Critical aspects in dissolution testing of nanomaterials in the oro-gastrointestinal tract: the relevance of juice composition for hazard identification and grouping

The dissolution of a nanomaterial (NM) in an <i>in vitro</i> simulant of the oro-gastrointestinal (OGI) tract is an important predictor of its biodurability <i>in vivo</i>. The cascade addition of simulated digestive juices (saliva, stomach and intestine), including inorganic/organic biomacromolecules and digestive enzymes (complete composition, referred to as "Type 1 formulation"), strives for realistic representation of chemical composition of the OGI tract. However, the data robustness requires consideration of analytical feasibility, such as the use of simplified media. Here we present a systematic analysis of the effects exerted by different digestive juice formulations on the dissolution% (or half-life values) of benchmark NMs (<i>e.g.</i>, zinc oxide, titanium dioxide, barium sulfate, and silicon dioxide). The digestive juices were progressively simplified by removal of components such as organic molecules, enzymes, and inorganic molecules (Type 2, 3 and 4). The results indicate that the "Type 1 formulation" augments the dissolution <i>via</i> sequestration of ions by measurable factors compared to formulations without enzymes (<i>i.e.</i>, Type 3 and 4). Type 1 formulation is thus regarded as a preferable option for predicting NM biodurability for hazard assessment. However, for grouping purposes, the relative similarity among diverse nanoforms (NFs) of a NM is decisive. Two similarity algorithms were applied, and additional case studies comprising NFs and non NFs of the same substance were included. The results support the grouping decision by simplified formulation (Type 3) as a robust method for screening and grouping purposes.