Biodistribution and radiation dosimetry of the carbonic anhydrase IX imaging agent [(18) F]VM4-037 determined from PET/CT scans in healthy volunteers

Molecular imaging for non-invasive risk stratification of renal masses

Anatomic imaging with contrast-enhanced computed tomography (CT) and magnetic resonance imaging (MRI) has long been the mainstay of renal mass characterization. However, those modalities are often unable to adequately characterize indeterminate, solid, enhancing renal masses - with some exceptions, such as the development of the clear-cell likelihood score on multi-parametric MRI. As such, molecular imaging approaches have gained traction as an alternative to anatomic imaging. Mitochondrial imaging with 99mTc-sestamibi single-photon emission computed tomography/CT is a cost-effective means of non-invasively identifying oncocytomas and other indolent renal masses. On the other end of the spectrum, carbonic anhydrase IX agents, most notably the monoclonal antibody girentuximab - which can be labeled with positron emission tomography radionuclides such as zirconium-89 - are effective at identifying renal masses that are likely to be aggressive clear cell renal cell carcinomas. Renal mass biopsy, which has a relatively high non-diagnostic rate and does not definitively characterize many oncocytic neoplasms, nonetheless may play an important role in any algorithm targeted to renal mass risk stratification. The combination of molecular imaging and biopsy in selected patients with other advanced imaging methods, such as artificial intelligence/machine learning and the abstraction of radiomics features, offers the optimal way forward for maximization of the information to be gained from risk stratification of indeterminate renal masses. With the proper application of those methods, inappropriately aggressive therapy for benign and indolent renal masses may be curtailed.

High-Affinity NIR-Fluorescent Inhibitors for Tumor Imaging via Carbonic Anhydrase IX

Tumor imaging and delivery of therapeutic agents may be achieved by designing high-affinity and high-selectivity compounds recognizing a tumor cell-expressing biomarker, such as carbonic anhydrase IX (CA IX). The CAIX, overexpressed in many hypoxic solid tumors, helps adjust to the energy requirements of the hypoxic environment, reduces intracellular acidification, and participates in the metastatic invasion of adjacent tissues. Here, we designed a series of sulfonamide compounds bearing CAIX-recognizing, high-affinity, and high-selectivity groups conjugated via a PEG linker to near-infrared (NIR) fluorescent probes used in the clinic for optically guided cancer surgery. We determined compound affinities for CAIX and other 11 catalytically active CA isozymes by the thermal shift assay and showed that the affinity Kd value of CAIX was in the subnanomolar range, hundred to thousand-fold higher than those of other CA isozymes. Similar affinities were also observed for CAIX expressed on the cancer cell surface in live HeLa cell cultures, as determined by the competition assay. The NIR-fluorescent compounds showed excellent properties in visualizing CAIX-positive tumors but not CAIX-negative knockout tumors in a nude mice xenograft model. These compounds would therefore be helpful in optically guided cancer surgery and could potentially be developed for anticancer treatment by radiotherapy.

Preclinical and pilot clinical evaluation of a small-molecule carbonic anhydrase IX targeting PET tracer in clear cell renal cell carcinoma

PURPOSE

Clear cell renal cell carcinoma (ccRCC) highly expresses carbonic anhydrase IX (CAIX). The purpose of this study was to evaluate 68Ga-NY104, a small-molecule CAIX-targeting PET agent, in tumor models of ccRCC and patients diagnosed with confirmed, or suspicious, ccRCC.

METHODS

The in vivo and ex vivo biodistribution of 68Ga-NY104 was investigated in CAIX-positive OS-RC-2 xenograft-bearing models. The binding of the tracer was further validated using autoradiography for human ccRCC samples. In addition, three patients with confirmed or suspicious ccRCC were studied.

