Experiences with the OECD TG 308 with human pharmaceuticals
The OECD 308 water-sediment transformation test has been routinely conducted in Phase II Tier A testing of the environmental risk assessment (ERA) for human pharmaceutical marketing authorisation applications in Europe since finalisation of Environmental Medicines Agency (EMA) ERA guidance in June 2006.
An overview of 31 OECD 308 studies conducted by 4 companies with a focus on how pharmaceuticals behave in these water-sediment systems was presented. The mean parent total system half-life for the 31 pharmaceuticals was 56 days ± 79 days. The formation of non-extractable residues (NER) was considerable, averaging 44 ± 25%, with cationic substances averaging 51 ± 27% of the applied radioactivity, neutral substances averaging 32 ± 13% and anionic substances averaging 31 ± 23%. In general there was an inverse relationship to the amount of non-extractable residue and the amount of total transformation products observed at study termination. On the sixteen test materials with OECD 218 (OECD, 2004b)sediment toxicity data, ten reported a LOEC (Lowest Observed Effect Concentration) as the highest concentration tested (range of 1 to 150 mg/kg) and six reported a NOEC (No Observed Effect Concentration), mean value of 98 mg/kg (range 5 to 400 mg/kg). NER challenge extractions at study termination showed no more than 5 to 10% of the dosed 14 C-residues recovered during this procedure. This was consistent with the entire data set, though the approaches varied to include some or combinations of the following procedures: series of solvent extractions (polar to non-polar), adjustment of pH, soxhlet extraction and/or use of EDTA (Ethylenediaminetetraacetic acid). A review of whether a simplified one point analysis could reasonably estimate the parent total system half-life showed that the total amount of parent remaining in the water and sediment extracts at day 50 or day 100 correlated fairly well with total system half-life; correlation coefficient r 2 for day 50 and day 100 was 0.83 and 0.93 respectively. This relationship in particular was observed once the aqueous dissipation phase was completed, day 50 for the data set studied. This suggested that there may be some potential for an abbreviated / water-sediment screening study. An approach to water-sediment screening was also presented using voriconazole as a case study. The goal was to develop a short term method (1 week or less) that could screen for potential transformation products typically observed in an OCED 308 study. Such a screen would be helpful in: 1) optimising analytical conditions for the OECD 308; 2) generating transformation products for MS/NMR identification procedures should there be a need; and 3) investigating conditions that may better represent water-sediment conditions found in a STP release environment. Initial design focused on a stirred or agitated reactor using sediment generally following OECD 309 collection procedure with solids levels at 0.1 to 1 g/L, much less than what is seen in the 308 study. Sediment K d values for test substance was used to target conditions that result in approximately 50-75% of the test substance dissolved in solution. Temperature of 20°C and 30°C were investigated to assess how an elevated temperature would potentially enhance the kinetics without impacting the viability of sediment micro-organisms. Results from the preliminary study showed similar transformation products of the screen when compared to what was observed in the OECD 308. As voriconazole has a low sediment K d value of 9.7 for an high organic content sediment, it was not anticipated that lowering the solids level would improve the availability by much. Comparison of the rate of disappearance of voriconazole and the rate of appearance for its hydroxylated transformation product in the OECD 308 and screen showed very similar results. Raising the test conditions to 30°C approximately doubled the rate of disappearance of voriconazole and appearance of the transformation product. Further work is planned to test a substances at a higher K d boundary condition, and by investigating other approaches to enhance transformation rates by using P450 inducers and/or other co-factors.
Recommendations from this 4 company collaboration included: 1) the need to develop a more relevant water-sediment transformation test reflecting the conditions of the STP discharge scenario more representative of human pharmaceuticals; 2) potential use of a one point estimate of parent total system half-life in the EMA ERA screening phase of testing; 3) use of the parent total system biotransformation half-life in revising predicted environmental concentration (PEC) in ERA; 4) need to investigate approaches to water-sediment screening and 5) routinely conduct sediment toxicity testing in Phase II Tier A given the extent of sediment binding generally observed with pharmaceuticals.