Drug exposure and clinical effect of transdermal mirtazapine in healthy young cats: a pilot study

Abstract

Objectives

The objective of this study was to measure drug exposure and clinical effects after administration of transdermal mirtazapine (TMZ) in healthy cats.

Methods

Phase I: seven healthy research cats received (1) 3.75 mg and 7.5 mg TMZ once aurally with 48 h serum sampling (serum samples were obtained via the jugular catheter at 0, 0.5, 1, 2, 5, 9, 12, 24, 36 and 48 h); (2) 7.5 mg TMZ and placebo daily aurally for 6 days then 48 h serum sampling; (3) 1.88 mg mirtazapine orally once with serum sampling at 1, 4 and 8 h. Phase II: 20 client-owned cats were enrolled in a randomized, double-blind, placebo-controlled, three-way crossover clinical effect study. Treatments consisted of 6 days of aural 7.5 mg TMZ or placebo gel at home, and 1.88 mg mirtazapine orally once in the clinic. Owners documented appetite, rate of food ingestion, begging activity and vocalization daily at home. On day 6, food consumed, activity and vocalization were documented in hospital, and trough and peak serum mirtazapine levels were obtained. Serum mirtazapine and gel concentrations were measured using liquid chromatography/tandem mass spectrometry.

Results

Phase I: administration of TMZ achieved measureable serum mirtazapine concentrations. Area under the curve_0–48 of multidose 7.5 mg TMZ was significantly higher than single-dose 1.88 mg oral mirtazapine (OMZ) (P = 0.02). Phase II: client-owned cats administered TMZ had a significant increase in appetite (P = 0.003), rate of food ingestion (P = 0.002), activity (P = 0.002), begging (P = 0.002) and vocalization (P = 0.002) at home. In hospital there was a significant increase in food ingested with both TMZ and OMZ compared with placebo (P <0.05). Gel concentrations ranged from 87%–119% of target dose.

Conclusions and relevance

TMZ 7.5 mg daily achieves measureable serum concentrations and produces significant appetite stimulation despite variance in compounded gel concentrations, but side effects denote a lower dose is indicated.

Introduction

Although originally developed as a human antidepressant, mirtazapine has become widely used as an appetite stimulant in veterinary medicine. The mechanism of action for appetite stimulation is not well described and may involve antagonism of the 5HT_2c receptor, which is known for its appetite inhibition activity, as well as antagonism of the H₁ receptor, which also plays a role in appetite regulation.^1,2 Antagonism of the 5-HT₃ receptor, which is an important receptor in the physiology of emesis, is likely responsible for mirtazapine’s antiemetic properties, and may also serve to palliate inappetence. ³

Mirtazapine is frequently used in cats with acute and chronic diseases experiencing dysrexia. Not all cats are amenable to oral administration of medications and this often becomes a source of frustration for a cat owner, which ultimately affects the human–animal bond. The use of mirtazapine in a transdermal gel form is appealing in order to ease stress of application for owners and ultimately benefit more cats. The question then becomes if mirtazapine can be absorbed via the transdermal route. Lipinski’s ‘Rule of Five’ describes the four physiochemical parameters of an oral drug that predict the likelihood of its absorption and permeation. ⁴ Poor absorption or permeation is expected if: (1) more than five hydrogen bond donors are present; (2) the molecular weight (MW) is >500 Daltons; (3) the log P >5 (n-octanol/water partition coefficient, ie, lipophilicity); and (4) >10 hydrogen bond acceptors are present. ⁴

If two of the four parameters are out of range, then poor absorption and permeability is expected. ⁴ In a recent retrospective analysis of various ophthalmic, inhaled and transdermal medications, transdermal delivery systems appear to require more stringent properties for absorption as the stratum corneum is highly selective. ⁵ Based on this retrospective analysis, the investigators statistically developed stricter thresholds of the physiochemical parameters for transdermal drug systems which include the following modifications to the ‘rule of five’: (1) fewer than two hydrogen donors; (2) MW <335 Daltons; (3) log P <5; (4) fewer than five hydrogen acceptors. ⁵ Mirtazapine has no hydrogen donors, is of appropriate size at 265 Daltons, has a coefficient of log P 2.9–3.2 and has only three hydrogen acceptors, making it theoretically suitable to transdermal administration (http://www.drugbank.ca/drugs/DB00370).

