The CXCR4/CXCL12 ‘receptor-ligand pair’ plays a prominent role in cell proliferation and metastasis in at-least 30 different human cancers [15, 16]. 68Ga-Pentixafor -a CXCR4 targeting radioligand allows in vivo visualization non-invasively of tumors expressing these receptors. 10,12 The use of 68Ga-Pentixafor PET/CT imaging has proven the potential of this tracer in evaluating the whole-body disease burden of CXCR4 receptors in many haematological and solid human malignancies [17]. Further, high contrast PET images demonstrated by this tracer have led to the development of beta emitting 90Y/177Lu-Pentixather as a powerful 68Ga and 90Y/177Lu theranostic pair [18–20]. This theranostic pair has been introduced successfully for the treatment of advanced stage multiple myeloma, lymphoma and leukaemia [21–23].
CXCR4 stromal cell derived 1-α factor is critical in cancer growth and metastasis. Typically, the rising activity of this factor in lymph nodes, bone, bone marrow, lung and liver has been reported to trigger the metastasis of CXCR4 expressing tumor cells [24–25]. CXCR4 receptors’ over-expression thus has been recognized as an adverse prognostic factor in various malignancies including lung cancer [26–28]. Therefore, 68Ga-Pentixafor PET/CT based in vivo whole-body quantification of CXCR4 receptors is viewed as a very promising diagnostic or therapeutic imaging biomarker in a variety of cancer patients [29–30].
In this study, we present 68Ga-Pentixafor PET/CT imaging results in 94 lung cancer patients and validation of the quantitative PET parameters with simultaneous tissue characterization and quantification of CXCR4 receptors’ density. To the best of our knowledge, this is the first study reporting the tracer uptake as a function of CXCR4 receptors’ density identified by IHC and FACS in primary lung cancer tissue of different histologic types. We observed that all sub-types of lung cancer showed increased tracer uptake in the primary lung lesions on 68Ga-Pentixafor PET, which was indicative of tumor CXCR4 over-expression. The highest CXCR4 expression was seen in SCLC, which is the most aggressive lung cancer sub-type characterized by rapid doubling time, high growth fraction and early development of metastatic spread [31]. CXCR4 activation is also linked to metastatic behaviour of cancer cells metastasizing to organs by invasive and migratory responses and adhesion to marrow stromal cells in SCLC [32, 33]. SCLC swiftly metastasizes to other organs and much more rapidly than NSCLC types. Hence, the finding of 68Ga-Pentixafor SUVmax and MFI values highlight higher CXCR4 expression in SCLC than that in NSCLC variants which in turn validates the specificity of in-vivo CXCR4 PET imaging by 68Ga-Pentixafor.
Despite, the higher CXCR4 expression (MFI) and the tracer uptake (SUVmax) in SCLC, we did not find a significant correlation between these two parameters which is probably due to the small number of patients (n = 14) in this group. In an extensive meta-analysis of 24 studies and 2037 lung cancer patients, CXCR4 was not significantly related to the prognosis factors such as age, gender. tumor size, smoking, etc. [26]. However, these authors reported that CXCR4 expression correlated with some prognosis factors such as N-stage (N1, N2 vs. N0), M-stage (M1 vs. M0), tumor-stage, etc. It has been reported that 68Ga-Pentixafor PET/CT showed a higher CXCR4 receptors’ density (MFI = 142.0; SUVmax= 13.2;) in a SCLC patient than in a patient (MFI = 120.0; SUVmax=8.8) with NSCLC variant [28]. It was also observed that in the SCLC patient, 18F-FDG PET/CT showed SUVmax value of 8.0 as against the SUVmax value of 13.2 on 68Ga-Pentixafor PET/CT. And 68Ga-Pentixafor PET/CT picked up additional brain metastatic lesions in the NSCLC patient.
It is thus highlighted that 68Ga-Pentixafor PET/CT demonstrating higher tracer uptake (SUVmax) is supported by higher receptors’ density (MFI) in SCLC. The Receiver Operating Characteristic (ROC) curve analysis of 68Ga-Pentixafor SUVmax values provided a cut-off value of 7.24 to differentiate SCLC from NSCLC (sensitivity 87.5% and specificity 72.0%). However, no definitive trend for sensitivity and specificity with 18F-FDG PET/CT has been reported for this differentiation [34–35].
