In vitro activities of a new ketolide, ABT-773, alone and in combination with amoxicillin, metronidazole, or tetracycline against Helicobacter pylori

Newer, Older, and Alternative Agents for the Eradication of Helicobacter pylori Infection: A Narrative Review

Although discovered 40 years ago, Helicobacter pylori infection is still raising diagnostic and therapeutic problems today. The infection is currently managed based on statements in several guidelines, but implementing them in practice is a long process. Increasing antibiotic resistance and weak compliance of the patients limit the efficacy of eradication regimens, leaving much room for improvement. Third-generation proton pump inhibitors have added little to the results of the first two generations. Potassium-competitive acid blockers have a stronger and longer inhibitory action of acid secretion, increasing the intragastric pH. They obtained superior results in eradication when compared to proton pump inhibitors. Instead of innovative antibiotics, derivatives of existing antimicrobials were developed; some new fluoroquinolones and nitazoxanide seem promising in practice, but they are not recommended by the guidelines. Carbonic anhydrase inhibitors have both anti-secretory and bactericidal effects, and some researchers are expecting their revival in the treatment of infection. Capsules containing components of the eradication regimens have obtained excellent results, but are of limited availability. Probiotics, if containing bacteria with anti-Helicobacter pylori activity, may be useful, increasing the rates of eradication and lowering the prevalence and severity of the side effects.

The ketolides: a critical review

Ketolides are a new class of macrolides designed particularly to combat respiratory tract pathogens that have acquired resistance to macrolides. The ketolides are semi-synthetic derivatives of the 14-membered macrolide erythromycin A, and retain the erythromycin macrolactone ring structure as well as the D-desosamine sugar attached at position 5. The defining characteristic of the ketolides is the removal of the neutral sugar, L-cladinose from the 3 position of the ring and the subsequent oxidation of the 3-hydroxyl to a 3-keto functional group. The ketolides presently under development additionally contain an 11, 12 cyclic carbamate linkage in place of the two hydroxyl groups of erythromycin A and an arylalkyl or an arylallyl chain, imparting in vitro activity equal to or better than the newer macrolides. Telithromycin is the first member of this new class to be approved for clinical use, while ABT-773 is presently in phase III of development. Ketolides have a mechanism of action very similar to erythromycin A from which they have been derived. They potently inhibit protein synthesis by interacting close to the peptidyl transferase site of the bacterial 50S ribosomal subunit. Ketolides bind to ribosomes with higher affinity than macrolides. The ketolides exhibit good activity against Gram-positive aerobes and some Gram-negative aerobes, and have excellent activity against drug-resistant Streptococcus pneumoniae, including macrolide-resistant (mefA and ermB strains of S. pneumoniae). Ketolides such as telithromycin display excellent pharmacokinetics allowing once daily dose administration and extensive tissue distribution relative to serum. Evidence suggests the ketolides are primarily metabolised in the liver and that elimination is by a combination of biliary, hepatic and urinary excretion. Pharmacodynamically, ketolides display an element of concentration dependent killing unlike macrolides which are considered time dependent killers. Clinical trial data are only available for telithromycin and have focused on respiratory infections including community-acquired pneumonia, acute exacerbations of chronic bronchitis, sinusitis and streptococcal pharyngitis. Bacteriological and clinical cure rates have been similar to comparators. Limited data suggest very good eradication of macrolide-resistant and penicillin-resistant S. pneumoniae. As a class, the macrolides are well tolerated and can be used safely. Limited clinical trial data suggest that ketolides have similar safety profiles to the newer macrolides. Telithromycin interacts with the cytochrome P450 enzyme system (specifically CYP 3A4) in a reversible fashion and limited clinically significant drug interactions occur. In summary, clinical trials support the clinical efficacy of the ketolides in upper and lower respiratory tract infections caused by typical and atypical pathogens including strains resistant to penicillins and macrolides. Considerations such as local epidemiology, patterns of resistance and ketolide adverse effects, drug interactions and cost relative to existing agents will define the role of these agents. The addition of the ketolides in the era of antibacterial resistance provides clinicians with more options in the treatment of respiratory infections.

