Targeting Integrins with Radiolabeled RGD Analogues for Radiotheranostics of Metastatic Radioactive Iodine Nonresponsive Thyroid Cancer: New Avenues in Personalized Medicine

Targeting integrins with radioligands has gained a lot of attention in recent years in the field of oncology (1). Integrins are a group of cell adhesion receptors consisting of 24 transmembrane heterodimers generated from a combination of 18α integrin and 8β integrin subunits. Integrins can be classified into receptors recognizing Arg-Gly-Asp (RGD) peptide motifs, collagen receptors, laminin receptors, and leukocyte-specific integrins (2). The pattern of integrin expression on the cell surface determines cell response to microenvironmental factors and regulates proliferation, migration, polarity, local invasion, differentiation, and survival (2). All these processes are important in carcinogenesis, hence altered integrin expression patterns have been associated with many types of cancers (2). Integrin αvβ3 has been found to be a key element associated with the neovascularization of tumors, which is required for tumor growth. Integrin αvβ3 is also important in regulating the metastatic potential of cancer cells by affecting cell motility through its interactions with fibronectin and enhancing the ability of cancer cells to survive in the circulation by increasing resistance to detachment-induced cell death (anoikis) (1). In fact, in breast and lung cancer, increased αvβ3 integrin expression is associated with a cancer stem cell phenotype, which is associated with resistance to anoikis and increased metastases (2). Integrin αvβ3 has been considered as an attractive drug target in cancer. However, despite encouraging preclinical studies, a selective αvβ3 and αvβ5 integrin inhibitor (cilengitide), in combination with standard of care (radiotherapy with the chemotherapeutic agent temozolomide), failed to improve survival of patients with glioblastoma (3). Another approach is utilizing the RGD motif to bind radiolabeled analogues. Radiolabeling with positron emitters such as ⁶⁸Ga has been used for imaging with positron emission tomography/computed tomography (PET/CT), while radiolabeling with β-radiation emitters, such as ¹⁷⁷Lu, has been utilized for treatment. Most of the currently available integrin-targeting imaging and therapeutic probes are based on the RGD tripeptide sequence because of its high affinity and specificity for integrin αvβ3.

Based on RNA seq data from The Cancer Genome Atlas, the expression of αv and β3 integrin subunits is particularly high in thyroid cancer. αvβ3 integrin overexpression in thyroid cancer is likely due to its presence on both the activated endothelial cells of cancer neovasculature and the cancer cell surface, as shown by several reports documenting αvβ3 integrin expression in thyroid cancer cell lines (4,5).

In this issue of Thyroid, Parihar et al. evaluated the diagnostic accuracy of radiolabeled αvβ3 integrin analogue ⁶⁸Ga-DOTA-RGD2 PET/CT in patients with radioactive iodine (RAI) nonavid thyroid cancer (6). The authors compared the diagnostic performance of ⁶⁸Ga-DOTA-RGD2 PET/CT with ¹⁸F-FDG PET/CT—the standard of care imaging modality for patients with RAI nonavid thyroid cancer. While the two imaging modalities had similar sensitivity, the specificity of ⁶⁸Ga-DOTA-RGD2 PET/CT was significantly higher (6). This study is consistent with a case report by Vatsa et al., who presented a case report revealing the usefulness of ⁶⁸Ga-DOTA-RGD2 PET/CT in the detection of metastases in a patient with RAI-nonavid thyroid cancer (7). One of the first radiolabeled RGD analogues used for PET/CT, ¹⁸F-galacto-RGD, also showed similar sensitivity to ¹⁸F-FDG PET/CT (8). Similarly, dimeric RGD peptide tracers such as ¹⁸F-FPP-RGD2 and alfatide showed comparable sensitivity to ¹⁸F-FDG in other small studies (8). Parihar et al. showed that increased specificity of ⁶⁸Ga-DOTA-RGD2 PET/CT in thyroid cancer was associated with more accurate identification of metastatic cervical lymph nodes (6). The most likely explanation of the superiority of ⁶⁸Ga-DOTA-RGD2 PET/CT over ¹⁸F-FDG PET/CT is that the latter commonly detects enhanced glucose uptake in inflammatory tissues, such as reactive lymph nodes, while ⁶⁸Ga-DOTA-RGD2 PET/CT is more specific for tumor-related neovascularization. There are also data revealing a superiority of RGD-based tracers over ¹⁸F-FDG PET/CT for imaging of brain metastases. RGD-based imaging is characterized by a higher tumor-to-background ratio than ¹⁸F-FDG because of its low background uptake in normal brain tissue (8). Consistently, compared with the 30.2% positive predictive value of ¹⁸F-FDG PET/CT for the detection of metastases, another radiolabeled RGD analogue—⁶⁸Ga-NOTA-PRGD2 PET/CT—had a value as high as 90% (9). Moreover, ⁶⁸Ga-NOTA-PRGD2 PET/CT combined with ¹⁸F-FDG-PET/CT was accurate in differentiation between meningiomas associated with severe peritumoral brain edema mimicking high-grade glioma and high-grade gliomas (10). The increased diagnostic accuracy of ⁶⁸Ga-NOTA-PRGD2 PET/CT enabled an appropriate individualized therapeutic approach while avoiding overtreatment of benign meningiomas.