The purpose of this study was to assess whether transdermal mirtazapine (TMZ) achieves therapeutic serum concentrations and results in a clinical effect in healthy cats. Specific aims to explore this objective included: (1) to determine whether drug exposure after administration of single dose TMZ (3.75 mg and 7.5 mg) was similar to administration of single-dose oral mirtazapine in healthy research cats; (2) to determine how drug exposure changes after daily administration of TMZ in healthy research cats; (3) to assess the clinical effect of TMZ in client-owned cats in the home and hospital environment; and (4) to compare the drug exposure of client-owned cats to that of research cats after daily administration of TMZ.

Materials and methods

Drug preparation

All mirtazapine and placebo transdermal gels were prepared at the Colorado State University Veterinary Teaching Hospital (CSU VTH) Pharmacy by a single individual. TMZ gels for phase I were prepared using US Pharmacopeial Convention (USP)-grade mirtazapine powder (Sigma-Aldrich) in Lipoderm gel (Professional Compounding Centers of America). Owing to the significant cost of USP-grade powder, subsequent TMZ gels for phase II were prepared using pharmaceutical-grade mirtazapine powder (NDC# 75839-108-00; Batch 131218 certificate of analysis: 99.5%, meets specifications; Attix Pharmaceuticals) in Lipoderm gel. For dosing of oral mirtazapine (OMZ), generic mirtazapine (Aurobindo Pharma) was compounded into a 1.88 mg capsule by the CSU VTH Pharmacy according to the Professional Compounding Centers of America’s protocol. The method used is theoretically guaranteed to produce accurate compounding to within 10% of the target dose.

Phase I

The specific aims of phase I were: (1) to determine whether drug exposure after administration of single-dose TMZ (3.75 mg and 7.5 mg) was similar to administration of single-dose OMZ in healthy research cats; and (2) to determine how drug exposure changes after daily administration of TMZ in healthy research cats. Seven healthy research cats with unremarkable physical examination, complete blood count (CBC), chemistry and urinalysis were used. All portions of the project were approved by the Institutional Care and Use Committee at CSU. For all serum sampling studies a jugular catheter was placed under ketamine/butorphanol sedation (20 mg ketamine/cat IV, 0.1 mg/kg butorphanol IV) 16 h prior to study initiation for ease of sample collection.

Procedures were as follows. (1) TMZ 3.75 mg/0.1 ml was administered once aurally to four cats with 48 h serum sampling. Serum samples were obtained via jugular catheter at 0, 0.5, 1, 2, 5, 9, 12, 24, 36 and 48 h. All four cats received Lipoderm gel compounded with USP mirtazapine powder. (2) TMZ 7.5 mg/0.1 ml was administered once aurally to five cats with 48 h serum sampling. Serum samples were obtained via jugular catheter at 0, 0.5, 1, 2, 5, 9, 12, 24, 36 and 48 h. All five cats received Lipoderm gel compounded with USP mirtazapine powder. (3) Mirtazapine 1.88 mg was administered orally once to seven cats with limited serum sampling at 1, 4 and 8 h. (4) TMZ 7.5 mg/0.1 ml was administered daily aurally, rotating ears, to seven cats for six consecutive days then 48 h serum sampling was performed. Mirtazapine was last administered aurally on the morning the serum sampling started. Serum samples were obtained via jugular catheter at 0, 0.5, 1, 2, 5, 9, 12, 24, 36 and 48 h, and in four cats samples were also obtained at 60 and 72 h. All seven cats had Lipoderm gel compounded with bulk mirtazapine powder. A washout period of at least 1 week was present between each experiment in phase I.

