論文の公開元へStudies on the antibacterial activity of a novel fluoroquinolone, OPS-2071, against enteropathogenic bacteria
概要
Intestinal infections are diseases caused by the growth of pathogens in the intestines, causing diarrhea and other symptoms and most frequently resulting from consuming contaminated food or water. Although bacteria, viruses, and protozoa are the causative pathogens, this thesis focuses only on bacteria as the target pathogens. Intestinal infections are common in developing countries where inadequate sanitation is widespread. It is one of the most serious diseases, with 1.7 million deaths reported annually worldwide, especially in children and the elderly over 70 years of age 70) . The number of patients who have died from intestinal infections is still too high, even though economic development has significantly reduced it over the past decades by improving access to treatment (Figure 1) 2) .
Drug-resistant bacteria are becoming a therapeutic threat in intestinal infections. Drugresistant bacteria, such as typhoidal/non-typhoidal Salmonella, Shigella spp, Campylobacter spp, Vibrio spp, and diarrheal Escherichia coli, have been reported as a problem in their treatment 40, 62, 70) . The spread of Salmonella with ESBL (extended spectrum beta-lactamase), azithromycinresistant Shigella, and ciprofloxacin resistance among a wide range of enteric infectious bacteria has reduced the effectiveness of antimicrobial agents, resulting in a depletion of therapeutic agents 70) . Antibiotic resistance is associated with the widespread use and misuse of antibiotics in humans and agriculture. In developing countries, in addition to the widespread of infectious diseases, the abuse and misuse of antibiotics through the purchase without prescriptions, and the lack of restrictions on antibiotic use in agriculture further contribute to the emergence of resistant bacteria13, 49) .
Enteric infections are also an important public health problem in developed countries; the Centers for Disease Control and Prevention (CDC) reported on the threat of antibiotic resistance in the United States in 2019, listing 18 antimicrobial-resistant bacteria and fungi including intestinal infectious bacteria, such as Clostridioides difficile, Campylobacter, typhoidal/nontyphoidal Salmonella, and Shigella 9) . Resistance to therapeutic agents against these organisms is rapidly increasing, and if this trend continues, treatment options may be lost for patients who need treatment. For instance, Campylobacter spp, one of the most common intestinal infections in the developed world, is estimated at 1.5 million cases per year in the U.S. and an annual medical cost of $2.7 million. Ciprofloxacin and azithromycin are used for treatment, but their drug resistance is increasing every year. Ciprofloxacin-resistant strains have increased to 28% as of 2017 9) .
C. difficile is also one of the important pathogens that cause infections in the intestinal tract. C. difficile is a Gram-positive anaerobic rod that is resistant to many clinically used antimicrobial agents and is known to cause antimicrobial-associated diarrhea caused by disruption of the intestinal flora due to antimicrobial treatment, which leads to microbial substitution, followed by an increase in C. difficile (Figure 2) 12) . C. difficile is not becoming increasingly resistant to therapeutic agents, but in addition to being inherently resistant to almost all clinically used antimicrobials, it has an estimated 12,800 deaths and 2,230,900 hospitalizations in the U.S. in 2017 and $1 billion in annual health care costs, making it a major problem 9) . The risk factors include antimicrobial therapy, prolonged hospitalization, use of immunosuppressants or proton pump inhibitors, and being over 65 years of age; C. difficile is known as one of the most important nosocomial organisms. Vancomycin, metronidazole, and fidaxomicin are therapeutic drugs for C. difficile Infection (CDI), metronidazole, however, is considered less effective than the other two drugs. The therapeutic challenge of CDI is its frequent recurrence after treatment. Although treatment with antimicrobial agents is reported to be highly effective, exceeding 80%, recurrence occurs in 10-25% of patients after treatment, and further recurrence is reported in up to 65% of patients with recurrent disease 21) . C. difficile is a spore-forming bacterium and is thought to recur when bacteria remaining in the intestine as spores re-grow after treatment. Spores are resistant to heat, acid, and antibiotics. CDI is caused by inoculation with the spores, and the growth of C. difficile is normally inhibited by the intestinal flora. When the flora is disrupted by the administration of antimicrobial agents, C. difficile begins to proliferate, causing damage to the intestinal tract by two toxins called enterotoxin A and cytotoxin B, the pathogenic agents of CDI (Figure 2) 12) . The C. difficile BI/NAP1/027 strain is reported as a hypervirulent strain producing C. difficile transferase toxin (CDT; or binary toxin), with strong sporulation ability and increased production of enterotoxin A and cytotoxin B, which are said to be involved in the severe disease. Epidemics of this highly virulent strain have been reported in North America and are considered a new threat to C. difficile. Fidaxomicin has been available as a therapeutic agent since 2011 and showed lower results than vancomycin for relapse (25% vancomycin, 15% fidaxomicin), but this inhibition was not confirmed for the BI/NAP/027 strain. Another treatment option, Bezlotoxumab (a monoclonal antibody that binds to C. difficile cytotoxin B), was approved by the FDA in 2016. It has shown efficacy in reducing relapse, but its use has been limited by its high cost and potential side effects. Fecal microbiota transplantation (FMT) is also being investigated as a new treatment option for CDI. Since disruption of intestinal flora is responsible for the development of CDI, FMT, which transplants normal intestinal flora, is a promising treatment for CDI 36) . In fact, the therapeutic effect in combination with conventional antimicrobial agents has shown the lowest recurrence compared to any other therapy and has been reported as a promising treatment, but the therapeutic process has not yet been established, and more time is needed before it can be widely offered as a treatment. These facts have increased the need for further therapeutic agents for intestinal infections.
