorubicin itself yielded a score of -6.6103 kilocalories per mole, confirming the protocol and demonstrating that several novel derivatives match or exceed the reference drug’s computed affinity.

A multiple linear regression analysis connecting quantum-chemical descriptors to measured IC50 values produced determination coefficients between 0.9567 and 0.9823 across the four cell lines, indicating that frontier orbital energies largely account for the biological variation. Lower LUMO energies favoured cytotoxicity, consistent with the importance of electron-accepting ability in stabilizing interactions with biological targets, while the positive contribution of the HOMO-LUMO gap suggested that optimal activity requires balanced reactivity rather than excessive molecular softness. Electronegativity enhanced activity in all models, whereas hardness and softness effects varied by cell line, reflecting differences in how each tumour type responds to electronic flexibility.

In silico pharmacokinetic profiling with SwissADME added a further layer of assessment. The lead compound 8, together with analogues 9 and 10a, showed high gastrointestinal absorption, good aqueous solubility and full Lipinski compliance, although several were predicted to be P-glycoprotein substrates, which could modestly limit oral bioavailability. Importantly, none of the synthesized analogues were predicted to cross the blood-brain barrier, which the authors interpret as evidence that the series is best suited to peripheral, non-CNS diseases rather than central nervous system indications.

The authors caution that the cytotoxic data represent a preliminary screening; mechanistic studies involving apoptosis markers, cell cycle analysis and Western blotting on the most active analogues are the logical next step. Nevertheless, the convergence of potent and selective antiproliferative activity, docking affinities rivalling a clinical drug, and acceptable predicted drug-likeness positions the thienopyridazine-thiazole hybrids, and the chloroacetamide lead in particular, as promising starting points for further optimization in the search for safer and more effective anticancer agents.

Subject of Research: Synthesis, computational modelling and cytotoxic evaluation of thienopyridazine-thiazole hybrid compounds as potential anticancer agents

Article Title: Synthesis, molecular modelling, and cytotoxic activity of new thienopyridazine analogues clubbed thiazole ring systems

Article References: Mazi, W., & Alenazi, N. A. (2026). Synthesis, molecular modelling, and cytotoxic activity of new thienopyridazine analogues clubbed thiazole ring systems. Journal of Saudi Chemical Society, 30(4), Article 46. https://doi.org/10.1007/s44442-026-00076-8

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00076-8

Keywords: thienopyridazine, thiazole, anticancer agents, cytotoxicity, DFT, molecular docking, Hsp90alpha, drug-likeness, molecular hybridization, HT-29, MCF-7, structure-activity relationship