The pharmacokinetics of pyrethrins in lactating and non-lactating ewes after oral and subcutaneous administration

Abstract

Pyrethrins were administered orally and subcutaneously (SC) at 150 mg/kg body weight to 10 lactating and non-lactating ewes in a cross over experimental design. A gas chromatographic method was used for analysis of Pyre-thrins in serum and milk samples from the experimental animals. The disposition curves were bi-exponential after first-order absorption and fitted 1 and 2 compartmental models. The maximum plasma concentration (Crnax) of Pyrethrins following oral and SC administra¬tion was 0.08 fig/ml and 0.1} fig/ml respectively with the corresponding time to maximum concentrations (Trnax) being 1 hr and 2 hrs respectively. At 48 hours, serum Pyrethrins levels were below the limit of detec¬tion of 0.005 fig/ml. Absorption was significantly higher for SC route compared to oral routes (p<0.05) but half¬lives (11/2B) were not significantly different for the two routes (p>0.05). The mean residence time (MRT) was 9.7 hours. Total clearance was 4,337 and 3,180 litres/ kg/hr for oral and SC routes respectively. Pyrethrins were secreted in milk at levels of up to 0.005 fig/ml. It was concluded that in ewes, Pyrethrins are rapidly absorbed after orally and SC, widely distributed and eliminated from the body within 24 hours, Secretion into milk occcurs in ewes but the residues in milk are too low to toxic effects in humans.