PEDIATRICS Vol. 118 No. 5 November 2006, pp. e1563-e1568 (doi:10.1542/peds.2006-0904)
EXPERIENCE & REASON |
Pharmacokinetics of Pyridostigmine in a Child With Postural Tachycardia Syndrome
a Department of Pediatrics, Children's Hospital of Eastern Ontario, University of Ottawa, Ottawa, Ontario, Canada
b Clinical Research and Development, Department of Anesthesiology, University of Colorado Health Sciences Center, Denver, Colorado
ABSTRACT
Pyridostigmine has been proposed for the treatment of postural orthostatic tachycardia syndrome in adults at a dose of 60 mg twice daily, but no dosing recommendation exists for children. With the approval of our local ethics board, we tested the pharmacokinetics of pyridostigmine in 6 children with myasthenia and a pediatric index patient with severe postural orthostatic tachycardia syndrome whose condition failed all conventional therapy and who had developed significant postural hypertension. Pyridostigmine was quantified by using a validated, semiautomated, and specific high-performance liquid chromatography/tandem mass spectrometry assay in combination with online column-switching extraction and turbo electrospray ionization. The patient with postural orthostatic tachycardia syndrome showed a dose-dependent favorable response to oral pyridostigmine. Pharmacokinetic evaluation revealed a short half-life of 2.29 hours, similar to the 2.0 ± 0.63 hours in the patients with myasthenia. The patient with postural orthostatic tachycardia syndrome has subsequently been treated at a dose of 45 mg in the morning, 30 mg at lunchtime, and 15 mg at bedtime; after 9 months, there has been persistent positive effect and without additional blood pressure medication. No major adverse effects occurred. Pyridostigmine has been a safe and effective treatment modality for this child with postural orthostatic tachycardia syndrome. The short half-life suggests that dosing 3 times per day is preferable.
Key Words: tachycardia • acetylcholine • nervous system • autonomic • pharmacokinetics • children
Abbreviations: POTS, postural orthostatic tachycardia syndrome • HPLC, high-performance liquid chromatography • LC/LC-MS/MS, high-performance liquid chromatography/tandem mass spectrometry assay in combination with online column-switching extraction and turbo electrospray ionization • AUC, area under the time-concentration curve • MRT, mean residence time • Cmax, maximum concentration • tmax, time to maximum concentration
Postural orthostatic tachycardia syndrome (POTS) is a type of orthostatic intolerance characterized by excessive tachycardia and decreased cerebral blood flow in the upright position. This can result in disabling symptoms of dizziness and light-headedness that can eventually lead to syncope. This condition may also rarely affect children and severely affects the patients' ability to function in daily life. The diagnosis of POTS requires orthostatic heart rate acceleration in excess of 120 beats per minute or an absolute increase of 
30 beats per minute in the absence of significant orthostatic hypotension.1 Many treatment strategies have been used with variable success, including high water intake,2 salt supplements, drugs that expand the plasma volume such as fludrocortisone,3 and 
-adrenergic drugs such as midodrine hydrochloride,4 which is the only drug that has been shown to be effective in a randomized placebo-controlled trial. The effects of these drugs are variable, and adverse effects are frequent. Midodrine hydrochloride causes supine hypertension in a dose-dependent manner.5 Administration of fludrocortisone may cause water and salt retention and baseline sympathetic activity. Administration of -adrenergic drugs increases the blood pressure, total peripheral resistance, and venomotor activity. Both drugs may lead to supine hypertension, which may even cause end-organ damage.6 Recent publications about the use of acetylcholinesterase inhibition for the treatment of POTS have identified a different therapeutic approach that may avoid such adverse effects.7 The rationale for its use lies in the fact that the efferent limb of the baroreflex is a 2-neuron system synapsing at the autonomic ganglion, where acetylcholine is the neurotransmitter, which is hydrolyzed by the enzyme acetylcholinesterase.8 To date, the use of pyridostigmine for the treatment of POTS has not been reported in children. Here we report the use of pyridostigmine in an adolescent with debilitating symptoms in whom all conventional therapeutic approaches failed to ameliorate the symptoms and fludrocortisone induced significant postural hypertension. Pediatric dosing has not been established yet, so we developed an assay for the determination of pyridostigmine plasma concentrations. We hypothesized that the pharmacokinetics of pyridostigmine may require child-specific dosing, possibly different than 30 to 60 mg twice daily. We also determined the pharmacokinetics of the drug in children who receive pyridostigmine for myasthenia.
