Chloroquine (CQ) and hydroxychloroquine (HCQ) are derivatives of the heterocyclic aromatic compound quinoline. These economical compounds have been used as antimalarial agents for many years. Currently, they are used as monotherapy or in conjunction with other therapies for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren's syndrome (SS) and antiphospholipid antibody syndrome (APS). Based on its effects on the modulation of the autophagy process, various clinical studies suggest that CQ and HCQ could be used in combination with other chemotherapeutics for the treatment of various types of cancer. Furthermore, the antiviral effects showed against Zika, Chikungunya, and HIV are due to the annulation of endosomal/lysosomal acidification. Recently, CQ and HCQ were approved for the U.S. Food and Drug Administration (FDA) for the treatment of infected patients with the coronavirus SARSCoV- 2, causing the disease originated in December 2019, namely COVID-2019. Several mechanisms have been proposed to explain the pharmacological effects of these drugs: 1) disruption of lysosomal and endosomal pH, 2) inhibition of protein secretion/expression, 3) inhibition of antigen presentation, 4) decrease of proinflammatory cytokines, 5) inhibition of autophagy, 6) induction of apoptosis and 7) inhibition of ion channels activation. Thus, evidence has shown that these structures are leading molecules that can be modified or combined with other therapeutic agents. In this review, we will discuss the most recent findings in the mechanisms of action of CQ and HCQ in the immune system, and the use of these antimalarial drugs on diseases.

Tumour relapse, chemotherapy resistance, and metastasis continue to be unsolved issues in cancer therapy. A recent approach has been to scrutinise drugs used in the clinic for other illnesses and modify their structure to increase selectivity to cancer cells. Chloroquine (CQ) and hydroxychloroquine (HCQ), known antimalarials, have successfully treated autoimmune and neoplastic diseases. CQ and HCQ, well-known lysosomotropic agents, induce apoptosis, downregulate autophagy, and modify the tumour microenvironment. Moreover, they affect the Toll 9/NF-κB receptor pathway, activate stress response pathways, enhance p53 activity and CXCR4-CXCL12 expression in cancer cells, which would help explain their effects in cancer treatment. These compounds can normalise the tumourassociated vasculature, promote the activation of the immune system, change the phenotype of tumour-associated macrophages (from M2 to M1), and stimulate cancer-associated fibroblasts. We aim to review the historical aspects of CQ and its derivatives and the most relevant mechanisms that support the therapeutic use of CQ and HCQ for the treatment of cancer.