In vitro assays and biomarkers for drug-induced phospholipidosis.

Drug-induced phospholipidosis is the cytoplasmic accumulation of phospholipids as a result of xenobiotic exposure. This accumulation results in a unique histological effect in cells noted as electron-dense lamellar inclusions or whorls in the cytoplasm when observed with transmission electron microscopy. Electron microscopy has been the widely accepted standard for classification of the phospholipidosis effect. Molecules that have been prone to induce such an effect are made up of a lipophilic region and a positively charged region. Phospholipidosis has most commonly been associated with drugs that are cationic, amphiphilic drugs and can occur in a variety of tissues. Although phospholipidosis is not considered adverse in isolation, depending on the tissue affected or the occasional circumstance of concurrent toxicity, phospholipidosis can be perplexing if identified in early drug development. In most circumstances, characterisation of the effect with in vivo studies allows for determination of exposure and the magnitude of the effect. On occasion in drug development, there may be an interest to screen early stage compounds to minimise phospholipidosis. In such circumstances, in silico and in vitro assays can be employed in a strategy with in vivo assessments. In addition, there may be an interest to monitor for the potential development of phospholipidosis in longer-term animal studies. In such cases, biomarker approaches could be used. The challenge in the overall assessment of phospholipidosis remains the question of the possible relevance to any toxicity, and, therefore, any screening approach, while assessing the potential to induce phospholipidosis, must be considered in relation to prediction of findings in vivo. The status of current assays and biomarkers is presented with strategies for screening.