CASE REPORT
The index patient with POTS presented with chronic constipation from infancy onward. The etiology was never understood, and the patient required large doses of very potent laxatives all her life. At 4 years of age she was assessed for multiple infections and fatigue. Both weight and height percentiles dropped from the 50th percentile at age 4 to below the third percentile at age 12. Subsequently, the diagnosis of cortisol deficiency was made. Some of her symptoms improved after starting hydrocortisone, which she continues to take 3 times daily. Additional testing was interpreted as no evidence for mineralocorticoid deficiency.
The patient continued to experience persistent fatigue and intermittent episodes of dizziness or light-headedness. After a tilt-table test, she was diagnosed with postural hypotension and POTS on the basis of orthostatic intolerance and heart rates up to 200 beats per minute, but she never had actual syncope. Various physicians and complete neurophysiological testing confirmed this diagnosis. Several treatments included salt supplements, high fluid intake, and fludrocortisone. Initially, these treatments significantly reduced her symptoms, but they recurred within a matter of months. Additional treatment attempts with both clonidine (0.05 mg once daily) and midodrine (2.5 mg twice daily) were abandoned because of severe adverse effects from both medications. The patient's dose of fludrocortisone was gradually increased, because without it her baseline blood pressure would drop to 70/40 mm Hg. She developed supine/nocturnal hypertension as an adverse effect of fludrocortisone and, therefore, commenced on antihypertensive agents at night. A 24-hour ambulatory blood pressure–monitoring study revealed postural hypertension with an inverse nocturnal rise of blood pressure. Her hypertension was treated with carvedilol and fosinopril, but that resulted in lower daytime blood pressure.
In an attempt to treat her debilitating orthostatic intolerance, a treatment trial with pyridostigmine was considered. Pyridostigmine is an unlicensed drug for this indication in pediatric patients. After a few dose adjustments, the patient commenced on 45 mg of pyridostigmine in the morning, 30 mg 4 hours after the morning dose, and 15 mg 8 hours after the morning dose. This dose remained unchanged over a follow-up period of 8 months; however, at the last appointment the patient requested an increase in the evening dose to 30 mg. It was possible to adjust the fludrocortisone dose to 0.15 mg/day. Carvedilol could be reduced substantially, and at the time of this writing, the patient was only taking it very rarely at a minute dose of 6.25 mg when she experienced tachycardia at bedtime. Unfortunately, there was no improvement of her chronic constipation. The patient remained on a stable dose of 25 g of polyethylene glycol. The treatment with pyridostigmine resulted in a significant improvement in her quality of life and exercise tolerance, and activities such as climbing the stairs have become much easier. On previous attempts to increase the dose, she experienced adverse effects including double vision, twitching of her legs, and increased tear production.
SUPPORTING STUDIES: METHODS
Patients
The local ethics board approved the study, and informed consent was obtained from each patient. We studied a 16-year-old adolescent girl with severe POTS. We also measured pyridostigmine plasma concentrations in 1 male patient with myasthenia gravis and 5 children (2 female) with myasthenia gravis myasthenic syndrome. The patients with myasthenia studied had a mean age of 10.6 ± 2.5 years, an average absolute pyridostigmine dose of 338 ± 165 mg/day, and an average weight of 44.4 ± 26.9 kg and received a pyridostigmine dose per kilogram of body weight of 8.57 ± 4.57 mg/kg per day.
Pyridostigmine Pharmacokinetic Profiles
Pyridostigmine levels were determined before oral intake of the first morning dose and 0.5, 1, 1.5, 2, 3, and 4 hours after intake for 1 dosing interval. The frequent sampling for the first 2 hours was performed to enable accurate determination of the time to maximum concentration (tmax). This was anticipated to be short because of a short half-life (60–90 minutes) of the drug in a previously study that involved children.9 One patient with a longer dosing interval of 8 hours had additional pyridostigmine concentrations determined at 6 and 8 hours postintake. On a separate occasion, the patient with POTS also had a total of 27 levels determined over a 24-hour period.
Pyridostigmine Assay
Pyridostigmine was quantified by using a newly developed semiautomated and specific high-performance liquid chromatography (HPLC)/tandem mass spectrometry assay (see Appendix).
