A Prospective Observational Study to Evaluate the Effects of Long-Acting Somatostatin Analogs on 68Ga-DOTATATE Uptake in Patients with Neuroendocrine Tumors

Imaging of Neuroendocrine Neoplasms; Principles of Treatment Strategies. What Referring Clinicians Want to Know

ABSTRACT

Neuroendocrine neoplasms (NENs) are a diverse group of tumors that express neuroendocrine markers and primarily affect the lungs and digestive system. The incidence of NENs has increased over time due to advancements in imaging and diagnostic techniques. Effective management of NENs requires a multidisciplinary approach, considering factors such as tumor location, grade, stage, symptoms, and imaging findings. Treatment strategies vary depending on the specific subtype of NEN. In this review, we will focus on treatment strategies and therapies including the information relevant to clinicians in order to undertake optimal management and treatment decisions, the implications of different therapies on imaging, and how to ascertain their possible complications and treatment effects.

Altered biodistribution of [68Ga]Ga-DOTA-TOC during somatostatin analogue treatment

PURPOSE

The need for an interval between the administration of long-acting Somatostatin Receptor Analogues (SSA) and the [68Ga]Ga-DOTA-TATE PET has been questioned based on recent literature in the new EANM guidelines. Here an earlier studies showed that SSA injection immediately before SSTR PET had minimal effect on normal organ and tumor uptake (1). However, data are scarce and there are (small) differences between [68Ga]Ga-DOTA-TATE and [68Ga]Ga-DOTA-TOC binding affinity, and it remains unknown whether these findings can be directly translated to scans with [68Ga]Ga-DOTA-TOC as well. The purpose of this study was to assess the effect of SSA use on the biodistribution in a subsequent [68Ga]Ga-DOTA-TOC PET/CT and compare this intra-individually across several cycles of SSA treatments.

METHODS

Retrospectively, 35 patients with NENs were included. [68Ga]Ga-DOTA-TOC PET at staging and after the 1st and 2nd cycle of SSA were included. SUVmean and SUVmax of blood, visceral organs, primary tumor and two metastases were determined. Also, the interval between SSA therapy and the PET scan was registered.

RESULTS

Treatment with SSA resulted in a significantly higher bloodpool activity and lower visceral tracer uptake. This effect was maintained after a 2nd cycle of SSA therapy. Furthermore, there was an inverse relationship between bloodpool tracer availability and visceral tracer binding and a positive correlation between bloodpool tracer availability and primary tumor tracer uptake. With an interval of up to 5 days, there was a significantly higher bloodpool activity than at longer intervals.

CONCLUSION

Absolute comparison of the SUV on [68Ga]Ga-DOTA-TOC PET should be done with caution as the altered biodistribution of the tracer after SSA treatment should be taken into account. We recommend not to perform a scan within the first 5 days after the injection of lanreotide.

Advancements in Neuroendocrine Neoplasms: Imaging and Future Frontiers

Neuroendocrine neoplasms (NENs) are a diverse group of tumors with varying clinical behaviors. Their incidence has risen due to increased awareness, improved diagnostics, and aging populations. The 2019 World Health Organization classification emphasizes integrating radiology and histopathology to characterize NENs and create personalized treatment plans. Imaging methods like CT, MRI, and PET/CT are crucial for detection, staging, treatment planning, and monitoring, but each of them poses different interpretative challenges and none are immune to pitfalls. Treatment options include surgery, targeted therapies, and chemotherapy, based on the tumor type, stage, and patient-specific factors. This review aims to provide insights into the latest developments and challenges in NEN imaging, diagnosis, and management.

Changes in tumor-to-blood ratio as a prognostic marker for progression-free survival and overall survival in neuroendocrine tumor patients undergoing PRRT

BACKGROUND

Historically, patient selection for peptide receptor radionuclide therapy (PRRT) has been performed by virtue of somatostatin receptor scintigraphy (SRS). In recent years, somatostatin receptor positron emission tomography (SSTR-PET) has gradually replaced SRS because of its improved diagnostic capacity, creating an unmet need for SSTR-PET-based selection criteria for PRRT. Tumor-to-blood ratio (TBR) measurements have shown high correlation with the net influx rate Ki, reflecting the tumor somatostatin receptor expression, to a higher degree than standardized uptake value (SUV) measurements. TBR may therefore predict treatment response to PRRT. In addition, changes in semiquantitative SSTR-PET parameters have been shown to predate morphological changes, making them a suitable metric for response assessment.

METHODS

The institutional database of the Department of Nuclear Medicine (University Hospital Essen) was searched for NET patients undergoing ≥ 2 PRRT cycles with available baseline and follow-up SSTR-PET. Two blinded independent readers reported the occurrence of new lesions quantified tumor uptake of up to nine lesions per patient using SUV and TBR. The association between baseline TBR and changes in uptake/occurrence of new lesions with progression-free survival (PFS) and overall survival (OS) was tested by use of a Cox regression model and log-rank test.

