Omadacycline : development of a novel aminomethylcycline antibiotic for treating drug-resistant bacterial infections

Paratek Pharmaceuticals, 1000 First Avenue, Suite 400, King of Prussia, PA 19406, USA *Author for correspondence: Tel.: +1 267 364 5560; stephen.villano@paratekpharma.com Omadacycline is a first-in-class aminomethylcycline antibiotic that circumvents common tetracycline resistance mechanisms. In vitro omadacycline has potent activity against Grampositive aerobic bacteria including methicillin-resistant Staphylococcus aureus, penicillinresistant and multidrug-resistant Streptococcus pneumoniae, and vancomycin-resistant Enterococcus spp. It is also active against common Gram-negative aerobes, some anaerobes and atypical bacteria including Legionella spp. and Chlamydia spp. Ongoing Phase III clinical trials with omadacycline are investigating once daily doses of 100 mg intravenously followed by once daily doses of 300 mg orally for the treatment of acute bacterial skin and skin structure infections and community-acquired bacterial pneumonia. This paper provides an overview of the microbiology, nonclinical evaluations, clinical pharmacology and initial clinical experience with omadacycline.


Background
The tetracycline family of therapeutic agents has been in commercial use since the late 1940s, although there is evidence that they may have provided antibiotic protection (through ingestion of grain contaminated by Streptomyces bacteria) even in ancient times (Cook et al., 1989) [1].While the tetracyclines have represented a mainstay of broad-spectrum antibiotics for many years, but the increasing incidence of bacterial resistance has relegated older tetracyclines to a limited role for treating common infectious diseases [2,3].Growing resistance to tetracyclines encouraged research into mechanisms of resistance and discovery of new generations of tetracycline.Tigecycline, a glycylcycline tetracycline, was introduced in the past decade and has been successfully used clinically, in part, because it circumvents the common tetracycline resistance mechanisms.However, tigecycline is only available as an intravenous (iv.) formulation, and is associated with a high incidence of dose-related nausea and vomiting, and safety concerns including increased all-cause mortality [4].Thus, alternatives are needed for treating common community-and hospital-acquired infections.
Omadacycline is a first-in-class aminomethylcycline antibiotic that overcomes the most common mechanisms of tetracycline [5,6].Omadacycline is active in vitro against Gram-positive aerobes, many Gram-negative aerobes regardless of ESBL phenotype, some anaerobes and atypical bacteria including Legionella spp.and Chlamydia spp.[7].In addition, omadacycline is active in vitro against many resistant Gram-positive pathogens including methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant and multidrug-resistant Streptococcus pneumoniae and vancomycin-resistant enterococcus [7].
For reprint orders, please contact: reprints@futuremedicine.com review Villano Omadacycline is undergoing clinical development as once daily oral and intravenous monotherapy for the treatment of acute bacterial skin and skin structure infections (ABSSSI) and community-acquired bacterial pneumonia (CABP).This review provides an overview of the microbiology, nonclinical data, clinical pharmacology and initial clinical experience with omadacycline.

Chemistry
As with other members of the tetracycline class, omadacycline binds to the tetracycline binding site on the 30S subunit of the bacterial ribosome and inhibits bacterial protein synthesis [6,8].Omadacycline differs from tetracycline by modifications at both the C7 and presence of an aminomethyl group at the C9 position (Figure 1) [5].Modifications at the C7 position were added to overcome the tetracycline efflux mechanism of resistance, and modifications at the C9 position were added to overcome ribosome protection mechanism of resistance [7].Omadacycline is prepared by chemical modification of minocycline, and is a stable, well-characterized crystalline drug substance [5].Omadacycline is administered as the tosylate salt for the intravenous formulation.

