Pharmaceutically active diketopiperazines

A diketopiperazine of formula (A) wherein each of R14 and R15, which may be the same or different, is independently selected from hydrogen and C1-C6 alkyl provided at least one of R14 and R15 is C1-C6 alkyl; and R1 to R10, which may be the same or different, is independently selected from hydrogen, C1-C6 alkyl unsubstituted or substituted by one or more hydrogen atoms, C1-C6 alkoxy, C1-C6 alkylthio, halogen, hydroxy, nitro, optionally substituted phenyl, cyano, -CH2OH, -CH2COOH, -CO2R , -NHCOR , -NHSO2R , -SO2R , -CON(R )R , -SOR , -SO2N(R R ), -N(R R O(CH2)nN(R R ), O(CH2)nCO2R , -OCOR , -CH2OCOR , -CH2NHCOR , -CH2NHCOOR , -CH2SR , -CH2SCOR , -CH2S(O)mR wherein m is 1 or 2, -CH2NHCO(CH2)n CO2R , -N(R COR ), -NHCOCF3, NHCO(CH2)nCO2R , NHCO(CH2)nOCOR and -CNHCO(CH2)nOR wherein n is 0 or is an integer of from 1 to 6, each of R and R is independently H or C1-C6 alkyl or, when R and R are attached to the same nitrogen atom, they may alternatively form with the nitrogen atom a saturated five or six-membered ring; and R is C1-C6 allyl; or any of R1 and R2, R2 and R3, R3 and R4 and R4 and R5, or R6 and R7, R7 and R8, R8 and R9 and R9 and R10, form together with the carbon atoms to which they are attached a benzene ring which is optionally substituted; and pharmaceutically acceptable salts and esters thereof; are modulators of multiple drug resistance.