Eravacycline consists of the tetracyclic core scaffold, with two unique modifications in the tetracyclic D ring at position C7 (addition of fluorine atom) and C9 (addition of pyrrolidinoacetamo group) . As a consequence of these substitutions at C7 and C9, which are not present in any naturally occurring or semisynthetic tetracyclines, eravacycline exhibits in vitro activity against Gram-positive and -negative bacterial strains expressing certain tetracycline-specific acquired resistance mechanisms (Sect. 2.3) [11, 13, 14].
Like other tetracyclines, eravacycline exerts its antibacterial action by reversibly binding to the bacterial ribosomal 30S subunit, thereby preventing the incorporation of amino acid residues into elongating peptide chains and leading to disruption of bacterial protein synthesis . In vitro, eravacycline had a tenfold higher affinity for ribosomal binding and inhibited protein translation at fourfold lower drug concentrations than tetracycline . Tetracyclines, including eravacycline, typically exhibit bacteriostatic activity; however, eravacycline also exhibits bactericidal activity against certain strains of Acinetobacter baumannii, E. coli and K. pneumoniae in vitro [11, 12].
In Vitro Activity
This section focuses on the in vitro antibacterial activity of eravacycline against clinical isolates of Gram-positive and -negative aerobic and anaerobic microorganisms associated with cIAIs against which eravacycline has demonstrated efficacy in clinical trials. In the US prescribing information  and/or EU summary of product characteristics , specified aerobic pathogens are Citrobacter freundii , Enterobacter cloacae , E. coli [11, 13], Enterococcus faecalis [11, 13], Enterococcus faecium [11, 13], Klebsiella oxytoca , K. pneumoniae [11, 13], Staphylococcus aureus [11, 13], viridans Streptococcus spp. (includes the S. anginosus, S. mitis and S. salivarius groups)  and Streptococcus anginosus group (i.e. S. anginosus, S. constellatus and S. intermedius isolates) . Specified anaerobic pathogens are Clostridium perfringens, Bacteroides caccae, B. fragilis, Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteroides vulgaris and Parabacteroides distasonis . Emphasis is given to results from in vitro studies involving > 500 clinical isolates collected since 2012 [15,16,17,18,19,20,21,22,23,24,25] (year not identified in some studies [18, 20]). In vitro data indicate that Pseudomonas aeruginosa isolates are not susceptible to eravacycline [11, 13]. EUCAST defined minimum inhibitory concentration (MIC) breakpoints for susceptibility of eravacycline against E. coli, S. aureus, Enterococcus spp. and viridans Streptococcus spp. are 0.5, 0.25, 0.125 and 0.125 μg/mL, respectively .
Eravacycline exhibited potent in vitro activity (based on the MIC required to inhibit the growth of 90% of isolates; MIC90) against a broad spectrum of Gram-positive pathogens, including E. faecalis (vancomycin sensitive and resistant strains), E. faecium (vancomycin sensitive and resistant strains), S. aureus [both methicillin-susceptible (MSSA) and methicillin-resistant (MRSA)] and S. anginosus group isolates (Table 1) [15, 16, 18, 20, 21, 24, 25]. Eravacycline also demonstrated potent in vitro activity against common aerobic and anaerobic Gram-negative pathogens (Table 2) [15,16,17,18,19,20, 22, 23, 25], including carbapenem-resistant (CR), MDR and extended-spectrum (i.e. 3rd and 4th generation) cephalosporin-resistant Enterobacteriaceae (Table 2), and extended-spectrum β-lactamase (ESBL) producing Enterobacteriaceae (Table 2) [15,16,17,18,19,20, 22, 23, 25]. Relative to tigecycline, eravacycline MIC90 values were typically two- to fourfold lower against common aerobic Gram-positive (Table 1) and -negative (Table 2) pathogens.
A. baumannii is a Gram-negative opportunistic pathogen that is becoming an increasingly important pathogen associated with nosocomial infections. Against A. baumannii clinical isolates collected globally (in 2013–2015 ) and in Canada (CANWARD surveillance study; 2014–2015 ), Europe (2015 ; 2016 [19, 26]), Europe and Singapore (2005–2015 ), UK (2011 ), the USA (2013–2016 ; 2013–2014 ; year not reported ) or not specified , eravacycline exhibited very good in vitro activity, including against CR [19, 23, 27, 28] and MDR [23, 26] A. baumannii isolates. For example, in the largest study, the MIC90 value for eravacycline against 1097 A. baumannii isolates (MIC90 2 μg/mL) was twofold lower than that of tigecycline (MIC90 4 μg/mL), with no change in these respective MIC90 values against CR (n = 707) and MDR (n = 808) A. baumannii isolates .
In vitro studies have not demonstrated antagonism between eravacycline and other commonly used antibacterial drugs for specified pathogens against which eravacycline has shown efficacy in clinical trials .
In Vivo Activity
The in vitro activity of eravacycline is supported by evidence of its efficacy in animal models of Gram-positive and -negative infections (reviewed by Zhanel et al. ), including models of MRSA , tetracycline-resistant MRSA  and Enterobacteriaceae [30,31,32] infections.
