A compound of formula, or a salt thereof, for use in the treatment of cancer: wherein: R4, R5, R6, R7, R8 and R9 are independently selected from hydrogen, halogen, hydroxyl, thiol, lower alkyl, substituted lower alkyl, lower alkenyl, substituted lower alkenyl, lower alkynyl, substituted lower alkynyl, alkylalkenyl, alkylalkynyl, alkoxy, alkylthio, acyl, aryloxy, amino, amido, carboxyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, heteroalkyl, cycloalkyl, substituted cycloalkyl, alkylcycloalkyl, alkylcycloheteroalkyl, nitro, cyano, or -S 0-2 R wherein R is alkyl, substituted alkyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, or substituted heteroaryl; x is CR11; y is CR12 or N; z is CR13 or N; r is CR14 or N; x' is CR15; y' is CR16 or N; z' is CR17 or N; r' is CR18 or N; R10 is hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, or acyl; and R11, R12, R13, R14, R15, R16, R17, and R18 are independently hydrogen, halogen, hydroxyl, thiol, lower alkyl, substituted lower alkyl, alkenyl, alkenyl, alkylalkenyl, alkylalkynyl, alkoxy, alkylthio, acyl, aryloxy, amino, amido, carboxyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, heteroalkyl, cycloalkyl, substituted cycloalkyl, alkylcycloalkyl, alkylcycloheteroalkyl, nitro or cyano, wherein: lower alkyl refers to a straight chain or branched alkyl group of 1 to 10 carbon atoms or a cyclic alkyl group of 3 to 10 carbon atoms; lower alkenyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbon atoms or a cyclic hydrocarbon of 3 to 10 carbon atoms, having at least one carbon to carbon double bond; lower alkynyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbons having at least one carbon to carbon triple bond. R4, R5, R6, R7, R8 and R9 are independently selected from hydrogen, halogen, hydroxyl, thiol, lower alkyl, substituted lower alkyl, lower alkenyl, substituted lower alkenyl, lower alkynyl, substituted lower alkynyl, alkylalkenyl, alkylalkynyl, alkoxy, alkylthio, acyl, aryloxy, amino, amido, carboxyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, heteroalkyl, cycloalkyl, substituted cycloalkyl, alkylcycloalkyl, alkylcycloheteroalkyl, nitro, cyano, or -S 0-2 R wherein R is alkyl, substituted alkyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, or substituted heteroaryl; x is CR11; y is CR12 or N; z is CR13 or N; r is CR14 or N; x' is CR15; y' is CR16 or N; z' is CR17 or N; r' is CR18 or N; R10 is hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, or acyl; and R11, R12, R13, R14, R15, R16, R17, and R18 are independently hydrogen, halogen, hydroxyl, thiol, lower alkyl, substituted lower alkyl, alkenyl, alkenyl, alkylalkenyl, alkylalkynyl, alkoxy, alkylthio, acyl, aryloxy, amino, amido, carboxyl, aryl, substituted aryl, heterocycle, heteroaryl, substituted heterocycle, heteroalkyl, cycloalkyl, substituted cycloalkyl, alkylcycloalkyl, alkylcycloheteroalkyl, nitro or cyano, wherein: lower alkyl refers to a straight chain or branched alkyl group of 1 to 10 carbon atoms or a cyclic alkyl group of 3 to 10 carbon atoms; lower alkenyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbon atoms or a cyclic hydrocarbon of 3 to 10 carbon atoms, having at least one carbon to carbon double bond; lower alkynyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbons having at least one carbon to carbon triple bond. lower alkyl refers to a straight chain or branched alkyl group of 1 to 10 carbon atoms or a cyclic alkyl group of 3 to 10 carbon atoms; lower alkenyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbon atoms or a cyclic hydrocarbon of 3 to 10 carbon atoms, having at least one carbon to carbon double bond; lower alkynyl refers to a straight chain or branched hydrocarbon of 2 to 10 carbons having at least one carbon to carbon triple bond.