A pharmaceutical composition comprising a compound Polymyxin B and 4-oxoquinolizina formula IIIc: ** (See formula) ** or a pharmaceutically acceptable salt thereof, wherein R1 is hydrogen, halogen, cyano, C1-8 alkyl, C1-8 haloalkyl, -ORx, -N (RX) 2, -C (O) RX, -C (O) ORX or -C (O) N (RX) 2, wherein each RX is independently hydrogen, C1-8, C1-8 alkyl or haloalkyl and Y is aryl, or heteroaryl, each substituted with one to five groups are - N (RY1) 2 or -C 1-8 alkyl-RY where RY is - N (RY1) 2 and optionally further substituted with one to four groups are each independently halogen, C1-8 alkyl, C1-8 haloalkyl, C3-8 cycloalkyl, heterocyclyl, aryl, heteroaryl, C3-8cycloalkyl-alkyl (C1-8), heterocyclyl (C1-8) alkyl, aryl cycloalkyl- (C1-8) heteroaryl (C1-8) alkyl, -R, or -C 1-8 alkyl-RY, wherein RY is nitro, cyano, -ORY1, -SRY1, -N (RY1) 2 , -C (O) RY1, -C (O) ORY1, -C (O) N (RY1) 2, -OC (O) RY1, -OC (O) ORY1, -OC (O) N (RY1) 2 , - N (RY1) C (O) RY1, -N (RY1) C (O) ORY1, -N (RY1) C (O) N (RY1) 2, -S (O) 2RY1, -S (O) 2ORY1, -S (O) 2N (RY1) 2, -OS (O) 2RY1, - OS (O) 2ORY1, -OS (O) 2N (RY1) 2, -N (RY1) S (O) 2RY1, - N (RY1) S (O) 2ORY1, or -N (RY1) S (O) 2N (RY1) 2, wherein each RY1 independently is hydrogen, C1-8alkyl, or C1-8 haloalkyl.Una composición farmacéutica que comprende Polimixina B y un compuesto de 4-oxoquinolizina de fórmula IIIc: **(Ver fórmula)** o una sal farmacéuticamente aceptable del mismo, en la que R1 es hidrógeno, halógeno, ciano, alquilo C1-8, haloalquilo C1-8, -ORX, -N(RX)2, -C(O)RX, -C(O)ORX, o -C(O)N(RX)2, en la que cada RX es independientemente hidrógeno, alquilo C1-8, o haloalquilo C1-8 y Y es arilo, o heteroarilo, cada uno sustituido con uno a cinco grupos que son - N(RY1)2 o -alquil C1-8-RY donde RY es - N(RY1)2 y adicionalmente sustituido opcionalmente con uno a cuatro grupos que son cada uno independientemente halógeno, alquilo C1-8, haloalquilo C1-8, cicloalquilo C3-8, heterociclilo, arilo, heteroarilo, cicloalquil C3-8-alquilo (C1-8), heteroc
