Advances in Mass Spectrometry Within Drug Discovery

Mass spectrometry has evolved from a specialist technique, used initially almost exclusively for structure determination, to one where the bulk of applications now revolve around routine applications in analyte detection and quantification.

In the beginning, samples of pure compounds were introduced into the ion source of the mass spectrometer (MS) deposited on a probe. Complex mixture analysis using probes was impractical, but could be performed by interfacing the MS with a gas chromatograph (GC). This was an early and highly successful development that enabled the analysis of compounds via first packed column, and then capillary GC-MS, but the technology was, obviously, limited to volatile compounds (or those that could be rendered volatile by derivatization). This requirement for thermally stable volatile analytes represented a clear limitation when the need was for the analysis of biological samples containing thermally labile, involatile analytes present in aqueous samples.

From the point of view of those who struggled with the primitive, ineffective, and unreliable methods for interfacing MS with liquid chromatography (LC), such as the “moving belt” or direct liquid introduction methods, it sometimes seemed that the goal of producing a viable LC-MS system was going to be a step too far. However, this was before the development of the now dominant electrospray and atmospheric pressure chemical ionization (ESI and APCI) methods. These ionization techniques completely transformed the prospects for the analysis of involatile analytes in aqueous solution, which could now be performed using either direct liquid introduction (DLI) MS or LC-MS.

In parallel to the hyphenation of LC with MS developments in surface analysis, techniques such as matrix-assisted laser desorption ionization (MALDI) and desorption electrospray ionization (DESI) have produced tremendous opportunities for the direct analysis of samples deposited on a suitable support or for the high-resolution imaging of tissues.

These advances in the ionization techniques available for MS combined with other developments in instrumentation, such as the introduction of triple-quadrupole, time-of-flight, and high-mass-resolution instruments (e.g., Fourier transform ion cyclotron resonance [FTICR] and “orbitraps”), as well as ion mobility, have enabled a suite of MS-based solutions to address complex problems within biology. It is not hard to see why MS-based methods have become popular, because when carefully optimized, mass spectrometry offers highly sensitive qualitative and quantitative analysis and specific detection combined with speed. Indeed, such has been the success of MS-based methodology that it is now difficult to envisage how modern bioscience, and particularly drug discovery and development, could be so efficiently conducted in its absence.

Continual improvements in both the speed and sensitivity of MS-based techniques are facilitating an ever-increasing number of applications for high-throughput screening, and this is reflected in the content of this special issue of the Journal of Biomolecular Screening (JBS). The topics covered are wide ranging.

There are several papers that exemplify new applications for existing high-throughput MS platforms and new targets, such as identifying LRRK kinase inhibitors ¹ for use in the treatment of Parkinson’s disease, the direct determination of analyte concentrations in cells to provide information about cell permeability, ² and the covalent modification of reactive cysteine residues on a protein target. ³ The requirement of ESI-MS for a chromatography step has proved throughput limiting; however, the gas phase ionization of laser-based desorption techniques has potentially higher throughput. Laser diode thermal desorption MS (LDTD-MS) coupled to acoustic sample deposition, when applied for cytochrome P450 screening, not only reduces sampling times, but also allows accurate multiplexing of end points. ⁴ There is growing application of MADLI in support of ultra-high-throughput screening: 1536-well formats ⁵ are now possible.

The range of drug targets for which MS end points could be developed is demonstrated by a series of papers; there are methods for screening for DGAT2 inhibitors ⁶ and fatty acid synthase inhibitors ⁷ through their effects on cellular concentrations of malonyl-CoA. The coupling of a nano-LC system in-line with a microfluidic chip to identify inhibitors of thrombin and factor Xa within complex mixtures of snake venom ⁸ demonstrates the versatility of MS and how it can be integrated with other assay platforms to deliver high-value data.

Other papers discuss screening via bioaffinity-MS, ⁹ the use of a bead/lysate-based affinity capture method coupled to MS analysis for target identification in a cell-based phenotypic screen ¹⁰ and a novel targeted lipidomics platform for monitoring eicosanoid lipid modulation. ¹¹

MS imaging aspects are discussed with respect to the use of the technique compared to conventional quantitative whole-body autoradiography (QWBA). A major advantage of using MS is the ability to follow not only the distribution of primary compound within a tissue, but also metabolites of test compounds over time. While QWBA remains something of a gold standard, the requirement for radiolabeled compound means that this rarely represents a viable technique for screening. ¹²

Overall, JBS editor-in-chief Robert M. Campbell and guest editors Ian Wilson and Jonathan Wingfield believe that this collection of papers nicely illustrates the many screening applications that MS currently supports. However, as the technical capabilities of MS technologies increase, there can be no doubt that the applications will also expand to take advantage of them. Indeed, the rate of innovation in this area is staggering and the prospects for the future are enormous. We hope that the papers presented in this special issue may inspire readers to develop and apply novel MS-based screening approaches to the particular problems that they face in their research.

Jonathan Wingfield, PhDAstraZenecaDiscovery SciencesCambridge, UK Ian D. Wilson, PhDImperial CollegeSurgery and CancerLondon, UK