Differentiating Resistance from Formulation Failure: Isoniazid Instability and Poor Dissolution in Crushed Multi-Drug Paediatric Preparations

Background: Bedside manipulation of adult anti-tuberculosis tablets for paediatric dosing is common in low-resource settings, yet it can compromise drug stability. This study investigated how grinding and multi-drug co-suspension affect the supramolecular organisation, thermal stability, and dissolution of isoniazid (INH). Methods: INH raw, INH branded tablets (whole and ground), and multi‑drug combination mixtures (MCM) that simulate paediatric multi-dose-resistant Tuberculosis (MDR‑TB) regimens were assessed. Samples were analysed as solids and aqueous suspensions using hot‑stage microscopy (HSM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Raman spectroscopy, FTIR-ATR, USP dissolution, and HPLC (LOD 0.0015 mg mL⁻¹; LOQ 0.005 mg mL⁻¹). Results: Grinding and co‑mixing lowered melting points and masked typical INH events. Spectroscopy revealed the broadening and shifting of OH/NH and pyridine-ring bands, consistent with the formation of new hydrogen bonding networks, correlative with supramolecular rearrangements. In mul-ti-drug suspensions, INH fell below the HPLC quantification limit in both pH 1.2 and 6.8 media, despite visible residue, suggesting formation of non-dissociable supramolecular complexes. Conclusions: Co‑suspension of INH with companion Tuberculosis (TB) drugs disrupts its supramolecular integrity, leading to pre‑administration degradation and loss of quantifiable drug. These findings emphasise the critical need for paediatric formulations designed to preserve INH molecular stability and avoid bedside multi‑drug co‑suspension. Dissolution testing showed minimal INH release at pH 1.2 and none at pH 6.8, contrasting with intact tablets/API. These observations highlight that converting an immediate-release tablet into an aqueous suspension fundamentally alters its physicochemical environment and requires rational formulation design to preserve molecular stability, differentiating true resistance from formulation failure.