RESULTS

NY104 can be labeled with high radiochemical yield and purity. It quickly cleared through kidney with α-half-life of 0.15 h. Discernible uptake is noted in the heart, lung, liver, stomach, and kidney. The OS-RC-2 xenograft demonstrated intense uptake 5 min after injection and gradually increased until 3 h after injection with ID%/g of 29.29 ± 6.82. Significant binding was detected using autoradiography on sections of human ccRCC tumor. In the three patients studied, 68Ga-NY104 was well-tolerated and no adverse events were reported. Substantial accumulation was observed in both primary and metastatic lesions in patient 1 and 2 with SUVmax of 42.3. Uptake in the stomach, pancreas, intestine, and choroid plexus was noted. The lesion in third patient was correctly diagnosed as non-metastatic for negative 68Ga-NY104 uptake.

CONCLUSION

68Ga-NY104 can efficiently and specifically bind to CAIX. Given the pilot nature of our study, future clinical studies are warranted to evaluate 68Ga-NY104 for detection of CAIX-positive lesions in patients with ccRCC.

TRIAL REGISTRATION

The clinical evaluation part of this study was retrospectively registered at ClinicalTrial.gov (NCT05728515) as NYPILOT on 6 Feb, 2023.

PET radiotracers and fluorescent probes for imaging human carbonic anhydrase IX and XII in hypoxic tumors

Positron emission tomography (PET) and fluorescent imaging play a pivotal role in medical diagnosis, biomedical oncologic research, and drug development process, which include identification of target location, target engagement, but also prove on mechanism of action or pharmacokinetics of new drug candidates. PET estimates physiological changes at the molecular level using specific radiotracers containing a short-lived positron emitting radionuclide such as fluorine-18 or carbon-11, whereas fluorescent imaging techniques use fluorescent probes labeled with suitable drug candidates for detection at the molecular level. The human carbonic anhydrase (hCA) isoforms IX and XII are overexpressed in hypoxic cancer cells, promoting tumor growth by regulating extra/intracellular pH, ferroptosis, and metabolism, being recognized as promising targets for anticancer theranostic agents. In this review, we have focused on PET radiotracers as well as fluorescent probes for diagnosis and treatment of tumors expressing hCA IX and hCA XII.

Radiotracers for positron emission tomography (PET) targeting tumour-associated carbonic anhydrase isoforms

The tumour-associated, cell membrane-bound isoforms IX and XII of human carbonic anhydrase (CA, EC 4.2.1.1) are overexpressed in cancer cells contributing to the hypoxic tumour pH/metabolism regulating machinery and as thus, can serve as markers of malignant neoplastic tissue. Inhibitors of CAs can be employed both for the treatment of hypoxic tumours and in the design of radiotracers for positron emission tomography and imaging of such cancers. The present review provides a comprehensive summary of the progress achieved to-date in the field of developing PET-tracers based on monoclonal antibodies, biomolecules, and small-molecule ligands of CA IX and XII.

A Microdosing Study with 99mTc-PHC-102 for the SPECT/CT Imaging of Primary and Metastatic Lesions in Renal Cell Carcinoma Patients

^99mTc-PHC-102 is a ^99mTc-labeled derivative of acetazolamide, a high-affinity small organic ligand of carbonic anhydrase IX (CAIX). ^99mTc-PHC-102 has previously shown favorable in vivo biodistribution properties in mouse models of CAIX-positive clear cell renal cell carcinoma (ccRCC) and colorectal cancer. In this study, we aimed to explore the targeting performance of ^99mTc-PHC-102 in SPECT in patients with renal cell carcinoma while also assessing the safety and tolerability of the radiotracer. Methods: We studied 5 patients with localized or metastatic ccRCC in a microdosing regimen, after the administration of a 50-μg total of CAIX ligand and 600-800 MBq of ^99mTc-PHC-102. Tissue distribution and residence time in normal organs and tumors were analyzed by serial SPECT/CT scans at 3 time points (30 min, 2 h, and 6 h) after intravenous administration. Results: In the 5 patients studied, ^99mTc-PHC-102 was well tolerated and no study drug-related adverse events were recorded. In the stomach, kidneys, and gallbladder, the radiotracer showed a rapid initial uptake, which cleared over time. Localization of the study drug in primary tumors of 5 patients was observed, with favorable tumor-to-background ratios. ^99mTc-PHC-102 SPECT/CT allowed the identification of 4 previously unknown lung and lymph node metastases in 2 patients. Conclusion: ^99mTc-PHC-102 is a promising SPECT tracer for the imaging of patients with ccRCC. This tracer has the potential to identify primary and metastatic lesions in different anatomic locations. ^99mTc-PHC-102 might also serve as a companion diagnostic agent for future CAIX-targeting therapeutics.

