A compound of following formula (I') or a pharmaceutically acceptable salt thereof, a stereoisomer or mixture of stereoisomers thereof, in any proportions, particularly a mixture of enantiomers, and especially a racemic mixture, wherein: • X 1 represents CR 2; • X 2 represents CR 5; • R 1 represents H, (C1-C6)alkyl or halogen; • R 2 represents CONR11R12; wherein R11 represents H or (C1-C6)alkyl; R12 represents a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or 5- or 6-membered heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR35 and NR36R37; R35, R36 and R37 represent, independently of one another, H or (C1-C6)alkyl; • R 3 represents H, (C1-C6)alkyl or halogen; • R 4 represents H, Cl, CN, NO 2, NHR18 or OR19, wherein: R18 represents H, (C1-C6)alkyl, aryl, heteroaryl or (C1-C6)alkylcarbonyl; R19 represents H or (C1-C6)alkyl; • R 5 represents NR21R22 wherein: R21 represents H, or R41; R22 represents H, or R42; or R21 and R22 form together with the nitrogen atom bearing them a heterocycle optionally substituted with a (C1-C6)alkyl group; R41 and R42 represent, independently of one another, a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR43 and NR44R45; R43, R44 and R45 represent, independently of one another, H or (C1-C6)alkyl; • R 6 represents H, OH, (C1-C6)alkoxy or (C1-C6)alkyl; and • R 7 represents H or (C1-C6)alkyl; provided that when R12 = aryl optionally substituted, then R21 represents H or (C1-C6)alkyl and R22 represents H or (C1-C6)alkyl, or R21 and R22 form together with the nitrogen atom bearing them a heterocycle, preferably a saturated heterocycle, optionally substituted with a (C1-C6)alkyl group, and the pharmaceutically acceptable salts thereof, for use in the treatment of cancer. • X 1 represents CR 2; • X 2 represents CR 5; • R 1 represents H, (C1-C6)alkyl or halogen; • R 2 represents CONR11R12; wherein R11 represents H or (C1-C6)alkyl; R12 represents a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or 5- or 6-membered heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR35 and NR36R37; R35, R36 and R37 represent, independently of one another, H or (C1-C6)alkyl; R11 represents H or (C1-C6)alkyl; R12 represents a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or 5- or 6-membered heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR35 and NR36R37; R35, R36 and R37 represent, independently of one another, H or (C1-C6)alkyl; • R 3 represents H, (C1-C6)alkyl or halogen; • R 4 represents H, Cl, CN, NO 2, NHR18 or OR19, wherein: R18 represents H, (C1-C6)alkyl, aryl, heteroaryl or (C1-C6)alkylcarbonyl; R19 represents H or (C1-C6)alkyl; R18 represents H, (C1-C6)alkyl, aryl, heteroaryl or (C1-C6)alkylcarbonyl; R19 represents H or (C1-C6)alkyl; • R 5 represents NR21R22 wherein: R21 represents H, or R41; R22 represents H, or R42; or R21 and R22 form together with the nitrogen atom bearing them a heterocycle optionally substituted with a (C1-C6)alkyl group; R41 and R42 represent, independently of one another, a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR43 and NR44R45; R43, R44 and R45 represent, independently of one another, H or (C1-C6)alkyl; R21 represents H, or R41; R22 represents H, or R42; or R21 and R22 form together with the nitrogen atom bearing them a heterocycle optionally substituted with a (C1-C6)alkyl group; R41 and R42 represent, independently of one another, a (C1-C6)alkyl, aryl, aryl-(C1-C6)alkyl or heteroaryl group optionally substituted with one or more groups selected from halo, (C1-C6)alkyl, OR43 and NR44R45; R43, R44 and R45 represent, independently of one another, H or (C1-C6)alkyl; • R 6 represents H, OH, (C1-C6)alkoxy or (C1-C6)alkyl; and • R 7 represents H or (C1-C6)alkyl; provided that when R12 = aryl optionally substituted, then R21 represents H or (C1-C6)alkyl and R22 represents H or (C1-C6)alkyl, or R21 and R22 form together with the nitrogen atom bearing them a heterocycle, preferably a saturated heterocycle, optionally substituted with a (C1-C6)alkyl group, and the pharmaceutically acceptable salts thereof, for use in the treatment of cancer. provided that when R12 = aryl optionally substituted, then R21 represents H or (C1-C6)alkyl and R22 represents H or (C1-C6)alkyl, or R21 and R22 form together with the nitrogen atom bearing them a heterocycle, preferably a saturated heterocycle, optionally substituted with a (C1-C6)alkyl group, and the pharmaceutically acceptable salts thereof, for use in the treatment of cancer.