Advancement of Imidazo[1,2‑<i>a</i>]pyridines with Improved Pharmacokinetics and nM Activity vs. <i>Mycobacterium tuberculosis</i>

A set of 14 imidazo­[1,2-<i>a</i>]­pyridine-3-carboxamides was synthesized and screened against <i>Mycobacterium tuberculosis</i> H₃₇Rv. The minimum inhibitory concentrations of 12 of these agents were ≤1 μM against replicating bacteria and 5 compounds (<b>9</b>, <b>12</b>, <b>16</b>, <b>17</b>, and <b>18</b>) had MIC values ≤0.006 μM. Compounds <b>13</b> and <b>18</b> were screened against a panel of MDR and XDR drug resistant clinical <i>Mtb</i> strains with the potency of <b>18</b> surpassing that of clinical candidate <b>PA-824</b> by nearly 10-fold. The <i>in vivo</i> pharmacokinetics of compounds <b>13</b> and <b>18</b> were evaluated in male mice by oral (PO) and intravenous (IV) routes. These results indicate that readily synthesized imidazo­[1,2-<i>a</i>]­pyridine-3-carboxamides are an exciting new class of potent, selective anti-TB agents that merit additional development opportunities

The representative aminothiazole 20 possesses bactericidal activity.

<p><i>M</i>. <i>tuberculosis</i> was inoculated to a starting of OD₅₉₀ of 0.1 in medium containing compound 20. CFU/mL was enumerated at indicated time points by serial dilution onto solid medium. The limit of detection was 20. Note that the lines for 0.625, 1.25 and 2.5 overlap.</p

Chemical synthesis of C-4 ketone and carboxamide analogs.

<p>Reagents: (i) EDC.HCl, HOBt, NCH₃(OCH₃), DIPEA, CH₂Cl₂, 16 h; (ii) CH₃MgBr, THF, -78°C–r.t., 2 h; (iii) 1-adamantanoyl chloride, Et₃N, THF, r.t., 1 h.</p

Chemical synthesis of analogs comprised of thiazole core replacement.

<p>Reagents:: (i) 4-(t-butyl)PhCOCl, EtOH, 1 h; (ii) EtOH, reflux, 16 h; (iii) H₂SO₄, r.t., 16 h; (iv) EDC, CH₂Cl₂, r.t., 16 h.</p

SAR at C-4 position of thiazole core.

<p>^aMIC (μM) is the minimum concentration required to inhibit the growth of <i>M</i>. <i>tuberculosis</i> in liquid culture. MICs of active compounds are the average of two independent experiments. ^bToxic concentration (TC₅₀, in μM) is the concentration required to inhibit growth of Vero cells by 50%. Selectivity index (SI) is the ratio of TC₅₀ to MIC. Cpd = compound; Rif = rifampicin; ND = not determined.</p

Chemical synthesis of pyrazole-based analogs.

<p>Reagents: (i) KO^<i>t</i>Bu, BuOH, reflux, 3 h; (ii) RCOCl, THF, r.t., 1 h; (iii) RNCO, THF, 1 h.</p

SAR of thiazole core replacements.

<p>^aCompounds were tested for inhibition of <i>M</i>. <i>tuberculosis</i>. Minimum inhibitory concentration (MIC, in μM) is the minimum concentration required to completely inhibit the growth of <i>M</i>. <i>tuberculosis</i> in liquid culture. MICs of active compounds are the average of two independent experiments. ^bToxic concentration (TC₅₀, in μM) is the concentration required to inhibit growth of Vero cells by 50%. Selectivity index (SI) is the ratio of TC50 to MIC. Cpd = compound; Rif = rifampicin; ND = not determined.</p

Chemical synthesis of pyrimidine-based analogs.

<p>Reagents: (i) 2,6-dimethylaniline, THF, reflux, 16 h; (ii) 2-(tributylstannyl)pyridine, Pd(PPh₃), DMF, 100°C, 16 h.</p

Spectrum of activity against bacterial species.

<p>The MIC against four bacterial species was determined on solid medium using the agar serial proportion method. MIC₉₉ is the minimum concentration required to prevent growth 99% of bacteria.</p

Identification of Phenoxyalkylbenzimidazoles with Antitubercular Activity

We conducted an evaluation of the phenoxyalkylbenzimidazole series based on the exemplar 2-ethyl-1-(3-phenoxypropyl)-1<i>H</i>-benzo­[<i>d</i>]­imidazole for its antitubercular activity. Four segments of the molecule were examined systematically to define a structure–activity relationship with respect to biological activity. Compounds had submicromolar activity against Mycobacterium tuberculosis; the most potent compound had a minimum inhibitory concentration (MIC) of 52 nM and was not cytotoxic against eukaryotic cells (selectivity index = 523). Compounds were selective for <i>M. tuberculosis</i> over other bacterial species, including the closely related Mycobacterium smegmatis. Compounds had a bacteriostatic effect against aerobically grown, replicating <i>M. tuberculosis</i>, but were bactericidal against nonreplicating bacteria. Representative compounds had moderate to high permeability in MDCK cells, but were rapidly metabolized in rodents and human liver microsomes, suggesting the possibility of rapid in vivo hepatic clearance mediated by oxidative metabolism. These results indicate that the readily synthesized phenoxyalkylbenzimidazoles are a promising class of potent and selective antitubercular agents, if the metabolic liability can be solved