Background: Reports of the use of multiplex enzyme assay screening for Pompe disease, Fabry disease, Gaucher disease, Niemann-Pick disease types A and B, and Krabbe disease have engendered interest in the use of this assay in newborn screening. We modified the assay for high-throughput use in screening laboratories. Methods: We optimized enzyme reaction conditions and procedures for the assay, including the concentrations of substrate (S) and internal standard (IS), assay cocktail compositions, sample clean-up procedures, and mass spectrometer operation. The S and IS for each enzyme were premixed and bottled at an optimized molar ratio to simplify assay cocktail preparation. Using the new S:IS ratio, we validated the modified assay according to CLSI guidelines. Stability of the S, IS, and assay cocktails were investigated. Dried blood spots from 149 healthy adults, 100 newborns, and 60 patients with a lysosomal storage disorder (LSD) were tested using the modified assay. Results: In our study, the median enzyme activity measured in adults was generally increased 2–3–fold compared to the original method, results indicating higher precision. In the multiplex format, each of the 5 modified enzyme assays enabled unambiguous differentiation between samples from healthy individuals (adults and newborns) and the corresponding disease-specific samples. Conclusions: The modified multiplex enzyme assay with premixed S and IS is appropriate for use in high-throughput screening laboratories.

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DARPin® molecules are small engineered proteins, derived from natural ankyrin repeat proteins that are selected to bind to specific targets with high affinity. Individual DARPin® molecules can be linked together genetically in order to create multi-specific drug molecules. This versatility of DARPin® molecules makes them an attractive alternative to antibodies for the development of drug conjugates. We have developed DARPin® drug conjugates (DDCs) using a model EGFR multi-specific DARPin® molecule, consisting of four DARPin® domains linked together. DARPin® constructs were conjugated to the indolinobenzodiazepine mono-imine DGN549, a potent DNA alkylating payload. Biophysical characterization showed the DDCs to be well behaved in stability and solubility assays. The in vitro binding and cytotoxicity of these DDCs were evaluated. The DARPin® conjugates displayed in vitro potency in direct cytotoxicity assays across a panel of cell lines expressing EGFR. The impact of conjugation on PK parameters and in vivo efficacy is currently under evaluation. DARPin® drug conjugates combine the potency observed with antibody drug conjugates and the modular design of DARPin® molecules to create designer therapeutics. Citation Format: Laura A. Laviolette, Cynthia J. Guidi, Christian Reichen, Qifeng Qiu, Luke Harris, Patricia Schildknecht, Stefanie Fischer, Zita Arany, Tanja Hospodarsch, Anna Skaletskaya, Megan Fuller, Stephen Abbott, Rebecca McCarthy, Jenny Lee, Katherine Francisco, Kerstin Sinkevicius, Sharlene Adams, Christopher Espelin, Emily Reid, Wei Li, Carla Marashio, Kerry Donahue, Stuart Hicks, Dan Snell. Generation of site-specific DARPin® drug conjugates using EGFR as a model system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 215.

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