Tiagabine

Abstract

The short-term stability of two different concentrations of extemporaneously compounded tiagabine suspensions was studied. Six bottles each of 6-mg/mL and 2-mg/mL suspensions were compounded using commercially available 16-mg tiagabine tablets in a 1:1 mixture of Ora-Sweet and Ora-Plus. For each concentration, three bottles were stored in the dark in a stability chamber at 23 to 25°C, and three were stored in the dark in a refrigerator at 3 to 5°C, for 61 days. The stability of tiagabine in each of the concentrations and environmental conditions was determined after preparation and at 7, 14, 28, 42 and 61 days by means of a high-performance liquid Chromatographic method, which involved a reversephase column, a gradient mobile phase containing acetonitrile and aqueous phosphoric acid at a flow rate of 2.75 mL/minute. Detection was carried out by an ultraviolet spectrophotometer at 254 nm. The drug tiagabine, the internal standard (propylparaben) and degradant eluted at 8.4, 8.6 and 9.9 minutes, respectively. The high-performance liquid chromatrographic validation involved limit of detection, limit of quantitation, accuracy and precision and interday and intraday variation. There was no appearance of degradation products in either concentration at any time. The suspensions stored at 23 to 25°C became slightly discolored after day 30, turning slightly gray from blue. However, this change could not be attributed to tiagabine degradation. The refrigerated suspensions were found to be stable (retention of 90% potency) up to 40 days. The 6-mg/mL suspensions were stable up to the study period of 61 days; however, the 2-mg/mL suspension at 3 to 5°C showed a potency of 87.7% on day 61 of the study.

Extemporaneously compounded liquid oral preparations of tiagabine 6 mg/mL in a 1:1 mixture of Ora-Plus and Ora-Sweet were stable for 61 days at 23 to 25°C and 3 to 5°C. The extemporaneously compounded liquid oral preparations of tiagabine 2 mg/mL in a 1:1 mixture of Ora-Plus and Ora-Sweet were stable for 61 days at 23 to 25°C and for 40 days at 3 to 5°C.

Introduction

Tiagabine is an antiepileptic drug approved for use as add-on therapy in individuals with partial seizure disorders.1 Its antiepileptic effect is produced by the drug's binding to recognition sites associated with the [gamma]-aminobutyric acid (GABA) uptake carrier, thus blocking GABA uptake into presynaptic neurons.1-3 This allows more GABA to be present for receptor binding on the surfaces of the postsynaptic cells. Several trials and studies in adults have shown efficacy and safety as adjunctive therapy in the treatment of partial seizures.4-6 Its efficacy as monotherapy in adult seizure patients is also well documented.7,8

Data also suggest that children and adolescents may benefit from both adjunct and monotherapy, especially those who have refractory partial seizures.9-12 Tiagabine may also prove beneficial in treating infantile spasms and psychoaffective disorders.13,14 Because children are often unable to take solid oral dosage forms for a variety of reasons, it would be beneficial to have a liquid form of tiagabine. This would make administration of tiagabine to children of all ages much simpler, as it would allow more precise dosing. Some authors have suggested an initial dose of 0.1 mg/kg/day, with increases of 0.1 mg/kg/day every 2 weeks to clinical effect (doses of 0.25 to 1.5 mg/kg/day have been used).9,15

Materials and Methods

Preparation of Samples

Two concentrations (6 ing/mL and 2 mg/mL) of a liquid suspension of tiagabine (Gabitril, Lot 86-593-AA-21, Cephalon, Inc., West Chester, Pennsylvania) were prepared by crushing and levigating 22.5 and 7.5 tablets, respectively (16 mg per tablet), in a glass mortar. A small amount of Sterile Water for Injection USP (Lot 75-067-DK, Abbott Laboratories, North Chicago, Illinois) was added to the fine powder to make a paste. Equal amounts of Ora-Sweet (Lot 182963, Paddock Laboratories, Inc., Minneapolis, Minnesota) and Ora-Plus (Lot 062315, Paddock Laboratories, Inc.) were added to make a final volume of 60 mL. The compounding procedures for both the 6-mg/mL and 2-mg/mL concentrations are outlined in Table 1.

Six identical samples of each concentration were made and stored in 2-oz amber plastic prescription bottles (Owens-Brockway Prescription Products, Berlin, Ohio). Three samples of each concentration were stored in the dark in a stability chamber (Model I-30BLL, Percival Scientific, Inc., Perry, Iowa) at 23 to 25°C and in a refrigerator (Model SR-952, Sanyo, Chatsworth, California) at 3 to 5°C.