In a recent study, Buck et al. reported that a very high tracer (68Ga-Pentixafor) uptake (SUVmax>12.0) was found in multiple myeloma (n = 113) followed by adrenocortical carcinoma (n = 30), mantle cell lymphoma (MCL, n = 20), adrenocortical adenoma (n = 6) and SCLC (n = 12) [17]. They concluded that these results may provided a roadmap to detect patients who may benefit from CXCR4 targeted therapies. The suitability of 68Ga-Pentixafor for non-invasive high contrast imaging of CXCR4 over-expressing cancers has been demonstrated initially for haematological malignancies [36–40]. With the subsequent development of 177Lu-Pentixather as a therapeutic companion, the first CXCR4 targeted radiotheranostic concept has been translated into the clinic [21, 41]. An encouraging therapeutic response of 177Lu/90Y -Pentixather for radioligand therapy (RLT) in advance stage multiple myeloma and other lymphoproliferative diseases have been reported [18–20]. The other potential therapeutic applications of this theranostic pair are being explored in prospective clinical trials.
In NSCLC and lung NETs, the mean SUVmax and MFI values were lower than SCLC patients. Though, we did not find a significant correlation between these two parameters in SCLC, but the same exhibited a significant correlation in NSCLC variants (adenocarcinoma; n = 16; r = 0.538; p = 0.031; squamous cell; n = 54; r = 0.690; p = 0.0001). Similarly, a significant correlation (p < 0.05) was seen between SUVmax values and the percentage-stained cells (r = 0.296; p = 0.030) in NSCLC-squamous cell patients. Therefore, NSCLC variants with the evidence of significant CXCR4 over-expression may also be considered for RLT using alpha and beta labelled CXCR4 targeting radionuclide theranostics. In a recent study, Watts et al. reported that 68Ga-Pentixafor PET/CT allows non-invasive assessment of CXCR4 expression in rare lung cancers i.e. haemangioendothelioma, sarcomatoid carcinoma and hemangiopericytoma and in lung metastasis cases [42]. The highest SUVmax of 13.0 was noted in the case of haemangioendothelioma. Therefore, the lung cancer cases other than SCLC and NSCLC which express significant quantity of CXCR4 expression also holds great potential both for imaging and treatment using 68Ga-Pentixafor/177Lu-Pentixather theranostic pair. The precision radiomolecular oncology using such targeted radiotheranostic approach challenging the classical statistical evidence based medicine has been reported [43]. 68Ga-Pentixafor PET/CT could be of special clinical significance in response assessment to CXCR4 based radiotherapeutics. And in a recent study, the varied physiological distribution of 68Ga-Pentixafor in spleen has been reported to be of great prognostic significance [44].
68Ga-Pentixafor PET/CT scan findings indicated an increased tracer uptake (SUVmax= 5.23 ± 1.23) in all the 5 lung- NET patients. No significant correlation was seen between SUVmax and the percent stained cells (r=-0.293; p = 0.663). There is only a single study in the literature by Werner et al., who have investigated the role of 68Ga-Pentixafor in imaging GEP NET tumors [45]. These authors compared the diagnostic performance of three tracers i.e.18F-FDG, 68Ga-DOTATATE and 68Ga-Pentixafor in 12 GEP NET patients and found concordant (positive) findings between 68Ga-Pentixafor and 68Ga-DOTATATE in 4/5 poorly differentiated NETs. However, 68Ga-Pentixafor demonstrated superiority and picked up more number (n = 66) of metastatic lesions as compared to 68Ga-DOTATATE which detected only 12 lesions. In this regard, these authors reported that an increasing number of CXCR4 (+)/SSTR (-) metastasis were identified in patients with increasing tumor aggressiveness. The usefulness of 18F-FDG-PET/CT in poorly dedifferentiated NETs has been described previously. These NET variants pose a serious therapeutic challenge with the currently available 177Lu -DOTATATE therapy and thus, 177Lu-Pentixather RLT targeting CXCR4 receptors may be useful in NETs having poor SSTR expression.
Therefore, the demonstration of variable quantitative CXCR4 receptors’ expression supported by the matching pattern of 68Ga-Pentixafor tissue uptake in different LC sub-types provides a convincing data for using this imaging modality for radio-theranostic applications. This may potentially supplement the existing data for inclusion and expanding CXCR4 -based radioligand therapies in LC beyond haematological malignancies.