Phase II

The specific aims of phase II were: (1) to assess the clinical effect of TMZ in client-owned cats in the home and hospital environment; and (2) to compare the drug exposure of client-owned cats to that of research cats after daily administration of TMZ. This was a randomized, double-blinded, placebo-controlled, crossover study. Twenty apparently healthy cats recruited from the student pet population with unremarkable physical examination, CBC, chemistry and urinalysis were enrolled. All portions of the project were approved by the Institutional Care and Use Committee at CSU and owners gave informed consent before participation. Each cat received three total treatments in a pre-randomized order: transdermal placebo (TP), TMZ and OMZ with at least a 7 day wash-out period. Randomization was determined by assigning each treatment a number and using an online random order generator to determine each cat’s treatment sequence randomly (List Randomizor; Randomness and Integrity Services [https://www.random.org/lists/]).

For the two transdermal treatments, owners applied 0.1 ml gel containing either placebo or mirtazapine to alternating ears for six consecutive days (without wiping off the ears). Protective gloves were provided and were discarded after each use. Owners completed daily questionnaires to record appetite, activity level, begging and vocalization (Appendix 1 in the supplementary material). On day 7, cats were brought to the CSU VTH. A baseline behavioral assessment was performed by a single blinded observer for each cat (Appendix 2 in the supplementary material). A blood sample was collected for serum mirtazapine concentration. The gel was then applied as described above. After application of the gel, one half cup of the cat’s usual food was weighed with a gram scale (Ohaus Scout Pro SP202) and was provided in the cage. A second blood sample was collected 1 h after gel administration. Behavioral assessments and food weights were performed at 1 h intervals for 8 h by a single blinded observer.

For administration of OMZ, cats were brought to the CSU VTH and a baseline behavioral assessment performed. Cats received 1.88 mg OMZ followed by 3 ml water administered by a syringe. Blood was collected 1 h after OMZ capsule administration for serum mirtazapine concentration. The cat’s pre-weighed one half cup of food was provided after blood collection. Behavioral assessment and food weight was monitored at 1 h intervals for 8 h by a single blinded observer.

Administration of medication for all three treatments was performed by a single individual who was not involved in scoring the cats. Serum alanine aminotransferase (ALT) was performed at the CSU Diagnostic Laboratories on samples from the day of in hospital assessment after daily transdermal mirtazapine administration.

Mirtazapine analysis

Mirtazapine was measured using liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) as previously described. ⁶ The analysis was carried out in the Pharmacology Core at the CSU VTH.

Gel concentration analysis

Mirtazapine concentration was analyzed in all TMZ gels compounded for use in phase I and II studies. Additionally, the mirtazapine concentration of four TMZ gels purchased from commercial compounding pharmacies were analyzed. To confirm transdermal gel concentration, approximately 10 mg dosing solution gel was diluted 1:100 with solution of 50:50 ACN:Milli-Q (v:v). Samples were vortex mixed, sonicated for 5 mins and diluted an additional 1:10,000 with 50:50 ACN:Milli-Q. In total, samples were diluted 1:1 × 10⁻⁶ and quantified using a standard curve of mirtazapine prepared in 50:50, ACN:Milli-Q using the LC/MS/MS method described previously. ⁶

