Comparative Study of the Effects of Pyridoxine, Rifampin, and Renal Function on Hematological Adverse Events Induced by Linezolid

Hematological disturbances that develop during linezolid treatmentare a major concern when linezolid is administered for prolongedperiods of time. The aim of this study was to evaluate the influencesof pyridoxine, rifampin, and renal function on hematologicaladverse events. From January 2002 to April 2006, 52 patientsreceived a long-term course of linezolid. Blood cell countswere monitored weekly. Thrombocytopenia was defined as a decreaseto <75% of the baseline platelet count, and anemia was definedwhen the hemoglobin concentration decreased by $≥$ 2 g/liter fromthe baseline value. Twenty-four patients received linezolidalone, and 28 patients received linezolid plus 200 mg of pyridoxine.The Kaplan-Meier survival method, followed by the log-rank test,was used to estimate the cumulative probability of adverse events,and Cox regression analysis was performed to evaluate the independentpredictors of toxicity. The baseline characteristics of thepatients in both groups were similar. The cumulative probabilityof thrombocytopenia and anemia in patients who received pyridoxinewas not different from that in patients who did not receiveit. Hematological adverse events were less frequent in patientstaking rifampin and were more frequent in patients with renalfailure. However; the Cox regression analysis showed that rifampinwas the only independent predictor associated with a lower riskof thrombocytopenia (hazard ratio, 0.37; 95% confidence interval,0.14 to 0.98; P = 0.045). In conclusion, pyridoxine did notprevent linezolid-related hematological adverse events, andthe coadministration of rifampin was associated with a lowerrisk of thrombocytopenia.

INTRODUCTION

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INTRODUCTION

Linezolid belongs to a family of antimicrobials (oxazolidinones)that inhibit bacterial protein synthesis by preventing the fusionof 30S and 50S ribosomal subunits (14). Linezolid has shownexcellent efficacy against gram-positive cocci, including Staphylococcusaureus, coagulase-negative staphylococci, enterococci, and streptococci,with MICs ranging from 0.5 to 4 µg/ml (12). Furthermore,linezolid has a 100% oral bioavailability and reaches high concentrationsin different tissues (skin, synovial fluid, bone, cerebrospinalfluid, lung, and eye). Therefore, it seems to be a good alternativefor the treatment of orthopedic implant infections, ventricle-peritonealshunts, and other infections related to foreign bodies wheregram-positive cocci are the main pathogens and where prolongedcourses of antimicrobial therapy are needed. However, a majorconcern with this antibiotic is its safety profile, especiallywhen it is administered for more than 4 weeks (10).

The most important adverse events are hematological disturbances,especially thrombocytopenia and anemia (1, 4, 11, 17, 19, 22),but the underlying mechanisms that explain this toxicity arestill unknown. Spellberg et al. described that the administrationof pyridoxine (vitamin B₆) was able to revert linezolid-relatedthrombocytopenia and anemia in two patients (24). Although thereis no clear mechanism to explain this effect, it is reasonableto evaluate the potential preventive effect of pyridoxine onhematological disturbances in patients receiving prolonged coursesof linezolid. In the present study, we evaluated the influenceof pyridoxine on the frequency of hematological disturbancesin two consecutive cohorts of patients (those who received pyridoxineand those who did not receive pyridoxine) who had similar baselinecharacteristics and who received prolonged courses of linezolid,as well as the influences of other clinically relevant variables,such as renal function and the coadministration of rifampin.

MATERIALS AND METHODS

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MATERIALS AND METHODS

From January 2002 to April 2006, patients who received a long-termcourse ( $≥$ 3 weeks) of linezolid (Zyvoxoid; Pfizer) were identifiedand monitored in a tertiary-care University Hospital in Barcelona,Spain. The protocol included weekly blood cell count, serumcreatinine level, and serum glucose level determinations. FromDecember 2004 to April 2006, the oral administration of pyridoxine(Godabion; Merck Pharma Quimica) at a dosage of 200 mg oncedaily was added to the linezolid treatment. A total of 52 patientswere included in the protocol. Linezolid was administered aloneto the first 24 patients, and pyridoxine was added to regimenand was administered to the remaining 28 patients. The clinicalvariables gathered were, age, sex, type of infection, etiologicmicroorganism, comorbidity, baseline serum creatinine level,and estimated baseline glomerular filtration rate (GFR), thelength of linezolid treatment (days), whether rifampin was coadministered,the baseline level of platelet count (10⁹ platelets/liter),and the baseline hemoglobin concentration (g/liter). GFR wasobtained by using the formula of Cockcroft and Gault: GFR (ml/min)= [(140 – age in years) x weight (kg)]/[72 (or 85 forwomen) x serum creatinine level (mg/dl)]. Thrombocytopenia wasdefined as a decrease in the platelet count to <75% of thebaseline value, and anemia was defined as a hemoglobin concentrationthat decreased by $≥$ 2 g/liter from the baseline value withoutanother plausible explanation. No other medications with potentialhematological toxicity were administered to any patient. Theethical committee of the hospital approved the administrationof pyridoxine, and the patients signed an informed consent.