Amino Acids as Building Blocks for Carbonic Anhydrase Inhibitors

Carbonic anhydrases (CAs) are a superfamily of metalloenzymes widespread in all life, classified into seven genetically different families (α⁻θ). These enzymes catalyse the reversible hydration of carbonic anhydride (CO₂), generating bicarbonate (HCO₃-) and protons (H⁺). Fifteen isoforms of human CA (hCA I⁻XV) have been isolated, their presence being fundamental for the regulation of many physiological processes. In addition, overexpression of some isoforms has been associated with the outbreak or progression of several diseases. For this reason, for a long time CA inhibitors (CAIs) have been used in the control of glaucoma and as diuretics. Furthermore, the search for new potential CAIs for other pharmacological applications is a very active field. Amino acids constitute the smallest fundamental monomers of protein and, due to their useful bivalent chemical properties, are widely used in organic chemistry. Both proteinogenic and non-proteinogenic amino acids have been extensively used to synthesize CAIs. This article provides an overview of the different strategies that have been used to design new CAIs containing amino acids, and how these bivalent molecules influence the properties of the inhibitors.

Past, Present, and Future: Development of Theranostic Agents Targeting Carbonic Anhydrase IX

Theranostics is the integration of diagnostic information with pharmaceuticals to increase effectiveness and safety of cancer treatments. Nuclear medicine provides a non-invasive means to visualize drug target expression across primary and metastatic sites, and assess pharmacokinetics and efficacy of companion therapeutic agents. This is significant given the increasing recognition of the importance of clonal heterogeneity in treatment response and resistance. Carbonic anhydrase IX (CA-IX) has been advocated as an attractive diagnostic and therapeutic biomarker for targeting hypoxia in solid malignancies. CA-IX confers cancer cell survival under low oxygen tension, and is associated with increased propensity for metastasis. As such, CA-IX is overexpressed in a broad spectrum of cancers. Different classes of antigen recognition molecules targeting CA-IX including monoclonal antibodies, peptides, small molecule inhibitors, and antibody mimetics have been radiolabeled for imaging and therapeutic applications. cG250, a chimeric monoclonal antibody, has been labeled with an assortment of radionuclides (¹²⁴I, ¹¹¹In, ⁸⁹Zr, ¹³¹I, ⁹⁰Y, and ¹⁷⁷Lu) and is the most extensively investigated CA-IX radiopharmaceutical. In recent years, there have been tremendous advancements made by the research community in developing alternatives to cG250. Although still in preclinical settings, several small molecule inhibitors and antibody mimetics hold great promise in improving the management of aggressive and resistant cancers.

