OPC-67683, a nitro-dihydro-imidazooxazole derivative with promising action against tuberculosis in vitro and in mice

Abstract

Background: Tuberculosis (TB) is still a leading cause of death worldwide. Almost a third of the world's population is infected with TB bacilli, and each year approximately 8 million people develop active TB and 2 million die as a result. Today's TB treatment, which dates back to the 1970s, is long and burdensome, requiring at least 6 mo of multidrug chemotherapy. The situation is further compounded by the emergence of multidrug-resistant TB (MDR-TB) and by the infection's lethal synergy with HIV/AIDS. Global health and philanthropic organizations are now pleading for new drug interventions that can address these unmet needs in TB treatment.

Methods and findings: Here we report OPC-67683, a nitro-dihydro-imidazooxazole derivative that was screened to help combat the unmet needs in TB treatment. The compound is a mycolic acid biosynthesis inhibitor found to be free of mutagenicity and to possess highly potent activity against TB, including MDR-TB, as shown by its exceptionally low minimum inhibitory concentration (MIC) range of 0.006-0.024 microg/ml in vitro and highly effective therapeutic activity at low doses in vivo. Additionally, the results of the post-antibiotic effect of OPC-67683 on intracellular Mycobacterium tuberculosis showed the agent to be highly and dose-dependently active also against intracellular M. tuberculosis H37Rv after a 4-h pulsed exposure, and this activity at a concentration of 0.1 microg/ml was similar to that of the first-line drug rifampicin (RFP) at a concentration of 3 microg/ml. The combination of OPC-67683 with RFP and pyrazinamide (PZA) exhibited a remarkably quicker eradication (by at least 2 mo) of viable TB bacilli in the lung in comparison with the standard regimen consisting of RFP, isoniazid (INH), ethambutol (EB), and PZA. Furthermore, OPC-67683 was not affected by nor did it affect the activity of liver microsome enzymes, suggesting the possibility for OPC-67683 to be used in combination with drugs, including anti-retrovirals, that induce or are metabolized by cytochrome P450 enzymes.

Conclusions: We concluded that based on these properties OPC-67683 has the potential to be used as a TB drug to help combat the unmet needs in TB treatment.

Figure 1. Structure of CGI-17341, PA-824, and OPC-67683

OPC-67683: (R)-2-methyl-6-nitro-2-(4-[4-(4-trifluoromethoxyphenoxy)piperidin-1-yl]phenoxymethyl}-2,3-dihydroimidazo[2,1-b]oxazole.

Figure 2. Analysis of Metabolites Produced after Mixing OPC-67683 and M. bovis BCG

(A) 15 μl of ¹⁴C OPC-67683 (0.5mg/ml: 0.056 μCi/μl) was added to 585 μl of 7H9/TN-ADC broth or bacterial culture and incubated for 48 h. After incubation, a 2-fold volume of acetonitrile was added and mixed well. The lysate was centrifuged for 5 min at 15,000 rpm. The supernatant was analyzed using HPLC with flow scintillation analyzer to determine the metabolite pattern. (B) The identified metabolite (desnitro-imidazooxazole) was synthesized at Otsuka Pharmaceutical, and the fragment pattern of the metabolite by electrospray ionization mass spectroscopy was then compared with that of another compound newly synthesized based on the predicted structure.

Figure 3. Effect of Pulsed Exposures to OPC-67683, RFP, INH, and PA-824 on the Intracellular Growth of M. tuberculosis H37Rv within THP-1 Cells

Infected cells were incubated with the test compound for 4 h, washed, cultured until 68 h at 37 °C, plated on 7H11 agar, and counted for colonies after 16 d of growth at 37 °C. Values represent mean ± S.D (n = 3).

Figure 4. Effects of OPC-67683 in an Experimental Mouse Model of TB

(A) ICR mice were inoculated intravenously with M. tuberculosis Kurono. After 28 d, test compounds were administered orally once daily for 28 d (OPC-67683: 40–0.156 mg/kg, RFP: 20–1.25 mg/kg, INH: 20–1.25 mg/kg, EB: 160–20 mg/kg, SM: 160–20 mg/kg, PZA: 320–40 mg/kg, and PA-824: 40–1.25 mg/kg; 2-fold dilution). Mean value (n = 5) of log₁₀ CFU was plotted. (B) BALB/c standard and nude mice were inoculated intravenously with M. tuberculosis Kurono. From the following day, OPC-67683 was administered orally once daily for 10 d (OPC-67683: 10–0.313 mg/kg, 2-fold dilution). The bar was expressed as mean value and SD (n = 5) of log₁₀ CFU. (C) The doses of conventional drugs used for evaluating regimen are summarized in this table. The doses set up for using the plasma C_max achieved in mice TB model is equivalent to that achieved in humans at the clinical dose. (D) ICR mice were inoculated intratracheally with M. tuberculosis Kurono. After 28 d, mice were treated for 2 mo with a combination of OPC-67683, RFP, and PZA (ORZ), or RFP, INH, EB, and PZA (RHEZ) (intensive treatment), and for an additional 2 mo with OPC-67683 and RFP or 4 mo with RFP and INH (maintenance treatment) (OPC-67683: 2.5 mg/kg, RFP: 5 mg/kg, INH: 10 mg/kg, EB: 100 mg/kg, and PZA: 100 mg/kg). Mean value and SD bar (n = 6) of log₁₀ CFU was plotted. The fraction refers to the number of mice in which at least one colony was detected of the total number of surviving mice.