A 500-µL sample was withdrawn from each of the 12 bottles at days 1, 7, 14, 28, 42 and 61. Each bottle was shaken vigorously by hand for 2.5 minutes, then gently inverted three times before sample removal. In addition, pH was recorded by use of a standard pH meter (PHM 210 Standard pH Meter, Radiometer Analytical S.A., Lyon, France) on each bottle of suspension at each sampling day. The 6-mg/mL samples were diluted to an expected concentration of 2.4 mg/mL with sterile water. An extraction procedure was performed using ethyl acetate (Lot 39039911, EM Science, Gibbstown, New Jersey), mobile phase, internal standard and ammonium chloride (Lot 89H0170, Sigma Chemical Company, St. Louis, Missouri). After centrifugation, the ethyl acetate portion was removed and evaporated in a water bath under nitrogen, then reconstituted with the mobile phase. The samples were assayed in triplicate by means of a high-performance liquid Chromatographic (HPLC) method, which involved a reverse-phase column (Synergi 4 µm, Hydro RP, 4.6 x 75 mm, Phenomenex U.S.A., Torranee, California), a gradient mobile phase containing acetonitrile (Lot 39081, EM Science) and aqueous phosphoric acid (Lot 00467C0, Aldrich Chemical Company, Milwaukee, Wisconsin) at a flow rate of 2.75 mL/minute. Detection was carried out by an ultraviolet (UV) spectrophotometer (System Gold 166 Detector, Beckman Coulter, Fullerton, California) at 254 nm. The drug tiagabine, the internal standard, propylparaben (Lot 03002PS, Aldrich Chemical Company), and the degradation product eluted at 8.4, 8.6 and 9.9 minutes, respectively.

HPLC Assay

The HPLC method used was a modification of the method acquired from Abbott Laboratories (Montreal, Canada). The internal standard was added to account for small injection-volume differences in a manual injector.

Instrumentation included a constant-flow delivery system (System Gold 126 Solvent Module HPLC Pump, Beckman Coulter) and a 4-µm particle column (Phenomenex U.S.A.). Included in the apparatus were a volume injector (7725 Manual Sample Injector, Rheodyne, Rohnert Park, California), a UV light detector (System Gold 166 Detector) at 254 nm and a computer with 32 Karat software (Beckman Coulter). The gradient mobile phase consisted of acetonitrile and aqueous phosphoric acid (pH = 2) pumped through the column at a rate of 2.75 mL/minute (Table 2).

Preparation of Standard Solutions and Standard Curve

A 3-mg/mL stock solution of analytical-grade tiagabine (Lot 41915ACOO, Cephalon, Inc.) was prepared by dissolving 12 mg of tiababine in 4 mL of a 50:50 water:methanol mixture. Standard samples of tiagabine were prepared by diluting the stock solution with a sample diluent (50:50 water:ethanol) to concentrations of 1.2, 0.9, 0.6 and 0.3 mg/mL. Each sample contained a fixed amount of propylparaben internal standard (20 mg/mL). In addition, a 0.3-mg/mL stock solution of analytical-grade degradant (5-oxo-tiagabine, Lot PD10396.1, Cephalon, Inc.) was prepared. Standard samples of degradant were prepared by diluting the stock solution with sample diluent to concentrations of 1.2, 0.9, 0.6 and 0.3 µg/mL. A representative chromatogram with tiagabine, degradant and the internal standard is shown in Figure 1. The standard plots of drug:internal standard peak absorbance area ratios with the drug concentration are shown in Figure 2. Similarly, the standard plot for degradants is shown in Figure 3.

The standard curve was linear (r^sup 2^ > 0.999) over the range of concentrations used for both tiagabine and degradant.

To determine the limit of detection (LOD) and the limit of quantitation (LOQ) values, serial dilutions of tiagabine hydrochloride (HCl) were made from the stock standard solution in the range of 0.195 to 0.96 µg/mL. Samples of 5 µL were injected, and the measured signal from the samples was compared with those of blank samples. The LOD was determined at 15% offset, and the LOQ was determined at 5% offset.

To determine the accuracy and precision of the method, a dilution (0.313 mg/mL) of the tiagabine HCl stock standard solution was prepared; and 5-µL samples were assayed in triplicate for both accuracy and precision. Interday and intraday variability was determined by assaying samples of 0.6 mg/mL tiagabine HCl with 0.02mg/mL propylparaben in sets of six on the same day and different days.

Analysis of Samples

Each bottle of tiagabine was shaken by hand thoroughly for 2.5 minutes immediately prior to assay. A 500-µL sample was taken with a micropipette, and tiagabine was then extracted. Five microliters of each diluted sample was injected into the HPLC system. Each sample was assayed in triplicate at each sampling time. In addition, each sample was evaluated for color, odor and pH.

