Assessment of the relevance of osmolality measurement as a criterion for the stability of solutions

Abstract Objectives The measurement of osmolality is used by many authors as an additional stability criterion of a drug in solution. In the current state of knowledge, no scientific publication correlates the osmolality values and the stability of a solution. To study the relevance of this analytical technique by measuring the osmolality of injectable solutions whose instability has been chemically demonstrated by high performance liquid chromatography (HPLC). Methods Selection of 13 drug preparations whose chemical instability has been demonstrated in the literature. Realization of three identical samples per selected preparation and measurements of the osmolality of the freshly prepared solutions, then, at various storage times until a chemical degradation of the molecule validated by HPLC of at least 10% and possibly up to 40%. Results Measurements of the osmolality were performed on five antibiotics (amoxicillin/clavulanic acid, cefepime, cefoxitine, meropenem and temocillin and cefoxitin) and five anticancer drugs (azacitidine, bendamustine, busulfan, fotemustine and oxaliplatin). Osmolality varied from −6.30 to 11.10% for antibiotics and from 0.57 to 2.04%. Conclusions Among the preparations tested, only two formulations have a variation in osmolality in accordance with the chemical degradation. For the other 11 formulas, the variations in osmolality values where not correlated with the degradation measured by HPLC. In view of these results, osmolality does not seem to be a criterion of choice for the study of drug stability. In the majority of the unstable solutions studied, the variation of osmolality measurements does not correlate with the loss of concentration and the appearance of degradation products.