Veterinary Anaesthesia and Analgesia, 2014
doi:10.1111/vaa.12235
RESEARCH PAPER
Pharmacokinetic profiles of the analgesic flupirtine in dogs after the administration of four pharmaceutical formulations Virginia De Vito*, Beata Łebkowska-Wieruszewska†, Ahmed Shaban‡, Andrzej Lisowski§, Cezary J Kowaski† & Mario Giorgi* *Department of Veterinary Sciences, University of Pisa, San Piero a Grado, Italy †Department of Pharmacology, University of Life Sciences, Lublin, Poland ‡Department of Pharmacology, University of Zagazig, Zagazig, Egypt §Department of Animal Hygiene and Environment, University of Life Sciences, Lublin, Poland
Correspondence: Mario Giorgi, Department of Veterinary Sciences, University of Pisa, 56122 San Piero a Grado, Italy. E-mail: [email protected]
Abstract Objective Flupirtine (FLU) is a non-opioid analgesic with no antipyretic or anti-inflammatory effects which is used in the treatment of pain in humans. There is a substantial body of evidence on the efficacy of FLU in humans but this is inadequate for the recommendation of its off-label use in veterinary clinical practice. The aim of this study was to evaluate the pharmacokinetic profiles of FLU after intravenous (IV), oral immediate release (POIR), oral prolonged release (POPR) and rectal (RC) administrations in healthy dogs. Study design Four-treatment, single-dose, fourphase, unpaired, cross-over design (4 9 4 Latinsquare). Animals Six adult Labrador dogs. Methods Animals in groups 1, 2 and 4 received a single dose of 5 mg kg1 FLU administered by IV, POIR and RC routes. Group 3 received a single dose of 200 mg subject1 via the POPR route. The washout periods were 1 week. Blood samples (1 mL) were collected at assigned times for 48 hours and plasma FLU concentrations were analysed by a validated HPLC method.
Results Adverse effects including salivation, tremors and vomiting were noted in the IV group and resolved spontaneously within 10 minutes. These effects did not occur in the other groups. The FLU plasma concentrations were detectable in all of the treatment groups for 36 hours following administration. The pharmacokinetic profiles after extravascular administrations showed similar trends. The bioavailability values after POIR, POPR and RC were 41.93%, 36.78% and 29.43%, respectively. There were no significant differences in pharmacokinetic profiles between the POIR and POPR formulations. A 5 mg kg1 POIR dose or a 200 mg subject1 POPR dose gave plasma concentrations similar to those reported in humans after clinical dosing. Conclusion and clinical relevance This study provides pharmacokinetic data that can be used to design further studies to investigate FLU in dogs. Keywords analgesic, biopharmaceutics, flupirtine, pharmacokinetics.
dogs,
Introduction Companion animals are living longer and so are suffering from more age-related diseases that can be associated with pain such as cancer, arthritis and 1
Biopharmaceutics of flupirtine in dogs V De Vito et al.
metabolic disorders (Giorgi 2012a). Attentive and effective pain management in dogs facilitates recovery from painful conditions and a quicker return to physiological normality (Lamont 2008). The most popular analgesics licensed for dogs include nonsteroidal anti-inflammatory drugs and opioids (KuKanich 2013). The small number of drugs approved for dogs has been the impetus for the recent movement towards the development of more effective and innovative veterinary therapies (Giorgi & Yun 2012; Giorgi et al. 2012). An increasingly popular solution to the limited range of analgesics for dogs has been off-label drug use (Giorgi 2012b; Giorgi & Owen 2012b) but it is not prudent to extrapolate the use of drugs in one species to another. Pharmacokinetic and pharmacodynamic studies in the target species are important. Flupirtine (FLU) is one compound which has been suggested as having potential for use in veterinary medicine (Giorgi & Owen 2012a). It is an aminopyridine drug (ethyl {2-amino-6-[(4-fluorobenzyl) amino]pyridin-3-yl}carbamate) that was first approved in Europe in 1984 for the treatment of pain in humans (Kumar et al. 2013). FLU is a centrally acting analgesic with a mechanism of action unlike that of opiates. It has no antipyretic or anti-inflammatory effects and is well tolerated (Singal et al. 2012). It is the first drug to be recognised in the unique class of ‘Selective Neuronal Potassium Channel Openers’ (SNEPCO) (Kornhuber et al. 1999). FLU interacts with the G-proteinregulated, Inwardly Rectifying K+ channels (GIRKs), a novel family of K+ channels that are distinct from voltage-dependent K+ channels. They are regulated by neurotransmitters and are expressed in different parts of the brain. FLU activates GIRKs and stabilizes the resting membrane potential by activating KCNQ potassium channels and thus generating a neuronal hyperpolarizing current (M-current). The increased M-current due to the action of FLU decreases neuronal excitability (Kolosov et al. 2012). Moreover, FLU inhibits the NMDA receptor indirectly by acting as an oxidizing agent at the redox site of the NMDA receptor, maintaining the Mg2+ block on the NMDA receptor (Singal et al. 2012). FLU can be useful in the treatment of a wide range of pain states in humans. In line with its mechanism of action promoting neuronal rest, it has proven useful in conditions involving neuronal hyperexcitability such as chronic pain (non-malignant and malignant), migraines and neurogenic pain (Luben et al. 1994; W€ orz et al. 1996; Mueller-Schwefe 2
2003; Ringe et al. 2003; Li et al. 2008; Szelenyi 2013). Its muscle relaxant effects provide additional benefits in painful conditions associated with increased muscle tension, such as musculoskeletal back pain, myofascial pain and tension headaches (W€ orz 1991; W€ orz et al. 1995, 1996; Banerjee et al. 2012; Kumar et al. 2013). FLU has also been shown as beneficial in the short-term treatment of acute pain to pain of a moderate duration such as postoperative pain, trauma and dysmenorrhoea (Heusinger 1987). The approved indications of FLU differ between countries but mainly include the clinical management of the pain states mentioned above. It has possibly not been used to its full potential as an analgesic in the first decade of the 21st century. In recent years, there has been resurgence in FLU use after discovery of its powerful additive effects when used with opioids (Goodchild et al. 2008; Capuano et al. 2011; Kolosov et al. 2012) as well as its properties when used alone (Wilhelmi 2013). There is a substantial body of evidence on the efficacy of FLU in humans however this is inadequate to recommend its off-label use in dogs in veterinary clinical practice (Giorgi & Owen 2012a). The aim of this study was to evaluate the pharmacokinetic profiles of FLU after intravenous (IV), oral immediate release (POIR), oral sustained release (POPR) and rectal (RC) administration in healthy dogs. Materials and methods Animals and experimental design The animal experiment was approved by the animal welfare ethics committee of the University of Lublin (authorization # 62014) and carried out in accordance with the European law (EC council Directive 86/609 EEC). Six adult, intact Labradors, one male and five females, aged between 3 and 6 years, with a body mass in the range of 34–40 kg, were enrolled in the study. The dogs were determined to be clinically healthy based on physical examination and serum chemistry and haematological analyses. Animals were evaluated daily (up to 1 week after the completion of the study) for visible adverse effects by trained personnel. Two weeks after the end of the study the dogs underwent a health-check for physical and behavioural abnormalities. Dogs were randomly assigned to four treatment groups (six slips of paper marked with the numbers 1 to 6 in a box), using an open, single-dose,
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Biopharmaceutics of flupirtine in dogs V De Vito et al. four-treatment, four-phase, unpaired, cross-over design (4 9 4 Latin-square). All dogs were fasted for 10 hours overnight before each experiment. During the first phase, each dog in group 1 (n = 2) received a single dose of 5 mg kg1 FLU (Katadolon 100 mg 3 mL1 vials, FLU D-gluconate; AWD Pharma, Germany) injected IV into the jugular vein with a 1 mL minute1 injection rate. Group 2 (n = 2) received the same dose orally as an IR formulation (Efiret 100 mg hard capsules, FLU maleate, 100 mg; MedaPharma S.p.A., Italy). The doses were prepared by weighing and partitioning the marketed drug. Group 3 (n = 1) received a prolonged release formulation of FLU at 200 mg dog1 orally (Katadolon PR 400 mg Prolonged-Release tablets, FLU maleate, 400 mg; AWD Pharma). The prolonged release tablet was split manually according to its division engraving. Group 4 (n = 1) received FLU at 5 mg kg1 via the rectal (RC) route (Katadolon Z€ apfchen, suppositories, FLU maleate, 150 mg; AWD Pharma, Germany). The suppositories were dipped in cold water for 30 minutes before use to facilitate their division and insertion. The dogs were housed in individual cages and monitored for defecation for at least two hours after insertion of the suppositories to ensure that the drug was fully absorbed. Animals were given the suppository whilst in a prone position. A 1 week wash out period was observed between the four phases, the groups were rotated and the experiment was repeated. At the end of the study, each animal had received all of the formulations. To facilitate blood sampling, 30 minutes before the start of the study, an 18 gauge soft cannula (VasofixBraunule, Luer Lock; B Braun Melsungen AG, Germany) was inserted into the medial saphenous vein, and fixed in place with a cohesive flex wrap bandage (Andover Healthcare, Petflex, MA, USA). Blood samples (1 mL) were collected and transferred to tubes containing lithium heparin at 5, 15, 30, 45 minutes and 1, 1.5, 2, 4, 6, 8, 10, 24, 36 and 48 hours after administration of FLU. Samples were immediately centrifuged at 2000 g for 10 minutes and the harvested plasma was stored at 20 °C until analysis was performed, within 30 days from collection.