Tilt-Test Protocol
The patient with POTS was brought to the tilt-test laboratory in a fasting state. Previous examinations had shown a normal resting electrocardiogram and a normal echocardiogram. After resting supine for 10 minutes, the patient was tilted to 70°. Her heart rate and blood pressure (Finapres) were monitored continuously, with intermittent manual blood pressure measurements taken to check accuracy of the continuous recording. Her symptoms were noted throughout. The bed was returned to the supine position after 10 minutes. Tilt tests were performed before administration of oral pyridostigmine, after 60 mg of pyridostigmine (test 2) on 1 occasion, and after 45 mg of pyridostigmine on a second occasion (test 3).
Analysis of Tilt Test
Three consecutive intervals between cardiac complexes (RR intervals) were measured every 10 seconds for the first 4 minutes of the tilt test and every 20 seconds for the remainder of the 10 minutes. The RR intervals were averaged and converted to heart rate (beats per minute).
Statistical Analysis
All continuous contiguous data were tested for normal distribution by using the D'Agostini Pearson Omnibus test. For descriptive statistics, normally distributed data were expressed as mean ± 1 SD, and nonparametric distributions were expressed as median and range.
Serial heart rate measurements from the tilt tests were described as peak heart rate, time of peak heart rate, and the mean ± SD of all measurements. The geometric mean is reported because the SD increases as the mean increases. The area under the time-concentration curve (AUC) was calculated.10 GraphPad Prism 4 software (Graph Pad Software Inc, San Diego, CA) was used for all the statistical evaluations described above.
All pharmacokinetic parameters were estimated on the basis of a noncompartmental model using the algorithms implemented in the WinNonlin 4.1 Professional software package (Pharsight Corp, Mountain View, CA). The following pharmacokinetic parameters were calculated: AUC0-
, apparent terminal half-life (t), and mean residence time (MRT). Individual concentration-time profiles were plotted, and the terminal elimination constant was determined by the logarithmic regression of the last 3 time points that were judged to be in the terminal elimination phase. Maximum concentration (Cmax) and tmax were determined directly from the pharmacokinetic profiles.
SUPPORTING STUDIES: RESULTS
Tilt-Table Testing
The patient with POTS underwent 3 tilt-table tests, one without the pyridostigmine (tilt 1), one with 45 mg orally (tilt 3), and one with 60 mg orally (tilt 2). All 3 tilt-table tests were conducted under the same conditions. Treatment with fludrocortisone, hydrocortisone, carvedilol, potassium chloride, and sodium chloride remained unaltered. The results showed that peak heart rate, time of peak heart rate, and mean heart rate all decreased on tilt 2 compared with tilt 1. As expected, the tilt-3 response was intermediate between the other 2 tests. The patient described a subjective decrease in symptoms, and tolerated tilt 2 without the sense of tachycardia, nausea, and light-headedness that were prominent features of tilt 1. Tilt 3 was also well tolerated. The absolute heart rates of tilt 1 and tilt 2 are provided in Table 1. There was a substantial decrease in the initial tachycardia response to upright tilt that persisted into the plateau phase after intake of 60 mg of pyridostigmine.
|
Pyridostigmine Pharmacokinetics
In adult patients with POTS, dosing of pyridostigmine was proposed at 60 mg twice daily. In children, pyridostigmine has a very short half-life. We obtained pharmacokinetic profiles of the morning dose of pyridostigmine for all of the 6 patients with myasthenia and 2 profiles on the patient with POTS. Figure 1 shows the dose-normalized pyridostigmine concentrations.
|
The results of the pharmacokinetic analysis are provided in Table 2.
|
DISCUSSION
We describe the pediatric use of pyridostigmine for the treatment of POTS, which has been supplemented by pharmacokinetic monitoring. The drug had not been used previously in children for this indication. We developed and validated a specific high-throughput LC/LC-MS/MS assay for the determination of the pyridostigmine levels, which allowed us to study the pharmacokinetics. We found a short half-life of 2 hours, which is not different from pediatric patients with myasthenia. Because half-life is better determined from a single-dose profile to infinity, we compared MRTs. The MRT in the index patient (4.3 hours) was not significantly different from the patients with myasthenia (4.0 ± 0.8 hours; P = .4650, 1-sample t test). The short half-life is in keeping with the literature, which quotes 60 to 90 minutes.11 The tmax and range of the Cmax in our patients with myasthenia were similar to the 1 to 2 hours and 40 to 60 mg/L, respectively, that were reported previously.9 It is notable that 3 patients in our control group received slow-release pyridostigmine. Therefore, the half-life could be even shorter for regular pyridostigmine only. We did not test the use of slow-release pyridostigmine in our index patient.