Microbiology
Omadacycline demonstrates antimicrobial activity in vitro against a range of Gram-positive and Gram-negative aerobes and some anaerobic bacteria that are commonly associated with ABSSSI and CABP [7, [9][10].
In vitro activity with omadacycline was demonstrated against atypical bacteria including Legionella pneumophila (Table 6) and Chlamydia spp.(MIC 90 of 0.25 mcg/ml) [15].In vitro, omadacycline exhibits the following activity against the anaerobes tested: Bacteroides fragilis (MIC 90 = 2 mcg/ml), Clostridium difficile (MIC 90 = 0.12 mcg/ml), Clostridium perfringens (MIC 90 = 4 mcg/ml) and anaerobic Gram-positive cocci (MIC 90 = 0.5 mcg/ml) Two mechanisms, efflux pump and ribosomal protection, account for much of the resistance to tetracycline antibiotics [3,6].While omadacycline is known to inhibit protein synthesis with a greater potency than tetracycline, definitive experiments performed with functional assays and macromolecular synthesis demonstrated that omadacycline inhibited protein synthesis in tetracycline-resistant bacterial strains that expressed both the efflux pump and ribosomal protection mechanisms [3,6,7].Using a cell-free in vitro protein synthesis model, protein synthesis inhibition activity of omadacycline was investigated in both the presence and absence of the Tet(O), a ribosome protection protein.Omadacycline inhibited protein synthesis in a cell-free system regardless of whether Tet(O) was present or not.Importantly, omadacycline was able to overcome tetracycline resistance mechanisms and also was not affected by resistance mechanisms of other antibiotics [6].Surveillance data from 2015 demonstrate that omadacycline retains activity (MIC 90 value 0.25 mcg/ml) against tetracycline resistant strains of S. aureus, E. faecalis and S. pneumoniae Additionally, the potential for the emergence of resistance to omadacycline was assessed by both single and multistep pressure selection.Resistance to omadacycline was not observed either following a single exposure to drug or after serial passage at sub-MIC concentrations for any of the strains [Paratek Pharmaecuticals, data on file].

Metabolism
The potential for enzymatic metabolism of omadacycline was evaluated in vitro using either future science group www.futuremedicine.compooled human liver microsome preparations, S9, liver cytosol or recombinant flavin monooxygenases (FMO1, FMO3, FMO5) [16].CYP450 isozymes evaluated included CYP 1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 2J2 and 3A4/5.Omadacycline did not induce CYP isozymes and no or minimal (<40% of maximal positive control response) induction of their mRNAs was observed.Omadacycline exhibited no significant inhibition of CYP isozyme activity and demonstrated no significant binding to human microsomes.These results indicate a low potential for drug-drug interactions via e nzymatic metabolism.The cardiovascular risk potential for omadacycline was evaluated through a series of in vitro and in vivo studies, including mammalian pharmacologic receptor binding; human etherà-go-go-Related Gene (hERG) channel binding; effects on rabbit ex vivo sinoatrial node activity; and in vivo effects on cardiovascular function in the cynomolgus monkey [18].No significant binding to the hERG channel, b-adrenergic receptor or any other receptors was observed that could result in a direct stimulatory effect on heart rates.Omadacycline binds in vitro to the muscarinic-2 (M 2 ) receptor subtype but not the M 1 , M 3 or M 4 receptor subtypes and exhibited a concentration-dependent antagonism of the effect carbamylcholine (a muscarinic receptor agonist), which resulted in an increase in heart rate in the ex vivo sinoatrial node model that reached a peak at 4.5 h after the dose.Omadacycline exhibited no effect on hERG channel activity at a concentration of 100 ug/ml.In addition, omadacycline at doses up to 40 mg/kg had no effect on the QTc interval in conscious monkeys.Overall these nonclinical findings showed that omadacycline had a vagolytic effect on heart rate but had a low potential for cardiac arrhythmia or clinically significant cardiovascular toxicity.

Clinical pharmacokinetics & pharmacodynamics
The human pharmacokinetic profile of omadacycline was characterized in healthy subjects.Pharmacokinetic parameters are shown from a bioavailability study that compared single intravenous and oral administration using the formulations and doses being evaluated in ongoing Phase III studies (Table 7). •

