Targeting radiopharmaceuticals: comparative biodistribution studies of gallium and indium complexes of multidentate ligands

Head-to-head comparison of [68 Ga]Ga-P16-093 and [68 Ga]Ga-PSMA-617 in dynamic PET/CT evaluation of the same group of recurrent prostate cancer patients

PURPOSE

This study was prospectively designed to evaluate the early dynamic organ distribution and tumor detection capability of [⁶⁸ Ga]Ga-P16-093, which was compared with [⁶⁸ Ga]Ga-PSMA-617 in the same group of recurrent prostate cancer patients.

METHODS

Twenty patients with recurrent prostate cancer were enrolled. In 2 consecutive days, each patient underwent a 60-min dynamic PET/CT scan after intravenous administration of 148-185 MBq (4-5 mCi) [⁶⁸ Ga]Ga-P16-093 and [⁶⁸ Ga]Ga-PSMA-617, respectively. Following a low-dose CT scan, serial dynamic PET scans were performed from head to proximal thigh at 9 time points (30 s/bed at 4, 7, 10, 13, and 16 min; 1 min/bed at 20, 30, and 45 min; and 2 min/bed at 60 min). Standardized uptake values were measured for semi-quantitative comparison.

RESULTS

[⁶⁸ Ga]Ga-P16-093 PET/CT revealed a significantly higher tumor uptake at 4 min (SUVmax 7.88 ± 5.26 vs. 6.01 ± 3.88, P < 0.001), less blood pool retention at 4 min (SUVmean 5.12 ± 1.16 vs. 6.14 ± 0.98, P < 0.001), and lower bladder accumulation at 60 min (SUVmean 31.33 ± 27.47 vs. 48.74 ± 34.01, P = 0.042) than [⁶⁸ Ga]Ga-PSMA-617 scan. Significantly higher [⁶⁸ Ga]Ga-P16-093 uptakes were also observed in the parotid gland, liver, spleen, and kidney. Besides, [⁶⁸ Ga]Ga-P16-093 exhibited a better detectability of tumor than [⁶⁸ Ga]Ga-PSMA-617 (366 vs. 321, P = 0.009).

CONCLUSIONS

[⁶⁸ Ga]Ga-P16-093 showed advantages over [⁶⁸ Ga]Ga-PSMA-617 with higher tumor uptakes, tumor-to-blood pool ratio and detection capability, less blood pool, and bladder accumulation in recurrent prostate cancer patients.

TRIAL REGISTRATION

[⁶⁸ Ga]Ga-P16-093 and [⁶⁸ Ga]Ga-PSMA-617 PET/CT Imaging in the Same Group of Prostate Cancer Patients (NCT04796467, Registered 12 March 2021, retrospectively registered) URL of registry: https://clinicaltrials.gov/ct2/show/NCT04796467.

[68Ga]Ga-P16-093 as a PSMA-Targeted PET Radiopharmaceutical for Detection of Cancer: Initial Evaluation and Comparison with [68Ga]Ga-PSMA-11 in Prostate Cancer Patients Presenting with Biochemical Recurrence

PURPOSE

This study was undertaken to evaluate radiation dosimetry for the prostate-specific membrane antigen targeted [⁶⁸Ga]Ga-P16-093 radiopharmaceutical, and to initially assess agent performance in positron emission tomography (PET) detection of the site of disease in prostate cancer patients presenting with biochemical recurrence.

PROCEDURES

Under IND 133,222 and an IRB-approved research protocol, we evaluated the biodistribution and pharmacokinetics of [⁶⁸Ga]Ga-P16-093 with serial PET imaging following intravenous administration to ten prostate cancer patients with biochemical recurrence. The recruited subjects were all patients in whom a recent [⁶⁸Ga]Ga-PSMA-11 PET/X-ray computed tomography (CT) exam had been independently performed under IND 131,806 to assist in decision-making with regard to their clinical care. Voided urine was collected from each subject at ~ 60 min and ~ 140 min post-[⁶⁸Ga]Ga-P16-093 injection and assayed for Ga-68 content. Following image segmentation to extract tissue time-activity curves and corresponding cumulated activity values, radiation dosimetry estimates were calculated using IDAC Dose 2.1. The prior [⁶⁸Ga]Ga-PSMA-11 PET/CT exam (whole-body PET imaging at 60 min post-injection, performed with contrast-enhanced diagnostic CT) served as a reference scan for comparison to the [⁶⁸Ga]Ga-P16-093 findings.

