Preclinical toxicity evaluation of novel antibacterial contezolid acefosamil in rats and dogs.

Contezolid acefosamil (CZA) is an intravenous prodrug of oxazolidinone antibiotic contezolid (CZD). It is being developed to treat infections due to Gram-positive bacteria including multidrug-resistant pathogens, while addressing myelosuppression and neurotoxicity limitations associated with long-term use of this class of antibiotics. In vivo, CZA is rapidly deacylated into its first metabolite MRX-1352, which is then dephosphorylated to release active drug CZD. Four-week repeat-dose toxicity studies of intravenous CZA were conducted in Sprague-Dawley rats (40, 80, and 160/120 mg/kg/dose twice a day [BID]) and beagle dogs (25, 50, and 100/75 mg/kg/dose BID). The high doses administered to both rats and dogs were adjusted due to adverse effects including decreased body weight and food consumption. Additionally, a dose-dependent transient reduction in erythrocyte levels was recorded at the end of dosing phase. Importantly, no myelosuppressive reduction in platelet counts was observed, in contrast to the myelosuppression documented for standard-of-care oxazolidinone linezolid. The no-observed-adverse-effect level (NOAEL) of CZA was 80 and 25 mg/kg/dose BID in rats and dogs, respectively. Separately, 3-month neuropathological evaluation in Long-Evans rats (25, 37.5, and 50 mg/kg/dose, oral CZA, BID) demonstrated no neurotoxicity in the central, peripheral, and optical neurological systems. Toxicokinetic data from these studies revealed that CZD exposures at NOAELs were higher than or comparable with that for the intended clinical dose. These results confirm the favorable safety profile for CZA and support its clinical evaluation for long-term therapy of persistent Gram-positive infections, beyond the application for earlier oxazolidinones.

Pharmacokinetics and Disposition of Contezolid in Humans: Resolution of a Disproportionate Human Metabolite for Clinical Development.

Contezolid (MRX-I), a novel oxazolidinone antibiotic, was recently approved for the treatment of serious Gram-positive infections. The pharmacokinetics and disposition of [14C]contezolid were investigated in a single-dose human mass balance study. Cross-species comparison of plasma exposure for contezolid and metabolites was performed, and the safety of the disproportionate metabolite in human was evaluated with additional nonclinical studies. After an oral administration of 99.1 µCi/602-mg dose of [14C]contezolid, approximately 91.5% of the radioactivity was recovered in 0 to 168 h postdose, mainly in urine followed by that in feces. The principal metabolic pathway of contezolid in human comprised an oxidative ring opening of the 2,3-dihydropyridin-4-one fragment into polar metabolites MRX445-1 and MRX459, with recovery of approximately 48% and 15% of the dose, respectively, in urine and feces. Contezolid, MRX445-1, and MRX459 accounted for 68.0%, 19.5%, and 4.84% of the plasma exposure of the total radioactivity, respectively. Metabolites MRX445-1 and MRX459 were observed in disproportionately larger amounts in human plasma than in samples from rat or dog, the rodent and nonrodent species, respectively, used for the general nonclinical safety assessment of this molecule. This discrepancy was resolved with additional nonclinical studies, wherein the primary metabolite, MRX445-1, was further characterized. The no-observed-adverse-effect level (NOAEL) of MRX445-1 was determined as 360 mg/kg body weight/day in a 14-day repeat-dose test in pregnant and nonpregnant Sprague Dawley rats. Furthermore, MRX445-1 exhibited no antibacterial activity in vitro. Thus, MRX445-1 is not expected to exert clinically relevant pharmacology and toxicity.

Nonclinical Evaluation of Antibacterial Oxazolidinones Contezolid and Contezolid Acefosamil with Low Serotonergic Neurotoxicity.

Linezolid, the principal oxazolidinone antibiotic for therapy of Gram-positive infections, is limited by its myelosuppression and monoamine oxidase (MAO) inhibition, with the latter manifested as serotonergic neurotoxicity. The oral oxazolidinone contezolid and its injectable prodrug contezolid acefosamil are developed to overcome the above limitations. Serotonergic profiles for contezolid in vitro and for orally administered contezolid acefosamil in rodents are reported. Contezolid exhibited 2- and 148-fold reduction over linezolid reversible inhibition of MAO-A and MAO-B human enzyme isoforms. In the mouse head-twitch model, contezolid acefosamil was devoid of neurotoxicity at supratherapeutic oral doses of 40, 80, and 120 mg/kg. In the rat tyramine challenge model, no significant increase in arterial blood pressure was observed for contezolid acefosamil up to 120 mg/kg oral dosing. In these tests, the comparator linezolid has elicited serotonergic responses. Thus, contezolid and contezolid acefosamil exhibited an attenuated propensity to induce MAO-related serotonergic neurotoxicity. The data support a continued clinical evaluation of these agents, with potential to expand oxazolidinone therapies to patient populations on concurrent selective serotonin reuptake inhibitor medications or where MAO inhibitors are contraindicated.