Statistical analysis. Continuous variables were expressed as the mean and standarddeviation (SD), and the median and interquartile range (IQR)were compared by using an unpaired Student's t test. Categoricalvariables were compared by using Fisher's exact test or thechi-square test, when necessary. The cumulative probabilityof thrombocytopenia and anemia was estimated by univariate analysisby the Kaplan-Meier survival method, followed by the log-ranktest. Furthermore, a stepwise forward Cox regression analysiswas performed to evaluate independent predictors of hematologicaltoxicity. Variables with a P value less than 0.15 in the univariateanalysis were entered into the multivariate analysis, and variablesachieving P values <0.05 in the final model were consideredsignificant. The hazard ratio with the 95% confidence intervalwas calculated for each significant variable. The statisticalanalysis was performed by using the SPSS 12.0 package (SPSS,Chicago, IL).

RESULTS

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RESULTS

The baseline characteristics of the patients are shown in Table1. There were no significant differences in the baseline parametersbetween patients who received pyridoxine and patients who didnot receive pyridoxine.

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TABLE 1. Characteristics of patients at the baseline^a

The survival curves demonstrated that the cumulative probabilityof thrombocytopenia and anemia was not different between patientswho received pyridoxine and those who did not (Fig. 1a and b).Linezolid was stopped due to severe thrombocytopenia (<100x 10⁹ platelets/liter) in 7 of 52 (13.4%) patients: 3 of 24(12.5%) patients who received linezolid alone and 4 of 28 (14.2%)patients who received pyridoxine (P = 0.58). Linezolid was stoppeddue to severe anemia (< 8 g/liter) in 4 of 52 (7.7%) patients:3 of 24 (12.5%) patients who received linezolid alone and 1of 28 (3.5%) patients who received pyridoxine (P = 0.24).

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FIG. 1. Cumulative probability of hematological adverse events in patients who received or did not receive pyridoxine.

Rifampin was added to the linezolid regimen in 17 patients withan orthopedic implant infection from whom the implant was notremoved. The age, length of linezolid treatment, basal serumcreatinine level, GFR, platelet count, and hemoglobin concentrationwere not different between the patients who received rifampinand those who did not. The cumulative probability of thrombocytopeniawas significantly lower in patients who received rifampin thanin those that did not (P = 0.02) (Fig. 2a). Only 1 of 17 (5.8%)patients who received rifampin but 6 of 35 (17.1%) patientswho did not receive rifampin had severe thrombocytopenia (<100x 10⁹ platelets/liter). There was a trend toward a lower cumulativeprobability of anemia in the rifampin group, but the differencedid not reach statistical significance (P = 0.08) (Fig. 2b).The cumulative probability of hematological adverse events wasanalyzed according to the GFR. The risk of thrombocytopeniawas significantly higher when the basal GFR was <50 ml/min(P = 0.02) (Fig. 3a). There was a trend toward a higher cumulativeprobability of anemia when the basal GFR was <50 ml/min;however, the difference was not statistically significant (P= 0.14) (Fig. 3b). The Cox regression model showed that thecoadministration of rifampin was the only factor independentlyassociated with a lower risk of thrombocytopenia (hazard ratio,0.37; 95% confidence interval, 0.14 to 0.98; P = 0.045). Novariable was independently associated with the development ofanemia.

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FIG. 2. Cumulative probability of hematological adverse events in patients who received did or did not receive rifampin.

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FIG. 3. Cumulative probability of hematological adverse events according to GFR.

DISCUSSION

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DISCUSSION

The activity of linezolid against a broad range of gram-positivecocci, its high oral bioavailability (100%), and the good resultsdescribed in studies of linezolid treatment of bone infections(3, 20, 21) make linezolid an attractive oral alternative toglycopeptides for the treatment of infections that require prolongedantimicrobial therapy. However, thrombocytopenia and anemiaare common adverse events when linezolid is administered formore than 3 weeks. The administration of 50 mg pyridoxine (vitaminB₆) orally once a day to two patients who developed hematologicaldisturbances was useful in reverting these adverse events (24).In a recent study, Plachouras et al. (18) administered 125 mgof pyridoxine to 24 patients who received linezolid for boneinfections, and the rates of thrombocytopenia (<140 x 10⁹platelets/liter) and anemia (hematocrit, <30%) were 45.8%and 25%, respectively. These data suggest that pyridoxine doesnot prevent linezolid-related hematological adverse events,since the frequencies of hematological adverse events were similarto or even higher than those reported in patients who did notreceive pyridoxine (19, 22). The study of Plachouras et al.(18) was a noncomparative study in which variables that couldinfluence the rate of cytopenia, such as age, sex, comorbidity,the coadministration of other antibiotics, or renal function,were not controlled. In our study, the influence of pyridoxineon hematological adverse events was analyzed by comparing twoconsecutive cohorts with similar baseline characteristics andsimilar lengths of linezolid treatment. The cumulative probabilityof thrombocytopenia and anemia was similar in both groups; therefore,our findings support the lack of a protective role of pyridoxine.

