Abstract

Current drug therapies used for treatment of aspergillosis in avian species have been largely ineffective, primarily because of their fungistatic mechanism of action. Terbinafine (an allylamine) has fungicidal activity and might provide a more efficacious therapeutic option against aspergillosis. Current research efforts are assessing the pharmacokinetics of terbinafine, formulating safe therapeutic dosing regimens, and determining clinical efficacy of this drug for treatment of aspergillosis in African penguins (Spheniscus demersus) and red-tailed hawks (Buteo jamaicensis). Preliminary results are inconclusive at this stage. Subsequent opportunistic field trials to determine clinical efficacy of terbinafine in the treatment of aspergillosis will utilize dosage regimens deemed therapeutic based on pharmacokinetic results in these species.

Introduction

Aspergillosis is the most commonly occurring avian mycotic infection and typically causes chronic, debilitating disease and mortality in all representative species of psittacines, water birds and raptors.¹⁵ A variety of drugs have been used in various combinations for treatment of aspergillosis; however, current treatment regimens are not often effective primarily because of their fungistatic nature.^1,2,4,5,10 Not uncommonly, a chronic, low-grade infection is maintained in individual birds or bird colonies, is exacerbated during times of stress, and then results in mortalities. Terbinafine (Lamisil®, Novartis, New York, NY, USA) was released in 1996 for treatment of mycotic nail infections in human patients,⁹ and pharmacokinetic studies have not been conducted to determine its value as a therapeutic agent in avian species. This drug has reported fungicidal effects in vivo and in vitro against a broad range of fungi, including Aspergillus fumigatus,^11-13 and can be administered orally or topically. Because of these characteristics, terbinafine has excellent therapeutic potential for the treatment of aspergillosis in avian species. Our ongoing research will assess the pharmacokinetics of terbinafine, formulate safe therapeutic dosing regimens, and determine clinical efficacy of this drug for treatment of aspergillosis, specifically in African penguins (Spheniscus demersus) and red-tailed hawks (Buteo jamaicensis).

Methods

Several pharmacokinetic trials have been completed, and clinical field trials will be initiated based on preliminary results. Therapeutic doses for terbinafine (phase I) are being determined by using three different dosages (15, 30 and 60 mg/kg in raptors and 3, 7 and 15 mg/kg in penguins; n=10 birds/group) with a washout period between trials. Terbinafine was given orally to each bird, and blood samples were collected at -5, 15, 30, 45 minutes and 1, 2, 4, 10, 12 and 24 hours post-administration. Optimal dosing frequencies for terbinafine (phase II) are being determined by utilizing dosage requirements based on these results. A specific dosage will be given either once (SID) or twice (BID) daily for a total of four administrations to the same birds after an appropriate 1–2-week washout period (n=10 birds/group). Blood samples will be collected at 2, 4, and 8 hours after each administration, plus 1 hour prior to the next administration to detect peak and trough concentrations from birds (phase II). Plasma is being stored at -4°C until time of analysis by HPLC at the New Bolton Center.

Results and Discussion

Because the MIC range of terbinafine against A. fumigatus in humans is reported to be at 0.02 to 5 µg/ml,^3,6-8,13,14 we a priori defined peak therapeutic serum concentrations in birds as between 2–4 µg/ml. Single-dose pharmacokinetic parameters for raptors were calculated using non-compartmental analysis and demonstrated that terbinafine followed linear pharmacokinetics with peak concentrations of 0.5±0.41 (mean±SD), 1.3±0.39, and 2.7±1.77 mg/L at 7±4.7, 4±1.6, and 7±4.1 hours, respectively, for 15, 30 and 60 mg/kg dosages. The half-life averaged between 20–21 hours for all dosages, and the area under the curve (AUC) was 12.2±3.89, 23.2±7.79, and 44.4±27.0 hours mg/L, respectively, for 15, 30 and 60 mg/kg dosages.

Because the 60 mg/kg terbinafine dosage only resulted in mean serum concentrations of 2.7 mg/L (low end of the range for therapeutic levels we were targeting), another phase I trial using 120 mg/kg was conducted (n=10 birds). Serum concentrations averaged 3.4±0.37 mg/L (range: 1.5–7.4 mg/L) for this dose. However, birds started to regurgitate after multiple doses of 120 mg/kg/day were administered. Recently, 60 mg/kg/day dosages of terbinafine were given to four birds for four days with no apparent adverse responses. We would now like to conduct trials using 60 mg/kg SID for four days (n=10 birds), following the original phase II blood collection protocol. Three birds will then be sacrificed to collect lung, liver and muscle tissues and determine drug concentrations in these tissues during steady state. After a washout period, we will repeat this procedure using 60 mg/kg BID dosages for another three days (n=7 birds). Blood samples will again be drawn, and at least three birds will be sacrificed at the end to quantify tissue levels of terbinafine. We believe that terbinafine concentrations in these tissues might be adequate using 60 mg/kg dosages, even though serum concentrations during single-dose trials did not achieve therapeutic levels as currently defined, because the drug may be sequestered in these tissues. Additionally, it is possible that after a few days of dosing, serum concentrations may rise to the level desired.

Phase I penguin trials are currently being completed at the Seneca Park Zoo in New York. Single-dose pharmacokinetic parameters for 3 mg/kg dosages were calculated using non-compartmental analysis. Peak concentrations reached 0.082±0.06 mg/L at 2.7±0.9 hours post-administration (n=10). The half-life was 24.8±11.4 hours, AUC was 0.909±0.34, and the clearance/fraction of dose absorbed was 3.3±1.0 L/hour (n=4).

Subsequent field trials to determine clinical efficacy of terbinafine in the treatment of aspergillosis (phase III) will utilize dosage regimens deemed therapeutic based on pharmacokinetic results. Opportunistic trials with African penguins will be conducted at SeaWorld facilities in Orlando, FL, San Diego, CA, San Antonio, TX and Aurora, OH as well as at the Oregon Coast Aquarium. Field trials with red-tailed hawks will be conducted at the University of Minnesota Raptor Center. Treatment efficacy will be evaluated by evidence of remission based on evaluation of the same diagnostic parameters utilized (i.e., a combination of 1) history and predisposing factors, 2) radiographs, 3) blood samples for complete blood chemistry and serum chemistry panels, 4) ELISA, and 5) Aspergillus antigen and antibody levels). Efficacious treatment of aspergillosis cases in other avian species might require adjustments to dosing regimens and routes of administration because of differences in drug metabolism among avian species and individual variations in health status.

Acknowledgments

We thank Dr. Steven Brown (Oregon Coast Aquarium) for initial project planning assistance, and the Morris Animal Foundation for funding this research.

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