Significance: The proto-oncogene KRAS , a small GTPase, is frequently mutated in pancreatic, colorectal, and lung cancer. These mutations result in elevated levels of the activated guanosine triphosphate-bound form of KRAS. Localized at the plasma membrane, KRAS functions to recruit effectors, predominantly RAF kinase for activation and initiation of the MAPK signaling cascade. Combining computational and biophysical methods we identify a membrane-distal state of the KRAS G-domain that alternates with two previously described membrane-proximal states through dynamic reorganization of the hypervariable region. Comprising about 90% of the ensemble, this membrane-distal state of the G-domain dominates the proximal states and may facilitate KRAS to recruit cytosolic RAF kinase to the membrane by a fly-casting mechanism.

Fast photochemical oxidation of proteins (FPOP) employs laser photolysis of hydrogen peroxide to give OH radicals that label amino acid side-chains of proteins on the microsecond time scale. A method for quantitation of hydroxyl radicals after laser photolysis is of importance to FPOP because it establishes a means to adjust the yield of •OH, offers the opportunity of tunable modifications, and provides a basis for kinetic measurements. The initial concentration of OH radicals has yet to be measured experimentally. We report here an approach using isotope dilution gas chromatography/mass spectrometry (GC/MS) to determine quantitatively the initial •OH concentration (we found ~0.95 mM from 15 mM H₂O₂) from laser photolysis and to investigate the quenching efficiencies for various •OH scavengers.

MAbs undergo several post-translational modifications, including the formation of succinimide from the deamidation of asparagine or the isomerization of aspartic acid. Because of the potential impact of succinimide formation on the biological activity of mAbs, detection and quantification of this species is a key area of interest for the pharmaceutical industry. However, studies assessing succinimide stability have been limited, and methods developed to monitor succinimide are either product specific or not robust. Here, we report the development of a platform low-pH peptide-mapping method using a combination of low-pH-resistant Lys-C and modified trypsin to maintain succinimide stability, eliminate deamidation assay artifact, and achieve efficient mAb digestion equivalent to conventional tryptic peptide-mapping method under alkaline condition. Using this method, succinimide stability in serum was accurately assessed in vitro study and the half-life was determined to be 1.5 days. With potential patient exposure to succinimide intermediate, a reliable method was developed to measure site-specific deamidation and succinimide intermediate. Coupled with a single quadrupole mass detector, our method was automated from digestion to data processing and applicable in a good manufacturing practice environment. The method was fully qualified to demonstrate accuracy, precision, linearity, and robustness.

Combination therapy is a fast-growing strategy to maximize therapeutic benefits to patients. Co-formulation of two or more therapeutic proteins has advantages over the administration of multiple medications, including reduced medication errors and convenience for patients. Characterization of co-formulated biologics can be challenging due to the high degree of similarity in the physicochemical properties of co-formulated proteins, especially at different concentrations of individual components. We present the results of a deamidation study of one monoclonal antibody component (mAb-B) in co-formulated combination antibodies (referred to as COMBO) that contain various ratios of mAb-A and mAb-B. A single deamidation site in the complementarity-determining region of mAb-B was identified as a critical quality attribute (CQA) due to its impact on biological activity. A conventional charge-based method of monitoring mAb-B deamidation presented specificity and robustness challenges, especially when mAb-B was a minor component in the COMBO, making it unsuitable for lot release and stability testing. We developed and qualified a new, quality-control-friendly, single quadrupole Dalton mass detector (QDa)–based method to monitor site-specific deamidation. Our approach can be also used as a multi-attribute method for monitoring other quality attributes in COMBO. This analytical paradigm is applicable to the identification of CQAs in combination therapeutic molecules, and to the subsequent development of a highly specific, highly sensitive, and sufficiently robust method for routine monitoring CQAs for lot release test and during stability studies.