RESULTS

Patients with baseline TBR in the 1st quartile had a shorter PFS (14.4 months) than those in the 3rd (23.7 months; p = 0.03) and 4th (24.1 months; p = 0.02) quartile. Similarly, these patients had significantly shorter OS (32.5 months) than those with baseline TBR in the 2nd (41.8 months; p = 0.03), 3rd (69.2 months; p < 0.01), and 4th (42.7 months; p = 0.03) quartile. Baseline to follow-up increases in TBR were independently associated with shorter PFS when accounting for prognostic markers, e.g., RECIST response (hazard ratio = 2.91 [95%CI = 1.54-5.50]; p = 0.01). This was confirmed with regard to OS (hazard ratio = 1.64 [95%CI = 1.03-2.62]; p = 0.04). Changes in SUVmean were not associated with PFS or OS.

CONCLUSIONS

Baseline TBR as well as changes in TBR were significantly associated with PFS and OS and may improve patient selection and morphological response assessment. Future trials need to assess the role of TBR for therapy monitoring also during PRRT and prospectively explore TBR as a predictive marker for patient selection.

Peptide receptor radionuclide therapy combinations for neuroendocrine tumours in ongoing clinical trials: status 2023

A growing body of literature reports on the combined use of peptide receptor radionuclide therapy (PRRT) with other anti-tumuor therapies in order to anticipate synergistic effects with perhaps increased safety issues. Combination treatments to enhance PRRT outcome are based on improved tumour perfusion, upregulation of somatostatin receptors (SSTR), radiosensitization with DNA damaging agents or targeted therapies. Several Phase 1 or 2 trials are currently recruiting patients in combined regimens. The combination of PRRT with cytotoxic chemotherapy, capecitabine and temozolomide (CAPTEM), seems to become clinically useful especially in pancreatic neuroendocrine tumours (pNETs) with acceptable safety profile. Neoadjuvant PRRT prior to surgery, PRRT combinations of intravenous and intraarterial routes of application, combinations of PRRT with differently radiolabelled (alpha, beta, Auger) SSTR-targeting agonists and antagonists, inhibitors of immune checkpoints (ICIs), poly (ADP-ribose) polymerase-1 (PARP1i), tyrosine kinase (TKI), DNA-dependent protein kinase, ribonucleotide reductase or DNA methyltransferase (DMNT) are tested in currently ongoing clinical trials. The combination with [131I]I-MIBG in rare NETs (such as paraganglioma, pheochromocytoma) and new non-SSTR-targeting radioligands are used in the personalization process of treatment. The present review will provide an overview of the current status of ongoing PRRT combination treatments.

Peptide Receptor Radionuclide Therapy Performed Shortly After Administration of Long-Acting Octreotide

ABSTRACT

We present a case of a 68-year-old man with metastatic small bowel neuroendocrine tumor who underwent 4 cycles of peptide receptor radionuclide therapy with 177 Lu-DOTATATE. For his first 3 cycles, therapy was performed approximately 4 weeks after his last dose of octreotide LAR. Because of miscommunication in scheduling, his fourth cycle was performed only 48 hours after his last full dose of octreotide LAR. Despite this, we found that the tumoral uptake was not reduced at all, which may add to the increasing evidence on the nonnecessity of stopping somatostatin analogs before peptide receptor radionuclide therapy.

Effects of somatostatin analogs on uptake of radiolabeled somatostatin analogs on imaging: a systematic review and meta-analysis

Background

The imaging of somatostatin receptors (SSTRs) plays a significant role in imaging neuroendocrine tumors (NETs). However, there has been no clear definition on whether it is necessary to withdraw somatostatin analogs (SSAs) before SSTRs imaging. We aimed to assess whether nonradioactive SSAs affect the uptake of radiolabeled SSAs on imaging for NETs patients.

Methods

The databases of PubMed, Embase, and Web of Science (WoS) were searched until March 12, 2022 to identify eligible studies. Maximum standardized uptake values (SUVmax) in tumor and normal tissues were extracted, pooled, and compared before and after SSAs treatment. The change of tumor-to-background/liver ratio was also described. The quality of each study was assessed using the revised Quality Assessment of Diagnostic Accuracy Studies-2 tool.

Results

A total of 9 articles involving 285 patients were included and 5 studies using Gallium-68-labeled [1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid]-D-Phe1-Tyr3-Thr8-octreotide (68Ga-DOTATATE) were used for pooled evaluation. We found a significantly decreased SUVmax in the liver (9.56±2.47 vs. 7.62±2.12, P=0.001) and spleen (25.74±7.14 vs. 20.39±6.07, P=0.006) after SSAs treatment whereas no significant differences were observed in the uptake of thyroid, adrenal, and pituitary gland. For either primary tumor sites or metastases, the SUVmax did not change significantly before and after SSAs treatment. The tumor-to-liver/background ratio increased following SSAs therapy. High heterogeneity was observed across the studies, mainly due to inherent diversity of study design, sample size, and scanning technique.

