Visual Abstract
Abstract
68Ga-pentixafor PET/CT imaging allows noninvasive assessment of C-X-C chemokine receptor type 4 (CXCR4) expression in various malignancies, but its use in rare lung cancer variants has not been reported. Methods: 68Ga-pentixafor PET/CT imaging was performed on 6 patients (3 men, 3 women; mean age, 57.0 ± 16.8 y) with suspected lung masses. Whole-body PET/CT images were acquired 1 h after intravenous injection of 148.0–185.0 MBq of the tracer. PET/CT images were reconstructed and analyzed. The image findings were correlated with histopathologic and quantitative (CXCR4) fluorescence-activated cell sorting analysis. Results: Histopathologic diagnosis of hemangioendothelioma, sarcomatoid carcinoma, and hemangiopericytoma was confirmed in 1 patient each. Lung metastasis was diagnosed in the remaining 3 of 6 patients with primary sarcoma (n = 1), renal cell carcinoma (n = 1), and unknown primary (n = 1). Increased uptake in the primary lung mass, with an SUVmax of 3.0, 6.34, and 13.0, was noted in the hemangiopericytoma, sarcomatoid carcinoma and hemangioendothelioma cases, respectively. The mean SUVmax, mean fluorescence intensity, and percentage of stained cells were highest in hemangioendothelioma. Among 3 patients with lung metastases, the highest SUVmax, 9.5, was in the primary sarcoma patient. Conclusion: 68Ga-pentixafor selectively targets the in vivo whole-body disease burden of CXCR4 receptors. This approach thus holds promise for developing suitable radiotheranostics for lung cancers expressing these targets.
Despite ever-evolving research and advances in diagnostic and treatment strategies, lung carcinoma remains the most lethal type of cancer worldwide (1). The diagnostic work-up in suspected lung tumors involves tissue diagnosis, including histopathology and immunohistochemistry analysis and imaging. A presumptive differentiation between small cell lung cancer (SCLC) and non–small cell lung cancer (NSCLC) can be made on the basis of clinical presentation and radiologic findings (2). Functional tumor imaging using 18F-FDG PET/CT offers complementary information by assessing tumor burden and helps in staging (3). However, noninvasive PET/CT imaging of receptor expression and the heterogeneity of specific receptors can provide complementary information (4).
There is evidence that SCLC and NSCLC display C-X-C chemokine receptor type 4 (CXCR4) overexpression, which is associated with high tumor aggressiveness, metastasis, and recurrence (5). CXCR4 expression is analyzed using immunohistochemistry and fluorescence-activated cell sorting (FACS) analysis of biopsy samples (6). Noninvasive imaging using high-throughput PET probes targeting CXCR4 receptors may provide important diagnostic or prognostic information on such patients (7).
Few studies have described the feasibility of radiolabeling a cyclic pentapeptide (pentixafor) with 68Ga, and the recent use of 68Ga-pentixafor has yielded encouraging preclinical and clinical results for in vivo imaging of CXCR4 expression in solid tumors as well as in hematologic malignancies (8–10). The use of 68Ga-pentixafor PET/CT imaging for selective targeting of CXCR4 receptors has made much progress in hematologic malignancies, whereas its role in solid tumors has been seldom reported (11–13).
Because there are more than 30 human malignancies known to overexpress CXCR4 receptors, the imaging applications of 68Ga-pentixafor PET/CT for targeting these receptors are fast expanding in other malignancies. It has recently been reported that 68Ga-pentixafor PET/CT shows high tumor uptake in patients with lung cancer (SCLC and NSCLC), glioblastoma multiforme, and multiple myeloma. This tracer exhibited strong affinity and specificity for the in vivo localization and imaging of CXCR4 receptors in these malignancies (11,14–16).
In this study, we investigated—for the first time, to our knowledge—the diagnostic utility of 68Ga-pentixafor PET/CT imaging in a few cases of rare lung cancer variants and in lung metastasis cases with distant primaries.
MATERIALS AND METHODS
Six patients (3 men, 3 women; mean age, 57.00 ± 16.80 y; range, 33–73 y) with clinically and radiologically suspected lung cancer were recruited prospectively for the study. Written informed consent was obtained from all participants. The protocol was approved by the Institute Ethics Committee as the doctoral thesis of the first author. All patients underwent 68Ga-pentixafor PET/CT, bronchoscopic or PET/CT-guided lung biopsy, routine histopathology, immunohistochemistry, and quantitative CXCR4 receptor analysis by FACS.