Rifampin is the most active antibiotic against biofilm-formingmicroorganisms, but it should not be administered alone dueto the high risk of selection of resistant mutants (2, 27).It is of note that the administration of rifampin was independentlyassociated with a lower risk of thrombocytopenia. Although invitro studies have demonstrated that linezolid is not metabolizedby human cytochrome P450 (26), recently, Egle et al. (7) observedin eight healthy men a 35% decrease in the serum linezolid concentrationafter the administration of 600 mg of rifampin. They hypothesizedthat linezolid may be a substrate of P-glycoprotein, whose expressionis rapidly induced by rifampin, as a consequence of which theintestinal secretion of linezolid may be increased. On the otherhand, hematological adverse events were more frequent in patientswith renal failure, as previously described by other authors(13, 25). Although linezolid does not require dosage adjustmentin patients with renal failure, its area under the serum concentration-timecurve (AUC) is higher in patients with renal failure than inpatients with a normal renal function (5). These findings suggestthat hematological adverse events could be related to the serumlinezolid concentration.

The underlying mechanism of hematological adverse events isunknown. Our group has recently described that linezolid inhibitsthe mitochondrial ribosomes (9, 23), and this effect may bethe cause of hematological alterations. Since the efficacy oflinezolid (for the inhibition of bacterial ribosomes) is associatedwith the degree of exposure of the microorganism to the antibiotic,as measured by the AUC/MIC ratio (6), it is reasonable to assumethat inhibition of the mitochondrial ribosome may also be associatedwith the degree of linezolid exposure. These hypotheses aresupported by the relationship between the linezolid AUC andthe development of thrombocytopenia (8).

McKee et al. (16) reported that the linezolid concentrationthat inhibits 50% of mitochondrial protein synthesis in ratand rabbit heart and liver mitochondria is between 3.37 and5.26 µg/ml. By using the standard dosage of 600 mg/12h, the trough serum concentration at steady state is 6 µg/ml(15) and the expected AUC at 24 h is 260 mg·h/liter.When the MICs of the etiologic agent are 1 and 2 µg/ml,the AUC/MIC ratios obtained with the standard regimen are 260or 130, respectively. In both cases, the AUC/MIC ratio is >50to 80, which is the target associated with the highest linezolidefficacy against gram-positive cocci (6). Therefore, in orderto reduce the rates of adverse events, in those patients whorequire prolonged treatment (>21 days) with linezolid andwhen the MIC of the etiologic agent is $≤$ 2 µg/ml, the linezolidconcentration could be reduced to obtain an AUC/MIC ratio ofabout 100.

The present study has two major drawbacks. First, this was anonrandomized study; however; the principle baseline characteristicsthat could influence hematological adverse events, such as age,sex, comorbidity, the use of other antibiotics, baseline hematologicalparameters, and renal function, were similar in both cohorts.Second, the low number of patients reduces the statistical powerof the study.

In conclusion, our data showed that 200 mg of pyridoxine oncedaily does not prevent linezolid-related hematological adverseevents. The coadministration of rifampin was associated witha lower rate of hematological adverse events, while renal failurewas associated with a higher rate. Since these factors affectthe serum linezolid concentration, these findings suggest thathematological toxicity is directly related to the degree oflinezolid exposure. In the future, it will be necessary to evaluatewhether adjustment of the concentration in serum to obtain thepharmacodynamic target would be a reasonable strategy to avoidadverse events in patients receiving prolonged courses of linezolid.

ACKNOWLEDGMENTS

We thank Laura García and Marta Sala for their collaborationwith the protocol.

We declare no conflict of interest in connection with this article.

FOOTNOTES

* Corresponding author. Mailing address: Department of Infectious Diseases, Hospital Clínic of Barcelona, C/Villarroel 170, Barcelona 08036, Spain. Phone: 00 34 932275708. Fax: 00 34 934514438. E-mail: asoriano{at}clinic.ub.es

Published ahead of print on 30 April 2007.

REFERENCES

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