• Absorption, distribution, metabolism & excretion
In a mass balance study, six healthy male subjects received a single oral 300 mg dose of [14C] omadacycline (mean radioactivity 36.6 μCi) under fasting conditions [20].Mean recovery of the radioactive dose was 95.5% after 7 days.
In plasma, omadacycline and its C-4 epimer (which is formed spontaneously upon standing) accounted for 100% of the AUC.No enzymatically formed metabolites were detected.Based on radioactivity measurements, the main routes of elimination were fecal (81.1%) and urinary (14.4%).Given on an oral bioavailability of 35% for the tablet formulation, approximately 40% of the absorbed dose should be eliminated in the urine [20].In a separate experiment, in vitro determination of protein binding in human plasma found no dose dependency over concentrations ranging from 0.1 to 10 mcg/ml, and the mean bound protein fraction was 21% [21]. •

• intravenous dosing
After single intravenous doses of omadacycline from 25 to 600 mg, mean AUC 0-inf was linear and ranged from 1.3 to 36.0 mcg*h/ml [19].For single doses from 25 to 200 mg (0.5 mg/ml infused over  The earliest clinical studies of oral omadacycline were conducted with simple capsule formulations.Subsequently, various oral formulations have been evaluated in an effort to improve bioavailability and tolerability.The bioavailability of different oral formulations of omadacycline relative to an intravenous dose was investigated in an open-label, randomized, crossover study in healthy subjects [22].Subjects received omadacycline 100 mg iv., two 300-mg tablet formulations with different dissolution profiles and a 300 mg oral solution.Equivalent total exposure relative to the 100 mg iv.dose was observed with both 300 mg tablet formulations with geometric mean ratios (90% CI) for AUC 0-inf of 1.00 (0.93,1.07) and 0.96 (0.90,1.03), respectively.The coefficients of variation for AUC 0-inf for all studied formulations ranged from 16 to 24%.The absolute bioavailability of the tablet formulation selected for use in Phase III studies was approximately 35%.Thus, a 300 mg oral dose of the Phase III tablet formulation produced omadacycline exposure equivalent to that of a 100 mg iv.dose.With respect to pharmacodynamic assessments, animal models had identified AUC/MIC as the index that is most important for the efficacy of omadacycline [23].In humans, the serum concentrations achieved following a 100 mg iv. or 300 mg oral dose are associated with AUC values (Table 7) that are expected to provide clinical activity against the bacteria commonly a ssociated with ABSSSI and CABP.

•
• Food effect A Phase I, random-sequence, open-label, 4-period crossover study evaluated the effects of food on the pharmacokinetics of omadacycline in healthy subjects (Tzanis et al., 2016) [24].In each period subjects received a single 300 mg oral dose of omadacycline but the meal time varied relative to dosing: A) ≥6-h fast before dosing, B) standard, high fat, non-dairy meal 4 h before dosing, C) standard, high fat, non-dairy meal 2 h before dosing, and D) standard, high fat meal containing dairy 2 h before dosing.Compared with a fast of at least 6 h, omadacycline exposure (AUC and C max ) was reduced by 15-17% for the meal 4 h before dosing, 40-42% for the meal 2 h before dosing, and 59-63% for the meal with dairy 2 h before dosing.Thus, the food effect was more pronounced when a meal was consumed closer to oral dosing, with an even greater effect when dairy was included in the meal.The latter findings are consistent with the known tetracycline characteristic of binding to calcium as well as other multivalent cations.Accordingly, in general it is recommended that oral omadacycline should be administered in a fasted state, with avoidance of concomitant oral products containing calcium or other multivalent cations (e.g., dairy products, antacids, or multivitamins).Two Phase I studies were undertaken to evaluate the effect of age and gender on the PK of omadacycline after oral and intravenous administration in healthy volunteers [25].Both were double-blind and placebo-controlled studies of single doses of omadacycline.Study 1 included four groups: young males; young females; elderly males; and elderly females -all of whom received omadacycline 200 mg oral or placebo.Study 2 included healthy young male and female subjects who received a single 200 mg oral or 100 mg iv.dose of omadacycline or placebo.In Study 1, no effect of age on omadacycline absorption and PK profile was observed, but exposure (C max and AUC inf ) was at least 30% higher among females versus males in both age groups.In contrast, in study 2 omadacycline exposure based on AUCi nf was similar for both genders after the oral dose, but was approximately 30% higher among females versus males after the intravenous dose.Therefore, female subjects tended to have higher omadacycline exposure than male subjects, though this was not observed consistently in the two studies.In a population PK analysis of subjects across 10 different Phase I studies of omadacycline, the proportion of female subjects was low (19%), but the model showed that gender did not affect omadacycline clearance (the only subject characteristic that did so was renal function) [26].Overall, no dosage adjustment for omadacycline is necessary on the basis of patient age or gender.

