Evaluation of the bioavailability and pharmacokinetics of two extended-release theophylline formulations in dogs

Pharmacokinetics of a 503B outsourcing facility-produced theophylline in dogs

Theophylline is an important drug for treatment of canine chronic bronchitis and bradyarrhythmias, but new products require validation since pharmacokinetics in dogs can vary by formulation. A new, 503B outsourcing facility-produced theophylline product (OFT) is available for veterinary use. Outsourcing facilities have many advantages over traditional compounding sources including current good manufacturing practice compliance. The purpose of this study was to establish the pharmacokinetics of OFT in dogs. Eight healthy dogs received 11 mg/kg intravenous aminophylline and 10 mg/kg oral OFT followed by serial blood sampling in a two-way, randomized, crossover design with 7-day washout. Plasma theophylline concentrations were quantified by liquid chromatography-mass spectrometry. Bioavailability, maximum concentration, time to maximum concentration, half-life and area under the curve were: 97 ± 10%, 7.13 ± 0.71 μg/mL, 10.50 ± 2.07 h, 9.20 ± 2.87 h, and 141 ± 37.6 μg*h/mL, respectively. Steady-state predictions supported twice daily dosing of the OFT, but specific dosage recommendations are hindered by lack of a canine-specific therapeutic range for plasma theophylline concentration. These findings suggest that the OFT is well absorbed and can likely be dosed twice daily in dogs, but future pharmacodynamic and clinical studies are needed to establish a definitive therapeutic range for theophylline in this species.

Multidose pharmacokinetics and safety of a modified, compounded theophylline product in dogs

Theophylline is used in canine medicine for the management of chronic bronchitis and bradyarrhythmias, yet no species-validated commercial products are available. This study reports the single-dose and multidose pharmacokinetics and safety of a modified, compounded theophylline (MCT) product readily available from a well-established, USP-compliant compounding pharmacy, which may be a suitable and reliable source for theophylline for dogs. Eleven dogs underwent serial plasma theophylline measurement following 10 mg/kg MCT PO. After a 7 days washout, dogs received 10 mg/kg MCT PO q12h and serial plasma theophylline quantification was repeated after the ninth dose. Dogs were monitored for potential adverse effects. For the nine dogs that completed the study, plasma theophylline concentrations were between 5 and 30 μg/ml for 91 +/- 15% of the dosing interval. There was no significant difference in half-life between single-dose and multidose administration. The most common adverse effects reported were mild and included agitation, excitement, and increased activity. The results of this study support the use of 10 mg/kg MCT administered twice daily as a starting dosage in dogs. This regimen appears safe, achieves appropriate plasma drug concentrations in most dogs, and does not cause significant changes in pharmacokinetic properties at steady state. Because compounded drugs do not undergo consistent testing for identity, quality, strength, purity, and stability, results of research described in reports using compounded products may not be reproducible.

Coughing in Small Animal Patients

Cough is one of the most common complaints in human medicine. In veterinary medicine cough is equally frequent and plays a significant role in an owner's perception of their pet's quality of life. In human and veterinary medicine, therapy for chronic cough is often ineffective. The complexity of the cough pathway and species differences have made it difficult to develop an effective antitussive agent for veterinary patients. The few effective antitussive agents currently available are associated with significant adverse effects. Fortunately, several promising drugs currently being studied in human clinical trials may offer options for use of novel antitussive therapies in small animal patients. This article reviews current understanding about cough pathophysiology, diagnostic strategies used to uncover underlying etiology of cough, and examines available options for controlling cough, including novel antitussive therapies used in human medicine.

A retrospective study of theophylline-based therapy with tracheal collapse in small-breed dogs: 47 cases (2013-2017)

Theophylline acts as a bronchodilator and has an anti-inflammatory effect. In addition, theophylline can be applied in patients where there are concerns regarding the side-effects of corticosteroids. This retrospective case series evaluated theophylline-based therapy in tracheal collapse (TC) canine patients. Forty-seven dogs with TC that received theophylline-based therapy during 2013–2017 were investigated. A fluoroscopic examination was performed to diagnose and grade TC. Theophylline was prescribed (7.5–30 mg/kg PO q12h) and the theophylline serum concentrations were measured. Coughing was assessed using a coughing scoring scale. The mean coughing score decreased after the theophylline-based therapy compared with that observed before treatment. Clinical improvements were observed in 46/47 patients (97.9%). As the intrathoracic TC grading increased, the final theophylline dosage also increased (p value 0.019). The symptom-free period (SFP) with therapy was 189.7 ± 194.45 days (range, 0–720 days) and there was no statistically significant correlation between the SFP and age, sex, or TC grade on fluoroscopy. Although theophylline has generally been used as a third-line treatment, it was used as the main treatment in this study and most patients showed improvements. Dogs have a wider therapeutic index of serum concentrations than humans, and any undesirable effects were easily overcome. With further research, this therapy may prove to be a useful approach, but its safety for long-term use in the treatment of canine TC patients needs to be established.