acetonitrile (ACN), methanol (MeOH), dichloromethane (CH2Cl2) and ethyl acetate (AcOEt) were purchased from Merck (Germany). Ammonium acetate (AcONH4) was purchased from Carlo Erba (Italy). Deionised water was produced by a Milli-Q Milli-pore Water System (Millipore, MA, USA). All other reagents and materials were of analytical grade and supplied from commercial sources. The LC mobile phase was filtered through 0.2 lm cellulose acetate membrane filters (Sartorius Stedim Biotech S.A., France) with a solvent filtration apparatus. High performance liquid chromatography The analytical method was based on a method previously validated for dog plasma (De Vito et al. 2014a). Briefly, the HPLC system (LC Jasco, Italy) consisted of a quaternary gradient system (PU 980) and an in-line multilambda fluorescence detector (FP 1520). The chromatographic separation assay was performed with a Luna C18(2) analytical column (250 mm 9 4.6 mm inner diameter, 5 lm particle size [Phenomenex, Bologna, Italy]) preceded by a security guard column with the same stationary phase (C18(2) [Phenomenex]). The system was maintained at 25 °C. The mobile phase consisted of ACN:AcONH4 (20 mmol L1) solution, pH 6.8 (60:40, v v1) at a flow rate of 1 mL minute1. Excitation and emission wavelengths were set at 323 and 370 nm, respectively. The elution of the substances was carried out in isocratic mode. Sample extraction The procedure was performed in a 15 mL polypropylene vial. A 500 lL aliquot of plasma was added to 100 lL of IS (100 lg mL1) and vortexed for 60 seconds. Four millilitres of AcOEt:CH2Cl2 (7:3 v v1) was added, then the sample was vortexed (30 seconds), shaken (100 oscillations minute1, 10 minutes) and centrifuged at 3000 g for 10 minutes at 10 °C. Three millilitres of the supernatant was collected and transferred into a separate vial. The organic phase was evaporated under a gentle stream of nitrogen at 40 °C and reconstituted with 500 lL of the mobile phase. Twenty microlitres of this latter solution was injected onto the HPLC-FL.
Chemical and reagents Pure FLU maleate salt and the Internal Standard trazodone (IS) powders (both >99.0% purity) were supplied by Sigma-Aldrich (MO, USA). HPLC grade
Pharmacokinetic evaluation The pharmacokinetic calculations were carried out using WinNonlin v 5.3 (Pharsight Corp, NC,
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
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Figure 1 Individual semi logarithm plasma concentrations of flupirtine versus time curves following 5 mg kg1 IV (-○-), oral immediate release (POIR -●-), and rectal (RC ♢) and 200 mg dog1 oral prolonged release (POPR ♦) administration of flupirtine in six dogs. Letters (a)(f) each represent an individual dog.
USA). The AUC0–∞ was calculated using the log-linear trapezoidal rule. Systemic availability (F%) was calculated from the ratio of the areas under the plasma FLU concentration curve, after each single extravascular route and the respective IV administration, indexed to their respective dose: Fð%Þ ¼ ðAUCPOIR; POPR; RC DoseIV Þ ðAUCIV DosePOIR; POPR; RC Þ1 100 Cmax, the highest observed plasma concentration, and Tmax, the time required to reach Cmax, were obtained from the individual plasma concentration versus time curves. The half-life was calculated from the slope of the logarithm of concentration versus time profile. Changes in plasma FLU concentrations were evaluated by the use of standard noncompartmental analysis, and the relative pharmacokinetic parameters were determined using standard non-compartmental equations (Gabrielsson & Weiner 2002).
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Statistical analysis Pharmacokinetic data were evaluated using the ANOVA test to determine statistically significant differences. Both pharmacokinetic parameters and FLU plasma concentrations are presented as means standard deviation (normality tested by Shapiro– Wilk test). All analyses were conducted using GraphPadInStat (GraphPad Software, La Jolla, CA, USA). In all experiments, differences were considered significant if p < 0.05.
Results There were adverse effects in all dogs in the IV group, including salivation, tremors and vomiting. All the adverse effects resolved spontaneously within 10 minutes. No observable adverse effects were noted in the other groups; physiological signs and parameters were normal.
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Biopharmaceutics of flupirtine in dogs V De Vito et al. 100,000
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Figure 2 Mean semi logarithm plasma concentrations of flupirtine versus time curves following 5 mg kg1 IV (-○-), oral immediate release (POIR -●-), and rectal RC (♢) and 200 mg dog1 oral prolonged release (POPR ♦) administration of flupirtine in six dogs. Bars represent the standard deviations.
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The individual and average plasma concentrations versus time curves after the four administrations are reported in Figs 1a–f & 2, respectively. After IV administration plasma FLU concentration varied widely, especially at the initial time points. FLU was detectable in plasma for up to 36 hours following administration. At 48 hours after administration the drug concentrations dropped below the LOQ of the method. After oral (IR and PR) administrations, the FLU plasma concentrations were lower than after IV administration, but were detectable over the same range of time. After POIR administration, the Cmax (1549.67 916.36 ng mL1) was shown at a Tmax of 1.42 0.58 hours. The POIR bioavailability (F%) was 41.93 8.47%. After POPR administration, the
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pharmacokinetic trend was similar to that reported in the POIR group. After POPR administration, the Cmax (1256.19 353.04 ng mL1) was shown at a Tmax of 2.17 0.93 hours. The POPR bioavailability (F %) was 36.78 8.44%. The lowest Cmax was attained after RC administration (635.34 266.46 ng mL1) and was achieved at a Tmax of 2.17 0.93 hours. The RC bioavailability (F%) was 29.43 8.84%. The terminal phase of all the mean pharmacokinetic curves showed a similar trend of elimination. The half life values did not differ significantly between the treatment groups. The volume of distribution and clearance values were not statistically different between the treatments, after normalization for their F% values. The complete pharmacokinetic parameters are reported in Table 1.