Interestingly, Parihar et al. reported that the highest standard uptake values (SUVmax) of ⁶⁸Ga-DOTA-RGD2 were observed in bone lesions (6). Shao et al. found utility in application of another radiolabeled RGD analogue ^99mTc-3P-RGD2 whole body scintigraphy in the detection of osteolytic metastases from lung cancer and other solid tumors, including thyroid cancer. In this study, metastatic lesions from lung cancer were characterized by higher uptake than those derived from thyroid cancer, but the sample size of the latter was small (11). Another radiolabeled RGD analogue ¹⁸F-Alfatide II had excellent diagnostic sensitivity for osteolytic metastases and mixed bone metastases (1).

There are several studies utilizing different chelators for radiolabeling RGD. As described in this issue of Thyroid, DOTA is a commonly used chelator, but RGD could also be labeled with EB-DOTA, extending its half-life in the blood due to reversible binding with albumin (12). There are also studies focused on dual targeting of the gastrin-releasing peptide receptor and integrin αvβ3 with the tracer ⁶⁸Ga-BBN-RGD, which has been shown to discriminate between primary breast cancers, axillary lymph node metastases, and distant metastases (13). There is an ongoing clinical trial utilizing dual somatostatin receptor type 2 (SSTR2) and integrin αvβ3 targeting PET/CT imaging in lung cancer and neuroendocrine tumors (clinicaltrials.gov identifier NCT02817945).

Radiolabeled RGD analogues have also been used for cancer prognosis. For sarcomas and gliomas, RGD uptake was positively correlated with the grade of tumor differentiation (8). Kim et al. showed that radiolabeled RGD analogue uptake, which is associated with the increased angiogenic activity, could be potentially used as an early prognostic marker for the prediction of breast cancer recurrence (14).

Since there is a significant intra- and intertumor heterogenicity in αvβ3 expression and differential neovascularization supporting tumor growth, several studies have analyzed the ability of radiolabeled RGD analogues to predict and appropriately evaluate the response to therapy with antiangiogenic agents. There are data showing that baseline SUVmax of the metastatic lesions as well as its change over the course of treatment predicts short-term response to therapy with antiangiogenic agents better than the classic tumor volumetrics (1,8). However, the sample sizes in the reported studies are relatively small and warrant confirmation in larger studies. There are ongoing clinical trials looking at the contribution of the RGD-based PET/CT imaging for the characterization of residual masses of nonseminoma tumors at the end of chemotherapy (clinicaltrials.gov identifier NCT02317393). The other ongoing clinical trial is evaluating the association of RGD-based PET/CT imaging with angiogenesis in patients with neuroendocrine tumors (clinicaltrials.gov identifier NCT03271281).

Currently FDA-approved drugs for the management of metastatic RAI nonavid thyroid cancer include the tyrosine kinase inhibitors—lenvatinib and sorafenib—which also have activity against vascular endothelial growth factor. As one challenge of medical management of patients with metastatic RAI nonavid thyroid cancer is when to start therapy with tyrosine kinase inhibitors, RGD-based PET/CT, which is correlated with tumor neovascularization, might be useful in deciding when to use these agents.

Therapy for cancer may be associated with intratumoral inflammatory reaction, which may affect RGD tracer uptake. Chen et al. tested this hypothesis in a preclinical mouse model that documented synergistic effects of combination therapy with immune checkpoint inhibitors and radiolabeled ¹⁷⁷Lu RGD analogue (15). It might be also interesting to examine potential synergism between ¹⁷⁷Lu-labeled RGD analogues and antiangiogenic agents in thyroid cancer therapy.

Overall, the study by Parihar et al. in this issue of Thyroid suggests that RGD-based radiotheranostics might form a new avenue for the management of patients with metastatic RAI nonavid thyroid cancer characterized by high αvβ3 expression.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The study was supported by the Intramural Research Programs of NIDDK and NIBIB, NIH.