Antibiotic resistance is now often understood at the VIEWPOINTS IN DIGESTIVE DISEASES

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1 GASTROENTEROLOGY 1998;115: VIEWPOINTS IN DIGESTIVE DISEASES Antibiotic Resistance in Helicobacter pylori: Implications for Therapy DAVID Y. GRAHAM Department of Medicine, Veterans Affairs Medical Center; and Division of Molecular Virology, Baylor College of Medicine, Houston, Texas Helicobacter pylori is a conventional gram-negative bacteria that causes an infection of the gastric mucosa. The organism is sensitive to most common antibiotics and theoretically should be easy to eradicate, provided that the patient takes antibiotics to which the organism is sensitive using a sufficient dose and duration of therapy. Unfortunately, the infection has proved difficult to cure. There are many other examples of bacterial infections that resist treatment with antibiotics even when the individual organisms demonstrate antibiotic sensitivity in vitro (e.g., Salmonella in the biliary tract). Failure can be due to the presence of antibiotic-resistant organisms, features of the infection may make it inaccessible to the antibiotics despite having antibiotic-sensitive organisms (a resistant infection), or both. Acquired resistance of H. pylori to clarithromycin, metronidazole, tetracycline, and amoxicillin have all been described. Antibiotic resistance is now often understood at the molecular level. For example, macrolide antibiotics such as clarithromycin bind to bacterial ribosomes and prevent protein synthesis. The substitution in one amino acid at, or near, the binding site on the ribosome can prevent the macrolide from binding, rendering it ineffective. 1 Different mutations result in differences in binding, and this is reflected in the change in minimal inhibitory concentrations (MICs). 2 Amoxicillin resistance appears related to absence of a penicillin-binding protein. 3 The mechanism of metronidazole resistance remains unclear. Whatever the mechanism as a general rule, the outcome of therapy is adversely influenced when a Helicobacter pylori infection is treated with antibiotics to which it is resistant. Antibiotic resistance in H. pylori is a growing problem. In the United States, the frequency of resistance to metronidazole ranges from about 20% to more than 50%. On average, it is about 25% but is higher in women and immigrants from semitropical countries where metronidazole is widely used. Clarithromycin resistance has steadily increased and, at present, is in the range of 7% 14%. In the U.S. trials, pretreatment amoxicillin MICs of 0.25 µg/ml were found in approximately 3% of the patients (Dr. Linda Utrup, personal communication, June, 1998). The data regarding tetracycline resistance are too meager to provide a reliable estimate, but it is probably low. Resistance as a Statistical Phenomenon Acquired antibiotic resistance implies that the organism was originally sensitive but became resistant after exposure to an antibiotic. In reality, acquired resistance may actually represent emergence of resistant organisms that were already present in the stomach rather than actual development of resistance. Acquired antibiotic resistance can be thought of as a statistical phenomenon. Bacteria multiply rapidly, and the chance of developing resistance to an antibiotic is dependent on the frequency of the natural mutation that confers resistance as well as on whether the mutation resulting in the resistant phenotype enhances, impairs, or is neutral with regard to the ability of the resistant organism to compete with antibiotic-sensitive organisms. Consider an example in which the frequency of a mutation resulting in antibiotic resistance occurred in 1 in every 10 million bacteria. If the total number of bacteria in the stomach were , 500 resistant organisms would be present even if the patient had not received the antibiotic. The proportion of H. pylori exhibiting the resistant phenotype would also be so low ( 0.001%) that culture and susceptibility testing of a gastric mucosal biopsy would not identify that any resistant organisms were present. Failure of antibiotic therapy would be attributed to the organism having acquired antibiotic resistance. Importantly, if all 500 resistant organisms succumbed to the other antibiotics in the combination therapy, treatment Abbreviations used in this paper: MIC, minimal inhibitory concentration; PPI, proton pump inhibitor by the American Gastroenterological Association /98/$3.00