Pharmacokinetics of a modified, compounded theophylline product in dogs

Theophylline is a commonly used bronchodilator drug for treatment of chronic canine bronchitis, but no formulations validated in dogs are currently available. An oral, modified and compounded theophylline product (MCT), which could fulfil this need, is available through a USP-compliant, veterinary compounding pharmacy; however, its pharmacokinetic properties are unknown. The aim of this study was to determine the pharmacokinetics of MCT. Plasma drug concentrations were measured in seven healthy, fed dogs after single doses of intravenous aminophylline (8.6 mg/kg theophylline equivalent) and oral MCT (10 mg/kg). Systemic bioavailability of the MCT was 96.2 ± 32.9%. MCT times to maximum concentration, mean absorption time and terminal half-life were 8.85 ± 3.63, 6.95 ± 3.42, and 8.67 ± 1.62 hr, respectively. Based on simulations of 10 mg/kg and 12-hr dosing, steady-state plasma theophylline concentrations are expected to exceed the minimum therapeutic concentration for 71.7 ± 35.6% of the dosing interval. Overall, the MCT product investigated showed similar pharmacokinetic characteristics compared to previously validated extended-release theophylline products. An oral dose of 10 mg/kg q 12 hr is likely an appropriate dosage to begin therapy; however, therapeutic drug monitoring may be warranted because of inter-individual variation.

Adenosine receptors mediate the hypoxic ventilatory response but not the hypoxic metabolic response in the naked mole rat during acute hypoxia

Naked mole rats are the most hypoxia-tolerant mammals identified; however, the mechanisms underlying this tolerance are poorly understood. Using whole-animal plethysmography and open-flow respirometry, we examined the hypoxic metabolic response (HMR), hypoxic ventilatory response (HVR) and hypoxic thermal response in awake, freely behaving naked mole rats exposed to 7% O₂ for 1 h. Metabolic rate and ventilation each reversibly decreased 70% in hypoxia (from 39.6 ± 2.9 to 12.1 ± 0.3 ml O₂ min(-1) kg(-1), and 1412 ± 244 to 417 ± 62 ml min(-1) kg(-1), respectively; p < 0.05), whereas body temperature was unchanged and animals remained awake and active. Subcutaneous injection of the general adenosine receptor antagonist aminophylline (AMP; 100 mg kg(-1), in saline), but not control saline injections, prevented the HVR but had no effect on the HMR. As a result, AMP-treated naked mole rats exhibited extreme hyperventilation in hypoxia. These animals were also less tolerant to hypoxia, and in some cases hypoxia was lethal following AMP injection. We conclude that in naked mole rats (i) hypoxia tolerance is partially dependent on profound hypoxic metabolic and ventilatory responses, which are equal in magnitude but occur independently of thermal changes in hypoxia, and (ii) adenosine receptors mediate the HVR but not the HMR.

Applying Biopharmaceutical Classification System (BCS) Criteria to Predict Oral Absorption of Drugs in Dogs: Challenges and Pitfalls

