In Silico, In Vitro, and In Vivo Investigations on Adapalene as Repurposed Third Generation Retinoid against Multiple Myeloma and Leukemia

SIMPLE SUMMARY: Hematological neoplasms afflict millions of children and adults yearly and are often uncurable due to refractory illness and recurrence. Therefore, new treatment approaches are required. The proto-oncogene c-MYC has been linked to carcinogenesis, particularly in hematological malignancies. Consequently, to develop new and effective therapies for blood cancers, it is essential to target c-MYC, specifically that c-MYC inhibitors have not yet received clinical approval. In this research, the verification of adapalene as a c-MYC inhibitor may be a glimmer of hope, especially that it is an already Food and Drug Administration (FDA)-approved drug, and its toxicity profiles, pharmacokinetics, and pharmacodynamics are well established. Our study presents a rationale that the discovery of adapalene as a c-MYC inhibitor may significantly lower the drug development costs of new anticancer medications. It also provides further insights in the future of adapalene-based designs that could result in more effective and targeted innovative therapies for multiple myeloma. ABSTRACT: The majority of hematopoietic cancers in adults are incurable and exhibit unpredictable remitting-relapsing patterns in response to various therapies. The proto-oncogene c-MYC has been associated with tumorigenesis, especially in hematological neoplasms. Therefore, targeting c-MYC is crucial to find effective, novel treatments for blood malignancies. To date, there are no clinically approved c-MYC inhibitors. In this study, we virtually screened 1578 Food and Drug Administration (FDA)-approved drugs from the ZINC15 database against c-MYC. The top 117 compounds from PyRx-based screening with the best binding affinities to c-MYC were subjected to molecular docking studies with AutoDock 4.2.6. Retinoids consist of synthetic and natural vitamin A derivatives. All-trans-retinoic acid (ATRA) were highly effective in hematological malignancies. In this study, adapalene, a third-generation retinoid usually used to treat acne vulgaris, was selected as a potent c-MYC inhibitor as it robustly bound to c-MYC with a lowest binding energy (LBE) of −7.27 kcal/mol, a predicted inhibition constant (pKi) of 4.69 µM, and a dissociation constant (K(d) value) of 3.05 µM. Thus, we examined its impact on multiple myeloma (MM) cells in vitro and evaluated its efficiency in vivo using a xenograft tumor zebrafish model. We demonstrated that adapalene exerted substantial cytotoxicity against a panel of nine MM and two leukemic cell lines, with AMO1 cells being the most susceptible one (IC(50) = 1.76 ± 0.39 µM) and, hence, the focus of this work. Adapalene (0.5 × IC(50), 1 × IC(50), 2 × IC(50)) decreased c-MYC expression and transcriptional activity in AMO1 cells in a dose-dependent manner. An examination of the cell cycle revealed that adapalene halted the cells in the G(2)/M phase and increased the portion of cells in the sub-G(0)/G(1) phase after 48 and 72 h, indicating that cells failed to initiate mitosis, and consequently, cell death was triggered. Adapalene also increased the number of p-H3(Ser10) positive AMO1 cells, which is a further proof of its ability to prevent mitotic exit. Confocal imaging demonstrated that adapalene destroyed the tubulin network of U2OS cells stably transfected with a cDNA coding for α-tubulin-GFP, refraining the migration of malignant cells. Furthermore, adapalene induced DNA damage in AMO1 cells. It also induced apoptosis and autophagy, as demonstrated by flow cytometry and western blotting. Finally, adapalene impeded tumor growth in a xenograft tumor zebrafish model. In summary, the discovery of the vitamin A derivative adapalene as a c-MYC inhibitor reveals its potential as an avant-garde treatment for MM.