A compound of formula I or a pharmaceutically acceptable salt or ester thereof is provided for the treatment of cancer wherein (i) the cancer is one that has the characteristic of being a type prone to being or becoming refractory or resistant to platinum drug based therapy and (ii) the treatment is with a dose of between 1 mg/m2 and 30 mg/m2 of compound per patient body surface area per administion. Method of treatment and novel dosage forms are also provided. Particularly treated arc ovarian cancers, particularly those expressing a-folate receptors, including epithelial ovarian, fallopian tube or peritoneal cancer.
