The Efficacy and Safety of a Novel Lipophilic Formulation of Methimazole for the Once Daily Transdermal Treatment of Cats with Hyperthyroidism

Radioiodine treatment in hyperthyroid cats: insights into the characteristics of owners and their cats, and owner motivation and perceptions

OBJECTIVES

Radioiodine (131I) therapy is the most appropriate treatment option for many hyperthyroid cats, as it is minimally invasive and often curative. Nevertheless, 131I treatment is not always pursued by owners. Hence, it is important to obtain more insight into owner satisfaction during and after 131I treatment, and their decision-making process. In this study, we describe the characteristics of owners and their hyperthyroid cats referred for 131I therapy, and determine owners' motivation and how they experienced the 131I treatment of their cat.

METHODS

A survey was sent to owners whose cats underwent 131I therapy (n = 1071) between 2010 and 2017 at Ghent University. The survey contained 35 questions with tick-box or free-text answer options concerning family situation, pet insurance, previous therapy, comorbidities, motivation for 131I therapy and owner perception of this treatment.

RESULTS

In total, 438 owners completed 94% or more of the questionnaire. Over half of the cats (55%) had received previous medical, dietary or surgical treatment. Motivations for changing the initial therapy to 131I therapy included difficulties in administering medication (31%), insufficient improvement in clinical signs (23%), side effects (16%) and following the referring veterinarian's advice (16%). Almost a fifth of owners (18%) were not informed about the existence of 131I therapy by their veterinarian and found information on 131I treatment online or through friends. Hospitalising their cat was very distressing for 17% of owners. Most owners (92%) were satisfied with the treatment. Reasons for dissatisfaction were insufficient communication, iatrogenic hypothyroidism, persistent hyperthyroidism and comorbidities post-treatment.

CONCLUSIONS AND RELEVANCE

Our study stresses the importance of communication regarding the possible outcome of 131I treatment, the importance of managing underlying comorbidities before treatment and anticipating the stress of owners during their cat's hospitalisation period. The results of this study could help in improving client communication when advising on 131I treatment.

Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease

PRACTICAL RELEVANCE

Both hyperthyroidism and chronic kidney disease (CKD) are common long-term conditions in older cats, which might be diagnosed concurrently or develop at different times. Hyperthyroidism may mask the presence of CKD, and vice versa, by various mechanisms that are described in this review. Hyperthyroidism treatment options should be carefully considered when CKD has also been diagnosed.

CLINICAL CHALLENGES

Although it can be difficult to diagnose hyperthyroidism and CKD simultaneously, given that one condition may mask the other, it is important to consider the presence of both diseases when examining an older cat presenting with vomiting, weight loss, polyuria/ polydipsia, anorexia or sarcopenia. The concurrent presence of hyperthyroidism and CKD requires careful monitoring of glomerular filtration rate biomarkers, and adequate and prompt support of kidney function when normal thyroid function is re-established. Iatrogenic hypothyroidism is a recognised complication of all of the treatment options for hyperthyroidism, and increases the risk of azotaemia. Therapy with levothyroxine is recommended for cats that are hypothyroid and azotaemic.

EVIDENCE BASE

The information in this review draws on current literature and guidelines related to the pathophysiology, diagnosis and treatment recommendations for feline hyperthyroidism and CKD.

The Prevalence, Magnitude, and Reversibility of Elevated Liver Enzyme Activities in Hyperthyroid Cats Presenting for Iodine-131 Treatment

Objectives

The primary objective of this study was to report the prevalence and magnitude of elevated liver enzyme activity in feline hyperthyroidism using a large cohort of cats presenting for iodine-131 treatment. The secondary objective was to determine if elevated liver enzyme activity was a reversible process following successful iodine-131 treatment.

Methods

Cases that presented for a single iodine-131 treatment were retrospectively reviewed. Short-term and long-term follow-up clinicopathologic data was then reviewed for the secondary objective.

Results

Two hundred seventeen hyperthyroid cats met the inclusion criteria for the primary objective. In total, 123/217 (56.7%) of the cats had at least one liver enzyme elevation on their chemistry panel, with alanine transaminase activity being the most common. All cats who were successfully treated with iodine-131 had liver enzyme activity within the reference range at short-term follow-up and long-term follow-up points.

