Lysosomal Drug Sequestration Mediated by ABC Transporters and Drug Resistance

<b>Background</b>: Drug resistance (DR) mediated by ABC transporters in the cytoplasmic membrane has been one of the best studied mechanisms of DR in vitro. More recently, it has also been suggested that ABC transporters expressed on lysosomal membranes could increase the sequestration of anticancer drugs in lysosomes, thereby reducing their concentration at target sites, and causing DR. Unfortunately, convincing evidence that such a DR mechanism actually exists is lacking, even in the case of in vitro experiments. <b>Methods</b>: This hypothetical study using simplified models evaluates the effect of ABC transporter-mediated accumulation of anticancer drugs in lysosomes on their concentration at target sites under standard in vitro conditions. <b>Results</b>: Calculations show that an ABC transporter resident on the plasma membrane must create and maintain a relatively small concentration gradient between extracellular space and the target site to reduce the drug concentration at the target site by, for example, half. In contrast, if a lysosomal ABC transporter is to also halve the concentration of the drug at the target site, then it must create and maintain a huge concentration gradient between lysosomes and target sites. It is very likely that massive accumulation of drugs in lysosomes would have a negative effect on the function of the lysosomes themselves. <b>Conclusions</b>: The results of this hypothetical study strongly suggest that the mechanism of DR mediated by lysosomal ABC transporters is questionable, as it requires enormous accumulation of the drug in lysosomes, which would likely also impair their function. Therefore, it is highly unlikely that this hypothetical DR mechanism could actually be utilized by tumor cells to defend against the cytotoxic effects of chemotherapy in vitro.