a Department of Internal Medicine, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA b Saint John's Medical College and Hospital, Bangalore c Maulana Azad Medical College, New Delhi d Baroda Hospital, Baroda e Central Railway Hospital, Bombay, India f Bureau of Microbiology, Ottawa, Ontario, Canada g Pfizer Limited, Bombay, India h Central Research Division, Pfizer Inc., Groton, CT, USA
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Abstract |
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Introduction |
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Azithromycin is the first of a new class of broad-spectrum antibiotics called azalides, which contain a nitrogen atom in the macrolide aglycone ring.9,10 Azithromycin has an MIC of 416 mg/L against isolates of S. typhi.11 Rapid movement of azithromycin from blood into tissue results in significantly higher azithromycin concentrations in tissue than in plasma (up to 50100 times the maximum observed concentration in plasma). After oral administration, serum concentrations of azithromycin decline in a polyphasic pattern, resulting in an average terminal half-life of 68 h. The high values for apparent steady-state volume of distribution (31.1 L/kg) and plasma clearance (630 mL/min) suggest that the prolonged half-life is due to extensive uptake and subsequent release of drug from tissues.10,12 The prolonged concentration of azithromycin in cells is advantageous in the treatment of experimental Salmonella spp. infection in mice, which is intracellular,13 and may explain the good results obtained with azithromycin in patients with typhoid fever in Chile14 and Egypt.15
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Materials and methods |
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Eighty patients with blood cultures showing Salmonella spp. were selected for
randomization of approximately 40 patients each to azithromycin and chloramphenicol
treatment
groups. This sample size was large enough, with a power of 80% and with an level of 0.05,
to show equivalence in bacterial eradication from blood cultures at day 8, using the 94%
eradication rate observed in the chloramphenicol group, at a
(lower boundary of the 95%
CI on
the difference in eradication rates) of -14.9.16 Males
or females of at least 18 years of age with
suspected typhoid fever on the basis of histories of fever for 415 days were considered
and examined for the presence of two or more of the following features: oral temperature
>38.5°C, abdominal tenderness, hepatomegaly, splenomegaly, rose spots, coated tongue
with
sparing of margins, toxic physical appearance or relative bradycardia. Written consent was
obtained from each patient before inclusion. Patients were excluded if they were pregnant or
lactating; had allergies to chloramphenicol, erythromycin or other macrolide antibiotics; had
major complications of typhoid fever (pneumonia, gastrointestinal haemorrhage, intestinal
perforation, shock or coma); were unable to swallow medication; had been treated with
antimicrobial drugs within 7 days unless blood cultures remained positive for Salmonella
spp.; or had serious underlying disease affecting bone marrow, kidneys, liver, heart, lung or
nervous system. To eliminate selection bias, patients were screened for entry criteria and asked
for
signed consent without the knowledge by the patient or physician of their assigned treatments.
Randomized treatments were assigned after entry, by the physician's opening a sealed
envelope containing the coded treatment choice that was made from a table of random numbers.
Patients and physicians were unblinded to the treatment after patients were entered.
Clinical and laboratory testing
The admission evaluation included history and physical examination; two blood cultures; a
stool culture; blood for white blood cell count, haemoglobin, platelet count, prothrombin time,
creatinine, blood urea nitrogen, bilirubin, alkaline phosphatase, AST, and ALT and urinalysis.
After entry, symptoms and changes in physical examination were recorded daily during
treatment,
and blood cultures were obtained on days 4, 8 and 14 after start of treatment. Isolates of S.
typhi and Salmonella paratyphi were confirmed by slide agglutination using
specific
antisera (Difco Laboratories, Detroit, MI, USA). Antimicrobial susceptibilities at the
investigative
sites were determined by disc diffusion. Isolates were considered resistant when zone diameters
for azithromycin discs containing 15 µg were 13 mm and for chloramphenicol discs
containing 30 µg were
12 mm. Isolates were sent to a reference laboratory at Texas
Tech University Health Sciences Center for determination of MICs and to a reference laboratory
in Ottawa, Canada, for Vi phage typing.17 MICs of
azithromycin were measured by the tube dilution
method in cation-adjusted MuellerHinton broth (Difco Laboratories).
Treatment and follow-up visits
Azithromycin was administered as 500 mg po (two capsules of 250 mg) od for 7 days. Doses were given 1 h before or 2 h after eating food. Chloramphenicol was administered po as 23 g daily in four divided doses for 14 days, with the dose determined by the patient's weight (with those <60 kg receiving the lower dose). After clinical improvement in the hospital, patients were discharged home to complete their therapies. If discharged before 14 days after start of therapy, they were asked to return daily until 14 days had elapsed. Follow-up visits were scheduled on days 21 and 35 for physical examination, cultures of stool and for the same blood tests that were performed at the time of admission. Blood cultures were repeated on days 21 and 35 if fever (>38°C) had returned at these times. Physicians and other caregivers were unblinded about patients' treatments during follow-up.