Conclusion and Relevance

Our study demonstrates that elevated liver values are common in cats presenting for iodine-131 treatment. Additionally, our study demonstrates that even when liver values are markedly elevated prior to treatment, the liver enzyme activity will return to normal after successful resolution of hyperthyroidism using iodine-131 treatment. Investigation into hepatobiliary disease and liver function tests for cats with a diagnosis of hyperthyroidism may be unnecessary as the liver values will likely return to normal with successful iodine-131 treatment.

A pilot study of transdermal gabapentin in cats

Background

Clinical use of gabapentin has increased; transdermal delivery in cats is incompletely studied.

Objective

To evaluate if gabapentin permeates feline skin in vitro and in vivo and to determine if pain scores improve after administration.

Animals

In vitro: cadaver skin from 6 cats; phase 1: 8 young, healthy client‐owned cats; phase 2: 15 client‐owned geriatric cats.

Methods

In vitro, gabapentin applied every q12h to ear or cervical skin in diffusion cells. Samples collected at 0, 2, 4, 12, and 24 hours after application. Phase 1: Cats assigned to 1 of 4 groups: 5 mg/kg or 10 mg/kg applied q8h for 5 days to either ear or cervical skin. Serum samples collected predose, and after 1 and 5 days. Phase 2: 10 mg/kg applied q8h for 5 days. Two validated pain scores recorded predose, and after days 1, 5, and 8. Serum samples collected predose, and after days 1 and 5. Samples were frozen at −80°C for concentration analysis utilizing a validated high‐performance liquid chromatography mass‐spectrometry method.

Results

Gabapentin was identified in all samples. Significant differences in gabapentin concentrations were observed from day 1 to day 5 (P < .02) and in pain scores from predose to day 5 (P < .05) and day 1 to day 5 (P < .05). No differences in pain scores were observed from predose to day 8 (P = .3).

Conclusions and Clinical Relevance

Gabapentin in a transdermal base penetrates feline skin in vitro, is absorbed systemically in cats, and may help decrease pain scores.

Prospective crossover clinical trial comparing transdermal with oral phenobarbital administration in epileptic cats

OBJECTIVES

The aim of this study was to compare serum phenobarbital concentrations, adverse events and client satisfaction during 14 weeks of transdermal vs oral phenobarbital administration to epileptic cats.

METHODS

This was a prospective, fixed-order, crossover pilot study. Nine client-owned cats with presumptive or diagnosed idiopathic epilepsy were enrolled. Oral phenobarbital (PO-PB) was administered for weeks 1-14 (median starting dosage of 3.8 mg/kg [2.0-5.4 mg/kg/day] q12h); transdermal phenobarbital (TD-PB) was administered for weeks 14-28 (median starting dosage 18.8 mg/kg/day [17.6-24.0 mg/kg/day] q12h). Serum phenobarbital concentrations (S-PB) were measured at weeks 2, 14, 16 and 28. Client satisfaction questionnaires and biochemistry were evaluated at 14 and 28 weeks.

RESULTS

Median S-PB concentrations during oral administration were 21 µg/ml (observed range 11-40 µg/ml) at week 2 and 22 µg/ml (8-35 µg/ml) at week 14, and at the higher TD dosage were 18 µg/ml (0-42 µg/ml) at week 16 and 17 µg/ml (7-50 µg/ml) at week 28. Phenobarbital concentrations were significantly correlated with PO dosage at week 2 (r = 0.75, P = 0.03) but not at weeks 16 and 28. Significantly more dose adjustments were needed during the TD phase (P = 0.03), but 6/9 owners (67%) still preferred TD to PO administration. Adverse effects were mild and comparable in both groups.

CONCLUSIONS AND RELEVANCE

Therapeutic S-PB concentrations were achievable in some cats using TD-PB at 18 mg/kg/day q12h. Poor correlation between TD dosage and S-PB concentrations was observed and more dosage adjustments were required during TD administration. These findings necessitate close therapeutic drug monitoring if TD-PB is prescribed.

Assessment of compounded transdermal mirtazapine as an appetite stimulant in cats with chronic kidney disease

Objectives

The aim of this study was to assess the appetite stimulation properties of compounded transdermal mirtazapine (CTM) in cats with chronic kidney disease (CKD).