68Ga-Pentixafor PET/CT Data Acquisition and Analysis
Briefly, 148.0–185.0 MBq of 68Ga-pentixafor were injected intravenously. Whole-body PET (Discovery; GE Healthcare) and contrast-enhanced CT (using standard CT acquisition parameters) were performed consecutively 1 h after tracer administration. The PET was performed at a rate of 3 min/frame (7–8 frames) from the base of the skull to the proximal thighs. Attenuation-corrected PET images were reconstructed iteratively using ordered-subset expectation maximization. The reconstructed images were projected in 3 planes (cross-sectional, coronal, and sagittal) and used for visual and quantitative (SUVmax) analysis.
FACS Analysis
In FACS analysis, the freshly biopsied lung sample was processed, and 5.0 μL of fluorescein isothiocyanate–labeled CD184 (BD Pharmingen) was used to further label the CXCR4-positive tumor cells in the tissue suspension. A flow cytometer (FACS Calibur; BD) was used to analyze the stained and unstained cell population, and the results were expressed as mean fluorescence intensity (MFI) and percentage of CXCR4-positive stained cells.
The 68Ga-pentixafor PET/CT image findings (SUVmax) were compared with histopathology and with the quantitative parameters of the FACS assay—that is, the MFI and the percentage of stained cells.
RESULTS
The results of histopathology, FACS, and 68Ga-pentixafor PET are presented in Table 1. High uptake was noted in all lung lesions. In 3 patients (patients 1–3), rare primary lung pathologies were identified. The highest SUVmax, 13.04, with the highest corresponding MFI, 682.0, was noted in the patient (patient 2) with histopathologic evidence of hemangioendothelioma (Fig. 1). The SUVmax and MFI were 6.34 and 110.5, respectively, in patient 1 (sarcomatoid carcinoma) and 3.0 and 27.90, respectively, in patient 3 (hemangiopericytoma). The corresponding SUVmax and MFI were 9.5 and 191.20, respectively, in a case of secondary lung metastasis from sarcoma (patient 4), 6.0 and 62.0, respectively, in a case of renal cell carcinoma (patient 5), and 7.5 and 216.0, respectively, in a case of unknown primary (patient 6). The results of 68Ga-pentixafor PET/CT, FACS, and histopathologic analysis for patient 4 are presented in Figure 2.
Findings in 6 Patients with Rare Lung Tumors and Metastatic Lung Disease
A 73-y-old woman (patient 2) with primary lung hemangioendothelioma. (A, B, and D) 68Ga-pentixafor PET/CT maximum-intensity projection (A) and cross-sectional PET/CT image (B) showing increased uptake (SUVmax, 13.0), with corresponding CT image (D). (C, E, and F) FACS analysis using CD184-PE showing stained CXCR4-positive tumor cells in scatterplots (C), photomicrograph with histopathologic disease evidence showing epithelioid tumor cells (hematoxylin and eosin, ×40) (E) and immunohistochemistry staining with CD31 showing diffuse membranous positivity (F).
A 70-y-old woman (patient 4) with secondary lung cancer disease. (A) 68Ga-pentixafor PET/CT maximum-intensity projection showing increased uptake in lung and in multiple sarcomatoid lesions. (B, D, and F) Cross-sectional PET/CT image showing increased (SUVmax, 9.5) uptake in metastatic lung lesion (B), in posterior subcutaneous lesions and left lateral aspect of scalp (SUVmax, 4.6) (D), and in lytic expansile lesion with soft-tissue component involving lateral aspect of fifth rib (SUVmax, 5.4) (F). (C) FACS analysis using CD184-PE showing stained CXCR4-positive tumor cells in scatterplots. (E) Photomicrograph with histopathologic disease evidence (hematoxylin and eosin, ×40).
Pearson correlation analysis indicated a significant correlation between SUVmax and MFI (r = 0.90), between SUVmax and percentage of stained cells (r = 0.79), and between MFI and percentage of stained cells (r = 0.72).
