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. 2025 Nov 10;90(11):e202500406.
doi: 10.1002/cplu.202500406. Epub 2025 Sep 3.

Formosulfathiazole: A Structural Revision

Claudio Maestri  1   2 , Toni Grell  3 , Fabio Travagin  1 , Christian R Göb  4 , Michele Castaldi  5 , Ivana Miletto  1 , Geo Paul  6 , Silvia Zampini  3 , Marco Vandone  3 , Valentina Colombo  3 , Giovanni B Giovenzana  1
Affiliations

Formosulfathiazole: A Structural Revision

Claudio Maestri et al. Chempluschem. .

Abstract

Formosulfathiazole (FSTz) is a synthetic active pharmaceutical ingredient (API) prepared by condensation of sulfathiazole with formaldehyde. Originally described for the first time in 1948, it is currently used for the treatment of bacterial and protozoal infections in cattle and pets, acting as a prodrug slowly releasing the sulfamidic sulfathiazole and formaldehyde. A systematic analysis of FSTz allowed to revise the originally believed undefined polymeric structure and uncovered the intriguing cyclophane skeleton of a well-defined cyclodimeric condensation product.

Keywords: cyclodimer; cyclophane; electron diffraction; formosulfathiazole; structural revision.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
a) Crystal structure (asymmetric unit only) of the DMSO solvate (FSTz·2DMSO) showing the symmetrically independent formosulfathiazole dimer and two independent DMSO molecules. Color code: C, gray; N, blue; O, red; S, yellow; H, white. Selected H‐bonds distances: N3(‐H)···O5 = 2.846(2) Å, N6(‐H)···O6 = 2.897(2) Å. b) Chemical structure of FSTz·2DMSO.
Figure 2
Figure 2
a) Asymmetric unit of the crystallographic structure of formosulfathiazole tetrahydrate (FSTz·4H2O) showing the conformation of FSTz dimer and the four water molecules. b) Enlargement of a portion of the crystal packing showing the water molecules forming an H‐bonded octameric cluster (see SI for further details). c) Crystal packing viewed down the b axis highlighting the intricate pattern of water‐mediated hydrogen bonds. Color code: C, gray; N, blue; O, red; S, yellow; H, white. H‐bonds.
Scheme 1
Scheme 1
Formosulfathiazole (FSTz): synthesis, originally proposed and revised experimental structure.
See this image and copyright information in PMC

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