• • eCG QT evaluation
The effect of single therapeutic and supratherapeutic intravenous doses of omadacycline on ventricular repolarization and the relationship between plasma concentrations of omadacycline and QTc intervals was evaluated [27].In this double-dummy, randomized, crossover study, healthy subjects were randomized to omadacycline  100 mg iv., omadacycline 300 mg iv., moxifloxacin 400 mg or placebo.Mean AUC 0-24 and C max were dose proportional for omadacycline 100 mg and 300 mg.ECG results showed that omadacycline did not increase QTc; the largest one-sided upper 95% confidence bound (95% CB) on the difference between and omadacycline and placebo (ddQTcF) was 1.53 ms for omadacycline 100 mg and 0.83 ms for omadacycline 300 mg.Further, no relationship was observed between omadacycline plasma concentrations and ddQTcF.Assay sensitivity was confirmed with a >10 ms increase in ddQTcF with moxifloxacin.Within 1 h after dosing, mean peak increases in heart rate were observed for omadacycline (17 bpm for 100 mg iv. and 24 bpm for 300 mg iv.) versus 3 bpm review Villano, Steenbergen & Loh future science group for placebo and 5 bpm for moxifloxacin.These changes were asymptomatic, not associated with changes in blood pressure, and were comparable across all groups by 12-24 h after dosing.Overall, this study is consistent with the results of preclinical studies demonstrating a low potential for adverse cardiac effects with omadacycline.

• • Hepatic impairment
The PK of omadacycline was evaluated in subjects with varying degrees of hepatic impairment (mild, moderate and severe as determined by Child-Pugh classes A, B and C, respectively) and matched healthy subjects [28].Both intravenous and oral doses of omadacycline were evaluated in these subjects.Results showed no effect of any degree of hepatic impairment on the PK of oral or intravenous omadacycline; geometric mean ratios for C max and AUC ranged from 0.89 to 1.37.Further, pooled analysis of dosenormalized PK parameters demonstrated no clear relationship between exposure parameters and the severity of hepatic impairment.Thus, no dose adjustment for omadacycline is warranted in patients with hepatic impairment.

Clinical efficacy for treatment of skin infections
Two randomized, double-blind, multicenter studies (a Phase II study started in 2007 and a truncated Phase III study started in 2009) have been completed with omadacycline in patients with skin infections [29,30].At the time that these studies were conducted, these infections were classified as 'complicated skin and skin structure infections' (cSSSI).In the Phase II study, adult patients with cSSSI received omadacycline 100 mg iv.once daily followed by the option to switch to 200 mg oral once daily, or linezolid 600 mg iv.twice daily with the option to switch to 600 mg oral twice daily.Aztreonam 2 g iv.twice daily could be added to linezolid if an infection due to a Gram-negative pathogen was suspected.Treatment was administered for up to 14 days.A total of 219 patients were treated (111 omadacycline, 108 linezolid) for an average of 10 days in both treatment groups [29].The primary efficacy assessment was performed at the test of cure (TOC) visit, which was to occur 10-17 days after the last dose of study drug.Clinical success at that timepoint was defined (in abbreviated terms) as resolution of infection such that no additional antibiotics were needed for the skin infection at that time or used at any time between the end of study drug treatment and the TOC evaluation, and no antibiotics were given for another indication up to that time in the study.Clinical response in the intent-to-treat population was 88.3% with omadacycline and 75.9% with linezolid, and both drugs also were effective in patients known to be infected with MRSA.
In the truncated Phase III study, enrollment was stopped early because of a decision by the US FDA to change the primary end point in studies of www.futuremedicine.com the treatment of bacterial skin infection.However, patients who had been enrolled up to that point were followed as originally planned [30].In the study, adult patients with cSSSI received omadacycline 100 mg iv.once daily followed by the option to switch to 300 mg oral once daily, or linezolid using same dosing regimen as the Phase II study.Moxifloxacin (IV or oral) could be added to linezolid if an infection due to a Gram-negative pathogen was suspected.Treatment was administered for up to 14 days.A total of 140 patients with cSSSI were treated (68 omadacycline, 72 linezolid) for an average of 10 days in both treatment groups [30].Clinical success was defined similarly to that in the Phase II study.Clinical response at the TOC visit in the intent-to-treat population was comparable for omadacycline and linezolid (85 vs 89%), and again both drugs were effective in the patients known to be infected with MRSA.

