1 Division of Clinical Pharmacology, Department of Laboratory Medicine, Karolinska Institutet, Huddinge University Hospital, Stockholm, 2 Department of Nephrology, Karolinska University Hospital, Karolinska Institutet, Stockholm, 3 Department of Neurology, Huddinge University Hospital, Stockholm and 4 Center for Allogenic Stem Cell Transplantation, Huddinge University Hospital, Karolinska Institutet, Stockholm, Sweden
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Abstract |
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Methods. We conducted an observational study from 1991 to mid 1999 based on samples sent for analysis of ACV concentration from various hospital departments in Sweden. Patients with neuropsychiatric symptoms (NS+, n=49) were compared with patients without symptoms (NS-, n=44). ACV and CMMG concentrations were analysed by HPLC. Medical records were analysed for symptoms and compared with pertinent cases identified from Medline.
Results. The serum CMMG levels were significantly higher in the NS+ group (mean=34.1 µmol/l, 95% confidence interval 23.446.1) compared with the NS- group (mean=4.7 µmol/l, 95% confidence interval 3.36.6; P<0.001). CMMG was the strongest predictor in a receiver-operating characteristics curve analysis (ROC), based on 77 patients, of ACV-related neuropsychiatric symptoms. The ROC curve for CMMG demonstrated that neuropsychiatric symptoms could be predicted with a sensitivity of 91% and a specificity of 93% with the use of a cut-off value of 10.8 µmol/l of CMMG. Thirty-five of 49 patients in the NS+ group showed levels exceeding this concentration compared with only three of 44 of patients in the NS- group (P<0.001). ACV exposure, ACV concentration, creatinine clearance and creatinine concentration were weaker but statistically significant predictors. Haemodialysis reduced CMMG and ACV levels and relieved the symptoms.
Conclusions. The determination of CMMG levels in serum may be a useful tool in supporting the diagnosis of ACV-associated neuropsychiatric symptoms. Furthermore, the monitoring of CMMG levels may prevent the emergence of symptoms.
Keywords: acyclovir; adverse effects; 9-carboxymethoxymethylguanine; neuropsychiatric; renal failure
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Introduction |
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ACV is probably metabolized by alcohol dehydrogenase and aldehyde dehydrogenase to 9-carboxymethoxymethylguanine (CMMG) and to a smaller extent to 8-hydroxy-9-(2-hydroxyethoxymethyl) guanine (8-OH-ACV) [11]. In patients without renal failure, intravenously administered ACV is excreted in the urine 6291% unchanged and 814% as CMMG. A larger proportion of the ACV dose is excreted as CMMG in the urine when renal function is impaired [12]. The mean half-life of ACV is increased from 3.8 h in normal subjects to 19.5 h in patients with chronic renal failure [13]. Thus, the steady-state concentration of ACV is expected to increase as renal function declines. CMMG in patients with decreased renal function has not been evaluated.
The aim of this study was to determine if there was a correlation between increased serum concentrations of CMMG and neuropsychiatric symptoms.
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Subjects and methods |
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The NS+ group was also subject to further analysis of several aspects of the patient records including: the effect of dialysis treatment, the indication for ACV treatment, the lag time from start of treatment until appearance of symptoms, the pattern of symptoms and the outcome of the symptom episodes. We also analysed the results from the following clinical investigations: EEG, culture and PCR of cerebrospinal fluid, cranial computer tomography scan (CT) and magnetic resonance imaging (MRI). The research ethics committee of Huddinge University Hospital approved the study.
Assessment of adverse drug reactions
The neuropsychiatric symptoms compiled from the patient files were compared with published case reports and articles quoted in this article. The neuropsychiatric symptoms were categorized into six main groups reflecting specific functions of the nervous system. Other symptoms, such as disturbances in lung function and acute renal failure, were also collated. Measures of metabolic disturbances were not systematically collated, but were often part of the clinical investigations.
Samples
The first serum concentration obtained from each patient was used in the concentrationsymptom analysis. The first sample was selected for two reasons; many patients had only one sample drawn, secondly, subsequent concentrations may have been influenced by the result from the first sample due to e.g. changed dosing. Pre- and post-dialysis ACV and CMMG concentrations were obtained from nine patients. The serum concentrations were determined by solid phase extraction followed by high-performance liquid chromatography with fluorescence detection [14]. The limit of detection was 0.12 µmol/l.