Data Analysis

The stability of tiagabine in suspension was evaluated by determining the percentage of the initial concentration that remained at each sampling time. The concentrations at each time point were obtained by converting the absorbance from the standard curve shown in Figure 2.

Results

Throughout the 61-day study period, all but the 2-mg/mL suspension stored at 3 to 5°C retained at least 90% of the initial tiagabine concentration or potency (Table 3). No change in odor was noted, and there was no substantial change from the initial pH (3.22 ± 0.08 for 6 mg/mL and 3.82 ± 0.02 for 2 mg/mL) in any bottle or concentration. However, the suspensions stored at 23 to 25°C became slightly discolored after day 30, even though there was no significant degradation of drug. The change in color might have been due to the oxidation of one of the excipients. No visible microbiological or particulate growth was seen. The only change in potency of greater than 10% in any suspension was in the 2-mg/mL suspension stored at 3 to 5°C at day 42. The suspension did retain at least 90% potency ± standard deviation at 40 days.

Discussion

Although the bioavailability of this suspension has not been evaluated, it is unlikely that this formulation made from crushed tablets in suspension would be less than the original dosage form. Approximately 95% of ingested tiagabine is absorbed, with absolute bioavailability approaching 90%.1

Although all suspensions remained stable throughout the study period, (except the 2-mg/mL at 3 to 5°C), the 6-mg/mL preparations were noticeably more bitter than the 2-mg/mL; and the refrigerated suspensions were more palatable, regardless of concentration. Therefore, it is recommended that if taken by mouth, the 2-mg/mL suspension should be used and the suspension should be refrigerated.

The gradient method employed resolved the drug, degradant and internal-standard peaks very well, as shown in Figure 1. The retention times of tiagabine, internal standard and degradant were found to be 8.4, 8.6 and 9.9 minutes, respectively. The limit of detection of tiagabine was found to be 0.39 µg/mL at 15% offset, and the limit of quantitation was 0.48 µg/mL at 5% offset. The intraday and interday variation was found to be minimal, as indicated by a low residual standard deviation, as shown in Table 4.

Conclusion

Extemporaneously compounded liquid oral preparations of tiagabine 6 mg/mL in a 1:1 mixture of Ora-Plus and Ora-Sweet were stable for 61 days at 23 to 25°C and 3 to 5°C. The extemporaneously compounded liquid oral preparations of tiagabine 2 mg/mL in a 1:1 mixture of Ora-Plus and Ora-Sweet were stable for 61 days at 23 to 25°C and for 40 days at 3 to 5°C.

No visible changes were apparent in the refrigerated suspensions. However, because of the slight color change in the suspensions stored at 23 to 25°C after 30 days, it is recommended that the suspensions stored under these conditions not be used after that time period. The validated HPLC method can be used for stability studies of tiagabine preparations.

Recently, another method for extemporaneously compounding tiagabine in methylcellulose and Ora-Plus/Ora Sweet was validated and published.16 However, while this method allows for suspensions of 1 mg/mL, our data show stability for both 2-mg/mL and 6-mg/mL suspensions. This may be useful when volume issues arise.

Acknowledgment

The authors would like to acknowledge Allan Anderson, PharmD, for his valuable assistance in the development of the HPLC procedures.

References

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12. Pellock JM, Deaton R, Shu V et al. Meta-analysis of adjunctive tiagabine efficacy and safety in adolescents in controlled studies. Neurology 1997;48:A335. Abstract.

13. Pellock JM. Tiagabine experience in children. Epilepsia 2001;42 (Suppl 3):49-51.

14. Carta MG, Hardoy MC, Grunze H et al. The use of tiagabine in affective disorders. Pharmacopsychiatry 2002;35:33-34.

15. Pellock JM. Managing pediatric epilepsy syndromes with new antiepileptic drugs. Pediatrics 1999;104:1106-1116.

16. Nahata MC, Morosco RS. Stability of tiagabine in two oral liguid vehicles. Am J Health Syst Pharm 2003;60:75-77.

Mark R. Haase, PharrnD, BCPS

Mansoor A. Khan, RPh, PhD

Texas Tech University Health Sciences Center School of Pharmacy

Jeremiah Bonilla, BS

Texas Tech University Health Sciences Center

Amarillo, Texas

This study was financially supported by the Texas Tech University Health Sciences Center School of Pharmacy Seed Grant Program and by Cephalon, Inc., by the provision of tiagabine analyticalgrade powder and 5-oxo-tiagabine degradant crystals.

Address correspondence to: Mark Haase, PharmD, BCPS, Texas Tech University Health Sciences Center School of Pharmacy, 1200 S. Coulter, Suite 206, Amarillo, TX 79106. E-mail: mhaase@cortex.ama.ttuhsc.edu

Copyright International Journal of Pharmaceutical Compounding Nov/Dec 2003
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