Cell-based assays have shown promise in predicting the lung toxicity of silica nanomaterials. Two different tests provided a ranking of  the potential of different silica nanoparticles to cause persistent lung inflammation in rats. The research by the EU European Union (European Union ) project SAbyNa demonstrated that cell tests can play a vital role in comparing the hazards of various candidate nanomaterials within a Safe and Sustainable by Design (SSbD) framework, aiding in the selection of safer materials during product development. Further development of cell testing is needed for other silica particles, (nano)materials and different exposure conditions.

Cell-based assays show potential in predicting silica nanomaterial toxicity

Two cell-based assays have shown promise in predicting the lung toxicity of silica nanomaterials. Researchers from the EU project SAbyNa reached this conclusion after comparing various cell-based toxicity tests with outcomes of a 4-week inhalation study in rodents. They found that the haemolysis assay that measures red blood cell membrane damage and the release of a pro-inflammatory marker by macrophages provided a ranking like the potential of different silica nanoparticles to cause persistent lung inflammation in rats. The assays may be valuable for hazard screening, helping to guide innovation in a Safe and Sustainable by Design (SSbD) approach.

The value of cell-based testing for evaluating nanomaterials

Cell tests using cultured mammalian cells are models that represent specific parts of various organs in our body. These in vitro models can be valuable tools to understand how nanomaterials interact with cells. Additionally, comparing the outcomes of cell-based tests with those from animal studies can provide a useful reference point for assessing the hazard potential of new (nano)materials. 

In this comparative approach, results from a set of in vitro tests on new (nano)materials are compared with outcomes from both in vitro and in vivo studies of well-studied nanomaterials. To ensure accurate comparisons, it is crucial to characterise the physical and chemical properties of the nanomaterials and to measure the amount of material deposited on the cells during the in vitro test. The comparative approach currently presents the most practical method for hazard screening within the SSbD framework.

Silica particles as an example

In the study, silica nanoparticles were chosen for their diverse structures —such as crystalline, pyrogenic, colloidal, functionalized— and their well-established biological mechanisms that impact toxicity and disease. Inhalation of silica particles can cause lung inflammation and disease through various biological pathways. This can be assessed by measuring reactive oxygen species and red blood cell membrane damage. In addition, macrophages, which detoxify solid materials, can release cytokines that promote inflammation. 

Four silica particle types were ranked according to their effectiveness in inducing pulmonary inflammation using data from twenty-four distinct cell models. This ranking was compared with the potency of these particles to induce adverse pulmonary outcomes in rats. 

The ranking of the silica particles based on the release of a pro-inflammatory cytokine in macrophages and the destruction of red blood cells (haemolysis assay) was relatively similar to the ranking based on pulmonary adverse outcomes in rats. However, due to biological variation in the in vitro tests, a ranking (1,2,3,4) was not always achievable as for example no statical differences were noted. In more physiologically representative in vitro models, no apparent effects were observed at the tested levels. 

Reflection by RIVM

It is neither feasible nor ethically desirable to assess the hazards of all new candidate materials through animal toxicity testing. The research by SAbyNa demonstrated that cell tests can play a vital role in comparing the hazards of various candidate nanomaterials within an SSbD framework. This approach aids in the selection of safer materials during product development. The researchers demonstrated that certain tests can yield similar rankings for specific effects observed in an in vivo model. However, these findings are based on experiments involving a limited selection of silica particles only. The predictive power of these tests should be further evaluated for other silica particles, other (nano)materials, and under various exposure conditions. To make use of this type of data for regulatory purposes, acceptance of the test method itself is needed as well. This requires standardization of the test method. This standardization process can bring its own challenges (see Overcoming challenges in safety test method standardisation: Insights from the NanoHarmony project).