Tetramorphs of the Antibiotic Drug Trimethoprim: Characterization and Stability

Trimethoprim (TMP) is a well-known antifolate drug and one of the most widely used broad-spectrum antibiotics. TMP was first approved by the United States FDA in combination with sulfamethoxazole (SMZ) in 1973, and three polymorphs of TMP (named Form I, II, and III) and a hydrate form were reported in 1978. However, a complete characterization and molecular level analysis of polymorphic structures are not available. We report herein four polymorphs (Forms 1–4) of TMP and a hemihydrate form with complete structural and thermal characterization. The polymorphs were isolated in the pure state by different techniques such as antisolvent method, freeze-drying, and spray drying. Polymorphs 1 and 2 of our study match with Forms I and II reported, and polymorphs 3 and 4 are novel. The X-ray crystal structure of a novel TMP Form 2 is now reported nearly four decades after the first crystal structure of Form 1 was determined by neutron diffraction (J. Am. Chem. Soc. 1976, 98, 2074−2078). The crystal structure of Form 1 has two types of R₂²(8) motifs, and Form 2 has a third type R₂²(8) and R₃²(8) motif of N–H···N_arom hydrogen bonds. The polymorphs were characterized by FT-IR and Raman spectroscopy, differential scanning calorimetry (DSC), ¹⁵N ss-NMR, field emission scanning electron microscopy, powder X-ray diffraction, and also dynamic vapor sorption. Thermodynamic stability, phase transformations, and Hirshfeld surfaces are also discussed. The polymorphs were tested under accelerated International Conference on Harmonization (ICH) conditions of 40 °C and 75% RH for stability, and found that all the solid forms were stable for three months except TMP Form 3, which converted to Form 2. Slurry grinding and thermal experiments suggest that TMP Form 1 is the most stable modification among TMP crystal forms