Drug exposure analysis

Owing to the nature of the transdermal medication, standard pharmacokinetic analysis was not possible. Instead, a drug concentration curve was generated and Prism software (GraphPad Software) was used for calculation of area under the curve (AUC)_0–48 from administration of 3.75 mg single-dose TMZ, 7.5 mg single-dose TMZ and 7.5 mg multiple-dose TMZ. For development of a limited-sampling model of mirtazapine exposure following oral administration, the mirtazapine serum concentration vs time data for five cats administered a fixed 1.88 mg oral dose was utilized for calculation of drug exposure (AUC_0–∞) by non-compartmental methods. The resulting AUC values were analyzed as a response to time point mirtazapine concentration values as a predictor by best subset multiple linear regression. This method evaluates all single time points, as well as all possible combinations of multiple time points (2, 3, 4, 5, 6 or 7) as predictors of the outcome (AUC_0–∞). Data utilized in the best subset linear regression analysis were those time points corresponding to post-administration samples and were designated as 15, 30, 60, 240, 480 and 1140 corresponding to the number of minutes the samples were collected after administration. The combination of statistical correlation and number of samples required were considered in choosing the optimal limited-sampling scheme. The results of best subset multiple linear regression revealed that using three points as predictors of AUC_0–∞ (60, 240 and 480 mins) could provide the best combination of statistical correlation (r² = 0.995) while minimizing sample number. The final model utilizing the identified time points is described by the equation:

$$\mathrm{AU}{\mathrm{C}}_{0-\infty }=6870.2+106.4\left({\mathrm{C}}_{60\min }\right)-196.7\left({\mathrm{C}}_{240\min }\right)+1404.9\left({\mathrm{C}}_{480\min }\right)$$

where C_60min, C_240min and C_480min represent the serum concentrations at 60, 240 and 480 mins, respectively, following oral administration. AUC predictions for seven cats administered an oral dose of mirtazapine in phase I (4) were generated with the previously described limited sampling model.

Statistical analysis

Phase I

All regression analysis for the limited-sampling model development was carried out using Minitab v. 16 software. AUC_0–48 from administration of 3.75 mg single, 7.5 mg single and 7.5 mg multidose TMZ and OMZ was compared using Wilcoxon Matched pair test (ANOVA could not be performed as not all cats participated in each category of dose administration).

Phase II

Prism software (GraphPad) was used for analysis. Daily scores were calculated as follows: an increase in appetite or activity, begging or vocalization behaviors received a score of 1, a decrease received a score of −1 and no change received a score of 0. The individual observation scores for the six days were summed, resulting in a total score. Owner observation scores between TMZ and TP were compared using a Wilcoxon matched pair test. Percentage food ingested between TMZ, TP and OMZ while in hospital was compared using a Friedman test with Dunn’s post-hoc comparison. Data were assessed for normality using the D’Agostino and Pearson normality test and were not found to be normally distributed; hence, non-parametric tests were used for analysis. For all analyses, a P value <0.05 was considered to be statistically significant.

Results

Phase I

All seven cats were domestic shorthairs: four neutered males and three spayed females. Ages ranged from 1–2 years (median 2 years) and weights ranged from 3.2–6.2 kg (median 5.0 kg). Mirtazapine concentrations were measurable in serum after all TMZ administrations. TMZ did not display a typical drug concentration curve, but rather maintained a steady-state serum concentration for at least 48 h after administration (Figure 1). In four cats that had serum levels obtained at 60 and 72 h, mirtazapine was still detectable. The AUC_0–48 of multidose 7.5 mg TMZ was significantly higher than single-dose 1.88 mg OMZ (P = 0.02), and approached being significantly higher than single-dose 7.5 mg TMZ (P = 0.06) (Figure 2).There was no significant difference in AUC_0–48 between administration of single-dose 3.75 mg TMZ, single-dose 7.5 mg TMZ and single-dose 1.88 mg OMZ. The median trough and peak serum mirtazapine concentration after multidose 7.5 mg TMZ administration and the median peak serum mirtazapine concentration after single-dose 1.88 mg OMZ are summarized in Table 1.

Figure 1.

Serum mirtazapine concentration curve (± SD) after administration of single-dose 3.75 mg transdermal mirtazapine (TMZ), single-dose 7.5 mg TMZ, single-dose 1.88 mg oral mirtazapine (OMZ) and multiple-dose 7.5 mg TMZ to healthy research cats. Serum concentrations after administration of TMZ were relatively consistent over at least a 48 h period in comparison with OMZ, which shows an initial spike in drug concentration followed by a typical drug concentration curve

Figure 2.