Discovery of Antibacterial Contezolid Acefosamil: Innovative O-Acyl Phosphoramidate Prodrug for IV and Oral Therapies.

New oral antibiotic contezolid (CZD) is effective against Gram-positive infections but unsuitable for intravenous (IV) administration due to its modest solubility. To address the medical need for an IV form of CZD, its isoxazol-3-yl phosphoramidate derivatives have been explored, and contezolid acefosamil (CZA, 8), the first representative of a novel O-acyl phosphoramidate prodrug class, has been identified. CZA exhibits high aqueous solubility (>200 mg/mL) and good hydrolytic stability at media pH suitable for IV administration. CZA rapidly converts into the active drug CZD in vivo. In a pharmacokinetic (PK) rat model, the exposure of active drug CZD after IV administration of the prodrug CZA was similar to or higher than that from the IV administration of CZD. The prodrug CZA is bioequivalent to or better than CZD in several preclinical infection models. CZA is likewise active upon its oral administration. To date, CZA has been evaluated in Phase 1 and Phase 2 clinical trials in the USA. It is advancing into further clinical studies including step-down therapy with in-hospital intravenous CZA administration followed by outpatient oral CZD treatment.

Novel antibacterial azetidine lincosamides.

The synthesis and evaluation of novel azetidine lincosamides 1 are described. Eleven new (3-trans-alkyl)azetidine-2-carboxylic acids were synthesized via alkylation of N-TBS-4-oxo-azetidine-2-carboxylic acid and subsequent elaboration then coupled to 7-chloro-1-methylthio-lincosamine. The resulting lincosamides differ from the drug clindamycin in both the size of the ring and the position/structure of the alkyl side-chain. SAR within the series was explored with attention to alkyl variants in positions 1 and 3 of the azetidine ring.

Conformational constraint in oxazolidinone antibacterials. Synthesis and structure-activity studies of (azabicyclo[3.1.0]hexylphenyl)oxazolidinones.

The oxazolidinones are a new class of synthetic antibacterials effective against a broad range of pathogenic Gram-positive bacteria, including multi-drug-resistant strains. Linezolid is the first drug from this class to reach the market and has become an important new option for the treatment of serious infections, particularly those caused by methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enteroccocus faecium (VRE). In the search for novel oxazolidinones with improved potency and spectrum, we have prepared and evaluated the antibacterial properties of conformationally constrained analogues in which the morpholine ring of linezolid is replaced with various substituted azabicyclo[3.1.0]hexyl ring systems. Several classes of azabicyclic analogues were identified with activity comparable or superior to that of linezolid. These include analogues bearing hydroxyl, amino, amido, or carboxyl groups on the azabicyclic ring. The azabicyclic acid analogue 50 was 4 times more potent than linezolid against key Gram-positive and fastidious Gram-negative pathogens (S. aureus, Streptococcus pneumoniae, and E. faecalis MICs < or = 1 microg/mL; Haemophilus influenzae MIC = 4 microg/mL).

Synthesis of aza-, oxa-, and thiabicyclo[3.1.0]hexane heterocycles from a common synthetic intermediate.

[reaction: see text] An efficient and stereospecific approach to the synthesis of structurally constrained aza-, oxa-, and thiabicyclo[3.1.0]hexane heterocycles has been achieved through application of the intramolecular cyclopropanation reaction of diazoacetates. The various constrained heterocycles (X = N, O, or S) are conveniently prepared from a common diol intermediate accessible from readily available cinnamyl alcohols. Application of the methodology to the synthesis of conformationally constrained oxazolidinone antibacterials is also discussed.

Solid-Phase Synthesis of beta-Sultams.

Solid-phase synthesis of beta-sultams amenable for construction of sulfonyl beta-lactam analogue combinatorial libraries is reported. Imine intermediates generated from polymer-immobilized amino acids and aldehydes are reacted with (chlorosulfonyl)acetates in the presence of pyridine to afford the solid-phase-tethered beta-sultam products. The latter can be released from support by acidic cleavage (TFA) or photocleavage, depending on the nature of the linker employed (acid-labile or photolabile linkers). Immobilized 4-(9-fluorenyl)methoxycarbonyl beta-sultams are further functionalized on supports to afford, upon cleavage, the respective carboxy and amido thiazetidine derivatives. The method can be employed in production of beta-sultam libraries for identification of new antibacterial agents.

New potent antibacterial oxazolidinone (MRX-I) with an improved class safety profile.

Oxazolidinones comprise an important class of antibacterial protein synthesis inhibitors. Myelosuppression and monoamine oxidase inhibition (MAOI) are key independent causes for limiting adverse effects in therapy with the sole approved drug of this class, linezolid. This annotation describes a novel oxazolidinone agent, (S)-5-((isoxazol-3-ylamino)methyl)-3-(2,3,5-trifluoro-4-(4-oxo-3,4-dihydropyridin-1(2H)-yl)phenyl)oxazolidin-2-one (MRX-I), distinguished by its high activity against Gram-positive pathogens coupled with markedly reduced potential for myelosuppression and MAOI. The medical need, medicinal chemistry rationale, preclinical data, and phase I clinical trial summary for this new agent are reviewed herein.