2 November 1998 ANTIBIOTIC RESISTANT H. PYLORI 1273 would be a success. The importance of this phenomenon is discussed below. Pharmacological Resistance Pharmacological resistance describes a situation in which therapy fails because the antibiotics do not achieve the appropriate concentration or duration at the site(s) where the antibiotic-sensitive bacterium is residing and multiplying. H. pylori has many niches; it resides in the mucus that lines the gastric mucosa, it is found attached to the surface cells, and it can be seen within surface epithelial cells. 4 H. pylori also inhabits extragastric sites such as gastric metaplasia in the duodenal bulb. H. pylori organisms within the stomach have a potentially formidable survival advantage because they live in an environment where the diffusion of antibiotics is limited and where the ph is lower than required for the antibiotics to be effective. H. pylori are also relatively slow growers and, because most antibiotics are effective only with actively multiplying organisms, slow growth may provide a survival advantage. The topical effect of antibiotics is also limited by the short time they are in the stomach. The Stomach as a Difficult Environment for Successful Antimicrobial Therapy The gastric environment is very hostile for effective use of antimicrobial agents. Attempts to treat infections in the stomach have introduced parameters not usually considered when planning antimicrobial therapy for infections of other tissues. Important factors related to the physical environment of the stomach include low, but inconstant ph, active secretion, a thick mucous layer, constant emptying of contents, and regular shedding of mucous and surface mucosal cells. Many of the problems encountered are akin to those encountered in treating other mucosal sites such as gingival infection. The external location of the bacteria provides the physician with a theoretical advantage because topical therapy will provide a high concentration of the antimicrobial agents directly at the site of the infection. 5 Three important considerations in the stomach that can be influenced are gastric emptying, acidity, and distribution of the drug. Administration of a drug to the fasting patient can result in the majority of the agent passing into the small intestine before the formulation can disperse or possibly even dissolve. The presence of food delays gastric emptying, promotes dispersion of the drug, and buffers gastric acid. 5,6 One potentially negative aspect of administration of antibiotics with meals is that the drugs may interact with or bind to components of the meal and become less available either locally or for absorption. Food in the stomach also has the potentially beneficial effect of causing desquamation of surface cells and discharge of mucus, which may assist in shedding the organisms or may expose them to high intraluminal concentrations of the antibiotic. 7,8 The potential advantage of using the fed vs. the fasted state in the treatment of H. pylori infection remains largely unstudied. Head-to-head comparisons are needed to ascertain whether taking full advantage of this natural parameter has a significant effect on the results of antimicrobial therapy. Nevertheless, multidrug therapies that include topical therapy with bismuth or furazolidone have proven very useful. Topical therapy alone is ineffective probably because it does not have access to all the niches that H. pylori occupies. The relative importance of penetration of the mucosa from the gastric side vs. the blood stream is also unknown. The ideal therapy would be one that killed H. pylori and nothing else. It should probably be active only in the stomach and duodenal bulb. Results with topical therapies suggest that this ideal may not be effective in curing the infection. Adjuvants to Antibiotic Therapy Antimicrobial action could possibly be improved by cotherapies with mucolytics or antisecretory drugs. Although there are few data about mucolytics, there are a number of studies on antimicrobial agents and antisecretory agents. The potential advantages of coadministration of antisecretory drugs include a decrease in intragastric volume, which increases the concentration of the antibiotic, and an increase in intragastric ph, which may lower the MIC and possibly increase effectiveness. In addition, there is direct antimicrobial effect of proton pump inhibitors (PPIs). Low intragastric ph is a major barrier to effective antimicrobial therapy with many drugs. 6,9,10 Adding an antisecretory drug such as a PPI to an anti H. pylori regimen to reliably increase the ph in the stomach was therefore an attractive idea. Antibiotics vary in their need for ph control. The activity of a number of antimicrobials has been tested over the range of ph 5 to 8 11 ; ampicillin was 10-fold more active at neutral ph than at the more acidic phs. In contrast, there was only a slight improvement (from 0.5 mg/l at ph 5.5 to 0.12 mg/l at ph 7.5) in the activity of tetracycline. Bismuth and metronidazole were unaffected by the change in ph from 5.5 to ,12 Such studies allow the physician to tentatively divide antimicrobials into those that are ph sensitive and those that are not. The classification is tentative because the gastric ph is often far below 5 and it is difficult, if not impossible, to evaluate H. pylori sensitivity in vitro at these low phs.

