Screening of the Pandemic Response Box library identified promising compound candidate drug combinations against extensively drug-resistant Acinetobacter baumannii

Abstract

Infections caused by antimicrobial-resistant Acinetobacter baumannii pose a significant threat to human health, particularly in the context of hospital-acquired infections. As existing antibiotics lose efficacy against Acinetobacter isolates, there is an urgent need for the development of novel antimicrobial agents. In this study, we assessed 400 structurally diverse compounds from the Medicines for Malaria Pandemic Response Box for their activity against two clinical isolates of A. baumannii: A. baumannii 5075, known for its extensive drug resistance, and A. baumannii QS17-1084, obtained from an infected wound in a Thai patient. Among the compounds tested, seven from the Pathogen box exhibited inhibitory effects on the in vitro growth of A. baumannii isolates, with IC₅₀s ≤ 48 µM for A. baumannii QS17-1084 and IC₅₀s ≤ 17 µM for A. baumannii 5075. Notably, two of these compounds, MUT056399 and MMV1580854, shared chemical scaffolds resembling triclosan. Further investigations involving drug combinations identified five synergistic drug combinations, suggesting potential avenues for therapeutic development. The combination of MUT056399 and brilacidin against A. baumannii QS17-1084 and that of MUT056399 and eravacycline against A. baumannii 5075 showed bactericidal activity. These combinations significantly inhibited biofilm formation produced by both A. baumannii strains. Our findings highlight the drug combinations as promising candidates for further evaluation in murine wound infection models against multidrug-resistant A. baumannii. These compounds hold potential for addressing the critical need for effective antibiotics in the face of rising antimicrobial resistance.

Fig. 1

Characteristics of A. baumannii ATCC19606 (ATCC19606), A. baumannii 5075 (AB5075), and A. baumannii QS17-1084 (QS17-1084). Strain sequence type (ST) is indicated next to each strain. The assigned antimicrobial agent resistance phenotype is provided where the green, yellow and blue squares indicate a result of susceptible, intermediate, and resistant to the tested antimicrobial agents, respectively. The known antimicrobial agent resistant genes are provided in which each color represents each class or subclass of antimicrobial agent as well as mode of their inhibition. The figure was produced using the iTOL tool.

Fig. 2

Antibacterial activities against A. baumannii QS17-1084 in the Pandemic Response Box collection at 10 µM concentration. Activities of known antibacterials are represented by blue circles [colistin (12.6 µM), erythromycin (12.8 µM), doxycycline (8.3 µM), imipenem (12.6 µM), and ciprofloxacin (9.4 µM)]. Most of the compounds had activities below the 50% growth inhibition cutoff. Seven of the most active compounds were selected for further tests based on their ability to inhibit the growth of A. bamannii QS17-1084 in the 80–100% range. These compounds are indicated by red circles.

Fig. 3

Inhibition of A. baumannii in vitro growth by 7 MMV Pandemic response box compounds. (A) Half maximal inhibitory concentration (IC₅₀) of the compounds against A. baumannii QS17-1084 and 5075. (B) IC₅₀ comparison of MMV1580854 and MUT056399 against A. baumannii QS17-1084 and 5075. Statistically significant differences calculated by multiple unpaired t test are indicated with one (P < 0.05) and nd for no difference.

Fig. 4

Drug interactions in A. baumannii. Forty-six drug combinations were carried out against both A. baumannii QS17-1084 and 5075. The numbers represent ΣFIC₅₀ (50% Fractional Inhibitory Concentrations) values: ΣFIC₅₀, synergism when ΣFIC₅₀ ≤ 0.5; toward synergism when ΣFIC₅₀ < ; additive when ΣFIC₅₀ = 1; toward antagonism when ΣFIC₅₀ > 1; antagonism when ΣFIC₅₀ ≥ 2 to 4. The values show the mean ± S.D. of 3 independent assays for each A. baumannii strain.

Fig. 5

Time-kill curve. (A) A. baumannii QS17-1084 incubated without antibiotic (growth control), with MUT056399, brilacidin, MUT056399:brilacidin (ratio 1:2), and MUT056399:brilacidin (ratio 4:1). (B) A. baumannii 5075 incubated without antibiotic (growth control), with MUT056399, eravacycline: MUT056399:eravacycline (ratio 3:1), and MUT056399 : eravacycline (ratio 1:2). *P < 0.05 was analyzed by Dunnett's multiple comparisons test to compare between drug combination (MUT056399 and brilacidin at the ratio 1:2 for A. baumannii QS17-1084; and MUT056399 and eravacycline at ratio 1:2 for A. baumannii 5075) versus other tested compounds and growth control.

Fig. 6

Effects of drug combination on preventing biofilm formation. (A) A. baumannii QS17-1084 was treated with MUT056399 and brilacidin at the ratio 1:2. (B) A. baumannii 5075 was incubated with MUT056399 and eravacycline at ratio 1:2. The drug concentrations were similar to those used in the time-kill assay. A. baumannii ATCC19606 and E. coli ATCC25922 were included as positive and negative biofilm producing strains, respectively. P < 0.05 was analyzed by using Student’s t-test. The representative images are biofilms stained by crystal violet.