Comparative calculated serum mirtazapine exposure over 48 h after single-dose 3.75 mg transdermal mirtazapine (TMZ), single-dose 7.5 mg TMZ, single-dose 1.88 mg oral mirtazapine (OMZ) and multidose 7.5 mg TMZ in healthy research cats. The area under the curve (AUC)_0–48 of multidose 7.5 mg TMZ was significantly higher than single-dose 1.88 mg OMZ (P = 0.02), and approached being significantly higher than single-dose 7.5 mg TMZ (P = 0.06).There was no significant difference in AUC_0–48 between administration of single-dose 3.75 mg TMZ, single-dose 7.5 mg TMZ and single-dose 1.88 mg OMZ

Table 1.

Comparison of multidose 7.5 mg transdermal mirtazapine (TMZ) and single-dose 1.88 mg oral mirtazapine (OMZ) median trough and peak serum mirtazapine concentrations in healthy cats

Treatment	Median (range) trough (ng/ml)	Median (range) peak (ng/ml)
Healthy research cats
Multidose 7.5 mg TMZ (n = 7)	9.1 (6.3–13.7)	15.2 (7.8–19.8)
Single-dose 1.88 mg OMZ (n = 7)	NA	30.1 (20.1–64.7)
Client-owned cats
Multidose 7.5 mg TMZ (n = 16)	17.5 (3.1–42.5)	37.4 (7.9–86.0)
Single-dose 1.88 mg OMZ (n = 15)	NA	47.3 (0.5–112.0)

NA = not applicable

Overall TMZ gel concentrations for phase I varied by 75–116% of target dose. Gel concentrations of mirtazapine for each treatment and the four commercial compounding pharmacies are summarized in Table 2.

Table 2.

Summary of the average percentage (± SD) and range of mirtazapine gel concentrations in respect to target dose

Gel type and dose	Average (%) ± SD	Range (%)
Phase I  3.75 mg single dose (n = 4)  Lipoderm/USP mirtazapine	106 ± 11	94–116
7.5 mg single dose (n = 5)  Lipoderm/USP mirtazapine	95 ± 18	75–109
7.5 mg multidose (n = 7)  Lipoderm/bulk mirtazapine Phase II  7.5 mg multidose (n = 18)  Lipoderm/bulk mirtazapine	95 ± 1 107 ± 9	87–106 87–119
Commercial pharmacies  Lipoderm (n = 2)  PLO (n = 2)	65 ± 14 66 ± 31	55–75 44–87

USP = US Pharmacopeial Convention; PLO = pluronic lecithin organogel

Phase II

Of the 20 cats enrolled in the clinical effect study 12 were spayed females and eight were neutered males. Breeds included domestic shorthairs (n = 9), domestic longhairs (n = 10) and Siamese (n = 1). Ages ranged from 1–7 years (median 2 years). Weights ranged from 2.86–7.19 kg (median 4.65 kg). Four cats were excluded from blood collection for serum mirtazapine levels but participated in all other aspects of the study: one cat was excluded from blood collection owing to participation in the blood donor program and three cats were excluded owing to demeanor during venepuncture. Owing to incomplete questionnaire data some cats were excluded from some statistical analysis of the home scores. Two cats were excluded from the appetite and activity score analysis, one cat was excluded from the rate of ingestion score analysis and three cats were excluded from the begging score and vocalization analysis.

When 7.5 mg TMZ was given daily at home, there was a statistically significant increase in appetite scores (P = 0.003; Figure 3) and rate of food ingestion scores (P = 0.002; Figure 4). There was also a statistically significant increase in undesired effects such as begging scores (P = 0.002; Figure 5), vocalization scores (P = 0.002; Figure 6) and activity scores (P = 0.002) of TMZ when compared with TP. Two cats were observed by owners to have a significant increase in undesirable food-seeking behavior. While cats were observed in hospital after 6 days of 7.5 mg TMZ administration, there was a statistically significant increase in percentage food ingested compared with TP (P <0.05). A statistically significant increase in percentage food ingested was also seen after administration of OMZ in hospital compared with placebo (P <0.05) (Figure 7). There was no statistically significant difference between TMZ, TP and OMZ for interaction scores and vocalization scores in hospital. Two cats were significantly vocal regardless of treatment arm. No tremors or twitching were observed during the 8 h observation period.

