Insight into the Development of PET Radiopharmaceuticals for Oncology

Abstract

While the development of positron emission tomography (PET) radiopharmaceuticals closely follows that of traditional drug development, there are several key considerations in the chemical and radiochemical synthesis, preclinical assessment, and clinical translation of PET radiotracers. As such, we outline the fundamentals of radiotracer design, with respect to the selection of an appropriate pharmacophore. These concepts will be reinforced by exemplary cases of PET radiotracer development, both with respect to their preclinical and clinical evaluation. We also provide a guideline for the proper selection of a radionuclide and the appropriate labeling strategy to access a tracer with optimal imaging qualities. Finally, we summarize the methodology of their evaluation in in vitro and animal models and the road to clinical translation. This review is intended to be a primer for newcomers to the field and give insight into the workflow of developing radiopharmaceuticals.

Keywords: diagnostic imaging; personalized medicine; positron emission tomography; radiochemistry; radiopharmaceuticals.

Figure 1

Immuno-positron emission tomography (immuno-PET) imaging of prostate cancer with [89Zr]Zr-11B6. (a) Coronal projection images in mice bearing LNCaP tumor xenograft. Longitudinal imaging shows continued uptake in tumor (T) with progressive clearance from liver (L). (b) Ex vivo biodistribution of activity in tumor and normal organs at 320 h p.i. (c) Time-activity curves in %IA/g of tumors (squares) and blood (circles) for different doses of antibody. (d) Greater uptake observed in human kallikrein 2 producing VCaP model compared to LnCaP and nonproducing DU145 xenografts, indicating specificity. Uptake can also be blocked with excess antibody. Figure reproduced with permission from Sci. Transl. Med. 2016, 8(367): 367ra167 [114].

Figure 2

Examples of PET radiopharmaceuticals based on bioactive molecules.

Figure 3

Examples of ¹⁸F-labeled amino acid derivatives. (a) Representative structures of natural amino acids and the synthetic boramino acid variants. (b) In vivo PET projection images of [¹⁸F]FDG and [¹⁸F]Phe-BF₃ of the brain, U87MG tumor xenograft, and site of inflammation, respectively. Maximum intensity projection (MIP) images show activity accumulation in tumor, gallbladder, and bladder. Figure adapted with permissions from Sci. Adv. 2015, 1(8): e1500694 [120], under a Creative Commons Attribution-NonCommerical (CC BY-NC 4.0) License. (c) Radiofluorination of an unactivated C–H bond (i.e., lacking a leaving group or an activating proximal functional group) and radiofluorination of an activated C–H bond, via a triflate leaving group for nucleophilic substitution.

Figure 4

Somatostatin receptor imaging. (a) Chemical structure of [⁶⁸Ga]Ga-DOTA-TATE. (b) SSTR2 expression in normal tissues and neoplastic tissues. Reproduced with permissions from Pharmacol. Rev. 2018, 70(4): 763–835 [128]. (c) In vivo SSTR2 imaging in a patient with metastatic low-grade cecal NET. [¹¹¹In]In-pentetreotide scintigraphy (left) with [⁶⁸Ga]Ga-DOTA-TATE PET (right) was performed before radiotherapy. In liver, retroperitoneal and thoracic lymph nodes, and bones, PET shows multiple metastases, many of which are undetectable on scintigraphy. Figure reproduced with permission from J. Nucl. Med. 2016, 57(12): 1949–1956 [129]. Copyright 2016 Society of Nuclear Medicine and Molecular Imaging.

Figure 5

Prostate-specific membrane antigen imaging. (a) Chemical structures of several PSMA imaging agents. The four radiopharmaceuticals share a Glu-urea-Lys binding motif (in blue). (b) [⁶⁸Ga]Ga-PSMA-11 PET maximum intensity projection (MIP) images at baseline and 3 months after [¹⁷⁷Lu]Lu-PSMA-617 treatment in eight patients with PSA decline of ≥98% in a prospective phase II study. Lesions with standardized uptake value (SUV) over three are highlighted in red. PSA values (ng/mL) are indicated below MIP images. Figure reproduced with permission from J. Nucl. Med. 2019, jnumed.119.236414 [93]. Copyright 2019 Society of Nuclear Medicine and Molecular Imaging.

Figure 6

Fibroblast activation protein imaging. (a) Chemical structures of FAP-targeted radiopharmaceuticals, which were investigated in detail preclinically and/or clinically. Radionuclides in parentheses were used for preclinical studies. The compounds share a common binding motif (in blue). Figure reproduced with permission from EJNMMI Radiopharm. Chem. 2019, 4:16 [53], under a Creative Commons Attribution 4.0 International License. (b) Maximum-intensity projection (MIP) images of [⁶⁸Ga]Ga-FAPI-04 PET/CT in patients reflecting 15 different histologically proven tumor entities. Ca = cancer; CCC = cholangiocellular carcinoma; CUP = carcinoma of unknown primary; MTC = medullary thyroid cancer; NET = neuroendocrine tumor. Figure reproduced with permission from J. Nucl. Med. 2019, 60(6): 801–805 [54]. Copyright 2019 Society of Nuclear Medicine and Molecular Imaging.

Figure 7

Programmed cell death protein (PD-1)/programmed death-ligand 1 (PD-L1) imaging. (a) Blocking the binding of PD-L1 to PD-1 with an immune checkpoint inhibitor (anti-PD-L1 or anti-PD-1) allows T cells to kill tumor cells. Figure courtesy of Terese Winslow for the National Cancer Institute © (2020) Terese Winslow LLC, U.S. Govt. has certain rights. (b) PET scans of two patients imaged with [¹⁸F]FDG that measures glucose metabolism (left), [¹⁸F]BMS-986192 that measures PD-L1 expression (middle), and [⁸⁹Zr]Zr-nivolumab (right) that measures PD-1 expression. Heterogenous tracer uptake observed between and within lesions. Figure was reproduced with permission from Nat. Commun. 2018, 9: 4664 [100], under a Creative Commons Attribution 4.0 International License.

Figure 8

Integrin α₄β₁ targeting by LLP2A_. (a) The design of the diverse and focused library targeting integrin α₄β_1, leading to the identification of the LLP2A pharmacophore. (b) Chemical structure of LLP2A-CB- LLP2A-CB-TE1A1P, a precursor for ⁶⁴Cu-labeling currently being evaluated in Phase I clinical trials. (c) PET/CT images produced by [⁶⁴Cu]Cu-LLP2A-CB-TE1A1P in B16F10 xenograft mice acquired at 2, 4, and 24 h post-injection. Figure adapted with permission from J. Nucl. Med. 2014, 55(11): 1856–1863 [169]. Copyright 2014 Society of Nuclear Medicine and Molecular Imaging.

Figure 9

Radiopharmaceutical automation. (a) Photograph of a radiochemist setting up an automated synthesis module. (b) Example of a graphical user interface for radiopharmaceutical synthesis. Images courtesy of Trasis.

Figure 10

An overview of the radiopharmaceutical developmental pathway. Radiopharmaceuticals undergo comprehensive (radio)chemical, in vitro, and in vivo characterization before they can advance into clinical testing.