Small Molecules against Metastatic Tumors: Concrete Perspectives and Shattered Dreams

SIMPLE SUMMARY: Recent advances in anticancer drug research led to the approval of new small molecules with different mechanisms of action, therapeutic indications, and adverse reactions. In this review, small molecules recently approved for metastatic cancer therapy or in trials belonging to different pharmacological classes are described. Particularly, we focus on receptor tyrosine kinase inhibitors, which are the largest category, and on other small molecules interfering with cell cycle mechanisms. Special attention is devoted to mutated KRAS inhibitors, which probably represent the first dream come true in anti-metastatic cancer therapy of pharmacology researchers and open the way to potentially very broad therapeutic indications. A second section deals with another family of small molecules, integrin antagonists, that has gone through light and shade moments, and the molecules of still relevant interest are critically discussed. ABSTRACT: Metastasis is the main cause of anti-cancer therapy failure, leading to unfavorable prognosis for patients. The true challenge to increase cancer patient life expectancy by making cancer a chronic disease with periodic but manageable relapses relies on the development of efficient therapeutic strategies specifically directed against key targets in the metastatic process. Traditional chemotherapy with classical alkylating agents, microtubule inhibitors, and antimetabolites has demonstrated its limited efficacy against metastatic cells due to their capacity to select chemo-resistant cell populations that undergo epithelial-to-mesenchymal transition (EMT), thus promoting the colonization of distant sites that, in turn, sustain the initial metastatic process. This scenario has prompted efforts aimed at discovering a wide variety of small molecules and biologics as potential anti-metastatic drugs directed against more specific targets known to be involved in the various stages of metastasis. In this short review, we give an overview of the most recent advances related to important families of antimetastatic small molecules: intracellular tyrosine kinase inhibitors, cyclin-dependent kinase inhibitors, KRAS inhibitors, and integrin antagonists. Although the majority of these small molecules are not yet approved and not available in the drug market, any information related to their stage of development could represent a precious and valuable tool to identify new targets in the endless fight against metastasis.