Antitubercular nitrofuran isoxazolines with improved pharmacokinetic properties

Abstract

A series of tetracyclic nitrofuran isoxazoline anti-tuberculosis agents was designed and synthesized to improve the pharmacokinetic properties of an initial lead compound, which had potent anti-tuberculosis activity but suffered from poor solubility, high protein binding and rapid metabolism. In this study, structural modifications were carried on the outer phenyl and piperidine rings to introduce solubilizing and metabolically blocking functional groups. The compounds generated were evaluated for their in vitro antitubercular activity, bacterial spectrum of activity, solubility, permeability, microsomal stability and protein binding. Pharmacokinetic profiles for the most promising candidates were then determined. Compounds with phenyl morpholine and pyridyl morpholine outer rings were found to be the most potent anti-tuberculosis agents in the series. These compounds retained a narrow antibacterial spectrum of activity, with weak anti-Gram positive and no Gram negative activity, as well as good activity against non-replicating Mycobacterium tuberculosis in a low oxygen model. Overall, the addition of solubilizing and metabolically blocked outer rings did improve solubility and decrease protein binding as designed. However, the metabolic stability for compounds in this series was generally lower than desired. The best three compounds selected for in vivo pharmacokinetic testing all showed high oral bioavailability, with one notable compound showing a significantly longer half-life and good tolerability supporting its further advancement.

Figure 1

A. 1 Initial lead anti-tubercular nitrofuran. In this study targeted changes were made to the outer C and D ring systems to improve ADME properties. B. Comparator oxazolidinone antibacterial agents Linezolid and Sutezolid with similar linear structures.

Scheme 1

Reagents and conditions: a) tributyl (vinyl)tin, PdCl₂(PPh₃)₂, DMF, MW, 160°C, 5 min; b) morpholine, Pd(OAc)₂, 2-(Di-tert-butylphosphino) biphenyl, NaO^tBu, toluene, 80°C, 12 h; c) Et₃N, CHCl₃, RT, 2h

Scheme 2

Reagents and conditions: a) thiomorpholine, Pd(OAc)₂, 2-(Di-tert-butylphosphino) biphenyl, NaO^tBu, toluene, 80 °C, 12 h; b) 5, Et₃N, CHCl₃, RT, 2h; c) NalO₄, CH₃CN, MeOH, H₂O, RT, 12 h

Scheme 3

Reagents and conditions: a) N,N'-dimethyl ethylene diamine, CuI, K₂CO₃, toluene, reflux, 6 h; b) 5, Et₃N, CHCl₃, RT, 2h