# Chloroquine Potentiates the Chemotherapeutic Effect of Carboplatin and ATR/Chk1 Inhibitors by Increasing the Replication Stress

## TL;DR

Chloroquine enhances the effectiveness of chemotherapy drugs by increasing replication stress in tumor cells.

## Contribution

A novel explanation is proposed for how chloroquine potentiates chemotherapy by inducing replication stress and reducing nucleotide availability.

## Key findings

- Chloroquine increases apoptosis when combined with platinum-based drugs like carboplatin.
- Chloroquine induces replication stress, as shown by increased Chk1 phosphorylation and S-phase accumulation.
- Supplementing with dNTPs reverses the inhibitory effects of chloroquine and platinum drugs on cell proliferation.

## Abstract

Lysosomal inhibition by different agents like chloroquine and bafilomycin A is known to sensitize some tumor cells to chemotherapeutic drugs. The mechanism and signaling pathways are still under investigation. We showed that chloroquine sensitized tumor cells (MCF7, SKBR3, HCT116) to drugs (carboplatin, cisplatin) treatment. Treatment with the combination of platinum drugs and chloroquine resulted in the increased rate of apoptosis compared with single agent treatment. Moreover, we demonstrated the inhibition of the resumption of cell proliferation after cell cycle arrest induced by drugs treatment. Cells treated with the combination of carboplatin (or cisplatin) and chloroquine demonstrated the significant increase in Chk1 protein phosphorylation (Ser345), which together with S-phase increase indicated the induction of replication stress compared to cells treated with carboplatin (or cisplatin) alone. The rescue experiment performed by supplementation the combination of carboplatin and chloroquine with deoxyribonucleotides (dNTPs) demonstrated the reverse of inhibition of cells’ re-proliferation after cell cycle arrest caused by this combination of drugs. Treatment with carboplatin and ATR inhibitor (ceralasertib) greatly increased the level of phospho-Chk1 and induced the replication stress, which is consistent with previous studies. Supplementation of the above drug combination with chloroquine further increased Chk1 phosphorylation and decreased the number of cells able to re-proliferate after the induced stress. Here, we also demonstrated that dNTPs’ supplementation reversed the effect of chloroquine. Similar results were obtained with the combination of carboplatin and Chk1 inhibitor (prexasertib). It was also demonstrated that chloroquine could potentiate the effect of single agent treatment of tumor cells with ATRi/Chk1i in MCF7 cells. Here, we proposed a novel explanation for the chloroquine ability to potentiate the effect of chemotherapy. The results clearly demonstrated that stress induced by chloroquine is due to its ability to increase the replication stress and to reduce the availability of nucleotides.