Table 1 Pharmacokinetic parameters of flupirtine after intravenous (IV), oral immediate release (POIR) and rectal (RC) (5 mg kg1) and oral prolonged release (POPR) (200 mg dog1) administrations in healthy dogs (n = 6) IV
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R2 kz T1/2 kz Tmax Cmax AUC0–∞ Vz/F* CL/F* AUMC0–∞ MRT0–∞ MAT F%
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0.98 0.11 6.20 – – 23,614 2089 240.46 102,861 6.18 – –
0.98 0.10 7.49 1.42 1549.67 10,084 6633 604.60 84,654 8.45 2.27 41.93
0.03 0.02 0.88
9122 646 90.52 54,136 1.07
0.01 0.03 1.97 0.58 916.36 4676 4226 289.98 42,011 1.69 0.31 8.47
0.99 0.10 7.08 2.17 1256.19 9885 7390 721.19 88,222 8.38 2.20 36.78
0.01 0.01 0.82 0.93 353.04 5244 4043 388.17 58,959 1.28 0.28 8.44
0.99 0.09 7.78 2.17 635.34 7314 9464 921.25 87,045 10.28 4.10 29.43
0.01 0.02 1.98 0.93 266.46 4790 4157 513.18 89,557 3.18 0.44 8.84
R2, correlation coefficient of the terminal portion of the curve; kz, terminal phase rate constant; T1/2kz, terminal half-life; Tmax, time of peak concentration; Cmax, peak plasma concentration; Vz/F, apparent volume of distribution; CL/F, apparent clearance; AUC0–∞, area under the plasma concentration–time curve; AUMC0–∞, area under the first moment curve; MRT0–∞, mean resident time; MAT, mean absorption time; F%, bioavailability. *For IV administration these values are Vz and CL. © 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
Discussion FLU is a centrally acting, non-opioid analgesic that is available in a number of European countries for the treatment of a variety of pain states (Devulder 2010). The therapeutic benefits seen with FLU relate to its unique pharmacological properties. Recently its potential for use in veterinary medicine has been explored (Giorgi & Owen 2012a). Preclinical studies showed that FLU was more potent than paracetamol and as potent as pentazocine in the electrostimulated pain test in mice (Nickel 1987). FLU significantly prolonged the latency of the tail-flick test in rats (Szelenyi et al. 1989). FLU produced an efficacy profile superior to that of tramadol for clinical and experimentally induced cancer-associated pain (Luben et al. 1994; Kolosov et al. 2012). FLU produced a significant increase in morphine antinociception when the two drugs were administered in combination in different rat models of pain (Goodchild et al. 2008; Capuano et al. 2011). If the opioid sparing effect is also evident in dogs, this active ingredient could play an important role in analgesic protocols and reduce the doses of opioids required. FLU might also be an attractive alternative for dogs with a history of adverse drug reactions to nonsteroidal anti-inflammatory drugs (NSAIDS) (Papich 2008) because in humans it does not induce the gastrointestinal side effects associated with classical NSAIDs or the cardio-/cerebrovascular and renal side effects evoked with chronic therapy with COX-2 selective inhibitors (Treudler et al. 2011). The doses administered in the present study (5 mg kg1 IV, POIR and RC or 200 mg subject1 POPR) were approximately three times higher than the minimum reported in human clinical practice (100 mg subject1). However, they were still within the recommended dose range for humans (100– 400 mg subject1 day1) (Devulder 2010). The rationale for dose selection of 5 mg kg1 was that the ED50 of FLU after oral administration in the electrical tooth pulp stimulation test in dogs was 3.5 mg kg1 (Nickel 1987), and FLU at 5 mg kg1, in association with morphine, increased the antinociceptive activity of morphine four-fold, without increasing the adverse effects (Goodchild et al. 2008; Capuano et al. 2011). Some adverse effects were observed in all the dogs that received FLU by IV injection (salivation, tremors and vomiting). No side effects were reported after administration by the oral or rectal routes. It is
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possible that these adverse effects were associated with the high plasma concentrations that were detected at the first sampling points and when the drug plasma concentrations fell, the adverse effects resolved. The 5 mg kg1 dose did not produce any permanent side effects in any of the dogs in the current study (evaluated up to 1 week after completion of the study), a finding that supports the good safety profile of FLU earlier reported in humans (Friedel & Fitton 1993). However, the potential adverse effects of repeated dosing should be evaluated in dogs. FLU for IV injection is marketed as a d-glucuronide derivative. The plasma concentrations found in this study were slightly higher than those reported in cats (De Vito et al. 2014b) but the drug was detectable over the same range of time (up to 36 hours). No side effects after IV injection (at the same dose and rate of administration used in this study) were evident in cats. The half-life values after IV administration (6.20 hours) were slightly shorter than those following the extravascular administrations (range 7.1–7.8 hours) in dogs although the differences were not statistically significant. These values were in line with the mean terminal plasma elimination half-life in healthy humans which was reported to be approximately 6.5 hours (Abrams et al. 1988), whereas they were about half of the half-life observed in cats (13.6 hours) (De Vito et al. 2014b). The lack of one of the two N-acetyl-transferases enzymes (the NAT2) responsible for the FLU bio-transformation in the N-acetylated analogue D13223 in cats may have played a role in the differences in half-life found between cats and dogs. FLU is a water soluble compound in the form of a maleate salt (pKa 5.3) that is rapidly absorbed from the human gastro-intestinal tract (Klawe & Maschke 2009). The Tmax after POIR in dogs (1.42 hours) is similar to that reported for humans (range 1.6– 1.8 hours) (Abrams et al. 1988) and is shorter than that found in cats (2.78 hours) (De Vito et al. 2014b). This inconsistency could be due to speciesspecific differences. In contrast, the maximum FLU plasma concentration after POIR administration in dogs was almost half of that reported in cats (De Vito et al. 2014b) and humans (Abrams et al. 1988). The POIR bioavailability was similar to that reported in cats (39.3%), but about half of the value reported in humans (90%) (Hlavica & Niebch 1985). Large
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Biopharmaceutics of flupirtine in dogs V De Vito et al. differences in F% between humans and dogs have previously been demonstrated, indicating that F% values derived in dogs should not be extrapolated to humans, and vice versa (Chiou et al. 2000). Modified release formulations could potentially offer a number of advantages in dogs. Firstly, they could ensure that the dosing schedule is simpler and easier to manage because of the reduction in dosing frequency (once daily), with better tolerance and increased compliance from both owner and animal. Secondly, the likelihood of adverse effects due to abrupt peaks in plasma concentrations and relapses of symptoms (due to rapidly decreasing post-peak plasma concentrations) is reduced because of the uniformity of drug plasma concentrations (Leucuta 2012). Surprisingly, the pharmacokinetic profile after POPR was almost identical to that attained after POIR administration and there were no statistically significant differences in the pharmacokinetic parameters between these formulations. The marketed PR formulation of FLU is a tablet in which the active ingredient is dispersed in a polymeric inert matrix. Hence tablet splitting would not have affected the PR drug release. This is further supported by instructions on the package insert to administer half of a tablet (splitting long the engraved line) to patients with liver impairment. Unfortunately no pharmacokinetic studies concerning the FLU PR formulation have been reported in the literature so far and the reason for this behaviour in dogs remains obscure. However, this is not the first time that a PR formulation marketed for humans has as not performed as anticipated in dogs (Giorgi et al. 2009b). As the two oral formulations do not differ in pharmacokinetic characteristics the IR preparation is preferable to the PR preparation because it is less expensive. The main rationale for the use of suppositories in human medicine is the avoidance of the first pass effect caused by high hepatic clearance. FLU is minimally affected by hepatic clearance in humans therefore the main benefit of the RC formulation is a rapid uptake into the systemic circulation as a result of the fatty excipients liquefying at body temperature in the rectum. Another benefit of this formulation is that suppositories can be administered in patients that are difficult to give tablets to (Bradshaw & Price 2007). Rectal administration of 150 mg of FLU in healthy young human volunteers produced a Cmax of 0.89 mg L1 after 5.7 hours with a F% 72.5 (Devulder 2010). In the present study, RC administration of FLU resulted in a lower concentration
