Chroman derivatives, medicaments and use in therapy

1 - 19 . (canceled) 
     
     
         20 . A method of treating cancer, the method comprising administering to a subject in need thereof a compound of the formula (I-a): 
       
         
           
           
               
               
           
         
         wherein
 R 1  is methyl; 
 R 2  and R 3  are independently hydrogen, hydroxy, alkoxy, halo, or alkyl, with the exception that R 2  and R 3  are not both hydrogen; and 
 R 4  is hydroxy or alkoxy; and 
 R 5  and R 6  are each independently hydrogen, 
 or a pharmaceutically acceptable salt thereof. 
 
       
     
     
         21 . The method of  claim 20 , wherein R 2  is hydroxy or methoxy. 
     
     
         22 . The method of  claim 20 , wherein R 3  is hydrogen. 
     
     
         23 . The method of  claim 20 , wherein R 4  is methoxy. 
     
     
         24 . The method of  claim 20 , wherein the cancer is of epithelial, mesenchymal, or neural origin. 
     
     
         25 . The method of  claim 24 , wherein the cancer is ovarian cancer, breast cancer, pancreatic cancer, or melanoma. 
     
     
         26 . The method of  claim 20 , wherein administering the compound of formula (I-a) results in the cancer being sensitized to one or more chemotherapeutic agent or radiotherapy, and the method further comprises administering to the subject a chemotherapeutic agent or radiotherapy to which the cancer has been sensitized. 
     
     
         27 . The method of  claim 26 , wherein the chemotherapeutic agent is selected from the group consisting of cisplatin, carboplatin, paclitaxel, gemcitabine, doxorubicin, decarbazine, or topotecan. 
     
     
         28 . A method of sensitizing a cancer cell, the method comprising contacting the cancer cell with a compound of formula (I-a): 
       
         
           
           
               
               
           
         
         wherein
 R 1  is methyl; 
 R 2  and R 3  are independently hydrogen, hydroxy, alkoxy, halo, or alkyl, with the exception that R 2  and R 3  are not both hydrogen; 
 R 4  is hydroxy or alkoxy; and 
 R 5  and R 6  are each independently hydrogen, 
 or a pharmaceutically acceptable salt thereof. 
 
       
     
     
         29 . The method of  claim 28 , wherein R 2  is hydroxy or methoxy. 
     
     
         30 . The method of  claim 28 , wherein R 3  is hydrogen. 
     
     
         31 . The method of  claim 28 , wherein R 4  is methoxy. 
     
     
         32 . The method of  claim 28 , wherein the cancer is of epithelial, mesenchymal, or neural origin. 
     
     
         33 . The method of  claim 32 , wherein the cancer is ovarian cancer, breast cancer, pancreatic cancer, or melanoma. 
     
     
         34 . The method of claim of  28 , wherein the method of sensitizing comprises sensitizing the cancer cell to a chemotherapeutic agent to which, prior to the method of sensitizing, the cancer cell is not sensitive or is poorly sensitive. 
     
     
         35 . The method of  claim 28 , wherein the method of sensitizing comprises sensitizing the cancer cell to radiotherapy to which, prior to the method of sensitizing, the cancer cell is not sensitive or is poorly sensitive.