Figure 3.

Appetite score based on daily owner assessment after administering 7.5 mg transdermal mirtazapine (TMZ) or placebo daily for 5 days in a blinded crossover design. There is a statistically significant difference in appetite score between TMZ and placebo (*P = 0.003; n = 18)

Figure 4.

Rate of food ingestion score based on daily owner assessment after administering 7.5 mg transdermal mirtazapine (TMZ) or placebo daily for 5 days in a blinded crossover design. There is a statistically significant difference in rate of food ingestion score between TMZ and placebo (*P = 0.002; n = 19)

Figure 5.

Begging score based on daily owner assessment after administering 7.5 mg transdermal mirtazapine (TMZ) or placebo daily for 5 days in a blinded crossover design. There is a statistically significant difference in begging score between TMZ and placebo (*P = 0.002; n = 17)

Figure 6.

Vocalization score based on daily owner assessment after administering 7.5 mg transdermal mirtazapine (TMZ) or placebo daily for 5 days in a blinded crossover design. There is a statistically significant difference in vocalization score between TMZ and placebo (*P = 0.002; n = 17)

Figure 7.

Comparison of percent food ingested when client-owned cats were observed in the hospital environment after 6 days of transdermal mirtazapine (TMZ) or transdermal placebo and a single oral mirtazapine dose (OMZ). There was a statistically significant difference in percent food ingested between placebo and TMZ, and placebo and OMZ (*P <0.05)

The median trough and peak serum mirtazapine concentrations for client-owned cats after multidose 7.5 mg TMZ administration and median peak serum mirtazapine concentration after single-dose 1.88 mg OMZ are described in Table 1. Overall TMZ gel concentrations for phase II varied by 87–119% of target dose and are detailed for comparison in Table 2. Serum ALT for 17 of the cats after 6 days of daily TMZ administration ranged from 21–97 IU/l (reference interval 30–140 IU/l).

When trough and peak serum mirtazapine concentrations were compared between the healthy research cat multidose 7.5 mg drug exposure study and the client-owned cat multidose 7.5 mg clinical effect study, client-owned cats had significantly higher peak (P = 0.02) and trough (P = 0.02) serum mirtazapine levels (Figure 8). However, the analyzed TMZ gel concentration was also significantly higher in the clinical effect study than in the drug exposure study (P = 0.02).

Figure 8.

Trough and peak serum mirtazapine concentrations compared between the healthy research cat multidose 7.5 mg drug exposure study and the client-owned cat multidose 7.5 mg clinical effect study; client-owned cats had significantly higher peak (*P = 0.02) and trough (*P = 0.02) serum mirtazapine levels

Discussion

The purpose of this pilot study was to determine if mirtazapine is amenable to transdermal application and is effective in stimulating appetite in healthy cats. Results indicated that administration of a single 3.75 mg or 7.5 mg TMZ dose achieved serum concentrations similar to administration of 1.88 mg oral mirtazapine, while repeated daily dosing of 7.5 mg TMZ resulted in significantly higher drug exposure after 6 days. Serum concentrations after administration of TMZ were relatively consistent over at least a 48 h period in comparison with OMZ, which led to an initial spike in serum drug concentration followed by elimination (Figure 1). In the clinical effect study in client-owned cats the efficacy of TMZ as an appetite stimulant was demonstrated by a statistically significant increase in food consumption in both the home and hospital setting in comparison with placebo. At a dose of 7.5 mg given daily for six consecutive days, a significant number of the cats were observed by their owners to have an increase in their appetite and rate of food ingestion. However, cats also showed a significant increase in undesirable side effects such as increased vocalization, begging behavior and increased activity, with two cats subjectively showing a marked increase in their food-seeking behaviors, indicating a lower dose would be more appropriate. Significant variation from target dose was documented in the TMZ gel preparations, highlighting known concerns with compounding medications. However, this study still provides evidence that transdermal mirtazapine could be beneficial for sick cats that are not amenable to pill administration. Additional studies are necessary to determine optimal dosing.