RESULTS

[⁶⁸Ga]Ga-P16-093 PET images at 60 min post-injection provided diagnostic information that appeared equivalent to the subject's prior [⁶⁸Ga]Ga-PSMA-11 scan. With both radiopharmaceuticals, sites of tumor recurrence were found in eight of the ten patients, identifying 16 lesions. The site of recurrence was not detected with either agent for the other two subjects. Bladder activity was consistently lower with [⁶⁸Ga]Ga-P16-093 than [⁶⁸Ga]Ga-PSMA-11. The kidneys, spleen, salivary glands, and liver receive the highest radiation exposure from [⁶⁸Ga]Ga-P16-093, with estimated doses of 1.7 × 10^-1, 6.7 × 10^-2, 6.5 × 10^-2, and 5.6 × 10^-2 mGy/MBq, respectively. The corresponding effective dose from [⁶⁸Ga]Ga-P16-093 is 2.3 × 10^-2 mSv/MBq.

CONCLUSIONS

[⁶⁸Ga]Ga-P16-093 provided diagnostic information that appeared equivalent to [⁶⁸Ga]Ga-PSMA-11 in this limited series of ten prostate cancer patients presenting with biochemical recurrence, with the kidneys found to be the critical organ. Diminished tracer appearance in the urine represents a potential advantage of [⁶⁸Ga]Ga-P16-093 over [⁶⁸Ga]Ga-PSMA-11 for detection of lesions in the pelvis.

Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging is an essential tool for disease diagnosis and management; all marketed clinical magnetic resonance imaging (MRI) contrast agents (CAs) are gadolinium (Gd) chelates and most are extracellular fluid (ECF) agents. After intravenous injection, these agents rapidly distribute to the extracellular space and are also characterized by low serum protein binding and predominant renal clearance. Gd is an abiotic element with no biological recycling processes; low levels of Gd have been detected in the central nervous system and bone long after administration. These observations have prompted interest in the development of new MRI contrast agents based on biotic elements such as iron (Fe); Fe-HBED (HBED = N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid), a coordinatively saturated iron chelate, is an attractive MRI CA platform suitable for modification to adjust relaxivity and biodistribution. Compared to the parent Fe-HBED, the Fe-HBED analogs reported here have lower serum protein binding and higher relaxivity as well as lower relative liver enhancement in mice, comparable to that of a representative gadolinium-based contrast agent (GBCA). Fe-HBED analogs are therefore a promising class of non-Gd ECF MRI CA.

Metal-Based Complexes as Pharmaceuticals for Molecular Imaging of the Liver

This article reviews the use of metal complexes as contrast agents (CA) and radiopharmaceuticals for the anatomical and functional imaging of the liver. The main focus was on two established imaging modalities: magnetic resonance imaging (MRI) and nuclear medicine, the latter including scintigraphy and positron emission tomography (PET). The review provides an overview on approved pharmaceuticals like Gd-based CA and ^99mTc-based radiometal complexes, and also on novel agents such as ⁶⁸Ga-based PET tracers. Metal complexes are presented by their imaging modality, with subsections focusing on their structure and mode of action. Uptake mechanisms, metabolism, and specificity are presented, in context with advantages and limitations of the diagnostic application and taking into account the respective imaging technique.

Photoradiosynthesis of 68Ga-Labeled HBED-CC-Azepin-MetMAb for Immuno-PET of c-MET Receptors

In an alternative approach for radiotracer design, a photoactivatable HBED-CC-PEG₃-ArN₃ chelate was synthesized and photoconjugated to the anti-c-MET antibody MetMAb (onartuzumab). Photoconjugation gave the functionalized protein HBED-CC-azepin-MetMAb with a photochemical conversion of 18.5 ± 0.5% ( n = 2) which was then radiolabeled with ⁶⁸Ga³⁺ ions. The purified and formulated [⁶⁸Ga]GaHBED-CC-azepin-MetMAb radiotracer was evaluated in vitro and in vivo. Standard stability tests and cellular binding assays confirmed that the radiotracer remained radiochemically pure and immunoreactive after photochemical conjugation. [⁶⁸Ga]GaHBED-CC-azepin-MetMAb showed specific uptake in c-MET-positive MKN-45 (high-expression) and PC-3 (low/moderate expression) tumors with tumor-associated activities at 6 h post-administration of 10.33 ± 1.27 ( n = 5) and 3.88 ± 1.27 ( n = 3) %ID/g, respectively. In competitive blocking experiments, MKN-45 tumor uptake was reduced by approximately 55% ( P-value <0.001 compared with nonblocked experiments) confirming specific radiotracer binding to c-MET in vivo. Radiochemical, cellular, and in vivo experiments confirmed that the photoradiochemical approach is a viable tool to synthesize new radiotracers for immuno-PET.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

