Pharmacodynamics and pharmacokinetics of SQ109, a new diamine‐based antitubercular drug

SQ109 is a novel [1,2]-diamine-based ethambutol (EMB) analog developed from high-throughput combinatorial screening. The present study aimed at characterizing its pharmacodynamics and pharmacokinetics. The antimicrobial activity of SQ109 was confirmed in vitro (Mycobacterium tuberculosis-infected murine macrophages) and in vivo (M. tuberculosis-infected C57BL/6 mice) and compared to isoniazid (INH) and EMB. SQ109 showed potency and efficacy in inhibiting intracellular M. tuberculosis that was similar to INH, but superior to EMB. In vivo oral administration of SQ109 (0.1–25 mg kg−1 day−1) to the mice for 28 days resulted in dose-dependent reductions of mycobacterial load in both spleen and lung comparable to that of EMB administered at 100 mg kg−1 day−1, but was less potent than INH at 25 mg kg−1 day−1. Monitoring of SQ109 levels in mouse tissues on days 1, 14 and 28 following 28-day oral administration (10 mg kg−1 day−1) revealed that lungs and spleen contained the highest concentration of SQ109, at least 10 times above its MIC. Pharmacokinetic profiles of SQ109 in mice following a single administration showed its Cmax as 1038 (intravenous (i.v.)) and 135 ng ml−1 (p.o.), with an oral Tmax of 0.31 h. The elimination t1/2 of SQ109 was 3.5 (i.v.) and 5.2 h (p.o.). The oral bioavailability was 4%. However, SQ109 displayed a large volume of distribution into various tissues. The highest concentration of SQ109 was present in lung (>MIC), which was at least 120-fold (p.o.) and 180-fold (i.v.) higher than that in plasma. The next ranked tissues were spleen and kidney. SQ109 levels in most tissues after a single administration were significantly higher than that in blood. High tissue concentrations of SQ109 persisted for the observation period (10 h). This study demonstrated that SQ109 displays promising in vitro and in vivo antitubercular activity with favorable targeted tissue distribution properties. Keywords: Antituberculosis, ethambutol, SQ109, pharmacodynamics, pharmacokinetics Introduction Recent years have seen an increased incidence of tuberculosis in both developing and industrialized countries, widespread emergence of drug-resistant tubercular strains and a deadly synergy of tuberculosis with the human immunodeficiency virus (HIV) (Espinal et al., 2001). Increased susceptibility to tuberculosis is associated with early stages of HIV infection, and tuberculosis in turn accelerates the progression of HIV infection to AIDS. The current tuberculosis treatment regimens, although highly effective when administered faithfully for the full dosing period, are far from ideal (Barry III, 1997). Using the optimal combination of available drugs, the duration of treatment required for curing patients cannot be reduced below 6 months. In most low-income countries, an 8-month regimen is used. Furthermore, all four of the most effective oral drugs – isoniazid (INH), rifampin, ethambutol (EMB) and pyrazinamide – must be taken together during the first 2-month treatment (American Thoracic Society Documents, 2003). With an estimated 8.8 million new cases of tuberculosis in 2002, of which 3.9 million were smear-positive (2004 WHO report, www.whoint/tb/publications/global_report/), and an annual mortality of nearly two million (Dye et al., 1999), there is a pressing need for new antitubercular agents acting with greater potency and efficacy than the current existing drugs (O'Brien & Nunn, 2001). With the completion of the genome sequence and progress on proteomics of Mycobacterium tuberculosis (Cole et al., 1998; Mattow et al., 2001), the promise of a new generation of potent drugs to combat the emerging epidemic of the disease is foreseeable. EMB discovered in 1961 is still a first-line drug for treating all forms of tuberculosis (American Thoracic Society Documents, 2003). Due to its modest efficacy against M. tuberculosis and its chemical simplicity, EMB is amenable to optimization by combinatorial chemistry (Barry III et al., 2000). Identification of diamine analogs with enhanced efficacy over EMB is one approach to improving the existing treatment of tuberculosis, potentially making therapeutic implementation easier and of shorter duration. A diverse library of 63,238 EMB analogs with a 1,2-diamine pharmacophore (Figure 1) was synthesized on solid support using a novel acylation-reduction sequence, and screened to determine minimal inhibitory concentration (MIC) using two techniques employing whole M. tuberculosis, including one where the organism was engineered to express luciferase with disruption of the bacterial cell wall, the mechanism of action of the parent compound EMB (Lee et al., 2003). After the lead optimization and screening, three analogs were found to have an enhanced antitubercular activity compared to EMB. Based on the results obtained from both the efficacious comparison and high-throughput pharmacokinetic screening using cassette dosing combined with liquid chromatography tandem mass spectrometry (LC/MS/MS) (Jia et al., 2003a), we selected N-Geranyl-N′-(2-adamantyl)ethane-1,2-diamine (SQ109; MW 330.2; Figure 1) as a lead compound for advanced drug testing. Figure 1 Chemical structures of EMB and SQ109. The purpose of the present study was to determine, by comparison with EMB and INH, the effectiveness of SQ109 monotherapy on M. tuberculosis-infected murine macrophage cells and examine the therapeutic effects of SQ109 on M. tuberculosis-infected mouse model. In addition, pharmacokinetic characteristics of SQ109 were examined in mice, including its distribution into various tissues after a single administration and its disposition into targeted tissues after a 28-day repeated administration. The goal of these studies was to understand the relationship between SQ109 antitubercular efficacy, its achieved concentration in M. tuberculosis-susceptible tissues, and its pharmacokinetic profile as measured by LC/MS/MS.