A four-channel multiplexed electrospray interface on a triple quadrupole mass spectrometer was evaluated for the simultaneous validation of LC/MS/MS methods for the quantitation of loratadine and its metabolite, descarboethoxyloratadine, in four different biological matrixes. The assays were performed in rat, rabbit, mouse, and dog plasma from 1 to 1000 ng/mL using 96-well solid-phase extraction for sample preparation. The limit of quantitation of 1 ng/mL corresponded to 5.56 pg of each analyte injected on-column. For the drug, quality control samples (n = 6 at four concentrations) had precision ranging from 0.967 to 16.0% and accuracy ranging from -8.44 to 10.5% across all four species. For the metabolite, the precision ranged from 0.684 to 11.0% and the accuracy was between 6.36 and -9.06%. Intersprayer cross talk for the multiplexed electrospray ion source was evaluated as a function of analyte concentration and was less than 0.08% at concentrations as high as 1000 ng/mL. These results demonstrate the feasibility of using parallel analysis to reduce the time required for method validation and to increase sample throughput in drug development studies.

Visceral leishmaniasis is responsible for up to 30,000 deaths every year. Current treatments have shortcomings that include toxicity and variable efficacy across endemic regions. Previously, we reported the discovery of GNF6702, a selective inhibitor of the kinetoplastid proteasome, which cleared parasites in murine models of leishmaniasis, Chagas disease, and human African trypanosomiasis. Here, we describe the discovery and characterization of LXE408, a structurally related kinetoplastid-selective proteasome inhibitor currently in Phase 1 human clinical trials. Furthermore, we present high-resolution cryo-EM structures of the Leishmania tarentolae proteasome in complex with LXE408, which provides a compelling explanation for the noncompetitive mode of binding of this novel class of inhibitors of the kinetoplastid proteasome.

Several modifications of the oxazoline ring of WIN 54954, a broad spectrum antipicornavirus compound, have been prepared in order to address the acid lability and metabolic instability of this compound. We have previously shown that the oxadiazole analogue 3 displayed comparable activity against a variety of rhinoviruses and appeared to be stable to acid. A monkey liver microsomal assay was developed to examine the metabolic stability in vitro of both compounds, and it was determined that WIN 54954 displayed 18 metabolic products while 3 was converted to 8 products. Two major products of 3 were determined by LC-MS/MS to be monohydroxylated at each of the terminal methyl groups. Replacement of the methyl on the isoxazole ring with a trifluoromethyl group, while preventing hydroxylation at this position, did not reduce the sensitivity of the molecule to microsomal metabolism at other sites. However, the (trifluoromethyl)oxadiazole 9 not only prevented hydroxylation at this position but also provided protection at the isoxazole end of the molecule, resulting in only two minor products to the extent of 4%. The major product was identified as the monohydroxylated compound 23. The global metabolic protective effect of trifluoromethyl group on the oxadiazole ring was further demonstrated by examining a variety of analogues including heterocyclic replacements of the isoxazole ring. In each case, the trifluoromethyl analogue displayed a protective effect when compared to the corresponding methyl analogue.

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