Exploring new frontiers in molecular imaging: Emergence of 68Ga PET/CT

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Nuclear medicine imaging of endocrine neoplasms

Endocrine tumors are hormonally active benign or malignant neoplasms arising within endocrine organs or from specialized cells of the amine precursor uptake and decarboxylation system. The detection rate of these tumors is increasing as a result of sensitive biochemical tests and high-resolution diagnostic imaging. Medical imaging has become a key component in the diagnosis and staging of endocrine malignancies; however, despite the impressive advances in computed tomography (CT) and MRI, detection of small primary tumors and metastases continues to be a challenge. Functional imaging techniques use radiopharmaceuticals targeted at unique tumor cellular processes in order to provide sensitive and highly specific whole-body imaging. Functional imaging allows prediction of the efficacy of radionuclide or receptor-based therapies and surveillance after therapy. Advances in imaging have not been limited to radiopharmaceuticals. Hybrid scanner technology in the form of PET/CT and single photon emission computed tomography (SPECT)/CT, designed to combine functional images with anatomic maps, has further improved the diagnostic accuracy. High-resolution hybrid imaging when deployed with novel PET and SPECT radiopharmaceuticals has the potential to dramatically change, individualize, and optimize imaging plans based on the histological grade, degree of differentiation, and genetic profile of each patient's endocrine neoplasm.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Investigation of direct production of 68Ga with low energy multiparticle accelerator

Experimentally measured cross sections are presented for the natCu(α, xn)66,67,68Ga and 68Zn(p, xn)67,68Ga nuclear processes up to 36 and 20 MeV, respectively. Based on these results and the reliable cross section data available in the literature, the possible thick target yields were also calculated. Two different 68Ga production routes (Cu+α and Zn+p) are discussed in detail, especially with regard to the 66Ga and 67Ga contamination levels as a function of the target enrichment level and the incident bombarding energies. Both processes can be used for in-house 68Ga production with low (<1%) 66Ga and/or 67Ga EOB contamination using enriched 68Zn (>80%) or 65Cu (>95%) target. The maximum available yield on 100% enriched 68Zn and 65Cu (irradiation time: 2.25 h; bombarding energy: Ep=20 MeV and E α =18 MeV) is 352.45 mCi/μA (13.04 GBq/μA) and 14.28 mCi/μA (528.36 MBq/μA), respectively.

A steroid-conjugated magnetic resonance probe enhances contrast in progesterone receptor expressing organs and tumors in vivo

Progesterone receptor (PR) is a significant biomarker in diseases such as endometriosis and breast, ovarian, and uterine cancers that is associated with disease prognosis and therapeutic efficacy. While receptor status is currently determined by immunohistochemistry assays, the development of noninvasive PR imaging agents could improve molecular characterization, treatment decisions, and disease monitoring. ProGlo, a progesterone-conjugated magnetic resonance imaging (MRI) contrast agent, was evaluated in vivo to determine whether it targets and enhances signal intensity in organs and tumors that express high PR levels. A tissue distribution study indicated that ProGlo accumulates in the PR-rich uterus, which was confirmed by in vivo imaging studies. Ex vivo images of these organs revealed that ProGlo was distributed in the substructures that express high PR levels. In xenograft tumor models, ProGlo was taken up to a greater extent than the nonfunctionalized Gd-DO3A in tumors, particularly in PR(+) tumors. The ability to accumulate and enhance signal intensity in PR(+) organs and tumors suggests that ProGlo may be a promising MRI probe for PR(+) diseases.