Phthalazinone derivatives

A method of treatment for breast or ovarian cancer deficient in a HR dependent DNA DSB repair pathway comprising administering to a subject a therapeutically effective amount of a compound of formula (Ib):

• or an isomer or salt thereof, or a mixture of any thereof, wherein:
• A and B together represent a fused aromatic ring, optionally substituted with one or more substituent groups selected from halo, nitro, hydroxyl, ether, thiol, thioether, amino, C1-7 alkyl, C5-20 aryl and a heterocyclyl group having from 3 to 7 ring atoms, of which from 1 to 4 are selected from nitrogen, oxygen and sulfur;
• RX is selected from H, C1-20 alkyl, C5-20 aryl, amido, thioamido, ester, acyl, sulfonyl groups and a heterocyclyl group having from 3 to 7 ring atoms, of which from 1 to 4 are selected from nitrogen, oxygen and sulfur, wherein the acyl, C1-20 alkyl, C5-20 aryl or heterocyclyl group is optionally substituted with one or more substituent groups selected from C1-20 alkyl, C5-20 aryl, halo, hydroxyl, ether, nitro, cyano, acyl, carboxy, ester, amido, amino, acylamido, ureido, acyloxy, thiol, thioether, sulfoxide, sulfonyl, thioamido, sulfonamido and a heterocyclyl group having from 3 to 7 ring atoms, of which from 1 to 4 are selected from nitrogen, oxygen and sulfur;
• RC1 and RC2 are both hydrogen;
• and
• R1 is selected from H and halo;
• wherein the cancer is breast cancer or ovarian cancer;
• and
• wherein the cancer comprises one or more cancer cells having a reduced or abrogated ability to repair DNA DSB by HR relative to normal cells.