Formulation of an oral suspension of pyrimethamine for pediatric use

Introduction
Pyrimethamine (PYR) is the first-line treatment for congenital toxoplasmosis in newborns. In France, there is no pharmaceutical form adapted for this age group. The objective of this study was to develop a pediatric-appropriate formulation of PYR and conduct physicochemical and microbiological stability studies.

Materials and Methods
The developed formulation consists of PYR at 5 mg/mL, xanthan gum (Gx) 0.5% (thickening agent), sucralose 0.016% (sweetener), potassium sorbate (SK) 0.3% (preservative), and citrate buffer (pH 3.0 -0.2%).
A sedimentation study through supernatant assay was conducted on flasks stored at 20°C (n=3) and 5°C (n=3), with assays performed at D0, M1, M3, and M6, and compared to D0.
To validate the SK concentration, the "antimicrobial preservation efficacy" test (Ph. Eu. 5.1.3) was performed with 0.2% and 0.3% SK. Microorganism strains were separately added to PYR flasks, and the results were read at D0, D14, and D28. At D14, a reduction of 99.9% for aerobic germs and 90% for yeast and molds was expected.
Formulation stability was studied over 6 months at 20°C (n=3) and 5°C ± 3°C (n=3). Content, degradation products (DP), pH, osmolality, and appearance were monitored over time (n=2 measurements per flasks except for pH).
The samples were assayed using a validated HPLC-UV stability-indicating method: kinetex coreshell column thermostated at 40°C, acetonitrile/methanol/KH2PO4 buffer gradient (pH 3, 10 mM), fixed λ at 230 nm + scan 190-400 nm. The formulation was considered stable if the content is > 95% and DP < 0.1%.
A microbiological stability study was conducted at 5°C: unopened flasks (n=3) at D0, M1, M2, M3, and M5, and daily opened flasks for 3 months (n=3) at D0, M1, M2, and M3 (Ph. Eu. 2.6.12 test). Specific testing for Escherichia coli was also conducted on these same flasks (test 2.6.13).

Results
The sedimentation study showed a decrease of 4.6% of the content in the supernatant at 20°C and an increase of 9.6% at 4°C after 6 months, probably due to a reversible creaming phenomenon upon agitation. With 0.2% SK, the reductions in terms of CFU for Staphylococcus aureus and E. coli were not satisfactory. A concentration of 0.3% SK was chosen (reduction of 100% except for S. aureus: 89.5%). The formulation remained stable up to S8 at 20°C and S22 at 5°C, based on pH monitoring (average of 5.80 ± 0.10 at 20°C and 5.93 ± 0.11 at 5°C), osmolality (average of 44.6 ± 1.2 at 20°C and 42.5 ± 1.8 mOsm/kg H2O at 5°C), and PYR content (loss of content by 1.58 ± 0.21% at 20°C and 1.80 ± 0.19% at 5°C). No degradation products were detected. The microbiological quality (test 5.1.4) was maintained for 5 months before opening and after 3 months of daily sampling at 5°C.

Conclusion
Xanthan gum reduced sedimentation, and the chosen SK concentration limit microbiological contamination. The developed formulation is validated and can be stored at 5°C for 14 weeks.

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