Concerns with the compounding of veterinary and human medications have received a significant amount of press recently, but are not a new concept.^7,8 Consistency and purity of product, bioavailability and efficacy are largely unproven for literally hundreds of transdermal medications potentially available from commercial pharmacies. These issues were certainly prominent in the current study with marked variability in TMZ concentrations both in gels prepared at CSU VTH Pharmacy and by commercial pharmacies. However, once this issue was identified in phase I, phase II gels were able to be prepared much closer to target simply with awareness and attention to the duration of gel mixing during the compounding process. Earlier pilot studies involved making a compounded product with the Food and Drug Administration (FDA)-approved tablet in a pluronic lecithin organogel gel. Although this product achieved measurable serum concentrations, use of tablets resulted in inconsistencies in the gel owing to the percentage of excipient that made both accurate application and analysis of gel concentration challenging. For the purposes of the study and proof of concept it was therefore deemed necessary to use a bulk mirtazapine powder. Current FDA recommendations are that compounding of drugs from bulk substances is acceptable when the FDA-approved product is demonstrated to result in inefficacy. ⁹ However, the USP powder product was prohibitively expensive for large-scale use and could only be used for initial exploration studies. A pharmaceutical-grade powder was located and was successfully and effectively used, but subsequent to the finish of the study this product was recalled for possible penicillin contamination. Therefore, despite the promising efficacy of TMZ in these pilot studies, recommendations for cautious and judicious use (ie, only when no other option exists) of compounded transdermal medications are only strengthened by the study results.

Based on the limited pharmacokinetic data available for other veterinary drugs used for transdermal delivery, it is assumed that the bioequivalence and duration of action is not equivalent to the oral or intravenous form of the drug, owing to some degree of dermal metabolism and properties of the vehicle for delivery. ¹⁰ Previous single-dose pharmacokinetic studies of transdermal medications in cats have shown low bioavailability compared with a single oral dose.^11–13 Previous anecdotal experience with transdermal mirtazapine at the equivalent oral dose of 1.88 mg implied little clinical effect. Therefore, for this study, two higher doses of transdermal mirtazapine, 3.75 mg (equivalent to one-quarter of a 15 mg tablet) and 7.5 mg (equivalent to one-half of a 15 mg tablet) were explored. In addition, single-dose pharmacokinetics are likely not appropriate for evaluating transdermal delivery systems, a concept that was supported by a study assessing transdermal amlodipine in cats where repeated transdermal dosing resulted in a clinical response. ⁸ Therefore, in this study both single-dose and multiple-dose pharmacokinetics were evaluated. It was, indeed, confirmed that a multiple dose regimen significantly increases drug exposure. In retrospect, also assessing repeated dosing of 1.88 mg and 3.75 mg TMZ would have been ideal as, in comparison with other transdermal medications that have been studied, mirtazapine appears to be relatively more bioavailable, even with a single dose.

The dose for the clinical effect study in client-owned cats (7.5 mg) was initially chosen based on the results of the drug-exposure study in healthy research cats. However, the significant increase in mirtazapine serum levels in client-owned cats in comparison with healthy research cats was not anticipated and implies a lower dose would be more appropriate. Lower serum concentrations after daily dosing in healthy research cats could have been present for a number of reasons. Firstly, the presence of ear tattoos in the healthy research cats could have affected drug uptake and this was the reason the design of the clinical effect study in client-owned cats included obtaining serum levels, which, in retrospect, was quite important. Secondly, the blood samples were taken from the healthy research cats via indwelling jugular catheter and from the client-owned cats via direct jugular venepuncture in the hospital setting. Either the excitement of the journey to the hospital and/or the venepuncture process could have resulted in increased aural circulation and increased drug absorption. Lastly, there was a statistically significant difference in the gel concentrations between the two groups which could have resulted in the difference. Regardless of the cause, the end result was that the dose necessary for appetite stimulation was likely overestimated in the clinical effect study.

