The present invention provides novel ±-amino acid derivatives of formula (1): (wherein, R 1 , R 2 , R 3 , R 4 , X and Y are as defined in the claims) or pharmaceutically acceptable salts, prodrugs or solvates thereof. The derivatives of formula (1) have ²ARK1 inhibitory activity and are useful for preventing or treating heart failure. Moreover, the derivatives of formula (1) also have antitumor activity, particularly dual inhibitory activity on Aurora kinase and CDK, and are useful for cell proliferative diseases such as cancer.