An important aspect in the development of a therapeutic drug is not only its therapeutic efficacy but also considerations regarding the risk of the emergence of bacterial resistance. In the long history of antibiotic use, the emergence of resistant bacteria seems inevitable. However, the emergence of resistant bacteria must be prevented as much as possible by their proper use. Pharmacokinetic/pharmacodynamic (PK/PD) modeling is an important concept for its proper use (Figure 3) 4) . The PD parameter is sometimes the minimum inhibitory concentration (MIC), an indicator of antimicrobial activity, particularly MIC90, which is the MIC value widely evaluated in clinical isolates, or breakpoint, which is considered the clinically treatable MIC. However, these are only concentrations at which antimicrobial activity is observed, and they do not take into account whether they prevent the emergence of resistant bacteria. In contrast, mutant prevention concentration (MPC) is a parameter for inhibiting the emergence of resistant bacteria and is defined as the concentration that prevents the emergence of resistant bacteria. On the other hand, concentrations below the MPC but above the MIC are defined as a mutant selection window (MSW) because they inhibit the growth of susceptible bacteria but not of resistant bacteria, and thus have a high risk of selectively increasing the number of resistant bacteria, and treatment should avoid this MSW as much as possible 8) . Unfortunately, MPC is not currently widely used as an indicator of PD, as standardized test methods such as the Clinical and Laboratory Standards Institute (CLSI) method, like MIC, have not yet been defined and have not been widely evaluated through clinical isolates. The variability among strains has not been adequately studied. Nevertheless, the emergence of resistant bacteria is a major challenge common to the entire world, and this concept will become even more important in the future for the long use of antimicrobial agents.
OPS-2071 is a novel quinolone compound synthesized by Otsuka Pharmaceutical Co., Ltd. that targets intestinal infections (Figure 5) 54) . Quinolone antibiotic is one of the most important classes of antibiotics due to their wide spectrum and potent antimicrobial activity, and their favorable pharmacokinetics 20) . In particular, ciprofloxacin, a second-generation quinolone antibacterial agent, was launched in the late 1980s and has still been widely used to date including for intestinal infections. Despite reports of bacterial resistance, it remains a therapeutically important antimicrobial agent, listed by the WHO as an essential drug and an extremely important antibiotic, and one of the most commonly prescribed drugs in the world 79) . Although ciprofloxacin is also recommended as a treatment for intestinal infections, its longstanding use has led to increasing reports of quinolone-resistant strains, particularly in developing countries.
Quinolones target DNA gyrase and topoisomerase, which are essential for bacteria, and are known to work bactericidal by inhibiting these enzymes. There are two major mechanisms of quinolone resistance: one is direct mutations of the target molecule that reduces the binding affinity of the quinolone antimicrobial agent, and the other is mutations to induce inhibition of the uptake of quinolone antimicrobial agents into the bacteria or an accelerated efflux of the drug out of the bacteria. The combination of these two mechanisms is known to induce highly resistant bacteria, but the resistance mechanisms that induce inhibition of drug uptake or promotion of drug efflux do not induce highly resistant bacteria and do not lead to clinically significant resistance 20, 38) . In contrast, direct mutations to target molecules are known to induce high levels of resistance on their own. The region where resistance mutations are introduced is called the quinolone resistance determination region (QRDR), and similar mutations have been reported in many strains of different species 20) . Evaluation against these resistance mutations is useful in terms of predicting antibacterial activity against resistant strains and differentiating them from existing quinolone antimicrobials.
In my study, the potential use of OPS-2071 for various intestinal infections as a therapeutic drug was evaluated. Various aspects of the drug need to be tested in order to evaluate its potential as a therapeutic agent. In vitro antibacterial activity, in vivo pharmacokinetics, and in vivo therapeutic efficacy are essential for predicting therapeutic efficacy. In addition to estimating therapeutic efficacy, it is also important to consider the risk of the emergence of resistant strains, since the problem is that antimicrobial therapy may become ineffective due to the acquisition of resistance after an initial strong therapeutic effect. In addition, the evaluation of inhibitory activity against the target molecules is useful information for characterizing the drug.
This thesis is composed of two chapters and a conclusion. In CHAPTER I, to evaluate its potential as an antimicrobial agent against C. difficile infection, the in vitro activity, in vivo PK profile, and in vivo efficacy were examined. In addition, the frequency of spontaneous resistance and mutant prevention concentration was evaluated as the evaluation of the risk of the emergence of drug resistance. CHAPTER II, to see if OPS-2071 is effective against a wide range of intestinal infections, we evaluated the in vitro antibacterial activity and mechanism of action of OPS-2071 against a wide range of enteric infection-causing bacteria, excluding C. difficile, as well as the risk of emergence of drug-resistant strains.
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