New Polymorphic Forms of Pemetrexed Diacid and Their Use for the Preparation of Pharmaceutically Pure Amorphous and Hemipentahydrate Forms of Pemetrexed Disodium

The preparation of stable amorphous pemetrexed disodium of pharmaceutical purity as well as the process optimization for the preparation of the hemipentahydrate form of pemetrexed disodium are described. Analytical methods for the polymorphic and chemical purity studies of pemetrexed disodium and pemetrexed diacid forms were developed. The physicochemical properties of the amorphous and hydrate forms of pemetrexed disodium, as well as new forms of pemetrexed diacid (a key synthetic intermediate) were studied by thermal analysis and powder X-ray diffraction. High-performance liquid chromatography and gas chromatography methods were used for the chemical purity and residual solvents determination. In order to study the polymorphic and chemical stability of the amorphous and hemipentahydrate forms, a hygroscopicity test (25 °C, 80% RH) was performed. Powder diffraction and high-performance liquid chromatography analyses revealed that the amorphous character and high chemical purity were preserved after the hygroscopicity test. The hemipentahydrate form transformed completely to the heptahydrate form of pemetrexed disodium. Both pemetrexed disodium forms were produced with high efficiency and pharmaceutical purity in a small commercial scale. Amorphous pemetrexed disodium was selected for further pharmaceutical development. Two new polymorphs (forms 1 and 2) of pemetrexed diacid were used for the preparation of high purity amorphous pemetrexed disodium.

Analysis: separate headspace vials containing standard solution and test solution were prepared and headspace injection was performed. The concentration of ethanol in % was calculated.
Standard solution: 1 mL of ethanol solution (ethanol dissolved in dimethyl sulfoxide to obtain a concentration of about 0.5% with respect to sample preparation) and 1 mL of H2O.
Test solution: ca. 50 mg of examined substance dissolved in a 1 mL of dimethyl sulfoxide and 1 mL of H2O.
Analysis: separate headspace vials containing standard solution I, standard solution II and test solution were prepared and headspace injections were performed. The concentration of solvents in ppm was calculated.

S2
Standard solution I: 1 mL of cyclohexane solution (cyclohexane weighed in ethanol and dissolved in dimethyl sulfoxide to obtain a concentration of about 3880 ppm with respect to sample preparation) and 1 mL of H2O.
Standard solution II: 1 mL of solvents solution (methanol and ethanol dissolved in dimethyl sulfoxide to obtain a concentration of about 3000 ppm of methanol and 5000 ppm of ethanol with respect to sample preparation) and 1 mL of H2O.
Test solution: ca. 50 mg of examined substance dissolved in a 1 mL of dimethyl sulfoxide and 1 mL of H2O.
Analysis: 1 μL of standard solution (analytes dissolved in methanol to obtain a concentration of about 150 ppm of 4-methylomorpholine, 88 mg of N,N-dimethylformamide and 500 mg of dimethyl sulfoxide with respect to sample preparation) and 1 μL of test solution (ca. 30 mg of examined substance dissolved in 1.0 mL of methanol) were separately injected into chromatograph. The areas of peaks of analytes from the test solution must not be bigger than the mean areas from chromatograms obtained with standard solution.
Analysis: separate headspace vials containing test solution, standard solution I and standard solution II were prepared and headspace injections were performed. The concentration of ethanol in % was calculated. The areas of peaks of dichloromethane, ethyl acetate and tetrahydrofuran from the test solution must not be bigger than the mean areas from chromatograms obtained with standard solution II.
Standard solution I: 2 mL of ethanol solution (ethanol dissolved in dimethylacetamide to obtain a concentration of about 0.5% with respect to sample preparation) and 0.5 mL of H2O.
Standard solution II: 2 mL of solvents solution (solvents dissolved in dimethylacetamide to obtain a concentration of about 60 ppm of dichloromethane, 500 ppm of ethyl acetate and 72 ppm of tetrahydrofuran with respect to sample preparation) and 0.5 mL of H2O.
Test solution: ca. 50 mg of examined substance was dissolved in a 2 mL of dimethylacetamide and 0.5 mL of H2O.           Results of elemental analysis performed on pemetrexed disodium hemipantahydrate (HP-PE) and heptahydrate (HT-PE), confirm assumed element percentage compositions in the samples.
(HP-PE) C20H19N5Na2O6·2.5H2O, molecular weight 516.39. DMSO bands are very well visible in FTIR spectra of both forms. S14 Figure S16a. The FTIR spectra comparinon of forms I and II with DMSO. Figure S16b. The FTIR spectra comparinon of forms I and II with DMSO.