Evaluation of gallium-68 tris(2-mercaptobenzyl)amine: a complex with brain and myocardial uptake

Previous research into development of a gallium-radiolabeled agent that crosses the blood-brain barrier has met with limited success. In this study, we focused our attention on a Ga(III) complex of a 4-coordinate amine trithiolate tripod ligand, tris(2-mercaptobenzyl) amine (S3N). The Ga(III) S3N complex is small, neutral, and lipophilic, meeting the requirements for a potential brain imaging agent. The Ga-68 complex was easily formed with a radiochemical purity of >95%. In vitro stability of the Ga-S3N complex, determined in rat serum incubated at 37 degrees C, was greater than 95% intact at 2 h by silica gel and reversed-phase radio-thin layer chromatography. Biodistribution studies conducted in female Sprague-Dawley rats showed the complex cleared rapidly from the blood with initial high liver uptake followed by rapid washout. Significant uptake was observed in the brain, with brain:blood ratios increasing from 0.11 at 2 min postinjection to 3.8 at 60 min postinjection. Uptake was also observed in the heart going from a heart:blood ratio of 2.3 at 2 min postinjection to 11 at 60 min postinjection. Molecular mechanics were used to determine the coordination number, and demonstrated that the Ga(III) complex prefers to be 4-coordinate. Imaging studies with 68Ga-S3N in a Nemestrina macaque showed significant brain uptake, similar to other lipophilic agents. The extraction of 68Ga-S3N into the brains of both rodents and primates, higher than any 68Ga agent reported in the literature, suggests that this compound may have potential as a brain imaging agent for positron emission tomography.

Immunoscintigraphy with a new indium-111-labeled monoclonal antibody (MAb 1A3) in patients with colorectal cancer

PURPOSE

This study was designed to evaluate a new anticolorectal carcinoma monoclonal antibody (1A3), conjugated with the bifunctional chelating agent N,N'-bis(2-hydroxybenzyl)1(4-bromoacetamidobenzyl)1,2-ethylenediam ine-N,N'- diacetic acid and labeled with indium-111, in a Phase I/II study involving 38 patients with localized or advanced colorectal cancer.

METHODS

Patients were injected with indium-111-N,N'-bis(2-hydroxybenzyl) 1(4-bromoacetamidobenzyl)1,2-ethylenediamine-N,N'-diacetic acid-monoclonal antibody 1A3 (1-50 mg, 1-5 mCi) and imaged at two or three sessions one to five days later. Scintigraphic findings were compared with radiologic, pathologic, surgical, and other clinical findings to assess the accuracy of radioimmunoscintigraphy.

RESULTS

At least one known tumor site was clearly defined by planar scintigraphy in 29 (76 percent) patients. Increased radioactivity was seen in 40 of 63 known tumor sites (37/43 abdominal-pelvic, 3/15 hepatic, and 0/5 pulmonary sites) without any apparent dose-related effects. Nineteen previously undetected sites were considered positive by imaging, and, of these, six were biopsy-proven tumor sites, four were probable tumor sites, three were definitely false positive sites, and six were probable false positive sites. Radioimmunoscintigraphy detected proven tumor in 15 of 16 patients with negative or equivocal computed tomography results. Of of the 28 patients with rectosigmoid cancer, 25 (89 percent) had positive studies with 34 of 47 tumor sites showing definite uptake on the scintigrams. This included 3 of 9 hepatic metastases. The only adverse reaction occurred in one patient who developed transient hives. Human anti-mouse antibody responses occurred in approximately one-half of the patients injected with doses of 10 or 50 mg.

CONCLUSION

This study shows that radioimmunoscintigraphy with this indium-111-labeled monoclonal antibody is safe, it can detect most nonhepatic abdominal-pelvic tumors with a positive predictive value of 83 (44/53) percent, and it should prove to be useful, particularly in the diagnosis of recurrent rectal carcinoma.