ACV exposure
To estimate ACV exposure in each patient an oral bioavailability (F) of 20% for 200-mg tablets, 13% for 400-mg tablets and 9% for 800-mg tablets, was assumed, based on earlier studies [15]. VACV was assumed to be 54% bioavailable [16]. Exposure was calculated using the equation for steady-state concentration (Css):
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Here, F is bioavailability, D is dose (in µmol), ACVCLn is the ACV clearance of an individual with normal renal function, CrCL is estimated creatinine clearance (in ml/min) and t is the dose interval (in minutes). In this equation it is assumed that ACV clearance (CL) is proportional to the sum of creatinine clearance (CrCL) and a non-renal clearance component. The results are presented as a percentage of the exposure in a patient with CrCL=100 ml/min who is treated with ACV 800 mg five times daily. In this way the exact value of ACVCLn need not be specified as it is eliminated in this calculation of relative exposure.
Statistics
Data on the concentrations of CMMG and ACV and the ACV exposure were analysed by Student's t-test. Doses were compared by MannWhitney U-test. Descriptive statistics are given as means±SD and in the abstract as means with 95% confidence intervals. The frequency data was analysed using 2 test. The computer program Statistica version 6.0 (Statsoft, New York, NY) was used in the study. Conventional receiver-operating characteristic (ROC) curves were used to determine the cut-off point of CMMG and ACV concentrations, ACV exposure, creatinine concentration and creatinine clearance with the highest sensitivity and specificity with respect to distinguishing patients with neuropsychiatric symptoms from those without such symptoms. A calculated area under the curve of 1.0 indicates that the laboratory test is a perfect indicator of neuropsychiatric symptoms, and an area of 0.5 is equivalent to chance.
Creatinine clearance
Each adult patient's creatinine clearance was calculated using the MDRD method [17], the children's using Schwartz's estimate [18].
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Results |
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CMMG and ACV concentration and ACV exposure
The mean CMMG concentration in the NS+ and NS- group was 34.1±39.4 and 4.7±4.7 µmol/l, respectively (P<0.001) (Figure 1A). The mean ACV concentration for the NS+ and NS- patients was 21.0±30.7 and 7.2±6.7 µmol/l, respectively (P=0.004) (Figure 1B
). Excluding the 16 patients in the NS+ mixed subgroup, a ROC curve had a cut-off value of 10.8 µmol/l of CMMG as an indicator of neuropsychiatric symptoms with a sensitivity of 91% and a specificity of 93%. The positive and negative predicted values were 91 and 93%, respectively. The results from ROC curve analyses are shown in Figure 2
and are summarized in Table 2
. CMMG is a significantly stronger predictor than ACV exposure, ACV concentration, creatinine clearance and creatinine. The latter four variables were statistically significant predictors not differing from one another with respect to the AUC of the ROC curves. The ACV doses administered are shown in Table 1
. Patients with neuropsychiatric symptoms had a significantly higher exposure (P<0.001) to ACV than patients without symptoms (Figure 3
).
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Symptoms
A total of 74 different symptoms were described in our patient's charts and in previous reports. The symptoms found included, e.g. agitation, confusion, pronounced tiredness, lethargy, coma, dysarthria, myoclonus and visual and/or auditory hallucinations (Table 3). The time from the start of the treatment to the appearance of the symptoms was 12 days (n=35).
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Outcome
Thirty-nine patients recovered; 12/39 shortly after haemodialysis and 24/39 after withdrawal of ACV, dose reduction or treatment with i.v. fluid therapy when appropriate. Three patients with herpes encephalitis recovered after ACV treatment. Six patients died. Four of these patients died while unconscious and on ACV therapy. None of the six patients had signs of CNS infections. Three of them developed cerebral emboli, which was reported to be the cause of death. In addition, two surviving patients developed cerebral emboli and one patient with herpes encephalitis developed cerebral haemorrhage. One additional patient with unknown encephalitis did not improve despite ACV treatment.
Haemodialysis
An average of 34 h of haemodialysis decreased CMMG and ACV concentrations by 64±9 and 57±9%, respectively, and symptoms subsided (Figure 4).
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Discussion |
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Approximately three publications have been presented per year between 1982 and 2001, in which severe neuropsychiatric events have been associated with ACV therapy. Apparently, neither current treatment recommendations nor available diagnostic tools to discriminate between ACV-associated side effects and herpes virus encephalitis are sufficient to avoid this clinical problem. This is clearly demonstrated by the higher ACV exposure in patients with neuropsychiatric symptoms (Figure 3). However, there were several patients for whom the dose had been adjusted for renal function according to the manufacturer in the Swedish Physicians Desk Reference (the valaciclovir doses are higher in Sweden than in the US). Our data suggest that determination of CMMG levels may be used in the differential diagnosis between neuropsychiatric symptoms due to ACV and other causes with sufficient sensitivity and specificity.
