Breaking Through the Barrier

Ambient mass spectrometry in clinical applications; state of the art

Ambient ionization mass spectrometry (MS) refers to techniques in which samples are analyzed under ambient conditions (ie, atmospheric pressure, room temperature) with little or no sample preparation, allowing high-throughput analysis of samples under native conditions. In the context of future clinical applications, ambient MS methods allow direct access to biological tissues without lengthy sample preparation. ¹ A typical ambient MS analysis workflow involves untargeted interrogation of tissue small molecule content and comparison (via multivariate statistical analysis) of a query m/z profile to a previously established and validated (via anatomic or molecular pathology) library of m/z profiles for various tissue disorders. Capitalizing on the close relationship between cancer formation and changes in lipid metabolism, ² analysis of the lipidome by ambient MS methods to predict disorder has constituted the bulk of those reported in the literature. ^1 , 345678910111213141516 Although most reported applications use untargeted analysis and changes in the entire mass range profile to highlight disorder, certain targeted applications using oncometabolites have been reported as well. 8910 ^, 17 , 18 Although ambient MS methods allow unprecedented access to rapid (within few seconds), on-the-spot information regarding tissue disorder with high (>98%) specificity in certain cases, ^{1 , 15 , 16} the added value of using this real-time information in clinical decision making is not yet validated against patient outcomes. However, ambient MS methods in diagnostics have been used in 2 broad categories of point sampling and imaging modes. Point sampling approaches often use a handheld desorption source (or probe) affixed to a mass spectrometer for processing desorbed bulk tissue (either in or ex vivo). In contrast, the imaging mode commonly involves using an x-y translation stage to raster the desorption probe over an ex vivo tissue piece (often in the form of a flat slice) to generate a two-dimensional, spatially resolved image of the tissue molecular content. Although point sampling methods may be combined with three-dimensional tracking technologies to obtain spatially resolved information from in vivo tissue sampled in situ, ¹⁹ this hybrid mode has just been reported and is not widely used or evaluated.

Although a large number of different ambient MS sources exist, ^{15 , 16 , 20} including aerosols produced by surgical aspirators, ²¹ most are based on either liquid extraction (eg, desorption electrospray ionization, ²² MasSpec Pen ¹⁴ ) or direct coupling between laser ablation (eg, picosecond infrared laser [PIRL] MS, ⁷ SpiderMass ¹² ) and MS or that of electrocautery ¹¹ or rapid evaporative ionization MS (REIMS). ^23 , 24 Ambient MS sources that use strong solvents or require significant electric fields at the probe tip to facilitate desorption or ionization have limitations that preclude them from immediate use in vivo without further modifications. However, recent developments of water-based extraction methods, such as the MasSpec Pen, ¹⁴ have revived interest in revisiting the role of solvent in ambient MS methods. A detailed description of ambient MS sources and technological advancements that have given rise to their developments are available elsewhere ^{1 , 15 , 16 , 25} and are not discussed further herein.

Ambient MS has experienced a significant horizontal growth throughout the recent decade, with no single source or method having broken through the regulatory barrier for approved clinical use. This article describes challenges associated with standardizing a validation workflow, and reviews concerns that must be addressed to facilitate approval, using 3 major clinical use case scenarios selected from the literature: assessment of biopsy adequacy, and clinical decision making using in vivo and ex vivo sampling. ¹ The regulatory considerations that each envisioned scenario may require are highlighted to also draw attention to the potential of ambient MS to form the core technology of future clinical devices. Parallels are drawn from recent approvals of nonambient MS methods for microbial identification ^26 , 27 as a potential alternative avenue to the laboratory-developed test (LDT) strategies that have formed the core of many targeted MS-based assays. ²⁸ Guidelines from the US Pharmacopeia ²⁹ (USP Pharmacopeial Convention, 2016, pp. 2053–2067) for validation of untargeted methods in food quality control (QC) are also discussed because they represent a close match with where ambient MS currently stands with respect to the need for harmonized validation protocols. This article further provides suggestions and additional considerations that may be relevant in systematic evaluation of ambient MS methods in clinical settings, especially for surgical guidance, which is reviewed elsewhere. ¹