Conclusions

Based on current evidence, long-acting SSAs therapy before imaging has no effect on the uptake of radiolabeled SSAs at tumor primary sites and metastatic lesions, but results in a significant reduction of uptake in the liver and spleen. These findings may implicate the unnecessary discontinuation of SSAs before radiolabeled SSAs imaging.

The Influence of Long-Acting Somatostatin Analogs on 68 Ga-DOTATATE Uptake in Patients With Neuroendocrine Tumors

PURPOSE

A high SUV max tumor-to-liver ratio (TLR) of 68 Ga-DOTATATE can be used to select patients with neuroendocrine tumors (NETs) for peptide receptor radionuclide therapy (PRRT). In addition, an SUV max TLR ≥ 8.1 is associated with increased progression-free survival in NET patients treated with somatostatin analogs (SSAs). To avoid a theoretical interaction, several guidelines recommend performing PET/CT just before the monthly administration of long-acting SSAs. We aimed to investigate the effect of SSA on the SUV max of 68 Ga-DOTATATE in patients with NET and to identify independent predictors for high SUV max TLR.

PATIENTS AND METHODS

For this retrospective study, 192 68 Ga-DOTATATE PET/CT scans of 165 patients without (n = 115) and with (n = 77) SSA (octreotide or lanreotide) in the 3 months before PET/CT were collected and reviewed. The effect of SSA on SUV max values was analyzed by a maximum likelihood mixed model.

RESULTS

Patients with SSA had a significantly higher median SUV max TLR than patients without SSA (4.7 [IQR], 3.1-7.7) versus 3.2 [IQR, 2.0-5.4]; P < 0.001). Multivariable logistic regression analysis showed that SSA use was an independent predictor for SUV max TLR ≥ 8.1 (odds ratio, 2.91; 95% confidence interval, 1.26-6.72; P = 0.012).

CONCLUSIONS

Our data suggest that higher SSA concentrations do not have a negative effect on 68 Ga-DOTATATE uptake in tumor lesions. In addition, we found that only SSA use was associated with SUV max TLR ≥ 8.1. Our results are consistent with previously conducted studies and in line with the recently published guideline that suggests that the relatively recent use of SSA does not necessitate any delay in 68 Ga-DOTATATE PET/CT imaging.

AI-based quantification of whole-body tumour burden on somatostatin receptor PET/CT

BACKGROUND

Segmenting the whole-body somatostatin receptor-expressing tumour volume (SRETVwb) on positron emission tomography/computed tomography (PET/CT) images is highly time-consuming but has shown value as an independent prognostic factor for survival. An automatic method to measure SRETVwb could improve disease status assessment and provide a tool for prognostication. This study aimed to develop an artificial intelligence (AI)-based method to detect and quantify SRETVwb and total lesion somatostatin receptor expression (TLSREwb) from [68Ga]Ga-DOTA-TOC/TATE PET/CT images.

METHODS

A UNet3D convolutional neural network (CNN) was used to train an AI model with [68Ga]Ga-DOTA-TOC/TATE PET/CT images, where all tumours were manually segmented with a semi-automatic method. The training set consisted of 148 patients, of which 108 had PET-positive tumours. The test group consisted of 30 patients, of which 25 had PET-positive tumours. Two physicians segmented tumours in the test group for comparison with the AI model.

RESULTS

There were good correlations between the segmented SRETVwb and TLSREwb by the AI model and the physicians, with Spearman rank correlation coefficients of r = 0.78 and r = 0.73, respectively, for SRETVwb and r = 0.83 and r = 0.81, respectively, for TLSREwb. The sensitivity on a lesion detection level was 80% and 79%, and the positive predictive value was 83% and 84% when comparing the AI model with the two physicians.

CONCLUSION

It was possible to develop an AI model to segment SRETVwb and TLSREwb with high performance. A fully automated method makes quantification of tumour burden achievable and has the potential to be more widely used when assessing PET/CT images.

Clinical utility of somatostatin receptor positron emission tomography imaging biomarkers for characterization of meningioma among incidental central nervous system lesions

OBJECTIVES

Somatostatin receptor (SSTR) PET imaging is utilized with increasing frequency in the clinical management of neuroendocrine tumors. Incidental PET-avid CNS lesions are commonly noted and presumed to be meningiomas. However, SSTR PET lacks specificity for meningioma identification. This study aimed to clarify the role of SSTR-based imaging in the classification of incidental CNS lesions based on current clinical practice.

METHODS

Patients who underwent both Ga-68-DOTATATE PET and brain MRI and had an incidental CNS lesion identified with a radiographic prediction of meningioma via one (discordant prediction) or both (concordant prediction) imaging modalities were retrospectively analyzed. Imaging indication, semiquantitative measures, and clinical history were recorded.