DISCUSSION
In the present study, 68Ga-pentixafor PET/CT imaging demonstrated high uptake (SUVmax, 13.0; MFI, 682.0) in hemangioendothelioma. The SUVmax in the other 2 pathologies—that is, sarcomatoid carcinoma and hemangiopericytoma—varied as a function of MFI. Interestingly, among the 3 cases of lung metastases, the highest SUVmax, 9.5 (MFI, 191.0), was seen in the lung metastasis with sarcoma as the primary disease. These findings presented a positive correlation (r = 0.90) between 68Ga-pentixafor uptake and CXCR4 receptor expression and density, which in turn indicated the high specificity of the tracer for these receptors. Likewise, SUVmax also correlated (r = 0.79) with the percentage of stained cells.
To our knowledge, we were the first to report the normal biodistribution of 68Ga-pentixafor in a healthy volunteer; the highest SUVmean and SUVmax were in the urinary bladder (146.0 and 239.0, respectively) and spleen (6.80 and 10.10, respectively), followed by the kidneys (4.99 and 20.55, respectively) (17). Variable physiologic uptake of 68Ga-pentixafor was seen in the spleen in different imaging studies and was found to have an association with stage of disease and clinical outcome, as reported in a study on 145 solid-tumor patients (18). A positive correlation was found between 68Ga-pentixafor splenic uptake and platelet or leukocyte counts in lung cancer and neuroendocrine tumors, suggesting that splenic uptake might play a role in systemic immunity or inflammation (18).
We previously reported that uptake of 68Ga-pentixafor in SCLC patients was higher than in NCSLC and other lung cancer variants and that the uptake varied as a function of CXCR4 receptor density (11,14). However, the pattern of uptake and the in vivo evidence of CXCR4 expression in rare lung malignancies have not been studied before.
68Ga-pentixafor PET tracer was shown to have excellent affinity for CXCR4 receptors in preclinical and clinical studies (19,20). According to the available literature, noninvasive imaging of CXCR4 expression in SCLC is feasible, and 68Ga-pentixafor as a novel PET tracer might serve as a readout for confirming the CXCR4 expression (20). Watts et al. reported that 68Ga-pentixafor uptake denoting CXCR4 expression is higher in SCLC than NSCLC patients (11,14). Evaluation of CXCR4 expression is a prerequisite for potential CXCR4-directed radiotherapies and chemotherapies in lung cancer and especially in SCLC, which has higher CXCR4 expression compared with all other variants of lung cancer.
The reports on CXCR4 expression in rare lung tumors included in this study are not available. However, the role of overexpression of CXCR4 receptors in tumor growth and progression in sarcoma and renal cell carcinoma primaries and in metastasis to lungs has been demonstrated (21,22). 68Ga-pentixafor PET imaging may thus be expanded beyond SCLC and NSCLC to unravel the CXCR4 receptor density and to understand the process of metastatic spread and the intra- or interindividual heterogeneity of these tumors (23). In a recent study, an urgent clinical need to develop novel therapeutics for devastating NSCLC disease targeting the CXCR4/CXCL12 axis has been advocated (24). The study has further stressed that this is the time to move forward and attempt to incorporate CXCR4 inhibitors into novel immune-based therapeutic protocols for lung cancer.
CONCLUSION
68Ga-pentixafor selectively targets and accurately maps the in vivo whole-body disease burden of CXCR4 receptors, a task that is not possible by tissue sampling methods. This technique thus holds great promise for translating this approach by labeling the vector with α- or β-emitters to a therapeutic scenario in such aggressive lung cancer variants having limited treatment options.
DISCLOSURE
Baljinder Singh of the Department of Nuclear Medicine received extramural research funding from the Department of Science and Technology under the DST-FIST program (grant SR/FST/LSI-548/2012) for the purchase of the automated chemistry module and other chemicals and peptides. No other potential conflict of interest relevant to this article was reported.
KEY POINTS
QUESTION: Can the in vivo expression of CXCR4 receptors be shown in rare lung cancers noninvasively by 68Ga-pentixafor PET/CT?
PERTINENT FINDINGS: 68Ga-pentixafor PET/CT detected the presence of CXCR4 receptors in rare lung cancers and metastases. The uptake varied as a function of receptor density, showing high specificity for in vivo imaging of CXCR4 receptor disease burden in lung cancers.
IMPLICATIONS FOR PATIENT CARE: This technique holds great promise for translating this approach by labeling the vector with α- or β-emitters for therapeutic applications in aggressive lung cancer variants having limited treatment options.
Footnotes
Published online May 24, 2022.
REFERENCES
- Received for publication March 22, 2022.
- Accepted for publication April 27, 2022.
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