Analysis of data
Patients were considered evaluable if their admission blood cultures grew S. typhi or S. paratyphi susceptible to the assigned antibiotic and they took azithromycin for at least 4 days or chloramphenicol for at least 4 days for the day 8 analysis and at least 7 days for the day 14 analysis. Patients with negative cultures or with other infections requiring additional antibiotics were removed from the study. Patients were considered clinically cured if fever disappeared and all signs and symptoms of typhoid fever resolved, and were considered improved if fever disappeared and signs and symptoms of typhoid fever had partially resolved. Clinical failure was defined as lack of improvement or worsening of signs and symptoms or need to change antibiotic therapy. Patients were considered afebrile when the daily maximum temperature was <38°C for 2 days or longer. Bacteriological eradication was defined as negative blood cultures for S. typhi and S. paratyphi on day 8 after start of therapy, with all cultures remaining negative to the final assessment on day 35. Clinical relapse was defined as return of fever after day 14, and bacteriological recurrence was defined as a blood culture positive for S. typhi or S. paratyphi on day 21 or 35 after start of therapy. Prolonged faecal carriage was defined as a stool culture showing S. typhi or S. paratyphi on day 21 or 35 after start of therapy. Clinical and bacteriological response rates were calculated for each treatment group and 95% CI were calculated for the difference in population response rates between the two treatments. Proportions of patients reporting adverse experiences and showing abnormal laboratory values after start of therapy were compared between treatment groups. Equivalence of response rates was defined as a 95% CI of the two response rates that covered zero, with the lower value being greater than -10%. A P value of <0.05 indicated a significant difference between the two groups.
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Results |
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Ninety-two patients with blood cultures showing growth of S. typhi or S.
paratyphiA were enrolled. Isolates were confirmed by biochemical reactions and
agglutination by antisera as S. typhi in 82 cases and S. paratyphiA in 10 cases.
The most predominant Vi phage type of S. typhi was E1 (Table I). Using zone diameters
13 mm for azithromycin, resistance to azithromycin was reported in 11 of the 92 isolates
(12%) by the Indian laboratories. Using zone diameters of
12 mm, resistance to
chloramphenicol was reported also in 11 isolates (12%). Only one isolate of S. typhi
was
resistant to both azithromycin and chloramphenicol. Chloramphenicol resistance occurred only
among S. typhi isolates, whereas azithromycin resistance occurred in both S. typhi and S. paratyphiA (Table I). Fifty-seven isolates that had
been stored and were
viable at the end of the study were sent to Texas for further susceptibility testing. Five of these
isolates had been reported by the Indian laboratories as resistant to azithromycin and seven
resistant to chloramphenicol. All were susceptible to azithromycin by disc diffusion method and
46 (81%) isolates were susceptible to chloramphenicol. Ten of the 11 chloramphenicol-resistant
isolates were also resistant to ampicillin and co-trimoxazole, whereas one isolate was resistant
only to chloramphenicol. All isolates were susceptible to ciprofloxacin.
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Patient enrolment, randomization and treatment
At the four sites in India, 109 patients met inclusion criteria, signed consent and were randomized to therapies. The distribution by site was 50 patients at Bangalore, 29 patients at New Delhi, 21 patients at Baroda and nine patients at Bombay. Fifty-six patients were randomized to receive azithromycin and 53 to receive chloramphenicol. Study drug was actually received initially by 106 patients.
Patients with negative blood cultures were removed from the study and offered other appropriate therapies. Blood cultures positive for S. typhi or S. paratyphi A were present in 92 patients (48 assigned to azithromycin, 44 to chloramphenicol). Four patients in each group were excluded because their isolates showed resistance to the study drug. Seven patients treated with azithromycin had isolates resistant to chloramphenicol and six patients treated with chloramphenicol had isolates resistant to azithromycin. Additionally, one patient assigned to azithromycin never took it, and five patients assigned to chloramphenicol stopped therapy early or never took it because of choices of individual physicians and their patients and not because of adverse events. Finally, one azithromycin patient was excluded from the analysis because of insufficient follow-up data. The remaining 77 patients were clinically and bacteriologically evaluable. Demographic characteristics, clinical findings and laboratory features of patients in the two treatment groups were comparable (Table II).
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In the patients treated with azithromycin, blood cultures on day 4 of treatment showed no growth in 83% of patients, and on days 8 and 14 after start of treatment showed no growth in all patients (Table III). In patients treated with chloramphenicol, blood cultures showed no growth in 94% of patients on days 4 and 8 after start of treatment, and in 97% of patients on day 14. Stool cultures on days 21 and 35 after start of treatment showed no growth of Salmonella spp. in all patients in both treatment groups.