RESULTS

Among 48 patients with a CNS lesion identified on both imaging modalities, most scans were performed for a history of neuroendocrine tumor (64.6%). Cases with concordant lesion-type prediction of meningioma between imaging modalities ( N = 24) displayed a significantly higher SUV max (median 7.9 vs. 4.0; P = 0.008) and Krenning score (median 3.0 vs. 2.0; P = 0.005) on Ga-68-DOTATATE PET compared with cases with a discordant prediction of meningioma ( N = 24). In cases with lower SUV max values, Ga-68-DOTATATE was more likely to discordantly predict meningioma without agreement by the corresponding MRI. Prior cranial radiation or use of somatostatin mimetics did not affect quantitative radiographic measures, and MRI-based tumor size was similar across groups.

CONCLUSION

Lesions with increased avidity may be more confidently predicted as meningioma in Ga-68-DOTATATE PET scans, whereas there is more discrepancy in prediction among low SUV cases.

Impact of Cold Somatostatin Analog Administration on Somatostatin Receptor Imaging: A Systematic Review

PURPOSE

The interactions between the administration of cold somatostatin analogs (cSAs) and their radiolabeled counterpart remain unclear, and discontinuation before imaging is still advised as a precaution. The aim of this systematic review is to evaluate the consequences of cSA administration on tumoral and surrounding healthy organs' uptake at somatostatin receptor (SSTR) imaging with SPECT or PET.

METHODS

After registration of the study on Prospero (CRD42022360260), an electronic search of PubMed and Scopus databases was performed. Inclusion criteria were as follows: human patients referred for SSTR imaging for oncological purposes; at least 1 examination performed either before cSA administration or after a long-enough withdrawal of cSA treatment; at least 1 examination was performed under cSA treatment. Included articles were independently appraised by 2 authors using the standardized protocol provided by the Quality Assessment of Diagnostic Accuracy Studies. Discrepancies were solved by consensus.

RESULTS

A total of 12 articles were included, 4 using 111In-pentetreotide and 8 using 68Ga-DOTA peptides. Administration of cSAs consistently resulted in decreased spleen and liver uptake (from 6.9% to 80% for spleen, 10% to 60% for liver) and increased tumor-to-background or tumor-to-healthy organ ratios. After cSA treatment, tumor uptake alone was unchanged or moderately decreased. Similar results were noted whether patient was octreotide-naive.

CONCLUSION

Impairment in SSTR imaging quality after cSA administration has not been demonstrated. On the contrary, the administration of cSAs seems to improve the contrast between tumoral lesions and the surroundings.

The effect of long-acting somatostatin analogues on the uptake of [177Lu]Lu-HA-DOTATATE

PURPOSE

According to IAEA/EANM/SNMMI guidelines, long-acting somatostatin analogues (LA-SSAs) should be discontinued 4-6 weeks prior to peptide receptor radionuclide therapy (PRRT) to prevent somatostatin receptor saturation. The aim of this study was to determine the effect of continued use of long-acting SSAs during PRRT on the uptake of [177Lu]Lu-HA-DOTATATE on SPECT/CT.

METHODS

Consecutive patients with neuroendocrine tumours who were treated with PRRT receiving 7.4 GBq of [177Lu]Lu-HA-DOTATATE were included. Patients were divided into 3 groups: (1) control (LA-SSA stopped > 6 weeks prior to PRRT), or continued treatment with (2) long-acting octreotide < 6 weeks prior to PRRT, or (3) long-acting lanreotide < 6 weeks prior to PRRT. The uptake of [177Lu]Lu-HA-DOTATATE was quantified in healthy tissues (spleen, liver, kidneys, bone marrow) and tumour lesions on SPECT/CT performed 24 h after PRRT. A Mann-Whitney U test was used to determine differences in uptake between the long-acting octreotide and long-acting lanreotide groups compared to the control group.

RESULTS

Forty-two patients with 135 cycles of PRRT were included: 28 with lanreotide, 50 with octreotide, and 57 cycles without LA-SSAs. Uptake of [177Lu]Lu-HA-DOTATATE was significantly decreased in liver parenchyma in patients with lanreotide (p < 0.001) and in the spleen in patients with either octreotide or lanreotide (both p < 0.001). No differences were observed for uptake in kidneys, bone marrow, and blood pool. Uptake of [177Lu]Lu-HA-DOTATATE in tumours was the same in patients with lanreotide compared to the control (p = 0.862) and in patients with octreotide compared to the control (p = 0.201), independent of tumour location.

CONCLUSION

Long-acting octreotide and lanreotide do not interfere with the uptake of [177Lu]Lu-HA-DOTATATE in tumour lesions 24 h post-injection. Uptake in healthy liver parenchyma significantly decreases after lanreotide administration prior to PRRT, while uptake in healthy spleen tissue significantly decreases with both octreotide and lanreotide administration.

