The authors would like to thank P. Nickolas Shepel for performing HPLC analysis. This work was supported by the Medical Research Council (MRC) of Canada. F.E.P. is an MRC/RPP Investigator. C.J.D.S. is the recipient of a studentship award from the Manitoba Health Research Council and the Natural Sciences and Engineering Research Council of Canada. C.G.L. is the recipient of an MRC/PMAC student stipend.

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