Systematic exploration of essential yeast gene function with temperature-sensitive mutants

Conditional temperature-sensitive (ts) mutations are valuable reagents for studying essential genes in the yeast Saccharomyces cerevisiae. We constructed 787 ts strains, covering 497 (∼45%) of the 1,101 essential yeast genes, with ∼30% of the genes represented by multiple alleles. All of the alleles are integrated into their native genomic locus in the S288C common reference strain and are linked to a kanMX selectable marker, allowing further genetic manipulation by synthetic genetic array (SGA)-based, high-throughput methods. We show two such manipulations: barcoding of 440 strains, which enables chemical-genetic suppression analysis, and the construction of arrays of strains carrying different fluorescent markers of subcellular structure, which enables quantitative analysis of phenotypes using high-content screening. Quantitative analysis of a GFP-tubulin marker identified roles for cohesin and condensin genes in spindle disassembly. This mutant collection should facilitate a wide range of systematic studies aimed at understanding the functions of essential genes.

Forcing interactions as a genetic screen to identify proteins that exert a defined activity

The interaction of proteins to form macromolecular complexes is the basis for most biological processes. Approaches have been described that employ artificial constructs to promote such complexes and assess the consequences. For example, a protein interaction scheme has been described that examines the effects of a specific phosphorylation event catalyzed by a protein kinase via the provision of an artificial protein binding interface between a modified version of the kinase and a single substrate. We have generalized this type of approach to form the basis for a genetic selection to identify proteins that exert an activity when recruited to a target protein. The assay uses the leucine zipper domains from the mammalian transcription factors Fos and Jun to force the interaction of two proteins. With a target protein fused to the Jun zipper and a library of open reading frames fused to the Fos zipper, we demonstrate this approach in yeast with both a selection to identify membrane-associated proteins and a selection to identify candidate components of the filamentous growth MAP kinase pathway.

Abstract 48: Non-clinical pharmacokinetics of XMT-1522, a HER2 targeting auristatin-based antibody drug conjugate

The ADC XMT-1522 consists of a novel human IgG1 anti-HER2 monoclonal antibody and a novel, auristatin-based cytotoxic payload (Auristatin F-hydroxypropylamide, AF-HPA). An average DAR of 12 AF-HPA molecules is achieved via a biodegradable polymer conjugation platform. The non-clinical DMPK properties of XMT-1522 have been characterized in vitro in plasma and microsomal stability studies, and in vivo in plasma and tissue disposition and excretion studies. Sample analysis for total AF-HPA drug payload and released (free) AF-HPA and its metabolites was performed by ESI+ LC/MS/MS; total antibody was determined by ELISA. The half-life for AF-HPA release in plasma was found to be greater than 120 hours in all species tested. Microsomal stability studies showed that AF-HPA was further converted to other metabolites including the carboxylic acid auristatin F (AF), as well as monomethyl auristatin F-HPA (MMAF-HPA) and MMAF. The pharmacokinetic profiles of XMT-1522 were evaluated in mouse, rat and cynomolgus monkey. The antibody of XMT-1522 is cross-reactive with monkey, but not rodent, HER2. In mouse and rat, XMT-1522 exposure was dose-proportional; exposure was slightly greater than dose-proportional in monkey consistent with saturation of target-mediated clearance. All species showed extended exposure to total AF-HPA drug payload, with measured clearance and volume of distribution similar for total AF-HPA and the antibody component of XMT-1522. Exposure to free AF-HPA and AF was less than 1/1000th the exposure of total AF-HPA. These data indicate the vast majority of AF-HPA in plasma is antibody-conjugated, indicating high stability of the ADC in systemic circulation. XMT-1522 tissue disposition was studied in NCI-N87 HER2-positive gastric cancer xenograft tumor bearing mice. After a single 3 mg/kg dose of XMT-1522, free AF-HPA and its metabolite AF were measurable in tumor tissue until the last time point measured (2 weeks). Total AF-HPA and free AF-HPA achieved peak tumor concentrations 48 hours after dosing. In contrast, AF achieved peak tumor concentration 7 days after dosing and showed only a slight decline in tumor concentration at 14 days, consistent with intracellular trapping of this poorly cell-permeable metabolite. Exposure to free AF-HPA or AF in other tissues was at least an order of magnitude lower than in tumor; in tissues with measurable free drug, AF was the predominant species. XMT-1522 excretion studies, conducted in rat, indicated that the AF-HPA payload was mainly excreted by the gastrointestinal route. In the first 96 hours after administration 33% of the AF-HPA dose was excreted in feces, compared to 3% excreted in urine. The major contributing metabolites both in feces and urine were conjugated AF-HPA, AF, and free AF-HPA. In conclusion, the plasma kinetics, tissue distribution and excretion profile of XMT-1522 are acceptable for clinical evaluation in cancer patients.

Citation Format: Alex Yurkovetskiy, Dmitry Gumerov, Elena Ter-Ovanesyan, Patrick Conlon, Michael Devit, Charlie Bu, Natalya Bodyak, Timothy Lowinger, Donald Bergstrom. Non-clinical pharmacokinetics of XMT-1522, a HER2 targeting auristatin-based antibody drug conjugate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 48. doi:10.1158/1538-7445.AM2017-48