Methods

Two sequential double-blind placebo-controlled crossover prospective studies were performed in client-owned cats with stable stage 2 or 3 CKD and a history of decreased appetite. In the first study nine CKD cats were randomized to receive 3.75 mg/0.1 ml CTM gel or placebo on the inner pinna every other day for 3 weeks, then, after a 4 day washout period, the cats were crossed over to the alternate 3 week treatment. In a second study, 10 CKD cats were randomized to receive 1.88 mg/0.1 ml CTM or placebo on the same schedule. Physical examination and serum biochemistry were performed before and after each treatment period, and owners kept daily logs of appetite, activity and eating behaviors. Mirtazapine concentrations in CTM gels and steady-state mirtazapine serum concentrations were measured using liquid chromatography/tandem mass spectrometry.

Results

Administration of both 3.75 mg and 1.88 mg CTM resulted in a statistically significant increase in weight (P = 0.002 for both), increase in appetite (P = 0.01 and P = 0.005, respectively), and increase in rate of food consumption (P = 0.03 and P = 0.008, respectively). No significant difference in activity or vocalization was seen at either dose; however, individual cats experienced excessive meowing. Median weight increase for the 3.75 mg arm was 0.22 kg (range 0.04–0.44 kg), while median weight increase for the 1.88 mg arm was 0.26 kg (range –0.25 to 0.5 kg). Improvement in body condition score was seen in 5/9 cats in the 3.75 mg arm (P = 0.04) and 6/10 cats in the 1.88 mg arm (P = 0.004).

Conclusions and relevance

CTM increased appetite and resulted in weight gain in CKD cats despite significant inconsistencies in compounding, and may benefit cats in countries where an approved product is not available.

Serum theophylline after multiple dosing with transdermal gels in cats

OBJECTIVES

Our objectives were, first, to determine if therapeutic serum theophylline concentrations could be achieved using long-term, once-daily dosing of transdermal theophylline and, secondarily, to evaluate the difference between two transdermal theophylline formulations.

METHODS

Seven healthy cats, 1-10 years of age, were evaluated in a two-way, randomized, double-blinded, crossover study. Participants received transdermal theophylline at 15 mg/kg for 21 days in either pluronic lecithin organogel (PLO) or Lipoderm formulation. On day 22, blood was collected 2, 6, 14 and 24 h after dosing. After a 14 day washout period, blood was collected to verify non-detectible theophylline concentrations. The alternate formulation was administered for 21 days, and sampling was repeated. Serum theophylline concentrations were determined using an automated immunoassay. Serum concentrations were compared between formulations using a two-way random-measures ANOVA and over time within a formulation using a repeated-measures ANOVA.

RESULTS

Therapeutic serum theophylline concentrations were achieved for 2/7 cats in each group. Of 56 serum theophylline measurements obtained, only seven (13%) were within the therapeutic range. No significant difference was detected in drug concentrations achieved by the transdermal formulations at any time point. In addition, no significant difference in serum theophylline concentrations was noted between time points for PLO ( P = 0.751) or Lipoderm ( P = 0.107).

CONCLUSIONS AND RELEVANCE

Once-daily transdermal dosing of theophylline does not reliably achieve therapeutic concentrations. Individual cats may achieve therapeutic concentrations. No significant difference was noted between PLO and Lipoderm formulations. Therefore, transdermal theophylline formulations should not be considered as a first-line therapy in feline asthma patients.

Serum levetiracetam concentrations after transdermal levetiracetam administration, 3 times daily, to healthy cats

Background

Repeated oral administration of antiepileptic drugs can be challenging for cat owners, resulting in reduced compliance, poor seizure control, and reduced quality of life for cats. Levetiracetam (LEV) has several properties that make it an appealing drug for transdermal application.

Objectives

The aims were to (1) determine if transdermal LEV, in a lipophilic, liposomic cream vehicle, resulted in serum concentrations above 5 μg/mL; (2) identify clinical adverse effects; and (3) evaluate the concentration of LEV in a lipophilic liposomic cream at set intervals.

Animals

Six healthy, client‐owned cats weighing ≤5 kg.

Methods

Prospective clinical trial. Transdermal LEV was applied to the inner pinna at a dosage of 60 mg/kg (400 mg/mL concentration) at home for 6 days. Day 7, cats were hospitalized for blood sample collection for LEV concentration at times 0 (before dose administration), 0.5, 1, 2, 3, and 4 hours after administration.