Alternative and rescue treatment regimens for Helicobacter pylori eradication

Eradication therapy has been incorporated into clinical practice. The regimens currently recommended for first-line treatment include a 2-week bismuth-based triple therapy (mainly in developing countries), a 1 - 2 week proton pump inhibitor (PPI)-based triple therapy and a 1-week ranitidine bismuth citrate (RBC)-based triple therapy. However, these regimens fail to eradicate Helicobacter pylori in up to 20% of patients due to poor compliance, inadequate treatment duration, smoking, old age and bacterial resistance to nitroimidazoles and/or macrolides in particular. Therefore, alternative regimens that avoid nitroimidazoles and/or macrolides or overcome bacterial resistance to these drugs, improve compliance, minimise side effects and/or reduce costs have been evaluated. One-week quadruple therapy, which adds a PPI or histamine receptor 2-blocker to bismuth-based triple therapy, usually achieves an eradication rate of 90% when used as an alternative first-line therapy but the efficacy decreases when used as a rescue therapy. Several new triple therapies that may be used as alternative and/or rescue therapies have been evaluated. Among these are furazolidone-based (furazolidone plus an antibiotic and a bismuth salt, a PPI or RBC), fluoroquinolone-based (levofloxacin or moxifloxacin plus an antibiotic and a PPI) and ecabet sodium-based (ecabet plus two antibiotics) triple therapies. Recently, rifabutin has been used in combination with a PPI and amoxycillin as a rescue therapy, with satisfactory eradication rates. In addition, a number of new antimicrobial agents are currently under investigation in in vitro studies but the clinical values of these agents needs to be confirmed.

Review article: alternative antibacterial agents for Helicobacter pylori eradication

Standard eradication therapies against Helicobacter pylori appear to be effective in most cases, but in clinical practice a failure rate higher than the 5-10% reported in clinical trials is often observed. Among the various reasons responsible for therapeutic failure, antibiotic resistance is becoming a major issue in some countries. A range of different antibacterial agents is currently under investigation: several macrolides, new fluoroquinolones, furazolidone and rifabutin. Although not formally tested in refractory cases, azithromycin, spiromycin, levofloxacin and furazolidone represent the most promising antibacterial agents for possible inclusion in eradication regimens. Rifabutin has been evaluated in H. pylori infections resistant to standard therapies. Although very effective, the drug is expensive and its use should be restricted to the most difficult cases to avoid the development of rifabutin resistance in Mycobacterium spp.

Comparative antimicrobial activity of ABT-773, a novel ketolide, tested against drug-resistant Gram-positive cocci and Haemophilus influenzae

The antimicrobial activity of ABT-773, a novel ketolide, was tested against 618 Gram-positive strains collected from various surveillance programmes between 1997 and 2000. ABT-773 has potent activity against Streptococcus pneumoniae (MIC90, < or = 0.03-0.12 mg/l), beta-haemolytic streptococci (MIC90, < or = 0.03 mg/l) and viridans group streptococci (MIC90, < or = 0.03 mg/l), including erythromycin-resistant strains. In contrast, ABT-773 was less active against erythromycin-resistant Staphylococcus aureus (31% susceptible at < or = 0.25 mg/l), coagulase-negative staphylococci (41% susceptible) and enterococci (30% susceptible). Haemophilus influenzae (MIC90, 4 mg/l) was less inhibited by the two ketolides tested, and ABT-773 was generally two- to fourfold more potent than telithromycin. The ketolides appear to have potential clinical use against some Gram-positive species resistant to macrolides.

ABT-773: a new ketolide antibiotic

ABT-773 is a new semisynthetic derivative of erythromycin A, the ketolide class of broad spectrum antibacterial agents, in Phase II development by Abbott. With good broad spectrum activity against Gram-positive, some Gram-negative organisms and intracellular bacteria, ABT-773 is being developed as a respiratory agent. Structural changes in the ketolide class agents such as ABT-773 provides expanded activity in vitro against macrolide-resistant strains of Streptococcus pneumoniae and improved activity against MLS(B) (macrolide-lincosamide-streptogramin) constitutive expressing streptococci.