For adults, 30- to 60-mg twice-daily doses are recommended for the treatment of POTS.11 Given the short half-life demonstrated in our study, this dosing interval seems inappropriate for children; we found dosing 3 times daily more effective in controlling the patients' symptoms.
Similar to previous studies in adults, we found that pyridostigmine was effective and well tolerated.5,12 The effect of therapy was more marked in our patient when compared with a recent treatment trial in adults.12 Unfortunately, this recent trial did not comment in detail on adverse effects, whereas our report clearly points to the need for careful observation of cholinergic adverse effects. The determination of pyridostigmine levels may serve as a useful tool to determine if adverse effects are related to treatment and whether inefficacy is related to underdosing. Clearly, more data must be obtained in additional pediatric patients before more generalized pediatric dosing recommendations can be made.
CONCLUSIONS
This case report suggests that pyridostigmine can be used safely and effectively in an adolescent with POTS. The pharmacokinetics of pyridostigmine was comparable to children with myasthenia gravis, but much smaller doses are required. The half-life of the drug is short. The use of sustained-release pyridostigmine may be a feasible alternative. The adverse effects were acceptable. Additional studies in other children with careful pharmacokinetic monitoring are encouraged to allow for safe pediatric dosing recommendations.
APPENDIX
Validated, semiautomated, and specific LC/LC-MS/MS was performed using a similar approach as previously published.13
Extraction Procedure
The only manual step was protein precipitation. The protein-precipitation solution (methanol/0.2 M ZnSO4, 7:3 [vol/vol]) contained the internal standard neostigmine (Sigma, St Louis, MO) at a concentration of 500 ng/mL. Protein-precipitation solution (250 µL) was added to 500 µL of EDTA plasma. After vortexing (3 minutes) and centrifugation (5°C, 5000g, 25 minutes), the supernatant was transferred into a glass HPLC vial.
LC-MS/MS Analysis
Samples were analyzed by using an LC/LC-MS/MS system that consisted of an Agilent Technologies (Palo Alto, CA) 1100 high-performance liquid chromatograph connected to an API4000 (Applied Biosystems, Foster City, CA) tandem mass spectrometer. The HPLC system had the following series 1100 HPLC components: HPLC I: G1312A binary pump, G1322A degasser, and a G1367A autosampler in combination with a G1330A thermostat; HPLC II: G1312A binary pump, G1322A degasser, G1316A column thermostat, and G1946A mass selective detector (all from Agilent Technologies). The 2 HPLC systems were connected via a 7240 Rheodyne (Cotati, CA) 6-port switching valve mounted on a step motor (for details regarding the connections see ref 13). The system was controlled by Analyst 1.3.1 software (Applied Biosystems).
Four hundred microliters of the samples were injected onto a 4.6 x 12.5-mm extraction column filled with Eclipse XDB-C8 material of 5-µm particle size (Agilent Technologies). Samples were washed with a mobile phase of 20% methanol and 80% 0.01% formic acid. The flow was 5 mL/minute, and the temperature for the extraction column was set to 65°C. After 1 minute, the switching valve was activated, and the analytes were eluted in the backflush mode from the extraction column onto a 150 x 4.6-mm C8 3.5-µm analytical column (Zorbax XDB C8, Agilent Technologies). The mobile phase consisted of methanol and 0.01% formic acid. The following gradient was run: 0 minutes, 65% methanol; 1 minute, 65% methanol; 3.0 minutes, 95%; 4.0 minutes, 95% methanol. The MS/MS was run in the positive mode. Nitrogen (purity: 99.999%) was used as spray gas and zero air as collision-activated dissociation gas. The mass spectrometer was run in the positive multiple-reaction-monitoring mode. The declustering potential was set to 50 V. The interface was heated to 500°C. For pyridostigmine, the following ion pair was detected: m/z = 181.3 [M+H]+ 
72.2. The internal standard neostigmine was detected by using the transition m/z = 223.4 [M+H]+ 208.4.
Pyridostigmine concentrations were corrected on the basis of the internal standard and quantified by using the calibration curves that were included in each batch. The assay had the following performance: The lower limit of quantitation was 250 pg/mL, and the assay was linear from 0.25 to 50 ng/mL (r2 0.99). Intraday precision (CV%, n = 5 per day, 3 days) was 12.3% at 0.5 ng/mL, 2.0% at 10 ng/mL, and 0.8% at 50 ng/mL. Intraday accuracy (% deviation from nominal, n = 5 per day, 3 days) was –9.8% at 0.5 ng/mL, 3.4% at 10 ng/mL, and 0% at 50 ng/mL. No matrix interferences, ion suppression, or carryover were detected. Pyridostigmine in the extracts was stable in the autosampler at 4°C for at least 24 hours, and plasma samples could undergo at least 3 freeze-thaw cycles.