A personal acquisition time regimen of 68Ga-DOTATATE total-body PET/CT in patients with neuroendocrine tumor (NET): a feasibility study

BACKGROUND

The injection activity of tracer, acquisition time, patient-specific photon attenuation, and large body mass, can influence on image quality. Fixed acquisition time and body mass related injection activity in clinical practice results in a large difference in image quality. Thus, this study proposes a patient-specific acquisition time regimen of ⁶⁸ Ga-DOTATATE total-body positron emission tomography-computed tomography (PET/CT) to counteract the influence of body mass (BM, kg) on image quality, and acquire an acceptable and constant image of patients with neuroendocrine tumors (NETs).

METHODS

The development cohort consisting of 19 consecutive patients with full activity (88.7-204.9 MBq, 2.0 ± 0.1 MBq/kg) was to establish the acquisition time regimen. The liver SNR (signal-to-noise ratio, SNR_L) was normalized (SNR_norm) by the product of injected activity (MBq) and acquisition time (min). Fitting of SNR_norm against body mass (BM, kg) in linear correlation was performed. Subjective assessment of image quality was performed using a 5-point Likert scale to determine the acceptable threshold of SNR_L, and an optimized acquisition regimen based on BM was proposed, and validated its feasibility through the validation cohort of 57 consecutive NET patients with half activity (66.9 ± 11.3 MBq, 1.0 ± 0.1 MBq/kg) and a fixed acquisition time regimen.

RESULTS

The linear correlation (R² = 0.63) between SNR_norm and BM (kg) was SNR_norm = -0.01*BM + 1.50. The threshold SNR_L of acceptable image quality was 11.2. The patient-specific variable acquisition time regimen was determined as: t (min) = 125.4/(injective activity)*(-0.01*BM + 1.50)². Based on that proposed regimen, the average acquisition time for acceptable image quality in the validation cohort was 2.99 ± 0.91 min, ranging from 2.18 to 6.35 min, which was reduced by 36.50% ~ 78.20% compared with the fixed acquisition time of 10 min. Subjective evaluation showed that acceptable image quality could be obtained at 3.00 min in the validation group, with an average subjective score of 3.44 ± 0.53 (kappa = 0.97, 95% CI: 0.96 ~ 0.98). Bland-Altman analysis revealed good agreement between the proposed regimen and the fixed acquisition time cohort.

CONCLUSION

A patient-specific acquisition time regimen was proposed in NET patients in development cohort and validated its feasibility in patients with NETs in validation cohort by ⁶⁸ Ga-DOTATATE total-body PET/CT imaging. Based on the proposed regimen, the homogenous image quality with optimal acquisition time was available independent of body mass.

Relationships between uptake of [68Ga]Ga-DOTA-TATE and absorbed dose in [177Lu]Lu-DOTA-TATE therapy

BACKGROUND

Somatostatin receptor ⁶⁸Ga PET imaging is standard for evaluation of a patient's suitability for ¹⁷⁷Lu peptide receptor radionuclide therapy of neuroendocrine tumours (NETs). The ⁶⁸Ga PET serves to ensure sufficient somatostatin receptor expression, commonly evaluated qualitatively. The aim of this study is to investigate the quantitative relationships between uptake in ⁶⁸Ga PET and absorbed doses in ¹⁷⁷Lu therapy.

METHOD

Eighteen patients underwent [⁶⁸Ga]Ga-DOTA-TATE PET imaging within 20 weeks prior to their first cycle of [¹⁷⁷Lu]Lu-DOTA-TATE. Absorbed doses for therapy were estimated for tumours, kidney, spleen, and normal liver parenchyma using a hybrid SPECT/CT-planar method. Gallium-68 activity concentrations were retrieved from PET images and also used to calculate SUVs and normalized SUVs, using blood and tissue for normalization. The ⁶⁸Ga activity concentrations per injected activity, SUVs, and normalized SUVs were compared with ¹⁷⁷Lu activity concentrations 1 d post-injection and ¹⁷⁷Lu absorbed doses. For tumours, for which there was a variable number per patient, both inter- and intra-patient correlations were analysed. Furthermore, the prediction of ¹⁷⁷Lu tumour absorbed doses based on a combination of tumour-specific ⁶⁸Ga activity concentrations and group-based estimates of the effective half-lives for grade 1 and 2 NETs was explored.

RESULTS

For normal organs, only spleen showed a significant correlation between the ⁶⁸Ga activity concentration and ¹⁷⁷Lu absorbed dose (r = 0.6). For tumours, significant, but moderate, correlations were obtained, with respect to both inter-patient (r = 0.7) and intra-patient (r = 0.45) analyses. The correlations to absorbed doses did not improve when using ⁶⁸Ga SUVs or normalized SUVs. The relationship between activity uptakes for ⁶⁸Ga PET and ¹⁷⁷Lu SPECT was stronger, with correlation coefficients r = 0.8 for both inter- and intra-patient analyses. The ¹⁷⁷Lu absorbed dose to tumour could be predicted from the ⁶⁸Ga activity concentrations with a 95% coverage interval of - 65% to 248%.