Selection and characterisation of Staphylococcus aureus mutants with reduced susceptibility to the investigational oxazolidinone MRX-I.

MRX-I is a new oxazolidinone antimicrobial under development. In this study, the potential for development of resistance to MRX-I in Staphylococcus aureus was investigated and key mutations were characterised. Determination of spontaneous resistance frequency and the mutant selection window (MSW) were performed with meticillin-susceptible S. aureus (MSSA) ATCC 29213, meticillin-resistant S. aureus (MRSA) ATCC 33591 and two clinical MRSA isolates SA 0016 and SA 0017. Selected resistant mutants were sequenced for 23S rRNA as well as genes encoding the ribosomal proteins L3, L4 and L22. Resistance frequencies for the aforementioned strains were <8.25×10(-12), <6.33×10(-12), <2.96×10(-13) and <4.52×10(-13), respectively, and the MSW of MRX-I was 2-4, 1-4, 1-2 and 1-4 mg/L, respectively. After 30 serial passages, MRX-I minimum inhibitory concentrations (MICs) increased up to 8- to 16-fold both against MSSA and MRSA, whilst linezolid MICs increased 128-fold against MSSA and 16- to 32-fold against MRSA. MRX-I resistance mutations were clustered mainly in 23S rRNA and L3 protein regions. The U2504A transversion in 23S rRNA dominated in MRX-I-resistant mutants. No mutations in L4 and L22 proteins were observed. MRX-I exhibits a low potential to develop resistance in S. aureus, with a reduced resistance propensity compared with linezolid.

A distal methyl substituent attenuates mitochondrial protein synthesis inhibition in oxazolidinone antibacterials.

Oxazolidinone analogs bearing substituted piperidine or azetidine C-rings are described. Analogs with a methyl group at the 3-position of the azetidine ring or the 4-position of the piperidine ring exhibited reduced mitochondrial protein synthesis inhibition while retaining good antibacterial potency.

Recent developments in the identification of novel oxazolidinone antibacterial agents.

The oxazolidinones are a promising new class of synthetic antibacterial agents. Here, we review recent efforts directed at the discovery of new antibacterial compounds of this class. New structures and structure-activity relationships (SAR) are discussed in the context of earlier work in the field. Key issues of potency, spectrum, selectivity, in vivo efficacy, and pharmacokinetic profile of the new analogs are addressed.

Conformational constraint in oxazolidinone antibacterials. Part 2: Synthesis and structure-activity studies of oxa-, aza-, and thiabicyclo[3.1.0]hexylphenyl oxazolidinones.

A new class of oxazolidinone antibacterials incorporating oxygen-, nitrogen-, or sulfur-containing heterobicyclic C-rings is described. The in vitro potency and in vivo efficacy of these conformationally constrained oxazolidinone analogs are discussed.

Synthesis and structure-activity studies of antibacterial oxazolidinones containing dihydrothiopyran or dihydrothiazine C-rings.

A new series of antimicrobial oxazolidinones bearing unsaturated heterocyclic C-rings is described. Dihydrothiopyran derivatives were prepared from the saturated tetrahydrothiopyran sulfoxides via a Pummerer-rearrangement/elimination sequence. Two new synthetic approaches to the dihydrothiazine ring system were explored, the first involving a novel trifluoroacetylative-detrifluoroacetylative Pummerer-type reaction sequence and the second involving direct dehydrogenation of tetrahydrothiopyran S,S-dioxide intermediates. Final analogs such as 4 and 13 represent oxidized congeners of recent pre-clinical and clinical oxazolidinones.

New antibacterial tetrahydro-4(2H)-thiopyran and thiomorpholine S-oxide and S,S-dioxide phenyloxazolidinones.

Combinatorial libraries of N-acylated 5-(S)-aminomethyloxazolidinone derivatives of S-oxide and S,S-dioxide tetrahydro-4(2H)-thiopyranyl and thiomorpholine phenyloxazolidinone series have been synthesized on a solid phase and evaluated for antimicrobial activity. Several novel potent leads have been identified, including orally active oxazolidinones with enhanced activity against respiratory tract infection pathogens Haemophilus influenzae and Moraxella catarrhalis.

Novel oxazolidinone-quinolone hybrid antimicrobials.

Antimicrobial compounds incorporating oxazolidinone and quinolone pharmacophore substructures have been synthesized and evaluated. Representative analogues 2, 5, and 6 display an improved potency versus linezolid against gram-positive and fastidious gram-negative pathogens. The compounds are also active against linezolid- and ciprofloxacin-resistant Staphylococcus aureus and Enterococcus faecium strains. The MOA for these new antimicrobials is consistent with a combination of protein synthesis and gyrase A/topoisomerase IV inhibition, with a structure-dependent degree of the contribution from each inhibitory mechanism.