Mass spectrometry technique may speed drug discovery and development
Correlation of drug distribution and protein changes sheds light on drug efficacy
Pharmaceutical companies spend an average of eight years and roughly $500 million to get a new drug to market.
They could avoid much of this time and expense if there were a way to identify unsuccessful
drugs before they reach the late stage of clinical trials. This would require further
development of the early stages of drug discovery — to gain better understanding
of drug distributions, for example, or of the relationship between a drug’s
distribution and its pharmacological or toxicological effects.
A group with Vanderbilt University in Nashville,
Tenn., has developed a technique that may help. In an Analytical Chemistry
paper published online on Aug. 3, the researchers described use of matrix-assisted
laser desorption/ionization (Maldi) imaging mass spectrometry to analyze whole-organ
and whole-body animal tissue sections, successfully measuring the concentrations
of drugs as well as the associated protein changes in tissue — the latter representing the therapeutic response to the drugs. Correlating these can reveal important information about the efficacy of a drug much earlier than
otherwise would be possible.
The technique also enables measurement
of protein changes in tissue, reflecting a therapeutic response to drugs. Correlating
these with the drug and metabolite distributions can tell researchers a great deal
about drug efficacy, much earlier than would be possible using conventional methods.
Shown here are optical (left) and protein (right) images of rat liver (blue in the
protein image), kidney (pink) and brain tissue (red; with white and green representing
substructures of the brain).
Perhaps the most important aspect of the method
is that it allows direct monitoring of the drug. “No [other] technique allows
you to visualize the molecular distribution of a compound,” said Sheerin Khatib-Shahidi,
the first author of the paper. “You can only follow a label.” Because
other methods track the distribution of a label, researchers can never be entirely
certain that they are looking at the original drug: The label may have been maintained
by a metabolite encountered along the way or simply cleaved away from the drug.
For this reason, she added, the techniques offer only limited ability to connect
the signal with the molecular identity.
used an imaging mass spectrometry technique to track drug (top; organs are outlined
in red) and metabolite (bottom) distributions in animal models, which could contribute
to the drug-discovery process. Unlike other techniques used for a similar purpose,
this method allows direct monitoring of the drug, and not of the label associated
This applies also to the current gold
standard for characterization of new drugs. Whole-body autoradiography, use of which
is required for FDA approval of pharmaceuticals, provides
spatial and quantitative information about the absorption and distribution of radiolabeled
drugs by tracking the radioactive substance as it diffuses through tissue. However,
it cannot distinguish between the originally labeled compound and any part of the
radiolabel that may have separated from it. The Maldi technique described by the
researchers can, but does not yet offer quantitative information. For this reason,
Khatib-Shahidi said, it could serve as an important complement to autoradiography.
The technique images these distributions using
Maldi mass spectrometry to desorb and detect compounds, relying on a matrix that
co-crystallizes with the analytes in question. The matrix and analytes alike desorb
from the sample surface when irradiated with ultraviolet light. This produces ions
that can be analyzed by a time-of-flight mass spectrometer. Researchers can then reconstruct a molecular image of the tissue using an ordered array of the mass spectra.
To demonstrate the method’s potential
for drug discovery and development, the investigators used it to measure the relative
concentrations of drugs in tissue as well as the distribution of several metabolites,
label-free. They also used it to detect protein changes in tissue, enabling evaluation
of the drugs’ efficacy. For the former experiments, they analyzed whole-body
tissue sections from male Fischer 344 rats, which had been given oral doses of Olanzapine,
a drug doctors use to treat mood disorders, including schizophrenia and acute mania
in bipolar patients. They used a time-of-flight mass spectroscopy system made by
MDS Sciex of Concord, Ontario, Canada, outfitted with a 337-nm nitrogen laser operating
at 20 Hz. They acquired spectra by irradiating the sample in 400-μm steps.
Imaging whole-body tissue sections
with Maldi imaging mass spectrometry was not easy. “We’re taking a very
large sample size, having to break it down into sections that we can analyze and
then reconstructing it,” Khatib-Shahidi said. “It’s at times laborious,
but the payoff is very big.”
Indeed, the investigators were able
to directly image drug and metabolites throughout the rats two and six hours after
administration. At two hours, they could see the drug in almost all tissues, with
especially high concentrations in certain organs.
To measure the protein changes, they
imaged tissue sections from the liver, kidney and brain, using either a Maldi-Tof/Tof-MS
system made by Bruker Daltonics of Billerica, Mass., outfitted with a solid-state
Smartbeam laser operating at 200 Hz, or a Maldi-Tof-MS system made by Applied Biosystems
of Foster City, Calif., equipped with a 337-nm nitrogen laser operating at 20 Hz.
Here, the technique detected protein signals from each of the tissue types, in some
cases enabling organ- or region-specific images.
By imaging the distributions of drugs and metabolites and providing for correlations
of these and protein changes, the imaging mass spectrometry method offers a broad
overview of the many complex interactions following
administration of pharmaceutical compounds. It can, therefore, advance drug discovery
and development, providing insight into drug efficacy as well as pharmacological
and toxicological effects, long before it reaches the late stage of clinical trials.
The researchers are monitoring drug accumulation
to target tissues — to determine how well a cancer therapy drug localizes
to a brain tumor, for example. In addition, they are exploring whether they can
identify early protein biomarkers indicative of early drug efficacy or toxicity.
For validation, they are following
conventional quantitative methods used by the pharmaceutical industry. They also
are comparing the relative information obtained from the imaging mass spectrometry
images to quantitative information provided by whole-body autoradiography.
“Ultimately, we would like to
develop the technique to allow for absolute quantitation of each analyte directly
from the organs analyzed,” Khatib-Shahidi said.
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