(635.34 ng mL1) and an F% of 29.43 (the lowest among the formulations tested). The hepatic clearance of flupirtine in dogs is unknown therefore position of the drug within the rectum could potentially influence bioavailability. Venous drainage of the caudal rectum in dogs occurs via caudal and middle rectal veins through the caudal vena cava, thus bypassing the liver, while cranial rectal veins drain into the liver by way of the portal vein (Evans & De Lahunta 2012). In this study, suppository may often have migrated to the cranial rectum because it was technically difficult to ensure that the suppository remained in the first cm of the caudal rectum. Another specific concern with RC administration of FLU and a likely explanation for the low systemic availability seen in this study is the possibility for sequestration of drug in faecal matter, a general disadvantage of this route for drug administration. These two phenomena might explain the low bioavailability (29.4%) produced by this route of administration. An earlier study reported that suppository formulations (marketed for humans) showed reduced bioavailability in dogs (Giorgi et al. 2009a). Although FLU has been used in the treatment of acute and chronic states in humans for 25 years, no minimal effective concentration for pain relief has been reported to date. However, it is noteworthy that in dogs (despite the low oral F%) a dose of 5 mg kg1 orally (IR and PR) produced plasma FLU concentrations similar to those produced by the POIR clinical dose (100 mg man1) in humans (Hlavica & Niebch 1985). Therefore, if the clinical plasma concentrations achieved in humans are assumed to be as effective in dogs, an oral dose of 5 mg kg1 would be a valid dose to begin efficacy studies with. Conclusion To the authors’ knowlege, this is the first published study reporting the pharmacokinetics of FLU in dogs. The dose of 5 mg kg1 by IV administration resulted in a high initial plasma concentration of FLU which may have been responsible for the adverse effects that were observed. Rectal administration gave the lowest bioavailability, while the oral formulations (IR and PR) were similar, both in pharmacokinetic profiles and parameters. Although the oral administrations gave lower bioavailability values than those produced in humans, a 5 mg kg1 dose of the IR formulation or a 200 mg subject1 dose of the PR
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
formulation resulted in plasma concentrations similar to those reported in humans after clinical dosing. Further studies need to be undertaken to evaluate the clinical efficacy of FLU in dogs. Conflict of interest None of the authors of this paper have a financial or personal relationship with other people or organizations that could inappropriately influence or bias the content of the paper. The authors thank Dr H. Owen (University of Queensland, Australia) for editing the English in the manuscript. References Abrams SM, Baker LR, Crome P et al. (1988) Pharmacokinetics of flupirtine in elderly volunteers and in patients with moderate renal impairment. Postgrad Med J 64, 361–363. Banerjee M, Bhattacharyya K, Sarkar RN et al. (2012) Comparative study of efficacy and tolerability of flupirtine versus tramadol in non-steroidal anti-inflammatory drug intolerant mechanical low back pain. Indian J Rheumatol 7, 135–140. Bradshaw A, Price L (2007) Rectal suppository insertion: the reliability of the evidence as a basis for nursing practice. J Clin Nurs 16, 98–103. Capuano A, De Corato A, Treglia M et al. (2011) Flupirtine antinociception in the rat orofacial formalin test: an analysis of combination therapies with morphine and tramadol. Pharmacol Biochem Behav 97, 544–550. Chiou WL, Jeong HY, Chung SM et al. (2000) Evaluation of using dog as an animal model to study the fraction of oral dose absorbed of 43 drugs in humans. Pharm Res 17, 135–140. De Vito V, Saba A, Owen H et al. (2014a) Bioanalytical method validation and quantification of flupirtine in canine plasma by HPLC with spectrofluorimetric detection. Biomed Chromatogr, BMC-13-0763R2. De Vito V, Łebkowska-Wieruszewska B, Owen H et al. (2014b) Pharmacokinetic profiles of the analgesic drug flupirtine in cats. Vet J DOI: 10.1016/j.tvjl.2014.06.011 Devulder J (2010) Flupirtine in pain management. CNS Drugs 24, 867–881. Evans HE, De Lahunta A (2012) Miller’s Anatomy of the Dog (4th edn). Elsevier, St. Louis, MO. Friedel HA, Fitton A (1993) Flupirtine. A review of its pharmacological properties, and therapeutic efficacy in pain states. Drugs 45, 548–569. Gabrielsson J, Weiner D (2002) Pharmacokinetic and Pharmacodynamic Data Analysis: Concepts and Applications (4th edn). Swedish Pharmaceutical Press, Stockholm, Sweden.
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Giorgi M (2012a) Veterinary pharmacology: is it still pharmacology’s Cinderella? Clin Exp Pharmacol 2, 2. Giorgi M (2012b) Tramadol vs Tapentadol: a new Horizon in pain treatment? Am J Anim Vet Sci 7, 7–11. Giorgi M, Owen H (2012a) Flupirtine: a human drug with potential for use in the veterinary field. Am J Anim Vet Sci 7, 213–217. Giorgi M, Owen H (2012b) Mirtazapine in veterinary medicine a pharmacological rationale for its application in chronic pain. Am J Anim Vet Sci 7, 42–47. Giorgi M, Yun H (2012) Pharmacokinetics of mirtazapine and its main metabolites in Beagle dogs: a pilot study. Vet J 192, 239–241. Giorgi M, Del Carlo S, Saccomanni G et al. (2009a) Pharmacokinetics of tramadol and its major metabolites following rectal and intravenous administration in dogs. N Z Vet J 57, 146–152. Giorgi M, Saccomanni G, Lebkowska-Wieruszewska B et al. (2009b) Pharmacokinetic evaluation of tramadol and its major metabolites after single oral sustained tablet administration in the dog: a pilot study. Vet J 180, 253–255. Giorgi M, Meizler A, Mills PC (2012) Pharmacokinetics of the novel atypical opioid tapentadol following oral and intravenous administration in dogs. Vet J 194, 309–313. Goodchild CS, Kolosov A, Tucker AP et al. (2008) Combination therapy with flupirtine and opioid: studies in rat pain models. Pain Med 9, 928–938. Heusinger JH (1987) Efficacy and tolerance of flupirtine and pentazocine in two multicentre trials. Postgrad Med J 63, 71–79. Hlavica P, Niebch G (1985) Pharmacokinetics and biotransformation of the analgesic flupirtine in humans. Arzneimittel forschung 35, 67–74. Klawe C, Maschke M (2009) Flupirtine: pharmacology and clinical applications of a nonopioid analgesic and potentially neuroprotective compound. Expert Opin Pharmacother 10, 1495–1500. Kolosov A, Goodchild CS, Williams ED et al. (2012) Flupirtine enhances the anti-hyperalgesic effects of morphine in a rat model of prostate bone metastasis. Pain Med 13, 1444–1456. Kornhuber J, Maler M, Wiltfang J et al. (1999) Neuronal potassium cannel opening with flupirtine. Fortschr Neurol Psychiatr 67, 466–475. KuKanich B (2013) Outpatient oral analgesics in dogs and cats beyond non steroidal anti-inflammatory drugs: an evidence-based approach. Vet Clin North Am Small Anim Pract 43, 1109–1125. Kumar R, Keshri UP, Sharma J (2013) Flupirtine: a mini review. J Drug Deliv Ther 3, 113–116. Lamont LA (2008) Multimodal pain management in veterinary medicine: the physiologic basis of pharmacologic therapies. Vet Clin North Am Small Anim Pract 38, 1173–1186.