The other predictors of ACV-associated neuropsychiatric symptoms, i.e. ACV exposure, ACV concentration, creatinine clearance and creatinine concentration had sensitivities and specificities lower than CMMG and are clearly not as clinically useful in differential diagnosis. However, the analysis shows that the risk of developing the syndrome is high whenever a patient with poor renal function is exposed to large doses of ACV, either through high doses, i.v. administration or the use of VACV.
An important methodological issue concerns the pharmacokinetics of ACV and CMMG and the data used in this study. First, the samples were collected with a longer interval from the administered dose in patients with symptoms as compared with those without symptoms. Secondly, as almost all patients with symptoms had renal insufficiency the half-lives of both ACV and CMMG were prolonged. We found that the CMMG concentrations were more than seven times higher in patients with CNS symptoms than in those without, a difference that is probably underestimated due to the differences in sampling time. We therefore conclude that the sensitivity and the specificity of CMMG levels for diagnosis of ACV-induced side effects may be even higher. Possibly, the CMMG cut-off value found, 10.8 µmol/l, in the ROC analysis, is slightly overestimated. It is here important to point out that the interpretation of CMMG levels depends on the treatment indication. ACV is vital in herpes encephalitis treatment in which case higher CMMG levels may be acceptable while the limited value of ACV in post-herpetic neuralgia prophylaxis makes even lower CMMG levels unacceptable. In the individual case, the utility of ACV must be evaluated, weighing treatment efficacy and the severity of the condition against the risk of side effects. In this connection it is important to discuss the predicted values of CMMG. The figures given for the positive and negative predicted value depends on the incidence in the population studied which here consists of patients for whom ACV therapy was considered to be so complicated that a concentration was asked for. In the general population the numbers will be much lower.
Another methodological issue is the classification of patients. The retrospective nature of the data reduces the precision with respect to presence of CNS symptoms. However, the evaluation of the symptoms was blinded to avoid bias.
It is most unlikely that the present findings are a random event since the difference in CMMG levels between patients with ACV as the only identified risk factor for CNS symptoms and those without symptoms is so large. A prospective study of the presence of CNS symptoms in relation to CMMG levels may sharpen the difference.
The mechanism by which ACV induces neuropsychiatric side effects is not clear. It is tempting to suggest that CMMG is the cause of this side effect. The evidence for such a causal relation is presently epidemiological. Considerable experimental work remains to be performed to test this hypothesis. An alternative hypothesis is that the other metabolite 8-OH-ACV is the causative agent.
The potential usefulness of the present findings has its basis in the difficulty to differentiate between symptoms of herpes encephalitis and symptoms due to ACV. The reality of the problem is illustrated by earlier case reports and also by cases among our patients, in whom symptoms or worsening of symptoms, apparently associated with ACV, have been misinterpreted as herpes encephalitis and the dosage has been increased, aggravating the clinical condition.
We also found that a single haemodialysis session resulted in a clinical improvement and a reduction in CMMG and ACV concentrations. Thus, haemodialysis can be used both to corroborate the diagnosis of ACV-associated side effects and to treat the condition. This clinical efficiency of haemodialysis has been shown earlier [19,20]. It should be noted that peritoneal dialysis only gave slow recovery and low ACV clearance [10].
In order to provide recommendations for optimal dosing of ACV in renal failure, a systematic investigation of the kinetics of both CMMG and ACV in patients with renal failure is required. The aim is to obtain a maximum production of ACV-tri-phosphate in virus-infected cells at the smallest possible cost in terms of CMMG levels. The present data is not a sufficient basis for such dose recommendations. Obviously, ACV treatment can and should be used to treat herpes encephalitis, even in patients with renal failure, but a monitoring of serum concentrations of CMMG and of CNS symptomathology is warranted. Naturally the doses of ACV should generally be reduced in this patient group. It is probably advisable to avoid the use of VACV in patients with renal dysfunction, as unacceptably high concentrations of CMMG are easily attained.
It is concluded that determination of serum concentrations of CMMG may help differentiate between neuropsychiatric side effects of ACV and symptoms of any form of encephalitis. Haemodialysis effectively relieves CNS side effects and decreases serum concentrations of ACV and CMMG. The monitoring of CMMG concentration is suggested in patients with renal dysfunction or when ACV-induced neuropsychiatric side effects are suspected.
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Acknowledgments |
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Conflict of interest statement. None declared.
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Notes |
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References |
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