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

<b>Background:</b> 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). <b>Methods</b>: INH raw, INH branded tablets (whole and ground), and multi-drug combination mixtures (MCMs) that simulate paediatric multi-drug-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). <b>Results</b>: 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 multi-drug suspensions, INH fell below the HPLC quantification limit in both pH 1.2 and 6.8 media, despite visible residue, suggesting the formation of non-dissociable supramolecular complexes. Using a validated HPLC assay, no quantifiable INH was detected from the crushed multi-drug suspensions in either pH 1.2 or pH 6.8, whereas intact API/tablets showed measurable release. <b>Conclusions</b>: Co-suspension of INH with companion tuberculosis (TB) drugs disrupts its supramolecular integrity, leading to pre-administration degradation and a loss of quantifiable drug. 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.