Optimization of the choline transporter (CHT) inhibitor ML352: Development of VU6001221, an improved in vivo tool compound.

This Letter describes the further lead optimization of the CHT inhibitor probe, ML352 (VU0476201), and the development of VU6001221, an improved in vivo tool. A multi-dimensional optimization effort encountered steep SAR, and ultimately, subtle tuning of the electronics of the central phenyl core provided VU6001221, a CHT inhibitor with comparable potency for choline uptake inhibition as ML352, yet improved PK and CNS penetration. Moreover, VU6001221 enabled evaluation, for the first time, of a CHT inhibitor in a standard preclinical rodent cognition model, novel object recognition (NOR). We observed VU6001221 to elicit a dose-responsive increase in NOR, raising the possibility of agonism of synaptic α7 nicotinic ACh receptors by elevated extracellular choline, that if confirmed would represent a novel molecular strategy to enhance cognition.

Method Development for a Short-Term 7-Day Toxicity Test with Unionid Mussels.

The US Environmental Protection Agency's short-term freshwater effluent test methods include a fish (Pimephales promelas), a cladoceran (Ceriodaphnia dubia), and a green alga (Raphidocelis subcapitata). There is a recognized need for additional taxa to accompany the three standard species for effluent testing. An appropriate additional taxon is unionid mussels because mussels are widely distributed, live burrowed in sediment and filter particles from the water column for food, and exhibit high sensitivity to a variety of contaminants. Multiple studies were conducted to develop a relevant and robust short-term test method for mussels. We first evaluated the comparative sensitivity of two mussel species (Villosa constricta and Lampsilis siliquoidea) and two standard species (P. promelas and C. dubia) using two mock effluents prepared by mixing ammonia and five metals (cadmium, copper, nickel, lead, and zinc) or a field-collected effluent in 7-day exposures. Both mussel species were equally or more sensitive (more than two-fold) to effluents compared with the standard species. Next, we refined the mussel test method by first determining the best feeding rate of a commercial algal mixture for three age groups (1, 2, and 3 weeks old) of L. siliquoidea in a 7-day feeding experiment, and then used the derived optimal feeding rates to assess the sensitivity of the three ages of juveniles in a 7-day reference toxicant (sodium chloride [NaCl]) test. Juvenile mussels grew substantially (30%-52% length increase) when the 1- or 2-week-old mussels were fed 2 ml twice daily and the 3-week-old mussels were fed 3 ml twice daily. The 25% inhibition concentrations (IC25s) for NaCl were similar (314-520 mg Cl/L) among the three age groups, indicating that an age range of 1- to 3-week-old mussels can be used for a 7-day test. Finally, using the refined test method, we conducted an interlaboratory study among 13 laboratories to evaluate the performance of a 7-day NaCl test with L. siliquoidea. Eleven laboratories successfully completed the test, with more than 80% control survival and reliable growth data. The IC25s ranged from 296 to 1076 mg Cl/L, with a low (34%) coefficient of variation, indicating that the proposed method for L. siliquoidea has acceptable precision. Environ Toxicol Chem 2021;40:3392-3409. © 2021 SETAC.

Influence of water hardness and sulfate on the acute toxicity of chloride to sensitive freshwater invertebrates.

Total dissolved solids (TDS) represent the sum of all common ions (e.g., Na, K, Ca, Mg, chloride, sulfate, and bicarbonate) in freshwater. Currently, no federal water quality criteria exist for the protection of aquatic life for TDS, but because the constituents that constitute TDS are variable, the development of aquatic life criteria for specific ions is more practical than development of aquatic life criteria for TDS. Chloride is one such ion for which aquatic life criteria exist; however, the current aquatic life criteria dataset for chloride is more than 20 years old. Therefore, additional toxicity tests were conducted in the current study to confirm the acute toxicity of chloride to several potentially sensitive invertebrates: water flea (Ceriodaphnia dubia), fingernail clams (Sphaerium simile and Musculium transversum), snail (Gyraulus parvus), and worm (Tubifex tubifex), and determine the extent to which hardness and sulfate modify chloride toxicity. The results indicated a significant ameliorating effect of water hardness (calcium and magnesium) on chloride toxicity for all species tested except the snail; for example, the 48-h chloride median lethal concentration (LC50) for C. dubia at 50 mg/L hardness (977 mg Cl(-) /L) was half that at 800 mg/L hardness (1,836 mg Cl(-) /L). Conversely, sulfate over the range of 25 to 600 mg/L exerted a negligible effect on chloride toxicity to C. dubia. Rank order of LC50 values for chloride at a given water hardness was in the order (lowest to highest): S. simile < C. dubia < M. transversum < G. parvus < T. tubifex. Results of the current study support the contention that the specific conductivity or TDS concentration of a water body alone is not a sufficient predictor of acute toxicity and that knowledge of the specific ion composition is critical.