The two primary mechanisms known to confer acquired resistance to tetracyclines are the acquisition of genes encoding efflux pumps and the presence of ribosomal protection proteins (RPPs) (reviewed by Nguyen et al. ). To date, 28 different tetracycline resistance efflux pumps have been identified, which are categorized into seven groups based mainly on sequence homology. The most common efflux pumps are encoded by tetA and tetB in Gram-negative bacteria and by tetK and tetL in Gram-positive bacteria. Typically, tetracycline-specific efflux pumps are most effective for conferring resistance to tetracycline (first generation) and, to a lesser extent, second generation tetracyclines such as doxycycline and minocycline, whereas certain efflux pumps confer minimal or no resistance to later generation tetracyclines such as tigecycline and eravacycline (i.e. for eravacycline, tetA, tetB and tetK encoded efflux pumps [11, 13]). To date, 12 distinct classes of RPPs have been identified, with the most common encoded by tetO and tetM. RPPs act to weaken the interactions between tetracycline and its binding site on the ribosomal protein, thereby conferring resistance to first- and second-generation tetracyclines, whilst having minimal or no effect on the in vitro antibacterial activity of third- or fourth-generation tetracyclines (i.e. for eravacycline, RPPs encoded by tetM and tetQ [11, 13]). The other key mechanisms associated with acquired tetracycline resistance are mutations within the 16S ribosomal RNA (rRNA; e.g. G1058C, A926T, G927T) and in genes encoding mono-oxygenases that consequently promote the degradation of tetracyclines (e.g. tetX, tet34 and tet37 genes [12, 14]), with these mechanisms much less prevalent than those for efflux pumps and RPPs. Innate resistance to tetracyclines may also occur due to differences in bacterial cell membrane permeability and/or the presence of small molecule transporters . With the exception of gene mutations that alter drug degradation, which only occur in Gram-negative bacteria, all other common tetracycline acquired resistance mechanisms occur in both Gram-positive and -negative bacteria (reviewed by Zhanel et al. ).
Eravacycline resistance in some bacteria is associated with up-regulation of non-specific intrinsic MDR efflux [11, 13] and target-site modifications such as to the 16S RNA or certain 30S ribosomal proteins (e.g. S10) . Resistance to eravacycline has also been observed in Enterococcus spp. isolates harbouring mutations encoded by rpsJ . There is no target-based cross-resistance between eravacycline and other classes of antibacterials such as fluoroquinolones, penicillins, cephalosporins and carbapenems .
The in vitro activity of eravacycline against S. aureus strains with up-regulated expression of the MepA (MIC 0.016 vs. 0.004 μg/mL in parent S. aureus strain) or NorA (MIC 0.004 vs. 0.004 μg/mL) efflux pumps was consistent with that observed in the respective parent strain . By contrast, the presence of these respective efflux pumps confers resistance to tigecycline and fluoroquinolones.
There was no change (for tetM, tetB and tetK genes) or a minimal change (tetA gene) in the in vitro activity of eravacycline against E. coli strains expressing common efflux pump (tetA, tetB and tetK) and RPP (tetM) tetracycline resistance genes compared with its in vitro activity against parent E. coli strains . In an E. coli strain expressing the TetA efflux pump, there was a fourfold increase in the MIC value for eravacycline (MIC 0.25 vs. 0.063 μg/mL against the control E. coli strain), although this fold increase in MIC was markedly lower than that observed for tigecycline (MIC 1 vs. 0.063 μg/mL; 16-fold increase) or other tetracyclines (≥ 16-fold increase) .
In vitro studies indicate that eravacycline exhibits potent activity against naturally occurring mcr-1 positive bacterial strains, which confers resistance to polymixin antibacterials such as colistin, and against carbapenem-resistant Enterobacteriaceae clinical isolates engineered to over-express mcr-1 . Eravacycline also exhibited very good in vitro activity against colistin-resistant mcr-1 positive E. coli and mcr-1 positive K. pneumoniae .
The best pharmacokinetic/pharmacodynamic (PK/PD) predictor of eravacycline efficacy is the area under the plasma concentration–time curve (AUC) divided by the MIC . Based on the flat exposure–response relationship observed in clinical studies, eravacycline exposure achieved with the recommended dosage regimen (Sect. 6) appears to be on the plateau of the exposure/response curve . Supportive evidence for the AUC/MIC ratio as the best predictor of the efficacy of eravacycline comes from murine thigh infection models [32, 35, 36]. For example, in an E. coli thigh infection model, the best PK/PD parameter for predicting the efficacy of eravacycline was the 24 h free-drug AUC (fAUC)/MIC (correlation co-efficient 0.80), with mean fAUC/MIC values for net stasis and 1 log10 of bacterial killing of 27.97 and 32.60 .
Effects on Cardiac Electrophysiology
In a thorough QT study in 60 healthy adult volunteers, a single intravenous infusion of eravacycline 1.5 mg/kg (i.e. 1.5 × the maximum approved recommended dose ) had no clinically relevant effect on the corrected QT (QTc) interval or any other ECG parameters .