Predicting the mechanism of phospholipidosis

Since the observation of phospholipidosis by Nelson and Fitzhugh in 1948 [1], many attempts have been made at understanding the underlying mechanism(s) [2,3]. Phospholipidosis is the excess accumulation of phospholipids induced in several cell types by numerous cationic amphiphilic drugs (CADs). The most reliable way of determining whether a compound has induced phospholipidosis is by electron microscopy. This analysis is important in the drug development process where the occurrence of phospholipidosis can cause delays and possibly termination of a project (as more tests need to be carried out to satisfy regulatory bodies). It is still unclear whether an accumulation of phospholipids is harmful to human health [4], the process is often reversible upon withdrawal of the compound, and despite attempts to understand the mechanism of phospholipidosis there is still no mechanistic understanding of how CADs can induce the accumulation of phospholipids in various cell types across different species.A build-up of phospholipids can be explained by an inhibition of the breakdown or an increase in the synthesis of the phospholipids. Early studies supported the idea that inhibition of the breakdown of phospholipids was a possible mechanism. Hostetler et al. [2] showed strong support for the theory that the action of CADs was located in the lysosomes and that inhibition of the lysosomal phospholipases A and C caused a build-up of phospholipids. However, there was no way to distinguish between a drug-enzyme or drug-phospholipid binding event as the cause of the inhibition. Joshi et al. [5] tried to address this problem by measuring binding of phospholipidosis-inducing drugs to L-α-dipalmitoyl phosphatidylcholine vesicles. This suggested that if a drug was found to bind, then drug-phospholipid binding would be the cause of the inhibition of the phospholipases. While most of the drugs tested did bind to L-α-dipalmitoyl phosphatidylcholine vesicles, chloroquine (a phospholipidosis-