Clinical safety & tolerability
In Phase I studies, single doses of omadacycline have been administered across a wide dose range (25-600 mg iv., and 50-600 mg oral).Multiple doses of up 200 mg iv.once daily (7 days) and 300 mg oral once daily (10 days) also have been investigated.Both the intravenous and oral formulations have been generally well tolerated in these studies.In the intravenous administration studies, modest and reversible aminotransferase increases were seen most notably with intravenous doses of 300 mg or greater.Following oral administration of omadacycline, mild nausea was observed most commonly at oral doses of 400 mg or greater.However, the different oral formulations evaluated in early studies (e.g., capsule vs tablet) may have influenced the gastrointestinal profile.
In both oral and intravenous Phase I studies, dose-dependent, transient increases in heart rate were observed for several hours following administration of omadacycline.The increases in heart rate were rarely reported as adverse events (palpitations) and were not associated with any ECG changes or other cardiac findings.Receptor binding studies suggest that omadacycline binds to the M2 subtype of the muscarinic receptor of the vagus nerve and this results in a short-lived nonadrenergic, vagolytic effect on heart rate, which is likely to be most notable in healthy volunteer subjects with relatively high vagal tone and lower resting heart rates (see the 'Nonclinical cardiovascular effects' section).Importantly, omadacycline did not increase ECG QTc intervals (see the 'Electrocardiogram QT evaluation' section).
In the Phase II and truncated Phase III studies in cSSSI, the incidence and type of adverse event (AE) was comparable between omadacycline and linezolid (Table 8) [29,30].In the Phase II study, gastrointestinal AEs were most common overall (19% omadacycline, 17% linezolid).Premature discontinuation of treatment due to an AE was very infrequent in both groups (1% omadacycline, 2% linezolid).There was no pattern of adverse changes in laboratory safety parameters among patients treated with omadacycline; linezolid patients showed a modest decrease in platelet count, which is a known potential effect of that drug.Increased serum transaminases were reported as AEs in 3% of omadacycline patients and 7% of linezolid patients.Measurement of vital signs was performed less frequently than in the Phase I studies, but changes from baseline in heart rate and blood pressure at the end of the course of treatment were clinically insignificant and similar between the treatment groups.Three omadacycline patients (3%) had AEs of tachycardia and one other patient reported palpitations; all of these AEs were mild in intensity, were assessed as unrelated to study drug, and none resulted in discontinuation.
Overall, the target therapeutic doses of omadacycline were very well tolerated in both oral and intravenous formulations.There were no serious AEs that were related to study drug in any of the completed clinical studies.Across both of the studies in cSSSI patients, nausea was the most common AE; all such events were of mild or moderate intensity and did not lead to treatment discontinuation in any of the completed studies.In contrast, dose-limiting nausea and vomiting occurs with intravenous tigecycline and with both intravenous and oral administration of eravacycline [31][32][33][34][35][36].Because it is well tolerated, especially with regard to GI effects of nausea and vomiting that are common with many antibiotics, omadacycline may be particularly well suited for treatment of community-acquired bacterial infections, whether they are managed in hospital or as outpatients.

Discussion
Omadacycline is being evaluated in two Phase III randomized, double-blind studies in ABSSSI and CABP.The primary objective of these studies is to demonstrate the noninferiority of omadacycline to active comparators.The ABSSSI study is expected enroll approximately 650 patients with skin infections known or suspected to be due to Gram-positive pathogens.This study will evaluate the following two regimens, each of which is to be administered for 7-14 days: • Omadacycline 100 mg iv.every 12 h for two doses and then 100 mg iv.every 24 h through at least Day 3, then an option to switch to 300 mg PO every 24 h; • Linezolid 600 mg iv.every 12 h through at least Day 3, then an option to switch to 600 mg PO every 12 h.