CONCLUSIONS

On a group level, a high uptake of [⁶⁸Ga]Ga-DOTA-TATE is associated with high absorbed doses at ¹⁷⁷Lu-DOTA-TATE therapy, but the relationship has a limited potential with respect to individual absorbed dose planning. Using SUV or SUV normalized to reference tissues do not improve correlations compared with using activity concentration per injected activity.

Differences in tumor-to-normal organ SUV ratios measured with 68 Ga-DOTATATE PET compared with 177 Lu-DOTATATE SPECT in patients with neuroendocrine tumors

BACKGROUND

Our goal is to quantitatively compare radiotracer biodistributions within tumors and major normal organs on pretherapy 68 Ga-DOTATATE PET to post-therapy 177 Lu-DOTATATE single-photon emission computed tomography (SPECT) in patients receiving peptide receptor radionuclide therapy (PRRT).

METHODS

PET/CT at ~ 60 min postinjection of Ga-68 DOTATATE and research 177 Lu-SPECT/CT imaging ~ at 4 h (SPECT1) and ~ 24 h (SPECT2) post-cycle#1 were available. Manual contours of lesions on baseline CT or MRI were applied to co-registered SPECT/CT and PET/CT followed by deep learning-based CT auto-segmentation of organs. Tumor-to-normal organ ratios (TNR) were calculated from standardized uptake values (SUV) mean and SUV peak for tumor, and SUV mean for non-tumoral liver (nliver), spleen and kidney.

RESULTS

There were 90 lesons in 24 patients with progressive metastatic neuroendocrine tumor. The correlation between PET and SPECT SUV TNRs were poor/moderate: PET versus SPECT1 R 2  = 0.19, 0.21, 0.29; PET versus SPECT2 R 2  = 0.06, 0.16, 0.33 for TNR nliver ,TNR spleen ,TNR kidney , respectively. Across all patients, the average value of the TNR measured on PET was significantly lower than on SPECT at both time points ( P  < 0.001). Using SUV mean for tumor, average TNR values and 95% confidence intervals (CI) were PET: TNR nliver  = 3.5 [CI: 3.0-3.9], TNR spleen  = 1.3 [CI, 1.2-1.5], TNR kidney  = 1.7 [CI: 1.6-1.9]; SPECT1: TNR nliver  = 10 [CI: 8.2-11.7], TNR spleen  = 2.9 [CI: 2.5-3.4], TNR kidney  = 2.8 [CI: 2.3-3.3]; SPECT2: TNR nliver  = 16.9 [CI: 14-19.9], TNR spleen  = 3.6 [CI: 3-4.2], TNR kidney  = 3.6 [CI: 3.0-4.2]. Comparison of PET and SPECT results in a sphere phantom study demonstrated that these differences are not attributed to imaging modality.

CONCLUSIONS

Differences in TNR exist for the theranostic pair, with significantly higher SUV TNR on 177 Lu SPECT compared with 68 Ga PET. We postulate this phenomenon is due to temporal differences in DOTATATE uptake and internalization in tumor as compared to normal organs.

Peptide Receptor Radionuclide Therapy Targeting the Somatostatin Receptor: Basic Principles, Clinical Applications and Optimization Strategies

Simple Summary

Peptide receptor radionuclide therapy (PRRT) is a systemic treatment consisting of the administration of a tumor-targeting radiopharmaceutical into the circulation of a patient. The radiopharmaceutical will bind to a specific peptide receptor leading to tumor-specific binding and retention. This will subsequently cause lethal DNA damage to the tumor cell. The only target that is currently used in widespread clinical practice is the somatostatin receptor, which is overexpressed on a range of tumor cells, including neuroendocrine tumors and neural-crest derived tumors. Academia played an important role in the development of PRRT, which has led to heterogeneous literature over the last two decades, as no standard radiopharmaceutical or regimen has been available for a long time. This review focuses on the basic principles and clinical applications of PRRT, and discusses several PRRT-optimization strategies.