3 1274 DAVID Y. GRAHAM GASTROENTEROLOGY Vol. 115, No. 5 The final arbitrator about whether ph control is needed must come from clinical trials. It has become recognized that after (and probably during) treatment, the nocturnal ph is not as effectively controlled with PPIs as in infected individuals. H 2 -receptor antagonists remain effective in the cured or partially treated individual, suggesting that the combination of twice-a-day PPI and a nocturnal H 2 -receptor antagonist will provide better 24-hour ph control. Clinical trials to test whether this theoretical advantage is associated with enhanced clinical effectiveness are not available. Methods to Optimize Antibiotic Therapy Factors that deserve systematic investigation to optimize therapy are shown in Table 1. Formulation may be extremely important because capsules tend to be trapped within folds and to release the drug locally. Drugs administered as tablets generally show good intragastric dissolution and distribution. 13 Liquids may provide the best intragastric distribution but also may be rapidly emptied from the stomach. Liquids also provide no depot effect compared with the contents of capsules or tablets, which are deposited in gastric mucus. 13 Nevertheless, bismuth compounds may precipitate or be bound to mucus and provide prolonged activity despite being administered in liquid form. Solubility of the drug may be important as the solubility of many compounds (e.g., erythromycin) is markedly reduced in an acidic environment. Few studies are available; changing the formulation of amoxicillin from capsules to liquids did not either improve or reduce effectiveness. 14 Tetracycline is a relatively acid-stable antimicrobial whose concentration in the gastric mucosa exceeds the MIC 90 for H. pylori for several hours. 12,15 It is not surprising that the addition of a PPI to tetracycline or to the combination of tetracycline and bismuth did not Table 1. Factors Relating to Resistance and Outcome of Therapy Presence of resistant bacteria Innate Acquired Presence of resistant infection Biofilm phenomenon Inoculum effect (large number of bacteria present) Gastric environments and niches Factors influencing effectiveness of antimicrobial therapy Drug formulation (tablet, liquid, colloid, granule, etc.) Administration in relation to meals Frequency, dosage, duration of drug administration Bismuth salt (e.g., citrate, nitrate, salicylate) Cotherapy to raise ph (PPI vs. H 2 -receptor antagonist) Administration of mucolytics result in an improved cure rate. 16 Because both bismuth and tetracycline reach effective concentrations in the gastric mucosa, the failure to achieve a benefit from the addition of a PPI suggests that the major effect of the PPI may be more related to the improvement in ph than to the inherent antimicrobial activity. Head-to-head comparisons are needed to address whether the antimicrobial effect of PPIs plays an important role in in vivo effectiveness. 17 A number of studies now suggest that H 2 -receptor antagonists may give equivalent results to PPIs with triple therapy Nevertheless, the addition of a PPI to the ph-insensitive combination of tetracycline, bismuth, and metronidazole has been suggested to improve efficacy with metronidazole-resistant strains. 26,27 Whether this is a synergistic effect of the PPI or an effect on ph remains to be determined. Biofilm and Inoculum Effects Successful treatment must overcome additional barriers such as the inoculum effect and the biofilm phenomenon. Attachment to a surface may be associated with an increase in the MIC of antibiotics (the biofilm phenomenon). This phenomenon has been shown with H. pylori in tissue culture 28 and is also likely to be present in vivo. Prevention of adhesion is another worthy goal either alone or to enhance antimicrobial therapy, but because H. pylori possess multiple different adhesins, successful in vivo application of this concept may be difficult, if not impossible. The inoculum effect describes a phenomenon in which an antibiotic loses its effectiveness when the inoculum of bacteria is large. There is frequently an extremely large number of H. pylori in the stomach. Several analogies apply. For example, it requires at least 10 5 bacteria per milliliter to see bacteria in an unspun urine; silver-stained gastric mucosal biopsy specimens typically show hundreds of bacteria per section. H. pylori are distributed relatively uniformly over the surface of the stomach. 32 The mucosal surface area of the stomach is very large such that there are a tremendous number of bacteria present (in the range of 10 7 to ), which increases the likelihood that a few naturally resistant mutants are always present. One approach to overcome this problem would be to use antibiotics associated with an extremely low rate of spontaneous mutation to a resistant phenotype. Another would be to rapidly reduce the total number of bacteria with a drug with low or absent resistance potential (e.g., bismuth). The odds of success can be improved by maneuvers that increase the hospitableness of the gastric contents for antimicrobial action (formulation, antisecretory therapy, etc.).

