Due to the poor permeability across Gram-negative bacterial membranes and the troublesome bacterial efflux mechanism, only a few GyrB/ParE inhibitors with potent activity against Gram-negative pathogens have been reported. Among them, pyrimido[4,5-<i>b</i>]indole derivatives represented by GP-1 demonstrated excellent broad-spectrum antibacterial activity against both Gram-positive and Gram-negative bacteria but were limited by hERG inhibition and poor pharmacokinetics profile. To improve their drug-like properties, we designed a series of novel pyrimido[4,5-<i>b</i>]indole derivatives based on the tricyclic scaffold of GP-1 and the C-7 moiety of acorafloxacin. These efforts have culminated in the discovery of a promising compound <b>18r</b> with reduced hERG liability and an improved PK profile. Compound <b>18r</b> exhibited superior broad-spectrum <i>in vitro</i> antibacterial activity compared to GP-1, including a variety of clinical multidrug G<sup>-</sup> pathogens, especially <i>Acinetobacter baumannii</i>, and the <i>in vivo</i> efficacy was also demonstrated in a neutropenic mouse thigh model of infection with multidrug-resistant <i>A. baumannii</i>.