Abstract

Peptide receptor radionuclide therapy (PRRT) consists of the administration of a tumor-targeting radiopharmaceutical into the circulation of a patient. The radiopharmaceutical will bind to a specific peptide receptor leading to tumor-specific binding and retention. The only target that is currently used in clinical practice is the somatostatin receptor (SSTR), which is overexpressed on a range of tumor cells, including neuroendocrine tumors and neural-crest derived tumors. Academia played an important role in the development of PRRT, which has led to heterogeneous literature over the last two decades, as no standard radiopharmaceutical or regimen has been available for a long time. This review provides a summary of the treatment efficacy (e.g., response rates and symptom-relief), impact on patient outcome and toxicity profile of PRRT performed with different generations of SSTR-targeting radiopharmaceuticals, including the landmark randomized-controlled trial NETTER-1. In addition, multiple optimization strategies for PRRT are discussed, i.e., the dose–effect concept, dosimetry, combination therapies (i.e., tandem/duo PRRT, chemoPRRT, targeted molecular therapy, somatostatin analogues and radiosensitizers), new radiopharmaceuticals (i.e., SSTR-antagonists, Evans-blue containing vector molecules and alpha-emitters), administration route (intra-arterial versus intravenous) and response prediction via molecular testing or imaging. The evolution and continuous refinement of PRRT resulted in many lessons for the future development of radionuclide therapy aimed at other targets and tumor types.

Receptor depletion and recovery in small-intestinal neuroendocrine tumors and normal tissues after administration of a single intravenous dose of octreotide measured by 68Ga-DOTATOC PET/CT

BACKGROUND

Low-grade neuroendocrine tumors (NETs) are characterized by an abundance of somatostatin receptors (SSTR) that can be targeted with somatostatin analogs (SSA). When activated with a single dose of SSA, the receptor-ligand complex is internalized, and the receptor is by default recycled within 24 h. Ongoing medication with long-acting SSAs at ⁶⁸Ga-DOTA-SSA-PET has been shown to increase the tumor-to-normal organ contrast. This study was performed to investigate the time-dependent extended effect (7 h) of a single intravenous dose of 400 µg short-acting octreotide on the tumor versus normal tissue uptake of ⁶⁸Ga-DOTATOC.

METHODS

Patients with small-intestinal NETs received a single intravenous dose of 400 µg octreotide and underwent dynamic abdominal ⁶⁸Ga-DOTATOC-PET/CT at three sessions (0, 3 and 6 h) plus static whole-body (WB) PET/CT (1, 4 and 7 h), starting each PET/CT session by administering 167 ± 21 MBq, 23.5 ± 4.2 µg (mean ± SD, n = 12) of ⁶⁸Ga-DOTATOC. A previously acquired clinical whole-body ⁶⁸Ga-DOTATOC scan was used as baseline. SUV and net uptake rate K_i were calculated in tumors, and SUV in healthy organs.

RESULTS

Tumor SUV decreased significantly from baseline to 1 h post-injection but subsequently increased over time and became similar to baseline at 4 h and 7 h. The tumor net uptake rate, K_i, similarly increased significantly over time and showed a linear correlation both with SUV and tumor-to-blood ratio. By contrast, the uptake in liver, spleen and pancreas remained significantly below baseline levels also at 7 h and the receptor turn-over in tumors thus exceeded that in the normal tissue, with restitution of tumor ⁶⁸Ga-DOTATOC uptake mainly completed at 7 h. These results however differed depending on tumor size, with significant increases in K_i and SUV between the 1st and 2nd PET, in large tumors (≥ 4 mL) but not in small (> 1 to  < 4 mL) tumors.

CONCLUSION

SSTR recycling is faster in small-intestinal NETs than in liver, spleen and pancreas. This opens the possibility to protect normal tissues during PRRT by administering a single dose of cold peptide hours before peptide receptor radionuclide therapy (PRRT), and most likely additionally improve the availability and uptake of the therapeutic preparation in the tumors.

Overcoming nephrotoxicity in peptide receptor radionuclide therapy using [177Lu]Lu-DOTA-TATE for the treatment of neuroendocrine tumours

Peptide receptor radionuclide therapy (PRRT) is used for the treatment of patients with unresectable or metastasized somatostatin receptor type 2 (SSTR2)-expressing gastroenteropancreatic neuroendocrine tumours (GEP-NETs). The radiolabelled somatostatin analogue [177Lu]Lu-DOTA-TATE delivers its radiation dose to SSTR2-overexpressing tumour cells, resulting in selective cell killing during radioactive decay. While tumour control can be achieved in many patients, complete remissions remain rare, causing the majority of patients to relapse after a certain period of time. This raises the question whether the currently fixed treatment regime (4 × 7.4 GBq) leaves room for dose escalation as a means of improving therapy efficacy. The kidneys have shown to play an important role in defining a patient's tolerability to PRRT. As a consequence of the proximal tubular reabsorption of [177Lu]Lu-DOTA-TATE, via the endocytic megalin/cubilin receptor complex, the radionuclides are retained in the renal interstitium. This results in extended retention of radioactivity in the kidneys, generating a risk for the development of radiation nephropathy. In addition, a decreased kidney function has shown to be associated with a prolonged circulation of [177Lu]Lu-DOTA-TATE, causing increased irradiation to the bone marrow. This can on its turn lead to myelosuppression and haematological toxicity, owing to the marked radio sensitivity of the rapidly proliferating cells in the bone marrow. In contrast to external beam radiotherapy (EBRT), the exact absorbed dose limits for these critical organs (kidneys and bone marrow) in PRRT with [177Lu]Lu-DOTA-TATE are still unclear. Better insights into these uncertainties, can help in optimizing PRRT to reach its maximum therapeutic potential, while avoiding severe adverse events, like nephropathy and hematologic toxicities. In this review we focus on the nephrotoxic effects of PRRT with [177Lu]Lu-DOTA-TATE for the treatment of GEP-NETs. If the absorbed dose to the kidneys can be lowered, higher activities can be administered, enlarging the therapeutic window for PRRT. Therefore, we evaluated the renal protective potential of current and promising future strategies and discuss the importance of (renal) dosimetry in PRRT.