ACKNOWLEDGMENTS
This work was supported by National Institutes of Health/National Institute of Diabetes and Digestive and Kidney Diseases grant P30 DK048520-10 (Clinical Nutrition Research Unit, Mass Spectrometry Core).
We thank Michelle Mullen for assistance with the ethical review and Darlene Poulin for excellent editorial assistance.
FOOTNOTES
Accepted Jun 5, 2006.
Address correspondence to Guido Filler, MD, PhD, FRCPC, Department of Pediatrics, Children’s Hospital of Western Ontario, University of Western Ontario, 800 Commissioners Rd E, London, Ontario, Canada N6A 5W9. E-mail: guido.filler{at}lhsc.on.ca
The authors have indicated they have no financial relationships relevant to this article to disclose.
REFERENCES
1. Makai A, Tahin T, Simor T, Csanadi Z, Rudas L. Postural tachycardia syndrome [in Hungarian] Orv Hetil. 2005;146 :515 –520[Medline]
2. Jordan J. Effect of water drinking on sympathetic nervous activity and blood pressure. Curr Hypertens Rep. 2005;7 :17 –20[Web of Science][Medline]
3. Freitas J, Santos R, Azevedo E, Costa O, Carvalho M, de Freitas AF. Clinical improvement in patients with orthostatic intolerance after treatment with bisoprolol and fludrocortisone. Clin Auton Res. 2000;10 :293 –299[CrossRef][Web of Science][Medline]
4. Low PA, Gilden JL, Freeman R, Sheng KN, McElligott MA. Efficacy of midodrine vs placebo in neurogenic orthostatic hypotension: a randomized, double-blind multicenter study. Midodrine Study Group [published correction appears in JAMA. 1997;278:388].
JAMA. 1997;277
:1046
–1051
5. Wright RA, Kaufmann HC, Perera R, et al. A double-blind, dose-response study of midodrine in neurogenic orthostatic hypotension.
Neurology. 1998;51
:120
–124
6. Shannon JR, Jordan J, Diedrich A, et al. Sympathetically me-diated hypertension in autonomic failure.
Circulation. 2000;101
:2710
–2715
7. Raj SR, Black BK, Biaggioni I, Harris PA, Robertson D. Acetylcholinesterase inhibition improves tachycardia in postural tachycardia syndrome.
Circulation. 2005;111
:2734
–2740
8. Singer W, Opfer-Gehrking TL, McPhee BR, Hilz MJ, Bharucha AE, Low PA. Acetylcholinesterase inhibition: a novel approach in the treatment of neurogenic orthostatic hypotension.
J Neurol Neurosurg Psychiatry. 2003;74
:1294
–1298
9. Aquilonius SM, Hartwig P. Clinical pharmacokinetics of cholinesterase inhibitors. Clin Pharmacokinet. 1986;11 :236 –249[Web of Science][Medline]
10. Mathews JNS, Altman DG, Campbell MJ, Royston P. Analysis of serial measurements in medical research.
BMJ. 1990;300
:230
–235
11. Grubb BP, Kanjwal Y, Kosinski DJ. The postural tachycardia syndrome: a concise guide to diagnosis and management. J Cardiovasc Electrophysiol. 2006;17 :108 –112[Web of Science][Medline]
12. Singer W, Sandroni P, Opfer-Gehrking TL, et al. Pyridostigmine treatment trial in neurogenic orthostatic hypotension.
Arch Neurol. 2006;63
:513
–518
13. Zhang YL, Bendrick-Peart J, Strom T, Haschke M, Christians U. Development and validation of a high-throughput assay for quantification of the proliferation inhibitor ABT-578 using LC/LC-MS/MS in blood and tissue samples. Ther Drug Monit. 2005;27 :770 –778[CrossRef][Web of Science][Medline]
PEDIATRICS (ISSN 1098-4275). ©2006 by the American Academy of Pediatrics
CiteULike 
Connotea 
Del.icio.us 
Digg 
Facebook 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  B. J Gales and M. A Gales Pyridostigmine in the Treatment of Orthostatic Intolerance Ann. Pharmacother., February 1, 2007; 41(2): 314 - 318. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