The Biopharmaceutical Classification System (BCS) has been a prognostic tool for assessing the potential effects of formulation on the human drug oral bioavailability. When used in conjunction with in vitro dissolution tests, the BCS can support the prediction of in vivo product performance and the development of mechanistic models that support formulation assessments through the generation of "what if" scenarios. To date, the applicability of existing human BCS criteria has not been evaluated in dogs, thereby limiting its use in canine drug development. Therefore, we examined 50 drugs for which absolute bioavailability (F) was available both in dogs and humans. The drugs were also evaluated for any potential association between solubility (calculated from the dose number, Do) or lipophilicity (LogP) and F in dogs. In humans, solubility is determined in 250 mL of fluid. However, the appropriate volume for classifying drug solubility in dogs has not been established. In this analysis, the estimated volume of a water flush administered to fasted dogs (6 mL) and a volume of 250 mL scaled to a Beagle dog (35 mL) were examined. In addition, in humans, a Do value greater than 1.0 is used to define a compound as highly soluble and a LogP value greater than 1.72 as high permeability. These same criteria were applied for defining highly soluble and highly permeable in dogs. Whether using 35 or 6 mL to determine Do, the canine solubility classification remained unchanged for all but seven compounds. There were no clear associations between a drug's F in dogs and humans or between the canine value of F and either its human BCS classification, its LogP value, or the canine Do estimate. There was a tendency for those drugs with canine values of F equal to or greater than 80% to have LogP values equal to or greater than 1.0. Exceptions to this observation tended to be those compounds known to be absorbed via mechanisms other than passive diffusion (e.g., via transporters or paracellular transporters). Although there are limitations to the approach used in this study, the results of our assessment strongly suggest that the human BCS classification system requires substantial modification before it can be reliably applied to dogs.

Invivo absorption behaviour of theophylline from starch-methyl methacrylate matrix tablets in beagle dogs

This study evaluates in vivo the drug absorption profiles from potato starch-methyl methacrylate matrices(*) using theophylline as a model drug. Healthy beagle dogs under fasting conditions were used for in vivo studies and plasma samples were analyzed by a fluorescence polarization immunoassay analysis (FPIA method). Non-compartmental and compartmental (population approach) analysis was performed to determine the pharmacokinetic parameters. The principle of superposition was applied to predict multiple dose plasma concentrations from experimental single dose data. An in vitro-in vivo correlation (IVIVC) was also assessed. The sustained absorption kinetics of theophylline from these formulations was demonstrated by comparison with two commercially available oral sustained-release theophylline products (Theo-Dur(®) and Theolair(®)). A one-compartment model with first order kinetics without lag-time best describes the absorption/disposition of theophylline from the formulations. Results revealed a theophylline absorption rate in the order FD-HSMMA≥Theo-Dur(®)≥OD-CSMMA>Theolair(®)≥FD-CSMMA. On the basis of simulated plasma theophylline levels, a twice daily dosage (every 12h) with the FD-CSMMA tablets should be recommended. A Level C IVIVC was found between the in vitrot50% and the in vivo AUC/D, although further optimization of the in vitro dissolution test would be needed to adequately correlate with in vivo data.

Canine chronic bronchitis

Chronic bronchitis is a syndrome defined by cough on most days for at least 2 months where no specific cause can be identified. Older small breed dogs are most commonly affected, but bronchitis is also documented in midsized and larger breed dogs. Diagnostic testing includes physical examination, laboratory testing, radiography, and airway evaluation via bronchoscopy, cytology, and culture. Treatment is directed at reducing exposure to irritants, reducing airway inflammation, and controlling cough.

Pharmacokinetics of the cytochrome P-450 substrates phenytoin, theophylline, and diazepam in healthy Greyhound dogs

The purpose of this study was to determine the pharmacokinetics of phenytoin, theophylline, and diazepam in six healthy Greyhound dogs. Additionally, the pharmacokinetics of the diazepam metabolites, oxazepam and nordiazepam, after diazepam administration was determined. Phenytoin sodium (12 mg/kg), aminophylline (10 mg/kg), and diazepam (0.5 mg/kg) were administered IV on separate occasions, and blood was collected at predetermined time points for the quantification of plasma drug concentrations by fluorescence polarization immunoassay (phenytoin, theophylline) or mass spectrometry (diazepam, oxazepam, and nordiazepam). The terminal half-life was 4.9, 9.2, and 1.0 h, respectively, for phenytoin, theophylline, and diazepam, and 6.2 and 2.4 h for oxazepam and nordiazepam after IV diazepam. The clearance was of 2.37, 0.935, and 27.9 mL · min/kg, respectively, for phenytoin, theophylline, and diazepam. The C(MAX) was 44.7 and 305.2 ng/mL for oxazepam and nordiazepam, respectively, after diazepam administration. Temazepam was not detected above 5 ng/mL in any sample after IV diazepam.

