A quantitative definition for poorly soluble particles

Poorly soluble low toxicity particles (PSLT) have long been a concept in particle toxicology and regulatory frameworks addressing inhalation hazards. The term PSLT refers to particles that exhibit low toxicity and minimal solubility in biological fluids, leading to prolonged retention in the lungs and potential overload effects. Historically, PSLT has been used to distinguish materials whose adverse effects are primarily driven by particle burden rather than intrinsic chemical toxicity. While the "low toxicity" (LT) component of the definition has been examined to a certain degree (Driscoll and Borm in Inhal Toxicol 32(2):53-62, 2020), the poorly soluble (PS)-criterion does not yet have a precise definition although it is critically influencing the interpretation of toxicological inhalation repeated dose studies and hazard classifications. An ECETOC Task Force (TF) was formed to define criteria for "poorly soluble" particles (PSPs). This paper presents a quantitative non-animal approach for defining PSP using a model particle, with a focus on its potential to cause volumetric lung overload and affect macrophage clearance mechanisms. The analysis allows to calculate a dissolution rate that would lead to a lung burden of about 1 µL/g of lung tissue (lung overload threshold according to Morrow (Morrow in Fundam Appl Toxicol 10(3):369, 1988)). Below this threshold dissolution rate, this specific model particle would qualify as PSP. In addition, a formula was presented to translate abiotic dissolution rates into biotic (rat) dissolution rates to allow a PS-assessment in an animal-free system. As proof of concept, the TF collected existing in vivo data from member companies, publicly available literature of presumed PS-substances, and reference materials. The collected data revealed that most substances exhibited dissolution rates below the critical threshold and that the lung burden at no observed adverse effect concentrations (NOAECs) remained below the lung overload limit. Importantly, this threshold dissolution rate can differ from particle to particle, depending on factors such as agglomerate density, particle size distribution, and expected concentration. Thus, it should be evaluated on a case-by-case basis.