4 November 1998 ANTIBIOTIC RESISTANT H. PYLORI 1275 Effect of Resistance on Success of Therapy Although the effect of resistance on the effectiveness of therapy should be completely predictable, it is not largely due to the incomplete understanding of the mechanisms involved. The results of combination therapy are generally what would be expected when the other components of a combination therapy were used alone but may also be greater than expected. For example, when the combination of a PPI, clarithromycin, and metronidazole is administered to a patient whose H. pylori strain is resistant to clarithromycin, one would expect the outcome to be identical to treatment with the two effective drugs, the PPI and metronidazole. If this simple approach failed to provide a complete understanding to the problem of variability, one must look elsewhere. With clarithromycin, in vitro resistance generally equates with in vivo resistance. Clinically, clarithromycin resistance presents as a bimodal distribution with isolates being characterized as sensitive or resistance, and clarithromycin resistance can be thought of as a yes or no phenomena with outcome meeting our expectations. Clarithromycin resistance has the effect of the antibiotic being exchanged for a placebo. Metronidazole-resistant H. pylori does not exhibit a bimodal distribution but rather shows a continuous spectrum of MICs. 33 This pattern of resistance suggests that there are many different pathways responsible for resistance and is consistent with the clinical results showing that in vitro resistance does not always predict in vivo effectiveness. Consider the combination of metronidazole, clarithromycin, and a PPI. Clarithromycin resistance leaves the PPI and metronidazole and results in a cure rate of 25%. Metronidazole resistance results in an outcome similar to the local cure rate with a PPI plus clarithromycin (e.g., 70%). This expected result contrasts markedly with the results with the combination of bismuth, tetracycline, metronidazole, plus an antisecretory drug that may or may not show reduced effectiveness in the presence of metronidazole resistance. Many more studies are needed to unravel this conundrum; metaanalyses and other techniques that combine results from different trials have provided little useful information. For example, if bismuth/metronidazole/tetracycline triple therapy in one country resulted in a cure of 90%, the fact that it only cured 30% in another country would not help the clinician understand which group his patients were likely to emulate, nor to understand the mechanism responsible for the widely disparate results. The different mechanisms of metronidazole resistance probably vary in importance and the predominant mechanism may differ not only among patients but also among different geographic areas. We must conclude that, if metronidazole is an effective antibiotic when the bacteria are judged to be resistant, resistance assessed in vitro and assessed in vivo are different. The reason why in vitro assessment of resistance may not equal resistance in vivo may be either simple or complex. For example, the assessment of metronidazole resistance in vitro is based on the concentration of the antibiotic in tissue after absorption and resistance is judged to be present when the MIC is 8 mg/l. The local concentrations of an antibiotic in the stomach may be extremely high; many multiples of the MIC 5,34 and 500 mg of metronidazole given to a fasting individual may achieve levels of 5000 mg/l. Finally, methods to assess metronidazole resistance in vitro are not yet standardized, nor have break points been determined based on clincial trials. Some of the apparent variability with respect to metronidazole probably represents misinformation regarding susceptibility. We still do not know the best duration or dose for any of the popular therapies. Increasing the dosage of metronidazole from 800 mg to 1600 mg in a PPI triple therapy increased the cure rate in both sensitive and in resistant H. pylori, 35 consistent with the results with quadruple therapy (bismuth, metronidazole, tetracycline, and a PPI), which had improved cure rates when the dose of metronidazole was increased to 500 mg three times daily. 26 Why is there such variability in outcome with sensitive and resistant H. pylori? There are a number of reasons why therapies could perform better than anticipated based on resistance patterns. The simplest explanation is that the laboratories ability to identify resistance is flawed. As noted above, the concentration of the drug in the stomach is generally much higher than the MIC at which resistance is determined, so that in vitro testing may incorrectly assign an organism a resistant phenotype. There also may be synergism between the various components that would not be identified by in vitro studies of individual components or different mechanisms because metronidazole resistance may be predominant in different regions. The Future One problem is that gastroenterologists have had to learn to think like infectious disease specialists. We have all been guilty of attempting to define the characteristics of an ideal therapy. Targets have been established for effectiveness, duration, number of tablets, etc. Clinicians accepted armchair targets as a valid method to separate good from unacceptable therapies. The best therapy is