Prospective Within-Patient Assessment of the Impact of an Unlabeled Octreotide Pre-dose on the Biodistribution and Tumor Uptake of 68Ga DOTATOC as Assessed by Dynamic Whole-body PET in Patients with Neuroendocrine Tumors: Implications for Diagnosis and Therapy

PURPOSE

Gastroenteropancreatic neuroendocrine tumors (GEP NETs) are often associated with high expression of somatostatin receptors (SSTRs) which allows for PET/CT imaging with radiolabeled somatostatin analogs such as ⁶⁸Ga-DOTATOC. The interplay between ⁶⁸Ga-DOTATOC and the synthetic somatostatin analogs commonly used to manage patient symptoms may lead to competition between the labelled and unlabeled peptides for receptor binding sites and current product labelling recommends patients be taken off somatostatin analogs before imaging. In this study, we prospectively investigated in human patients the effect of a pre-dose of octreotide, a short-acting somatostatin analog, on the distribution of ⁶⁸Ga-DOTATOC in GEP NETs and normal organs.

PROCEDURE

Research participants with GEP NETs were studied on two occasions using dynamic whole-body ⁶⁸Ga-DOTATOC PET/CT. The two imaging studies were performed within 21 days of each other, using an identical acquisition protocol except for the administration of 50 μg of short-acting octreotide (pre-dose) immediately before the second PET/CT. Paired t-tests were used to compare tracer uptake with and without octreotide, for tumor and various normal organs.

RESULTS

Seven participants with a mean age of 53 ± 10 years were studied. Octreotide pre-dosing decreased radiotracer uptake in the normal liver and spleen by 25 % (p = 0.04) and 47 % (p = 0.05) respectively but did not significantly change uptake in tumor (p = 0.53), red marrow (p = 0.12), kidneys (p =0.57), or pituitary gland (p = 0.27).

CONCLUSIONS

Our data indicate SSTR imaging can be improved with a pre-dose of unlabeled octreotide given just prior to injection of the radiotracer. These data suggest there may be no need to discontinue somatostatin analog therapy prior to PET/CT with ⁶⁸Ga-DOTATOC, allowing for a simpler, less disruptive patient protocol. This approach warrants further study in a variety of settings.

Strategies Towards Improving Clinical Outcomes of Peptide Receptor Radionuclide Therapy

PURPOSE OF REVIEW

Peptide receptor radionuclide therapy (PRRT) with [177Lu-DOTA0,Tyr3] octreotate is an effective and safe second- or third-line treatment option for patients with low-grade advanced gastroenteropancreatic (GEP) neuroendocrine neoplasms (NEN). In this review, we will focus on possible extensions of the current use of PRRT and on new approaches which could further improve its treatment efficacy and safety.

RECENT FINDINGS

Promising results were published regarding PRRT in other NENs, including lung NENs or high-grade NENs, and applying PRRT as neoadjuvant or salvage therapy. Furthermore, a diversity of strategic approaches, including dosimetry, somatostatin receptor antagonists, somatostatin receptor upregulation, radiosensitization, different radionuclides, albumin binding, alternative renal protection, and liver-directed therapy in combination with PRRT, have the potential to improve the outcome of PRRT. Also, novel biomarkers are presented that could predict response to PRRT. Multiple preclinical and early clinical studies have shown encouraging potential to advance the clinical outcome of PRRT in NEN patients. However, at this moment, most of these strategies have not yet reached the clinical setting of randomized phase III trials.

Peptide Receptor Radionuclide Therapy - Prospects for Personalised Treatment

Peptide receptor radionuclide therapy is a type of molecular radiotherapy that has been used in the treatment of patients with neuroendocrine tumours for over two decades. It is not until recently, however, that it has achieved regulatory approval. The currently approved treatment regimen is a one-size-fits-all scheme, i.e. all patients receive a fixed activity of the radiopharmaceutical (¹⁷⁷Lu-DOTATATE) and a fixed number of treatment cycles. Several research groups around the world have studied different approaches of further improving on the results of peptide receptor radionuclide therapy, with many promising retrospective and prospective clinical studies having been published over the years. In this overview, we summarise some of the most promising strategies identified so far.