Mechanism of action
• Omadacycline exerts its primary effect by binding to the 30S subunit of the bacterial ribosome and inhibiting protein synthesis.
• Omadacycline is active against bacterial strains expressing efflux and ribosomal protection, which are the two main forms of tetracycline resistance.

Microbiology
• Omadacycline demonstrates antimicrobial activity in vitro against a wide range of Gram-positive and Gram-negative pathogens.
• Omadacycline is active against atypical bacteria including Legionella pneumophila and Chlamydia spp.

Pharmacokinetics
• Following intravenous (iv.) administration, omadacycline exhibits a linear pharmacokinetic profile over the dose range of 25-600 mg.
• The oral tablet formulation is 35% bioavailable; a 300 mg oral dose is bioequivalent to a 100 mg iv.dose.
• Omadacycline has low plasma protein binding (21%) and no active metabolites have been identified; in vitro studies indicate a low potential for drug-drug interactions.
• Omadacycline is eliminated predominantly by fecal elimination of parent drug; approximately 40% of an absorbed dose is excreted in the urine.

Clinical efficacy
• In both a Phase II and a truncated Phase III clinical study in patients with complicated skin and skin structure infections, the efficacy of omadacycline was noninferior to linezolid.

Safety & tolerability
• In the completed studies in patients with complicated skin and skin structure infections, the target therapeutic doses of omadacycline were well tolerated in both oral and intravenous formulations.In these studies the adverse event (AE) profile of omadacycline was comparable to linezolid; the most common AE was nausea, which occurred at similar rates for both omadacycline and linezolid and did not lead to any treatment discontinuations.
• Transient increases in heart rate (due to a vagolytic effect) were observed most notably in healthy volunteers with lower resting heart rates; there were no associated changes in blood pressure or other cardiac findings.Omadacycline does not prolong QTc intervals.
• Reversible increases in liver enzymes have been observed at relatively high doses.

Dosage & administration
• For the indications of treatment of acute bacterial skin and skin structure infections and community-acquired bacterial pneumonia, omadacycline is being evaluated as once-daily doses of 100 mg iv.followed by once daily doses of 300 mg orally.
• Oral omadacycline should be administered in a fasted state.The CABP study is expected to enroll approximately 750 patients with known or suspected bacterial pneumonia classified as Patient Outcomes Research Team (PORT) Risk Class II-IV.This study will evaluate the following two regimens, each of which is to be administered for 7-14 days: • Omadacycline 100 mg iv.every 12 h for two doses and then 100 mg iv.every 24 h through at least Day 3, then an option to switch to 300 mg PO every 24 h; • Moxifloxacin 400 mg iv.every 24 h through at least Day 3, then an option to switch to 400 mg PO every 24 h.
In addition to the Phase III studies described above, additional clinical pharmacology studies of omadacycline are ongoing to quantify lung penetration and urinary excretion, to evaluate the PK of omadacycline in subjects with renal impairment, and to assess the safety profile of multiple doses higher than those being used in the ongoing Phase III studies.These evaluations will inform decisions about potential development of omadacycline for indications beyond ABSSSI and CABP.

Conclusion & future perspective
Omadacycline may represent a novel antibiotic with a broad spectrum of activity against community-associated bacterial pathogens, once daily oral and intravenous dosing, favorably pharmacokinetics, low plasma protein binding, low potential for drug-drug interactions, and early evidence of efficacy and tolerability.Ongoing and future clinical studies with both oral and intravenous formulations will help define the place of omadacycline in the armamentarium for treating common, serious bacterial infectious diseases originating in the community.

Table 7 . Summary of pharmacokinetic parameters for omadacycline after single intravenous and oral doses.
Values are mean ± standard deviation, except for T max , which is median.AUC: Area under the concentration-time curve; CL: Clearance; CV: Coefficient of variation; CL/F: Total clearance after oral administration; iv.: Intravenous.Data taken fromSun et al. (2011) [19].

•
Among adults, no dosage adjustment is required for age, gender, or hepatic impairment.
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