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The 10 patients infected with S. paratyphi A responded in ways comparable with the patients infected with S. typhi. Four of the patients infected with S. paratyphi A were treated with azithromycin, and all showed bacteriological eradication after treatment and all were clinically cured. The six patients treated with chloramphenicol all showed bacteriological eradication and were clinically cured.
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Discussion |
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The two study drugs were very different in regard to their administration, pharmacokinetics, therapeutic principles and side effects. Azithromycin was given once daily in a dose of 500 mg a day for 7 days, whereas chloramphenicol was given four times a day in doses of 23 g a day for 14 days. Both antibiotics penetrate into cells effectively, and this intracellular penetration explains the effective therapeutic activity against the predominantly intracellular pathogen S. typhi. On the other hand, serum concentrations of azithromycin reported to be in the range of 0.040.4 mg/L during treatment12,27 are less than the MIC of azithromycin against S. typhi and are less than the serum concentrations of 5.557 mg/L reported for chloramphenicol during treatment of typhoid fever.4 The ability of azithromycin to achieve intracellular concentrations in monocytes 231 times greater than the serum concentrations and in polymorphonuclear leucocytes 83 times the serum concentrations,28,29 as well as a long half-life of 23 days of the intracellular concentration,10 appears to be essential for its therapeutic activity in typhoid fever.
Adverse events, including gastrointestinal symptoms, were reported by five patients treated with azithromycin in this trial, but these events were not serious and did not require discontinuation of therapy. Although none of the patients treated with chloramphenicol reported adverse events, more patients randomized to chloramphenicol therapy than to azithromycin refused therapy or stopped therapy early (five versus one) and were excluded from analysis. The reasons were that individual patients, after reconsidering their decisions to enter the study, chose not to take chloramphenicol because of concern about possible adverse events. Additionally, in view of the fact that this was an open study, there may have been less reporting of adverse events with chloramphenicol because of investigator bias toward an older, more tried agent. The known haematological side effects of chloramphenicol cause physicians in some countries to select alternative therapies routinely, and baseline leucopenia in one of the patients in this trial was the reason his physician did not initiate treatment with chloramphenicol.
Emergence of antimicrobial drug resistance in S. typhi is a global problem1,2,3 and was the compelling reason for us to undertake this trial of a new antibiotic for typhoid fever. In-vitro resistance was reported by the Indian laboratories to chloramphenicol in 12% of isolates and to azithromycin in 12% of isolates. Chloramphenicol resistance was expected because of the high prevalence of multidrug resistance to ampicillin, chloramphenicol and co-trimoxazole on the Indian subcontinent.2,30,31 Azithromycin resistance was not expected because this drug has not been used extensively yet in India. When 57 of the isolates of S. typhi were tested in Texas for azithromycin MICs, all isolates except one were susceptible, with MICs of 48 mg/L. This discrepancy between results of susceptibility testing by disc zone diameters and MICs suggests that there was less azithromycin resistance in our isolates of S. typhi than was reported initially from results of disc diffusion. A tendency for zones around the azithromycin discs to show a trailing of light growth rather than the sharp demarcation noted in the Texas laboratory is a technical difficulty in obtaining reproducible results by disc diffusion. Our results of azithromycin MICs against 10 isolates of S. paratyphi A showed that seven were relatively resistant, with MICs of 16 mg/L. This is the first report of azithromycin resistance in S. paratyphi A. Although our patients with S. paratyphi A infection all did well when treated with azithromycin, this in-vitro resistance deserves attention in future studies of typhoid fever.
The place of azithromycin in the treatment of typhoid fever needs to be defined by further clinical studies in adults and children. Once-daily oral treatment for 7 days is convenient and should be favourable for out-patient compliance. At the time of this study a parenteral form of azithromycin was not available. Although parenteral azithromycin is now available, it has not yet been tested in typhoid fever. The fluoroquinolones ciprofloxacin and ofloxacin have been tested in adults in geographical areas with multidrug resistance and gave good results.5,18,19 However, the fluoroquinolones are generally not approved for use in children because of the potential for these drugs to damage cartilage in growing bones in animals. Children are affected by typhoid fever more frequently than adults.32 Children with multidrug-resistant typhoid fever have been treated successfully with furazolidone, ceftriaxone, cefixime and aztreonam.33,34,35 The availability of a paediatric suspension of azithromycin provides an opportunity to examine the efficacy and safety of this drug in young children with typhoid fever.
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Acknowledgments |
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Notes |
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References |
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Received 23 November 1998; returned 9 February 1999; revised 3 March 1999; accepted 6 April 1999