Emergence of new technologies will further accelerate drug metabolism
studies in both drug discovery and development. In drug discovery settings,
where rapid metabolite characterization is required to speed up lead
optimization and candidate selection for development, software-assisted data
acquisition and processing are extremely valuable and will continue to play a
pivotal role. FTICRMS and FT-OrbitrapMS are rapidly gaining acceptance as
tools of choice for drug metabolism studies, due to their ultrahigh mass
accuracy, which make them highly attractive for identification of unknown
metabolites. The ability of DESI–MS technique to desorb intact molecule ions
directly from the surface is an exciting new feature and can be used to detect,
identify, and quantitate drugs and metabolites directly from tissue slices
without the need for extraction of drug-derived material.
In drug development, the availability of human metabolism data early in the
program has significant merit from a business as well as regulatory perspective
(Baillie et al., 2002). Understanding of metabolism in humans early in
the clinical development program allows for defining the enzymology
responsible for major in vivo human biotransformation pathway(s).
Traditionally, these findings have been obtained from a single-dose metabolism
study with radiolabeled drug that provides both mass balance (excretion of
radioactivity in urine and feces) and metabolism information from plasma and
excreta. Unfortunately, due to the significant cost and effort involved,
radiolabeled studies are not conducted prior to establishing a therapeutic
‘‘proof of principle’’ in humans. Although the detection of metabolites from
nonradiolabeled first-in-human (FIH) studies can be achieved with the current
LC/MS technologies, it is far more difficult to obtain relative amounts of
metabolites present in any matrix. Metabolism often results in structural
changes that can dramatically change LC/MS response of a metabolite. Chip-
based NSI systems (e.g., NanoMateTM) may overcome this issue. Recent
studies (Hop et al., 2005) indicated that the degree of variability associated
with the ion intensities of a variety of compounds is much smaller with NSI
compared to that with ESI, which makes metabolite identification studies with
NSI potentially semiquantitative in nature.
There is also a demand for a universal detector that will allow quantification
of drug metabolites without the need for radioisotopes or authentic reference
standards. Online coupling of LC with inductively coupled plasma mass
spectrometry (LC/ICPMS) offers the ability to quantify metabolites. ICPMS is
an element-specific detector with almost uniform response independent of
molecular structure. The response is only related to the molar content of the
detected element (Axelsson et al., 2001). Unfortunately, the use of ICPMS for
metabolite profiling and identification is limited to compounds containing
specific elements such as P, I, F, Br, S, and selected metals. We predict that in
not too distant future technologies will be available to reliably detect, identify,
and quantitate ‘‘major’’ human metabolites routinely from first-in-human
studies.
358 APPLICATION OF LIQUID CHROMATOGRAPHY/MASS SPECTROMETRY