5 1276 DAVID Y. GRAHAM GASTROENTEROLOGY Vol. 115, No. 5 one that cures the infection in all patients, in all geographic regions, irrespective of the requirements regarding dosing interval, number of tablets, or duration. Only after we have identified an excellent therapy and understand its effectiveness in the face of sensitive and resistant H. pylori can we undertake appropriately designed head-to-head comparisons to make the therapy easier, simpler, or less costly. In retrospect, it seems foolish that we continue to, or ever, provide cure rates with different treatment combinations while disregarding the results of antibiotic sensitivity testing. This is akin to trying to understand the use of new penicillin for Streptococcus pneumoniae in a population with variable percentages of penicillin-resistant organisms. We need to know the effectiveness of different combinations separately for sensitive and resistant organisms, and this information should be required of all treatment trials. The rise in antibiotic resistance emphasizes the need for community surveillance of H. pylori sensitivity as in other important infectious diseases. These data will allow physicians to calculate what they should expect and to choose therapy appropriate for their patients. 36 Culture and susceptibility testing of H. pylori is not difficult and can be easily done in any hospital laboratory. The major difficulty and expense is in obtaining the specimens for culture. Less invasive and less expensive methods to reliably obtain specimens for culture are needed if culture and sensitivity testing is to become widely available. Currently, such testing is only practical and cost-effective for treatment failures. If antibiotic resistance continues to increase, pretherapy antibiotic sensitivity testing may become necessary in many regions. References 1. Versalovic J, Shortridge D, Kibler K, Griffy MV, Beyer J, Flamm RK, Tanaka SK, Graham DY, Go MF. Mutations in 23S rrna are associated with clarithromycin resistance in Helicobacter pylori. Antimicrob Agents Chemother 1996;40: Versalovic J, Osato MS, Spakovsky K, Dore MP, Reddy R, Stone GG, Shortridge D, Flamm RK, Tanaka SK, Graham DY. Point mutations in the 23S rrna gene of Helicobacter pylori associated with different levels of clarithromycin resistance. J Antimicrob Chemother 1997;40: Dore MP, Graham DY, Sepulveda A. PBP-D, a novel penicillinbinding protein is involved in amoxicillin resistance in Helicobacter pylori (abstr). Gastroenterology 1998;114:A Engstrand L, Graham DY, Schelnius A, Genta RM, El-Zaatari FAK. Is the sanctuary where Helicobacter pylori avoids antibacterial treatment intracellular? Am J Clin Pathol 1997;108: Graham DY, Evans DG. Prevention of diarrhea caused by enterotoxigenic Escherichia coli: lessons learned with volunteers. Rev Infect Dis 1990;12(suppl 1):S68 S Graham DY, Börsch GM. The who s and when s of therapy for Helicobacter pylori. Am J Gastroenterol 1990;85: Grant R, Grossman MI, Ivy AC. Histological changes in the gastric mucosa during digestion and their relationship to mucosal growth. Gastroenterology 1953;25: Willems G. Trophicity of gastric epithelium and its regulation. In: Mignon M, Galmiche J-P, eds. Safe and effective control of acid secretion. Paris: John Libbey Eurotext, 1988: Börsch GM, Graham DY. Helicobacter pylori. Pharmacology of peptic ulcer disease. In: Collen MJ, Benjamin SB, eds. Handbook of experimental pharmacology. Volume 99. Berlin: Springer- Verlag, 1991: Hunt RH. Hp and ph: implications for the eradication of Helicobacter pylori. Scand J Gastroenterol Suppl 1993;196: Grayson ML, Eliopoulos GM, Ferraro MJ, Moellering RC Jr. Effect of varying ph on the susceptibility of Campylobacter pylori to antimicrobial agents. Eur J Clin Microbiol Infect Dis 1989;8: Goodwin CS, McNulty CAM. Bacteriological and pharmaceutical basis for the treatment of Helicobacter pylori infection. In: Rathbone B, Vealtley V, eds. Helicobacter pylori and gastrointestinal disease. 