Evaluation of Nonpeptidic Ligand Conjugates for the Treatment of Hypoxic and Carbonic Anhydrase IX-Expressing Cancers

The majority of tumors contain regions of hypoxia, which cause marked phenotypic changes to resident cells. This altered gene expression often leads to increased resistance to anticancer treatments. Therefore, elimination of these resistant hypoxic cells is crucial to prevent disease recurrence. Herein, we describe the selective delivery of imaging and chemotherapeutic agents to cells expressing carbonic anhydrase IX (CA IX), a highly upregulated hypoxia receptor. These agents were conjugated to a potent divalent CA IX ligand through a hydrophilic PEG linker. These conjugates are shown to bind CA IX-expressing cells in a receptor-dependent manner in vitro with mid-nanomolar affinities and in vivo with good tumor selectivity. In a mouse xenograft tumor model using HT-29 cells, a cytotoxic tubulysin B conjugate completely inhibited tumor growth. Overall, the targeting of a hypoxia marker, such as CA IX, to selectively deliver imaging or chemotherapeutic agents may lead to better treatment options for solid, hypoxic tumors. In addition, the combination of standard chemotherapeutics that are most potent in normoxic dividing cells and drugs specifically designed to eliminate hypoxic nondividing cells may elicit a superior clinical outcome. Mol Cancer Ther; 16(3); 453-60. ©2016 AACR.

Imaging of carbonic anhydrase IX with an 111In-labeled dual-motif inhibitor

We developed a new scaffold for radionuclide-based imaging and therapy of clear cell renal cell carcinoma (ccRCC) targeting carbonic anhydrase IX (CAIX). Compound XYIMSR-01, a DOTA-conjugated, bivalent, low-molecular-weight ligand, has two moieties that target two separate sites on CAIX, imparting high affinity. We synthesized [¹¹¹In]XYIMSR-01 in 73.8–75.8% (n = 3) yield with specific radioactivities ranging from 118 – 1,021 GBq/μmol (3,200–27,600 Ci/mmol). Single photon emission computed tomography of [¹¹¹In]XYIMSR-01 in immunocompromised mice bearing CAIX-expressing SK-RC-52 tumors revealed radiotracer uptake in tumor as early as 1 h post-injection. Biodistribution studies demonstrated 26% injected dose per gram of radioactivity within tumor at 1 h. Tumor-to-blood, muscle and kidney ratios were 178.1 ± 145.4, 68.4 ± 29.0 and 1.7 ± 1.2, respectively, at 24 h post-injection. Retention of radioactivity was exclusively observed in tumors by 48 h, the latest time point evaluated. The dual targeting strategy to engage CAIX enabled specific detection of ccRCC in this xenograft model, with pharmacokinetics surpassing those of previously described radionuclide-based probes against CAIX.

[(18)F]VM4-037 MicroPET Imaging and Biodistribution of Two In Vivo CAIX-Expressing Tumor Models

Purpose

[¹⁸F]VM4-037 was recently developed as a positron emission tomography (PET) tracer for the detection of carbonic anhydrase IX (CAIX), a tumor-specific protein upregulated under hypoxic conditions. In this study, the accumulation of [¹⁸F]VM4-037 was determined in two CAIX-expressing preclinical human tumor models.

Procedures

U373 and HT29 tumor-bearing animals were injected with [¹⁸F]VM4-037 and underwent microPET imaging up to 4 h post-injection (p.i.). Biodistribution throughout the different organs was assessed at 2 and 4 h p.i. using gamma counting.

Results

MicroPET imaging showed high [¹⁸F]VM4-037 uptake in the abdominal region, and biodistribution revealed high radioactivity in the kidney, ileum, colon, liver, stomach, and bladder. Although high CAIX expression was confirmed in both tumor models, tumor uptake assessed with microPET and biodistribution experiments was comparable to background tissues.

Conclusions

In this study, [¹⁸F]VM4-037 does not specifically accumulate in CAIX-expressing tumors, indicating that the tracer is not suitable for the detection of CAIX.

PET/CT imaging of renal cell carcinoma with (18)F-VM4-037: a phase II pilot study

BACKGROUND

Carbonic anhydrase IX (CA-IX) is a potential imaging biomarker of clear cell renal cell carcinoma (ccRCC). Here, we report the results of a phase II clinical trial of a small molecule radiotracer targeting CA-IX ((18)F-VM4-037) in ccRCC.