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Biopharmaceutics of flupirtine in dogs V De Vito et al. Leucuta SE (2012) Drug delivery systems with modified release for systemic and biophase bioavailability. Curr Clin Pharmacol 7, 282–317. Li C, Ni J, Wang Z et al. (2008) Analgesic efficacy and tolerability of flupirtine vs. tramadol in patients with subacute low back pain: a double-blind multicentre trial. Curr Med Res Opin 24, 3523–3530. Luben V, Muller H, Lobisch M et al. (1994) Treatment of tumor pain with flupirtine: results of a double-blind study versus tramadol. Fortschr Med 112, 282–286. Mueller-Schwefe G (2003) Flupirtine in acute and chronic pain associated with muscle tenseness: results of a postmarket surveillance study. Fortschr Med Orig 121, 11– 18. Nickel B (1987) The antinociceptive activity of flupirtine: a structurally new analgesic. Postgrad Med J 63, 19–28. Papich MG (2008) An update on nonsteroidal antiinflammatory drugs (NSAIDs) in small animals. Vet Clin North Am: Small Anim Pract 38, 1243–1266. Ringe JD, Miethe D, Pittrow D (2003) Analgesic efficacy of flupirtine in primary care of patients with osteoporosis related pain: a multivariate analysis. Arzneimittel forschung 53, 496–502. Singal R, Gupta P, Jain N et al. (2012) Role of flupirtine in the treatment of pain – chemistry and its effects. Mædica J Clin Med 7, 163–166.
Szelenyi I (2013) Flupirtine, a re-discovered drug, revisited. Inflamm Res 62, 251–258. Szelenyi I, Nickel B, Borbe HO et al. (1989) Mode of antinociceptive action of flupirtine in the rat. Br J Pharmacol 97, 835–842. Treudler R, Pohle K, Simon JC (2011) Flupirtine is a safe alternative drug in patients with hypersensitivity to NSAIDs. Eur J Clin Pharmacol 67, 961–963. Wilhelmi E (2013) Flupirtine retard – recommended by the German society for pain management practice guideline on low back pain. J Pharmacol Ther 22, 72–73. W€ orz R (1991) Flupirtine in chronic myofascial pain conditions. Fortschr Med 109, 158–160. W€ orz R, Lobisch M, Schwittmann B et al. (1995) Effectiveness of flupirtine in chronic tension headache: results of a double-blind study versus placebo. Fortschr Med 113, 463–468. W€ orz R, Bolten W, Heller B et al. (1996) Flupirtine in comparison with chlormezanone in chronic musculoskeletal back pain: results of a multicenter randomized double-blind study. Fortschr Med 114, 500–504. Received 8 April 2014; accepted 25 July 2014.
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doi:10.1111/vaa.12235
RESEARCH PAPER
Pharmacokinetic profiles of the analgesic flupirtine in dogs after the administration of four pharmaceutical formulations Virginia De Vito*, Beata Łebkowska-Wieruszewska†, Ahmed Shaban‡, Andrzej Lisowski§, Cezary J Kowaski† & Mario Giorgi* *Department of Veterinary Sciences, University of Pisa, San Piero a Grado, Italy †Department of Pharmacology, University of Life Sciences, Lublin, Poland ‡Department of Pharmacology, University of Zagazig, Zagazig, Egypt §Department of Animal Hygiene and Environment, University of Life Sciences, Lublin, Poland
Correspondence: Mario Giorgi, Department of Veterinary Sciences, University of Pisa, 56122 San Piero a Grado, Italy. E-mail: [email protected]
Abstract Objective Flupirtine (FLU) is a non-opioid analgesic with no antipyretic or anti-inflammatory effects which is used in the treatment of pain in humans. There is a substantial body of evidence on the efficacy of FLU in humans but this is inadequate for the recommendation of its off-label use in veterinary clinical practice. The aim of this study was to evaluate the pharmacokinetic profiles of FLU after intravenous (IV), oral immediate release (POIR), oral prolonged release (POPR) and rectal (RC) administrations in healthy dogs. Study design Four-treatment, single-dose, fourphase, unpaired, cross-over design (4 9 4 Latinsquare). Animals Six adult Labrador dogs. Methods Animals in groups 1, 2 and 4 received a single dose of 5 mg kg1 FLU administered by IV, POIR and RC routes. Group 3 received a single dose of 200 mg subject1 via the POPR route. The washout periods were 1 week. Blood samples (1 mL) were collected at assigned times for 48 hours and plasma FLU concentrations were analysed by a validated HPLC method.
Results Adverse effects including salivation, tremors and vomiting were noted in the IV group and resolved spontaneously within 10 minutes. These effects did not occur in the other groups. The FLU plasma concentrations were detectable in all of the treatment groups for 36 hours following administration. The pharmacokinetic profiles after extravascular administrations showed similar trends. The bioavailability values after POIR, POPR and RC were 41.93%, 36.78% and 29.43%, respectively. There were no significant differences in pharmacokinetic profiles between the POIR and POPR formulations. A 5 mg kg1 POIR dose or a 200 mg subject1 POPR dose gave plasma concentrations similar to those reported in humans after clinical dosing. Conclusion and clinical relevance This study provides pharmacokinetic data that can be used to design further studies to investigate FLU in dogs. Keywords analgesic, biopharmaceutics, flupirtine, pharmacokinetics.
dogs,
Introduction Companion animals are living longer and so are suffering from more age-related diseases that can be associated with pain such as cancer, arthritis and 1
Biopharmaceutics of flupirtine in dogs V De Vito et al.