Future studies aimed at dose titration are warranted to decrease undesirable side effects (vocalization, begging behavior, activity level, obsessive food-seeking behavior). While the doses explored in this study did achieve adequate serum levels and appetite stimulation in healthy cats, side effects seen in the clinical effect study in the home environment indicate that a lower dose would be more appropriate. Additionally, more information is needed to determine the appropriate dose for cats with illnesses and geriatric cats. Cats with chronic kidney disease have been shown to have a prolonged half-life of mirtazapine and therefore require a lower dose and increased dosing interval and the doses used in this pilot study are unlikely to be appropriate for this patient group. ¹⁴ Other factors that could contribute to alterations in transdermal drug delivery include concurrent illness, hydration status and stratum corneum health. These factors may play an additional role when exploring the use of transdermal mirtazapine in geriatric ill cats. ¹⁰

This study has a few limitations which should be taken into account when interpreting the results. Serum samples to determine drug exposure were obtained via jugular catheter or jugular venepuncture and may have been falsely elevated owing to proximity to the ear, a finding described in a recent study of buccal mucosal absorption of buprenorphine utilizing sampling from 10 cm jugular catheters in cats. ¹⁵ In phase I of the current study, significantly longer 32 cm long-line jugular catheters were utilized in the healthy research cats, while direct venepuncture was utilized in client-owned cats in phase II. It is unknown if the longer jugular catheters used in the current study would produce similar biased results as the previous report. However, the difference between use of jugular catheters in the healthy research cats and direct venepuncture in client-owned animals does provide an additional explanation for why drug exposure concentrations may have been higher in client-owned cats. Placement of jugular or other long-line catheters in client-owned cats for the purpose of the study was not clinically feasible.

Phase I procedures were performed as sequential pilot studies and a randomized crossover of treatment arms was not performed. This may have had an unappreciated effect on the results of this portion of the study. Despite non-randomization and the use of multiple sources of mirtazapine, it was decided to include the data gathered in this portion of the study because it demonstrates that transdermal mirtazapine reaches serum concentrations resulting in a clinical effect, regardless of the source of mirtazapine or gel utilized. Current commercially available transdermal mirtazapine gels use a variety of mirtazapine sources and gel types.

Despite repeated efforts of the CSU VTH pharmacy staff and utilization of techniques approved by Professional Compounding Centers of America as standard protocol for gel compounding, not all tested gel samples fell within the stipulated guidelines of 10% of target dose (however, accuracy was superior to products tested from commercial pharmacies). As the purpose of this study was to inform the practitioner of the possible utility of the currently available transdermal mirtazapine gel product, and not technological development, further effort was not made to develop a solution to this problem. The end result of this study was appreciable drug levels in the cat and a significant clinical effect which may be beneficial to patients with no other feasible option for appetite stimulation. Given the inherent variability of delivery methodology (varied syringe dispensing and application with glove to pinna by a pet owner) it should also be considered that testing performed on the gel is likely not representative of what is available for absorption by the cat; transdermal gel application is currently an inexact science, at best.

Conclusions

Mirtazapine is amenable to administration in a transdermal gel and achieves therapeutic serum levels. Despite variability in compounded gel concentrations, transdermal mirtazapine administered at a dose of 7.5 mg daily resulted in significant increases in appetite, but also resulted in undesirable dose-related side effects. It was therefore concluded that the transdermal dose necessary for appetite stimulation was likely overestimated and future dose titration studies for elderly and diseased patient populations are necessary to optimize efficacy, while minimizing side effects.