METHODS

Between October 2012 and May 2013, 11 patients with kidney masses underwent (18)F-VM4-037 PET/CT prior to surgery. Dynamic imaging was performed for the first 45 min post injection and whole-body imaging was obtained at 60 min post injection. Tumors were surgically excised or biopsied within 4 weeks of imaging.

RESULTS

All patients tolerated the radiotracer well with no adverse events. Ten of the 11 patients had histologically confirmed malignancy. One patient had a Bosniak Type 3 cyst with no tumor found at surgery. Two patients had extrarenal disease and 9 had tumors only in the kidney. Primary ccRCC lesions were difficult to visualize on PET alone due to high uptake of the tracer in the adjacent normal kidney parenchyma, however when viewed in conjunction with CT, the tumors were easily localized. Metastatic lesions were clearly visible on PET. Mean SUV for primary kidney lesions was 2.55 in all patients; in patients with histologically confirmed ccRCC, the mean SUV was 3.16. The time-activity curves (TAC) are consistent with reversible ligand binding with peak activity concentration at 8 min post injection followed by washout. Distribution Volume Ratio (DVR) of the lesions was measured using the Logan graphical analysis method. The mean DVR value across the 9 kidney lesions was 5.2 ± 2.8, (range 0.68-10.34).

CONCLUSION

18F-VM4-037 is a well-tolerated PET agent that allows same day imaging of CA-IX expression. The agent demonstrated moderate signal uptake in primary tumors and excellent visualization of CA-IX positive metastases. While the evaluation of primary ccRCC lesions is challenging due to high background activity in the normal kidney parenchyma, 18F-VM4-037 may be most useful in the evaluation of metastatic ccRCC lesions.

Radiopharmaceuticals as probes to characterize tumour tissue

Tumour cells exhibit several properties that allow them to grow and divide. A number of these properties are detectable by nuclear imaging methods. We discuss crucial tumour properties that can be described by current radioprobe technologies, further discuss areas of emerging radioprobe development, and finally articulate need areas that our field should aspire to develop. The review focuses largely on positron emission tomography and draws upon the seminal 'Hallmarks of Cancer' review article by Hanahan and Weinberg in 2011 placing into context the present and future roles of radiotracer imaging in characterizing tumours.

Hypoxia-induced carbonic anhydrase IX as a target for cancer therapy: from biology to clinical use

The tumor microenvironment includes a complicated network of physiological gradients contributing to plasticity of tumor cells and heterogeneity of tumor tissue. Hypoxia is a key component generating intratumoral oxygen gradients, which affect the cellular expression program and lead to therapy resistance and increased metastatic propensity of weakly oxygenated cell subpopulations. One of the adaptive responses of tumor cells to hypoxia involves the increased expression and functional activation of carbonic anhydrase IX (CA IX), a cancer-related cell surface enzyme catalyzing the reversible conversion of carbon dioxide to bicarbonate ion and proton. Via its catalytic activity, CA IX participates in regulation of intracellular and extracellular pH perturbations that result from hypoxia-induced changes in cellular metabolism producing excess of acid. Through the ability to regulate pH, CA IX also facilitates cell migration and invasion. In addition, CA IX has non-catalytic function in cell adhesion and spreading. Thus, CA IX endows tumor cells with survival advantages in hypoxia/acidosis and confers an increased ability to migrate, invade and metastasize. Accordingly, CA IX is expressed in a broad range of tumors, where it is associated with prognosis and therapy outcome. Its expression pattern and functional implications in tumor biology make CA IX a promising therapeutic target, which can be hit either by immunotherapy with monoclonal antibodies or with compounds inhibiting its enzyme activity. The first strategy has already reached the clinical trials, whereas the second one is still in preclinical testing. Both strategies indicate that CA IX can become a clinically useful anticancer target, but urge further efforts toward better selection of patients for immunotherapy and deeper understanding of tumor types, clinical situations and synthetic lethality interactions with other treatment approaches.