**4. Summary**

tope labelling is preferred for the less abundant 13C, and 15N nuclei. HR-MAS can be ap‐ plied for many natural product studies[118,120] commonly used to study metabolic changes in diseased and treated tissues,[121,122] metabolism,[123] combinatorial chemis‐ try[124] and whole cells[125]. Intensities in the HR-MAS NMR spectra are dependent on the environment of the analyte and as such molecules that are in rigid environments with completely restricted mobility are not detected. Macro molecular assemblies on-theother-hand can be examined for profiling and quantitation of algae lipid content and pol‐

MRI is a nuclear MR technique applicable for natural product development in particular during *in vivo* testing and diagnostic stages. MRI is capable of producing 3D images that can be used to monitor changes in brain activity in response to application of a natural product *via* fMRI techniques[126-128], indirectly monitor the effects of a natural product on tumours[129], or directly monitor the bio-distribution and bio-accumulation of a natu‐

Direct monitoring of the natural product is one of the most typical methods for drug de‐ velopment. For direct monitoring however, the molecules should be tagged with a MRI

[134] This rapid relaxation is exploited with MRI and is depicted as "dark spots" within the image whereas unaffected water molecules remain as bright areas. In order to "tag" a natural product with a MRI contrast agent such as super-paramagnetic iron oxide (SPIO), the complete structure and pharmacophore identification is valuable (see Section 2.2) as it allows one to pick a functional group distant from the active site that can be chemically coupled to the contrast agent. Many SPIO contrast agents are commercially available with a variety of functional groups amenable to chemical coupling under aque‐

Once the natural product is conjugated to the paramagnetic particle, it can be adminis‐ tered to an organism and imaged. The bio-distribution and bio-accumulation of the tag‐ ged molecule is monitored and compared against a control particle that does not contain the active natural product.[133,135-137] The difference in clearance time is considered as conformation that the molecule is associated with the tissue being examined. As an ex‐ ample the peptide SOR-C27, a 27 amino acid fragment of the paralytic natural product peptide SOR-54 from the Northern Short Tail shrew (*Blarina brevicauda*) was found to bind the calcium ion channel TRPV6 which is highly over-expressed by breast, prostate and ovarian cancers.[138,139] The SOR-C27 peptide was chemically bonded to a malei‐ mide functionalized SPIO particle through the sulfur centre of the Cys-14 residue. From MRI investigations on an ovarian cancer xenograft mouse model, the SPIO-peptide parti‐

H nuclei on water in close

H nuclei distant from the paramagnetic centre.

ral product by tagging the compound with an MRI contrast agent.[130-133].

contrast agent which contains a paramagnetic centre causing 1

80 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

proximity to "relax" much faster than 1

ous conditions.[134]

ysaccharides or metabolites.[100]

**3. MRI**

A comprehensive overview of MR techniques for natural product development is well beyond the scope of a single chapter or book. The fundamental experiments have been briefly outlined and the typical information that is gleaned from the experiments pre‐ sented. Many other opportunities were not covered such as determining equilibrium dis‐ sociating constants, or use of spin labels for lead drug optimization. MRI has tremendous application for later stage *in vivo* applications of the drug development pipe‐ line. Together the MR technologies of NMR and MRI can cover the full range of natural product drug development from discovery through to clinical testing (Fig. 4).


**Figure 4.** Applications for nuclear MR technologies cover the entire range of natural product development from fun‐ damental molecular sciences through to practical health care. Spectroscopic applications (red) are delineated from imaging applications (blue).