Factors influencing the gastric residence of dosage forms in dogs

An appreciation of the variables influencing canine gastric transit time is of interest both because of the push to develop pharmaceutical products that meet the therapeutic needs of the veterinary patient and because of efforts to improve our understanding of the strengths and weaknesses associated with the use of the dog as a preclinical model to support human product development. The gastric transit time of monogastric species is influenced by many factors. Physiological variables include the time of dosing relative to the phase of the interdigestive migrating myoelectric current (IMMC), the sieving properties of the pylorus, the presence or absence of food, and the inherent crushing force of the stomach. Pharmacological factors include particle size, shape and density, drug solubility, and the hardness of the tablet. Despite the importance of understanding the factors influencing gastric residence time in dogs, an in-depth examination of currently available information on this topic has not as yet been published. Therefore, this review provides an examination of each of these factors and their potential impact on canine oral drug absorption characteristics.

Advances in respiratory therapy

Effective respiratory therapy depends on obtaining a definitive diagnosis and following established recommendations for treatment. Unfortunately, many respiratory conditions are idiopathic in origin or are attributable to nonspecific inflammation. In some situations, disorders are controlled rather than cured. Recent advances in pulmonary therapeutics include the use of new agents to treat common diseases and application of local delivery of drugs to enhance drug effect and minimize side effects.

Pharmacokinetics of an extended-release theophylline product in cats

OBJECTIVE

To evaluate the pharmacokinetics of a brand of extended-release theophylline tablets and capsules in healthy cats.

DESIGN

Randomized 3-way crossover study.

ANIMALS

6 healthy cats.

PROCEDURES

A single dose of aminophylline (10 mg/kg [4.5 mg/lb], IV), a 100-mg extended-release theophylline tablet, or a 125-mg extended-release theophylline capsule was administered to all cats. Plasma samples were collected via preplaced central catheters throughout a 36-hour period. Plasma samples were frozen until analyzed by use of a fluorescence polarization monoclonal immunoassay.

RESULTS

All cats tolerated drug administration and plasma collection with no adverse effects. Peak concentrations were reached for both orally administered products between 8 and 12 hours after administration. Bioavailability was excellent. Plasma concentrations were within the human therapeutic concentration of 5 to 20 microg/mL.

CONCLUSIONS AND CLINICAL RELEVANCE

Daily administration of the brand of theophylline tablets and capsules used in this study at 15 mg/kg (6.8 mg/lb) and 19 mg/kg (8.6 mg/lb), respectively, maintained plasma concentrations within the desired therapeutic range in healthy cats.

Syncope secondary to transient atrioventricular block in a German shepherd dog with dilated cardiomyopathy and atrial fibrillation

This case report describes transient atrioventricular block as the etiology for syncopal events in a 6-year-old male German shepherd dog with atrial fibrillation and dilated cardiomyopathy. The arrhythmia diagnosis was obtained via Holter monitoring. Medical treatment with a sustained-release preparation of theophylline, as an additive to the standard congestive heart failure treatment (benazepril, furosemide and pimobendan) may have contributed to temporary remission of the syncopal events. However, the congestive heart failure progressed and the dog was euthanized. Veterinarians should be aware of the possibility of transient atrioventricular block causing syncope in dogs with DCM and AF and should be careful in empirically lowering the ventricular response rate if these dogs present with syncopal episodes.

Pharmacokinetics of morphine and plasma concentrations of morphine-6-glucuronide following morphine administration to dogs

The purpose of this study was to evaluate the pharmacokinetics of morphine and morphine-6-glucuronide (M-6-G) following morphine administered intravenously and orally to dogs in a randomized crossover design. Six healthy 3-4-year-old Beagle dogs were administered morphine sulfate (0.5 mg/kg) as an i.v. bolus and extended release tablets were administered orally as whole tablets (1.6 +/- 0.1 mg/kg) in a randomized crossover design. Plasma concentrations of morphine and M-6-G were determined using high-pressure liquid chromatography and electrochemical coulometric detection. Following i.v. administration all dogs exhibited dysphoria and sedation, and four or six dogs vomited. Mean +/- SE values for half-life, apparent volume of distribution, and clearance after i.v. administration were 1.16 +/- 0.15 h, 4.55 +/- 0.17 L/kg, and 62.46 +/- 10.44 mL/min/kg, respectively. One dog vomited following oral administration and was excluded from the oral analysis. Oral bioavailability was 5% as determined from naïve-averaged analysis. The M-6-G was not detected in any plasma samples following oral or i.v. administration of morphine at a 25 ng/mL the limit of quantification. Computer simulations concluded morphine sulfate administered 0.5 mg/kg intravenously every 2 h would maintain morphine plasma concentrations consistent with analgesic plasma concentrations in humans. Oral morphine is poorly and erratically absorbed in dogs.