metabolic disorders (Giorgi 2012a). Attentive and effective pain management in dogs facilitates recovery from painful conditions and a quicker return to physiological normality (Lamont 2008). The most popular analgesics licensed for dogs include nonsteroidal anti-inflammatory drugs and opioids (KuKanich 2013). The small number of drugs approved for dogs has been the impetus for the recent movement towards the development of more effective and innovative veterinary therapies (Giorgi & Yun 2012; Giorgi et al. 2012). An increasingly popular solution to the limited range of analgesics for dogs has been off-label drug use (Giorgi 2012b; Giorgi & Owen 2012b) but it is not prudent to extrapolate the use of drugs in one species to another. Pharmacokinetic and pharmacodynamic studies in the target species are important. Flupirtine (FLU) is one compound which has been suggested as having potential for use in veterinary medicine (Giorgi & Owen 2012a). It is an aminopyridine drug (ethyl {2-amino-6-[(4-fluorobenzyl) amino]pyridin-3-yl}carbamate) that was first approved in Europe in 1984 for the treatment of pain in humans (Kumar et al. 2013). FLU is a centrally acting analgesic with a mechanism of action unlike that of opiates. It has no antipyretic or anti-inflammatory effects and is well tolerated (Singal et al. 2012). It is the first drug to be recognised in the unique class of ‘Selective Neuronal Potassium Channel Openers’ (SNEPCO) (Kornhuber et al. 1999). FLU interacts with the G-proteinregulated, Inwardly Rectifying K+ channels (GIRKs), a novel family of K+ channels that are distinct from voltage-dependent K+ channels. They are regulated by neurotransmitters and are expressed in different parts of the brain. FLU activates GIRKs and stabilizes the resting membrane potential by activating KCNQ potassium channels and thus generating a neuronal hyperpolarizing current (M-current). The increased M-current due to the action of FLU decreases neuronal excitability (Kolosov et al. 2012). Moreover, FLU inhibits the NMDA receptor indirectly by acting as an oxidizing agent at the redox site of the NMDA receptor, maintaining the Mg2+ block on the NMDA receptor (Singal et al. 2012). FLU can be useful in the treatment of a wide range of pain states in humans. In line with its mechanism of action promoting neuronal rest, it has proven useful in conditions involving neuronal hyperexcitability such as chronic pain (non-malignant and malignant), migraines and neurogenic pain (Luben et al. 1994; W€ orz et al. 1996; Mueller-Schwefe 2
2003; Ringe et al. 2003; Li et al. 2008; Szelenyi 2013). Its muscle relaxant effects provide additional benefits in painful conditions associated with increased muscle tension, such as musculoskeletal back pain, myofascial pain and tension headaches (W€ orz 1991; W€ orz et al. 1995, 1996; Banerjee et al. 2012; Kumar et al. 2013). FLU has also been shown as beneficial in the short-term treatment of acute pain to pain of a moderate duration such as postoperative pain, trauma and dysmenorrhoea (Heusinger 1987). The approved indications of FLU differ between countries but mainly include the clinical management of the pain states mentioned above. It has possibly not been used to its full potential as an analgesic in the first decade of the 21st century. In recent years, there has been resurgence in FLU use after discovery of its powerful additive effects when used with opioids (Goodchild et al. 2008; Capuano et al. 2011; Kolosov et al. 2012) as well as its properties when used alone (Wilhelmi 2013). There is a substantial body of evidence on the efficacy of FLU in humans however this is inadequate to recommend its off-label use in dogs in veterinary clinical practice (Giorgi & Owen 2012a). The aim of this study was to evaluate the pharmacokinetic profiles of FLU after intravenous (IV), oral immediate release (POIR), oral sustained release (POPR) and rectal (RC) administration in healthy dogs. Materials and methods Animals and experimental design The animal experiment was approved by the animal welfare ethics committee of the University of Lublin (authorization # 62014) and carried out in accordance with the European law (EC council Directive 86/609 EEC). Six adult, intact Labradors, one male and five females, aged between 3 and 6 years, with a body mass in the range of 34–40 kg, were enrolled in the study. The dogs were determined to be clinically healthy based on physical examination and serum chemistry and haematological analyses. Animals were evaluated daily (up to 1 week after the completion of the study) for visible adverse effects by trained personnel. Two weeks after the end of the study the dogs underwent a health-check for physical and behavioural abnormalities. Dogs were randomly assigned to four treatment groups (six slips of paper marked with the numbers 1 to 6 in a box), using an open, single-dose,
© 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia
Biopharmaceutics of flupirtine in dogs V De Vito et al. four-treatment, four-phase, unpaired, cross-over design (4 9 4 Latin-square). All dogs were fasted for 10 hours overnight before each experiment. During the first phase, each dog in group 1 (n = 2) received a single dose of 5 mg kg1 FLU (Katadolon 100 mg 3 mL1 vials, FLU D-gluconate; AWD Pharma, Germany) injected IV into the jugular vein with a 1 mL minute1 injection rate. Group 2 (n = 2) received the same dose orally as an IR formulation (Efiret 100 mg hard capsules, FLU maleate, 100 mg; MedaPharma S.p.A., Italy). The doses were prepared by weighing and partitioning the marketed drug. Group 3 (n = 1) received a prolonged release formulation of FLU at 200 mg dog1 orally (Katadolon PR 400 mg Prolonged-Release tablets, FLU maleate, 400 mg; AWD Pharma). The prolonged release tablet was split manually according to its division engraving. Group 4 (n = 1) received FLU at 5 mg kg1 via the rectal (RC) route (Katadolon Z€ apfchen, suppositories, FLU maleate, 150 mg; AWD Pharma, Germany). The suppositories were dipped in cold water for 30 minutes before use to facilitate their division and insertion. The dogs were housed in individual cages and monitored for defecation for at least two hours after insertion of the suppositories to ensure that the drug was fully absorbed. Animals were given the suppository whilst in a prone position. A 1 week wash out period was observed between the four phases, the groups were rotated and the experiment was repeated. At the end of the study, each animal had received all of the formulations. To facilitate blood sampling, 30 minutes before the start of the study, an 18 gauge soft cannula (VasofixBraunule, Luer Lock; B Braun Melsungen AG, Germany) was inserted into the medial saphenous vein, and fixed in place with a cohesive flex wrap bandage (Andover Healthcare, Petflex, MA, USA). Blood samples (1 mL) were collected and transferred to tubes containing lithium heparin at 5, 15, 30, 45 minutes and 1, 1.5, 2, 4, 6, 8, 10, 24, 36 and 48 hours after administration of FLU. Samples were immediately centrifuged at 2000 g for 10 minutes and the harvested plasma was stored at 20 °C until analysis was performed, within 30 days from collection.
acetonitrile (ACN), methanol (MeOH), dichloromethane (CH2Cl2) and ethyl acetate (AcOEt) were purchased from Merck (Germany). Ammonium acetate (AcONH4) was purchased from Carlo Erba (Italy). Deionised water was produced by a Milli-Q Milli-pore Water System (Millipore, MA, USA). All other reagents and materials were of analytical grade and supplied from commercial sources. The LC mobile phase was filtered through 0.2 lm cellulose acetate membrane filters (Sartorius Stedim Biotech S.A., France) with a solvent filtration apparatus. High performance liquid chromatography The analytical method was based on a method previously validated for dog plasma (De Vito et al. 2014a). Briefly, the HPLC system (LC Jasco, Italy) consisted of a quaternary gradient system (PU 980) and an in-line multilambda fluorescence detector (FP 1520). The chromatographic separation assay was performed with a Luna C18(2) analytical column (250 mm 9 4.6 mm inner diameter, 5 lm particle size [Phenomenex, Bologna, Italy]) preceded by a security guard column with the same stationary phase (C18(2) [Phenomenex]). The system was maintained at 25 °C. The mobile phase consisted of ACN:AcONH4 (20 mmol L1) solution, pH 6.8 (60:40, v v1) at a flow rate of 1 mL minute1. Excitation and emission wavelengths were set at 323 and 370 nm, respectively. The elution of the substances was carried out in isocratic mode. Sample extraction The procedure was performed in a 15 mL polypropylene vial. A 500 lL aliquot of plasma was added to 100 lL of IS (100 lg mL1) and vortexed for 60 seconds. Four millilitres of AcOEt:CH2Cl2 (7:3 v v1) was added, then the sample was vortexed (30 seconds), shaken (100 oscillations minute1, 10 minutes) and centrifuged at 3000 g for 10 minutes at 10 °C. Three millilitres of the supernatant was collected and transferred into a separate vial. The organic phase was evaporated under a gentle stream of nitrogen at 40 °C and reconstituted with 500 lL of the mobile phase. Twenty microlitres of this latter solution was injected onto the HPLC-FL.
Chemical and reagents Pure FLU maleate salt and the Internal Standard trazodone (IS) powders (both >99.0% purity) were supplied by Sigma-Aldrich (MO, USA). HPLC grade
Pharmacokinetic evaluation The pharmacokinetic calculations were carried out using WinNonlin v 5.3 (Pharsight Corp, NC,
© 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
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Figure 1 Individual semi logarithm plasma concentrations of flupirtine versus time curves following 5 mg kg1 IV (-○-), oral immediate release (POIR -●-), and rectal (RC ♢) and 200 mg dog1 oral prolonged release (POPR ♦) administration of flupirtine in six dogs. Letters (a)(f) each represent an individual dog.
USA). The AUC0–∞ was calculated using the log-linear trapezoidal rule. Systemic availability (F%) was calculated from the ratio of the areas under the plasma FLU concentration curve, after each single extravascular route and the respective IV administration, indexed to their respective dose: Fð%Þ ¼ ðAUCPOIR; POPR; RC DoseIV Þ ðAUCIV DosePOIR; POPR; RC Þ1 100 Cmax, the highest observed plasma concentration, and Tmax, the time required to reach Cmax, were obtained from the individual plasma concentration versus time curves. The half-life was calculated from the slope of the logarithm of concentration versus time profile. Changes in plasma FLU concentrations were evaluated by the use of standard noncompartmental analysis, and the relative pharmacokinetic parameters were determined using standard non-compartmental equations (Gabrielsson & Weiner 2002).