Results

Median (range) timed serum concentrations were 16.6 (8.6‐39.6) μg/mL, 16.1 (6.8‐34.4) μg/mL, 15.4 (10.1‐36.7) μg/mL, 17.4 (9.2‐32.7) μg/mL, 15.1 (8.3‐25.9) μg/mL, and 14.8 (11.9‐28.4) μg/mL, respectively. Adverse events were limited to sedation (1/6 cats) and pinna crusting (1/6 cats). The LEV, in the proposed vehicle, retained concentration above 95% at 400 mg/mL up to 5 weeks.

Conclusions and Clinical Importance

Thrice daily transdermal LEV resulted in median serum concentrations ≥5 μg/mL throughout the sampling period and clinical adverse events were minimal. Transdermal LEV can provide an alternative for cats resistant to administration of other forms of anticonvulsant medication.

A double-blind, placebo-controlled, randomized study to evaluate the weight gain drug, mirtazapine transdermal ointment, in cats with unintended weight loss

Mirtazapine is classified as a weight gain drug in cats, and the purpose of this study was to evaluate its efficacy in cats experiencing unintended weight loss. This was a multi-center, double-blind, placebo-controlled, randomized clinical study in client-owned cats ≥1 year of age, weighing ≥2 kg, with a documented loss (≥5%) in body weight. Cats were treated once daily with either 2 mg/cat mirtazapine transdermal ointment (n = 83) or placebo (n = 94) (Per Protocol population) applied to the inner surface of the pinna for 14 ± 3 days. Physical examination, body weight, complete blood count, serum chemistry, and urinalysis were performed prior to treatment and on Day 14. Changes in body weight between the mirtazapine and placebo groups were evaluated from Day 1 to Day 14 and compared using a two-sample t test. The mean percent change in body weight was +3.9% (standard deviation ±5.4%) in the mirtazapine group and +0.4% (±3.3%) in the placebo group (p < 0.0001). The most common adverse event was mild erythema at the application site in 17.4% of placebo and 10.4% of mirtazapine-treated cats. Application of mirtazapine transdermal ointment was well tolerated both topically and systemically and resulted in significant weight gain in cats experiencing unintended weight loss associated with various underlying diseases.

Evaluation of Transdermal Administration of Phenobarbital in Healthy Cats

The purpose was to determine the safety and achievable serum concentrations of transdermally administered phenobarbital in healthy cats. The hypothesis was that transdermal phenobarbital would achieve therapeutic serum concentrations (15-45 µg/mL) with minimal short-term adverse effects. Enrolled cats had normal physical and neurologic exams and unremarkable bloodwork. Transdermal phenobarbital in a pluronic lecithin organogel-based vehicle was administered at a dosage of 3.0-3.1 mg/kg per ear pinna (total of 6.0-6.2 mg/kg) every 12 hr for 14 days. Serum phenobarbital concentrations were measured 3-6 hr after dosing at seven different times over 15 days. The mean and median serum concentration of phenobarbital at study completion were 5.57 and 4.08 µg/mL, respectively. Mean peak concentration and mean time to peak concentration were 5.94 µg/mL and 13.3 days, respectively. Mild adverse effects were observed. Potency was analyzed in three replicates of the transdermal phenobarbital gel administered; potencies ranged from 62.98 to 82.02%. Transdermal application of phenobarbital in healthy cats achieves a detectable, but subtherapeutic, serum concentration and appears safe in the short term. The use of therapeutic drug monitoring is recommended when this formulation of phenobarbital is used to ensure therapeutic serum concentrations are achieved.

Single and multiple dose pharmacokinetics of a novel mirtazapine transdermal ointment in cats

Single and multiple dose pharmacokinetics (PK) of mirtazapine transdermal ointment applied to the inner ear pinna of cats were assessed. Study 1 was a randomized, cross-over single dose study (n = 8). Cats were treated once with 0.5 mg/kg of mirtazapine transdermal ointment applied topically to the inner ear pinna (treatment) or administered orally (control) and then crossed over after washout. Plasma was collected predose and at specified intervals over 96 hr following dosing. Study 2 was a multiple dose study (n = 8). Cats were treated daily for 14 days with 0.5 mg/kg of mirtazapine transdermal ointment applied topically to the inner pinna. Plasma was collected on Day 13 predose and at specified intervals over 96 hr following the final dose. In Study 1, single transdermal administration of mirtazapine resulted in mean T_max = 15.9 hr, C_max = 21.5 ng/mL, AUC_0-24 = 100 ng*hr/mL, AUC_0-∞ = 260 ng*hr/mL and calculated half-life = 26.8 hr. Single oral administration of mirtazapine resulted in mean T_max = 1.1 hr, C_max = 83.1 ng/mL, AUC_0-24 = 377 ng*hr/mL, AUC_0-∞ = 434 ng*hr/mL and calculated half-life = 10.1 hr. Mean relative bioavailability (F) of transdermal to oral dosing was 64.9%. In Study 2, daily application of mirtazapine for 14 days resulted in mean T_max = 2.1 hr, C_max = 39.6 ng/mL, AUC_0-24 = 400 ng*hr/mL, AUC_0-∞ = 647 ng*hr/mL and calculated half-life = 20.7 hr. Single and repeat topical doses of a novel mirtazapine transdermal ointment achieve measurable plasma concentrations in cats.