2nd ed. London: Blackwell Scientific, 1992: Graham DY, Smith JL, Bouvet AA. What happens to tablets and capsules in the stomach: endoscopic comparison of disintegration and dispersion characteristics of two microencapsulated potassium formulations. J Pharm Sci 1990;79: Graham KS, Malaty H, el-zimaity HM, Genta RM, Cole RA, Al-Assi MT, Yousfi MM, Neil GA, Graham DY. Variability with omeprazoleamoxicillin combinations for treatment of Helicobacter pylori infection. Am J Gastroenterol 1995;90: Marshall BJ. Treatment strategies for Helicobacter pylori infection. Gastroenterol Clin North Am 1993;22: Al-Assi MT, Genta RM, Graham DY. Short report: omeprazoletetracycline combinations are inadequate as therapy for Helicobacter pylor infection. Aliment Pharmacol Ther 1994;8: Graham DY, Genta R, Evans DG, Reddy R, Clarridge JE, Olson CA, Edmonds AL, Siepman N. Helicobacter pylori does not migrate from the antrum to the corpus in response to omeprazole. Am J Gastroenterol 1996;91: Gotz JM, Veenendaal RA, Veselic M, Bernards S, Lamers CB. Triple therapy with ranitidine, clarithromycin, and metronidazole in the treatment of Helicobacter pylori. Scand J Gastroenterol Suppl 1995;212: Yousfi MM, el-zimaity HM, Cole RA, Genta RM, Graham DY. Metronidazole, ranitidine and clarithromycin combination for treatment of Helicobacter pylori infection (modified Bazzoli s triple therapy). Aliment Pharmacol Ther 1996;10: Miederer S. Triple ranitidine therapy in Helicobacter pylori infection: recommendation for effective, safe and cost effective combination for Helicobacter pylori eradication. Fortschr Med 1996;114: Kihira K, Satoh K, Saifuku K, Taniguchi Y, Takimoto T, Yamamoto H, Ido K, Yoshida Y, Kimura K. Comparison of ranitidine and lansoprazole in short-term low-dose triple therapy for Helicobacter pylor infection. Aliment Pharmacol Ther 1997;11: Lazzaroni M, Bargiggia S, Porro GB. Triple therapy with ranitidine or lansoprazole in the treatment of Helicobacter pylori associated duodenal ulcer. Am J Gastroenterol 1997;92: Hentschel E, Brandstatter G, Dragosics B, Hirschl AM, Nemec H, Schutze K, Taufer M, Wurzer H. Effect of ranitidine and amoxicillin plus metronidazole on the eradication of Helicobacter pylori and the recurrence of duodenal ulcer. N Engl J Med 1993;328: Adamek RJ, Opferkuch W, Wegener M. Modified short-term triple therapy ranitidine, clarithromycin, and metronidazole for cure of Helicobacter pylori infection (letter). Am J Gastroenterol 1995; 90: Tham TC, Collins JS, Molloy C, Sloan JM, Bamford KB, Watson RG. Randomised controlled trial of ranitidine versus omeprazole

6 November 1998 ANTIBIOTIC RESISTANT H. PYLORI 1277 in combination with antibiotics for eradication of Helicobacter pylori. Ulster Med J 1996;65: de Boer WA. How to achieve a near 100% cure rate for H. pylori infection in peptic ulcer patients: a personal viewpoint. J Clin Gastroenterol 1996;22: van der Hulst RWM, Keller JJ, Rauws EA, Tytgat GNJ. Treatment of Helicobacter pylori infection in humans: a review of the world literature. Helicobacter 1996;1: Megraud F, Trimoulet P, Lamouliatte H, Boyanova L. Bactericidal effect of amoxicillin on Helicobacter pylori in an in vitro model using epithelial cells. Antimicrob Agents Chemother 1991;35: Evans DG, Evans DJ, Jr. Adhesion properties of Helicobacter pylori. Methods Enzymol 1995;253: Boren T, Normark S, Falk P. Helicobacter pylori: molecular basis for host recognition and bacterial adherence. Trends Microbiol 1994;2: Simon PM, Goode PL, Mobasseri A, Zopf D. Inhibition of Helicobacter pylori binding to gastrointestinal epithelial cells by sialic acid containing oligosaccharides. Infect Immun 1997;65: Genta RM, Huberman RM, Graham DY. The gastric cardia in Helicobacter pylor infection. Hum Pathol 1994;25: Midolo PD, Turnidge J, Lambert JR, Bell JM. Validation of a modified Kirby Bauer disk diffusion method for metronidazole susceptibility testing of Helicobacter pylori. Diagn Microbiol Infect Dis 1995;21: Graham DY, Lew GM, Evans DG, Evans DJ, Jr., Klein PD. Effect of triple therapy (antibiotics plus bismuth) on duodenal ulcer healing: a randomized controlled trial. Ann Intern Med 1991;115: Bardhan KD, Bayerdörffer E, Delchier J, Hellblom M, Megraud F, Stubberod A, Veldhuyzen van Zanten SJ, Lind T, Burman C, Gromark P. H. pylori (Hp) eradication with omeprazole (O), metronidazole (M) and amoxicillin (A): the impact of drug dosing & resistance on efficacy the Homer story (abstr). Gastroenterology 1998;114:A Graham DY, de Boer WA, Tytgat GN. Choosing the best anti Helicobacter pylori therapy: effect of antimicrobial resistance. Am J Gastroenterol 1996;91: Received December 24, Accepted March 31, Address requests for reprints to: David Y. Graham, M.D., Veterans Affairs Medical Center (111D), 2002 Holcombe Boulevard, Houston, Texas dgraham@bcm.tmc.edu; fax: (713) Supported by the Department of Veterans Affairs and by the generous support of Hilda Schwartz.