4
Statistical analysis Pharmacokinetic data were evaluated using the ANOVA test to determine statistically significant differences. Both pharmacokinetic parameters and FLU plasma concentrations are presented as means standard deviation (normality tested by Shapiro– Wilk test). All analyses were conducted using GraphPadInStat (GraphPad Software, La Jolla, CA, USA). In all experiments, differences were considered significant if p < 0.05.
Results There were adverse effects in all dogs in the IV group, including salivation, tremors and vomiting. All the adverse effects resolved spontaneously within 10 minutes. No observable adverse effects were noted in the other groups; physiological signs and parameters were normal.
© 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia
Biopharmaceutics of flupirtine in dogs V De Vito et al. 100,000
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Figure 2 Mean semi logarithm plasma concentrations of flupirtine versus time curves following 5 mg kg1 IV (-○-), oral immediate release (POIR -●-), and rectal RC (♢) and 200 mg dog1 oral prolonged release (POPR ♦) administration of flupirtine in six dogs. Bars represent the standard deviations.
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The individual and average plasma concentrations versus time curves after the four administrations are reported in Figs 1a–f & 2, respectively. After IV administration plasma FLU concentration varied widely, especially at the initial time points. FLU was detectable in plasma for up to 36 hours following administration. At 48 hours after administration the drug concentrations dropped below the LOQ of the method. After oral (IR and PR) administrations, the FLU plasma concentrations were lower than after IV administration, but were detectable over the same range of time. After POIR administration, the Cmax (1549.67 916.36 ng mL1) was shown at a Tmax of 1.42 0.58 hours. The POIR bioavailability (F%) was 41.93 8.47%. After POPR administration, the
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pharmacokinetic trend was similar to that reported in the POIR group. After POPR administration, the Cmax (1256.19 353.04 ng mL1) was shown at a Tmax of 2.17 0.93 hours. The POPR bioavailability (F %) was 36.78 8.44%. The lowest Cmax was attained after RC administration (635.34 266.46 ng mL1) and was achieved at a Tmax of 2.17 0.93 hours. The RC bioavailability (F%) was 29.43 8.84%. The terminal phase of all the mean pharmacokinetic curves showed a similar trend of elimination. The half life values did not differ significantly between the treatment groups. The volume of distribution and clearance values were not statistically different between the treatments, after normalization for their F% values. The complete pharmacokinetic parameters are reported in Table 1.
Table 1 Pharmacokinetic parameters of flupirtine after intravenous (IV), oral immediate release (POIR) and rectal (RC) (5 mg kg1) and oral prolonged release (POPR) (200 mg dog1) administrations in healthy dogs (n = 6) IV
POIR
POPR
RC
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Mean SD
Parameter
Unit
Mean SD
Mean SD
R2 kz T1/2 kz Tmax Cmax AUC0–∞ Vz/F* CL/F* AUMC0–∞ MRT0–∞ MAT F%
1 hour1 hour hour ng mL1 hour ng mL1 mL kg1 mL hour1 kg1 hour2 ng mL1 hour hour %
0.98 0.11 6.20 – – 23,614 2089 240.46 102,861 6.18 – –
0.98 0.10 7.49 1.42 1549.67 10,084 6633 604.60 84,654 8.45 2.27 41.93
0.03 0.02 0.88
9122 646 90.52 54,136 1.07
0.01 0.03 1.97 0.58 916.36 4676 4226 289.98 42,011 1.69 0.31 8.47
0.99 0.10 7.08 2.17 1256.19 9885 7390 721.19 88,222 8.38 2.20 36.78
0.01 0.01 0.82 0.93 353.04 5244 4043 388.17 58,959 1.28 0.28 8.44
0.99 0.09 7.78 2.17 635.34 7314 9464 921.25 87,045 10.28 4.10 29.43
0.01 0.02 1.98 0.93 266.46 4790 4157 513.18 89,557 3.18 0.44 8.84
R2, correlation coefficient of the terminal portion of the curve; kz, terminal phase rate constant; T1/2kz, terminal half-life; Tmax, time of peak concentration; Cmax, peak plasma concentration; Vz/F, apparent volume of distribution; CL/F, apparent clearance; AUC0–∞, area under the plasma concentration–time curve; AUMC0–∞, area under the first moment curve; MRT0–∞, mean resident time; MAT, mean absorption time; F%, bioavailability. *For IV administration these values are Vz and CL. © 2014 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
Discussion FLU is a centrally acting, non-opioid analgesic that is available in a number of European countries for the treatment of a variety of pain states (Devulder 2010). The therapeutic benefits seen with FLU relate to its unique pharmacological properties. Recently its potential for use in veterinary medicine has been explored (Giorgi & Owen 2012a). Preclinical studies showed that FLU was more potent than paracetamol and as potent as pentazocine in the electrostimulated pain test in mice (Nickel 1987). FLU significantly prolonged the latency of the tail-flick test in rats (Szelenyi et al. 1989). FLU produced an efficacy profile superior to that of tramadol for clinical and experimentally induced cancer-associated pain (Luben et al. 1994; Kolosov et al. 2012). FLU produced a significant increase in morphine antinociception when the two drugs were administered in combination in different rat models of pain (Goodchild et al. 2008; Capuano et al. 2011). If the opioid sparing effect is also evident in dogs, this active ingredient could play an important role in analgesic protocols and reduce the doses of opioids required. FLU might also be an attractive alternative for dogs with a history of adverse drug reactions to nonsteroidal anti-inflammatory drugs (NSAIDS) (Papich 2008) because in humans it does not induce the gastrointestinal side effects associated with classical NSAIDs or the cardio-/cerebrovascular and renal side effects evoked with chronic therapy with COX-2 selective inhibitors (Treudler et al. 2011). The doses administered in the present study (5 mg kg1 IV, POIR and RC or 200 mg subject1 POPR) were approximately three times higher than the minimum reported in human clinical practice (100 mg subject1). However, they were still within the recommended dose range for humans (100– 400 mg subject1 day1) (Devulder 2010). The rationale for dose selection of 5 mg kg1 was that the ED50 of FLU after oral administration in the electrical tooth pulp stimulation test in dogs was 3.5 mg kg1 (Nickel 1987), and FLU at 5 mg kg1, in association with morphine, increased the antinociceptive activity of morphine four-fold, without increasing the adverse effects (Goodchild et al. 2008; Capuano et al. 2011). Some adverse effects were observed in all the dogs that received FLU by IV injection (salivation, tremors and vomiting). No side effects were reported after administration by the oral or rectal routes. It is
6
possible that these adverse effects were associated with the high plasma concentrations that were detected at the first sampling points and when the drug plasma concentrations fell, the adverse effects resolved. The 5 mg kg1 dose did not produce any permanent side effects in any of the dogs in the current study (evaluated up to 1 week after completion of the study), a finding that supports the good safety profile of FLU earlier reported in humans (Friedel & Fitton 1993). However, the potential adverse effects of repeated dosing should be evaluated in dogs. FLU for IV injection is marketed as a d-glucuronide derivative. The plasma concentrations found in this study were slightly higher than those reported in cats (De Vito et al. 2014b) but the drug was detectable over the same range of time (up to 36 hours). No side effects after IV injection (at the same dose and rate of administration used in this study) were evident in cats. The half-life values after IV administration (6.20 hours) were slightly shorter than those following the extravascular administrations (range 7.1–7.8 hours) in dogs although the differences were not statistically significant. These values were in line with the mean terminal plasma elimination half-life in healthy humans which was reported to be approximately 6.5 hours (Abrams et al. 1988), whereas they were about half of the half-life observed in cats (13.6 hours) (De Vito et al. 2014b). The lack of one of the two N-acetyl-transferases enzymes (the NAT2) responsible for the FLU bio-transformation in the N-acetylated analogue D13223 in cats may have played a role in the differences in half-life found between cats and dogs. FLU is a water soluble compound in the form of a maleate salt (pKa 5.3) that is rapidly absorbed from the human gastro-intestinal tract (Klawe & Maschke 2009). The Tmax after POIR in dogs (1.42 hours) is similar to that reported for humans (range 1.6– 1.8 hours) (Abrams et al. 1988) and is shorter than that found in cats (2.78 hours) (De Vito et al. 2014b). This inconsistency could be due to speciesspecific differences. In contrast, the maximum FLU plasma concentration after POIR administration in dogs was almost half of that reported in cats (De Vito et al. 2014b) and humans (Abrams et al. 1988). The POIR bioavailability was similar to that reported in cats (39.3%), but about half of the value reported in humans (90%) (Hlavica & Niebch 1985). Large
