Determining Distribution of Erlotinib in Pancreatic Tumors by MALDI Using an LTQ Linear Ion Trap

Use imaging mass spectrometry to determine the biodistribution of erlotinib (Tarceva®) in pancreatic tumors from treated and untreated mice.

Imaging Mass Spectrometry (IMS) using matrix-assisted laser desorption ionization (MALDI) is being investigated using a linear ion trap with the overall goal of analyzing the distribution of various targeted therapies within patients' tumors. The drug chosen for evaluation is erlotinib (Figure 1) which targets the epidermal growth factor receptor (EGFR), a type I receptor tyrosine kinase (TK) involved in cellular differentiation and proliferation, by binding to the ATP pocket and inhibiting the autophosphorylation of the receptor. Erlotinib has demonstrated clinical activity in nonsmall cell lung cancer, head and neck cancer, and ovarian cancer in Phase II studies. The sensitivity and MS-MS capabilities of the LTQ XL are exploited for the unambiguous determination of the distribution of this drug within human pancreatic tumors.

Figure 1

Methods

SCID mice bearing patients' pancreas tumors grown as xenografts were dosed once with 2.5 mg erlotinib in 6% Captisol, Captisol alone, or left untreated. Tissue was sectioned to 12 μm thick, placed on glass microscope slides and 6-aza-2-thiothymine (ATT) MALDI matrix applied with an airbrush.

Mass Spectrometry:

The laser was set to raster at 100 μm spacing and Xcalibur™ software was used for unattended data collection. Two and three dimensional maps were generated with ImageQuest™ 1.0 Software.

Results

Single Reaction Monitoring (SRM) experiments were used to monitor the m/z 278 fragment characteristic of both erlotinib (m/z 394) and its metabolite (m/z 380). This fragment ion appeared to be distributed evenly over the tumor. All expected erlotinib fragments (m/z 278, 304, 322, 336) were detected directly off tissue and images generated. Neither the drug nor the metabolite were found at significant levels in three untreated tissue sections that were analyzed with two different MALDI matrices (selection shown in Figure 2).

Figure 2

Conclusion

The LTQ with MALDI source makes for an outstanding platform for the study of drug distribution and its metabolites directly off tissue. The sensitivity of the linear ion trap, in particular, allows for an MS-MS spectrum with less than ten laser shots at each location.

MALDI as conducted in this study, without the use of isotopic labels or internal standards, is not considered a quantitative technique. Data processing for both treated and untreated tissues was kept consistent. A relative comparison of measurable drug between the two was therefore possible.

References

(1) Zhao, M, He, P. Rudek, M. A., Hidalgo, M., Baker, S. D. 2003. "Specific method for determination of OSI-774 and its metabolite OSI-420 in human plasma by using liquid chromatography-tandem mass spectrometry." J Chromatography B, 793, 413–420.

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