PET Imaging of Small Animals: Molecules for Molecular Imaging

1. PET Imaging of Small Animals:Molecules for Molecular Imaging Department of Chemistry and the McMaster Institute of Applied Radiation Sciences With the support of : The Department of Nuclear Medicine, Hamilton Health Sciences

2. Molecular (Radio) Imaging • Diagnostic techniques that allow for the study of • specific biochemical processes • Can be done in vivo (SPECT, planar imaging, PET) or • in vitro (fluorescence, autoradiography) • Requires the development of radiolabeled compounds • (contrast agents)

3. Radiotracers • Compounds which carry a radionuclide whose emissions can be tracked by an external detector • Gamma emitters (SPECT/planar imaging) • Positron emitters (PET) • Used to follow the distribution and biochemistry of a particular compound

4. The Tracer Principle The amount of tracer used in a study is so small that it does not perturb the biological system under investigation.

5. Areas of Research • The preparation and testing of radiopharmaceuticals and fluorescent compounds for: • Clinical diagnostic imaging (SPECT and PET) • Drug testing • Radiotherapy • Novel radiolabeling and drug discovery techniques

6. Specifics • Technetium (Tc) labeled peptides and hormones • Small molecule Tc complexes for molecular imaging • 18F labeled hormones for PET

7. Technetium-99m (99mTc) • Gives off gamma rays that can be detected by a scintillation camera • The most widely used radionuclide in diagnostic medicine (ideal nuclear properties, readily available, low cost)

8. Tc-Essential Compounds • “Simple” complexes-distribution determined based on charge, size, lipophilicity etc.

9. Molecular Radioimaging • Use a biomolecule to guide 99mTc to a particular receptor • Requirements: • Bond between Tc and the biomolecule will not degrade in vivo • Attachment of Tc will not alter the selectivity of the parent targeting agent

10. Tc is a metal

11. Bifunctional Chelates • Used to append Tc to biomolecules • The chelate must be able to form a Tc complex in water where the metal is in very low concentrations (10-8 M to 10-9 M) • All reactions begin with TcO4-

12. Problems • Existing Tc ligands are large • Metal complexes are susceptible to premature degradation in vivo • Limited synthetic flexibility

13. Project 1: Develop better ligands Linker site Metal binding • Stable • Prepared in high yield • Easily linked to targeting agents

14. Peptides • Small peptides have proven to be effective targeting agents 111In-OctreoScan

15. Amino Acid Derived Chelates lysine

16. Automated Synthesis

17. Library Construction • Use the automated synthesizer to prepare families of analogues in parallel

18. Radiolabel

19. Synthesis of the metal complex

20. Re and Fluorescence

21. Correlating in vivo and in vitro molecular imaging Screen RBA’s in vitro and in vivo Look for localization at the cellular level

22. Summary • Prepare libraries of any peptide (up to 30 amino acids) carrying a fluorescent tag or a radiolabel • Correlated in vitro and in vivo imaging • Molecular therapy using radioactive Re-isotopes

23. Small Molecule Tc Radiopharmaceuticals Designed to image biological processes • Amino acid metabolism • Carbohydrate utilization • Challenge: You need a small Tc-compound that is stable in vivo and easily functionalized

24. Carboranes

25. Synthetic Versatility

26. Functionalized Carboranes Amines, amino acids Carbohydrates ER agonists/antagonists

27. Molecular Imaging • Targeting receptors that exits in low concentrations is a real challenge • You must not violate the tracer principle

28. The Excess Ligand Problem Large excess Must be removed

29. First Year Chemistry “Like dissolves like”, “Oil and Water don’t mix” Polar compounds have affinity for each other Non-polar compounds have affinity for each other Fluorine rich compounds have affinity for each other

30. Fluorous Labeling Strategy R = Biomolecule X= Radionuclide

31. Removing the Fluorous Material Solid-Phase Extraction

32. 125I Labeling Used to tag proteins, antibodies

33. Purification

34. Radiochromatogram-Crude rxn mixture

35. What about the excess starting material? Crude Rxn 1 Rxn 2

36. The Fluorous Labeling System • Rapid and “hands free” synthesis and labeling of compounds • Applicable to a significant number of different isotopes: • Iodine-123 (SPECT) • Iodine-125 (Therapy and animal imaging) • Iodine-124 (PET) • Fluorine-18 (PET) • Carbon-11 (PET)

37. Imaging in Drug Development • Look at the distribution of a labeled version of a new drug candidate • Study receptor specificity of new compounds (screening in vivo) • Evaluate different drug delivery systems

38. Radiolabeled Compounds for Drug Development PET SPECT

39. The Future • Animal Imaging (animal PET and SPECT/CT) • Cancer imaging and therapy • Targeted MRI and CT contrast agents

40. Acknowledgements Syracuse University Molecular Insight Pharma. Inc.