Assessment of absorption of transdermal ondansetron in normal research cats

Objectives The objective of this study was to assess the absorption of transdermal ondansetron in healthy cats. Methods Five research cats with unremarkable complete blood count, biochemistry and urinalysis were used for both single- and multiple-dose application studies. For single-dose application, 4 mg ondansetron in 0.1 ml Lipoderm gel was applied once to the internal ear pinna. Blood samples were collected via jugular catheter over a 48 h period following administration (0, 15 mins, 30 mins, 1 h, 2 h, 4 h, 8 h, 12 h, 24 h and 48 h). For multiple-dose application, 4 mg ondansetron in 0.1 ml Lipoderm gel was applied for five consecutive days before blood samples were obtained in the same manner. Serum was separated and frozen prior to analysis. Ondansetron was measured via liquid chromatography coupled to tandem mass spectrometry. Results Analysis revealed no clinically relevant drug levels in serum after either single- or multiple-dose administration of 4 mg transdermal ondansetron. Conclusions and relevance Transdermal application of 4 mg ondansetron does not result in clinically relevant serum concentrations of drug. Despite characteristics of the drug that imply suitability for transdermal application, this does not appear to be an acceptable method of drug delivery for this medication at this dose. This study highlights the importance of assessing the suitability of each medication for transdermal administration.

Management and monitoring of hyperthyroid cats: a survey of Australian veterinarians

Objectives This study sought to evaluate how Australian veterinarians approach management and monitoring of feline hyperthyroidism and compare these results with a similar survey recently performed in the UK. Methods An invitation to complete an online survey was sent to veterinarians in all states and territories of Australia. The survey comprised questions relating to management of hyperthyroidism, use of antithyroid drugs vs radioiodine treatment vs surgical thyroidectomy, in addition to demographic information for respondents. Results A total of 546 clinicians completed the survey. The most commonly preferred treatments for long-term management of feline hyperthyroidism were antithyroid medications (305/546; 56%) and radioiodine (210/546; 38%), with substantially more respondents selecting radioiodine when cost was removed as a consideration (425/546; 78%). However, most respondents had treated or referred few cases for radioiodine (median 2). Most veterinarians (500/546; 92%) used antithyroid medications either long term or prior to definitive treatment of hyperthyroidism. For medical management, 45% (244/546) of veterinarians used twice-daily carbimazole. Half of respondents (274/546) aimed to maintain the total thyroxine concentration anywhere within the laboratory reference interval in hyperthyroid cats without chronic kidney disease. Blood pressure monitoring was uncommon. Surgical thyroidectomy was rarely performed. Conclusions and relevance Radioiodine was more frequently preferred by Australian veterinarians compared with those in the UK, likely associated with greater availability, reduced cost and shorter hospitalisation times in this jurisdiction, although antithyroid medications were the most frequently used treatment modality. Barriers remain to its utilisation, however, including perceived cost, misconceptions with regard to expected success rate and accessibility. Recent changes to recommendations on the management and monitoring of hyperthyroid cats do not appear to have been widely adopted by veterinarians at this time.

