Provided herein are compounds (e.g., compounds of Formulae (I), (II), (III) and (IV), or of Table 1 or Table 4), and pharmaceutically acceptable salts thereof, pharmaceutical compositions thereof, and kits comprising the same. The compounds provided herein are activin receptor-like kinase (e.g., ALK-5) inhibitors and are, therefore, useful, for example, for treating and/or preventing diseases (e.g., proliferative diseases, such as cancer) in a subject, inhibiting tumor growth in a subject, or inhibiting the activity of an activin receptor-like kinase (e.g., ALK-5) in vitro or in vivo. Also provided herein are methods and synthetic intermediates useful in the preparation of compounds described herein.