Amitriptyline accumulation and elimination in Calliphora vicina larvae

Calliphora vicina larvae reared on artificial foodstuffs spiked with human equivalent therapeutic (100 ng/g), toxic (300 ng/g), lethal (500 ng/g), and 10 x lethal (5,000 ng/g) concentrations of amitriptyline and nortriptyline, alone and in various combinations, were harvested at various stages of development and analysed for drug content by high-pressure liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). Mean (range) larval amitriptyline concentrations (ng/g) in larvae reared on foodstuffs containing 100 ng/g, 500 ng/g, and 5,000 ng/g amitriptyline were 3.21 (<1-5.72), 21.3 (14.2-27.4), and 50.1 (38.8-64.3), respectively, on day 5; 6.62 (5.98-7.72), 22.5 (16.4-32.4), and 38 (22.8-50.9) on day 8; and 4.45 (3.45-5.93), 25.2 (18.6-38.4), and 26.2 (22.7-29.7) on day 11. Nortriptyline concentrations (ng/g) in larvae reared on foodstuffs containing 100 ng/g, 500 ng/g, and 5,000 ng/g nortriptyline were 6.86 (4.48-8.96), 14.1 (11.9-17.8), and 18.5 (16.7-20.6), respectively, on day 5; 8.32 (4.9-11.7), 12.9 (11.5-14.2), and 18.8 (11.5-23) on day 8; and 5.06 (3.27-7.25), 19.4 (17.8-22.4), and 26.6 (11.7-44.7) on day 11. Among 45 separate larval rearings fed on the same foodstuff, mean larval weight ranged from 24-96 mg and larval amitriptyline concentration from <1-148 ng/g. Biological variability in larval drug concentrations were greatest in larvae reared on high drug concentrations. Such variability makes quantitative extrapolation back to the drug concentration in foodstuff unreliable. Larval drug accumulation became unpredictable when larvae encounter more than one drug or different concentrations of a single drug. Drug concentrations measured were partly due to surface contamination with drug-rich putrefactive residue and they also depend partly on the analytical method used. Fly larvae are unreliable samples for quantitative toxicological analysis.