2016 AAFP Guidelines for the Management of Feline Hyperthyroidism

CLINICAL CONTEXT

Since 1979 and 1980 when the first reports of clinical feline hyperthyroidism (FHT) appeared in the literature, our understanding of the disease has evolved tremendously. Initially, FHT was a disease that only referral clinicians treated. Now it is a disease that primary clinicians routinely manage. Inclusion of the measurement of total thyroxine concentration in senior wellness panels, as well as in diagnostic work-ups for sick cats, now enables diagnosis of the condition long before the cat becomes the classic scrawny, unkempt, agitated patient with a bulge in its neck. However, earlier recognition of the problem has given rise to several related questions: how to recognize the health significance of the early presentations of the disease; how early to treat the disease; whether to treat FHT when comorbid conditions are present; and how to manage comorbid conditions such as chronic kidney disease and cardiac disease with treatment of FHT. The 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism (hereafter referred to as the Guidelines) will shed light on these questions for the general practitioner and suggest when referral may benefit the cat.

SCOPE

The Guidelines explain FHT as a primary disease process with compounding factors, and provide a concise explanation of what we know to be true about the etiology and pathogenesis of the disease.The Guidelines also:Distill the current research literature into simple recommendations for testing sequences that will avoid misdiagnosis and separate an FHT diagnosis into six clinical categories with associated management strategies.Emphasize the importance of treating all hyperthyroid cats, regardless of comorbidities, and outline the currently available treatments for the disease.Explain how to monitor the treated cat to help avoid exacerbating comorbid diseases.Dispel some of the myths surrounding certain aspects of FHT and replace them with an evidence-based narrative that veterinarians and their practice teams can apply to feline patients and communicate to their owners.

EVIDENCE BASE

To help ensure better case outcomes, the Guidelines reflect currently available, evidenced-based knowledge. If research is lacking, or if a consensus does not exist, the expert panel of authors has made recommendations based on their extensive, cumulative clinical experience.

Trans-pinnal movement of methimazole: an in vitro study showing that methimazole can cross from the inner to outer pinna of cats

OBJECTIVES

The aim of the study was to determine if methimazole applied in a transdermal formulation to the internal pinna will cross to the external pinna in an in vitro Franz cell model.

METHODS

The ears from six cats were harvested soon after death. Whole ears were mounted onto Franz-type diffusion cells with the stratum corneum of the inner pinnae uppermost. A commercial transdermal preparation containing methimazole (0.1 ml/10 mg) was applied to the inner pinnae. At 1, 2, 4, 6, 8, 12, 18, 24 and 30 h, a 200 µl sample of reservoir solution was removed to determine the methimazole concentration by high-performance liquid chromatography. The ears were then dissected, separating the internal pinna from the cartilage and the external pinna, before the methimazole concentration was measured at each site. The thickness of the different regions of the ear was measured on paraffin histology sections.

RESULTS

Mean ± SD methimazole concentrations at 30 h for the right and left ear, respectively, were: inner ear, 1.25 ± 0.53 mg/g, 0.39 ± 0.26 mg/g; cartilage, 1.36 ± 0.47 mg/g, 0.33 ± 0.20 mg/g; and outer ear, 1.0 ± 0.32 mg/g, 0.33 ± 0.14 mg/g. There was a difference between the left and right ears (P <0.001). Minimal methimazole concentrations were detected in the receptor fluid. The mean methimazole concentration absorbed by the skin after application of 10 mg was, for the right ear, 3.65 ± 1.27 mg/g and, for the left, 1.08 ± 0.27 mg/g. There was no correlation between methimazole concentrations and thickness of each region of the ear.

CONCLUSIONS AND RELEVANCE

Methimazole in a lipophilic vehicle applied to the inner pinna will penetrate to the outer pinna of cats in an in vitro model, which may have safety implications for humans associated with cats treated with transdermal methimazole. Substantial inter-individual variation was found. Further research is required in the area of transdermal penetration of drugs in cats.

Regional variations in percutaneous absorption of methimazole: an in vitro study on cat skin

The use of transdermal gel medications in cats has become popular in veterinary medicine due to the ease of administration compared to oral medication. The research to support systemic absorption of drugs after transdermal gel administration and the preferred skin region to apply these drugs in cats is limited. The aim of this study was to characterize the effect of different skin regions on the percutaneous absorption pharmacokinetics of a commercially available transdermal methimazole after a finite dose was applied to feline skin in vitro. A commercial formulation of methimazole (10 mg) was applied to four skin regions (the inner stratum corneum of the ear, groin, neck, and thorax regions) from six cats. The receptor medium was sampled up to 36 h postapplication, and methimazole concentrations were measured using high-performance liquid chromatography. Methimazole was absorbed more completely across the pinnal skin, compared to the groin, neck, and thorax (P < 0.001), which justifies application to the pinna to maximize efficacy and also to minimize the effects of grooming.

