Tulathromycin is a novel member of the triamilide class of antibiotics that was developed as a safe and effective single-dose treatment of bovine and porcine respiratory disease. An accurate and precise analytical method was developed for the extraction and measurement of tulathromycin in livestock plasma and lung homogenates. Analytes were solid-phase extracted onto a weak cation exchanger after aqueous dilution of samples and addition of heptadeutero-tulathromycin as an internal standard. Following HPLC with a narrow bore C8 column, quantitative detection of tulathromycin was accomplished by monitoring the transition of a doubly charged precursor ion to a singly charged product ion by tandem mass spectrometry using a triple quadrupole mass spectrometer. Procedures were validated for a dynamic range of 0.1 to 25 ng on column. Observed accuracies were between 90 and 110% of nominal and precision (RSD) varying 7% or less. Response and stability experiments showed that deuterated tulathromycin did not parallel the chemical behavior of tulathromycin. Applicability of the method to livestock studies was tested with plasma and lung samples from cattle and swine dosed with tulathromycin at multiple doses. The results demonstrated that the analytical method performed well in a range of sample concentrations spanning over 3 orders of magnitude and provided dose-exposure relationships for cattle and swine.

As part of our efforts to develop new classes of tubulin inhibitor payloads for antibody-drug conjugate (ADC) programs, we developed a tubulysin ADC that demonstrated excellent in vitro activity but suffered from rapid metabolism of a critical acetate ester. A two-pronged strategy was employed to address this metabolism. First, the hydrolytically labile ester was replaced by a carbamate functional group resulting in a more stable ADC that retained potency in cellular assays. Second, site-specific conjugation was employed in order to design ADCs with reduced metabolic liabilities. Using the later approach, we were able to identify a conjugate at the 334C position of the heavy chain that resulted in an ADC with considerably reduced metabolism and improved efficacy. The examples discussed herein provide one of the clearest demonstrations to-date that site of conjugation can play a critical role in addressing metabolic and PK liabilities of an ADC. Moreover, a clear correlation was identified between the hydrophobicity of an ADC and its susceptibility to metabolic enzymes. Importantly, this study demonstrates that traditional medicinal chemistry strategies can be effectively applied to ADC programs.

Antibody-drug conjugates (ADC) are currently an active area of research, focused primarily on oncology therapeutics, but also to a limited extent on other areas such as infectious disease. The success of this type of targeted drug delivery is dependent upon many factors, one of which is the performance of the linker in releasing an active drug moiety under the appropriate conditions. As a tool in the development of linker/payload chemistry, we have developed an in vitro method for the identification of payload species released from ADCs in the presence of lysosomal enzymes. This method utilizes commercially available human liver S9 fraction as the source of these enzymes, and this has certain advantages over lysosomal fractions or purified enzymes. This article describes the characterization and performance of this assay with multiple ADCs composed of known and novel linkers and payloads. Additionally, we report the observation of incomplete degradation of mAb protein chains by lysosomal enzymes in vitro, believed to be the first report of this phenomenon involving an ADC therapeutic.

Myeloperoxidase (MPO) is known to be inactivated and covalently modified by treatment with hydrogen peroxide and agents similar to 3-(2-ethoxypropyl)-2-thioxo-2,3-dihydro-1H-purin-6(9H)-one (1), a 254.08 Da derivative of 2-thioxanthine. Peptide mapping by liquid chromatography and mass spectrometry detected modification by 1 in a labile peptide-heme-peptide fragment of the enzyme, accompanied by a mass increase of 252.08 Da. The loss of two hydrogen atoms was consistent with mechanism-based oxidative coupling. Multistage mass spectrometry (MS(4)) of the modified fragment in an ion trap/Orbitrap spectrometer demonstrated that 1 was coupled directly to heme. Use of a 10 amu window delivered the full isotopic envelope of each precursor ion to collision-induced dissociation, preserving definitive isotopic profiles for iron-containing fragments through successive steps of multistage mass spectrometry. Iron isotope signatures and accurate mass measurements supported the structural assignments. Crystallographic analysis confirmed linkage between the methyl substituent of the heme pyrrole D ring and the sulfur atom of 1. The final orientation of 1 perpendicular to the plane of the heme ring suggested a mechanism consisting of two consecutive one-electron oxidations of 1 by MPO. Multistage mass spectrometry using stage-specific collision energies permits stepwise deconstruction of modifications of heme enzymes containing covalent links between the heme group and the polypeptide chain.