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Biopharmaceutics of flupirtine in dogs V De Vito et al. differences in F% between humans and dogs have previously been demonstrated, indicating that F% values derived in dogs should not be extrapolated to humans, and vice versa (Chiou et al. 2000). Modified release formulations could potentially offer a number of advantages in dogs. Firstly, they could ensure that the dosing schedule is simpler and easier to manage because of the reduction in dosing frequency (once daily), with better tolerance and increased compliance from both owner and animal. Secondly, the likelihood of adverse effects due to abrupt peaks in plasma concentrations and relapses of symptoms (due to rapidly decreasing post-peak plasma concentrations) is reduced because of the uniformity of drug plasma concentrations (Leucuta 2012). Surprisingly, the pharmacokinetic profile after POPR was almost identical to that attained after POIR administration and there were no statistically significant differences in the pharmacokinetic parameters between these formulations. The marketed PR formulation of FLU is a tablet in which the active ingredient is dispersed in a polymeric inert matrix. Hence tablet splitting would not have affected the PR drug release. This is further supported by instructions on the package insert to administer half of a tablet (splitting long the engraved line) to patients with liver impairment. Unfortunately no pharmacokinetic studies concerning the FLU PR formulation have been reported in the literature so far and the reason for this behaviour in dogs remains obscure. However, this is not the first time that a PR formulation marketed for humans has as not performed as anticipated in dogs (Giorgi et al. 2009b). As the two oral formulations do not differ in pharmacokinetic characteristics the IR preparation is preferable to the PR preparation because it is less expensive. The main rationale for the use of suppositories in human medicine is the avoidance of the first pass effect caused by high hepatic clearance. FLU is minimally affected by hepatic clearance in humans therefore the main benefit of the RC formulation is a rapid uptake into the systemic circulation as a result of the fatty excipients liquefying at body temperature in the rectum. Another benefit of this formulation is that suppositories can be administered in patients that are difficult to give tablets to (Bradshaw & Price 2007). Rectal administration of 150 mg of FLU in healthy young human volunteers produced a Cmax of 0.89 mg L1 after 5.7 hours with a F% 72.5 (Devulder 2010). In the present study, RC administration of FLU resulted in a lower concentration
(635.34 ng mL1) and an F% of 29.43 (the lowest among the formulations tested). The hepatic clearance of flupirtine in dogs is unknown therefore position of the drug within the rectum could potentially influence bioavailability. Venous drainage of the caudal rectum in dogs occurs via caudal and middle rectal veins through the caudal vena cava, thus bypassing the liver, while cranial rectal veins drain into the liver by way of the portal vein (Evans & De Lahunta 2012). In this study, suppository may often have migrated to the cranial rectum because it was technically difficult to ensure that the suppository remained in the first cm of the caudal rectum. Another specific concern with RC administration of FLU and a likely explanation for the low systemic availability seen in this study is the possibility for sequestration of drug in faecal matter, a general disadvantage of this route for drug administration. These two phenomena might explain the low bioavailability (29.4%) produced by this route of administration. An earlier study reported that suppository formulations (marketed for humans) showed reduced bioavailability in dogs (Giorgi et al. 2009a). Although FLU has been used in the treatment of acute and chronic states in humans for 25 years, no minimal effective concentration for pain relief has been reported to date. However, it is noteworthy that in dogs (despite the low oral F%) a dose of 5 mg kg1 orally (IR and PR) produced plasma FLU concentrations similar to those produced by the POIR clinical dose (100 mg man1) in humans (Hlavica & Niebch 1985). Therefore, if the clinical plasma concentrations achieved in humans are assumed to be as effective in dogs, an oral dose of 5 mg kg1 would be a valid dose to begin efficacy studies with. Conclusion To the authors’ knowlege, this is the first published study reporting the pharmacokinetics of FLU in dogs. The dose of 5 mg kg1 by IV administration resulted in a high initial plasma concentration of FLU which may have been responsible for the adverse effects that were observed. Rectal administration gave the lowest bioavailability, while the oral formulations (IR and PR) were similar, both in pharmacokinetic profiles and parameters. Although the oral administrations gave lower bioavailability values than those produced in humans, a 5 mg kg1 dose of the IR formulation or a 200 mg subject1 dose of the PR
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Biopharmaceutics of flupirtine in dogs V De Vito et al.
formulation resulted in plasma concentrations similar to those reported in humans after clinical dosing. Further studies need to be undertaken to evaluate the clinical efficacy of FLU in dogs. Conflict of interest None of the authors of this paper have a financial or personal relationship with other people or organizations that could inappropriately influence or bias the content of the paper. The authors thank Dr H. Owen (University of Queensland, Australia) for editing the English in the manuscript. References Abrams SM, Baker LR, Crome P et al. (1988) Pharmacokinetics of flupirtine in elderly volunteers and in patients with moderate renal impairment. Postgrad Med J 64, 361–363. Banerjee M, Bhattacharyya K, Sarkar RN et al. (2012) Comparative study of efficacy and tolerability of flupirtine versus tramadol in non-steroidal anti-inflammatory drug intolerant mechanical low back pain. Indian J Rheumatol 7, 135–140. Bradshaw A, Price L (2007) Rectal suppository insertion: the reliability of the evidence as a basis for nursing practice. J Clin Nurs 16, 98–103. Capuano A, De Corato A, Treglia M et al. (2011) Flupirtine antinociception in the rat orofacial formalin test: an analysis of combination therapies with morphine and tramadol. Pharmacol Biochem Behav 97, 544–550. Chiou WL, Jeong HY, Chung SM et al. (2000) Evaluation of using dog as an animal model to study the fraction of oral dose absorbed of 43 drugs in humans. Pharm Res 17, 135–140. De Vito V, Saba A, Owen H et al. (2014a) Bioanalytical method validation and quantification of flupirtine in canine plasma by HPLC with spectrofluorimetric detection. Biomed Chromatogr, BMC-13-0763R2. De Vito V, Łebkowska-Wieruszewska B, Owen H et al. (2014b) Pharmacokinetic profiles of the analgesic drug flupirtine in cats. Vet J DOI: 10.1016/j.tvjl.2014.06.011 Devulder J (2010) Flupirtine in pain management. CNS Drugs 24, 867–881. Evans HE, De Lahunta A (2012) Miller’s Anatomy of the Dog (4th edn). Elsevier, St. Louis, MO. Friedel HA, Fitton A (1993) Flupirtine. A review of its pharmacological properties, and therapeutic efficacy in pain states. Drugs 45, 548–569. Gabrielsson J, Weiner D (2002) Pharmacokinetic and Pharmacodynamic Data Analysis: Concepts and Applications (4th edn). Swedish Pharmaceutical Press, Stockholm, Sweden.
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