The clinical value of the anthracyclines is reflected in their inclusion in chemotherapy regimens for a wide range of cancers. The realisation of their full benefit, however, is compromised by the occurrence of dose-limiting cardiotoxicity. Three areas of investigation relating to the anthracyclines were pursued: the pharmacokinetics of doxorubicin and its major metabolite, doxorubicinol, in sulphur-crested cockatoos (Cacatua galerita); the pharmacokinetics and single-dose toxicity of this metabolite in dogs; and the use of digitised M-mode echocardiography for the detection of anthracycline cardiotoxicity in paediatric oncology patients.
A novel microscale method was developed for the determination of doxorubicin and doxorubicinol in small volumes (100 µL) of parrot, dog and human plasma, and parrot and dog ultrafiltrate. The method used a protein precipitation-liquid extraction workup and reverse-phase HPLC with fluorescence detection. Separations were performed on a phenyl-hexyl column and daunorubicin was used as the internal standard. All peaks were baseline resolved with no interference. The assay imprecision was ≤ 17.8 % (25 µg/L) and ≤ 5.7 % (250 µg/L). Inaccuracy was ≤ 7.9 % (25 µg/L) and ≤ 2.5 % (250 µg/L). Calibration plots were linear (r2 > 0.998) and recovery was 62 % - 87 % from 20 µg/L to 400 µg/L.
The pharmacokinetics of doxorubicin was investigated in four sulphur-crested cockatoos after a single, 2 mg/kg, 20 min intravenous infusion. The infusion and all associated procedures were tolerated well, with no local or systemic adverse effects observed. Pharmacokinetic analysis was conducted using established non-parametric methods. Considerable inter-subject variability was observed in all pharmacokinetic parameters. The mean ± SD systemic clearance of doxorubicin from plasma (2.7 ± 1.1 L/h/kg) was two to three times higher than reported in humans and rats, but similar to that seen in the dog. The β-phase half-life (1.28 ± 1.01 h) was consistent with values reported in humans and dogs for non-terminal P-phases, while the volume of distribution at steady state (0.24 ±0.13 L/kg) was much smaller. The metabolite, doxorubicinol, was identified in plasma samples from all birds, but reached quantifiable levels only in a single bird whose profile gave limited evidence in support of formation rate-limited kinetics.
In a crossover design with doxorubicin administration, an intravenous infusion of doxorubicinol (2 mg/kg) was administered over 20 min to the same four cockatoos. Each arm of the study was separated by 4-weeks. As for doxorubicin, there were no adverse effects noted as a result of doxorubicinol administration, nor was there any apparent effect of order of administration on pharmacokinetics or toxicity. The mean + SD values for plasma clearance (0.94 ± 0.47 L/h/kg), steady state volume of distribution (0.12 ± 0.05 L/kg) and P-phase half-life (0.66 + 0.61 h) of doxorubicinol were all lower than that of doxorubicin in cockatoos. These observations were consistent with the greater polarity of the metabolite. No evidence of the parent drug was seen in plasma samples after the administration of doxorubicinol.
The administration of doxorubicin (2 mg/kg) to three cockatoos via the intraosseous route was well tolerated and did not result in any apparent adverse effects. The pharmacokinetic profiles were characterised by markedly lower peak plasma concentrations and a shorter β-phase half-life (0.13 ± 0.05 h) than from intravenous administration. Comparison of areas under plasma concentration-time curves after intravenous and intraosseous administration revealed a bioavailability of the intraosseous route of between 6 % and 44 %. Further work is needed to establish and optimise the efficacy of doxorubicin administered intraosseously.
A pharmacokinetic study of doxorubicinol in dogs was conducted in order to isolate its disposition and single-dose toxicity fi-om that of the parent drug. A 20 min intravenous infusion of doxorubicinol (30 mg/m^) administered to four dogs was tolerated well with no overt toxicity. A two-compartment pharmacokinetic model was fitted to the doxorubicinol concentration-time data from each dog using NONMEM. Mean ± SD estimates of clearance (1.18 ± 0.22 L/h/kg), and the volume of distribution of the central (0.14 ± 0.04 L/kg) and peripheral compartments (0.33 ± 0.13 L/kg) and at steady state (0.46 ± 0.17 L/kg) were obtained.The β-phase half-life was 0.79 ±0.11 h. From levels of doxorubicinol in the urine, the renal clearance was determined to be 0.15 ± 0.06 L/h/kg.
A study undertaken into two methods used for cardiac monitoring of anthracycline-treated paediatric oncology patients demonstrated no apparent advantage of digitised M-mode echocardiography over traditional 2-dimensional echocardiography in routine monitoring for cardiotoxicity.
This work has resulted in a new method of measuring doxorubicin and doxorubicinol in parrot, dog and human samples. The pharmacokinetics of doxorubicin (intravenous and intraosseous) in birds has been discerned and paves the way for the establishment and optimisation of avian chemotherapy schedules incorporating this agent.
Information on the pharmacokinetics of, and clinical reaction to, administration of the metabolite, doxorubicinol, in birds and dogs has also been gained and will facilitate the design of future studies into the toxicity of this metabolite.