Percutaneous absorption of methimazole: an in vitro study of the absorption pharmacokinetics for two different vehicles

The use of transdermal medications in cats has become popular in veterinary medicine due to the ease of administration compared to oral medication. However, the research to support systemic absorption of drugs applied to the pinna after transdermal administration in cats is limited. The aim of this study was to characterize the percutaneous absorption pharmacokinetics of methimazole in a lipophilic vehicle compared to methimazole in Pluronic(®) lecithin organogel (PLO) using a finite dose applied to feline ear skin in an in vitro Franz cell model. The two formulations of methimazole (10 mg) were applied to the inner stratum corneum of six pairs of feline ears. The receptor medium was sampled up to 30 h post-administration, and methimazole concentrations were measured using high-performance liquid chromatography (HPLC). Histological examination of all ears was undertaken as small differences in the thickness of ear skin may have contributed to inter-individual differences in methimazole absorption between six cats. Methimazole was absorbed more completely across the pinnal skin when administered in the lipophilic vehicle compared to administration in the PLO gel (P < 0.001).

Diagnosis and management of feline hyperthyroidism: current perspectives

Previous and ongoing research has provided insights to the pathophysiology and diagnosis of hyperthyroidism as well as new treatment modalities. This paper reviews the etiology, clinical presentation, and clinicopathologic changes associated with hyperthyroidism, and provides a thorough explanation of confirmatory testing and treatment options.

Efficacy and safety of methimazole ointment for patients with hyperthyroidism

Oral methimazole has been widely used to treat hyperthyroidism, but its usage is restricted by its adverse systemic effects. The aim of this study was to investigate the efficacy and safety of methimazole ointment for the treatment of hyperthyroidism. One hundred forty-four subjects with hyperthyroidism were initially enrolled. These patients were initially divided into two groups and given the following treatments for 12 weeks: patients in group A received 5% methimazole ointment applied to the skin around the thyroid and an oral placebo; and patients in group B received methimazole tablets and placebo ointment. One hundred thirty-one subjects were included in the final analysis. Therapeutic efficacy was assessed via the levels of free triiodothyronine and thyroxine in the serum and by biweekly monitoring of the symptoms of thyrotoxicosis. Adverse effects were recorded. Fifty-nine (89.40%) patients in group A and 57 (87.69%) patients in group B were euthyroid and experienced alleviation of thyrotoxicosis symptoms (complete control; p>0.05). The median times required to achieve complete control for the patients in the two groups were 6.5 weeks and 6.4 weeks for groups A and B, respectively (p>0.05). Systemic adverse effects (e.g., rash, liver dysfunction, leucopenia, etc.) were significantly less common in group A (1.5%) than in group B (12.3%; p<0.05). This study showed that methimazole ointment has a clinical efficacy similar to that of oral tablets, but methimazole ointment caused fewer systemic adverse effects in patients with hyperthyroidism.

Duration of t4 suppression in hyperthyroid cats treated once and twice daily with transdermal methimazole

BACKGROUND

Transdermal methimazole is an acceptable alternative to oral treatment for hyperthyroid cats. There are, however, no studies evaluating the duration of T4 suppression after transdermal methimazole application. Such information would be valuable for therapeutic monitoring.

OBJECTIVE

To assess variation in serum T4 concentration in hyperthyroid cats after once- and twice-daily transdermal methimazole administration.

ANIMALS

Twenty client-owned cats with newly diagnosed hyperthyroidism.

METHODS

Methimazole was formulated in a pluronic lecithin organogel-based vehicle and applied to the pinna of the inner ear at a starting dose of 2.5 mg/cat q12h (BID group, 10 cats) and 5 mg/cat q24h (SID group, 10 cats). One and 3 weeks after starting treatment, T4 concentrations were measured immediately before and every 2 hours after gel application over a period of up to 10 hours.

RESULTS

Significantly decreased T4 concentrations were observed in week 1 and 3 compared with pretreatment concentrations in both groups. All cats showed sustained suppression of T4 concentration during the 10-hour period, and T4 concentrations immediately before the next methimazole treatment were not significantly different compared with any time point after application, either in the BID or SID groups.

CONCLUSIONS

Because transdermal methimazole application led to prolonged T4 suppression in both the BID and SID groups, timing of blood sampling does not seem to be critical when assessing treatment response.